Table of contents

Several investigations in the study of cosmological structure formation use numerical simulations in both two and three dimensions. In this paper we address the subtle question of ambiguities in the nature of two-dimensional gravity in an expanding background. We take a detailed and formal approach by deriving the equations describing gravity in (D+1) dimensions using the action principle of Einstein. We then consider the Newtonian limit of these equations and finally obtain the necessary fluid equations required to describe structure formation. These equations are solved for the density perturbation in both the linearized form and in the spherical top-hat model of nonlinear growth. We find that, when the special case of D=2 is considered, no structures can grow. We therefore conclude that, within the frame work of Einstein's theory of gravity in (2+1) dimensions, formation of structures cannot take place. Finally, we indicate the different possible ways of getting around this difficulty, so that growing structures can be obtained in two-dimensional cosmological gravitational simulations, and discuss their implications.

We study density cusps in the center of clusters of galaxies in order to reconcile X-ray mass estimates with gravitational lensing masses. For various mass-density models with cusps, we compute X-ray surface brightness distributions and fit them to observations to measure the range of parameters in the density models. The Einstein radii estimated from these density models are compared with Einstein radii derived from the observed arcs for Abell 2163, Abell 2218, and RX J1347.5-1145. The X-ray masses and lensing masses corresponding to these Einstein radii are also compared. While steeper cusps give smaller ratios of lensing mass to X-ray mass, the X-ray surface brightnesses estimated from flatter cusps are better fits to the observations. For Abell 2163 and Abell 2218, although the isothermal sphere with a finite core cannot produce giant arc images, a density model with a central cusp can produce a finite Einstein radius that is smaller than the observed radii. We find that a total mass-density profile that declines as ~r^-1.4 produces the largest radius in models that are consistent with the X-ray surface brightness profile. As a result, the extremely large ratio of the lensing mass to the X-ray mass is improved from 2.2 to 1.4 for Abell 2163 and from 3 to 2.4 for Abell 2218. For RX J1347.5-1145, which is a cooling flow cluster, we cannot reduce the mass discrepancy.

Gamma-ray bursts are now known to be a cosmological population of objects, which are often accompanied by X-ray and optical afterglows. The total energy emitted in the afterglow can be similar to the energy radiated in the gamma-ray burst itself. If a galaxy containing a large column density of dust is near the line of sight to a gamma-ray burst, small-angle scattering of the X-rays due to diffraction by the dust grains will give rise to an X-ray echo of the afterglow. A measurement of the angular size of the echo at a certain time after the afterglow is observed yields a combination of the angular diameter distances to the scattering galaxy and the gamma-ray burst that can be used to constrain cosmological models in the same way as a time delay in a gravitational lens. The scattering galaxy will generally cause gravitational lensing as well, and this should modify the shape of the X-ray echo from a circular ring. The main difficulty in detecting this phenomenon is the very low flux expected for the echo. The flux can be increased when the gamma-ray burst is highly magnified by gravitational lensing, or when the deflecting galaxy is at low redshift. X-ray echoes of continuous (but variable) sources, such as quasars, may also be detectable with high-resolution instruments and would allow similar measurements.

The empirical coupling relation in M_d=γM^1/2r between the halo dark mass distribution M_d and the visible mass M at radius r, previously demonstrated for spiral galaxies, is extrapolated to the case of a simple cluster core with mass distribution constrained by a lens mass model. In the at present unique case of MS 2137-2353, where the lens model density profile may be constrained from ~10 h₅₀^-1 kpc to ~100 h₅₀^-1 kpc because of the presence of a radial arc, the density profile deduced from the coupling relation is found to be consistent with that deduced from lens modeling. The coupling relation predicts very sharp cluster cores. In the case of a cluster such as Abell 2218, where a lens model cannot be tested in the innermost region, the density distribution deduced from the structural relation is more sharply peaked than an isothermal lens model, but the density distributions in the overlap region are consistent.

The results of a spectroscopic investigation of a complete sample of objects with K_s<20 mag in a 2' by 73 field at J005325+1234 are reported. Redshifts were successfully obtained for 163 of the 195 objects in the sample; these redshifts lie in the range [0.173, 1.44] and have a median of 0.58 (excluding 24 Galactic stars). The redshift identifications are believed to be almost complete for z<0.8. Approximately one-half of the galaxies lie in five narrow redshift features with local velocity dispersions of ~300 km s^-1. These narrow redshift "peaks" are primarily populated both by absorption-line galaxies and the most luminous galaxies in the sample, although the incidence of emission lines in the luminous galaxies increases with redshift. The estimated dynamical masses of these redshift peaks, and the sky distribution of the galaxies within them, appear similar to groups or poor clusters of galaxies in the local universe at various stages of virialization. Some groups of galaxies therefore form at epochs z>1.5, and the galaxies in such groups appear to be coeval and to show little sign of ongoing star formation. The galaxies outside the redshift peaks are also clustered, albeit more weakly, are less luminous and more frequently exhibit strong emission lines. These "isolated" galaxies therefore appear, on average, to form stars at later epochs than the strongly clustered galaxies. The galaxy spectral energy distributions (SEDs) derived from our UBVRIK photometry are also very closely correlated with the galaxy spectral types and luminosities. These results have strong implications for the analysis of redshift surveys at intermediate redshift. The sample is used to investigate the evolution of the combined galaxy luminosity function back to z=0.8. No significant change is found in the characteristic luminosity L*, and only weak color changes are detected, consistent with passive evolution. The blue galaxy-luminosity function is more dwarf rich than the red galaxy-luminosity function. No significant change in the comoving density is found in this sample out to z~1.4, assuming that the objects without redshifts (16% of the sample) are galaxies, essentially all of which have z>0.8. This suggests that mergers are not important among the objects in this sample. A population of extremely red objects with (R-K)>5 mag exists in the infrared-selected sample; all four such objects with redshifts are found to be absorption-line galaxies with z~1. Most of the very red objects therefore appear to be galaxies with z≳1 that are not heavily reddened by dust. A measure of the UV extinction at 2400 Å for the emission-line galaxies of a factor of 2 is obtained, implying only modest UV extinction in high-redshift star-forming galaxies.

The distance to NGC 4725 has been derived from Cepheid variables as part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale. Thirteen F555W (V) and four F814W (I) epochs of cosmic-ray-split Wide Field and Planetary Camera 2 observations were obtained. Twenty Cepheids were discovered, with periods ranging from 12 to 49 days. Adopting a Large Magellanic Cloud distance modulus and extinction of 18.50 ± 0.10 mag and E(V-I)=0.13 mag, respectively, a true reddening-corrected distance modulus (based on an analysis employing the ALLFRAME software package) of 30.50 ± 0.16 (random) ± 0.17(systematic) mag was determined for NGC 4725. The corresponding of distance of 12.6 ± 1.0 (random) ± 1.0 (systematic) Mpc is in excellent agreement with that found with an independent analysis based upon the DoPHOT photometry package. With a foreground reddening of only E(V-I)=0.02, the inferred intrinsic reddening of this field in NGC 4725, E(V-I)=0.19, makes it one of the most highly reddened fields encountered by the HST Key Project to date.

Hot interstellar gas in elliptical galaxies has two sources: mass lost from evolving stars and a much older component that accompanied galaxy formation or arrived subsequently by secondary cosmic infall toward the galaxy group containing the elliptical. We present here an approximate but comprehensive study of the dynamical evolution of the hot gas in massive elliptical galaxies born into a simple flat universe. Baryonic and dark matter are both conserved. We use NGC 4472 as a prototypical massive elliptical having a well-observed hot interstellar medium. We allow for star formation in a simple single burst using a Salpeter initial mass function but treat the gasdynamics in detail. The galaxy has a de Vaucouleurs stellar core and a Navarro-Frenk-White dark halo surrounded by inflowing cosmic matter. Using rather standard assumptions and parameters, we are able to successfully reproduce the gas-density and temperature distributions—n(r) and T(r)—in the hot interstellar gas determined from recent X-ray observations. Our model is sensitive to the baryon fraction of the universe, the Type II supernova energy released per unit stellar mass, and the time of galaxy formation. However, there is some degeneracy; as each of these parameters is varied, the effect on model fits to n(r) and T(r) is similar. Nevertheless, secondary inflow of cosmic gas is essential for successful fits to n(r) and T(r). Some gas is expelled from the stellar galactic core at early times when the Type II supernova energy is released. As a result, the present day baryonic fraction has a deep minimum in the outer galactic halo. Interstellar gas that cooled since the time of maximum star formation cannot all have collected at the galactic center but must be spatially dispersed; otherwise both gas temperatures and stellar dispersions in the galactic center would be larger than those observed. Finally, when relatively gas-rich, X-ray luminous models are spatially truncated at early times, simulating tidal events that may have occurred during galaxy group dynamics, the current locus of truncated models lies just along the L-X-ray size correlation among well-observed ellipticals. This is another striking confirmation of our model of elliptical evolution.

The nonlinear asymptotic theory of bar-driven spiral density waves, developed by Yuan & Kuo, is tested by numerical hydrodynamical calculations using a relaxation scheme proposed by Jin & Xin. The scheme is of second-order accuracy and uses a local relaxation approximation. We have shown that the asymptotic theory, which is based on the assumption of tightly wound spirals, is valid even for relatively open spirals. We present two numerical results, one for waves excited at the outer Lindblad resonance (tightly wound spirals) and one for waves excited at the inner Lindblad resonance (relatively open spirals). Both are in good agreement with the asymptotic theory.

Six BL Lac objects from the complete 1 Jy radio-selected sample of 34 objects were observed in cycle 5 with the Hubble Space Telescope (HST) Wide-Field Planetary Camera 2 (WFPC2) to an equivalent limiting flux of μ~26 mag arcsec^-2. Here we report results for the second half of this sample, as well as new results for the first three objects, discussed previously by Falomo and coworkers. In addition, we have analyzed in the same way HST images of three X-ray-selected BL Lac objects observed by Jannuzi and coworkers. The ensemble of nine BL Lac objects spans the redshift range from z=0.19 to ~1. Host galaxies are clearly detected in seven cases, while the other two, at z~0.258 (redshift highly uncertain) and z=0.997, are not resolved. The HST images make up a homogeneous data set with unprecedented morphological information between a few tenths of an arcsecond and several arcseconds from the nucleus, allowing us to rule out definitively a pure disk light profile in six of the seven detected host galaxies. The host galaxies are luminous ellipticals with an average absolute magnitude of M~-24.6 mag (with dispersion 0.7 mag), more than a magnitude brighter than L and comparable to the brightest cluster galaxies. The morphologies are generally smooth and have small ellipticities (≲0.2). Given such roundness, there is no obvious alignment with the more linear radio structures. In the six cases for which we have HST WFPC2 images in two filters, the derived color profiles show no strong spatial gradients and are as expected for K-corrected passively evolving elliptical galaxies. The host galaxies of the radio-selected and X-ray-selected BL Lac objects for this very limited sample are comparable in both morphology and luminosity.

We critically assess the role of the Blandford-Znajek mechanism in the powering of outflows from accretion-disk-fed black holes. We argue that there is no reason to suppose that the magnetic field threading the central spinning black hole differs significantly in strength from that threading the central regions of the disk. In this case, we show that the electromagnetic output from the inner disk regions is expected in general to dominate over that from the hole. Thus the spin (or not) of the hole is probably irrelevant to the expected electromagnetic power output from the system. We also point out that the strength of the poloidal field in the center of a standard accretion disk has been generally overestimated, and we discuss scenarios that might give rise to more significant central poloidal fields.

We introduce a simple and economical but effective method for including relativistic electron transport in multidimensional simulations of radio galaxies. The method is designed to follow explicitly diffusive acceleration at shocks, and, in smooth flows, second-order Fermi acceleration, plus adiabatic and synchrotron losses for electrons in the energy range responsible for radio emission in these objects. We are able to follow both the spatial and the energy (momentum) distributions of the electrons, so that direct synchrotron emission properties can be modeled in time-dependent simulated flows of this type for the first time. That feature is essential if simulations are to bridge successfully the fundamental physical gap between flow dynamics and observed emissions. As an initial step toward that goal, we present results from some axisymmetric MHD simulations of Mach 20 light jet flows. These explicitly explore the effects of shock acceleration, as well as adiabatic expansion and synchrotron aging in smooth flows. The simulations demonstrate the importance of the fact that even for steady inflows jet terminal shocks are not simple, steady plane structures. Most importantly, this should play a very major role in determining the properties of synchrotron emission within the terminal hot spot and in the lobes generated by the jet back flow. In fact, the outflows are inherently complex, because of the basic "driven" character of a jet flow. Consequently, the nonthermal electron population emerging from the jet may encounter a wide range of shock types and strengths, as well as of magnetic field environments. We may expect to find a complex range in synchrotron spectral and brightness patterns associated with terminal hot spots and lobes. These include the possibility of steep spectral gradients (of either sign) within hot spots, the potential in lobes for islands of flat-spectrum electrons within steeper spectral regions (or the reverse), and spectral gradients coming from the dynamical history of a given flow element rather than from synchrotron aging of the embedded electrons. Finally, synchrotron "aging" in the lobes tends to proceed more slowly than one would estimate from regions of high emissivity. This is a consequence of the fact that those regions are ordinarily places where the magnetic fields are the strongest, so that the instantaneous rates of energy loss are atypical of the full history of the electron population. This feature supports earlier suggestions that nonuniform field structures may help to explain why dynamical ages of FR II sources often seem to be greater than the apparent age of the electrons radiating in the lobes, as measured in terms of spectral steepening, or the absence thereof.

We present the combined results of optical polarization surveys of QSOs showing broad absorption lines (BALQSOs) conducted at the Steward and McDonald Observatories. The merged list of 53 objects provides the first statistical justification for claims of the tendency of BALQSOs to show stronger than average polarization, with a typical BALQSO being polarized a factor 2.4 times greater than a QSO from an optical survey selected without regard for absorption lines. Spectropolarimetry of sufficient quality to distinguish the polarization of emission lines versus absorption troughs versus continuum is also presented for six objects from the survey. When taken together with published data on other sources, the results lead to several significant correlations that can be used to gain insight into the structure of a BALQSO. These include: (1) a reduced degree of polarization in the broad emission lines, (2) enhanced polarization in the absorption troughs, and (3) an increase in the degree of polarization toward shorter wavelengths. In addition, BALQSOs with more prominent absorption systems tend to be more strongly polarized, and there is evidence that the subclass of low-ionization absorbers is more strongly polarized than other BALQSOs. If the increased polarization of BALQSOs is due to attenuation of direct, as opposed to scattered, lines of sight to the nucleus, absorption-line objects are underrepresented in optical surveys, and their true incidence is 20%-30% that of all UV-bright QSOs. Nevertheless, BALQSOs on average are not as highly polarized as their more obscured radio-quiet cousins, the IRAS QSOs/hyperluminous IR galaxies. We are led to a consistent picture in which broad absorption is observed in BALQSOs because they are inclined at intermediate inclinations, where our line of sight passes through gas clouds located near the surface of a dusty torus.

We present two ROSAT Position Sensitive Proportional Counter (PSPC) observations of the radio-loud, lobe-dominated quasar 3C 351, which shows an "ionized absorber" in its X-ray spectrum. The factor of 1.7 change in flux in the ~2 years between the observations allows a test of models for this ionized absorber.

The absorption feature at ~0.7 keV (quasar frame) is present in both spectra but with a lower optical depth when the source intensity—and hence the ionizing flux at the absorber—is higher, in accordance with a simple, single-zone, equilibrium photoionization model. Detailed modeling confirms this agreement quantitatively. The maximum response time of 2 years allows us to limit the gas density: n_e>2×10⁴ cm^-3; and the distance R of the ionized gas from the central source is less than 19 pc. This produces a strong test for a photoionized absorber in 3C 351: a factor of 2 flux change in ~1 week in this source must show nonequilibrium effects in the ionized absorber.

The 1.6 μm Near Infrared Camera and Multiobject Spectrometer image of the broad absorption line QSO PG 1700+518 clearly resolves the QSO host galaxy and a ringlike companion. The companion is most likely a ring galaxy produced in a collision with the QSO galaxy ~5×10⁷ yr ago. The morphology of the PG 1700+518 system is very similar to IRAS 04505-2958 (Boyce et al. 1996). Both objects were identified in a sample of eight QSOs selected by "warm" far-IR colors and extreme IR luminosities (Low et al. 1988). All eight QSOs show signs of strong interaction, and the presence of two head-on colliding systems in this sample may suggest that small impact parameters favor the energizing of IR-luminous QSOs.

Hubble Space Telescope Wide-Field Planetary Camera 2 imaging polarimetry of the hyperluminous infrared galaxy (and misdirected QSO) IRAS P09104+4109 reveals a highly polarized (p≈20%) giant (~5 kpc) bipolar reflection nebula centered on the nucleus. This, together with our previous detection of broad, polarized Mg II and newly detected broad, polarized Balmer emission lines in our ground-based spectropolarimetry, confirms that the lobes of the nebula are dominated by scattered light from the misdirected QSO and that the object would be indistinguishable from typical luminous QSOs if viewed from either pole. Comparison with previously published narrowband images in the light of [O III] λλ4959, 5007 and [O II] λ3727 shows that the northern lobe of the nebula is coincident with the ionization cone, thus the same light that impinges on the scattering material also ionizes the narrow-line gas. The biconical structure and high polarization suggest that the central UV continuum source is surrounded by a dusty torus of half-opening angle ~23° inclined ~37° with respect to our line of sight. The radio structure of this radio-intermediate object also indicates a long-lived axisymmetry to the central power source, but with a different axis relative to the scattering bicone. We propose that this difference in axes betrays the history of a cataclysmic event that altered the fundamental orientation of the central engine—the bicone defines the current axis of the system. Radio-emitting plasma is beginning to move outward along this new axis, depriving the old radio lobes of power. IRAS P09104+4109 is the first radio-quiet/intermediate object of QSO luminosity and hidden broad lines to show direct evidence that the axisymmetric torus inferred to exist in many Seyfert nuclei is also present in objects of high luminosity.

We present the first far-infrared (FIR) polarization results of the OVV quasar 3C 279 obtained with ISOPHOT for two epochs in 1996 and 1997. We describe its integral polarization properties at a wavelength of 170 μm, where the source shows a maximum in its energy distribution. After a γ-ray flare in 1996 January, a polarization of 23% closely aligned with the radio jet axis was measured in 1996 July. In 1997 June, the polarization degree had decreased to 6.5%, with a less good alignment. On the other hand, the total 170 μm flux is the same for both epochs. Our measurements provide additional constraints for the multiwavelength properties of synchrotron emission in radio jets and the temporal evolution of these properties: they show that the FIR radiation of 3C 279 is optically thin and that its origin is very close to the core. The variability of the FIR polarization without any change of the total FIR flux can be explained by a disordering of the magnetic field in between the core and the first stationary VLBI radio knot.

We present high spatial resolution (FWHM≈03-05) H (1.6 μm) and K' (2.1 μm) images of a complete sample (12) of ultraluminous infrared galaxies (ULIGs) chosen to have "warm" mid-infrared colors (f₂₅/f₆₀ > 0.2) known to be characteristic of active galaxies. The extended underlying galaxy is detected in each system at H and K', as are tidal features and many of the star-forming knots seen at optical wavelengths. While some of these knots have considerable near-infrared excesses, we show that they are likely to have bolometric luminosities more similar to those of the extended starbursts with similar optical morphology seen in less far-infrared-luminous interacting systems. We find that each ULIG has increasing contributions at long wavelengths by a very compact source, which we identify as an active galactic nucleus (AGN). We show that the optical/near-infrared colors of these putative nuclei are much more extreme than in the most active starburst IRAS galaxies, yet are identical to those in "far-infrared-loud" quasars, which are in turn similar to optical quasars combined with large quantities of hot (≈800 K) dust. Half of the ULIGs have nuclei with dereddened near-infrared luminosities comparable to those of QSOs, while the other half have dereddened luminosities more similar to Seyfert galaxies, although this may be an effect of patchy extinction and scattering.

Based on new Infrared Space Observatory midinfrared observations and ground-based H_α and near-infrared observations, we report the detection of a bright starburst in the intragroup medium (IGM) of the famous compact group of galaxies known as Stephan's Quintet (source A in Fig. 1). We demonstrate that this starburst is caused by a collision between a high-velocity (δV~1000 km s^-1) intruder galaxy (NGC 7318b) and the IGM of the group. While this is the only starburst known today that is induced by a galaxy/cold intergalactic medium collision, it provides new constraints to the theory for interaction-induced starbursts and may hint at a new mechanism for the star formation excess seen in more distant clusters.

In this paper we explore collisional ionization and time-evolving photoionization in the X-ray-discovered, ionized absorbers in Seyfert galaxies. These absorbers show temporal changes inconsistent with simple equilibrium models. We develop a simple code to follow the temporal evolution of nonequilibrium photoionized gas. As a result several effects appear that are easily observable and that, in fact, may explain otherwise paradoxical behavior. Specifically, we find the following:

1. In many important astrophysical conditions (O VII, O VIII dominant and high [≳10^22.5 cm^-2] column density) pure collisional and photoionization equilibria can be distinguished with moderate spectral resolution observations, because of a strong absorption structure between 1 and 3 keV. This feature is due mainly to iron L XVII-XIX and neon K IX-X absorption, which is much stronger in collisional models. This absorption structure may be misinterpreted as a flattening of the intrinsic emission spectrum above ~1 keV in low-resolution data.

2. In time-evolving nonequilibrium photoionization models the response of the ionization state of the gas to sudden changes of the ionizing continuum is smoothed and delayed at low gas densities (usually up to 10⁸ cm^-3), even when the luminosity increases. The recombination time can be much longer (up to orders of magnitude) than the photoionization timescale. Hence, a photoionized absorber subject to frequent, quick, and consistent changes of ionizing luminosity is likely to be overionized with respect to the equilibrium ionization state.

3. If the changes of the ionizing luminosity are not instantaneous, and the electron density is low enough (the limit depends on the average ionization state of the gas but is usually ~10⁷ to ~10⁸ cm^-3), the ionization state of the gas can continue to increase while the source luminosity decreases, so a maximum in the ionization state of a given element may occur during a minimum of the ionizing intensity (the opposite of the prediction of equilibrium models).

4. Different ions of different elements reach their equilibrium configuration on different timescales, so models in which all ions of all elements are in photoionization equilibrium so often fail to describe active galactic nucleus (AGN) spectral evolution.

These properties are similar to those seen in several ionized absorbers in AGNs, properties that had hitherto been puzzling. We applied these models to a high signal-to-noise ratio ROSAT PSPC observation of the Seyfert 1 galaxy NGC 4051. The compressed dynamical range of variation of the ionization parameter U and the ionization delays seen in the ROSAT observations of NGC 4051 may be simply explained by a nonequilibrium photoionization model, giving well-constrained parameters: n=(1.0)×10⁸ cm^-3 and R=(0.74)×10¹⁶ cm (~3 light-days).

We present Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) and ISOPHOT-S spectroscopy of the Seyfert galaxy NGC 4151. We detect a total of 17 fine-structure emission lines emitted by a wide range of low- and high-excitation ions, two rotational lines of molecular hydrogen, and the Brβ H I line. We find that the mid-IR fine-structure line profiles display blue asymmetries that are very similar to those observed in the optical lines produced in the narrow-line region. Because the mid-infrared lines are much less sensitive to extinction than the optical lines, this similarity places strong constraints on scenarios that have been invoked to explain the optical line asymmetries. For example, we are able to rule out the simplest radial motion plus dust scenarios for the production of the line asymmetries. Our preferred model is of a central, geometrically thin but optically thick, obscuring screen of subarcsecond extension, enclosing a total hydrogen gas mass of ≳5×10⁶M_☉. This mass may be molecular. The weakness of polycyclic aromatic hydrocarbon (PAH) emission features in the low-resolution spectrum is evidence that star formation plays a minor role in the circumnuclear region of NGC 4151. In a companion paper, we use the rich set of mid-infrared lines to determine the obscured photoionizing continuum produced by the active galactic nucleus.

The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC 4151, which are described in a companion paper, are used in conjunction with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow-line-emitting gas in the active galactic nucleus (AGN). We present a new method for determining the best-fitting photoionizing continuum and emission-line cloud model from a heterogeneous set of emission-line data. For NGC 4151, we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average, and that is consistent with the observed geometry of the optical narrow-line region (NLR). Our model consists of a clumpy, optically thick (ionization-bounded) gas distribution, with a hydrogen gas density of ~1000 cm^-3 and a volume-filling factor of 6.510^-4. Our best-fitting spectral energy distribution (SED) falls sharply beyond the Lyman limit and then rises sharply again toward 100 eV. In particular, it does not display the "big blue bump" signature of a hot accretion disk. We find that this SED, which best reproduces the NLR line emission on the 100-500 pc scale, does not have enough UV photons to produce the observed broad-line region (BLR) recombination emission from the inner 1 pc. This suggests that the BLR is photoionized by the intrinsic continuum source, which does have a strong UV component (perhaps a big blue bump), but that this UV component is absorbed by material located between the NLR and BLR. Our analysis suggests that the absorber consists of ~510¹⁹ cm^-2 of neutral hydrogen. Such an absorber has been independently detected by UV absorption lines. Using our new method, we confirm our previous conclusion that a big blue bump is present in the SED of the Seyfert 2 galaxy Circinus.

Spectropolarimetry of the 3.4 μm aliphatic C-H stretch feature, generally attributed to carbonaceous dust in the diffuse interstellar medium, has been carried out in the line of sight from the Galactic center source Sagittarius A IRS 7. The feature is unpolarized (Δp/Δτ<0.2): the upper limit for polarization is well below that expected on the basis of a model in which the carrier molecules are associated with the aligned silicate component of interstellar dust, for example, as an organic or carbonaceous mantle on a silicate core. The simplest explanation is that the 3.4 μm carrier resides in a population of small, nonpolarizing carbonaceous grains, physically separate from the silicates and sharing many characteristics with the carriers of the 217.5 nm extinction bump.

High-resolution VLA observations of 1720 MHz OH maser emission from Sagittarius A East and the circumnuclear disk with spatial and spectral resolutions of ≈25×13 and 0.27 km s^-1 are reported. This follow-up observational study focuses on the recent discovery of a number of such OH maser features and their intense, circularly polarized maser lines detected toward these Galactic center sources. The 1720 MHz maser line of OH arises from collisionally excited gas behind a C-type shock and is an important diagnostic of the interaction process that may occur between molecular clouds and associated X-ray-emitting shell-type supernova remnants. The present observations have confirmed that the observed Stokes V signal is due to Zeeman splitting, and the OH masers are angularly broadened by the scattering medium toward the Galactic center. The scale length of the magnetic field fluctuations in the scattering medium toward the Galactic center is estimated to be greater than 0.1-0.2 pc using the correlation of the position angles of the scatter-broadened maser spots. In addition, the kinematics of the maser spots associated with Sgr A East are used to place a 5 pc displacement between this extended radio structure and the Galactic center.

We present 2 μm narrowband images of the core H II region in the Galactic star-forming region Sagittarius D. The emission-line images are centered on 2.17 μm (Brγ) and 2.06 μm (He I). The H II region appears at the edge of a well-defined dark cloud, and the morphology suggests a blister geometry as pointed out in earlier radio continuum work. There is a deficit of stars in general in front of the associated dark cloud indicating the H II region is located in between the Galactic center and the Sun. The lesser spatial extent of the He I line emission relative to Brγ places the effective temperature of the ionizing radiation field below 40,000 K. The He I 2.06 μm to Brγ ratio and Brγ/far-infrared dust emission put T_eff at about 36,500-40,000 K as derived from ionization models.

The spectrum emitted by any astronomical plasma is sensitive to a variety of details, some of which may not be obviously important. This paper describes the sensitivity of the He I 2.06 μm line to the gas opacity at ionizing energies. The intensity of the line relative to a hydrogen line depends on the He⁺/H⁺ ratio, but also on the ratio of continuous to He I Lyα line opacity, since this determines whether the Lyα line can scatter often enough to be converted to the 2.06 μm line. The intensity of the infrared line relative to Hβ can change by factors of several depending on details of the radiative transfer of He I Lyα, the gas microturbulence, the dust-to-gas ratio, and the level of ionization.

We report deep near-infrared (HK) imaging observations of the dark cloud associated with IC 5146. With an order of magnitude greater sensitivity, we have imaged roughly half the region of the cloud originally surveyed by Lada and coworkers. Using measurements of ~2000 stars, we have employed techniques previously developed by Lada and coworkers to construct ordered, uniformly sampled maps of the extinction through this cloud. With the improved sensitivity, we detected approximately 5 times as many heavily extincted stars (i.e., A~20-50 mag) as found in the earlier survey of this same cloud area. Moreover, we were able to produce a Gaussian-smoothed extinction map of the cloud with an angular resolution (30'') somewhat more than a factor of 2 higher than achieved in the earlier study. With the increased sensitivity and angular resolution we were also able to measure the average radial column density profile orthogonal to the major axis of this filamentary cloud. Assuming cylindrical symmetry, we modeled this column density gradient and determined that the corresponding volume density profile of the cloud must smoothly fall off as r^-2.

To investigate the structure of the cloud on size scales smaller than the effective resolution of our maps, we constructed plots of the relation between the derived mean extinction and its measured dispersion for all the pixels in a series of maps made with varying angular resolution. We find, similar to Lada et al., that the dispersion increases linearly with mean A_V, independent of the angular resolution of our maps. However, although we quantitatively reproduce the earlier results at the same angular resolution (90''), we find the interesting result that the slope of the σ-A relation decreases in a systematic fashion with increasing angular resolution. We construct synthetic models of the cloud density distribution and use Monte Carlo techniques to produce artificial extinction maps and investigate the origin of the σ-A relations. These models show that both the observed form of the σ-A relation and its variation with angular resolution are the natural consequences of a smooth, radially decreasing volume density gradient in a cylindrically symmetric cloud. For a volume density gradient falling off as r^-2, the quantitative agreements between the model predictions and data are excellent. Apparently, these relations can be understood without the need for random fluctuations in the structure of the cloud on small spatial scales.

Using the VLA at 8.485 GHz, we have imaged the southern portion of the star-forming ridge of molecular gas in NGC 6334. The diffuse radio source G351.20+0.70, discovered by Moran et al. (1990), is now resolved into a roughly spherical shell of radius ~1' (0.5 pc). The distribution of molecular gas (traced by CO emission), of photodissociated gas (traced by [C II] 158 μm emission), and of ionized gas (traced by radio continuum emission), is precisely that expected for a photodissociation region—the ionized gas lies on the interior of the shell, the photodissociated gas just outside the ionized gas, and the molecular gas just outside the photodissociated gas.

We also detected faint radio counterparts to the strong infrared sources NGC 6334 IV IRS 20 and NGC 6334 V. If these objects are zero-age main-sequence stars, they produce far less radio free-free emission than would be expected for the observed infrared flux. Some possible explanations for this discrepancy are the following: (1) the radio free-free emission is optically thick, (2) the stellar ionizing radiation is obscured by dust, (3) the objects are not single OB stars but very compact clusters of later type stars, or (4) the objects are protostars. For both NGC 6334 V and NGC 6334 IV IRS 20, the radio spectrum for the unresolved sources is inconsistent with optically thick free-free emission or dust obscuration from a homogeneous H II region. The radio spectral index for NGC 6334 IV IRS 20 is consistent with the value of 0.6 expected for an optically thick H II region for a star undergoing mass loss, but that of NGC 6334 V is not. Because the IR sources in NGC 6334 V are very compact (≲0.02 pc), the stellar volume densities for a cluster of later-type stars would be unreasonably large. The objects in NGC 6334 V are probably protostars.

In order to derive the physical parameters of double-mode RR Lyrae stars in the Galactic globular clusters M15, M68, and IC4499, we fully utilize all observables (periods, apparent magnitudes, and colors) together with linear pulsation models. Except for the zero point of the color-temperature calibration and for the adopted metallicity of the double-mode stars in M15, our method is free from any systematic effects due to ambiguities in the various zero points (i.e., magnitude, bolometric correction, etc.). It is shown that for any "reasonable" chosen metallicity of M15, the so-derived absolute magnitudes are always brighter by some 0.2-0.3 mag than the corresponding Baade-Wesselink values. The difference becomes less than 0.1 mag only for extremely metal deficient compositions of [M/H]_M15 < -3.0. Masses are in the range 0.65-0.85 M, depending on the cluster and the above-mentioned parameters. These conclusions are essentially independent of the details of the chemical composition (e.g., enhancement of the heavy elements).

Numerical models of rotating neutron stars are constructed for four equations of state using the computer code RNS written by Stergioulas. For five selected values of the star's gravitational mass (in the interval between 1.0 and 1.8 solar masses) and for each equation of state, the star's angular momentum is varied from J=0 to the Keplerian limit J=J_max. For each neutron star configuration, we compute Q, the quadrupole moment of the mass distribution. We show that for given values of M and J, |Q| increases with the stiffness of the equation of state. For fixed mass and equation of state, the dependence on J is well reproduced with a simple quadratic fit, Q≃ -aJ²/Mc², where c is the speed of light and a is a parameter of order unity depending on the mass and the equation of state.

There are now about 50 known radio pulsars in binary systems, including at least five in double neutron star binaries. In some cases, the stellar masses can be directly determined from measurements of relativistic orbital effects. In others, only an indirect or statistical estimate of the masses is possible. We review the general problem of mass measurement in radio pulsar binaries and critically discuss all current estimates of the masses of radio pulsars and their companions. We find that significant constraints exist on the masses of 21 radio pulsars and on five neutron star companions of radio pulsars. All the measurements are consistent with a remarkably narrow underlying Gaussian mass distribution, m=1.35±0.04 M. There is no evidence that extensive mass accretion (Δm≳0.1 M) has occurred in these systems. We also show that the observed inclinations of millisecond pulsar binaries are consistent with a random distribution, and thus find no evidence for either alignment or counteralignment of millisecond pulsar magnetic fields.

We report the pulse-timing history of PSR B0540-69, based on all applicable data from the Ginga X-ray observatory. Using these data, together with other, overlapping optical and X-ray observations, we have determined an improved value for the braking index, n=2.080, as well as a new estimate for the celestial position of PSR B0540-69. Our results are based on an analysis of local pulse frequencies, using a methodology designed to efficiently utilize the sparsely sampled observations of PSR B0540-69. This strategy rests on iteratively determining a sequence of successively more precise pulse ephemerides from successively more precise estimates of local pulse frequencies. We also present improved X-ray and optical pulse profiles and evidence for a close alignment between profiles in these two energy regimes. We may have detected a small, Crab-like glitch in the pulse frequency of this pulsar in a reanalysis of previously published optical data.

The Be/X-ray binary transient pulsar EXO 2030+375 (P_s≈42 s) has been observed with the large-area detectors (LADs) of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO). Beginning in 1991 May, 22 outbursts were observed over 4 years. Thirteen outbursts between 1992 February and 1993 August occurred consecutively at intervals of ≈46 days, close to the orbital period determined by Parmar and colleagues using EXOSAT data. The pulse profiles from the BATSE data are double peaked and show no significant energy or luminosity dependence, unlike the EXOSAT observations of 1985 May-August. An exponential model was used to fit the observed hard X-ray energy spectra from the 13 consecutive outbursts. When EXOSAT discovered this pulsar during a giant outburst in 1985 May, the X-ray luminosity peaked at L_X=1.0×10³⁸ ergs s^-1 (1-20 keV), assuming a 5 kpc distance to the source. The BATSE outbursts are found to be weaker, 0.3×10³⁷ ≤ L_X(1-20 keV) ≤ 3.0×10³⁷ ergs s^-1 after extrapolating the observed flux (20-50 keV) to the EXOSAT energy band. Pulse phases derived from the 13 outbursts were fitted to two different models to determine a binary orbit. The new orbit is used to estimate 95% confidence limits for the mean peak spin frequency change during the outbursts observed with BATSE. This and the mean peak flux are compared to the spin-up rates and fluxes determined by EXOSAT from the 1985 giant outburst, where disk accretion was thought to have occurred. It is unclear whether these normal outbursts were driven by wind or disk accretion.

In this paper we provide a possible link between the structure of the bipolar nebula surrounding SN 1987A and the properties of its progenitor star. A wind-blown bubble (WBB) scenario is employed in which a fast, tenuous wind from a blue supergiant expands into a slow, dense wind expelled during an earlier red supergiant phase. The bipolar shape develops because of a pole-to-equator density contrast in the slow wind (i.e., the slow wind forms a slow torus). We use the wind compressed disk (WCD) model of Bjorkman & Cassinelli to determine the shape of the slow torus. In the WCD scenario, the shape of the torus is determined by the rotation of the progenitor star. We then use a self-similar semianalytical method for WBB evolution to determine the shape of the resulting bipolar nebula. We find that the union of wind-compressed disk and bipolar WBB models allows us to recover the salient properties of the circumstellar nebula of SN 1987A. In particular, the size, speed, and density of the inner ring of SN 1987A are easily reproduced in our calculations. An exploration of parameter space shows that the red supergiant progenitor must have been rotating at ≳0.3 of its breakup speed. We conclude that the progenitor was most likely spun up by a merger with a binary companion. Using a simple model for the binary merger, we find that the companion is likely to have had a mass of ≳0.5 M_☉.

We examine the processes that could lead to the observed enhancement of Li and possibly other light elements (Be, B) in the companions of a number of X-ray novae. We conclude that one of the most promising mechanisms is the spallation of CNO elements on the surface of the companion induced by the neutron flux produced in the hot accretion flow onto the compact object. Direct production of the observed Li and its deposition onto the dwarf companion seem less likely, mainly because of the possibility of its destruction in the production region itself and difficulties in its deposition associated with the configuration of the companion's magnetic field. We discuss other potential observables of the above scenario.

We model the UV/optical spectrum of the black hole binary Nova Muscae as a sum of blackbody emissions from the outer region of an accretion disk. We show for self-consistency that scattering effects in this region are not important. The black hole mass (M≈6 M), the inclination angle (μ≈0.5), and the distance to the source (D≈5 kpc) have been constrained by optical observations by Orosz and coworkers during quiescence. Using these values we find that the accretion rate during the peak was ≈8×10 g s and subsequently decayed exponentially. We define a radiative fraction (f) to be the ratio of the X-ray energy luminosity to the total gravitational power dissipated for a Keplerian accretion disk. We find that f≈0.1 and remains nearly constant during the ultrasoft and soft spectral states. Thus, for these states, the inner region of the accretion disk is advection-dominated; f probably increased to ≈0.5 during the hard state and finally decreased to ≈0.03 as the source returned to quiescence. We show that advective flow in the disk is optically thick because of high accretion rates during the outburst. This is in contrast to some theoretical models of advection-dominated disks that require optical thinness. We speculate that this optically thick advective disk could be the origin of the soft component if copious external cold photons are available. The soft component could also be due to a Keplerian nonadvective disk that terminates at around R≈30 Schwarzschild radius. However, in this case the inner advective flow has to be photon-starved. Theoretical models of inner hot accretion disks are generally parameterized in terms of the normalized accretion rate =/ , where is the Eddington accretion rate. Our results show that Nova Muscae was in the ultrasoft state when ≥50, in the soft state for 50>>2, and in the hard state for ≤2. Our results constrain present and future theoretical models for the inner regions of accretion disks around black holes. We highlight the need for multiwavelength observations of future black hole novae to confirm the results presented here.

Archival IUE spectra of U Sge have been used to study hot (~10⁵ K) circumstellar gas in this system. The observed spectra contain several UV resonance lines, of which the Si IV lines (λλ1394 and 1405) are the strongest. These lines are observed in absorption, so we must remove the photospheric absorption line contribution to the spectra in order to see the emission produced by the circumstellar gas. We have developed a robust method for creating such difference spectra from IUE data. In the observed spectra we see a variation in the line profiles and line centers with an orbital phase suggesting the presence of circumstellar gas. The residual emission seen in the difference spectra is strongest between phases ϕ=0.3 and ϕ=0.7, with a strength of up to 0.2 of the continuum flux. This is confirmed by the presence of an emission region visible in the Doppler tomogram of the λ1394 line. This is consistent with the location of an Hα "absorption zone" seen in tomograms of U Sge and U CrB, and with theoretical predictions of a high temperature accretion region.

We present new observations of the red supergiant VY CMa at 1.25, 1.65, 2.26, 3.08, and 4.8 μm. Two complementary observational techniques were utilized: nonredundant aperture masking on the 10 m Keck I telescope, yielding images of the innermost regions at unprecedented resolution, and adaptive optics imaging on the ESO 3.6 m telescope at La Silla, attaining an extremely high (~10⁵) peak-to-noise dynamic range over a wide field. For the first time the inner dust shell has been resolved in the near-infrared to reveal a one-sided extension of circumstellar emission within 01 (~15 R_*) of the star. The line-of-sight optical depths of the circumstellar dust shell at 1.65, 2.26, and 3.08 μm have been estimated to be 1.86±0.42, 0.85±0.20, and 0.44±0.11, respectively. These new results allow the bolometric luminosity of VY CMa to be estimated independent of the dust shell geometry, yielding L_* ≈ 2×10⁵L_☉. A variety of dust condensations, including a large scattering plume and a bow-shaped dust feature, were observed in the faint, extended nebula up to 4'' from the central source. While the origin of the nebulous plume remains uncertain, a geometrical model is developed assuming the plume is produced by radially driven dust grains forming at a rotating flow insertion point with a rotational period between 1200 and 4200 yr, which is perhaps the stellar rotational period or the orbital period of an unseen companion.

The gamma-ray burst (GRB) location algorithm used to produce the BATSE GRB locations is described. The general flow of control of the current location algorithm is presented, and the significant properties of the various physical inputs required are identified. The development of the burst location algorithm during the releases of the BATSE 1B, 2B, and 3B GRB catalogs is presented so that the reasons for the differences in the positions and error estimates between the catalogs can be understood. In particular, differences between the 2B and 3B locations are discussed for events that have moved significantly and the reasons for the changes explained. The locations of bursts located independently by the interplanetary network (IPN) are used to illustrate the effect on burst location accuracy of various components of the algorithm. IPN data and locations from other gamma-ray instruments are used to calculate estimates of the systematic errors on BATSE burst locations.

We present our NextGen Model Atmosphere grid for low-mass stars for effective temperatures larger than 3000 K. These LTE models are calculated with the same basic model assumptions and input physics as the VLMS part of the NextGen grid so that the complete grid can be used, e.g., for consistent stellar evolution calculations and for internally consistent analysis of cool star spectra. This grid is also the starting point for a large grid of detailed NLTE model atmospheres for dwarfs and giants. The models were calculated from 3000 to 10,000 K (in steps of 200 K) for 3.5 ≤ log g ≤ 5.5 (in steps of 0.5) and metallicities of -4.0 ≤ [M/H] ≤ 0.0.

We discuss the results of the model calculations and compare our results to the Kurucz grid. Some comparisons to standard stars like Vega and the Sun are presented and compared with detailed NLTE calculations.

An error analysis of the inverse photometric problem is performed. The uncertainties of the input parameters, which are the brightness measurements and the linear limb-darkening coefficients at the effective wavelength of the observations, are analytically propagated to compute the uncertainties of the solution parameters [size (r), latitude (β), inclination of rotation axis (i), and spot temperatures (T_s)] from the light curves of a spherical star with a cool circular spot. The uncertainties of the solution parameters are mostly caused by the uncertainties of the brightness. The error contribution from the uncertainty of the limb-darkening coefficients is negligible. According to the test models, with maculation wave of about 0.15 mag amplitude, for the light curves which are accurate ±0.005 mag in brightness and ±0.005 in linear limb-darkening coefficients, the uncertainty of the spot size is about the original spot size. The uncertainties of i and β are much bigger. Light curves with such uncertainties are not good at all for determining the inclinations and the latitudes because predicted uncertainties are even bigger than the allowed limits, that is, greater than ±90°. The spot temperatures, on the other hand, are estimated to be uncertain about ±500 K for the models which are assumed with a spot 1300 K cooler than photospheric temperature with T_eff=4820 K. In order to determine unique surface maps of spotted stars and the inclination of their rotation axis with a reasonable accuracy (≈±10°, highly accurate light curves (≈±0.0001 mag) are required. The uniqueness problem of the light curve modeling and its consequences are discussed.

We have modeled the injection and acceleration of pickup ions at the solar wind termination shock and investigated the parameters needed to produce the observed anomalous cosmic-ray (ACR) fluxes. A nonlinear Monte Carlo technique was employed that in effect solves the Boltzmann equation and is not restricted to near-isotropic particle distribution functions. This technique models the injection of thermal and pickup ions, the acceleration of these ions, and the determination of the shock structure under the influence of the accelerated ions. The essential effects of injection are treated in a mostly self-consistent manner, including effects from shock obliquity, cross-field diffusion, and pitch-angle scattering. Using recent determinations of pickup ion densities, we are able to match the absolute flux of hydrogen in the ACRs by assuming that pickup ion scattering mean free paths, at the termination shock, are much less than an AU and that modestly strong cross-field diffusion occurs. Simultaneously, we match the flux ratios He⁺/H⁺ or O⁺/H⁺ to within a factor ~5. If the conditions of strong scattering apply, no pre-termination-shock injection phase is required and the injection and acceleration of pickup ions at the termination shock are totally analogous to the injection and acceleration of ions at highly oblique interplanetary shocks recently observed by the Ulysses spacecraft. The fact that ACR fluxes can be modeled with standard shock assumptions suggests that the much discussed "injection problem" for highly oblique shocks stems from incomplete (either mathematical or computer) modeling of these shocks rather than from any actual difficulty shocks may have in injecting and accelerating thermal or quasi-thermal particles.

Motivated by observations of sunspot and active-region latitudes that suggest that the subsurface toroidal field in the Sun occurs in narrow latitude belts, we analyze the joint instability of solar latitudinal differential rotation and the concentrated toroidal field below the base of the convection zone, extending the work of Gilman & Fox (hereafter GF). We represent the profile of the toroidal field by Gaussian functions whose width is a variable parameter and solve the two-dimensional perturbation equations of GF by relaxation methods. We reproduce the results of GF for broad profiles, and we find instability for a wide range of amplitudes of differential rotation and toroidal fields (10³-10⁶ G fields at the base of the solar convection zone), as well as a wide range of toroidal-field bandwidths. We show that the combination of concentrated toroidal fields and solar-type latitudinal differential rotation is again unstable, not only to longitudinal wavenumber m=1 as in GF, but also to m>1 for sufficiently narrow toroidal-field profiles. For a fixed peak field strength, the growth rate first increases as the toroidal-field band is narrowed, reaching a peak for bandwidths between 10° and 20° in latitude, depending on the peak field strength, and then decreases to a cut-off in the instability for toroidal field bands of 3°-4°. Irrespective of bandwidth, the differential rotation is the primary energy source for the instability for weak fields, and the toroidal field is the primary source for strong fields. The weaker (stronger) the peak toroidal field is, the narrower (broader) is the bandwidth for which the toroidal field becomes the primary energy source. The Reynolds, Maxwell, and mixed stresses required to extract energy from the differential rotation and toroidal field are most active in the neighborhood of the singular or turning points of the perturbation equations. This first study focuses on toroidal fields that peak near 45° latitude, as in GF; later papers will place the toroidal-field peak at a wide variety of latitudes, as we might expect to occur at different phases of a sunspot cycle.

We describe an approach for finding the eigenfrequencies of solar acoustic modes (p-modes) in a convective envelope in the Wentzel-Kramers-Brillouin limit. This approximation restricts us to examining the effects of fluid motions that are large compared with the mode wavelength but allows us to treat the three-dimensional mode as a localized ray. The method of adiabatic switching is then used to investigate the frequency shifts resulting from simple perturbations to a polytropic model of the convection zone as well as from two basic models of a convective cell. We find that although solely depth-dependent perturbations can give frequency shifts that are first order in the strength of the perturbation, models of convective cells generate downward frequency shifts that are second order in the perturbation strength. These results may have implications for resolving the differences between eigenfrequencies derived from solar models and those found from helioseismic observations.

Observationally, there is a small fraction of solar white-light flares (WLFs), the so-called type II WLFs, showing an increased visible continuum but no significant Balmer jump and less strong chromospheric line emission in comparison with type I WLFs. The classical point of view, that the flare energy is initially released in the corona and then transported downward, can hardly explain WLFs of this kind. In this paper we explore the possibility that type II WLFs originate from a deeper layer. Assuming an in situ energy release, in particular in the form of high-energy particles, in a region around the temperature minimum, the continuum emission is computed in different time stages during the flare evolution. At first, nonthermal excitation and ionization of hydrogen atoms caused by bombarding particles result in a decline of the visible continuum. Later on, the lower atmosphere is gradually heated through radiative transfer, mitigating the continuum decline. In the final stage, when the particle bombardment stops while the atmosphere still keeps a heated state, we obtain a positive continuum contrast without an obvious Balmer jump. This meets the condition required for type II WLFs. The presence or absence of a continuum decline in the early stage of the flare provides a diagnostic tool for nonthermal processes in the lower atmosphere.

Ring diagram analysis of solar oscillation power spectra obtained from Michelson Doppler Imager data is carried out to study the velocity fields in the outer part of the solar convection zone. The three-dimensional power spectra are fitted to a model that has a Lorentzian profile in frequency and includes the advection of the wave front by horizontal flows in order to obtain the two components of the subsurface flows as a function of the horizontal wave number and radial order of the oscillation modes. This information is then inverted using the optimally localized averages method and regularized least squares method to infer the variation in horizontal flow velocity with depth. The average rotation velocity at different latitudes obtained by this technique agrees reasonably with helioseismic estimates made using frequency-splitting data. The shear layer just below the solar surface appears to consist of two parts, with the outer part measuring up to a depth of 4 Mm where the velocity gradient does not show any reversal up to a latitude of 60°. In the deeper part the velocity gradient shows reversal in sign around a latitude of 55°. The zonal flow velocities inferred in the outermost layers appear to be similar to those obtained by other measurements. A meridional flow from equator poleward is found. It has a maximum amplitude of about 30 m s^-1 near the surface, and the amplitude is nearly constant in the outer shear layer.

A method is proposed to solve for the linear axisymmetric oscillations of a general, axisymmetric, potential, magnetostatic sunspot equilibrium. The basic approach is to express the solution as a series of terms that are products of a prescribed radial planform times an unknown function of the vertical spatial coordinate and time. If the potential sunspot magnetic field is strictly uniform and aligned with the prevailing gravitational stratification, then a single term in the proposed series solution suffices, and the familiar problem first considered by Ferraro & Plumpton is readily recovered. For less trivial magnetic fields, which possess both vertical and radial gradients, the proposed series solution does not truncate after a finite number of terms, but the equations that determine the unknown functions of the vertical coordinate and time enjoy the advantage of being separable partial differential equations, which can be attacked through the solution of subordinate ordinary differential equations by the method of separation of variables. It is also demonstrated that the proposed series solution encompasses the thin flux tube expansion. Consequently, a rigorous mathematical basis is provided for this popular method employed to describe the dynamics of slender magnetic flux tubes, which proves useful in understanding the intrinsic astrophysical limitations of the approach. Whether this proposed method of solution is also a practical and efficient means to calculate the oscillation modes of axisymmetric sunspot equilibria is not answered here but will be addressed in a forthcoming companion paper.

We present observations of two coronal mass ejections (CMEs) observed by the LASCO and EIT instruments on board the Solar and Heliospheric Observatory. One was observed on 1997 April 30 and the other on 1997 February 23. The latter CME is accompanied by a spectacular prominence eruption and reaches velocities of about 900 km s^-1, while the former has no apparent accompanying prominence eruption and attains velocities of only about 300 km s^-1. However, the two CMEs are similar in appearance, having bright circular rims that can be interpreted as marking the apexes of expanding magnetic flux ropes, and both can be tracked from their origins near the surface of the Sun out to great distances. We compare the kinematic and morphological properties of these CMEs with an MHD model of an erupting flux rope and find that the CMEs can be successfully modeled in this manner.

This paper reports the identification of 14 lines in the solar coronal spectrum as high-excitation 2s²2p²3s-2s²2p²3p and 2s²2p²3p-2s²2p²3d transitions in Si VIII and S X (N I isoelectronic sequence). The analysis combines solar spectra, recorded by the SUMER (Solar Ultraviolet Measurement of Emitted Radiation) instrument on board the Solar and Heliospheric Observatory (SOHO) satellite, with laboratory data obtained with the beam-foil technique. Theoretical gA values and branching ratios for the observed lines, based on semiempirical calculations, are also presented. Intensity ratios between the newly identified high-excitation transitions and known forbidden transitions within the 2s²2p³ ground configuration can be used in determining coronal plasma temperatures in the 70-120 eV range.

Following a previous experimental study of the photostability of a series of polycyclic aromatic hydrocarbons (PAHs) this work extends to irregularly shaped PAHs, methyl-substituted PAHs, and related compounds. A photostability index R has been devised, which measures the propensity for H-loss of PAH monocations relative to that for regular catacondensed or pericondensed PAHs containing the same number of atoms. This index is also valid, to a good approximation, for neutral PAHs. Structural effects (presence of a methyl group; dihydro groups; H-H steric hindrance; degree of π conjugation; isomers) and size-dependent effects on R are demonstrated and discussed. Astrophysical applications of the propensity of PAH photodissociation include an estimation of the maximum amount of energy that can be deposited in the nascent parent PAH ion in H I regions, and an evaluation of the lower limit to the size of photostable PAHs in these regions, for various types of PAHs and their derivatives. The results provide a possible explanation for the existence of a limited number of PAHs in the ISM as required by the interstellar carbon budget determined by Snow & Witt. The information obtained is also relevant to interpretation of the presence and formation of PAHs and their methyl derivatives in meteoritic, micrometeoritic, cometary, and interplanetary particle materials.

The optical transient (OT) associated with GRB 980329 was remarkably red. It has previously been concluded that this was the result of dust extinction in the host galaxy (Taylor et al.; Reichart et al.; Palazzi et al.). However, an extinction model can only agree with the data if the I-band observations, taken about 0.8 days after the outburst, are discounted (Klose, Meusinger, & Lehmann; Reichart et al.); the flux density ratio between the I and the R band of a factor ~7 is too great to be explained by extinction, given the relatively blue K-I color. Here it is shown that the entire observed optical/infrared spectrum is consistent with that which is expected from an unextincted OT at z~5. At this redshift, the light in the observer's R band is strongly suppressed by absorption in the Lyα forest—an effect that has been seen clearly in galaxies in the Hubble Deep Field (Weymann et al.; Spinrad et al.). In spite of its potentially high redshift, GRB 980329 was an unusually bright burst. If GRB 980329 was indeed at z~5, and its gamma rays were radiated isotropically, the implied energy of the burst would be 5 × 10⁵⁴ ergs. Should GRB 980329 have a host galaxy, deep imaging could confirm or reject the conclusion that this burst was at z~5.

We have detected two high ionization phases of gas in the z~2.77 partial Lyman-limit system (LLS) toward QSO 1157+3143. We detect the first phase by C IV and Si IV absorption and the second phase—which is either warmer or undergoing larger random bulk motions than the first—via O VI absorption. Both phases of gas are present in similar column density ratios in each of the five velocity components, making it appear that this LLS is constructed of five very similar building blocks. We find that this system displays some of the properties expected of a hierarchical merging event, although published models may have trouble explaining the Si IV absorption we observe. When different ions show similar velocity structure, we commonly assume that they arise in the same gas, and we compare their column densities to derive the ionization and abundances. For this one absorption system, the different ions have similar velocity, but they do not arise in the same gas.

A model is constructed for the mass and dynamics of M87 and the Virgo Cluster. Existing surface photometry of the galaxy, mass estimates from X-ray observations of the hot intracluster gas, and the velocity dispersions of early-type Virgo galaxies are all used to constrain the run of dark matter density over 0 ≤ r ≲ 2 Mpc in the cluster. The "universal" halo advocated by Navarro, Frenk, & White, ρ_dm ∝ (r/r_s)^-1(1 + r/r_s)^-1, provides an excellent description of the combined data, as does a Hernquist profile with ρ_dm ∝ (r/r_s)^-1(1+r/r_s)^-3. These models are favored over isothermal spheres, and their central structure is preferred to density cusps either much stronger or much weaker than r^-1. The galaxies and gas in the cluster trace its total mass distribution (ρ_gal ∝ ρ_gas ∝ ρ_dm), the galaxies' velocity ellipsoid is close to isotropic, and the gas temperature follows the virial temperature profile of the dark halo. The virial radius and mass and the intracluster gas fraction of Virgo are evaluated.

If dark matter in the form of compact objects comprises a large fraction of the mass of the universe, then gravitational lensing effects on gamma-ray bursts are expected. We utilize BATSE and Ulysses data to search for lenses of different mass ranges, which cause lensing in the milli, pico, and femto regimes. Null results are used to set weak limits on the cosmological abundance of compact objects in mass ranges from 10^-16 to 10^-9M_☉. A stronger limit is found for a much-discussed Ω=0.15 universe dominated by black holes of mass ~10^6.5M_☉, which is ruled out at the ~90% confidence level.

We have recently discovered that the modified Newtonian dynamics (MOND) implies some universal upper bound on the acceleration that can be produced by a "dark halo," which, in a Newtonian analysis, is assumed to account for the effects of MOND. Not surprisingly, the limit is on the order of the acceleration constant of the theory. This can be contrasted directly with the results of structure-formation simulations. The new limit is substantial and different from earlier MOND acceleration limits (discussed in connection with the MOND explanation of the Freeman law for galaxy disks and the Fish law for elliptical galaxies): it pertains to the "halo" and not to the observed galaxy; it is absolute and independent of further physical assumptions on the nature of the galactic system; and it applies at all radii, whereas the other limits apply only to the mean acceleration in the system.

Two groups have recently discovered a statistically significant deviation in the fluxes of high-redshift Type Ia supernovae from the predictions of a Friedmann model with a zero cosmological constant. In this Letter, I argue that bright, dusty, starburst galaxies would preferentially eject a dust component with a shallower opacity curve (hence less reddening) and a higher opacity/mass than the observed galactic dust that is left behind. Such dust could cause the falloff in flux at high redshift without violating constraints on reddening or metallicity. The specific model presented is of needle-like dust, which is expected from the theory of crystal growth and has been detected in samples of interstellar dust. Carbon needles with conservative properties can supply the necessary opacity and would very likely be ejected from galaxies as required. The model is not subject to the arguments given in the literature against gray dust but may be constrained by future data from supernova searches done at higher redshift, in clusters, or over a larger frequency range.

A large sample of X-ray-emitting clusters of galaxies is considered in the context of Milgrom's modified Newtonian dynamics (MOND). The virial discrepancy (the ratio of the dynamical to observable mass) is reduced, on average, from a factor of 4 in the Newtonian analysis to a factor of 2 in the MOND analysis. The reason for this modest reduction is that the internal accelerations in the inner 1 Mpc of the sample clusters are not very small in terms of the critical MOND acceleration: a ≈ 0.5a₀. Although the uncertainty is large, there remains a discrepancy that is not yet accounted for by the directly observed matter in galaxies and diffuse gas. This may be consistent with the fact that modified dynamics cannot resolve the strong lensing discrepancy in those clusters where this phenomenon occurs. If MOND is correct, then the implication is that the budget of possibly detectable matter is not yet complete, particularly in the central regions of rich clusters.

We present sensitive, high-resolution Very Long Baseline Array observations of the central 03 of PKS 2322-123 at 1.3 and 5 GHz. These observations reveal straight and symmetric jets emerging from both sides of an inverted spectrum core. The 21 cm line of atomic hydrogen is detected in absorption against the core and eastern jet with substantial opacities, but it is not seen toward the equally strong western jet. Both a narrow (110 km s^-1 FWHM) and a very broad (735 km s^-1 FWHM) line are seen, although the very broad line is seen only against the core. Both lines are redshifted (~220±100 km s^-1) with respect to the systemic velocity. The most likely explanation for the observed H I kinematics are an atomic torus centered on the nucleus with considerable turbulence and inward streaming motions. The scale height of this torus is less than 20 pc. Although rare in flux-limited samples of compact radio sources, symmetric parsec-scale structure appears nearly ubiquitous among radio galaxies with H I absorption, probably because they are viewed more nearly edge-on through the torus.

Using the Rossi X-ray Timing Explorer, we have measured lags of the 9-33 keV photons relative to the 2-9 keV photons in the timing noise between 0.01 and 100 Hz in the accreting neutron stars 4U 0614+09 and 4U 1705-44. We performed similar measurements on the accreting black hole candidates Cyg X-1 and GX 339-4 as a comparison. During the observations, these sources were all in low (hard) states. We find phase lags of between 0.03 and 0.2 rad in all of these sources, with a variation in frequency much less than expected for a lag constant in time. We also measure a coherence consistent with unity in all sources. As already noted for the black hole candidates, these data are inconsistent with simple Comptonization models invoking a constant time delay. Comptonization in a nonuniform medium can perhaps explain the lags. However, the magnitudes of the lags imply that the hot electron gas extends to more than 10³ Schwarzschild radii. This may constitute an energy problem. We argue that while a large hot cloud is possible for black holes, which may hide some of their accretion energy in advection, such a distribution may not be possible for neutron stars, in which all the accretion energy is eventually released at the neutron star surface. This casts doubt on the Comptonization model, although the energy problem may be resolved, for example, by a wind from the inner disk.

We report on a Rossi X-Ray Timing Explorer observation of Aquila X-1 during its outburst in 1997 March in which, immediately following a type I burst, the broadband 2-10 keV flux decreased by about 10% and the kilohertz quasi-periodic oscillation (kHz QPO) frequency decreased from 813±3 to 776±4 Hz. This change in kHz QPO frequency is much larger than expected from a simple extrapolation of a frequency-flux correlation that was established in data before the burst. Meanwhile, a very low frequency noise component in the broadband fast Fourier transform power spectra, with a fractional rms amplitude of 1.2% before the burst, ceased to exist after the burst. All these changes were accompanied by a change in the energy spectral shape. If we characterize the energy spectra with a model composed of two blackbody (BB) components and a power-law component, almost all the decrease in flux was in the two BB components. We attribute the two BB components to the contributions from a region very near the neutron star, or even from the neutron star itself, and from the accretion disk, respectively.

We have discovered a 7.06 ± 0.08 Hz quasi-periodic oscillation (QPO) in the X-ray flux of the low-luminosity low-mass X-ray binary (LMXB) and atoll source 4U 1820-30. This QPO was only observable at the highest observed mass accretion rate, when the source was in the uppermost part of the banana branch, at a 2-25 keV luminosity of 5.4×10³⁷ ergs s^-1 (for a distance of 6.4 kpc). The QPO had an FWHM of only 0.5 ± 0.2 Hz during small time intervals (32 s of data) and showed erratic shifts in the centroid frequency between 5.5 and 8 Hz. The rms amplitude over the energy range 2-60 keV was 5.6% ± 0.2%. The amplitude increased with photon energy from 3.7% ± 0.5% between 2.8 and 5.3 keV to 7.3% ± 0.6% between 6.8 and 9.3 keV, above which it remained approximately constant at ~7%. The time lag of the QPO between 2.8-6.8 and 6.8-18.2 keV was consistent with being zero (-1.2 ± 3.4 ms). The properties of the QPO (i.e., its frequency and its presence only at the highest observed mass accretion rate) are similar to those of the 5-20 Hz QPO observed in the highest luminosity LMXBs (the Z sources) when they are accreting near the Eddington mass accretion limit. If this is indeed the same phenomenon, then models explaining the 5-20 Hz QPO in the Z sources, which require the near-Eddington accretion rates, will not hold. Assuming isotropic emission, the 2-25 keV luminosity of 4U 1820-30 at the time of the 7 Hz QPOs is at maximum at only 40% (for a companion star with cosmic abundances), but most likely at ~20% (for a helium companion star), of the Eddington accretion limit.

We report the discovery of strong aperiodic X-ray variability and quasi-periodic oscillation (QPO) in the X-ray light curves of a new X-ray nova, XTE J1550-564, and the evolution of the observed temporal properties during the rise of the recent X-ray outburst. The power spectral analysis of the first observation reveals strong aperiodic X-ray variability of the source (~28%) as well as the presence of a QPO at ~82 mHz with fractional rms amplitude ~14% over the 2-60 keV energy range. Also apparent is the first harmonic of the QPO with the amplitude ~9%. As the X-ray flux increases, the source tends to become less variable, and the QPO frequency increases rapidly, from 82 mHz to 4 Hz, over the flux (2-50 keV) range of 1.73 - 5.75 × 10^-8 ergs cm^-2 s^-1. The amplitude of the fundamental component of the QPO varies little, while that of the harmonic follows a decreasing trend. The fundamental component strengthens toward high energies, while its harmonic weakens. Initially, the power spectrum is roughly flat at low frequencies and turns into a power law at high frequencies, with the QPO harmonic sitting roughly at the break. In later observations, however, the high-frequency portion of the continuum can actually be better described by a broken power law (as opposed to a simple power law). This effect becomes more apparent at higher energies. The overall amplitude of the continuum shows a similar energy dependence to that of the fundamental component of the QPO. Strong rapid X-ray variability, as well as a hard energy spectrum, makes XTE J1550-564 a good black hole candidate. We compare its temporal properties with those of other black hole candidates.

We calculate radio-to-X-ray light curves for afterglows caused by nonthermal emission from a highly relativistic blast wave, which is inferred from the γ-ray flux detected in GRB 980425 and from the very bright radio emission detected in SN 1998bw. We find that the observed γ-ray and radio light curves are roughly reproduced by the synchrotron emission from a relativistic fireball. The optical flux predicted for the nonthermal emission is well below that of the thermal emission observed for SN 1998bw so that it will not be seen at least for a few years. The model predicts the X-ray flux just above the detection limit of BeppoSAX for the epoch when it was pointed to the field of GRB 980425. Therefore, the nondetection of X-ray and optical afterglows is consistent with the model. The models presented here are consistent with the physical association between SN 1998bw and GRB 980425 and lend further support to the idea that this object might correspond to an event similar to the "hypernova" or "collapsar"—events in which the collapse of a massive star forms a rotating black hole surrounded by a disk of the remnant stellar mantle.

The supernova SN 1978K has been noted for its lack of emission lines broader than a few thousand kilometers per second since its discovery in 1990. Modeling of the radio spectrum of the peculiar SN 1978K indicates the existence of H II absorption along the line of sight. To determine the nature of this absorbing region, we have obtained a high-dispersion spectrum of SN 1978K at the wavelength range 6530-6610 Å. The spectrum shows not only the moderately broad Hα emission of the supernova ejecta but also narrow nebular Hα and [N II] emission. The high [N II] λ6583/Hα ratio, 0.8-1.3, suggests that this radio-absorbing region is a stellar ejecta nebula. The expansion velocity and emission measure of the nebula are consistent with those seen in ejecta nebulae of luminous blue variables. Previous low-dispersion spectra have detected a strong [N II] λ5755 line, indicating an electron density of (3–12) × 10⁵ cm^-3. We argue that this stellar ejecta nebula is probably part of the preshock dense circumstellar envelope of SN 1978K. We further suggest that SN 1997ab may represent a young version of SN 1978K.

We present thermal infrared images of the unusual multiple stellar system HD 98800, which contains two double-star systems (A and B) separated by ≈08 in the north/south direction. This system may have formed in a recent burst of star formation in a very low mass cloud in the local solar neighborhood. Our images resolve the A and B components. Both A and B emit roughly equal fluxes at 4.71 μm, where radiation is primarily photospheric. The northern component (B) is about 3.7 times brighter than the southern component (A) at 9.78 μm where the radiation is due to thermal emission from dust, showing that 78% of the circumstellar dust in the system resides around component B. The two primary components are nearly identical K5 V pre-main-sequence stars, both with very significant amounts of dust emission. The dust system around the B component may have size, temperature, and possible structure analogous to those of a primitive zodiacal cloud.

Among the ~20 isolated neutron stars that emit in more than one energy band, only Geminga was radio quiet. Very recently, three groups at the Pushchino Radio Astronomy Observatory reported independently the detection of a weak pulsed radio emission from this source at ~100 MHz, bringing Geminga back into the standard group of radio-loud, isolated neutron stars. Here we report a series of observations of Geminga performed with the Arecibo radio telescope at various frequencies between 318 and 1400 MHz, performed from 1982 to 1984. No pulsed emission was detected. The high sensitivity of the Arecibo telescope allowed us to put a stringent upper limit to the spectral index in the radio band: α≤-3.6. If the 100 MHz pulsed emission is confirmed, Geminga will retain its peculiar nature of a radio pulsar with an extremely steep radio spectrum. This would severely constrain the models for the radio emission from isolated rotating neutron stars.

We report the discovery of five T Tauri star systems, two of which are resolved binaries, in the vicinity of the nearest known region of recent star formation: the TW Hydrae Association. The newly discovered systems display the same signatures of youth (namely high X-ray flux, large Li abundance, and strong chromospheric activity) and the same proper motion as the original five members. These similarities firmly establish the group as a bona fide T Tauri association, unique in its proximity to Earth and its complete isolation from any known molecular clouds. At an age of ~10 Myr and a distance of ~50 pc, the association members are excellent candidates for future studies of circumstellar disk dissipation and the formation of brown dwarfs and planets. Indeed, as an example, our speckle imaging revealed a faint, very likely companion 2'' north of CD -33° 7795 (TWA 5). Its color and brightness suggest a spectral type of ~M8.5 which, at an age of ~10⁷ yr, implies a mass ~20M_Jup.

We present the discovery of a candidate substellar object in a survey of young stars in the solar vicinity using the sensitivity and spatial resolution afforded by the NICMOS coronagraph on the Hubble Space Telescope. The H = 12.1 mag object was discovered approximately 2'' from the TW Hydrae association member CD -33°7795 (TWA 5), and the photometry implies a spectral type M8-M8.5, with a temperature of ~2600 K. We estimate that the probability of a chance alignment with a background object of this nature is less than 2 × 10^-5 and therefore postulate that the object (TWA 5B) is physically associated at a projected separation of 100 AU. Given the likely youth of the primary (~10 Myr), current brown dwarf cooling models predict a mass of ≈20M_Jup for TWA 5B.

Unidentified molecular emission bands first observed in optical spectra of the plasma tail of comet Halley (1P/1982 U1) have been discovered in pure ion tail spectra (Δλ~4700-6300 Å, λ/Δλ~1000-4400) of comet Hyakutake (C/1996 B2) and three other recent comets (Nakamura-Nishimura-Machholz P/1994 N1, Austin C/1989 X1, and Brorsen-Metcalf 32P/1989 N1), but not in comet Hale-Bopp (C/1995 O1). The strengths of the bands near 4940, 5310, and 6000 Å are correlated among the comets observed, indicative that the unidentified bands arise from a single molecular species. The bands have an asymmetric spatial distribution that extends in the antisolar direction, resembling molecular ions observed in comet plasma tails. Thus, the source of the unidentified bands is probably a single species of molecular ions. Of known laboratory molecular ion spectra, none can be attributed to the unidentified cometary emission bands. The absence of the unidentified ions in the tail of Hale-Bopp may be explained by a parent species of the unidentified ions that mimics the coma dynamics and photochemistry of H₂O more closely than CO. This is inferred from the large tail CO⁺/H₂O⁺ ratio observed in Hale-Bopp, which arises from the comet's large production rate that extends the ionopause and coma collision zone. Our observations provide evidence that the source region for the ions in plasma tails of comets lies external to the magnetic field-free ionopause.