A GLIMPSE into the Nature of Galactic Mid-IR Excesses

We investigate the nature of the mid-IR excess for 31 intermediate-mass stars that exhibit an 8 μm excess in either the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire or the Mid-Course Space Experiment using high-resolution optical spectra to identify stars surrounded by warm circumstellar dust. From these data we determine projected stellar rotational velocities and estimate stellar effective temperatures for the sample. We estimate stellar ages from these temperatures, parallactic distances, and evolutionary models. Using MIPS [24] measurements and stellar parameters we determine the nature of the infrared excess for 19 GLIMPSE stars. We find that 15 stars exhibit Hα emission and four exhibit Hα absorption. Assuming that the mid-IR excesses arise in circumstellar disks, we use the Hα fluxes to model and estimate the relative contributions of dust and free-free emission. Six stars exhibit Hα fluxes that imply free-free emission can plausibly explain the infrared excess at [24]. These stars are candidate classical Be stars. Nine stars exhibit Hα emission, but their Hα fluxes are insufficient to explain the infrared excesses at [24], suggesting the presence of a circumstellar dust component. After the removal of the free-free component in these sources, we determine probable disk dust temperatures of T_disk≃ 300–800 K and fractional infrared luminosities of L_IR/L_*≃ 10⁻³. These nine stars may be pre-main-sequence stars with transitional disks undergoing disk clearing. Three of the four sources showing Hα absorption exhibit circumstellar disk temperatures ≃300-400 K, L_IR/L_*≃ 10⁻³, IR colors K − [ 24] < 3.3, and are warm debris disk candidates. One of the four Hα absorption sources has K − [ 24] > 3.3 implying an optically thick outer disk and is a transition disk candidate.