The Ultracompact H II Region G5.97−1.17: An Evaporating Circumstellar Disk in M8

     We present new high spatial resolution optical, infrared, and radio observations of G5.97-1.17, an ultracompact H II region 27 distant from the O7 star Herschel 36 in the center of M8, the Lagoon Nebula. Infrared imaging was obtained using the adaptive optics system ADONIS on the ESO 3.6 m telescope and a thermal-IR camera on the Hale 5 m; optical images taken with the Hubble Space Telescope Wide Field Planetary Camera 2 were extracted from the ST-ECF data archive; and radio continuum images were made using the VLA. The narrowband optical images show G5.97-1.17 to be a bow shock–shaped structure with its apex pointed toward Her 36, with the Hα flux distributed over 06 and consistent with the appearance at 2 cm. The optical continuum and near-infrared images show a star displaced from the peak of the bow shock by 0125 in the direction away from Her 36. This star exhibits a thermal-IR excess, indicating hot circumstellar dust, the presence of which can be reconciled with the optical visibility of the star if the dust is in a disklike (rather than spherical) distribution. Therefore, contrary to previous hypotheses that G5.97-1.17 is an ultracompact H II region intrinsically ionized by an embedded B0 star, our observations strongly suggest that it is actually a young star surrounded by a circumstellar disk that is being photoevaporated by Her 36, similar to the so-called proplyds seen near θ¹ Ori C in the Orion Nebula. At a distance of 1.8 kpc, this is the most distant known proplyd. These new observations confirm our previous claims based on earlier adaptive optics and deconvolved WF/PC images.