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 2 ".17 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 H alpha flux distributed over O ".6 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 0".125 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 BO 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 theta(1) 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.