IRAS-23545+6508, A DISSOCIATING STAR - PROTOTYPE FOR A NEW OBSERVATIONAL CLASS

We present observations of a compact H I source coincident with a strong, far-infrared source, IRAS 23545 + 6508. We have followed up the original discovery of this object with observations in the lambda-6 cm radio continuum, optical, near-infrared, 3 mm CO, and 18 cm OH bands. The infrared source is coincident with a star and a weak 6 cm continuum source. The more extended H I source also peaks at this position. We take this coincidence to imply a physical relation among these various components. These objects are in turn found in a region of CO line emission which is about 30'-40' in diameter and agrees reasonably well in velocity with H I line emission. The CO emission clearly traces a molecular cloud, and around its periphery we have detected a shell of H I which we suspect is a dissociated exterior layer. We have also found a faint reflection nebula which follows a partial arc around the star. These results imply the following interpretation: a B3 or B4 star behind about 11 mag of visual obscuration at the near side of a molecular cloud is responsible for dissociating and heating a large region (1 pc x 0.5 pc) of gas and dust surrounding it, but creating only a very small (0.05 pc) H II region. The mass of atomic gas is about 1.4 M ., and the ratio of atomic to ionized mass is about 2200. The whole complex is probably at a distance of about 1 kpc. In contrast with O and earlier B stars ("ionizing stars") which generated substantial H II regions, we have called these objects "dissociating stars," since in the extreme case of a late B4 or B5 star, its surroundings would be almost entirely dissociated atomic gas, embedded in an environment of molecular gas. We think that this object is representative of a new observational class which traces stars in the range of spectral types from B1 to B5 which are embedded in molecular clouds. Using methods based on birthrate statistics and other data, we estimate that there are of order 10(3) such sources within a radius of 2 kpc of the Sun. A prominent H I filament appears at the same velocity as the molecular cloud. We discuss whether filaments of this type, common on high-resolution H I images, are cylindrical or sheetlike in three dimensions.