Observations of a High-mass Protostar in Ngc 7538 S

Abstract

We present high angular resolution continuum observations of the high-mass protostar NGC 7538 S with BIMA and CARMA at 3 and 1.4 mm, Very Large Array (VLA) observations at 1.3, 2, 3.5, and 6 cm, and archive Infrared Array Camera (IRAC) observations from the Spitzer Space Observatory, which detect the star at 4.5, 5.8, and 8 mu m. The star looks rather unremarkable in the mid-IR. The excellent positional agreement of the IRAC source with the VLA free-free emission, the OH, CH3OH, H2O masers, and the dust continuum confirms that this is the most luminous object in the NGC 7538 S core. The continuum emission at millimeter wavelengths is dominated by dust emission from the dense cold cloud core surrounding the protostar. Including all array configurations, the emission is dominated by an elliptical source with a size of similar to 8 '' x 3 ''. If we filter out the extended emission we find three compact millimeter sources inside the elliptical core. The strongest one, S-A, coincides with the VLA/IRAC source and resolves into a double source at 1.4 mm, where we have subarcsecond resolution. The measured spectral index, a, between 3 and 1.4 mm is similar to 2.3, and steeper at longer wavelengths, suggesting a low dust emissivity or that the dust is optically thick. We argue that the dust in these accretion disks is optically thick and estimate a mass of an accretion disk or infalling envelope surrounding S-A to be similar to 60 M-circle dot.