ALMA Observations of Orion Source I at 350 and 660 GHz

R. L. Plambeck, M. C. H. Wright

Published 2016-08-02Version 1

Orion Source I ('SrcI') is the protostar at the center of the Kleinmann-Low Nebula. ALMA observations of SrcI at 350 and 660 GHz failed to detect the H26$\alpha$ and H21$\alpha$ recombination lines, ruling out the possibility that SrcI is a hypercompact HII region. The deconvolved size of the continuum source is approximately 0.23 x 0.05" (100 x 20 AU); it is interpreted as a disk viewed almost edge-on. The continuum flux density is proportional to $\nu^2$ from 43 GHz to 350 GHz, then increases slightly faster than $\nu^2$ from 350 GHz to 660 GHz. Optically thick thermal emission from ~500 K dust is the most plausible source of the continuum, even at frequencies as low as 43 GHz; the disk mass is most likely in the range 0.02-0.2 Mo. A rich spectrum of molecular lines is detected, mostly from sulfur- and silicon-rich molecules like SO, SO$_2$, and SiS, but also including vibrationally excited CO and several unidentified transitions. Lines with upper energy levels E$_U$ > 500 K appear in emission and are symmetric about the source's LSR velocity of 5 km/sec, while lines with E$_U$ < 500 K appear as blueshifted absorption features against the continuum, indicating that they originate in outflowing gas. The emission lines exhibit a velocity gradient along the major axis of the disk that is consistent with rotation around a 5-7 Mo central object. The relatively low mass of SrcI and the existence of a 100 AU disk around it are incompatible with the model in which SrcI and the nearby Becklin-Neugebauer Object were ejected from a multiple system 500 years ago.