ALMA Observations of Orion Source I at 350 and 660 GHz

TL;DR

This study uses ALMA observations at 350 and 660 GHz to analyze Orion Source I, revealing a nearly edge-on disk, molecular outflows, and rotation consistent with a 5-7 solar mass protostar, challenging previous ejection models.

Contribution

First high-resolution ALMA imaging of Orion Source I at these frequencies, providing detailed disk structure, molecular composition, and kinematics that inform star formation theories.

Findings

Detected a ~100 AU edge-on disk around SrcI.

Observed molecular lines indicating outflows and rotation.

Disputed ejection scenario based on mass and disk presence.

Abstract

Orion Source I (`SrcI') is the protostar at the center of the Kleinmann-Low Nebula. ALMA observations of SrcI with 0.2" angular resolution were made at 350 and 660 GHz to search for the H26alpha and H21alpha hydrogen recombination lines and to measure the continuum flux densities. The recombination lines were not detected, ruling out the possibility that SrcI is a hypercompact HII region. The deconvolved size of the continuum source is approximately 0.23 x 0.07" (100 x 30 AU); it is interpreted as a disk viewed almost edge-on. 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 to 0.2 Mo. A rich spectrum of molecular lines is detected, mostly from sulfur- and silicon-rich molecules like SO, SO2, 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 difficult to reconcile with the model in which SrcI and the nearby Becklin-Neugebauer Object were ejected from a multiple system 500 years ago.