High Resolution VLA Observations of the Orion SiO Maser and Its Exciting Star (IRc~2?)

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Session 96 -- YSOs and Masers
Oral presentation, Wednesday, 11, 1995, 2:00pm - 3:30pm

[96.02] High Resolution VLA Observations of the Orion SiO Maser and Its Exciting Star (IRc~2?)

Karl M. Menten (CfA), Mark J. Reid (CfA)

While hundreds of mass-losing evolved stars have SiO maser emission arising from the innermost regions of their circumstellar envelopes, only three star-forming regions are known to contain SiO maser sources. One of these is the Orion BN/KL region, where the SiO maser emission arises from the proximity of one of the most prominent infrared sources, IRc~2, possibly from the circumstellar disk surrounding a massive young stellar object as suggested by Plambeck et al. (1990, ApJ, 348, L65). Luminosities exceeding $10^4 L_{\odot}$ are required for the excitation of SiO masers. It is thus interesting to investigate the relationship of the Orion SiO maser to its environment, since its exciting source is likely to be one of the dominant energy sources in the region.

In order to study the small scale structure of the SiO emission and to determine its location relative to the compact radio continuum source associated with IRc~2, we have used the VLA for simultaneous observations of the 7 mm radio continuum emission and the $J = 1\to 0$ rotational transition within the $v= 1$ vibrationally excited state of SiO with a spatial resolution of $0{\rlap.}{''}3$. By using the strong maser line as a phase reference, we are able to self-calibrate the continuum emission from the compact radio source and determine its position relative to the SiO maser emission to within a few milli-arcseconds. We find the continuum emission to be located in between the two regions containing strong SiO emission, which are separated by $\approx 0{\rlap.}{''}15$. This very strongly suggests that the object giving rise to the radio emission is also powering the SiO masers.

Previous 1.3 cm VLA observations with $0{\rlap.}{''}1$ resolution indicate that the radio continuum source is elongated along the line connecting the SiO emission regions. This may indicate that the high SiO abundance necessary to produce the maser emission is created in the shocked interaction region of a stellar jet with ambient circumstellar material. We also find a significant offset of the SiO/radio source from the infrared position(s) of IRc~2.

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