Very compact radio emission from high-mass protostars. I. CRL 2136: Continuum and water maser observations

TL;DR

This study presents radio observations of the high-mass protostar CRL 2136, revealing compact emission likely from a jet and water masers originating close to the star, suggesting a different maser excitation mechanism than typical outflows.

Contribution

First detailed radio continuum and water maser observations of CRL 2136, indicating a potential jet origin and a unique maser excitation environment close to the protostar.

Findings

Detected optically thick continuum emission up to 22 GHz

Water masers are spatially coincident with the continuum emission

Maser emission likely arises from infalling gas near the protostar

Abstract

We report 5-43 GHz radio observations of the CRL 2136 region at 0.6-6 arcsec resolution. We detect weak (mJy intensity) radio emission from the deeply embedded high-mass protostar IRS 1, which has an optically thick spectrum up to frequencies of 22 GHz, flattening at higher frequencies, which might be explained by emission from a jet. Water maser mapping shows that the strong emission observed redshifted relative to the systemic velocity is spatially coincident with the optically thick continuum emission. The H2O maser emission from this object (and others we know of) seems to have a different origin than most of these masers, which are frequently tracing bipolar high-velocity outflows. Instead, the CRL 2136 H2O emission arises in the close circumstellar environment of the protostar (within 1000 AU). We speculate that most of it is excited in the hot, dense infalling gas after the accretion shock, although this cannot explain all the H2O emission. An accretion shock nature for the continuum emission seems unlikely.