Distribution and excitation of thermal methanol in 6.7 GHz maser bearing star-forming regions

Abstract

Context. Candidate high-mass star-forming regions can be identified through the occurrence of 6.7 GHz methanol masers. In these sources the methanol abundance of the gas must be enhanced, because the masers require a considerable methanol path length. The place and time of origin of this enhancement is not well known. Similarly, it is debated in which of the physical components of the high-mass star-forming region the masers are located. Aims. The aim of this study is to investigate the distribution and excitation of the methanol gas around Cep A and to describe the physical conditions of the region. In addition the large-scale abundance distribution is determined to understand the morphology and kinematics of star-forming regions in which methanol masers occur. Methods. The spatial distribution of methanol is studied by mapping the line emission, as well as the column density and excitation temperature, which are estimated using rotation diagrams. For a limited number of positions the parameters are checked with non-LTE models. Furthermore, the distribution of the methanol abundance is derived in comparison with archival dust continuum maps. Results. Methanol is detected over a 0.3 × 0.15 pc area centred on the Cep A HW2 source and shows an outflow signature. Most of the gas can be characterized by a moderately warm rotation temperature (30−60 K). At the central position two velocity components are detected with different excitation characteristics, the first related to the large-scale outflow. The second component, uniquely detected at the central location, is probably associated with the maser emission on much smaller scales of 2 . A detailed analysis reveals that the highest densities and temperatures occur for these inner components. In the inner region the dust and gas are shown to have different physical parameters. Conclusions. Abundances of methanol in the range 10 −9 −10 −7 are inferred, with the abundance peaking at the maser position. The geometry of the large-scale methanol is in accordance with previous determinations of the Cep A geometry, in particular those from methanol masers. The dynamical and chemical time-scales are consistent with a scenario where the methanol originates in a single driving source associated with the HW2 object and the masers in its equatorial region.

Radiative Transfer and Molecular Data for Astrochemistry

“…The coverage of the first torsionally excited state also enables to constrain the intensity of the far-infrared radiation field within astrophysical sources. Torsionally excited emission has already been observed in some objects (e.g., Torstensson et al 2011) and will become routine with ALMA.…”

“…The advantage of this improved method (sometimes called the 'population diagram') is that it also provides an estimate of the size of the emitting region. An example of combining the two methods is the analysis of CH 3 OH line images of the Cep A region, where classic rotation diagrams work well for the diffuse extended gas, while optical depth correction are needed close to the central protostar (Torstensson et al 2011).…”

Masers in star forming regions

“…Such ring-like characteristics were also seen in water masers associated with slightly more advanced stages, where masers were likely tracing an accretion disc or its remnant (Motogi et al 2011a). Torstensson et al (2011b) analysed some of these ring-like maser sources using thermal emission at arcsec scale and found that mostly the distribution of the methanol gas peaks at the maser position with the larger scale gas showing a modest outflow velocity. They argued that the methanol gas has a single origin in these sources, possibly associated with an accretion shock.…”

Tracing Embedded Stellar Populations in Clusters and Galaxies Using Molecular Emission: Methanol as a Signature of the Low-Mass End of the Imf

“…These transitions are not excited in the cloud where mostly low-J transitions emit (E up /k 10 B~K ), and they are only moderately excited in hot cores where emission from higher-J transitions is more dominant (E up / k 100 B  K). The J = 7 K -6 K transitions of methanol around 338 GHz are particularly well suited, both because they are good outflow tracers and because instruments exist to map emission efficiently (e.g., Kristensen et al 2010;Torstensson et al 2011).…”

“…The J=7 K -6 K transitions of methanol around 338 GHz are particularly well suited, both because they are good outflow tracers, and because instruments exist to map emission efficiently (e.g. Kristensen et al 2010, Torstensson et al 2011.…”