The pumping mechanisms for astrophysical masers are often complex, involving infrared line opacities, collisions, and infrared line overlaps. Hence, the interpretation of the observed emission requires detailed modelling. In spite of these potential problems the masering lines can, however, give useful information on the physical conditions of the regions where the masers are produced, i.e., the innermost zones of circumstellar envelopes.
While SiO, OH, and H2O masers are a common feature of O-rich stars and were detected more than forty years ago, strong maser emission in C-rich stars has been detected only in some vibrational lines of HCN, and in some high-J lines of the ground state of SiS.
In the line survey of IRC+10216 with HIFI we have discovered several lasers involving the pair of vibrational levels 4v2/v2+v1. The strongest ones are above 1 THz and carry information of the 1-3 stellar radii zone. Several masers have been also found in some lines of the v2 bending mode between 500 and 900 GHz.
We would like to carry a systematic search for HCN masers in C-rich evolved stars by observing selected frequency ranges covering from the v1 to the 3v2 vibrational states (typically a range of 15-20 GHz) for each rotational line within the SIS receivers of HIFI (from J=6-5 up to J=13-12). The pummping mechanism can be different in each object depending on the dust opacity. High mass loss rate stars will have their 3 micron emission absorbed near the star by the large amount of dust produced in these objects. The pumping of HCN will be done mainly through the bending mode at 14 microns. However, in low mass loss rate the 3 micron photons will populate the v1, v3 and combination bands allowing a much complex pumping pattern.
This study will permit to characterize HCN laser emission in C-rich stars and their pu...mping mechanisms. It will also complement our recently granted high angular resolution observations of HCN with ALMA in cycle 0.
Publication
Instrument
HIFI_HifiFS_freq
Temporal Coverage
2012-05-03T20:23:28Z/2012-11-30T22:48:27Z
Version
SPG v14.2.0
Mission Description
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.
European Space Agency, 2013, A Search For Far-Infrared Hcn Lasers In C-Rich Evolved Stars, SPG v14.2.0, European Space Agency, https://doi.org/10.5270/esa-rawtllj
