Vibrationally excited HCN transitions in circumstellar envelopes of carbon-rich AGB stars

Context. HCN is the most abundant molecule after H$_{2}$ and CO in the circumstellar envelopes (CSEs) of carbon-rich asymptotic giant branch (AGB) stars. Its rotational lines within vibrationally excited states are exceptional tracers of the innermost region of C-rich CSEs. Aims. We aim to constrain the physical conditions of CSEs of C-rich stars using thermal lines of the HCN molecule. Additionally, we also search for new HCN masers and probe the temporal variations for HCN masers, which should shed light on their pumping mechanisms. Methods. We observed 16 C-rich AGB stars in various HCN rotational transitions within the ground and 12 vibrationally excited states with the APEX 12 m submillimeter telescope. Results. We detect 68 vibrationally excited HCN lines from 13 C-rich stars, including 39 thermal transitions and 29 maser lines, which suggests that vibrationally excited HCN lines are ubiquitous in C-rich stars. Population diagrams constructed, for two objects from the sample, for thermal transitions from different vibrationally excited states give excitation temperature around 800-900 K, confirming that they arise from the hot innermost regions of CSEs (i.e., r10$^{8}$cm$^{-3}$ are required to excite the HCN masers. In some ways, HCN masers in C-rich stars might be regarded as an analogy of SiO masers in O-rich stars.
36 pages, 26 figures, accepted for publication in A&A