2022A&A...666A..69J

Context. The most abundant molecule after H₂ and CO in the circumstellar envelopes (CSEs) of carbon-rich asymptotic giant branch (AGB) stars is HCN. Its rotational lines within vibrationally excited states are exceptional tracers of the innermost region of carbon-rich CSEs.
Aims. We aim to constrain the physical conditions of CSEs of carbon-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 carbon-rich AGB stars in various HCN rotational transitions, within the ground and 12 vibrationally excited states, with the Atacama Pathfinder Experiment (APEX) 12-metre sub-millimetre telescope.
Results. We detect 68 vibrationally excited HCN lines from 13 carbon-rich stars, including 39 thermal transitions and 29 maser lines, suggesting that vibrationally excited HCN lines are ubiquitous in carbon-rich stars. Population diagrams constructed for two objects from the sample, for thermal transitions from different vibrationally excited states, give excitation temperatures around 800-900 K, confirming that they arise from the hot innermost regions of CSEs (i.e. r <20 R_*). Among the detected masers, 23 are newly detected, and the results expand the total number of known HCN masers lines towards carbon-rich stars by 47%. In particular, the J = 2-1 (0, 3^le, 0), J = 3-2 (0, 2, 0), and J = 4-3 (0, 1^1f, 0) masers are detected in an astronomical source for the first time. Our observations confirm temporal variations of the 2-1 (0, 1^1e, 0) maser on a timescale of a few years. Our analysis of the data suggests that all detected HCN masers are unsaturated. A gas kinetic temperature of ≳700 K and an H₂ number density of >10⁸ cm^–3 are required to excite the HCN masers. In some ways, HCN masers in carbon-rich stars might be regarded as an analogy of SiO masers in oxygen-rich stars.