New sub-millimetre HCN lasers in carbon-rich evolved stars

Strong laser emission from hydrogen cyanide (HCN) at 805 and 891 GHz has been discovered towards carbon-rich (C-rich) asymptotic giant branch (AGB) stars. Both lines belong to the Coriolis-coupled system between the (1,1^ 1e,0) and (0,4^0,0) vibrational states, which has been extensively studied in early molecular spectroscopy in the laboratory. However, the other lines in this system with frequencies above ∼900 GHz, which are challenging to observe with ground-based telescopes, have remained unexplored in astronomical contexts. We aim to (1) search for new HCN transitions that show laser activity in the (0,4^0,0), J=10-9 line at 894 GHz, the (1,1^ 1e,0)--(0,4^0,0), J=11-10 line at 964 GHz, the (1,1^ 1e,0), J=11-10 at 968 GHz, and the (1,1^ 1e,0), J=12-11 line at 1055 GHz towards C-rich AGB stars; (2) study the variability of multiple HCN laser lines, including the two known lasers at 805 and 891 GHz; and (3) construct a complete excitation scenario to the Coriolis-coupled system. We conducted SOFIA/4GREAT observations and combined our data with Herschel /HIFI archival data to construct a sample of eight C-rich AGB stars, covering six HCN transitions (i.e. the 805, 891, 894, 964, 968, and 1055 GHz lines) in the Coriolis-coupled system. We report the discovery of HCN lasers at 964, 968, and 1055 GHz towards C-rich AGB stars. Laser emission in the 805, 891, and 964 GHz HCN lines was detected in seven C-rich stars, while the 968 GHz laser was detected in six stars and the 1055 GHz laser in five stars. Notably, the 894 GHz line emission was not detected in any of the targets. Among the detected lasers, the emission of the cross-ladder line at 891 GHz is always the strongest, with typical luminosities of a few 10^44 photons s^-1. The cross vibrational state 964 GHz laser emission, which is like a twin of the 891 GHz line, is the second strongest. The 1055 GHz laser emission always has a stronger 968 GHz counterpart. Towards IRC+10216, all five HCN laser transitions were observed in six to eight epochs and exhibited significant variations in line profiles and intensities. The 891 and 964 GHz lines exhibit similar variations, and their intensity changes do not follow the near-infrared light curve (i.e. they have non-periodic variations). In contrast, the variations in the 805, 968, and 1055 GHz lines appear to be quasi-periodic, with a phase lag of 0.1 -- 0.2 relative to the near-infrared light curve. A comparative analysis indicates that these HCN lasers may be seen as analogues to vibrationally excited SiO and H_2O masers in oxygen-rich stars. We suggest that chemical pumping and radiative pumping could play an important role in the production of the cross-ladder HCN lasers, while the quasi-periodic behaviour of the rotational HCN laser lines may be modulated by additional collisional and radiative pumping driven by periodic shocks and variations in infrared luminosity.