Detection of a Second, Strong Submillimeter HCN Laser Line toward Carbon Stars

We have searched for and found strong laser action at a frequency near 891 GHz in the J=10-9 transition between the (11¹0) and (04⁰0) vibrationally excited states of HCN toward the mass-losing carbon stars IRC +10216, CIT6, and Y CVn. This line is part of a Coriolis-coupled system that has been well studied in the laboratory during the early years of molecular laser spectroscopy. This system also includes the 805 GHz J=9-8 transition within the (04⁰0) state that was discovered toward IRC +10216 by Schilke, Mehringer, & Menten and that we also find to be lasing in CIT6. Toward both stars, the 891 GHz line is about an order of magnitude stronger than the 805 GHz line, and observations spaced about half a year apart provide clear evidence for temporal variability. As was concluded for the latter, given that the lines' lower energy levels are 4200 K over the ground state, they must arise from the innermost parts of the stars' circumstellar envelopes. Future high-resolution interferometric observations with the Atacama Large Millimeter Array of the HCN laser lines will yield important information on the dust formation zone of carbon stars.