A Submillimeter HCN Laser in IRC +10216

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Abstract
We report the detection of a strong submillimeter-wavelength HCN laser line at a frequency near 805 GHz toward the carbon star IRC +10216. This line, the J = 9-8 rotational transition within the (0400) vibrationally excited state, is one of a series of HCN laser lines that were first detected in the laboratory in the early days of laser spectroscopy. Since its lower energy level is 4200 K above the ground state, the laser emission must arise from the innermost part of IRC +10216's circumstellar envelope. To better characterize this environment, we observed other, thermally emitting, vibrationally excited HCN lines and found that they, like the laser line, arise in a region of temperature ≈1000 K that is located within the dust formation radius; this conclusion is supported by the line width of the laser. The (0400), J = 9-8 laser might be chemically pumped and may be the only known laser (or maser) that is excited both in the laboratory and in space by a similar mechanism.
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