Submillimeter‐ and Millimeter‐Wavelength Observations of SiO and HCN in Circumstellar Envelopes of AGB Stars

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Abstract
We report molecular line observations with the Heinrich Hertz Submillimeter Telescope of a sample of 30 asymptotic giant branch stars with spectral types M, S, and C. Measured lines include SiO J = 5-4, 8-7, 10-9, and HCN J = 3-2 and 4-3 transitions, which arise from energy levels ranging from 26 to 115 K above ground. The observed transitions were detected in almost all stars observed, regardless of spectral type. The HCN J = 3-2 and 4-3 lines in the M stars are bright compared with previous observations of the J = 1-0 line. We calculated emergent spectra for model circumstellar envelopes and compare these with the observed line intensity ratios. We conclude that (1) the HCN line intensity ratios for the M stars are inconsistent with chemical models in which HCN is produced via photochemistry in the outer circumstellar envelope. HCN must be formed by a nonequilibrium chemical process in the inner envelope or extended stellar atmosphere. (2) The HCN/SiO intensity ratios of lines with similar excitation energies clearly separate the carbon stars from the M and S stars. The M and S stars show a trend of increasing HCN/SiO intensity ratios with increasing mass-loss rate. (3) These data support the idea that pulsation-driven shocks result in the formation of organic molecules like HCN in the envelopes of M stars. Observations of these molecules could give useful constraints on hydrodynamic models for stellar mass loss driven by pulsational shocks in the stellar atmosphere. We detected emission from vibrationally excited HCN in the v = (0,11c,0), J = 3-2 and 4-3 transitions toward four carbon stars. In one star, V Cyg, the lines are strong and narrow, and are most likely a result of maser amplification.