CIRCUMSTELLAR ION-MOLECULE CHEMISTRY: OBSERVATIONS OF HCO{sup +} IN THE ENVELOPES OF O-RICH STARS AND IRC+10216
- Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States)
Millimeter-wave observations of HCO{sup +} have been conducted toward oxygen-rich circumstellar envelopes, as well as IRC+10216, using the facilities of the Arizona Radio Observatory (ARO). The J = 1 {yields} 0 and 2 {yields} 1 transitions of this molecule were measured with the ARO 12 m antenna, while the J = 3 {yields} 2 and 4 {yields} 3 lines were observed using the ARO Sub-Millimeter Telescope. HCO{sup +} was detected toward the supergiant NML Cyg and the asymptotic giant branch (AGB) stars IK Tau, TX Cam, and W Hya in at least two transitions. The J = 2 {yields} 1 and 3 {yields} 2 lines of this ion were also detected toward IRC+10216, confirming the identification of HCO{sup +} in this object. The line profiles measured for HCO{sup +} toward NML Cyg consist of red- and blueshifted components, suggesting a non-spherical shell. Based on a radiative transfer analysis, the abundances in the O-rich envelopes were f(HCO{sup +}/H{sub 2}) {approx} 0.15-1.3 Multiplication-Sign 10{sup -7}, with the AGB stars typically exhibiting the higher values. In IRC+10216, f(HCO{sup +}/H{sub 2}) {approx} 4.1 Multiplication-Sign 10{sup -9}, lower than the O-rich counterparts. The abundances of HCO{sup +} were also found to peak at considerable distances from the star, indicative of an outer envelope molecule. Comparison with H{sub 2}O and CO, the main precursor species, suggests that HCO{sup +} is more prevalent in envelopes that have substantial water, but CO also plays a role in its formation. The abundance of HCO{sup +} appears to increase inversely with mass-loss rate, provided the rate is >10{sup -6} M{sub Sun} yr{sup -1}. The common appearance of HCO{sup +} in circumstellar gas indicates that, at some level, ion-molecule reactions influence the chemistry of evolved stellar envelopes.
- OSTI ID:
- 22004544
- Journal Information:
- Astrophysical Journal, Vol. 743, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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