MILLIMETER DETECTION OF AlO (X$^2 \Sigma ^+$): METAL OXIDE CHEMISTRY IN THE ENVELOPE OF VY CANIS MAJORIS
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Date
2009
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Ohio State University
Abstract
A new circumstellar molecule, the radical AlO (X$^2 \Sigma ^+$), has been detected toward the envelope of the oxygen-rich supergiant star VY Canis Majoris (VY CMa) using the Arizona Radio Observatory (ARO). The N = 7 $\rightarrow$ 6 and 6 $\rightarrow$ 5 rotational transitions of AlO at 268 and 230 GHz were observed at 1 mm using the ARO Submillimeter Telescope (SMT) and the N = 4 $\rightarrow$ 3 line was detected at 2 mm using the ARO 12 m. Based on the shape of the line profiles, AlO most likely arises from the dust-forming region in the spherical outflow of VY CMa, as opposed to the blue- or red-shifted winds, with a source size of $\theta _s \sim$ 0.5$^{\prime \prime}$. Given this source size, the column density of AlO was found to be N$_{tot} \sim 2 \times 10^{15}$ cm$^{-2}$ for T$_{rot}$ $\sim$ 230 K, with a fractional abundance, relative to H$_2$, of $\sim$ 10$^{-8}$. Gas-phase thermodynamic equilibrium chemistry is the likely formation mechanism for AlO in VY CMa, but shocks may disrupt the condensation process into Al$_2$O$_3$, allowing AlO to survive to a radius of $\sim$ 20 R$_*$. The detection of AlO in VY CMa is additional evidence of an active gas-phase refractory chemistry in oxygen-rich envelopes, and suggests such objects may be fruitful sources for other new oxide identifications.
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Author Institution: University of Arizona, Steward Observatory, Department of Chemistry, Arizona Radio Observatory Tucson, AZ 85721