MILLIMETER DETECTION OF AlO (X 2 Σ + ): METAL OXIDE CHEMISTRY IN THE ENVELOPE OF VY CANIS MAJORIS

A new circumstellar molecule, the radical AlO (X 2 Σ + ), 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 → 6 and 6 → 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 → 3 line was detected at 2 mm using the ARO 12 m telescope. 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 redshifted winds, with a source size of θ s ∼ 0 5. Given this source size, the column density of AlO was found to be N tot ∼ 2 × 10 15 cm −2 for T rot ∼ 230 K, with a fractional abundance, relative to H 2 , of ∼10 −8 . Gas-phase thermodynamic equilibrium chemistry is the likely formation mechanism for AlO in VY CMa, but either shocks disrupt the condensation process into Al 2 O 3 , or chemical "freezeout" occurs. The species therefore survives further into the circumstellar envelope to a radius of r ∼ 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.