Millimeter detection of alo (X ²Σ⁺): Metal oxide chemistry in the envelope of VY canis majoris

A new circumstellar molecule, the radical AlO (X ²Σ ⁺), 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 05. Given this source size, the column density of AlO was found to be N _tot 2 × 10¹⁵ cm^-2 for T _rot 230 K, with a fractional abundance, relative to H₂, 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 ₂O₃, 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.