Aluminium oxide in the optical spectrum of VY Canis Majoris

TL;DR

This study reports the first optical detection of AlO in VY CMa, revealing gas conditions and dynamics that challenge existing equilibrium chemistry models, suggesting shock-driven processes.

Contribution

First identification of AlO optical bands in VY CMa, providing new insights into circumstellar chemistry and gas dynamics in this star.

Findings

AlO emission centered at stellar velocity

Gas temperature around 700K at ~20 stellar radii

Narrowest thermal emission feature observed in VY CMa

Abstract

We report the first identification of the optical bands of the B-X system of AlO in the red supergiant VY CMa. In addition to TiO, VO, ScO, and YO, which were recognized in the optical spectrum of the star long time ago, AlO is another refractory molecule which displays strong emission bands in this peculiar star. Simulating the bands of AlO, we derive a rotational temperature of the circumstellar gas of Trot=700K. By resolving individual rotational components of the bands, we derive the kinematical characteristics of the gas, finding that the emission is centered at the stellar radial velocity and its intrinsic width is 13.5 km/s (full width at half maximum). It is the narrowest emission among all (thermal) features observed in VY CMa so far. The temperature and line widths suggest that the emission arises in gas located within ~20 stellar radii, where the outflow is still being accelerated. This result contradicts equilibrium-chemistry models which predict substantial AlO abundances only to within a few stellar radii. We argue that non-equilibrium models involving propagation of shocks are needed to explain the observations.