We report the first identification of the optical bands of the B ^ { 2 } \Sigma ^ { + } – X ^ { 2 } \Sigma ^ { + } 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 T _ { rot } =700 K. By resolving individual rotational components of the bands , we derive the kinematical characteristics of the gas , finding that the emission is centred at the stellar radial velocity and its intrinsic width is 13.5 km s ^ { -1 } ( 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 \sim 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 .