ALMA observations of TiO${}_2$ around VY Canis Majoris

Abstract

Titanium dioxide, TiO${}_2$, is a refractory species that could play a crucial role in the dust-condensation sequence around oxygen-rich evolved stars. To date, gas phase TiO${}_2$ has been detected only in the complex environment of the red supergiant VY CMa. We aim to constrain the distribution and excitation of TiO${}_2$ around VY CMa in order to clarify its role in dust formation. We analyse spectra and channel maps for TiO${}_2$ extracted from ALMA science verification data. We detect 15 transitions of TiO${}_2$, and spatially resolve the emission for the first time. The maps demonstrate a highly clumpy, anisotropic outflow in which the TiO${}_2$ emission likely traces gas exposed to the stellar radiation field. A roughly east-west oriented, accelerating bipolar-like structure is found, of which the blue component runs into and breaks up around a solid continuum component. A distinct tail to the south-west is seen for some transitions, consistent with features seen in the optical and near-infrared. We find that a significant fraction of TiO${}_2$ remains in the gas phase outside the dust-formation zone and suggest that this species might play only a minor role in the dust-condensation process around extreme oxygen-rich evolved stars like VY CMa.

Accepted for publication in Astronomy & Astrophysics, 25 pages, 20 figures