ALMA observations of TiO₂ around VY Canis Majoris^⋆

¹ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
e-mail: elvire.debeck@chalmers.se
² Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
³ Université de Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁴ CNRS, LAB, UMR 5804, 33270 Floirac, France
⁵ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
⁷ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany

Received: 28 February 2015
Accepted: 25 May 2015

Abstract

Context. Titanium dioxide, TiO₂, is a refractory species that could play a crucial role in the dust-condensation sequence around oxygen-rich evolved stars. To date, gas phase TiO₂ has been detected only in the complex environment of the red supergiant VY CMa.

Aims. We aim to constrain the distribution and excitation of TiO₂ around VY CMa in order to clarify its role in dust formation.

Methods. We analyse spectra and channel maps for TiO₂ extracted from ALMA science verification data.

Results. We detect 15 transitions of TiO₂, and spatially resolve the emission for the first time. The maps demonstrate a highly clumpy, anisotropic outflow in which the TiO₂ emission likely traces gas exposed to the stellar radiation field. An accelerating bipolar-like structure is found, oriented roughly east-west, 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.

Conclusions. We find that a significant fraction of TiO₂ 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.