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. Appendix A is available in electronic form at http://www.aanda.org
Astronomy and Astrophysics
- Pub Date:
- August 2015
- stars: individual: VY Canis Majoris;
- stars: mass-loss;
- circumstellar matter;
- submillimeter: stars;
- Astrophysics - Solar and Stellar Astrophysics
- Accepted for publication in Astronomy &