The cool circumstellar environment of IRAS 08182−6000: radiative transfer modelling of TiO absorption bands

IRAS 08182−6000 has very red infrared colours, with an excess at K and L, and a silicate absorption feature at 9.7 μm. Optical spectra show strong, narrow absorption bands of TiO and AlO and weaker bands of VO that are indicative of formation in a very cool region, superimposed on a late F-type or early G-type spectrum. It is similar to the unusual star U Equ = IRAS 20547+0247 and both are high-velocity stars. These characteristics, together with spectroscopic indications of low surface gravity, suggest that these stars are of low mass and are possibly at a post-AGB stage of evolution. IRAS 08182−6000 brightened by several magnitudes during 12 yr of photometric observation. Photospheric and some circumstellar atomic absorption lines are evident, as well as atomic lines seen in emission that differ in radial velocity from those in absorption by c. 50 km s⁻¹. Molecular bands of the γ, γ′ and α systems of circumstellar TiO have been modelled using a one-dimensional radiative transfer code that makes use of the most recent laboratory and theoretical spectroscopic data on TiO. It was found necessary to invoke a non-local thermodynamic equilibrium model, which resulted in the determination of different rotational, spin–orbit and vibrational temperatures for the TiO gas, all of which lie well below the temperature at which TiO begins to deplete from the gas phase due to grain formation.