Suggestion for the detection of TiO₂ in interstellar medium

Abstract

Since all the carbon in oxygen-rich stars is locked into carbon monoxide (CO), how the formation of dust takes place in their environment is a matter of great interest. Being a refractory species, the titanium dioxide (TiO₂) is thought to play important role in the dust-condensation sequence. The TiO₂ is detected in the environment of red supergiant VY Canis Majoris through sub-millimeter wavelengths. All these lines are between the levels lying at high energies for which large kinetic temperature in the region is required. Based on the detailed study of transfer of radiation, we propose for the identification of TiO₂ through its transitions between low lying levels. Using spectroscopic data, we have calculated energies of 100 rotational levels of para-TiO₂ (up to $82~\mbox{cm}^{-1}$) and the Einstein $A$-coefficients for radiative transitions between the levels. These Einstein $A$-coefficients along with the scaled values of collisional rate coefficients, we have solved a set of 100 statistical equilibrium equations coupled with 436 equations of radiative transfer. We have found 9 transitions having anomalous absorption and 6 transitions showing emission features. These transitions may help in identification of TiO₂ in a cosmic object.