Rotational spectra of vibrationally excited AlO and TiO in oxygen rich stars

Abstract

Rotational transitions in vibrationally excited AlO and TiO -- two possible precursors of dust -- were observed in the 300 GHz range (1 mm wavelength) towards the oxygen rich AGB stars R Dor and IK Tau with ALMA, and vibrationally excited AlO was observed towards the red supergiant VY CMa with the SMA. The $J=11 \to 10$ transition of TiO in the $v=1~{\rm{and}}~2$ levels, and the $N = 9 \to 8$ transition in the $v=2$ level of AlO were identified towards R Dor; the $J=11 \to 10$ line of TiO was identified in the $v=1$ level towards IK Tau; and two transitions in the $v=1~{\rm{and}}~2$ levels of AlO were identified towards VY CMa. The newly-derived high vibrational temperature of TiO and AlO in R Dor of $1800 \pm 200$ K, and prior measurements of the angular extent confirm that the majority of the emission is from a region within $\lesssim2R_{\star}$ of the central star. A full radiative transfer analysis of AlO in R Dor yielded a fractional abundance of $\sim$3% of the solar abundance of Al. From a similar analysis of TiO a fractional abundance of $\sim78$% of the solar abundance of Ti was found. The observations provide indirect evidence that TiO is present in a rotating disk close to the star. Further observations in the ground and excited vibrational levels are needed to determine whether AlO, TiO, and TiO$_2$ are seeds of the Al$_2$O$_3$ dust in R Dor, and perhaps in the gravitationally bound dust shells in other AGB stars with low mass loss rates.