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 dustwere 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 → 10 transition of TiO in the v = 1 and 2 levels, and the N = 9 → 8 transition in the v = 2 level of AlO were identified towards R Dor; the J = 11 → 10 line of TiO was identified in the v = 1 level towards IK Tau; and two transitions in the v = 1 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 ± 200 K, and prior measurements of the angular extent confirm that the majority of the emission is from a region within 2 R of the central star. A full radiative transfer analysis of AlO in R Dor yielded a fractional abundance of ∼3% of the solar abundance of Al. From a similar analysis of TiO a fractional abundance of ∼ 78% 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.

ATOMIUM: halide molecules around the S-type AGB star W Aquilae

“…These abundances are around one to two orders of magnitude lower than the Al 35 Cl abundance we found for W Aql. Gaseous AlO and AlOH have also been detected around the same two oxygen-rich stars (Decin et al 2017;Danilovich et al 2020), but at low enough abundances that we would not expect their presence to inhibit the production of AlCl.…”

“…In both cases the abundance was scaled until the lines predicted by the model were equal to and/or did not exceed the rms values given in Table A.1 (using the same extraction apertures). The molecular data used here is taken from Decin et al (2017) and Danilovich et al (2020) for the AlOH and AlO models respectively. For the constant abundance models, we found upper limits of f AlO ≤ 6 × 10 −9 and f AlOH ≤ 3 × 10 −8 , relative to H 2 .…”

ATOMIUM: ALMA tracing the origins of molecules in dust forming oxygen rich M-type stars: Motivation, sample, calibration, and initial results

“…Significant advances have been made in the past few years in characterising the physical and chemical properties of the dust in the inner wind owing to: (1) the polarimetric direct imaging of the dust in the visible at high angular resolution with VLT/SPHERE by Khouri et al (2016aKhouri et al ( , 2018Khouri et al ( , 2020, Ohnaka et al (2016Ohnaka et al ( , 2017, and Adam & Ohnaka (2019); and (2) parallel observations of the rotational spectra of potential Ti and Al bearing precursors of the dust (Kamiński et al 2016(Kamiński et al , 2017Decin et al 2017;Takigawa et al 2017;Danilovich et al 2020a). However, very little is known about the physicochemical processes in the intermediate wind where dust-gas interactions occur, and tiny dust grains formed in the inner wind, grow in size by accretion of small abundant gaseous molecules onto the grains (for a comprehensive overview see the review by Decin 2021, and references therein).…”

“…The metal oxides and hydroxides AlO, AlOH, TiO, OHand most prominently SiO -are the key molecules we are using to study the impact of higher density clumps and correlated density structures on the time scales for dust growth in the inner region, and the efficiency of ice deposition in the intermediate region of the 17 stars in the ATOMIUM survey. The abundance structures are being examined with the recent radiative transfer analysis of vibrationally excited AlO and TiO in R Dor which has provided a new view of the formation of Al 2 O 3 dust (Danilovich et al 2020a) -and the same approach is also being applied to CO, HCN, SO, SO 2 , SiS, AlCl, NaCl, and PO which are observed in non-maser emission in the ground and the excited vibrational levels within a couple of R of a number of the stars in the ATOMIUM sample.…”

“…The parameters for R Aql resemble the stellar parameters in Table 1 in the main text. The temperature profile is assumed to be similar to that of Danilovich et al (2017Danilovich et al ( , 2020a: T (r) = T (R /r) 0.65 ; and the collisional rates and Einstein A coefficients were taken from the LAMDA database (van der Tak et al 2020;Yang et al 2010;Schöier et al 2005).…”