Search for aluminium monoxide in the winds of oxygen-rich AGB stars

Abstract

Aluminium monoxide, AlO, is likely efficiently depleted from the gas around oxygen-rich evolved stars to form alumina clusters and dust seeds. Its presence in the extended atmospheres of evolved stars has been derived from optical spectroscopy. More recently, AlO gas was also detected at long wavelengths around the supergiant VY CMa and the oxygen-rich asymptotic giant branch (AGB) star o Cet (Mira A). In search of AlO, we mined data obtained with APEX, the IRAM 30m telescope, Herschel/HIFI, SMA, and ALMA, which were primarily aimed at studying other molecular species. We report here on observations of AlO towards a sample of eight oxygen-rich AGB stars in different rotational transitions, up to seven for some stars. We present definite detections of one rotational transition of AlO for o Cet and R Aqr, and tentative detections of one transition for R Dor and o Cet, and two for IK Tau and W Hya. The presented spectra of WX Psc, R Cas, and TX Cam show no signature of AlO. For o Cet, R Aqr, and IK Tau, we find that the AlO(N=9-8) emission likely traces the inner parts of the wind, out to only a few tens of AU, where the gas has not yet reached its terminal velocity. The conclusive detections of AlO emission in the case of o Cet and R Aqr confirm the presence of AlO gas in outflows of AGB stars. The tentative detections further support this. Since most of the observations presented in this study were obtained with stronger emission from other species than AlO in mind, observations with higher sensitivity in combination with high angular resolution will improve our understanding of the presence and behaviour of AlO. From the current data sets we cannot firmly conclude whether there is a direct correlation between the wind properties and the detection rate of AlO emission. We hope that this study can serve as a stimulus to perform sample studies in search of AlO in oxygen-rich outflows.

arXiv comment9 pages, 9 figures, Accepted for publication in Astronomy & Astrophysics

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• ALMA spectral line and imaging survey of a low and a high mass-loss rate AGB star between 335 and 362 GHz
• The ASAS-SN Catalog of Variable Stars IX: The Spectroscopic Properties of Galactic Variable Stars
• Bottom-up dust nucleation theory in oxygen-rich evolved stars I. Aluminium oxide clusters
• A high-resolution line list for AlO
• Aluminium oxide in the atmosphere of hot Jupiter WASP-43b
• Circumstellar environment of the M-type AGB star R Dor. APEX spectral scan at $159.0-368.5$ GHz
• Constraints on metal oxide and metal hydroxide abundances in the winds of AGB stars - Potential detection of FeO in R Dor
• The abundance of S- and Si-bearing molecules in O-rich circumstellar envelopes of AGB stars
• Multiwavelength properties of Miras
• CO envelope of the symbiotic star R Aqr seen by ALMA
• A CO-multilayer outer atmosphere for eight evolved stars revealed with VLTI/AMBER
• Molecules and Chemistry in Red Supergiants