The aluminum monoxide, AlO, radical, has been detected in three rotational transitions in the 2 and 1 mm regions belonging to N“ = 3, 5, 6 with hyperfine structure probably somewhat resolved for the lowest transition:
E. D. Tenenbaum and L. M. Ziurys,
Millimeter Detection of AlO (X <sup>2</sup>Σ<sup>+</sup>): Metal Oxide Chemistry in the Envelope of VY Canis Majoris
Astrophys. J. 694, L59–L63 (2009).
It appears as if only a very minute amount of Al is in the gas phase around O-rich circumstellar envelopes, in contrast to C-rich circumstellar envelopes, such as that of IRC +10216, in which more volatile molecules such as AlF, AlCl, and AlNC have been detected.

More recently, the same autors have detected aluminum hydroxide, AlOH, in the same source in three rotational transitions in the 2 and 1 mm regions belonging to J” = 4, 6, 8:
E. D. Tenenbaum and L. M. Ziurys,
Exotic Metal Molecules in Oxygen-rich Envelopes: Detection of AlOH (X <sup>1</sup>Σ<sup>+</sup>) in VY Canis Majoris
Astrophys. J. 712, L93–L63 (2010).
The molecule is approximately 17 times as abundant as AlO.

T. Kamiński, K. T. Wong, M. R. Schmidt, H. S. P. Müller, C. A. Gottlieb, I. Cherchneff, K. M. Menten, D. Keller, S. Brünken, J. M. Winters, and N. A. Patel,
reported on
An Observational Study of Dust Nucleation in Mira (ο Ceti) I. Variable Features of AlO and Other Al-bearing Species
Astron. Astrophys. 592, Art. No. A42 (2016).
Using dedicated as well as archival data obtained with Herschel, APEX, and ALMA, several transitions of AlO and AlOH were observed in the circumstellar shell of Mira. The AlO signals originate largely from the dust formation zone. Hence, AlO may well be (one of) the molecule(s) with which dust formation starts in O-rich AGB stars.

T. Danilovich, C. A. Gottlieb, L. Decin, A. M. S. Richards, K. L. K. Lee, T. Kamiński, N. A. Patel, K. H. Young, and K. M. Menten
Rotational Spectra of Vibrationally Excited AlO and TiO in Oxygen-rich Stars
Astrophys. J. 904, Art. No. 110 (2020).
N = 8 → 7 and 9 → 8 transitions of AlO were found in the circumstellar envelope of VY Canis Majoris up to v = 2. The N = 9 → 8, v = 2 transition was also detected toward R Doradus and IK Tauri.

Contributor(s): H. S. P. Müller; 03, 2009; 02, 2010; 08, 2016; 12, 2020

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