Absorption bands of TiO in the visible have been identified in spectra of stars more than hundred years ago. In fact, these features are instrumental to classify M- and S-type stars. Moreover, TiO is thought to be responsible for the pulsation of Mira stars, one class of variable O-rich late-type stars. However, these features originate from the atmospheres of these stars, not from the circumstellar shells of these stars.

A. R. Hyland, E. E. Becklin, G. Neugebauer, and G. Wallerstein,
reported on the
Observations of the Infrared Object, VY Canis Majoris
Astrophys. J. 158, 619–628 (1969).
They detected several “unidentified emission lines” toward this O-rich late-type star, which they called to be “associated with TiO band heads” (six times, one possibly overlapping with atomic Rb), “strong VO band here” (once), “probably ScO” (four times). Additional emission lines were firmly assigned to TiO (and VO) by
G. Wallerstein,
Spectroscopic Observations of VY Canis Majoris during 1969-1971
Astrophys. J. 169, 195–197 (1971).
The rotational temperature appears to vary greatly between about 500 K or even lower and about 900 K and suggests that the emission has its origin in the circumstellar envelope of the star rather than the photosphere.

More recently,TiO emission lines in the optical have also seen toward U Equ by
C. Barnbaum, A. Omont, and M. Morris,
The Unusual Circumstellar Environment of the Evolved Star, U Equulei
Astron. Astrophys. 310, 295–270 (1996).
The temperature was estimated to be between 750 and 1500 K.
TiO emission lines were also seen toward several other types of stars, among them comparatively prominently in:
Optical TiO and VO Band Emission in Two Embedded Protostars: IRAS 04369+2539 and IRAS 05451+0037
L. A. Hillenbrand, G. R. Knapp, D. L. Padgett, L. M. Rebull, and P. M. McGehee,
Astron. J. 143, Art. No. 37 (2012).

More recently, the
Discovery of a TiO Emission Band in the Infrared Spectrum of the S star NP Aurigae
Astron. Astrophys. 543, Art. No. L2 (2012);
and tentatively toward V899 Aql and RX Psc were reported by
K. Smolders, T. Verhoelst, P. Neyskens, J. A. D. L. Blommaert, L. Decin, H. van Winckel, S. van Eck, G. C. Sloan, J. Cami, S. Hony, P. De Cat, J.Menu, and J. Vos.
A temperature of 600 K was estimated. Even if it is rather uncertain, it is again compatible with circumstellar origin of the emission.

Finally, the detection of rotational transitions in the circumstellar envelope of VY CMa (and others) will be described in the TiO2 section.

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).
J = 11 → 10 transitions of TiO with Ω = 1, 2, and 3 were found in the circumstellar envelope of R Doradus up to v = 2 and that of IK Tauri up to v = 1.

The detection of TiO2 between 279 and 355  and around 222.5 GHz employing the SubMillemeter Array (SMA) and the Plateau de Bure Interferometer(PdBI), respectively, has been described by
T. Kamiński, C. A. Gottlieb, K. M. Menten, N. A. Patel, K. H. Young, S. Brünken, H. S. P. Müller, M. C. McCarthy, J. M. Winters, and L. Decin,
Pure Rotational Spectra of TiO and TiO<sub>2</sub> in VY Canis Majoris
Astron. Astrophys. 551 (2013), Art. No. A113.
27 transitions with J and Ka up to 42 and 9, respectively, upper state energies between 25 and about 730 K, and signal-to-noise ratios between 3 and 37 have been detected in emission. In addition, rotational transitions of TiO were detected in that study for the first time. 8 fine structure components of four rotational transitions with J' between 7 and 11 were detected with significance. Two additional high-lying Ω = 3 components had signal-to-noise ratios smaller than three, albeit just barely for one.
Only an estimate of about 1000 K is available for the rotational temperature of TiO, whereas a rather accurate value of 225 ± 24 K was derived for TiO2. The dioxide emission has its origin certainly in the circumstellar envelope of VY CMa. The situation is less clear for the monoxide, but it is rather likely that most of the TiO emission is of circumstellar nature and from outside the dust formation zone. Both molecules have fairly high column densities of around 7 × 1014 cm–2.

T. Kamiński, H. S. P. Müller, M. R. Schmidt, I. Cherchneff, K. T. Wong, S. Brünken, K. M. Menten, J. M. Winters, C. A. Gottlieb, and N. A. Patel,
reported on
An observational study of dust nucleation in Mira (ο Ceti) – II. Titanium oxides are negligible for nucleation at high temperatures
Astron. Astrophys. 599 (2017), Art. No. A59.
The authors detect several emission lines of TiO and TiO2 mostly with ALMA and with APEX. They detect not only lines of the main isotopolog (with 48Ti), but also lines of minor Ti isotopic species, of which some appear to be unblended.

Contributor(s): H. S. P. Müller; 03, 2013; 03, 2017; 12, 2020

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