Magellanic Cloud stars with TiO bands in emission: binary post-RGB/AGB stars or young stellar objects?

P. R. Wood, D. Kamath, H. Van Winckel

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)

Abstract

14 stars from a sample of Magellanic Cloud objects selected to have a mid-infrared flux excess have been found to also show TiO bands in emission. The mid-infrared dust emission and the TiO band emission indicate that these stars have large amounts of hot circumstellar dust and gas in close proximity to the central star. The luminosities of the sources are typically several thousand L⊙, while the effective temperatures are ∼4000–8000 K which puts them bluewards of the giant branch. Such stars could be post-asymptotic giant branch (post-AGB) stars of mass ∼0.4–0.8 M⊙ or pre-main-sequence stars (young stellar objects) with masses in the range ∼7–19 M⊙. If the stars are pre-main-sequence stars, they are substantially cooler and younger than stars at the birth line where Galactic protostars are first supposed to become optically visible out of their molecular clouds. They should therefore be hidden in their present evolutionary state, although this problem may be overcome if asymmetries are invoked or if the reduced metallicity of the Small Magellanic Cloud and Large Magellanic Cloud compared to the Galaxy makes the circumstellar material more transparent. The second explanation for these stars is that they are post-AGB or post-red giant branch stars that have recently undergone a binary interaction when the red giant of the binary system filled its Roche lobe. Being oxygen-rich, they have gone through this process before becoming carbon stars. Most of the stars vary slowly on time-scales of 1000 d or more, suggesting a changing circumstellar environment. Apart from the slow variations, most stars also show variability with periods of tens to hundreds of days. One star shows a period that is rapidly decreasing and we speculate that this star may have accreted a large blob of gas and dust on to a disc whose orbital radius is shrinking rapidly. Another star has Cepheid-like pulsations of rapidly increasing amplitude, suggesting a rapid rate of evolution. Seven stars show quasi-periodic variability and one star has a light curve similar to that of an eclipsing binary.
Original languageEnglish
Pages (from-to)355-367
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Volume435
Issue number1
DOIs
Publication statusPublished - 2013
Externally publishedYes

Keywords

  • stars: AGB and post-AGB
  • stars: emission-line, Be
  • stars: pre-main-sequence

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