EXors and the stellar birthline

We assess the evolutionary status of EXors. These low-mass, pre-main-sequence stars repeatedly undergo sharp luminosity increases, each a year or so in duration. We place into the HR diagram all EXors that have documented quiescent luminosities and effective temperatures, and thus determine their ma...

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Bibliographic Details
Published in:Astronomy and astrophysics (Berlin) 2017-04, Vol.600, p.A133
Main Authors: Moody, Mackenzie S. L., Stahler, Steven W.
Format: Article
Language:English
Subjects:
accretion
accretion disks
Astronomy
Evolution
Herbig Ae/Be
Infrared sources (astronomy)
Luminosity
Protostars
Stars
stars: formation
stars: pre-main sequence
stars: protostars
stars: variables: T Tauri
Stellar age
T Tauri stars
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Summary:We assess the evolutionary status of EXors. These low-mass, pre-main-sequence stars repeatedly undergo sharp luminosity increases, each a year or so in duration. We place into the HR diagram all EXors that have documented quiescent luminosities and effective temperatures, and thus determine their masses and ages. Two alternate sets of pre-main-sequence tracks are used, and yield similar results. Roughly half of EXors are embedded objects, i.e., they appear observationally as Class I or flat-spectrum infrared sources. We find that these are relatively young and are located close to the stellar birthline in the HR diagram. Optically visible EXors, on the other hand, are situated well below the birthline. They have ages of several Myr, typical of classical T Tauri stars. Judging from the limited data at hand, we find no evidence that binarity companions trigger EXor eruptions; this issue merits further investigation. We draw several general conclusions. First, repetitive luminosity outbursts do not occur in all pre-main-sequence stars, and are not in themselves a sign of extreme youth. They persist, along with other signs of activity, in a relatively small subset of these objects. Second, the very existence of embedded EXors demonstrates that at least some Class I infrared sources are not true protostars, but very young pre-main-sequence objects still enshrouded in dusty gas. Finally, we believe that the embedded pre-main-sequence phase is of observational and theoretical significance, and should be included in a more complete account of early stellar evolution.
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/201630196