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On the nature of candidate luminous blue variables in M33

Clark, J. S.; Castro, N.; Garcia, M.; Herrero, A.; Najarro, F.; Negueruela, I.; Ritchie, B. W. and Smith, K. T. (2012). On the nature of candidate luminous blue variables in M33. Astronomy & Astrophysics, 541, article no. A146.

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Context. Luminous blue variables (LBVs) are a class of highly unstable stars that have been proposed to play a critical role in massive stellar evolution as well as being the progenitors of some of the most luminous supernovae known. However the physical processes underlying their characteristic instabilities are currently unknown.

Aims. In order to provide observational constraints on this behaviour we have initiated a pilot study of the population of (candidate) LBVs in the Local Group galaxy M 33.

Methods. To accomplish this we have obtained new spectra of 18 examples within M 33. These provide a baseline of ≥4 yr with respect to previous observations, which is well suited to identifying LBV outbursts. We also employed existing multi-epoch optical and mid-IR surveys of M 33 to further constrain the variability of the sample and search for the presence of dusty ejecta.

Results. Combining the datasets reveals that spectroscopic and photometric variability appears common, although in the majority of cases further observations will be needed to distinguish between an origin for this behavour in short lived stochastic wind structure and low level photospheric pulsations or coherent long term LBV excursions. Of the known LBVs we report a hitherto unidentified excursion of M 33 Var C between 2001-5, while the transition of the WNLh star B517 to a cooler B supergiant phase between 1993−2010 implies an LBV classification. Proof-of-concept quantitative model atmosphere analysis is provided for Romano’s star; the resultant stellar parameters being consistent with the finding that the LBV excursions of this star are accompanied by changes in bolometric luminosity. The combination of temperature and luminosity of two stars, the BHG [HS80] 110A and the cool hypergiant B324, appear to be in violation of the empirical Humphreys-Davidson limit. Mid-IR observations demonstrate that a number of candidates appear associated with hot circumstellar dust, although no objects as extreme as η Car are identified. The combined dataset suggests that the criteria employed to identify candidate LBVs results in a heterogeneous sample, also containing stars demonstrating the B[e] phenomenon. Of these, a subset of optically faint, low luminosity stars associated with hot dust are of particular interest since they appear similar to the likely progenitor of SN 2008S and the 2008 NGC 300 transient (albeit suffering less intrinsic extinction).

Conclusions. The results of such amultiwavelength observational approach, employing multiplexing spectrographs and supplemented with quantitative model atmosphere analysis, appears to show considerable promise in both identifying and characterising the physical properties of LBVs as well as other short lived phases of massive stellar evolution.

Item Type: Journal Item
Copyright Holders: 2012 ESO
ISSN: 1432-0746
Project Funding Details:
Funded Project NameProject IDFunding Body
Not SetAYA2010-21697-C05-01/05Spanish Ministerio de Ciencia e Innovación (MICINN)
Not SetCSD2006-70Consolider-Ingenio 2010 project
Not SetPID2010119Gobierno de Canrias
Not SetAYA2008-06166-C03-02/03Spanish Ministerio de Ciencia e Innovación (MICINN)
Keywords: stellar evolution; circumstellar matter; binaries; stellar winds, outflows; early-type stars
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Physical Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Item ID: 35955
Depositing User: J. Simon Clark
Date Deposited: 02 Jan 2013 14:25
Last Modified: 07 Dec 2018 15:50
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