On the nature of candidate luminous blue variables in M 33

Abstract

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 a multiwavelength 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.

The evolution of massive stars and their spectra

“…It would be misleading to classify these stars as BHGs, since LBVs have spectral and wind properties markedly distinct from BHGs (Clark et al 2012b). Despite having similar T eff as BHGs, LBVs typically have lower υ ∞ and higherṀ than BHGs (Clark et al 2012a). However, there seems to be a smooth transition (and even some overlap) between BHG and LBV properties, which makes it challenging to draw a firm line dividing the two classes.…”

“…While we recognize that formally there is no "LBV" spectral classification, we opted to use this classification since there is no objective spectral classification criteria of stars with winds denser than those of BHGs and that have 8000 K T eff 25000 K. The spectra of these stars have been commonly referred to in the literature as "P Cygnitype", "iron", and "slash" stars (see, e.g., Walborn & Fitzpatrick 2000;Clark et al 2012a). In addition, we subdivide the LBVs in hot LBV" and "cool LBV" according to the spectral morphology, following in general lines the classification scheme from Massey et al (2007) and Clark et al (2012a). Hot LBVs have strong or moderate He  lines showing P Cygni profiles.…”

A Spectroscopic Survey of Massive Stars in M31 and M33*

“…Some were observed spectroscopically because the stars were known to have emission as shown by narrow-band Hα imaging (e.g., King et al 1998;Massey et al 2007a), or due to UV-excess (Massey et al 1996), or simply discovered "accidentally" as part of spectroscopic surveys, such as that of Massey (2006) and the present paper. Many of these "candidate" LBVs have now been shown to have photometric and/or spectral variability characteristic of true LBVs (Clark et al 2012;Lee et al 2014;Humphreys et al 2015;Sholukhova et al 2015). It is not clear what these cLBVs would have to do to be sanctified as true LBVs; the criteria needed seem to have been applied inconsistently and somewhat idiosyncratically within the literature.…”

Fundamental properties of core-collapse supernova and GRB progenitors: predicting the look of massive stars before death

“…Models that have spectrum similar to observed bona-fide LBVs, such as AG Car, P Cygni, and HR Car, have their spectral type listed as LBVs. While we recognize that formally there is no "LBV" spectral classification, we opted to use this classification since there is no objective spectral classification criteria of stars with dense winds that have 8000 K < ∼ T eff < ∼ 25 000 K. The spectra of these stars have been commonly referred to in the literature as "P Cygni-type", "iron", and "slash" stars (see, e.g., Walborn & Fitzpatrick 2000;Clark et al 2012a). The results of our spectral classification of the SN progenitors are summarized in the last column of Table 2.…”