Variable C – “a typical” LBV in M 33?

Abstract

One of the original Hubble-Sandage variables, Variable C in M33 is thought to be a very typical Luminous Blue Variable (LBV). An observational signature of LBVs is a variable brightness which is coupled to a change in spectral type. We compiled a 110 year long light curve of Var C and a set of spectra covering several decades. Analyzing both data sets, various astonishing changes of Var C, some very recent, emerged. Is Var C a typical or an atypical LBV?

LUMINOUS AND VARIABLE STARS IN M31 AND M33. II. LUMINOUS BLUE VARIABLES, CANDIDATE LBVs, Fe II EMISSION LINE STARS, AND OTHER SUPERGIANTS

“…In addition to the evidence for circumstellar nebulae discussed by Weis et al (2014), we find that many of the stars described in §2 have warm circumstellar dust including several of the Fe II emission line stars, but conclude that the known LBVs do not.…”

“…The spectrum of M31-004526.62, with Fe II emission lines and prominent P Cygni profiles in the multiplet 42 Fe II lines, closely resembles that of M31-004444.52, one of the warm hypergiants described in Paper I. M31-004526.62 has strong hydrogen emission lines with broad wings and P Cygni profiles, [N II] emission, and the absorption line spectrum of an early A-type supergiant. Weis et al (2014) conclude that it has a high density circumstellar nebula, although it does not have the [Ca II] emission nor an infrared excess due to dust. Its blue and red spectra are shown with M31-004444.52 in Figure 7.…”

“…Given the lack of or weak [O III] and [O II] emission in their spectra, the signature emission lines of an H II region, the red [N II] and [S II] emission lines may be formed in their circumstellar ejecta and can be used to estimate the gas density. This special subclass of our stars are discussed in a separate paper by Weis et al (2014).…”

“…Many of the hot supergiants show weak nebular emission in their spectra due to contamination from emission nebulosity in the Hectospec aperture. Fortunately, the relative strengths of the [N II] lines (Weis et al 2014) allow us to separate a likely circumstellar nebula from an H II region. The stars with H II emission contamination in their spectra distinct from a circumstellar nebula are not included in Table 3.…”

A New Luminous Blue Variable in M311

“…11 in Humphreys et al [2014]). However, the relative strengths of the [N II] λ5755 and λ6584 emission lines compared to the [S II] lines in the 2010 spectrum indicate the presence of a relatively dense circumstellar nebula, much denser than expected for an H II region (Weis et al 2015). The outflow velocities from the stellar wind measured from the absorption minima in the P Cygni profiles in these spectra are summarized in Table 2.…”