A New LBV Candidate in M33

The evolutionary relationships and mechanisms governing the behavior of the wide variety of luminous stars populating the upper H-R diagram are not well established. Luminous blue variables (LBVs) are particularly rare, with only a few dozen identified in the Milky Way and nearby galaxies. Since 2012, the Barber Observatory Luminous Stars Survey has monitored more than 100 luminous targets in M33, including M33C-4119 which has recently undergone photometric and spectroscopic changes consistent with an S Doradus eruption of an LBV.


1.
M33C-4119 (LGGS J013312.81+303012.6) is a new Luminous Blue Variable (LBV) candidate discovered in an on-going survey of M33 . LBVs are rare and difficult to identify due to the infrequency of their characteristic photometric and spectroscopic variability. Only a few dozen have been identified in the Milky Way and nearby galaxies (Humphreys et al. 2016). Their eruptive mechanism and connection to other classes of massive stars including B[e] supergiants, warm hypergiants, and supernova impostors are poorly understood.
From 2012 to the present its brightness has been recorded several times a year by a BVRI CCD survey of M33 conducted with the University of Illinois Springfield Barber Observatory 20-inch telescope . From 2012 -2018 it brightened ≈ 1.0 mag in all observed bands, including a rapid rise beginning in 2017. The initial stage, 2012-2016, is confirmed by CCD photometry recorded by the Tautenburg Landessterwarte 2-m telescope (Burggraf 2015;Gottschling 2017). It maintained peak brightness (≈ 0.5 magnitude brighter than seen previously) for more than a year before a more rapid decline to its minimum brightness by 2021 ( Figure 1 and Table 1). Throughout the event the star's color was correlated with its change in brightness as expected during an S Doradus eruption, being significantly bluer/hotter when fainter and redder/cooler when visually brighter.
Spectra of M33C-4119 were obtained with the MMT Hectospec Multi-Object Spectrograph in 2010.76 and 2014.88 with the 600-line grating in the blue and red covering 3600 to 8300A (Figure 2 and 3) and also with the LBT MODS spectrograph on 2011.75 (Humphreys et al. 2013(Humphreys et al. , 2014. The 2010 and 2011 spectra closely resemble each other. The 2014 spectrum shows a significant change and a shift to a cooler apparent temperature. A fourth spectrum recorded in 2007.76 by Burggraf (2015) using the CAFOS spectrograph on the Calar Alto 2.2-m telescope is similar in appearance to the 2010 and 2011 spectra with lower resolution and a higher noise level which affects the clear detection of weaker emission and absorption features.
Both In 2014 when the star was halfway through its period of brightening, the He I emission lines are replaced by absorption and the He I lines previously present in absorption are weaker. Absorption lines of Ca II K, the Na I D lines and Mg II are significantly stronger, indicating a shift to cooler temperatures consistent with a late B-type spectral type (≈ B8).
To estimate the star's total bolometric luminosity and place on an HR Diagram, we determined the visual extinction. Since M33C-4119 has strong emission lines, we adopt A V = 1.10 from two nearby OB stars. Although we lack a spectrum at maximum light, we argue that the observed shift in color supports an equivalent late A or F spectral type with little or no bolometric correction. A maximum V ≈ 16.8 mag at a distance modulus of 24.5 mag (Scowcroft et al. 2009) implies M v = M bol ≈ -8.8 (Figure 4. This is also consistent with the luminosity estimated from the spectral types and brightness observed in 2010 and 2014.
LBVs are defined by S Doradus eruptive episodes characterized by a 1-2 mag increase in visual brightness accompanied by an apparent shift to cooler temperatures and change in spectral type to late-A to F type with little or no appreciable change in luminosity. Many hot supergiants have emission lines and the B[e] supergiants are spectroscopically like LBVs, thus observing an S Dor event is the only way to confirm a star is an LBV. The brightening of M33C-4119 is consistent with an S Dor eruption including the time scale, the rise in visual brightness, the reddening of the colors near peak brightness, and the shift to later spectral type during the brightening. Although there is no spectrum at maximum, the case is compelling that M33C-4119 is an LBV.   The UIS Barber Observatory survey of luminous stars in M33 was initiated under and supported by NSF grant AST-1108890 with additional support from the University of Illinois Springfield Henry R. Barber Astronomy Endowment funded by the people of Central Illinois. We also thank Brigita Burggraf and Niels Gottschling for the photometry and spectroscopy they contributed.
This work also made use of the Pan-STARRS1 Surveys (PS1) and the PS1 public science archive which has been made possible through contributions of a number of organizations credited in Chambers et al. (2016).