The Extreme LBV Star GR290 (Romano's Star) in M33. Optical Spectrophotometric Monitoring

TL;DR

This study monitors the spectral and photometric variability of the LBV star GR290 in M33, revealing its spectral evolution, possible transition phase, and the relationship between its luminosity and spectral type, with implications for massive star evolution.

Contribution

It provides the first detailed spectrophotometric monitoring of GR290 over several years, identifying spectral type variations and proposing a transitional evolutionary phase from LBV to WN star.

Findings

GR290 shows spectral variability between WN11 and WN8-9 types.

A correlation exists between the star's temperature and visual brightness.

During minimum brightness, the bolometric luminosity decreases significantly.

Abstract

We study the long term, S Dor-type variability and the present hot phase of the LBV star GR290 (Romano's Star) in M33 in order to investigate possible links between the LBV and WNL stages of very massive stars. We use intermediate resolution spectra, obtained with WHT in December 2008, when GR290 was at minimum (V = 18.6), as well as new low resolution spectra and B V R I photometry obtained with the Loiano and Cima Ekar telescopes during 2007-2010. We identify more than 80 emission lines in the 3100-10000 A range, belonging to different species and to forbidden transitions. Many lines, especially the HeI triplets, show a P Cygni profile with an a-e radial velocity difference from -300 to -500 km/s. The shape of the 4630-4713 A emission blend and of other emission lines resembles that of WN9 stars; the blend deconvolution shows that the HeII 4686 A has a strong broad component with FWHM \simeq 1700 km/s. During 2003-2010 the star underwent large spectral variations, best seen in the 4630-4686 A emission feature. Using the late-WN spectral types of Crowther & Smith (1997), GR290 apparently varied between the WN11 and WN8-9 spectral types, the hotter being the star the fainter its visual magnitude. This spectrum-visual luminosity anticorrelation of GR290 is reminiscent of the behaviour of the best studied LBVs. During the 2008 minimum we find a significant decrease in bolometric luminosity, which could be attributed to absorption by newly formed circumstellar matter. We suggest that, presently, the broad 4686 A line and the optical continuum are formed in a central WR region, while the narrow emission line spectrum originate in an extended, slowly expanding envelope, that is composed by matter ejected during previous high luminosity phases, and ionized by the central nucleus. GR290 could have just entered in a phase preceeding the transition from the LBV state to late WN type.