Optical spectrophotometric monitoring of the extreme luminous blue variable star GR 290 (Romano's Star) in M 33

摘要

We study the long-term, S Dor-type variability and the present hot phase of the luminous blue variable (LBV) star GR 290 (Romano's Star) in M 33 in order to investigate possible links between the LBV and the late, nitrogen sequence Wolf-Rayet Stars (WNL) stages of very massive stars. We use intermediate- resolution spectra, obtained with the William Herschel Telescope (WHT) in 2008 December, when GR 290 was at minimum (V = ～18.6), as well as new low-resolution spectra and BVRI photometry obtained with the Loiano and Cima Ekar telescopes during 2007-2010. We identify more than 80 emission lines in the 3100-10000 ? range covered by the WHT spectra, belonging to different species: the hydrogen Balmer and Paschen series, neutral and ionized helium, C III, NII-III, SIV, Si III-IV, and many forbidden lines of N II, O III, S III, A III, Ne III, and Fe III. Many lines, especially the He i triplets, show a P Cygni profile with an α-e radial velocity difference of -300 to -500 km s-1. The shape of the 4630-4713 ? emission blend and of other emission lines resembles that of WN9 stars; the blend deconvolution shows that the He ii 4686 ? has a strong broad component with FWHM ? 1700 km s-1. During 2003-2010 the star underwent large spectral variations, best seen in the 4630-4686 ? emission feature. Using the late-WN spectral types of Crowther Smith, GR 290 apparently varied between the WN11 and WN8-9 spectral types; the hotter the star was the fainter its visual magnitude was. This spectrum-visual luminosity anticorrelation of GR 290 is reminiscent of the behavior of the best-studied LBVs, such as S Dor and AG Car. During the 2008 minimum, we found a significant decrease in bolometric luminosity, which could be attributed to absorption by newly formed circumstellar matter. We suggest that the broad 4686 ? line and the optical continuum formed in a central Wolf-Rayet region, while the narrow emission line spectrum originated in an extended, slowly expanding envelope which is composed by matter ejected during previous high luminosity phases and ionized by the central nucleus. We argue that GR 290 could have just entered a phase preceding the transition from the LBV state to a late-WN type.