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تسجيل مستخدم جديدThe 2006 hot phase of Romanos star (GR 290) in M33
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Understanding the nature of the instabilities of LBVs is important to understand the late evolutionary stages of very massive stars. We investigate the long term, S Dor-type variability of the luminous blue variable GR290 (Romanos star) in M33, and its 2006 minimum phase. New spectroscopic and photometric data taken in November and December 2006 were employed in conjunction with already published data on GR290 to derive the physical structure of GR290 in different phases and the time scale of the variability. We find that by the end of 2006, GR 290 had reached the deepest visual minimum so far recorded. Its present spectrum resembles closely that of the Of/WN9 stars, and is the hottest so far recorded in this star (and in any LBV as well), while its visual brightness decreased by about 1.4 mag. This first spectroscopic record of GR290 during a minimum phase confirms that, similarly to AG Car and other LBVs, the star is subject to ample S Dor-type variations, being hotter at minimum, suggesting that the variations take place at constant bolometric luminosity.
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We have built the historical light curve of the luminous variable GR 290 back to 1901, from old observations of the star found in several archival plates of M 33. These old recordings together with published and new data show that for at least half a
century the star was in a low luminosity state, with B ~18. After 1960, five large variability cycles of visual luminosity were recorded. The amplitude of the oscillations was seen increasing towards the 1992-1994 maximum, then decreasing during the last maxima. The recent light curve indicates that the photometric variations have been quite similar in all the bands, and that the B-V color index has been constant within +/-0.1 m despite the 1.5m change of the visual luminosity. The spectrum of GR 290 at the large maximum of 1992-94, was equivalent to late-B type, while, during 2002-2014, it has varied between WN10h-11h near the visual maxima to WN8h-9h at the luminosity minima. We have detected, during this same period, a clear anti-correlation between the visual luminosity, the strength of the HeII 4686 A emission line, the strength of the 4600-4700 A lines blend and the spectral type. From a model analysis of the spectra collected during the whole 2002-2014 period we find that the Rosseland radius R_{2/3}, changed between the minimum and maximum luminosity phases by a factor of 3, while T_eff varied between about 33,000 K and 23,000 K. The bolometric luminosity of the star was not constant, but increased by a factor of ~1.5 between minimum and maximum luminosity, in phase with the apparent luminosity variations. In the light of current evolutionary models of very massive stars, we find that GR 290 has evolved from a ~60 M_Sun progenitor star and should have an age of about 4 million years. We argue that it has left the LBV stage and is moving to a Wolf-Rayet stage of late nitrogen spectral type.
We present the light curve of Luminous Blue Variable candidate star GR290 (Romanos star) in M33. The photographic photometry was made in photographic plates taken in B band of the M33 galaxy and cover an eight year period, 1982 - 1990. Twenty five pl
ates, separated in seven groups, have been used. CCD B magnitude of the star is also presented. The analysis of our data together with the Romanos magnitudes (1978) shows normal eruptions with amplitude of more than 1 mag and timescale of about 20 years and smaller oscillations with amplitude 0.5 mag and a period of about 320 days. This is a typical photometrical behavior for LBVs.
We study the long term, S Dor-type variability and the present hot phase of the LBV star GR290 (Romanos 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.
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