Gr 290 (Romano’s Star). Ii. Light History and Evolutionary State

Abstract

We have investigated the past light history of the luminous variable star GR 290 (M33/V532, Romano’s Star) in the M33 galaxy, and collected new spectrophotometric observations in order to analyze links between this object, the LBV category, and the Wolf–Rayet stars of the nitrogen sequence. We have built the historical light curve of GR 290 back to 1901, from old observations of the star found in several archival plates of M33. These old recordings together with published and new data on the star allowed us to infer that for at least half a century the star was in a low luminosity state, with B ≃ 18–19, most likely without brighter luminosity phases. After 1960, five large variability cycles of visual luminosity were recorded. The amplitude of the oscillations was seen increasing toward 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 1.5 m 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 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 He ii 4686 Å emission line, the strength of the 4600–4700 Å 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 three while T eff varied between about 33,000 and 23,000 K. We confirm that the bolometric luminosity of the star has not been constant, but has increased by a factor of ∼1.5 between minimum and maximum luminosity, in phase with the apparent luminosity variations. Presently, GR 290 falls in the H–R diagram close to WN8h stars and is probably younger than them. In the light of current evolutionary models of very massive stars, we find that GR 290 has evolved from an ∼60 M ☉ progenitor star and should have an age of about four million years. From its physical charcteristics, we argue that GR 290 has left the LBV stage and is presently moving from the LBV stage to a Wolf–Rayet stage of a late nitrogen spectral type.

A new and unusual LBV-like outburst from a Wolf–Rayet star in the outskirts of M33

“…GR 290 had a major eruptive episode that peaked in 1994, although its photometric evolution was much slower than that of MCA-1B and HD 5980. GR 290 shows a late WN-type spectrum at most epochs, although at the peak of its eruption in 1994 it showed a mid to late Btype spectrum (Polcaro et al 2016), somewhat cooler than MCA-1B and more like HD 5980. As it faded for many years after its eruption, it slowly cascaded through late WN types, from WN10/11 to WN8, very much as MCA-1B did.…”

“…There are some known examples of extragalactic LBV eruptions and SN impostors that show relatively hot temperatures throughout their evolution. These include V1 in NGC 2366 (Drissen et al 2001), HD 5980 in the SMC (Drissen et al 2001;Barba et al 1995), GR 290 in M33 (Polcaro et al 2016), SN2000ch (Smith et al 2011), and the progenitor outbursts of SN 2009ip (Smith et al 2010), although most of these objects did not exhibit WN-type spectra at maximum brightness. In a decade-long HST study of bright variables in M51, Conroy et al (2018) detected several bright blue variable stars that brighten or fade by ∼1 mag without a strong colour change (see their Figure 13).…”

“…Romano's star (Romano 1978), also called GR 290 or v532, is a luminous and peculiar LBV that is sometimes referred to as an LBV candidate because it does not quite fit expectations for typical LBVs -namely, it stays hot and shows little or no colour variation during eruptions, like MCA-1B. This LBV has been studied in detail; see the recent overview by Maryeva et al (2019) and several other studies (Romano 1978;Polcaro et al 2003Polcaro et al , 2016Kurtev et al 2001;Fabrika et al 2005;Viotti et al 2006Viotti et al , 2007.…”

Wind and nebula of the M 33 variable GR 290 (WR/LBV)

“…Few studies have been performed of H ii regions surrounding individual stars in M 33, and fewer still of the ISM environment of an LBVs in this galaxy. Thus, the study of GR 290 (M 33 V0532 or Romano's star), classified as an LBV candidate (Romano 1978;Humphreys & Davidson 1994), but suspected of being a post-LBV star transitioning into the WR phase (Polcaro et al 2016), is extremely relevant. Its long term photometric behavior includes a rise to maximum brightness (B ∼16.1 mag) in the 1990's and a slow decline interrupted by brief brightness enhancements (Polcaro et al 2016).…”

“…Thus, the study of GR 290 (M 33 V0532 or Romano's star), classified as an LBV candidate (Romano 1978;Humphreys & Davidson 1994), but suspected of being a post-LBV star transitioning into the WR phase (Polcaro et al 2016), is extremely relevant. Its long term photometric behavior includes a rise to maximum brightness (B ∼16.1 mag) in the 1990's and a slow decline interrupted by brief brightness enhancements (Polcaro et al 2016). Since 2013, GR 290 is at its historically faintest state with V = 18.7 − 18.8 mag (Calabresi et al 2014;Maryeva et al 2019).…”