2018A&A...617A..51M

Context. GR 290 (M 33/V532 = Romano's Star) is a suspected post-luminous blue variable star located in M 33 galaxy that shows a rare Wolf-Rayet (WR) spectrum during its minimum light phase.In spite of many studies, its atmospheric structure, its circumstellar environment, and its place in the general context of massive stars' evolution is poorly known.
Aims. We present a detailed study of this star's wind and mass loss, and a study of the circumstellar environment associated to the star.
Methods. Long-slit spectra of GR 290 were obtained during its present minimum luminosity phase with the Gran Telescopio Canarias covering the ∼3600-7500Å wavelength range together with contemporaneous photometry using B, V, R andI filters. The data were compared with non-local thermodynamical equilibrium (non-LTE) model atmosphere synthetic spectra computed with CMFGEN code and with models for ionized interstellar medium regions computed with CLOUDY code.
Results. The current m_V=18.8mag is the faintest at which this source has ever been observed. The non-LTE models indicate effective temperatures of T_eff=27000-30000K at radius R_2/3=27-21R_☉ and mass-loss rate M=1.5x10^–5M_☉/yr. The terminal wind speed v_{inf}=620km/s is faster than ever before recorded, while the current luminosity L_*=(3.1-3.7)x10⁵L_☉ is the lowest ever deduced. The star is overabundant in He and N and underabundant in C and O.It is surrounded by an unresolved compact HII region with dimensions ≤4pc, from where H-Balmer, HeI lines, and [OIII] and [NII] are detected.In addition, we find emission from a more extended interstellar medium (ISM) region, which appears to be asymmetric, with a larger extent to the east (16-40pc) than to the west.
Conclusions. In the present long lasting visual minimum, GR 290 is in a lower bolometric luminosity state with higher mass-loss rate. The nearby nebular emission seems to suggest that the star has undergone significant mass loss over the past 10⁴-10⁵yr and is nearing the end stages of its evolution.