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

Abstract

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 ∼3600–7500 Å wavelength range together with contemporaneous photometry using B, V, R and I 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 mV = 18.8 mag is the faintest at which this source has ever been observed. The non-LTE models indicate effective temperatures of Teff = 27 000–30 000 K at radius R2/3 = 27−21 R⊙ and mass-loss rate Ṁ = 1.5 × 10−5 M⊙yr−1. The terminal wind speed v∞ = 620 km s−1 is faster than ever before recorded, while the current luminosity L* = (3.1–3.7) × 105L⊙ 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 H II region with dimensions ≤4 pc, from where H-Balmer, He I lines, and [O III] and [N II] 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 (16–40 pc) 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 104–105 yr and is nearing the end stages of its evolution.