Modelling the optical spectrum of Romano's star

Abstract

We consider the luminous blue variable (LBV) star V532 in M33, also known as Romano's star, in two different spectral states: in the optical minimum of 2007/2008 and during a local brightening in 2005. Optical spectra of low and moderate resolution are modelled using the non-local thermodynamic equilibrium model atmosphere code cmfgen. All the observed properties of the object in the minimum are well described by a late WN (nitrogen-sequence Wolf-Rayet) star model with a relatively high hydrogen abundance (H/He = 1.9), while the spectrum during the outburst corresponds to the spectral class WN11 and is similar to the spectrum of P Cyg. The atmosphere is enriched in nitrogen by about a factor of 6 in both states. Most of the heavy-element abundances are consistent with the chemical composition of M33. Bolometric luminosity is shown to vary between the two states by a factor of ∼1.5. This makes V532 another example of an LBV that shows variations in its bolometric luminosity during an outburst. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.