Modelling the optical spectrum of Romano's star

TL;DR

This study models the optical spectrum of Romano's star in different states, revealing its spectral classification, chemical composition, and luminosity variations, using non-LTE atmosphere modeling to understand its LBV behavior.

Contribution

First detailed non-LTE modeling of Romano's star across different spectral states, linking spectral features to stellar evolution and LBV variability.

Findings

Spectrum during minimum matches WN star with high hydrogen

Spectrum during outburst resembles WN11, similar to P Cyg

Bolometric luminosity varies by a factor of 1.5 between states

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-LTE model atmosphere code CMFGEN. All the observed properties of the object in the minimum are well described by a late nitrogen-sequence Wolf-Rayet (WN) star model with 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.