Spectral Variability of LBV star V532 (Romano's star)

TL;DR

This study analyzes the spectral and photometric variability of LBV star V532 over several decades, revealing correlations between brightness, spectral type, and wind properties, and comparing its evolution with similar massive stars.

Contribution

It provides a detailed long-term spectral and photometric analysis of V532, highlighting its variability patterns and proposing a new perspective on LBV star spectral classification.

Findings

V532 exhibits maximum brightness in 1992-1994 and minimum in 2007-2008.

Spectral type varies from WN11 in high/cold state to WN8.5h in low/hot state.

Wind velocity correlates with stellar brightness, increasing as brightness decreases.

Abstract

(abridged) We present the results of studying the spectral and photometric variability of the luminous blue variable star V532 in M33. The photometric variations are traced from 1960 to 2010, spectral variations - from 1992 to 2009. The star has revealed an absolute maximum of visual brightness (1992-1994, high/cold state) and an absolute minimum (2007-2008, low/hot state). The variability of the spectrum of V532 is fully consistent with the temperature variations in its photosphere, while both permitted and forbidden lines are formed in an extended stellar atmosphere. Broad components of the brightest lines were found, the broadening of these components is due to electron scattering in the wind parts closest to the photosphere. The wind velocity clearly depends on the size of the stellar photosphere or on the visual brightness, when brightness declines, the wind velocity increases. In the absolute minimum a kinematic profile of the V532 atmosphere was detected. The wind velocity increases and its temperature declines with distance from the star. In the low/hot state, the spectral type of the star corresponds to WN8.5h, in the high/cold state - to WN11. We studied the evolution of V532 along with the evolution of AG Car and the massive WR binary HD5980 in SMC. During their visual minima, all the three stars perfectly fit with the WNL star sequence by Crowther and Smith (1997). However, when visual brightness increases, all the three stars form a separate sequence. It is possible that this reflects a new property of LBV stars, namely, in the high/cold states they do not pertain to the bona fide WNL stars.