Two New LBV Candidates in the M33 Galaxy

TL;DR

This paper reports the discovery of two new luminous blue variable (LBV) star candidates in the M33 galaxy, characterized by their spectral features, luminosities, and potential variability, contributing to the understanding of massive star evolution.

Contribution

The study identifies and characterizes two new LBV candidates in M33, providing detailed spectral and photometric analysis, and highlighting their potential variability and dust activity, which is novel in this context.

Findings

Both stars exhibit broad Halpha emission with extended wings.

Star N45901 shows infrared excess and probable variability.

Star N125093 has high luminosity and signs of recent eruptions.

Abstract

We present two new luminous blue variable (LBV) candidate stars discovered in the M33 galaxy. We identified these stars (Valeev et al. 2010) as massive star candidates at the final stages of evolution, presumably with a notable interstellar extinction. The candidates were selected from the Massey et al. (2006) catalog based on the following criteria: emission in Halpha, V<18.5 and 0.35<(B-V)<1.2. The spectra of both stars reveal a broad and strong Halpha emission with extended wings (770 and 1000 km/s). Based on the spectra we estimated the main parameters of the stars. Object N45901 has a bolometric luminosity log(L/Lsun)=6.0-6.2 with the value of interstellar extinction Av=2.3+-0.1. The temperature of the star's photosphere is estimated as Tstar~13000-15000K its probable mass on the Zero Age Main Sequence is M~60-80Msun. The infrared excess in N45901 corresponds to the emission of warm dust with the temperature Twarm~1000K, and amounts to 0.1% of the bolometric luminosity. A comparison of stellar magnitude estimates from different catalogs points to the probable variability of the object N45901. Bolometric luminosity of the second object, N125093, is log(L/Lsun)=6.3-6.6, the value of interstellar extinction is Av=2.75+-0.15. We estimate its photosphere's temperature as Tstar~13000-16000K, the initial mass as M~90-120Msun. The infrared excess in N125093 amounts to 5-6% of the bolometric luminosity. Its spectral energy distribution reveals two thermal components with the temperatures Twarm~1000K and Tcold~480K. The [CaII] lines (7291A and 7323A), observed in LBV-like stars VarA and N93351 in M33, are also present in the spectrum of N125093. These lines indicate relatively recent gas eruptions and dust activity linked with them. High bolometric luminosity of these stars and broad Halpha emissions allow classifying the studied objects as LBV candidates.