Wind and nearby stellar environment of the quiescent luminous blue variable Var 2 in the Galaxy M33

TL;DR

This study models Var 2 in M33 with self‑consistent hydrodynamic non‑LTE extended‑atmosphere simulations to derive its fundamental parameters and wind properties. Using CMFGEN, the authors obtain $L=6.5\times10^{5}\,L_\odot$, $M_*\approx23$–$24\,M_\odot$, ${\rm X}_{\rm H}\approx43\%$, $\dot{M}=2.1\times10^{-5}\,M_\odot\,\mathrm{yr}^{-1}$, $v_{\infty}\approx230\,\mathrm{km\,s^{-1}}$, and $T_{\rm ph}\approx24.4\,\mathrm{kK}$, with a wind that is moderately clumped and requires a two‑zone acceleration governed by Fe IV/Fe III. The best self‑consistent wind models place Var 2 at an evolutionary stage with $M_{\rm init}\approx50\,M_\odot$ and $t_{\rm age}\sim5\,\mathrm{Myr}$, transitioning toward the WR phase, while its wind shows characteristics intermediate between OB and late‑WN winds. Environment analysis suggests Var 2 most likely formed in a local sparse group rather than being ejected from a distant cluster, informing LBV formation scenarios. Overall, the work advances understanding of LBV wind driving and evolution by linking detailed atmosphere/wind modeling with environmental context.

Abstract

This paper is dedicated to the study of the luminous blue variable (LBV) Var 2 in the galaxy M33, which is currently in a "dormant" state and is observed as an Of/late-WN star. We had calculated self-consistent hydrodynamic non-LTE models of the extended atmosphere, taking into account the balance of radial forces in the wind of the star. This allowed us to obtain reliable estimates of the fundamental parameters of Var 2. The derived luminosity ($L=6.5\times10^{5}\,L_\odot$), current mass ($M_* \approx23\,M_\odot$) and hydrogen abundance on the surface (${\rm X}_{\rm H}\approx43\%$) correspond well to a star with the initial mass $M_{\rm init}\approx50\,M_\odot$ and age $t_{\rm age}\approx5.0\times10^6\,$years that is evolving from lower temperatures towards the Wolf-Rayet stars. A study of dynamic properties of the wind showed that the shape of the wind velocity profile of the Var 2 is close to the one of OB supergiants. In contrast, for the obtained mass loss rate $\dot{M}=2.1\times10^{-5}\,M_{\odot}\,\text{yr}^{-1}$, the ratio of the wind momentum to the luminosity of Var 2 is in good agreement with the values found for late-WN stars. At the same time, Var 2 has a lower terminal wind velocity $v_\infty\approx230\,$km$\,$s$^{-1}$ compared to typical early-type supergiants or late-WN stars. Given the obtained age estimates, Var 2 could potentially have been ejected from the cluster associated with the nearest large star-forming region located at a distance of $\sim100\,$pc. However, statistical analysis of the projected distribution of stars in the vicinity of Var 2 suggests that this LBV could have formed in a local low-populated group.

Figures (7)

• Figure 1: Image of the Var 2 vicinity in the $F814W$ filter obtained with ACS/WFC camera on HST.
• Figure 2: Comparison of the line profiles for several lines in models with different distributions of wind inhomogeneties: (a) He II $\lambda4686$; (b) H$_\beta$; (c) N II $\lambda\lambda$4994--5007, He I $\lambda5015$; (d) He I $\lambda5876$; (e) H$_\alpha$ (f) He I $\lambda$6678. The black dashed and green dotted lines correspond to models A and B (see text). Normalized spectrum of Var 2 is shown with red solid line.
• Figure 3: The observed normalized spectrum of Var 2 (red solid line) and the best-fit wind model (black dashed line). The He I and N II lines are indicated by short blue and long black dashes, respectively.
• Figure 4: Wind velocity profiles for the baseline extended atmosphere model of Var 2 (red solid line) presented in Section \ref{['modeling_section']} and two self-consistent models calculated on its basis with different distributions of inhomogeneities in the wind. The lower boundary of the atmosphere for the self-consistent models corresponds to the Rossenland opacity $\tau_{\rm Ross} \approx 150$ similar to the initial baseline model.
• Figure 5: Comparison of the line profiles between models with analytical velocity distribution (black dashed) and self-consistent velocity structure (green dotted). The normalized observed spectrum of Var 2 is shown with red solid line. The set of lines is similar to Figure \ref{['clump_param']}.