On the spatial distribution of luminous blue variables in the M33 galaxy

Abstract

In the current paper, we present a study of the spatial distribution of luminous blue variables (LBVs) and various LBV candidates (cLBVs) with respect to OB associations in the M33 galaxy. The identification of blue star groups was based on the LGGS data and was carried out by two clustering algorithms with initial parameters determined during simulations of random stellar fields. We have found that the distribution of distances to the nearest OB association obtained for the LBV/cLBV sample is close to that for massive stars with $M_{\rm init}>20\,M_\odot$ and Wolf-Rayet stars. This result is in good agreement with the standard assumption that luminous blue variables represent an intermediate stage in the evolution of the most massive stars. However, some objects from the LBV/cLBV sample, particularly Fe$\,$II-emission stars, demonstrated severe isolation compared to other massive stars, which, together with certain features of their spectra, implicitly indicates that the nature of these objects and other LBVs/cLBVs may differ radically.

Figures (8)

• Figure 1: Hertzsprung-Russell diagram with evolutionary tracks for Z=0.006 Eggenberger2021. Black lines of the evolutionary tracks represent the main sequence stage, colored --- later stages of the stellar evolution followed after the end of hydrogen burning in the core (red) and in the shell (cyan). Green lines show the region of the diagram with the most massive unevolved stars. The color to the effective temperature $Q-T_{\text{eff}}$ transformation and bolometric corrections are calculated according to the relations presented in Massey1995b and Massey2005 correspondingly.
• Figure 2: Distribution diagram of $\langle d_{\rm min} \rangle$ (gradient fill) and $\zeta$ (red contour lines) values depending on the initial parameters of the DBSCAN (left) and OPTICS (right) algorithms.
• Figure 3: The results of the spatial clustering of a sample of hot massive stars using DBSCAN (left) and OPTICS (right) algorithms. The green dots correspond to the clusters members, the background objects are grey dots. The mass centers of the star groups are indicated by red crosses. LBV and LBV candidates are shown by blue stars.
• Figure 4: The cumulative distribution functions of the projected distances from stars of different samples to the nearest centers of OB associations, identified by DBSCAN (top) and OPTICS (bottom) clustering algorithms. The 70% confidence interval for LBV/cLBV distribution is shown by grey color.
• Figure 5: The cumulative distribution functions as in figure \ref{['fig4']}. The 95% confidence intervals for distance distributions of blue stars, WR stars and red supergiants are shown by corresponding colors.