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

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.

💡 Research Summary

**
The paper investigates the spatial relationship between luminous blue variables (LBVs) and their candidate counterparts (cLBVs) and the OB associations in the nearby galaxy M33. Using photometric data from the Local Group Galaxy Survey (LGGS), the authors first construct two samples of massive blue stars based on color–magnitude criteria. The primary sample (2 912 stars) targets the most luminous, hottest objects (initial masses ≳ 20 M⊙), while a secondary sample (5 376 stars) contains slightly less massive early‑type stars (≈ 10–20 M⊙). These samples serve as tracers of the youngest stellar clusters.

To identify OB associations, the authors apply two clustering algorithms (a density‑based method such as DBSCAN and a distance‑threshold method). They calibrate the algorithms’ parameters (search radius, minimum number of members) through Monte‑Carlo simulations of random stellar fields, ensuring that spurious groups are minimized and that the recovered associations reflect the hierarchical structure known from previous studies of M33. The resulting catalog contains several hundred OB associations across the galaxy.

The LBV/cLBV list comprises four confirmed LBVs and nineteen candidates drawn from Humphreys et al. (2017), supplemented by three recently reported objects. The sample includes a variety of spectral types, notably Fe II‑emission stars and a B