Painting a Family Portrait of the Yellow Super- and Hypergiants in the Milky Way I. Constraining the Distances and Luminosities

Abstract

Context. Distances to evolved massive stars in the Milky Way are not well constrained by Gaia parallaxes due to their brightness and variability. This makes it difficult to determine their fundamental stellar parameters, such as radius or luminosity, and infer their evolutionary states. Aims. We aim to improve the distance estimates of Yellow Hypergiants (YHGs) and Yellow Supergiants (YSGs) by identifying possible cluster and association memberships. Using these distances, we derived updated luminosities and revised their positions in the Hertzsprung-Russell diagram. Methods. We compiled from the literature a sample of 35 luminous yellow massive stars (YHGs and the most luminous YSGs). We used Gaia DR3 astrometry to identify possible membership in clusters and OB associations. We derived distances by combining the parallaxes of nearby co-moving stars. We independently validated these distances by comparing the stellar radial velocities to the Galactic H I kinematic map. We combined angular diameters and effective temperature values from the literature with the new distances to estimate luminosities. Results. We improved the distance estimates for 28 of the 35 stars through association with co-moving stellar groups. For an additional six stars, we provided distance estimates based on the H I kinematic map. For one star, the distance remains unclear. Most YSGs are members of young stellar populations, while the environments of the YHGs are more diverse, and for some of them, their origin populations remain unclear. We derived updated luminosities for a subset of 20 stars. Most YHGs have luminosities above log L/L = 5.4, while YSGs occupy a wider range of luminosities and the most luminous YSGs have luminosities similar to YHGs.

Figures (11)

• Figure 1: Sky region around 6 Cas (red cross). Top panel: projected offsets on sky. Bottom panel: proper motions. Field stars from Gaia are marked in grey (one in ten has been plotted), open clusters from hunt_improving_2024 are marked in coloured points, members of the OB associations Cas OB5 from melnik_kinematics_2017 and no. 141 chemel_search_2022 are marked with open symbols (some members overlap between the two catalogues). Arrows in the top panel indicate the motion of cluster and OB association stars over the past 0.1 Myr based on their proper motions.
• Figure 2: Gaia effective temperatures and bailer-jones_estimating_2021 distances for hot stars within 10 of 6 Cas. Stars meeting our astrometric quality criteria are highlighted in orange. The final selection of stars used for the distance estimation is marked with black crosses.
• Figure 3: Selection of stars near 6 Cas (red cross) based on proper motion, using the same sample as in Fig. \ref{['fig:6cas_example_teff_dist']}. The proper motion cut with a radius of 0.2 mas yr$^{-1}$ is marked with a red circle.
• Figure 4: Comparison of group-based distances derived in this work with distances derived from Gaia parallaxes by bailer-jones_estimating_2021. Stars associated with clusters, OB association, and stars without an origin group are marked with different symbols and colours. The grey dashed line marks the one-to-one relation. Outliers are marked. Both axes are logarithmic.
• Figure 5: Radial velocity of HI gas along the lines of sight towards six YHGs. The thin blue lines show the sampled posterior velocity distribution of HI at each distance bin within a 0.5 deg region around each star. The solid black line indicates the mean velocity. Red horizontal lines mark the observed stellar radial velocity, with the shaded region showing the approximate variability amplitude.