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

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.

💡 Research Summary

The paper tackles the long‑standing problem of poorly constrained distances for luminous yellow supergiants (YSGs) and yellow hypergiants (YHGs) in the Milky Way, a difficulty that hampers reliable determinations of fundamental parameters such as radius, luminosity, and evolutionary status. Gaia DR3 provides parallaxes for billions of stars, but for these bright, variable, and often heavily reddened objects the parallaxes are either highly uncertain (>20 % errors) or even negative, largely because of saturation, photocentric jitter, and possible unresolved companions. To overcome these limitations the authors employ two complementary, largely independent methods.

Method 1 – Cluster and OB association membership:
The authors first compile a list of 35 target stars (25 YSGs and 10 YHGs) from the literature, excluding known post‑AGB objects and stars with reliable Gaia distances that would place them too close to be truly luminous. They then cross‑match the positions and proper motions of each target with modern catalogues of open clusters (Hunt & Reffert 2024) and OB associations (Mel’nik & Dambis 2017; Chemel et al. 2022). A star is considered a member if it lies within the projected cluster radius and its proper motion is within three sigma of the cluster mean; for OB associations a looser criterion (at least two association members within 1° and 0.5 mas yr⁻¹) is used.

Having identified a plausible group, the authors select nearby hot stars (effective temperatures 8 700–18 000 K, corresponding roughly to B‑type and early‑A‑type dwarfs) within a 10‑arcminute radius that have high‑quality Gaia astrometry (ruwe < 1.4, G < 18 mag, parallax/error > 5, five‑parameter solution). These hot stars are assumed to share the same distance as the target because they belong to the same birth environment. The Gaia proper motions are corrected for known bright‑star biases (Cantat‑Gaudin & Brandt 2021) and the parallaxes are zero‑point corrected (Lindegren et al. 2021). The authors then apply a proper‑motion cut (typically 0.1–1.5 mas yr⁻¹ depending on the dispersion of the group) to isolate co‑moving stars, discard outliers beyond 2 σ in parallax, and finally combine the remaining parallaxes using the weighted‑average scheme of Campillay et al. (2019) that incorporates external systematic uncertainties and the angular covariance term. The resulting group parallax is transformed into a geometric distance with the generalized gamma‑distribution prior of Bailer‑Jones et al. (2021). This procedure yields robust distances for 28 of the 35 targets; an illustrative case is 6 Cas, for which 31 co‑moving hot stars give a mean parallax ϖ = 0.3586 ± 0.0088 mas, corresponding to a distance of 2790 + 70 − 71 pc, in excellent agreement with earlier independent estimates.

Method 2 – HI kinematic comparison:
For stars where a convincing cluster/association cannot be identified (e.g., because the star has drifted away or its natal group has dissolved), the authors compare the star’s measured radial velocity with the Galactic HI longitude‑velocity (ℓ‑v) map. The velocity‑distance relation derived from the HI rotation curve provides a kinematic distance estimate. If the star’s radial velocity matches the expected HI velocity at a given distance, that distance is adopted as a secondary estimate. Using this approach, six additional stars receive distance estimates, while one star remains ambiguous.

Luminosity recalculation:
With the new distances in hand, the authors combine literature angular diameters (θ) and effective temperatures (Teff) to compute bolometric luminosities via L = 4πd²σTeff⁴θ². Updated luminosities are presented for 20 stars (the subset with reliable angular diameter measurements). The results show that most YHGs have log L/L☉ > 5.4, confirming their status as some of the most luminous cool massive stars. YSGs span a broader range, from log L/L☉ ≈ 4.5 up to ≈ 5.8; the most luminous YSGs overlap the YHG luminosity regime, suggesting a possible evolutionary connection.

Key findings and implications:

1. Distance improvement: By leveraging co‑moving hot stars, the authors reduce distance uncertainties from >20 % (or undefined) to typically a few percent for the majority of the sample. The independent HI kinematic check validates many of these distances, increasing confidence.

2. Environmental diversity: Most YSGs are firmly linked to young clusters or OB associations, indicating they are still near their birthplaces. YHGs display a more heterogeneous picture: some are associated with clusters, others with loose associations, and a few appear isolated, hinting at possible runaway status or dissolution of their natal groups.

3. Revised HR diagram positions: The new luminosities shift several objects upward in the HR diagram, reinforcing the view that YHGs occupy the uppermost part of the cool supergiant branch. The overlap between the brightest YSGs and YHGs supports the hypothesis that the two classes represent a continuum of evolutionary stages, perhaps with YSGs evolving into YHGs as they approach the Humphreys–Davidson limit and experience enhanced mass loss.

4. Methodological contribution: The study demonstrates that, even when Gaia parallaxes are unreliable, cluster/association membership combined with high‑quality astrometry of nearby hot stars provides a powerful distance estimator for bright, evolved objects. The HI kinematic method offers an independent sanity check, especially valuable for stars lacking a clear group.

5. Future prospects: The authors note that upcoming Gaia data releases (DR4) will improve bright‑star astrometry, potentially allowing direct parallax distances for many of these objects. Until then, the presented methodology fills a critical gap. Precise distances will enable tighter constraints on mass‑loss rates, pulsation properties, and ultimately the fate of these stars as supernova progenitors.

In summary, the paper delivers a substantial refinement of distances and luminosities for a representative sample of Galactic yellow supergiants and hypergiants, clarifies their environmental contexts, and provides a robust framework for future studies of massive star evolution in the Milky Way.