Inferring the parallax of Westerlund 1 from Gaia DR2

Abstract

Westerlund 1 (Wd1) is potentially the largest star cluster in the Galaxy. That designation critically depends upon the distance to the cluster, yet the cluster is highly obscured, making luminosity-based distance estimates difficult. Using Gaia Data Release 2 (DR2) parallaxes and Bayesian inference, we infer a parallax of 0.31 ± 0.04 mas corresponding to a distance of 3.2 ± 0.4 kpc. To leverage the combined statistics of all stars in the direction of Wd1, we derive the Bayesian model for a cluster of stars hidden among Galactic field stars; this model includes both the zero-point and astrometric excess noise. We infer the exponential length scale, L, for field stars; L = 0.84 +0.02 −0.03 kpc. This is ∼1.7 times smaller than the models used for Bailer-Jones et al. (2018). Previous estimates for the distance to Wd1 ranged from 1.2 to 5.5 kpc, although values around 5 kpc have usually been adopted. The Gaia DR2 parallaxes reduce the uncertainty from a factor of 3 to 15% and rules out the most often quoted value of 5 kpc with 99% confidence. This new distance allows for more accurate mass and age determinations for the stars in Wd1. For example, the previously inferred initial mass at the main sequence turnoff was around 40 M ; the new Gaia DR2 distance shifts this down to about 25 M . This has important implications for our understanding of late stages of stellar evolution, including the initial mass of the magnetar and the LBV in Wd1. Similarly, the new distance suggests that the total cluster mass is about three times lower than previously calculated.

Multiplicity of Galactic luminous blue variable stars

“…6. We stress however that independent studies have shown that, for Cl * Westerlund 1 W 243, its real distance is more between 2.6 and 4.1 kpc (Aghakhanloo et al 2020;Beasor et al 2021). However, considering different distances does affect the measurements of the stellar radii but would not change the conclusions of our study.…”

Unlocking Galactic Wolf–Rayet stars with Gaia DR2 – II. Cluster and association membership

“…We estimate a cluster distance of 3.78 +0.56 −0.46 kpc (though this excludes many stars with high excess noise). This is more distant than the 2.6 +0.6 −0.4 kpc obtained from the full bayesian combination of cluster member parallaxes from Aghakhanloo et al (2020). The difference may stem from the fact we excluded some stars, via a parallax cut, as they seemed to be foreground objects.…”

Comparison of Gaia and Hipparcos parallaxes of close visual binary stars and the impact on determinations of their masses

“…Which may result in multiple systems higher than binaries in the case of massive proto-stellar cores and discs, because of the gradually increasing likelihood that more massive stars will fragment (Kratter & Matzner, 2006). We expect a strong contribution from Gaia data in solving the mysteries of formation of multiple stars, where recent stellar evolution theories concentrate on the study of massive stars (Aghakhanloo et al, 2020).…”

“…In general, the currently most accepted theory of the formation of BS is the fragmentation of protostellar cores or circumstellar discs (Bate, Bonnell, & Price 1995;Kroupa 1995;Bate, Bonnell, & Bromm 2002;Tohline 2002;Kratter & Matzner 2006;Clarke 2009;Offner et al 2010;Kratter Lodato 2016;Moe & Di Stefano 2017;Moe, Kratter, & Badenes 2019;Tokovinin & Moe 2020) which may result in multiple systems higher than binaries in the case of massive protostellar cores and discs because of the gradually increasing likelihood that more massive stars will fragment (Kratter & Matzner 2006). We expect a strong contribution from Gaia data in solving the mysteries of formation of multiple stars, where recent stellar evolution theories concentrate on the study of massive stars (Aghakhanloo et al 2020).…”