# The distances to star-clusters hosting Red Supergiants: $\chi$ Per, NGC 7419, and Westerlund 1

## TL;DR

This paper estimates distances to three star clusters rich in Red Supergiants using Gaia DR2 data, highlighting the importance of accurate parallax zero-point calibration for precise measurements.

## Contribution

The study provides new distance estimates for three key star clusters using Gaia DR2 astrometry and emphasizes the impact of Gaia's zero-point offset on measurement accuracy.

## Key findings

- Distances are 2.25, 3.00, and 3.87 kpc for the clusters.
- Gaia's zero-point parallax offset is the main source of error.
- Improved zero-point calibration is needed for more precise distances.

## Abstract

Galactic, young massive star clusters are approximately coeval aggregates of stars, close enough to resolve the individual stars, massive enough to have produced large numbers of massive stars, and young enough for these stars to be in a pre-supernova state. As such these objects represent powerful natural laboratories in which to study the evolution of massive stars. To be used in this way, it is crucial that accurate and precise distances are known, since this affects both the inferred luminosities of the cluster members and the age estimate for the cluster itself. Here we present distance estimates for three star clusters rich in Red Supergiants ($\chi$ Per, NGC 7419 and Westerlund 1) based on their average astrometric parallaxes $\bar{\pi}$ in Gaia Data Release 2, where the measurement of $\bar{\pi}$ is obtained from a proper-motion screened sample of spectroscopically-confirmed cluster members. We determine distances of $d=2.25^{+0.16}_{-0.14}$kpc, $d=3.00^{+0.35}_{-0.29}$kpc, and $d=3.87^{+0.95}_{-0.64}$kpc for the three clusters respectively. We find that the dominant source of error is that in Gaia's zero-point parallax offset $\pi_{\rm ZP}$, and we argue that more precise distances cannot be determined without an improved characterization of this quantity.