We are progressing with the analysis paper that aims to identify all potential young stars in the target clusters and to study their variability properties. Step one is to find all the objects that are potentially part of the cluster. For this we use the latest release of the Gaia data. Amongst other things, it contains parallaxes and and proper motions of all stars it can detect. As an example, the above figure shows selection process of cluster members:
We first only select stars with accurate parallax values. I.e. their parallax value is much larger than the uncertainty of the measurement. We then convert these parallaxes to distances – the distance in parsec is one divided by the parallax measured in arcsecond. Then a histogram of all distances is plotted – left part of the figure. In this case there are a large number of stars at roughly the same distance that represent the cluster. We highlight the ones that are between 250 and 350pc in red.
We then plot the proper motions in Right Ascension and Declination for all stars in blue and the ones we selected based on their distances in red – right plot. As one can see the main group of stars in the cluster shares roughly the same proper motion – we have marked those with the black circle. One can see that there are some stars with about the same distance but different proper motion. These are obviously objects that are not part of the cluster.
We then select only the stars in the above mentioned distance range and that are within the black circle in the proper motion plot. This allows us to determine the median cluster distance as 318.1+-1.2pc. The median proper motion of the cluster is 4.48+-0.04 milli arcseconds per year in Right Ascension and -6.35+-0.04mas/yr in Declination. Note that the typical scatter from this median value of the individual stars is 0.52mas/yr. This is not caused by the uncertainty in the measurements. These are the typical random projected motions of the individual stars in the cluster – note that they are all having their own random orbits around the centre of mass of the cluster, and the cluster as a whole is moving with the above mentioned value across the sky.
There is a second grouping of stars at a proper motion of 6.72+-0.10mas/yr in RA and -9.74+-0.06mas/yr in DEC. One finds that their median distance is 297+-4pc. Thus, these stars do not just have a different proper motion, but are also about 20pc closer than the main cluster. Indeed, this is a known, widely distributed foreground population of young stars towards the Perseus molecular cloud. They are slightly older than the main cluster, but many of these stars are still moving with roughly the same speed across the sky. This is the leftover speed from the molecular cloud they have formed from.