A method for determining the radius of an open cluster from stellar proper motions

DOI: 
10.1093/mnras/sty088
Publication date: 
01/01/2018
Main author: 
Sánchez, Néstor
IAA authors: 
Alfaro, Emilio J.
Authors: 
Sánchez, Néstor;Alfaro, Emilio J.;López-Martínez, Fátima
Journal: 
Monthly Notices of the Royal Astronomical Society
Refereed: 
Yes
Publication type: 
Article
Pages: 
4122-4132
Abstract: 
We propose a method for calculating the radius of an open cluster in an objective way from an astrometric catalogue containing, at least, positions and proper motions. It uses the minimum spanning tree (hereinafter MST) in the proper motion space to discriminate cluster stars from field stars and it quantifies the strength of the cluster-field separation by means of a statistical parameter defined for the first time in this paper. This is done for a range of different sampling radii from where the cluster radius is obtained as the size at which the best cluster-field separation is achieved. The novelty of this strategy is that the cluster radius is obtained independently of how its stars are spatially distributed. We test the reliability and robustness of the method with both simulated and real data from a well-studied open cluster (NGC 188), and apply it to UCAC4 data for five other open clusters with different catalogued radius values. NGC 188, NGC 1647, NGC 6603 and Ruprecht 155 yielded unambiguous radius values of 15.2 ± 1.8, 29.4 ± 3.4, 4.2 ± 1.7 and 7.0 ± 0.3 arcmin, respectively. ASCC 19 and Collinder 471 showed more than one possible solution but it is not possible to know whether this is due to the involved uncertainties or to the presence of complex patterns in their proper motion distributions, something that could be inherent to the physical object or due to the way in which the catalogue was sampled.
Database: 
ADS
SCOPUS
URL: 
https://ui.adsabs.harvard.edu/#abs/2018MNRAS.475.4122S/abstract
ADS Bibcode: 
2018MNRAS.475.4122S
Keywords: 
open clusters and associations: general;open clusters and associations: individual: ASCC 19;Collinder 471;NGC 1647;NGC 188;NGC 6603;Ruprecht 175;stars: kinematics and dynamics;Astrophysics - Astrophysics of Galaxies