A proper motion study of the globular cluster M55

TL;DR

This study measures the absolute proper motion of globular cluster M55 using CCD images over 11 years, identifying cluster members, variable stars, and background galaxy motions to understand its dynamics and membership.

Contribution

The paper provides the first precise proper motion measurement of M55 and a detailed catalog of cluster members, variables, and background objects based on extensive CCD data.

Findings

Proper motion of M55: (-3.31, -9.14) mas/yr

Catalog of 16,945 stars with membership status

Detection of background Sgr dSph galaxy stars

Abstract

We have derived the absolute proper motion (PM) of the globular cluster M55 using a large set of CCD images collected with the du Pont telescope between 1997 and 2008. We find (PM_RA*cos(DEC), PM_DEC) = (-3.31 +/- 0.10, -9.14 +/- 0.15) mas/yr relative to background galaxies. Membership status was determined for 16 945 stars with 14<V<21 from the central part of the cluster. The PM catalogue includes 52 variables of which 43 are probable members of M55. This sample is dominated by pulsating blue straggler stars but also includes 5 eclipsing binaries, three of which are main sequence objects. The survey also identified several candidate blue, yellow and red straggler stars belonging to the cluster. We detected 15 likely members of the Sgr dSph galaxy located behind M55. The average PM for these stars was measured to be (PM_RA*cos(DEC), PM_DEC)=(-2.23 +/- 0.14, -1.83 +/- 0.24) mas/yr.