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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.
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