Journal Article

Mercer 5: a probable new globular cluster in the Galactic bulge

A. J. Longmore, R. Kurtev, P. W. Lucas, D. Froebrich, R. de Grijs, V. D. Ivanov, T. J. Maccarone, J. Borissova and L. M. Ker

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 1, pages 465-478
Published in print September 2011 | ISSN: 0035-8711
Published online August 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19056.x
Mercer 5: a probable new globular cluster in the Galactic bulge

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We present a detailed study of a dust-obscured Galactic star cluster Mercer 5 ([MCM2005b] 5) in an extremely crowded field in the Milky Way. Near-infrared (near-IR) photometry from United Kingdom Infrared Digital Sky Surveys (UKIDSS) and the Son of ISAAC on the New Technology Telescope (SofI/NTT), combined with near-IR spectroscopy also from SofI, indicates that it is almost certainly a Galactic globular cluster, located at the edge of the Galactic bulge. The cluster suffers ∼9 mag of visual extinction, with strong evidence for an extinction gradient across the cluster. A simulation of the differential reddening in the cluster using empirical data from NGC 6539 (chosen because it had high signal-to-noise ratio data and low field star contamination) as a template mimics the observations extremely well. This simulation and other arguments are used to indicate that the most prominent clump of stars in the colour–magnitude diagrams is a horizontal branch clump. On this basis we conclude that the cluster is at a distance of ∼5.5 kpc and suffers from visual extinction ranging from ∼8.5 to ∼12.5 mag. Alternative explanations for its nature, such as a young cluster or an old open cluster, are much less likely, on the grounds of no visible main sequence or stars with IR excesses for the former and location versus lifetime arguments for the latter.

Keywords: stars: abundances; stars: distances; Galaxy: abundances; globular clusters: general; globular clusters: individual: Mercer 5: [MCM2005b] 5

Journal Article.  9763 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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