TY - JOUR
T1 - The ultraviolet colour of globular clusters in M31
T2 - A core density effect?
AU - Peacock, Mark B.
AU - Maccarone, Thomas J.
AU - Dieball, Andrea
AU - Knigge, Christian
PY - 2011/2
Y1 - 2011/2
N2 - We investigate the effect of stellar density on the ultraviolet (UV) emission from M31's globular clusters (GCs). Published far-UV (FUV) and near-UV (NUV) colours from Galaxy Evolution and Explorer (GALEX) observations are used as a probe into the temperature of the horizontal branch (HB) stars in these clusters. From these data, we demonstrate a significant relationship between the core density of a cluster and its FUV-NUV colour, with dense clusters having bluer UV colours. These results are consistent with a population of (FUV bright) extreme-HB (EHB) stars, the production of which is related to the stellar density in the clusters. Such a relationship may be expected if the formation of EHB stars is enhanced in dense clusters due to dynamical interactions. We also consider the contribution of low-mass X-ray binaries (LMXBs) to the integrated FUV luminosity of a cluster. We note that two of the three metal-rich clusters, (recently) identified by Rey et al. as having a FUV excess, are known to host LMXBs in outburst. Considering the FUV luminosity of Galactic LMXBs, we suggest that a single LMXB is unlikely to produce more than 10 per cent of the observed FUV luminosity of clusters that contain a significant population of blue-HB stars.
AB - We investigate the effect of stellar density on the ultraviolet (UV) emission from M31's globular clusters (GCs). Published far-UV (FUV) and near-UV (NUV) colours from Galaxy Evolution and Explorer (GALEX) observations are used as a probe into the temperature of the horizontal branch (HB) stars in these clusters. From these data, we demonstrate a significant relationship between the core density of a cluster and its FUV-NUV colour, with dense clusters having bluer UV colours. These results are consistent with a population of (FUV bright) extreme-HB (EHB) stars, the production of which is related to the stellar density in the clusters. Such a relationship may be expected if the formation of EHB stars is enhanced in dense clusters due to dynamical interactions. We also consider the contribution of low-mass X-ray binaries (LMXBs) to the integrated FUV luminosity of a cluster. We note that two of the three metal-rich clusters, (recently) identified by Rey et al. as having a FUV excess, are known to host LMXBs in outburst. Considering the FUV luminosity of Galactic LMXBs, we suggest that a single LMXB is unlikely to produce more than 10 per cent of the observed FUV luminosity of clusters that contain a significant population of blue-HB stars.
KW - Binaries: general
KW - Globular clusters: general
KW - Stars: horizontal branch
KW - X-rays: binaries
UR - http://www.scopus.com/inward/record.url?scp=78751685461&partnerID=8YFLogxK
U2 - 10.1111/j.1365-2966.2010.17691.x
DO - 10.1111/j.1365-2966.2010.17691.x
M3 - Article
AN - SCOPUS:78751685461
SN - 0035-8711
VL - 411
SP - 487
EP - 494
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 1
ER -