Mass segregation in globular clusters |
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Authors: | K K Scaria M K V Bappu |
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Institution: | (1) Indian Institute of Astrophysics, 560034 Bangalore |
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Abstract: | Photoelectric aperture-photometry ofω Cen inU, B, V, R andI bands has established that the cluster is bluer between 2 arcmin and 4 arcmin from the centre, than it is elsewhere. The
difference inB - I colour between the centre and this blue zone is ≃ 0.45 mag. The core radius is found to be dependant on the wavelength band
chosen for observation, the smallest core radius being for theI band. Equidensitometry ofω Cen inB, V and infrared bands shows a wavelength dependence with the cluster being nearly spherical in the infrared band. It shows a
maximum ellipticity around 3 arcmin from the cluster centre. The blue contribution in this zone comes from both a diffuse
background of unresolved stars and an increase in the relative abundance of horizontal branch (HB) stars. The similarity between
the diffuse background and the HB stars has been demonstrated. A photographic subtraction technique is used to study the distribution
of HB stars in the cluster.
Results of equidensitometry of the cluster 47 Tuc, obtained in the present study, are compared with the earlier results of
photoelectric photometry. Here too an increase in ellipticity is associated with an increase in the blueness of the cluster.
All globular clusters studied so far for ellipticity show a similarity in the dependence of the ellipticity on the distance
from the centre. The ellipticity has small values near the centre and in the outer regions, with the maximum value in between.
We suggest that the red stars in globular clusters have a nearly spherical distribution. The blue stars form a bulge around
the core with a more elliptical distribution and a different orientation. A similarity between the ellipticity aspects of
both the globular clusters and rotation in the nucleus of M 31 is pointed out; the M 31 nucleus may thus show a bluer colour
and smaller UV excess around the region where the rotation curve shows a peak.
An erratum to this article is available at . |
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Keywords: | globular clusters ω Centauri photoelectric photometry of globular clusters equidensitometry horizontal branch stars |
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