Galactic And Extragalactic Globular Clusters
Working Group Members:
- Annapurni Subramaniam, IIA, Bangalore
- Gajendra Pandey, IIA, Bangalore
- B. Kumar, ARIES, Nainital
- B Shylaja, J. Nehru Planetarium, Bangalore
Globular clusters science areas:
A number of research areas in globular clusters studies have been
identified that will be studied with priority using the TAUVEX
observations. The globular clusters (GCs) are test bed for the
studies of stellar evolution of low mass stars. Since GCs have a
very high stellar density, they are also excellent stellar crash
test labs. Violent encounters between binaries
and single stars in dense cluster cores give
rise to exotic stellar population such as Blue Stragglers (BSs), Cataclysmic
Variables (CVs), Low mass x-ray binaries (LMXBs) and millisecond pulsars.
GCs also provide a good testing ground to study the late evolutionary stages
of low mass stars, like white dwarfs and horizontal branch stars.
The UV advantage of the GCs
The above mentioned exotic stars are mostly populated in the cores
of the GCs where the stellar crowding makes it impossible to identify them.
The HB stars show lesser concentration when compared to the blue stragglers.
These stars are very bright in the UV when compared to the MS stars, which
make them easy to identify. The main advantage lies in the fact that the
stellar crowding gets dramatically reduced in the UV.
The analysis of stellar population in clusters in done with the help
of colour-magnitude diagram, which is the observational equivalent of the
theoretical H-R diagram. Similar to the optical CMD, the UV CMD also
delineates the location of stars in various evolutionary stages. The BSs,
WD sequence and HB stars stand out. Hence it is easy to identify them
and study them in the UV. This is evident from the CMDs presented in
Ferraro et al. (2001).
Why to study exotic stars?
A widely accepted hypothesis explains the origin of BSs based on the merger
of two stars. This has two variants. In a binary system, the components
gradually spiral together to form a single star. The other scenario is the
collisional merger, where the remnant of a collision between single stars
or a single star and a binary results in the formation of BSs. The BSs made
from collisions may have excess He abundance outside the core compared to
those made from binary mergers. These two merger mechanisms will create
products with different chemistry and hence different position in and through
the CMD. This effect may be observable, particularly in the UV colours
(Baylin 1992).
LMXBs and CVs
The LMXBs and CVs are observed in the field as well in the GCs. The LMXBs
are much more abundant in GCs than in the field. This is explained by the
tidal capture of stars by the degenerate stars. The same would result in
the production of CVs also. The CVs are thought to be mostly in the quiet
phase and their detection is possible through the UV. There are models
which predict a large number of CVs in GCs, but there are also models
which predict the opposite. This is because the lifetime of these binary
systems and their evolution are not known (Shara et al. 1996). Hence a
it is very necessary to identify and study these systems in the GCs, which
can be done by imaging in the UV.
The optical counter part of most of the LMXBs in GCs are not known.
This is due to the crowding. In the UV images, it should be possible
to identify the LMXBs. The resolution of the TAUVEX is adequate for
this purpose.
Horizontal branch stars
HB stars are the core-helium burning stars, which span a range in
temperature, but with similar luminosity. The GCs are known to exhibit
variation in the HB morphology, called the second parameter problem,
in which two clusters with same metallicity exhibit difference in the
population of the red and blue HB stars.
The Blue HB stars (BHBs) can be easily observed in the UV, where they
are easily identified. In clusters with vertical BHB distribution, UV CMD
can be used to estimate the temperature range covered by them. The study
of BHBs can throw light into the evolutionary scenario as well as the reason
for their presence in only a few GCS.
Previous studies of GC stars in the UV was carried out to limited extend
using UIT. In particular well studied GCs like, M13 were imaged
using UIT (Parise et al. 1998). The optical and
UV morphology of the HB stars in the
UV CMDs show the lifetime and the temperature range of HB stars. The HST
study of this object (Ferraro et al. 1998) covers less area which
could result in incompleteness. Hence the study of GCs in the UV is very minimal.
With the advancement of the theory in the last decade, there is a necessity to
observationally confirm the proposed theories, many of which can be done
only in the UV.
Extra galactic GCs
The globular cluster systems (GCSs) in external galaxies are very widely
studied in the optical. There has been very little study in the UV, only a
few objects are observed in M31. The GCSs can be used as a tool to probe
the formation history of the parent galaxy. The age and metallicity
distribution of the GCS is a good pointer in this direction. These parameters
are estimated using particular features in a CMD, since the individual stars
can be resolved in a Galactic GC. For GCs in external galaxies one has to
rely on their integrated colour or spectra.
The metallicity vs age in Galactic GCs
Model isochrones as a function of HB morphology can be used to
differentiate between the age and metallicity effects. Lee and Lee (2002)
demonstrate this procedure. This type of figure can decouple the age
and metallicity of the GCs. This method is not possible for the extragalactic
GCs, simply because the individual stars in the HB are not resolved.
Though it was realised that the HB stars and the post-AGB stars have a
significant contribution in the UV region, population synthesis models
were not available till recently (Lee and Lee 2002). The models
from Lee and Lee (2002) provide the integrated colour over a wide range:
UV to near IR. Comparison of their models with observations indicate that
reliable age estimation is not possible using the currently available
optical and IR data. Their models indicate that the relation between
the metallicity and (15-V) colour can in fact be used as an age discriminant.
The GC system in M31 was studied using UIT. On the other hand, the FUV
measurements of M31 GCs are not available and the near UV-V colour does not
indicate the age range. Therefore a systematic and complete study of the
GCs in M31 is necessary.
Requirements to study extra galactic GCs
It is needed to understand the bimodal colour distribution seen from
many early-type galaxies (Larson et al. 2001) as well as some spirals
(Forbes, Brodie and Larson 2001). The origin of the red and blue
subpopulations and the implications for the formation of their host
galaxies remains unclear. The existence of any relative age difference
among the subgroups can be identified using (FUV-V) colour. Previous
studies of the GCs have been carried out mainly by UIT and
HST. The UIT
has poor resolution and hence the identification of objects near the
core of GCs is not possible. The studies using the HST concentrated on
the core of the GCs. In the case of HST, most of the filters have red
leaks, which makes the interpretation of the results difficult
(Brosch et al 1999).
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