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The study of faint galactic open clusters

Babu, G S D

The study of faint galactic open clusters [Ph.D Thesis] G S D Babu - Bangalore, Indian Institute of Astrophysics 1985 - 240p.

The topic of this thesis was originally suggested by
Late Prof. M. K. V. Bappu and was undertaken with two major aims.
The first was to obtain the luminosities of stars contained
in the very distant and hitherto not well-studied clusters.
The second aim was to use the younger of these clusters as
spiral arm tracers to substantiate the existing optical picture
of our Galaxy's spiral structure. In this thesis, the portion
covered was restricted to the anticenter direction of the Galaxy
(1600 < gal. long. < 2800 ) so th at an attempt may be made to
optically study the remotest galactic parts. This is made
slightly easier because in this direction, the complications
due to the effects of interstellar extinction are less than
those in the direction of the galactic center.
As the first step, one hundred and four not well-studied
clusters were chosen from the catalogue of open clusters ; n
the given range of galactic longitudes. Then thirty two of
them were observed with the modified objective grating method
(Hoag & Schroeder, 1970), so as to get an approximate estimate
of spectral types of the individual stars in each cluster region.
These spectral types were combined with their respective visual
magnitudes determined from the image sizes on the already existing
sky survey charts, to construct the approximate HR-diagram
of each cluster. Out of them, six were found to have stars
of spectral type B3 or earlier on the respective main-sequence
bands and were categorised as young. Six more bluer clusters were selected based on the visual inspection of the blue and
red prints of the sky survey charts.
All these twe lve clusters were photometrically observed
using the facilities at Kavalur Observatory in India and at
Siding Spring Observatory in Australia. In general, the stars which
appeared to be members, out of the observed ones, in each of
the programme clusters, ,have been found to be in the range
of 10 to 30, with one exception, for which 55 members have
been counted.
Each one of these clusters is found to be showing some
amount of luminosity spread, which is mainly caused by stars
of differing ages, supporting the earlier suggestion that star
formation in clusters is not coeval.
range of 6.0 x 106 to 5.9 x 108 years.
The ranges are in the
Their distances are found to be ranging from 0.6 kpc
to 7.1kpc and their colour excesses, E(B-V), are in the range
of 0.00 mag to 1.20 mag. It is clearly seen from the present
work that as the cluster distance increases, the value of
E(B-V) increases. That is, the amount of interstellar matter
increases in the line of sight column with the increasing distance
from the Sun ;n the plane of the Galaxy.
Out of the twelve clusters studied here, four are located
along the Cygnus-Orion arm and onecs a definite member of the
Sagittarius-Carina arm. The existence of the outer Peruses
spiral feature is substantiated by four clusters at distances
of about 4 to 5.S kpc in the direction of the galactic longitude 170° to 220°, some evidence for which was found earlier by
Moffat & Vogt (1973). There is a clear indication that this
outer feature is probably extending into the Puppis group of
clusters which seems to be the junction for the merging of
the loc a 1 arm with the outer Peruse arm. Thisis supported
by one of the presently studied clusters which is located at
a distance of about 6.5 kpc in the galactic longitude of about
245° ,
There is an indication of a feature which is originating
from the Orion-Cygnus arm at a distance of about 4.5 kpc and
extending upto a distance of about 8 kpc towards the galactic
longitude of 280°, This inference is based on the location
of one of the programme clusters at a distance of about 7 kpc
in the direction of galactic longitude 2700 along with two
other already known clusters in the same area. Even though
there is some suggestion of this branching off feature from
the radio studies, this is perhaps its first indication from
the optical observations.

Astronomy and Astrophysics Thesaurus
Clusters of Galaxies
Open Clusters
Ph.D Thesis

043:52 / BAB

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