@misc{oai:ir.soken.ac.jp:00003573,
 author = {〓, 正〓 and ザオ, ジェンシイ and ZHAO, Zhengshi},
 month = {2016-02-17},
 note = {Blue straggler stars (BSSs) lie above the current main sequence turn off
on the color-magnitude diagram of star cluster. Their positions on the
color-magnitude diagram imply that they are more massive than current
main sequence turn-off stars in the same cluster. Thus, their presence of
such an upper main sequence was a challenge for standard stellar evolution
theory, as normal stars with the same mass have already evolved into white
dwarfs. To explain their extraordinary long lifespan on the main sequence,
various mechanisms have been suggested, all involving in additional hydrogen
material input to normal stars. Two of the most popular mechanisms
are merger by direct collisions in single stars and coalescence or mass transfer
in close binary systems. As products of dynamical processes in the host
cluster and stellar evolution itself, studying BSSs can shed a light on the
process of merger and/or mass transfer, and be a tracer of the dynamical
evolution of the host cluster.
The first discovery of BSSs in a globular cluster M3 dates back to six
decades ago. Since then, many observational ndings suggest that BSSs
are present in globular clusters, open clusters, as well as dwarf spheroidal
galaxies (dSphs), and even in the elds and the bulge of the Milky Way
and imply that dierent formation mechanisms may operate in different
dynamical environments. Though we are still facing with problems in understanding
these objects clearly and completely. The aim of this thesis is
to derive a comprehensive interpretation for origins of BSS by investigating
the BSS properties in dierent star systems, such as open clusters, globular
clusters and dSphs based on observational data of the highest quality up to
the present day.
We have collected homogeneous deep photometric data for 9 dSphs
taken from Subaru/ Suprime-Cam. The sample includes 3 classical dSphs,
Draco, Ursa Minor (UMi) and Sextans and 6 ultra faint dwarf galaxies. For
these classical dSphs, we have analyzed the BSSs' radial distributions and
luminosity functions. BSSs radial distribution shows a lack of strong central
peak and luminosity function is independent of radius. These results imply
that in these dSphs, the blue plume stars are most likely to be real BSSs
formed mainly by mass transfer in primordial binaries. The frequency of
BSSs in the 9 galaxies does not correlate with galaxy parameters, such as
half light radius rh, metallicity [Fe/H], central surface brightness μV;C, concentration
parameter c, central density pc, velocity dispersion δV , half mass
relaxation time log trh and collisional rate Γ , as well as integrated absolute
magnitude, MV. This result suggests that the evolution of primordial binaries,
which would form BSSs, was not affected significantly by dynamical
interactions in such sparse environments. Thus, the observed BSS frequency
is almost the same in these galaxies under the simple hypothesis that the
primordial binary fraction was the same in all galaxy.
We have also collected homogeneous deep photometric data for 51 GC
obtained by ACS/HST. 8 GCs in our sample show bimodal BSSs frequency
radial distributions. Among them, the bimodal distribution is reported for
the first time in NGC 5286, NGC 5986, NGC 6362, NGC 6535, NGC6541
and NGC7099, in total 6 GCs. It has raised the number of this kind of
GCs by twice the number known before. Moreover, the significant anticorrelation
between BSS frequency and MV is confimed with our sample.
BSS frequency is also significantly anti-correlated to central density, central
velocity dispersion and collision rate. Furthermore, luminosity functions of
BSSs in bright (MV < -8.8) and faint (MV > -8.8) clusters do not show
significant differences. One possible interpretation for all these properties is
that BSSs formation from binaries is the dominant mechanism in globular
clusters. However, The effect of collision on BSS formation is still considerable
because collisions destroy primordial binaries and/or accelerate
evolution of binary systems.
Finally, we have observed 30 BSSs and 9 main sequence turn-off candidates
in three old Galactic open clusters, NGC 2682, NGC 2112 and NGC
6819. Among them, radial velocities (Vr) of all target stars in NGC 2682
and NGC 6819, and 5 BSSs and all TOs in NGC 2112 are consistent with
mean Vr of member stars in the clusters. Rotational velocity (vsini ) of
BSSs spans a wide range from few kms-1 to hundreds km_s-1, while those
of turn-off stars vary in small range from few kms-1 to 25 kms-1. The
high rotation velocity of current BSS is consistent with predictions both
from mass transfer and collision formation mechanism. Some turn-off stars
and a few of BSSs show normal Li abundances. For the other BSSs, only
the upper limit of Li abundance is estimated. Interestingly, BS1371 in NGC
2112 and BS 10010 in NGC 6819, show very high Li abundance, A(Li) 3.94
dex and A(Li) 3.52 dex. Two BSSs show depletion in oxygen abundance,
while the most of BSSs show similar oxygen abundance to turn-off stars.
Some BSSs show depletion in carbon abundance, while other BSSs show
the normal carbon abundance comparing to turn-off stars. These abundance
anomalies in light elements found in some BSSs suggest that mass
transfer from such highly evolved stars is the origin of these BSSs. On the
other hand, the progenitor of other BSSs with normal carbon and oxygen
abundances and low lithium abundances might be normal red giant stars., 総研大甲第1543号},
 title = {Origins of Blue Straggler Stars in Open Clusters, Globular Clusters and Dwarf Spheroidal Galaxies},
 year = {}
}