13 research outputs found
Discovery of double BSS sequences in the old Galactic open cluster Berkeley 17
Blue straggler stars (BSS) are peculiar objects which normally appear as a
single broad sequence along the extension of the main sequence. Only four
globular clusters (GCs) have been observed to have two distinct and parallel
BSS sequences. For the first time for any open cluster (OC), we report double
BSS sequences in Berkeley 17. Using the machine-learning based membership
algorithm ML-MOC on Gaia EDR3 data, we identify 627 cluster members, including
21 BSS candidates out to 15 arcmin from the cluster center. Both the BSS
sequences are almost equally populated and parallel to one another in Gaia as
well as in Pan-STARRS colour-magnitude diagram (CMD). We statistically confirm
their presence and report that both BSS sequences are highly segregated
compared to the reference population out to 5.5 arcmin and not segregated
thereafter. The lower densities of OCs make BSS formation impossible via the
collisional channel. Therefore, mass transfer seems to be the only viable
channel for forming candidates of both sequences. The gap between the red and
blue BSS sequences, on the other hand, is significant and presents a great
opportunity to understand the connection between BSS formation and internal as
well as external dynamics of the parent clusters.Comment: Accepted for publication at MNRAS Letter
The blue straggler population of the old open cluster Berkeley 17
Blue Straggler Stars (BSSs) are observed in Galactic globular clusters and
old open clusters. The radial distribution of BSSs has been used to diagnose
the dynamical evolution of globular clusters. For the first time, with a
reliable sample of BSSs identified with Gaia DR2, we conduct such an analysis
for an open cluster. We identify members, including BSSs, of the oldest known
Galactic open cluster Berkeley 17 with the Gaia DR2 proper motions and
parallaxes. We study the radial distribution of the BSS population to
understand the dynamical evolution of the cluster. We select cluster members to
populate the colour magnitude diagram in the Gaia filters. Cluster parameters
are derived using the brightest members. The BSSs and giant branch stars are
identified, and their radial distributions are compared. The segregation of
BSSs is also evaluated with respect to the giant branch stars using the Minimum
Spanning Tree analysis. We determine Berkeley 17 to be at
pc. We find 23 BSS cluster members, only two of
which were previously identified. We find a bimodal radial distribution of BSSs
supported by findings from the MST method. The bimodal radial distribution of
BSSs in Berkeley 17 indicates that they have just started to sink towards the
cluster center, placing Berkeley 17 with globular clusters of intermediate
dynamical age. This is the first such determination for an open cluster.Comment: 6 pages, 6 figures, Accepted for publication in Astronomy and
Astrophysic
Determination of dynamical ages of open clusters through the A parameter -- II
Blue straggler stars (BSS), one of the most massive members of star clusters,
have been used for over a decade to investigate mass segregation and estimate
the dynamical ages of globular clusters (GCs) and open clusters (OCs). This
work is an extension of our previous study, in which we investigated a
correlation between theoretically estimated dynamical ages and the observed
values, which represent the sedimentation level of BSS with
respect to the reference population. Here, we use the ML-MOC algorithm on
\textit{Gaia} EDR3 data to extend this analysis to 23 OCs. Using cluster
properties and identified members, we estimate their dynamical and physical
parameters. In order to estimate the values, we use the
main sequence and main sequence turnoff stars as the reference population. OCs
are observed to exhibit a wide range of degrees of dynamical evolution, ranging
from dynamically young to late stages of intermediate dynamical age. Hence, we
classify OCs into three distinct dynamical stages based on their relationship
to and . NGC 2682 and King 2 are
discovered to be the most evolved OCs, like Familly III GCs, while Berkeley 18
is the least evolved OC. Melotte 66 and Berkeley 31 are peculiar OCs because
none of their dynamical and physical parameters correlate with their BSS
segregation levels.Comment: Accepted for publication at MNRA
UOCS-XI. Study of blue straggler stars in open cluster NGC 7142 using UVIT/AstroSat
We present a study of blue straggler stars (BSSs) of open cluster NGC 7142
using AstroSat/UVIT data and other archival data. Using a machine
learning-based algorithm, ML-MOC, on Gaia DR3 data, we find 546 sources as
cluster members. Based on the location on the Gaia color-magnitude diagram, we
identify ten BSS candidates, also detected in UVIT/F148W filter. We study the
variable nature of BSSs by constructing their light curves using the TESS data.
Two BSSs reported as eclipsing binaries in Gaia DR3 are confirmed to be
eclipsing binaries based on our analysis and also show the presence of hot
companions as per the multi-wavelength spectral energy distributions (SEDs).
The physical parameters of the hot companions of these two BSSs derived by
fitting binary models to their light curves and those derived from the SEDs are
found to be in good agreement. Additionally, five more BSSs in the cluster
shows UV excess, four of which are likely to have a hot companion based on
SEDs. The hot companions with the estimated temperatures 14000 28000
K, radii 0.01 0.05 R, and luminosities 0.03 0.1
L, are inferred to be extremely low mass ( 0.2 M),
low-mass (0.2 0.4 M), normal-mass (0.4 0.6 M), and
high-mass ( 0.6 M) white dwarfs (WD). For the first time in an
open cluster, we find the entire range of masses in WDs found as hot companions
of BSSs. These masses imply that the Case-A/Case-B mass transfer as well as
merger are responsible for the formation of at least 60 of the BSSs of this
cluster
Field blue straggler stars: Discovery of white dwarf companions to blue metal-poor stars using UVIT/AstroSat
Blue metal-poor (BMP) stars are the main-sequence stars that appear bluer and
more luminous than normal turn-off stars of metal-poor globular clusters. They
are believed to be either field blue straggler stars (FBSS) formed via
post-mass transfer mechanism or accreted from dwarf satellite galaxies of the
Milky Way. A significant fraction of BMP stars are discovered to be potential
binaries. We observed 27 BMP stars using UVIT/\textit{AstroSat} in two FUV
filters, F148W and F169M. We report the discovery of white dwarf (WD)
companions of 12 BMP stars for the first time. The WD companions have estimated
temperatures T 10500 18250 K, and masses 0.17 M
0.8 M. Based on [Fe/H] and space velocity, we group the 12 BMP/FBSS
stars as the thick disk (5) and halo (5), whereas two stars appear to be
in-between. All the 5 thick disk BMP/FBSS have extremely low-mass (M 0.2
M) WDs as companions, whereas the 5 halo BMP/FBSS have low (0.2
M M 0.4 M), normal (0.4 M M
0.6M), and high mass (M 0.6 M) WD companions. Our
analysis suggests that at least 44 of BMP stars are FBSS, and these
stars hold the key to understand the details of mass transfer, binary
properties, and chemical enrichment among the FBSS
A Smoking Gun in the Carina Nebula
The Carina Nebula is one of the youngest, most active sites of massive star
formation in our Galaxy. In this nebula, we have discovered a bright X-ray
source that has persisted for ~30 years. The soft X-ray spectrum, consistent
with kT ~128 eV blackbody radiation with mild extinction, and no counterpart in
the near- and mid-infrared wavelengths indicate that it is a ~1e6-year-old
neutron star housed in the Carina Nebula. Current star formation theory does
not suggest that the progenitor of the neutron star and massive stars in the
Carina Nebula, in particular Eta Carinae, are coeval. This result suggests that
the Carina Nebula experienced at least two major episodes of massive star
formation. The neutron star may be responsible for remnants of high energy
activity seen in multiple wavelengths.Comment: 9 pages, 3 figures, accepted for publication to ApJ