445 research outputs found
Photometry of the Globular Cluster NGC 5466: Red Giants and Blue Stragglers
We present wide-field BVI photometry for about 11,500 stars in the
low-metallicity cluster NGC 5466. We have detected the red giant branch bump
for the first time, although it is at least 0.2 mag fainter than expected
relative to the turnoff. The number of red giants (relative to main sequence
turnoff stars) is in excellent agreement with stellar models from the
Yonsei-Yale and Teramo groups, and slightly high compared to Victoria-Regina
models. This adds to evidence that an abnormally large ratio of red giant to
main-sequence stars is not correlated with cluster metallicity. We discuss
theoretical predictions from different research groups and find that the
inclusion or exclusion of helium diffusion and strong limit Coulomb
interactions may be partly responsible.
We also examine indicators of dynamical history: the mass function exponent
and the blue straggler frequency. NGC 5466 has a very shallow mass function,
consistent with large mass loss and recently-discovered tidal tails. The blue
straggler sample is significantly more centrally concentrated than the HB or
RGB stars. We see no evidence of an upturn in the blue straggler frequency at
large distances from the center. Dynamical friction timescales indicate that
the stragglers should be more concentrated if the cluster's present density
structure has existed for most of its history. NGC 5466 also has an unusually
low central density compared to clusters of similar luminosity. In spite of
this, the specific frequency of blue stragglers that puts it right on the
frequency -- cluster M_V relation observed for other clusters.Comment: 51 pages, 21 figures, 1 electronic table, accepted to Ap
Planet Consumption and Stellar Metallicity Enhancements
The evolution of a giant planet within the stellar envelope of a
main-sequence star is investigated as a possible mechanism for enhancing the
stellar metallicities of the parent stars of extrasolar planetary systems.
Three-dimensional hydrodynamical simulations of a planet subject to impacting
stellar matter indicate that the envelope of a Jupiter-like giant planet can be
completely stripped in the outer stellar convection zone of a solar-mass star.
In contrast, Jupiter-like and less massive Saturn-like giant planets are able
to survive through the base of the convection zone of a 1.22 solar-mass star.
Although strongly dependent on details of planetary interior models, partial or
total dissolution of giant planets can result in significant enhancements in
the metallicity of host stars with masses between about 1.0 and 1.3 solar
masses. The implications of these results with regard to planetary orbital
migration are briefly discussed.Comment: 11 pages, 2 figures, accepted for ApJ Letter
Close binary stars in the solar-age Galactic open cluster M67
We present multi-colour time-series CCD photometry of the solar-age galactic
open cluster M67 (NGC 2682). About 3600 frames spread over 28 nights were
obtained with the 1.5 m Russian-Turkish and 1.2 m Mercator telescopes.
High-precision observations of the close binary stars AH Cnc, EV Cnc, ES Cnc,
the Scuti type systems EX Cnc and EW Cnc, and some long-period
variables belonging to M67 are presented. Three full multi-colour light curves
of the overcontact binary AH Cnc were obtained during three observing seasons.
Likewise we gathered three light curves of EV Cnc, an EB-type binary, and two
light curves of ES Cnc, a blue straggler binary. Parts of the light change of
long-term variables S1024, S1040, S1045, S1063, S1242, and S1264 are obtained.
Period variation analysis of AH Cnc, EV Cnc, and ES Cnc were done using all
times of mid-eclipse available in the literature and those obtained in this
study. In addition, we analyzed multi-colour light curves of the close binaries
and also determined new frequencies for the Scuti systems. The
physical parameters of the close binary stars were determined with simultaneous
solutions of multi-colour light and radial velocity curves. Finally we
determined the distance of M67 as 857(33) pc via binary star parameters, which
is consistent with an independent method from earlier studies.Comment: 12 pages, 9 Figures, 13 Table
Modelling Collision Products of Triple-Star Mergers
In dense stellar clusters, binary-single and binary-binary encounters can
ultimately lead to collisions involving two or more stars. A comprehensive
survey of multi-star collisions would need to explore an enormous amount of
parameter space, but here we focus on a number of representative cases
involving low-mass main-sequence stars. Using both Smoothed Particle
Hydrodynamics (SPH) calculations and a much faster fluid sorting software
package (MMAS), we study scenarios in which a newly formed product from an
initial collision collides with a third parent star. By varying the order in
which the parent stars collide, as well as the orbital parameters of the
collision trajectories, we investigate how factors such as shock heating affect
the chemical composition and structure profiles of the collision product. Our
simulations and models indicate that the distribution of most chemical elements
within the final product is not significantly affected by the order in which
the stars collide, the direction of approach of the third parent star, or the
periastron separations of the collisions. We find that the sizes of the
products, and hence their collisional cross sections for subsequent encounters,
are sensitive to the order and geometry of the collisions. For the cases that
we consider, the radius of the product formed in the first (single-single star)
collision ranges anywhere from roughly 2 to 30 times the sum of the radii of
its parent stars. The final product formed in our triple-star collisions can
easily be as large or larger than a typical red giant. We therefore expect the
collisional cross section of a newly formed product to be greatly enhanced over
that of a thermally relaxed star of the same mass.Comment: 20 pages, submitted to MNRA
Variable Stars in the Globular Cluster M5. Application of the Image Subtraction Method
We present -band light curves of 61 variables from the core of the
globular cluster M5 obtained using a newly developed image subtraction method
(ISM). Four of these variables were previously unknown. Only 26 variables were
found in the same field using photometry obtained with DoPHOT software. Fourier
parameters of the ISM light curves have relative errors up to 20 times smaller
than parameters measured from DoPHOT photometry. We conclude that the new
method is very promising for searching for variable stars in the cores of the
globular clusters and gives very accurate relative photometry with quality
comparable to photometry obtained by HST. We also show that the variable V104
is not an eclipsing star as has been suggested, but is an RRc star showing
non-radial pulsations.Comment: submitted to MNRAS, 9 pages, 4 figure
Search for giant planets in M67 I. Overview
Precise stellar radial velocities are used to search for massive (Jupiter
masses or higher) exoplanets around the stars of the open cluster M67. We aim
to obtain a census of massive exoplanets in a cluster of solar metallicity and
age in order to study the dependence of planet formation on stellar mass and to
compare in detail the chemical composition of stars with and without planets.
This first work presents the sample and the observations, discusses the cluster
characteristics and the radial velocity (RV) distribution of the stars, and
individuates the most likely planetary host candidates. We observed a total of
88 main-sequence stars, subgiants, and giants all highly probable members of
M67, using four telescopes and instrument combinations. We investigate whether
exoplanets are present by obtaining radial velocities with precisions as good
as 10 m/s. To date, we have performed 680 single observations (Dec. 2011) and a
preliminary analysis of data, spanning a period of up to eight years. Although
the sample was pre-selected to avoid the inclusion of binaries, we identify 11
previously unknown binary candidates. Eleven stars clearly displayed larger RV
variability and these are candidates to host long-term substellar companions.
The average RV is also independent of the stellar magnitude and evolutionary
status, confirming that the difference in gravitational redshift between giants
and dwarfs is almost cancelled by the atmospheric motions. We use the subsample
of solar-type stars to derive a precise true RV for this cluster. We finally
create a catalog of binaries and use it to clean the color magnitude diagram
(CMD). As conclusion, by pushing the search for planets to the faintest
possible magnitudes, it is possible to observe solar analogues in open
clusters, and we propose 11 candidates to host substellar companions.Comment: 11 pages, 10 figure
Composition Mixing during Blue Straggler Formation and Evolution
We use smoothed-particle hydrodynamics to examine differences between direct
collisions of single stars and binary star mergers in their roles as possible
blue straggler star formation mechanisms. We find in all cases that core helium
in the progenitor stars is largely retained in the core of the remnant, almost
independent of the type of interaction or the central concentration of the
progenitor stars.
We have also modelled the subsequent evolution of the hydrostatic remnants,
including mass loss and energy input from the hydrodynamical interaction. The
combination of the hydrodynamical and hydrostatic models enables us to predict
that little mixing will occur during the merger of two globular cluster stars
of equal mass. In contrast to the results of Proctor Sills, Bailyn, & Demarque
(1995), we find that neither completely mixed nor unmixed models can match the
absolute colors of observed blue stragglers in NGC 6397 at all luminosity
levels. We also find that the color distribution is probably the crucial test
for explanations of BSS formation - if stellar collisions or mergers are the
correct mechanisms, a large fraction of the lifetime of the straggler must be
spent away from the main sequence. This constraint appears to rule out the
possibility of completely mixed models. For NGC 6397, unmixed models predict
blue straggler lifetimes ranging from about 0.1 to 4 Gyr, while completely
mixed models predict a range from about 0.6 to 4 Gyr.Comment: AASTeX, 28 pg., accepted for ApJ, also available at
http://ucowww.ucsc.edu/~erics/bspaper.htm
Imaging the circumstellar envelopes of AGB stars
We report the results of an exploratory program to image the extended
circumstellar envelopes of asymptotic giant branch (AGB) stars in
dust-scattered galactic light. The goal is to characterize the morphology of
the envelopes as a probe of the mass-loss process. The observations consist of
short exposures with the VLT and longer exposures with 1-2m telescopes,
augmented with archival images from the Hubble Space Telescope. We observed 12
AGB stars and detected the circumstellar envelopes in 7. The detected envelopes
have mass loss rates more than about 5 10E-6 solar mass per year, and they can
be seen out to distances of about 1 kpc. The observations provide information
on the mass loss history on time scales up to about 10,000 years. For the five
AGB envelopes in which the circumstellar geometry is well determined by
scattered light observations, all except one (OH348.2-19.7) show deviations
from spherical symmetry. Two (IRC+10216 and IRC+10011) show roughly spherical
envelopes at large radii but asymmetry or bipolarity close to the star; one
(AFGL 2514) shows an extended, elliptical envelope, and one (AFGL 3068) shows a
spiral pattern. The non-spherical structures are all consistent with the
effects of binary interactions. Our observations are in accord with a scenario
in which binary companions play a role in shaping planetary nebulae, and show
that the circumstellar gas is already partly shaped on the AGB, before
evolution to the proto-planetary nebula phase.Comment: Accepted by AA 21 Feb 2006; 18 pages, 14 figs; for high resolution
images, contact mauron at graal.univ-montp2.f
A CCD photometric study of the late type contact binary EK Comae Berenices
We present CCD photometric observations of the W UMa type contact binary EK
Comae Berenices using the 2 metre telescope of Girawali Observatory,
India. The star was classified as a W UMa type binary of subtype-W by
\citet{sam1996}. The new V band photometric observations of the star reveal
that shape of the light curve has changed significantly from the one observed
by \citet{sam1996}. A detailed analysis of the light curve obtained from the
high-precision CCD photometric observations of the star indicates that EK Comae
Berenices is not a W-type but an A-type totally eclipsing W UMa contact binary.
The photometric mass ratio is determined to be 0.349 0.005. A temperature
difference of K between the components and an orbital
inclination of were obtained for the binary
system. Absolute values of masses, radii and luminosities are estimated by
means of the standard mass-luminosity relation for zero age main-sequence
stars. The star shows O'Connell effect, asymmetries in the light curve shape
around the primary and secondary maximum. The observed O'Connell effect is
explained by the presence of a hot spot on the primary component.Comment: 26 pages, 9 figures, 5 tables. Accepted for publication in New
Astronom
Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission
Asteroseismic constraints on K giants make it possible to infer radii, masses
and ages of tens of thousands of field stars. Tests against independent
estimates of these properties are however scarce, especially in the metal-poor
regime. Here, we report the detection of solar-like oscillations in 8 stars
belonging to the red-giant branch and red-horizontal branch of the globular
cluster M4. The detections were made in photometric observations from the K2
Mission during its Campaign 2. Making use of independent constraints on the
distance, we estimate masses of the 8 stars by utilising different combinations
of seismic and non-seismic inputs. When introducing a correction to the Delta
nu scaling relation as suggested by stellar models, for RGB stars we find
excellent agreement with the expected masses from isochrone fitting, and with a
distance modulus derived using independent methods. The offset with respect to
independent masses is lower, or comparable with, the uncertainties on the
average RGB mass (4-10%, depending on the combination of constraints used). Our
results lend confidence to asteroseismic masses in the metal poor regime. We
note that a larger sample will be needed to allow more stringent tests to be
made of systematic uncertainties in all the observables (both seismic and
non-seismic), and to explore the properties of RHB stars, and of different
populations in the cluster.Comment: 6 pages, 3 figures, accepted for publication in MNRA
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