167 research outputs found
HS Hya about to turn off its eclipses
Aims: We aim to perform the first long-term analysis of the system HS Hya.
Methods: We performed an analysis of the long-term evolution of the light
curves of the detached eclipsing system HS Hya. Collecting all available
photometric data since its discovery, the light curves were analyzed with a
special focus on the evolution of system's inclination. Results: We find that
the system undergoes a rapid change of inclination. Since its discovery until
today the system's inclination changed by more than 15 deg. The shape of the
light curve changes, and now the eclipses are almost undetectable. The third
distant component of the system is causing the precession of the close orbit,
and the nodal period is about 631 yr. Conclusions: New precise observations are
desperately needed, preferably this year, because the amplitude of variations
is decreasing rapidly every year. We know only 10 such systems on the whole sky
at present.Comment: 4 pages, 3 figures, published in 2012A&A...542L..23
First detailed analysis of multiple system V2083 Cyg
Main aim of this paper is the first detailed analysis of multiple system
V2083 Cyg and to reveal its basic physical properties. The system was studied
by method of the light and radial velocity curves analysis, together with the
interferometric data of the visual pair obtained during a last century. There
was found that the close subsystem contains two very similar stars of spectral
type A7-8. Moreover, the third body is orbiting around this pair with period of
about 177 years. Due to the discrepancy of total mass as derived from two
methods, there arises that the third body is maybe also a binary, or some
object with lower luminosity but higher mass than normal main-sequence star.
Another explanation is that the Hipparcos value of parallax is incorrect and
the system is much closer to the Sun.Comment: 6 pages, 4 figures, published in 2012MNRAS.421.1196
Core properties of alpha Cen A using asteroseismology
A set of long and nearly continuous observations of alpha Centauri A should
allow us to derive an accurate set of asteroseismic constraints to compare to
models, and make inferences on the internal structure of our closest stellar
neighbour. We intend to improve the knowledge of the interior of alpha Centauri
A by determining the nature of its core. We combined the radial velocity time
series obtained in May 2001 with three spectrographs in Chile and Australia:
CORALIE, UVES, and UCLES. The resulting combined time series has a length of
12.45 days and contains over 10,000 data points and allows to greatly reduce
the daily alias peaks in the power spectral window. We detected 44 frequencies
that are in good overall agreement with previous studies, and found that 14 of
these show possible rotational splittings. New values for the large and small
separations have been derived. A comparison with stellar models indicates that
the asteroseismic constraints determined in this study allows us to set an
upper limit to the amount of convective-core overshooting needed to model stars
of mass and metallicity similar to those of alpha Cen A.Comment: 8 pages, 11 figures, A&A accepte
Studies of multiple stellar systems - III. Modulation of orbital elements in the triple-lined system HD 109648
The triple-lined spectroscopic triple system HD 109648 has one of the
shortest periods known for the outer orbit in a late-type triple, 120.5 days,
and the ratio between the periods of the outer and inner orbits is small, 22:1.
With such extreme values, this system should show orbital element variations
over a timescale of about a decade. We have monitored the radial velocities of
HD 109648 with the CfA Digital Speedometers for eight years, and have found
evidence for modulation of some orbital elements. While we see no definite
evidence for modulation of the inner binary eccentricity, we clearly observe
variations in the inner and outer longitudes of periastron, as well as in the
radial velocity amplitudes of the three components. The observational results,
combined with numerical simulations, allow us to put constraints on the
orientation of the orbits.Comment: 11 pages, 7 figures, accepted by MNRA
Limits on the primordial stellar multiplicity
Most stars - especially young stars - are observed to be in multiple systems.
Dynamical evolution is unable to pair stars efficiently, which leads to the
conclusion that star-forming cores must usually fragment into \geq 2 stars.
However, the dynamical decay of systems with \geq 3 or 4 stars would result in
a large single-star population that is not seen in the young stellar
population. Additionally, ejections would produce a significant population of
hard binaries that are not observed. This leads to a strong constraint on star
formation theories that cores must typically produce only 2 or 3 stars. This
conclusion is in sharp disagreement with the results of currently available
numerical simulations that follow the fragmentation of molecular cores and
typically predict the formation of 5--10 seeds per core. In addition, open
cluster remnants may account for the majority of observed highly hierarchical
higher-order multiple systems in the field.Comment: A&A in press, 5 pages (no figures
The Zeta Herculis binary system revisited. Calibration and seismology
We have revisited the calibration of the visual binary system Zeta Herculis
with the goal to give the seismological properties of the G0 IV sub-giant Zeta
Her A. We have used the most recent physical and observational data. For the
age we have obtained 3387 Myr, for the masses respectively 1.45 and 0.98 solar
mass, for the initial helium mass fraction 0.243, for the initial mass ratio of
heavy elements to hydrogen 0.0269 and for the mixing-length parameters
respectively 0.92 and 0.90 using the Canuto & Mazitelli (1991, 1992) convection
theory. Our results do not exclude that Zeta Her A is itself a binary
sub-system; the mass of the hypothetical unseen companion would be smaller than
0.05 solar mass. The adiabatic oscillation spectrum of Zeta Her A is found to
be a complicated superposition of acoustic and gravity modes; some of them have
a dual character. This greatly complicates the classification of the non-radial
modes. The echelle diagram used by the observers to extract the frequencies
will work for ell=0, 2, 3. The large difference is found to be of the order of
42 mu Hz, in agreement with the Martic et al. (2001) seismic observations.Comment: 12 pages, A&A in pres
Interferometric Observations of the Hierarchical Triple System Algol
Algol is a triple stellar system consisting of a close semidetached binary
orbited by a third object. Due to the disputed spatial orientation of the close
pair, the third body perturbation of this pair is a subject of much research.
In this study, we determine the spatial orientation of the close pair orbital
plane using the CHARA Array, a six-element optical/IR interferometer located on
Mount Wilson, and state-of-the-art e-EVN interferometric techniques. We find
that the longitude of the line of nodes for the close pair is
\Omega_1=48\degr\pm2\degr and the mutual inclination of the orbital planes of
the close and the wide pairs is 95\degr\pm3\degr. This latter value differs
by 5\degr from the formerly known 100\degr which would imply a very fast
inclination variation of the system, not borne out by the photometric
observations. We also investigated the dynamics of the system with numerical
integration of the equations of motions using our result as an initial
condition. We found large variations in the inclination of the close pair (its
amplitude \sim 170\degr) with a period of about 20 millennia. This result is
in good agreement with the photometrically observed change of amplitude in
Algol's primary minimum.Comment: ApJ, in press. This is the accepted version; will be changed with the
final version later (minor language corrections
GAIA: Composition, Formation and Evolution of the Galaxy
The GAIA astrometric mission has recently been approved as one of the next
two `cornerstones' of ESA's science programme, with a launch date target of not
later than mid-2012. GAIA will provide positional and radial velocity
measurements with the accuracies needed to produce a stereoscopic and kinematic
census of about one billion stars throughout our Galaxy (and into the Local
Group), amounting to about 1 per cent of the Galactic stellar population.
GAIA's main scientific goal is to clarify the origin and history of our Galaxy,
from a quantitative census of the stellar populations. It will advance
questions such as when the stars in our Galaxy formed, when and how it was
assembled, and its distribution of dark matter. The survey aims for
completeness to V=20 mag, with accuracies of about 10 microarcsec at 15 mag.
Combined with astrophysical information for each star, provided by on-board
multi-colour photometry and (limited) spectroscopy, these data will have the
precision necessary to quantify the early formation, and subsequent dynamical,
chemical and star formation evolution of our Galaxy. Additional products
include detection and orbital classification of tens of thousands of
extra-Solar planetary systems, and a comprehensive survey of some 10^5-10^6
minor bodies in our Solar System, through galaxies in the nearby Universe, to
some 500,000 distant quasars. It will provide a number of stringent new tests
of general relativity and cosmology. The complete satellite system was
evaluated as part of a detailed technology study, including a detailed payload
design, corresponding accuracy assesments, and results from a prototype data
reduction development.Comment: Accepted by A&A: 25 pages, 8 figure
Analysis of alpha Centauri AB including seismic constraints
Detailed models of alpha Cen A and B based on new seismological data for
alpha Cen B by Carrier & Bourban (2003) have been computed using the Geneva
evolution code including atomic diffusion. Taking into account the numerous
observational constraints now available for the alpha Cen system, we find a
stellar model which is in good agreement with the astrometric, photometric,
spectroscopic and asteroseismic data. The global parameters of the alpha Cen
system are now firmly constrained to an age of t=6.52+-0.30 Gyr, an initial
helium mass fraction Y_i=0.275+-0.010 and an initial metallicity
(Z/X)_i=0.0434+-0.0020. Thanks to these numerous observational constraints, we
confirm that the mixing-length parameter alpha of the B component is larger
than the one of the A component, as already suggested by many authors (Noels et
al. 1991, Fernandes & Neuforge 1995 and Guenther & Demarque 2000): alpha_B is
about 8% larger than alpha_A (alpha_A=1.83+-0.10 and alpha_B=1.97+-0.10).
Moreover, we show that asteroseismic measurements enable to determine the radii
of both stars with a very high precision (errors smaller than 0.3%). The radii
deduced from seismological data are compatible with the new interferometric
results of Kervella et al. (2003) even if they are slightly larger than the
interferometric radii (differences smaller than 1%).Comment: 13 pages, 9 figures, accepted for publication in A&
The Effective Fragment Molecular Orbital Method for Fragments Connected by Covalent Bonds
We extend the effective fragment molecular orbital method (EFMO) into
treating fragments connected by covalent bonds. The accuracy of EFMO is
compared to FMO and conventional ab initio electronic structure methods for
polypeptides including proteins. Errors in energy for RHF and MP2 are within 2
kcal/mol for neutral polypeptides and 6 kcal/mol for charged polypeptides
similar to FMO but obtained two to five times faster. For proteins, the errors
are also within a few kcal/mol of the FMO results. We developed both the RHF
and MP2 gradient for EFMO. Compared to ab initio, the EFMO optimized structures
had an RMSD of 0.40 and 0.44 {\AA} for RHF and MP2, respectively.Comment: Revised manuscrip
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