263 research outputs found
Resolving Sirius-like binaries with the Hubble Space Telescope
We have imaged seventeen recently discovered Sirius-like binary systems with
HST/WFPC2 and resolved the white dwarf secondary in eight cases. Most of the
implied orbital periods are of order several hundred years, but in three cases
(56 Per, Zeta Cygni and REJ1925-566) the periods are short enough that it may
be possible to detect orbital motion within a few years. It will then be
possible to derive dynamically determined masses for the white dwarfs, and
potentially these stars could be used as stringent tests of the mass-radius
relation and initial-final mass relation.Comment: To appear in Proceedings of the 12th European Workshop on White
Dwarfs, eds. H. Shipman and J. Provenca
NLTE effects on Fe I/II in the atmospheres of FGK stars and application to abundance analysis of their spectra
We describe the first results from our project aimed at large-scale
calculations of NLTE abundance corrections for important astrophysical atoms
and ions. In this paper, the focus is on Fe which is a proxy of stellar
metallicity and is commonly used to derive effective temperature and gravity.
We present a small grid of NLTE abundance corrections for Fe I lines and
discuss how NLTE effects influence determination of effective temperature,
surface gravity, and metallicity for late-type stars.Comment: 6 pages, to be published in IOP The Journal of Physics: Conference
Series, proceedings of the Workshop: 'Stellar Atmospheres in the Gaia Era:
Quantitative Spectroscopy and Comparative Spectrum Modelling', Brussels, June
201
Grids of Stellar Models and Frequencies with CLES + LOSC
We present a grid of stellar models, obtained with the CLES evolution code,
following the specification of ESTA-Task1, and the corresponfing seismic
properties, computed with the LOSC code. We provide a complete description of
the corresponding files that will be available on the ESTA web-pages.Comment: 8 pages, accepted for publication in Astrophys. Space Sci.
(CoRoT/ESTA Volume
Impact of the new solar abundances on the calibration of the PMS binary system RS Cha
Context: In a recent work, we tried to obtain a calibration of the two
components of the pre-main sequence binary system RS Cha by means of
theoretical stellar models. We found that the only way to reproduce the
observational parameters of RS Cha with standard stellar models is to decrease
the initial abundances of carbon and nitrogen derived from the GN93 solar
mixture of heavy elements by a few tenths of dex.
Aims: In this work, we aim to reproduce the observational properties of the
RS Cha stars with stellar evolution models based on the new AGS05 solar mixture
recently derived from a three-dimensional solar model atmosphere. The AGS05
mixture is depleted in carbon, nitrogen and oxygen with respect to the GN93
mixture.
Methods: We calculated new stellar models of the RS Cha components using the
AGS05 mixture and appropriate opacity tables. We sought models that
simultaneously satisfy the observations of the two components (masses, radii,
luminosities, effective temperatures and metallicity).
Results: We find that it is possible to reproduce the observational data of
the RS Cha stars with AGS05 models based on standard input physics. From these
models, the initial helium content of the system is Y~0.255 and its age is
~9.13 +- 0.12 Myr.Comment: Research note accepted in A&A, 5 pages, 2 figure
On the linear fractional self-attracting diffusion
In this paper, we introduce the linear fractional self-attracting diffusion
driven by a fractional Brownian motion with Hurst index 1/2<H<1, which is
analogous to the linear self-attracting diffusion. For 1-dimensional process we
study its convergence and the corresponding weighted local time. For
2-dimensional process, as a related problem, we show that the renormalized
self-intersection local time exists in L^2 if .Comment: 14 Pages. To appear in Journal of Theoretical Probabilit
Uncertainties in stellar evolution models: convective overshoot
In spite of the great effort made in the last decades to improve our
understanding of stellar evolution, significant uncertainties remain due to our
poor knowledge of some complex physical processes that require an empirical
calibration, such as the efficiency of the interior mixing related to
convective overshoot. Here we review the impact of convective overshoot on the
evolution of stars during the main Hydrogen and Helium burning phases.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
New homogeneous iron abundances of double-mode Cepheids from high-resolution echelle spectroscopy
Aims: We define the relationship between the double-mode pulsation of
Cepheids and metallicity in a more accurate way, determine the empirical
metallicities of double-mode Cepheids from homogeneous, high-resolution
spectroscopic data, and study of the period-ratio -- metallicity dependence.
Methods: The high S/N echelle spectra obtained with the FEROS spectrograph were
analyzed using a self-developed IRAF script, and the iron abundances were
determined by comparing with synthetic spectra assuming LTE. Results: Accurate
[Fe/H] values of 17 galactic beat Cepheids were determined. All these stars
have solar or slightly subsolar metallicity. Their period ratio P1/P0 shows
strong correlation with their derived [Fe/H] values. The corresponding period
ratio -- metallicity relation has been evaluated.Comment: 10 pages, 7 figures, accepted in A&
Analysis of a Fragmenting Sunspot using Hinode Observations
We employ high resolution filtergrams and polarimetric measurements from
Hinode to follow the evolution of a sunspot for eight days starting on June 28,
2007. The imaging data were corrected for intensity gradients, projection
effects, and instrumental stray light prior to the analysis. The observations
show the formation of a light bridge at one corner of the sunspot by a slow
intrusion of neighbouring penumbral filaments. This divided the umbra into two
individual umbral cores. During the light bridge formation, there was a steep
increase in its intensity from 0.28 to 0.7 I_QS in nearly 4 hr, followed by a
gradual increase to quiet Sun (QS) values in 13 hr. This increase in intensity
was accompanied by a large reduction in the field strength from 1800 G to 300
G. The smaller umbral core gradually broke away from the parent sunspot nearly
2 days after the formation of the light bridge rendering the parent spot
without a penumbra at the location of fragmentation. The penumbra in the
fragment disappeared first within 34 hr, followed by the fragment whose area
decayed exponentially with a time constant of 22 hr. The depleted penumbra in
the parent sunspot regenerated when the inclination of the magnetic field at
the penumbra-QS boundary became within 40 deg. from being completely horizontal
and this occurred near the end of the fragment's lifetime. After the
disappearance of the fragment, another light bridge formed in the parent which
had similar properties as the fragmenting one, but did not divide the sunspot.
The significant weakening in field strength in the light bridge along with the
presence of granulation is suggestive of strong convection in the sunspot which
might have triggered the expulsion and fragmentation of the smaller spot.
Although the presence of QS photospheric conditions in sunspot umbrae could be
a necessary condition for fragmentation, it is not a sufficient one.Comment: Accepted for publication in ApJ; 15 pages, 15 figures, 1 tabl
Disentangling discrepancies between stellar evolution theory and sub-solar mass stars. The influence of the mixing length parameter for the UV Psc binary
Serious discrepancies have recently been observed between predictions of
stellar evolution models in the 0.7-1.1 M_sun mass range and accurately
measured properties of binary stars with components in this mass range. We
study one of these objects, the eclipsing binary UV Piscium, which is
particularly interesting because Popper (1997) derived age estimates for each
component which differed by more than a factor of two. In an attempt to solve
this significant discrepancy (a difference in age of 11 Gyr), we compute a
large grid of stellar evolution models with the CESAM code for each component.
By fixing the masses to their accurately determined values (relative error
smaller than 1% for both stars), we consider a wide range of possible
metallicities Z (0.01 to 0.05), and Helium content Y (0.25 to 0.34)
uncorrelated to Z. In addition, the mixing length parameter alpha_MLT is left
as another free parameter. We obtain a best fit in the T_eff-radius diagram for
a common chemical composition (Z, Y)=(0.012, 0.31), but a different MLT
parameter alpha_MLT_A = 0.95+-0.12(statistical)+0.30(systematic) and
alpha_MLT_B = 0.65+-0.07(stat)+0.10(syst). The apparent age discrepancy found
by Popper (1997) disappears with this solution, the components being coeval to
within 1%. This suggests that fixing alpha_MLT to its solar value (~1.6), a
common hypothesis assumed in most stellar evolutionary models, may not be
correct. Secondly, since alpha_MLT is smaller for the less massive component,
this suggests that the MLT parameter may decrease with stellar mass, showing
yet another shortcoming of the mixing length theory to explain stellar
convection. This trend needs further confirmation with other binary stars with
accurate data.Comment: 8 pages, accepted for publication in Astronomy & Astrophysic
Solution to the problem of the surface gravity distribution of cool DA white dwarfs from improved 3D model atmospheres
The surface gravities of cool (Teff < 13,000 K) hydrogen-atmosphere DA white
dwarfs, determined from spectroscopic analyses, are found to be significantly
higher than the canonical value of log g ~ 8 expected for these stars. It was
recently concluded that a problem with the treatment of convective energy
transport within the framework of the mixing-length theory was the most
plausible explanation for this high-log g problem. We pursue the investigation
of this discrepancy by computing model spectra of cool convective white dwarfs
from a small sequence (11,300 K < Teff < 12,800 K) of 3D hydrodynamical model
atmospheres, which feature a sophisticated treatment of convection and
radiative transfer. Our approach is to proceed with a differential analysis
between 3D and standard 1D models. We find that the 3D spectra predict
significantly lower surface gravities, with corrections of the right amplitude
as a function of effective temperature to obtain values of log g ~ 8 on
average. We conclude that the surface gravity distribution of cool convective
DA white dwarfs is much closer to that of hotter radiative objects when using,
for the treatment of the convection, 3D models instead of the mixing-length
framework.Comment: 5 pages, 5 figures, accepted for publication in the Astronomy &
Astrophysics Letter
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