26 research outputs found
Influence of local treatments of convection upon solar p mode excitation rates
We compute the rates P at which acoustic energy is injected into the solar
radial p modes for several solar models. The solar models are computed with two
different local treatments of convection: the classical mixing-length theory
(MLT hereafter) and Canuto et al (1996)'s formulation (CGM hereafter). Among
the models investigated here, our best models reproduce both the solar radius
and the solar luminosity at solar age and the observed Balmer line profiles.
For the MLT treatment, the rates P do depend significantly on the properties of
the atmosphere whereas for the CGM's treatment the dependence of P on the
properties of the atmosphere is found smaller than the error bars attached to
the seismic measurements. The excitation rates P for modes associated with the
MLT models are significantly underestimated compared with the solar seismic
constraints. The CGM models yield values for P closer to the seismic data than
the MLT models. We conclude that the solar p-mode excitation rates provide
valuable constraints and according to the present investigation clearly favor
the CGM treatment with respect to the MLT, although neither of them yields
values of P as close to the observations as recently found for 3D numerical
simulations.Comment: 11 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
Effect of convective outer layers modeling on non-adiabatic seismic observables of delta Scuti stars
The identification of pulsation modes in delta Scuti stars is mandatory to
constrain the theoretical stellar models. The non-adiabatic observables used in
the photometric identification methods depend, however,on convection modeling
in the external layers. Our aim is to determine how the treatment of convection
in the atmospheric and sub-atmospheric layers affects the mode identification,
and what information about the thermal structure of the external layers can be
obtained from amplitude ratios and phase lags in Str\"omgren photometric bands.
We derive non-adiabatic parameters for delta Scuti stars by using, for the
first time, stellar models with the same treatment of convection in the
interior and in the atmosphere. We compute classical non-gray mixing length
models, and as well non-gray ``Full Spectrum of Turbulence'' models.
Furthermore, we compute the photometric amplitudes and phases of pulsation by
using the colors and the limb-darkening coefficents as derived from the same
atmosphere models used in the stellar modeling. We show that the non-adiabatic
phase-lag is mainly sensitive to the thermal gradients in the external layers,
(and hence to the treatment of convection), and that this sensitivity is also
clearly reflected in the multi-color photometric phase differences.Comment: 14 pag. 19 figs. accepted for publication in Astronomy and
Astrophysic
Impact of granulation effects on the use of Balmer lines as temperature indicators
Balmer lines serve as important indicators of stellar effective temperatures
in late-type stellar spectra. One of their modelling uncertainties is the
influence of convective flows on their shape. We aim to characterize the
influence of convection on the wings of Balmer lines. We perform a differential
comparison of synthetic Balmer line profiles obtained from 3D hydrodynamical
model atmospheres and 1D hydrostatic standard ones. The model parameters are
appropriate for F,G,K dwarf and subgiant stars of metallicity ranging from
solar to 1/1000 solar. The shape of the Balmer lines predicted by 3D models can
never be exactly reproduced by a 1D model, irrespective of its effective
temperature. We introduce the concept of a 3D temperature correction, as the
effective temperature difference between a 3D model and a 1D model which
provides the closest match to the 3D profile. The temperature correction is
different for the different members of the Balmer series and depends on the
adopted mixing-length parameter in the 1D model. Among the investigated models,
the 3D correction ranges from -300K to +300K. Horizontal temperature
fluctuations tend to reduce the 3D correction. Accurate effective temperatures
cannot be derived from the wings of Balmer lines, unless the effects of
convection are properly accounted for. The 3D models offer a physically well
justified way of doing so. The use of 1D models treating convection with the
mixing-length theory do not appear to be suitable for this purpose. In
particular, there are indications that it is not possible to determine a single
value of the mixing-length parameter which will optimally reproduce the Balmer
lines for any choice of atmospheric parameters.Comment: 6 pages, 3 figures, accepted for publication in A&
Some Aspects of the calculation of Balmer lines in the sun and stars
We compare the results of Balmer-line calculations using recent theory and
improved computational algorithms with those from the widely-used SYNTHE and
BALMER9 routines. The resulting profiles are mostly indistinguishable. Good
fits to the normalized solar Balmer lines H through H are
obtained (apart from the cores) using the recent unified-broadening
calculations by Barklem and his coworkers provided that some adjustment for the
continuum is performed. We discuss a surprising linearity with temperature of
the Balmer line profiles in dwarfs.Comment: Submitted to Astronomy and Astrophysics Better figures at
http://astro.lsa.umich.edu/users/cowle
On the anomaly of Balmer line profiles of A-type stars. Fundamental binary systems
In previous work, Gardiner et al. (1999) found evidence for a discrepancy
between the Teff obtained from Balmer lines with that from photometry and
fundamental values for A-type stars. An investigation into this anomaly is
presented using Balmer line profiles of stars in binary system with fundamental
values of both Teff and log g. A revision of the fundamental parameters for
binary systems given by Smalley & Dworetsky (1995) is also presented. The Teff
obtained by fitting Halpha and Hbeta line profiles is compared to the
fundamental values and those obtained from uvby photometry. We find that the
discrepancy found by Gardiner et al. (1999) for stars in the range 7000 K <
Teff < 9000 K is no longer evident.Comment: 10 pages, 4 figures; Accepted by A&
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
Detailed analysis of Balmer lines in cool dwarf stars
An analysis of H alpha and H beta spectra in a sample of 30 cool dwarf and
subgiant stars is presented using MARCS model atmospheres based on the most
recent calculations of the line opacities. A detailed quantitative comparison
of the solar flux spectra with model spectra shows that Balmer line profile
shapes, and therefore the temperature structure in the line formation region,
are best represented under the mixing length theory by any combination of a low
mixing-length parameter alpha and a low convective structure parameter y. A
slightly lower effective temperature is obtained for the sun than the accepted
value, which we attribute to errors in models and line opacities. The programme
stars span temperatures from 4800 to 7100 K and include a small number of
population II stars. Effective temperatures have been derived using a
quantitative fitting method with a detailed error analysis. Our temperatures
find good agreement with those from the Infrared Flux Method (IRFM) near solar
metallicity but show differences at low metallicity where the two available
IRFM determinations themselves are in disagreement. Comparison with recent
temperature determinations using Balmer lines by Fuhrmann (1998, 2000), who
employed a different description of the wing absorption due to self-broadening,
does not show the large differences predicted by Barklem et al. (2000). In
fact, perhaps fortuitously, reasonable agreement is found near solar
metallicity, while we find significantly cooler temperatures for low
metallicity stars of around solar temperature.Comment: 17 pages, 9 figures, to appear in A&
Abundance analysis of targets for the COROT/MONS asteroseismology missions II. Abundance analysis of the COROT main targets
One of the goals of the ground-based support program for the COROT and MONS/RØMER satellite missions is to characterize suitable target stars for the part of the missions dedicated to asteroseismology. We present the detailed abundance analysis of nine of the potential COROT main targets using the semi-automatic software VWA. For two additional COROT targets we could not perform the analysis due to the high rotational velocity of these stars. For five stars with low rotational velocity we have also performed abundance analysis by a classical equivalent width method in order to test the reliability of the VWA software. The agreement between the different methods is good. We find that it is necessary to measure abundances extracted from each line relative to the abundances found from a spectrum of the Sun in order to remove systematic errors. We have constrained the global atmospheric parameters Teff log g, and [Fe/H] to within 70-100 K, 0.1-0.2 dex, and 0.1 dex for five stars which are slow rotators (ν sin i 60 km s-1) it is not possible to constrain the atmospheric parameters
Abundance analysis of targets for the COROT / MONS asteroseimology missions I. Semi-automatic abundance analysis of the gamma Dor star HD 49434
One of the goals of the ground-based support program for the COROT and
MONS/Roemer satellite missions is to select and characterise suitable target
stars for the part of the missions dedicated to asteroseismology. While the
global atmospheric parameters may be determined with good accuracy from the
Stromgren indices, careful abundance analysis must be made for the proposed
main targets. This is a time consuming process considering the long list of
primary and secondary targets. We have therefore developed new software called
VWA for this task. The VWA automatically selects the least blended lines from
the atomic line database VALD, and consequently adjusts the abundance in order
to find the best match between the calculated and observed spectra. The
variability of HD 49434 was discovered as part of COROT ground-based support
observations. Here we present a detailed abundance analysis of HD 49434 using
VWA. For most elements we find abundances somewhat below the Solar values, in
particular we find [Fe/H] = -0.13(14). We also present the results from the
study of the variability that is seen in spectroscopic and photometric time
series observations. From the characteristics of the variation seen in
photometry and in the line profiles we propose that HD 49434 is a variable star
of the gamma Doradus type.Comment: 10 pages, 6 figure
Prospects for population synthesis in the H band: NeMo grids of stellar atmospheres compared to observations
For applications in population synthesis, libraries of theoretical stellar
spectra are often considered an alternative to template libraries of observed
spectra, because they allow a complete sampling of stellar parameters. Most
attention in published theoretical spectral libraries has been devoted to the
visual wavelength range. We present a detailed comparison of theoretical
spectra in the range 1.57-1.67m, for spectral types from A to early M and
for giants and dwarf stars, with observed stellar spectra at resolutions around
3000, which would be sufficient to disentangle the different groups of late
type stars. We have selected the NeMo grids of stellar atmospheres to perform
such a comparison. We first demonstrate that after combining atomic and
molecular line lists, it is possible to match observed spectral flux
distributions with theoretical ones very well for almost the entire parameter
range covered by the NeMo grids at moderate resolution in the visual range. In
the infrared range, although the overall shape of the observed flux
distributions is still matched reasonably well, the individual spectral
features are reproduced by the theoretical spectra only for stars earlier than
mid F type. For later spectral types the differences increase and theoretical
spectra of K type stars have systematically weaker line features than those
found in observations. These discrepancies are traced back to stem primarily
from incomplete data on neutral atomic lines, although some of them are also
related to molecules. Improving atomic data in the near infrared is a key
element in making the construction of reliable libraries of stellar spectra in
the infrared feasible.Comment: 17 pages, 19 figures, accepted for publication in A&