1,837 research outputs found
Line formation in convective stellar atmospheres. I. Granulation corrections for solar photospheric abundances
In an effort to estimate the largely unknown effects of photospheric
temperature fluctuations on spectroscopic abundance determinations, we have
studied the problem of LTE line formation in the inhomogeneous solar
photosphere based on detailed 2-dimensional radiation hydrodynamics simulations
of the convective surface layers of the Sun. By means of a strictly
differential 1D/2D comparison of the emergent equivalent widths, we have
derived "granulation abundance corrections" for individual lines, which have to
be applied to standard abundance determinations based on homogeneous 1D model
atmospheres in order to correct for the influence of the photospheric
temperature fluctuations. In general, we find a line strengthening in the
presence of temperature inhomogeneities as a consequence of the non-linear
temperature dependence of the line opacity. For many lines of practical
relevance, the magnitude of the abundance correction may be estimated from
interpolation in the tables and graphs provided with this paper. The
application of abundance corrections may often be an acceptable alternative to
a detailed fitting of individual line profiles based on hydrodynamical
simulations. The present study should be helpful in providing upper bounds for
possible errors of spectroscopic abundance analyses, and for identifying
spectral lines which are least sensitive to the influence of photospheric
temperature inhomogeneities.Comment: Accepted by A&
The effects of numerical resolution on hydrodynamical surface convection simulations and spectral line formation
The computationally demanding nature of radiative-hydrodynamical simulations
of stellar surface convection warrants an investigation of the sensitivity of
the convective structure and spectral synthesis to the numerical resolution and
dimension of the simulations, which is presented here. With too coarse a
resolution the predicted spectral lines tend to be too narrow, reflecting
insufficient Doppler broadening from the convective motions, while at the
currently highest affordable resolution the line shapes have converged
essentially perfectly to the observed profiles. Similar conclusions are drawn
from the line asymmetries and shifts. In terms of abundances, weak FeI and FeII
lines show a very small dependence (~0.02 dex) while for intermediate strong
lines with significant non-thermal broadening the sensitivity increases (~0.10
dex). Problems arise when using 2D convection simulations to describe an
inherent 3D phenomenon, which translates to inaccurate atmospheric velocity
fields and temperature and pressure structures. In 2D the theoretical line
profiles tend to be too shallow and broad compared with the 3D calculations and
observations, in particular for intermediate strong lines. In terms of
abundances, the 2D results are systematically about 0.1 dex lower than for the
3D case for FeI lines. Furthermore, the predicted line asymmetries and shifts
are much inferior in 2D. Given these shortcomings and computing time
considerations it is better to use 3D simulations of even modest resolution
than high-resolution 2D simulations.Comment: Accepted for A&
The discrepancy in G-band contrast: Where is the quiet Sun?
We compare the rms contrast in observed speckle reconstructed G-band images
with synthetic filtergrams computed from two magneto-hydrodynamic simulation
snapshots. The observations consist of 103 bursts of 80 frames each taken at
the Dunn Solar Telescope (DST), sampled at twice the diffraction limit of the
telescope. The speckle reconstructions account for the performance of the
Adaptive Optics (AO) system at the DST to supply reliable photometry. We find a
considerable discrepancy in the observed rms contrast of 14.1% for the best
reconstructed images, and the synthetic rms contrast of 21.5% in a simulation
snapshot thought to be representative of the quiet Sun. The areas of features
in the synthetic filtergrams that have positive or negative contrast beyond the
minimum and maximum values in the reconstructed images have spatial scales that
should be resolved. This leads us to conclude that there are fundamental
differences in the rms G-band contrast between observed and computed
filtergrams. On the basis of the substantially reduced granular contrast of
16.3% in the synthetic plage filtergram we speculate that the quiet-Sun may
contain more weak magnetic field than previously thought.Comment: 16 pages, 8 figure
Oxygen isotopic ratios in cool R Coronae Borealis stars
We investigate the relationship between R Coronae Borealis (RCB) stars and
hydrogen-deficient carbon (HdC) stars by measuring precise 16O/18O ratios for
five cool RCB stars. The 16O/18O ratios are derived by spectrum synthesis from
high-resolution (R=50,000) K-band spectra. Lower limits to the 16O/17O and
14N/15}N ratios as well as Na and S abundances (when possible) are also given.
RCB stars in our sample generally display less 18O than HdC stars - the derived
16O/18O ratios range from 3 to 20. The only exception is the RCB star WX CrA,
which seems to be a HdC-like star with 16O/18O=0.3. Our result of a higher
16O/18O ratio for the RCB stars must be accounted for by a theory of the
formation and evolution of HdC and RCB stars. We speculate that a late
dredge-up of products of He-burning, principally 12C and 16O, may convert a
18O-rich HdC star into a 18O-poor RCB star as the H-deficient star begins its
final evolution from a cool supergiant to the top of the white dwarf cooling
track.Comment: Accepted for publication in the Astrophysical Journal; 21 pages, 3
Tables, 11 Figure
Numerical simulations of surface convection in a late M-dwarf
Based on detailed 2D and 3D numerical radiation-hydrodynamics (RHD)
simulations of time-dependent compressible convection, we have studied the
dynamics and thermal structure of the convective surface layers of a
prototypical late-type M-dwarf (Teff~2800K log(g)=5.0, solar chemical
composition). The RHD models predict stellar granulation qualitatively similar
to the familiar solar pattern. Quantitatively, the granular cells show a
convective turn-over time scale of ~100s, and a horizontal scale of 80km; the
relative intensity contrast of the granular pattern amounts to 1.1%, and
root-mean-square vertical velocities reach 240m/s at maximum. Deviations from
radiative equilibrium in the higher, formally convectively stable atmospheric
layers are found to be insignificant allowing a reliable modeling of the
atmosphere with 1D standard model atmospheres. A mixing-length parameter of
alpha=2.1 provides the best representation of the average thermal structure of
the RHD model atmosphere while alternative values are found when fitting the
asymptotic entropy encountered in deeper layers of the stellar envelope
alpha=1.5, or when matching the vertical velocity field alpha=3.5. The close
correspondence between RHD and standard model atmospheres implies that
presently existing discrepancies between observed and predicted stellar colors
in the M-dwarf regime cannot be traced back to an inadequate treatment of
convection in the 1D standard models. The RHD models predict a modest extension
of the convectively mixed region beyond the formal Schwarzschild stability
boundary which provides hints for the distribution of dust grains in cooler
(brown dwarf) atmospheres.Comment: 19 pages, 16 figures, accepted for publication in A&
Hydrodynamical model atmospheres and 3D spectral synthesis
We discuss three issues in the context of three-dimensional (3D)
hydrodynamical model atmospheres for late-type stars, related to spectral line
shifts, radiative transfer in metal-poor 3D models, and the solar oxygen
abundance. We include a brief overview about the model construction, taking the
radiation-hydrodynamics code CO5BOLD (COnservative COde for the COmputation of
COmpressible COnvection in a BOx of L Dimensions with L=2,3) and the related
spectral synthesis package Linfor3D as examples.Comment: 6 pages, 2 figures, to appear in the Proceedings of the
ESO/Lisbon/Aveiro Workshop "Precision Spectroscopy in Astrophysics", eds. L.
Pasquini, M. Romaniello, N.C. Santos, and A. Correi
Accounting for Convective Blue-Shifts in the Determination of Absolute Stellar Radial Velocities
For late-type non-active stars, gravitational redshifts and convective
blueshifts are the main source of biases in the determination of radial
velocities. If ignored, these effects can introduce systematic errors of the
order of ~ 0.5 km/s. We demonstrate that three-dimensional hydrodynamical
simulations of solar surface convection can be used to predict the convective
blue-shifts of weak spectral lines in solar-like stars to ~ 0.070 km/s. Using
accurate trigonometric parallaxes and stellar evolution models, the
gravitational redshifts can be constrained with a similar uncertainty, leading
to absolute radial velocities accurate to better than ~ 0.1 km/s.Comment: To appear in the proceedings of the Joint Discussion 10, IAU General
Assembly, Rio de Janeiro, August 10-11, 200
HE0107-5240, A Chemically Ancient Star.I. A Detailed Abundance Analysis
We report a detailed abundance analysis for HE0107-5240, a halo giant with
[Fe/H]_NLTE=-5.3. This star was discovered in the course of follow-up
medium-resolution spectroscopy of extremely metal-poor candidates selected from
the digitized Hamburg/ESO objective-prism survey. On the basis of
high-resolution VLT/UVES spectra, we derive abundances for 8 elements (C, N,
Na, Mg, Ca, Ti, Fe, and Ni), and upper limits for another 12 elements. A
plane-parallel LTE model atmosphere has been specifically tailored for the
chemical composition of {\he}. Scenarios for the origin of the abundance
pattern observed in the star are discussed. We argue that HE0107-5240 is most
likely not a post-AGB star, and that the extremely low abundances of the
iron-peak, and other elements, are not due to selective dust depletion. The
abundance pattern of HE0107-5240 can be explained by pre-enrichment from a
zero-metallicity type-II supernova of 20-25M_Sun, plus either self-enrichment
with C and N, or production of these elements in the AGB phase of a formerly
more massive companion, which is now a white dwarf. However, significant radial
velocity variations have not been detected within the 52 days covered by our
moderate-and high-resolution spectra. Alternatively, the abundance pattern can
be explained by enrichment of the gas cloud from which HE0107-5240 formed by a
25M_Sun first-generation star exploding as a subluminous SNII, as proposed by
Umeda & Nomoto (2003). We discuss consequences of the existence of HE0107-5240
for low-mass star formation in extremely metal-poor environments, and for
currently ongoing and future searches for the most metal-poor stars in the
Galaxy.Comment: 60 pages, 16 figures. Accepted for publication in Ap
A new approach to the solar oxygen abundance problem
In this work we present new data that sets strong constraints on the solar
oxygen abundance. Our approach, based on the analysis of spectro-polarimetric
observations, is almost model-independent and therefore extremely robust. The
asymmetry of the Stokes V profile of the 6300 A [OI] and NiI blend is used as
an indicator of the relative abundances of these two elements. The peculiar
shape of the profile requires a value of EO = 730+/-100 ppm (parts per
million), or logEO = 8.86+/-0.07 in the logarithmic scale commonly used in
Astrophysics. The uncertainty range includes the model dependence as well as
uncertainties in the oscillator strengths of the lines. We emphasize that the
very low degree of model dependence in our analysis makes it very reliable
compared to traditional determinations.Comment: Accepted for publication in The Astrophysical Journal Letters. 12
pages, 3 figures, referee format. This is the replacement of a previous
version of the paper. Our revised analysis takes into consideration the
formation of molecules, resulting in a substantially larger value for the
derived Oxygen abundanc
- …