10,630 research outputs found
3D Simulation of Convection and Spectral Line Formation in A-type Stars
We present first realistic numerical simulations of 3D radiative convection
in the surface layers of main sequence A-type stars with Teff = 8000 K and 8500
K, log g = 4.4 and 4.0, recently performed with the CO5BOLD radiation
hydrodynamics code. The resulting models are used to investigate the structure
of the H+HeI and the HeII convection zones in comparison with the predictions
of local and non-local convection theories, and to determine the amount of
"overshoot" into the stable layers below the HeII convection zone. The
simulations also predict how the topology of the photospheric granulation
pattern changes from solar to A-type star convection. The influence of the
photospheric temperature fluctuations and velocity fields on the shape of
spectral lines is demonstrated by computing synthetic line profiles and line
bisectors for some representative examples, allowing us to confront the 3D
model results with observations.Comment: 5 pages, 6 figures (17 figure files), 1 Tabl
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
The Uniqueness Problem of Sequence Product on Operator Effect Algebra
A quantum effect is an operator on a complex Hilbert space that satisfies
. We denote the set of all quantum effects by . In
this paper we prove, Theorem 4.3, on the theory of sequential product on which shows, in fact, that there are sequential products on which are not of the generalized L\"{u}ders form. This result answers a
Gudder's open problem negatively
Spectroscopic properties of a two-dimensional time-dependent Cepheid model II. Determination of stellar parameters and abundances
Standard spectroscopic analyses of variable stars are based on hydrostatic
one-dimensional model atmospheres. This quasi-static approach has theoretically
not been validated. We aim at investigating the validity of the quasi-static
approximation for Cepheid variables. We focus on the spectroscopic
determination of the effective temperature , surface gravity
, microturbulent velocity , and a generic metal
abundance -- here taken as iron. We calculate a grid of 1D
hydrostatic plane-parallel models covering the ranges in effective temperature
and gravity encountered during the evolution of a two-dimensional
time-dependent envelope model of a Cepheid computed with the
radiation-hydrodynamics code CO5BOLD. We perform 1D spectral syntheses for
artificial iron lines in local thermodynamic equilibrium varying the
microturbulent velocity and abundance. We fit the resulting equivalent widths
to corresponding values obtained from our dynamical model. For the
four-parametric case, the stellar parameters are typically underestimated
exhibiting a bias in the iron abundance of \approx-0.2\,\mbox{dex}. To avoid
biases of this kind it is favourable to restrict the spectroscopic analysis to
photometric phases using additional
information to fix effective temperature and surface gravity. Hydrostatic 1D
model atmospheres can provide unbiased estimates of stellar parameters and
abundances of Cepheid variables for particular phases of their pulsations. We
identified convective inhomogeneities as the main driver behind potential
biases. For obtaining a complete view on the effects when determining stellar
parameters with 1D models, multi-dimensional Cepheid atmosphere models are
necessary for variables of longer period than investigated here.Comment: accepted for publication in Astronomy & Astrophysic
Spectroscopic properties of a two-dimensional time-dependent Cepheid model I. Description and validation of the model
Standard spectroscopic analyses of Cepheid variables are based on hydrostatic
one-dimensional model atmospheres, with convection treated using various
formulations of mixing-length theory. This paper aims to carry out an
investigation of the validity of the quasi-static approximation in the context
of pulsating stars. We check the adequacy of a two-dimensional time-dependent
model of a Cepheid-like variable with focus on its spectroscopic properties.
With the radiation-hydrodynamics code CO5BOLD, we construct a two-dimensional
time-dependent envelope model of a Cepheid with K, , solar metallicity, and a 2.8-day pulsation period. Subsequently, we
perform extensive spectral syntheses of a set of artificial iron lines in local
thermodynamic equilibrium. The set of lines allows us to systematically study
effects of line strength, ionization stage, and excitation potential. We
evaluate the microturbulent velocity, line asymmetry, projection factor, and
Doppler shifts. The mean Doppler shift is non-zero and negative, -1 km/s, after
averaging over several full periods and lines. This residual line-of-sight
velocity (related to the "K-term") is primarily caused by horizontal
inhomogeneities, and consequently we interpret it as the familiar convective
blueshift ubiquitously present in non-pulsating late-type stars. Limited
statistics prevent firm conclusions on the line asymmetries. Our
two-dimensional model provides a reasonably accurate representation of the
spectroscopic properties of a short-period Cepheid-like variable star. Some
properties are primarily controlled by convective inhomogeneities rather than
by the Cepheid-defining pulsations
Pure-hydrogen 3D model atmospheres of cool white dwarfs
A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white dwarfs
is presented for a surface gravity of log g = 8 and effective temperatures from
6000 to 13,000 K. We show that convective properties, such as flow velocities,
characteristic granulation size and intensity contrast of the granulation
patterns, change significantly over this range. We demonstrate that these 3D
simulations are not sensitive to numerical parameters unlike the 1D structures
that considerably depend on the mixing-length parameters. We conclude that 3D
spectra can be used directly in the spectroscopic analyses of DA white dwarfs.
We confirm the result of an earlier preliminary study that 3D model spectra
provide a much better characterization of the mass distribution of white dwarfs
and that shortcomings of the 1D mixing-length theory are responsible for the
spurious high-log g determinations of cool white dwarfs. In particular, the 1D
theory is unable to account for the cooling effect of the convective overshoot
in the upper atmospheres.Comment: 14 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
Spectroscopic analysis of DA white dwarfs with 3D model atmospheres
We present the first grid of mean three-dimensional (3D) spectra for
pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use CO5BOLD
radiation-hydrodynamics 3D simulations instead of the mixing-length theory for
the treatment of convection. The simulations cover the effective temperature
range of 6000 < Teff (K) < 15,000 and the surface gravity range of 7 < log g <
9 where the large majority of DAs with a convective atmosphere are located. We
rely on horizontally averaged 3D structures (over constant Rosseland optical
depth) to compute spectra. It is demonstrated that our spectra can be
smoothly connected to their 1D counterparts at higher and lower Teff where the
3D effects are small. Analytical functions are provided in order to convert
spectroscopically determined 1D effective temperatures and surface gravities to
3D atmospheric parameters. We apply our improved models to well studied
spectroscopic data sets from the Sloan Digital Sky Survey and the White Dwarf
Catalog. We confirm that the so-called high-log g problem is not present when
employing spectra and that the issue was caused by inaccuracies in the 1D
mixing-length approach. The white dwarfs with a radiative and a convective
atmosphere have derived mean masses that are the same within ~0.01 Msun, in
much better agreement with our understanding of stellar evolution. Furthermore,
the 3D atmospheric parameters are in better agreement with independent Teff and
log g values from photometric and parallax measurements.Comment: 15 pages, 18 figures, 10 pages online appendix, accepted for
publication in Astronomy and Astrophysic
Convective line shifts for the Gaia RVS from the CIFIST 3D model atmosphere grid
To derive space velocities of stars along the line of sight from wavelength
shifts in stellar spectra requires accounting for a number of second-order
effects. For most stars, gravitational redshifts, convective blueshifts, and
transverse stellar motion are the dominant contributors. We provide theoretical
corrections for the net velocity shifts due to convection expected for the
measurements from the Gaia Radial Velocity Spectrometer (RVS). We used a set of
three-dimensional time-dependent simulations of stellar surface convection
computed with CO5BOLD to calculate spectra of late-type stars in the Gaia RVS
range and to infer the net velocity offset that convective motions will induce
in radial velocities derived by cross-correlation. The net velocity shifts
derived by cross-correlation depend both on the wavelength range and spectral
resolution of the observations. Convective shifts for Gaia RVS observations are
less than 0.1 km/s for late-K-type stars, and they increase with stellar mass,
reaching about 0.3 km/s or more for early F-type dwarfs. This tendency is the
result of an increase with effective temperature in both temperature and
velocity fluctuations in the line-forming region. Our simulations also indicate
that the net RVS convective shifts can be positive (i.e. redshifts) in some
cases. Overall, the blueshifts weaken slightly with increasing surface gravity,
and are enhanced at low metallicity. Gravitational redshifts amount up to 0.7
km/s and dominate convective blueshifts for dwarfs, but become much weaker for
giants.Comment: 13 pages, to appear in A&A; model fluxes available from
ftp://leda.as.utexas.edu/pub/callende/Gaia3D and soon from CD
- …