38 research outputs found
Excitation of solar-like oscillations across the HR diagram
We extend semi-analytical computations of excitation rates for solar
oscillation modes to those of other solar-like oscillating stars to compare
them with recent observations. Numerical 3D simulations of surface convective
zones of several solar-type oscillating stars are used to characterize the
turbulent spectra as well as to constrain the convective velocities and
turbulent entropy fluctuations in the uppermost part of the convective zone of
such stars. These constraints, coupled with a theoretical model for stochastic
excitation, provide the rate 'P' at which energy is injected into the p-modes
by turbulent convection. These energy rates are compared with those derived
directly from the 3D simulations. The excitation rates obtained from the 3D
simulations are systematically lower than those computed from the
semi-analytical excitation model. We find that Pmax, the excitation rate
maximum, scales as (L/M)^s where s is the slope of the power law and L and M
are the mass and luminosity of the 1D stellar model built consistently with the
associated 3D simulation. The slope is found to depend significantly on the
adopted form of the eddy time-correlation ; using a Lorentzian form results in
s=2.6, whereas a Gaussian one gives s=3.1. Finally, values of Vmax, the maximum
in the mode velocity, are estimated from the computed power laws for Pmax and
we find that Vmax increases as (L/M)^sv. Comparisons with the currently
available ground-based observations show that the computations assuming a
Lorentzian eddy time-correlation yield a slope, sv, closer to the observed one
than the slope obtained when assuming a Gaussian. We show that the spatial
resolution of the 3D simulations must be high enough to obtain accurate
computed energy rates.Comment: 14 pages ; 7 figures ; accepted for publication in Astrophysics &
Astronom
Radiative transfer with scattering for domain-decomposed 3D MHD simulations of cool stellar atmospheres
We present the implementation of a radiative transfer solver with coherent
scattering in the new BIFROST code for radiative magneto-hydrodynamical (MHD)
simulations of stellar surface convection. The code is fully parallelized using
MPI domain decomposition, which allows for large grid sizes and improved
resolution of hydrodynamical structures. We apply the code to simulate the
surface granulation in a solar-type star, ignoring magnetic fields, and
investigate the importance of coherent scattering for the atmospheric
structure. A scattering term is added to the radiative transfer equation,
requiring an iterative computation of the radiation field. We use a
short-characteristics-based Gauss-Seidel acceleration scheme to compute
radiative flux divergences for the energy equation. The effects of coherent
scattering are tested by comparing the temperature stratification of three 3D
time-dependent hydrodynamical atmosphere models of a solar-type star: without
scattering, with continuum scattering only, and with both continuum and line
scattering. We show that continuum scattering does not have a significant
impact on the photospheric temperature structure for a star like the Sun.
Including scattering in line-blanketing, however, leads to a decrease of
temperatures by about 350\,K below log tau < -4. The effect is opposite to that
of 1D hydrostatic models in radiative equilibrium, where scattering reduces the
cooling effect of strong LTE lines in the higher layers of the photosphere.
Coherent line scattering also changes the temperature distribution in the high
atmosphere, where we observe stronger fluctuations compared to a treatment of
lines as true absorbers.Comment: A&A, in pres
The origin of the x-ray emission from the high-velocity cloud MS30.7-81.4-118
A soft X-ray enhancement has recently been reported toward the high-velocity cloud MS30.7-81.4-118 (MS30.7), a constituent of the Magellanic Stream. In order to investigate the origin of this enhancement, we have analyzed two overlapping XMM-Newton observations of this cloud. We find that the X-ray enhancement is 6??? or 100 pc across, and is concentrated to the north and west of the densest part of the cloud. We modeled the X-ray enhancement with a variety of spectral models. A single-temperature equilibrium plasma model yields a temperature of and a 0.4-2.0 keV luminosity of 7.9 ?? 1033 erg s-1. However, this model underpredicts the on-enhancement emission around 1 keV, which may indicate the additional presence of hotter plasma (T &#8819; 107 K), or that recombination emission is important. We examined several different physical models for the origin of the X-ray enhancement. We find that turbulent mixing of cold cloud material with hot ambient material, compression or shock heating of a hot ambient medium, and charge exchange reactions between cloud atoms and ions in a hot ambient medium all lead to emission that is too faint. In addition, shock heating in a cool or warm medium leads to emission that is too soft (for reasonable cloud speeds). We find that magnetic reconnection could plausibly power the observed X-ray emission, but resistive magnetohydrodynamical simulations are needed to test this hypothesis. If magnetic reconnection is responsible for the X-ray enhancement, the observed spectral properties could potentially constrain the magnetic field in the vicinity of the Magellanic Stream.open1
Improved synthetic spectra of helium-core white dwarf stars
We examine the emergent fluxes from helium-core white dwarfs following their
evolution from the end of pre-white dwarf stages down to advanced cooling
stages. For this purpose, we include a detailed treatment of the physics of the
atmosphere, particularly an improved representation of the state of the gas by
taking into account non-ideal effects according to the so-called occupation
probability formalism. The present calculations also incorporate hydrogen line
opacity from Lyman, Balmer and Paschen series, pseudo-continuum absorptions and
new updated induced-dipole absorption from H-H, H-He and H-He
pairs. We find that the non-ideal effects and line absorption alter the
appearance of the stellar spectrum and have a significant influence upon the
photometric colours in the UBVRI-JHKL system. This occurs specially for hot
models T_{\rm eff}\ga 8000 due to line and pseudo-continuum opacities, and
for cool models T_{\rm eff}\la 4000 where the perturbation of atoms and
molecules by neighbour particles affects the chemical equilibrium of the gas.
In the present study, we also include new cooling sequences for helium-core
white dwarfs of very low mass (0.160 and 0.148 M) with metallicity
. These computations provide theoretical support to search for and
identify white dwarfs of very low mass, specially useful for recent and future
observational studies of globular cluster, where these objects have began to be
detected.Comment: 15 pages. Accepted for publication in MNRA
Perspectives in Global Helioseismology, and the Road Ahead
We review the impact of global helioseismology on key questions concerning
the internal structure and dynamics of the Sun, and consider the exciting
challenges the field faces as it enters a fourth decade of science
exploitation. We do so with an eye on the past, looking at the perspectives
global helioseismology offered in its earlier phases, in particular the
mid-to-late 1970s and the 1980s. We look at how modern, higher-quality, longer
datasets coupled with new developments in analysis, have altered, refined, and
changed some of those perspectives, and opened others that were not previously
available for study. We finish by discussing outstanding challenges and
questions for the field.Comment: Invited review; to appear in Solar Physics (24 pages, 6 figures