549 research outputs found
The polarization signature of photospheric magnetic fields in 3D MHD simulations and observations at disk center
Before using 3D MHD simulations of the solar photosphere in the determination
of elemental abundances, one has to ensure that the correct amount of magnetic
flux is present in the simulations. The presence of magnetic flux modifies the
thermal structure of the solar photosphere, which affects abundance
determinations and the solar spectral irradiance. We compare the polarization
signals in disk-center observations of the solar photosphere in quiet-Sun
regions with those in Stokes spectra computed on the basis of 3D MHD
simulations having average magnetic flux densities of about 20, 56, 112 and 224
G. This approach allows us to find the simulation run that best matches the
observations. The observations were taken with the Hinode SP, TIP, POLIS and
the GFPI, respectively. We determine characteristic quantities of full Stokes
profiles in a few photospheric spectral lines in the visible (630 nm) and
near-infrared (1083 and 1565 nm). We find that the appearance of abnormal
granulation in intensity maps of degraded simulations can be traced back to an
initially regular granulation pattern with numerous bright points in the
intergranular lanes before the spatial degradation. The linear polarization
signals in the simulations are almost exclusively related to canopies of strong
magnetic flux concentrations and not to transient events of magnetic flux
emergence. We find that the average vertical magnetic flux density in the
simulation should be less than 50 G to reproduce the observed polarization
signals in the quiet Sun internetwork. A value of about 35 G gives the best
match across the SP, TIP, POLIS and GFPI observations.Comment: 12 pages, 11 figures; accepted for publication in Ap
Solar Fe abundance and magnetic fields - Towards a consistent reference metallicity
We investigate the impact on Fe abundance determination of including magnetic
flux in series of 3D radiation-MHD simulations of solar convection which we
used to synthesize spectral intensity profiles corresponding to disc centre. A
differential approach is used to quantify the changes in theoretical equivalent
width of a set of 28 iron spectral lines spanning a wide range in lambda,
excitation potential, oscillator strength, Land\'e factor, and formation
height. The lines were computed in LTE using the spectral synthesis code LILIA.
We used input magnetoconvection snapshots covering 50 minutes of solar
evolution and belonging to series having an average vertical magnetic flux
density of 0, 50, 100 and 200 G. For the relevant calculations we used the
Copenhagen Stagger code. The presence of magnetic fields causes both a direct
(Zeeman-broadening) effect on spectral lines with non-zero Land\'e factor and
an indirect effect on temperature-sensitive lines via a change in the
photospheric T-tau stratification. The corresponding correction in the
estimated atomic abundance ranges from a few hundredths of a dex up to |Delta
log(Fe)| ~ 0.15 dex, depending on the spectral line and on the amount of
average magnetic flux within the range of values we considered. The
Zeeman-broadening effect gains relatively more importance in the IR. The
largest modification to previous solar abundance determinations based on
visible spectral lines is instead due to the indirect effect, i.e., the
line-weakening caused by a warmer stratification on an optical depth scale. Our
results indicate that the average solar iron abundance obtained when using
magnetoconvection models can be 0.03-0.11 dex higher than when using the
simpler HD convection approach. We demonstrate that accounting for magnetic
flux is important in state-of-the-art solar photospheric abundance
determinations based on 3D simulations.Comment: 12 pages, 7 figures, A&A in pres
Spectroscopy at the solar limb: II. Are spicules heated to coronal temperatures ?
Spicules of the so-called type II were suggested to be relevant for coronal
heating because of their ubiquity on the solar surface and their eventual
extension into the corona. We investigate whether solar spicules are heated to
transition-region or coronal temperatures and reach coronal heights (>6 Mm)
using multi-wavelength observations of limb spicules in different chromospheric
spectral lines (Ca II H, Hepsilon, Halpha, Ca II IR at 854.2 nm, He I at 1083
nm). We determine the line width of individual spicules and throughout the
field of view and estimate the maximal height that different types of off-limb
features reach. We derive estimates of the kinetic temperature and the
non-thermal velocity from the line width of spectral lines from different
chemical elements. We find that most regular spicules reach a maximal height of
about 6 Mm above the solar limb. The majority of features found at larger
heights are irregularly shaped with a significantly larger lateral extension
than spicules. Both individual and average line profiles in all spectral lines
show a decrease in their line width with height above the limb with very few
exceptions. Both the kinetic temperature and the non-thermal velocity decrease
with height above the limb. We find no indications that the spicules in our
data reach coronal heights or transition-region or coronal temperatures.Comment: Accepted for publication in Solar Physics, 52 pages, 32 figure
C, N, O Abundances in the Most Metal-Poor Damped Lyman alpha Systems
This study focuses on some of the most metal-poor damped Lyman alpha
absorbers known in the spectra of high redshift QSOs, using new and archival
observations obtained with UV-sensitive echelle spectrographs on the Keck and
VLT telescopes. The weakness and simple velocity structure of the absorption
lines in these systems allows us to measure the abundances of several elements,
and in particular those of C, N, and O, a group that is difficult to study in
DLAs of more typical metallicities. We find that when the oxygen abundance is
less than about 1/100 of solar, the C/O ratio in high redshift DLAs and
sub-DLAs matches that of halo stars of similar metallicity and shows higher
values than expected from galactic chemical evolution models based on
conventional stellar yields. Furthermore, there are indications that at these
low metallicities the N/O ratio may also be above simple expectations and may
exhibit a minimum value, as proposed by Centurion and her collaborators in
2003. Both results can be interpreted as evidence for enhanced production of C
and N by massive stars in the first few episodes of star formation, in our
Galaxy and in the distant proto-galaxies seen as QSO absorbers. The higher
stellar yields implied may have an origin in stellar rotation which promotes
mixing in the stars' interiors, as considered in some recent model
calculations. We briefly discuss the relevance of these results to current
ideas on the origin of metals in the intergalactic medium and the universality
of the stellar initial mass function.Comment: 17 pages, 9 Figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations
Numerical 3D radiative (M)HD simulations of solar convection are used to
understand the physical properties of the solar photosphere. To validate this
approach, it is important to check that no excessive thermodynamic fluctuations
arise as a consequence of the partially incomplete treatment of radiative
transfer. We investigate the realism of 3D convection simulations carried out
with the Stagger code. We compared the characteristic properties of several
spectral lines in solar disc centre observations with spectra synthesized from
the simulations. We degraded the synthetic spectra to the spatial resolution of
the observations using the continuum intensity distribution. We estimated the
necessary spectral degradation by comparing atlas spectra with averaged
observed spectra. In addition to deriving a set of line parameters directly, we
used the SIR code to invert the spectra. Most of the line parameters from the
observational data are matched well by the degraded simulation spectra. The
inversions predict a macroturbulent velocity below 10 m/s for the simulation at
full spatial resolution, whereas they yield ~< 1000 m/s at a spatial resolution
of 0.3". The temperature fluctuations in the inversion of the degraded
simulation do not exceed those from the observational data (of the order of
100-200 K rms for -2<log tau<-0.5). The comparison of line parameters in
spatially averaged profiles with the averaged values of line parameters in
spatially resolved profiles indicates a significant change of (average) line
properties at a spatial scale between 0.13" and 0.3". Up to a spatial
resolution of 0.3", we find no indications of the presence of excessive
thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that
simulations without spatial degradation contain fully realistic thermodynamic
fluctuations requires observations at even better spatial resolution.Comment: 21 pages, 15 figures + 2 pages Appendix, accepted for publication in
A&A; v2 version: corrected for an error in the calculation of stray-light
estimates, for details see the Corrigendum to A&A, 2013, 557, 109 (DOI:
10.1051/0004-6361/201321596). Corrected text and numbers are in bold font.
Apart from the stray-light estimates, nothing in the rest of the paper was
affected by the erro
Is there a reentrant glass in binary mixtures?
By employing computer simulations for a model binary mixture, we show that a
reentrant glass transition upon adding a second component only occurs if the
ratio of the short-time mobilities between the glass-forming component
and the additive is sufficiently small. For , there is no
reentrant glass, even if the size asymmetry between the two components is
large, in accordance with two-component mode coupling theory. For , on the other hand, the reentrant glass is observed and reproduced only by
an effective one-component mode coupling theory.Comment: 4 pages, 3 figure
A view of the Galactic halo using beryllium as a time scale
Beryllium stellar abundances were suggested to be a good tracer of time in
the early Galaxy. In an investigation of its use as a cosmochronometer, using a
large sample of local halo and thick-disk dwarfs, evidence was found that in a
log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components.
One is consistent with predictions of evolutionary models while the other is
chemically indistinguishable from the thick-disk stars. This is interpreted as
a difference in the star formation history of the two components and suggests
that the local halo is not a single uniform population where a clear
age-metallicity relation can be defined.Comment: To appear in Proceedings of the International Astronomical Union, IAU
Symposium, Volume 265, Chemical abundances in the Universe: connecting first
stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure
Molecular mode-coupling theory for supercooled liquids: Application to water
We present mode-coupling equations for the description of the slow dynamics
observed in supercooled molecular liquids close to the glass transition. The
mode-coupling theory (MCT) originally formulated to study the slow relaxation
in simple atomic liquids, and then extended to the analysis of liquids composed
by linear molecules, is here generalized to systems of arbitrarily shaped,
rigid molecules. We compare the predictions of the theory for the -vector
dependence of the molecular nonergodicity parameters, calculated by solving
numerically the molecular MCT equations in two different approximation schemes,
with ``exact'' results calculated from a molecular dynamics simulation of
supercooled water. The agreement between theory and simulation data supports
the view that MCT succeeds in describing the dynamics of supercooled molecular
liquids, even for network forming ones.Comment: 22 pages 4 figures Late
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