10,302 research outputs found
Alkali Line Profiles in Degenerate Dwarfs
Ultracool stellar atmospheres show absorption by alkali resonance lines
severely broadened by collisions with neutral perturbers. In the coolest and
densest atmospheres, such as those of T dwarfs, Na I and K I broadened by
molecular hydrogen and helium can come to dominate the entire optical spectrum.
Their profiles have been successfully modelled with accurate interaction
potentials in the adiabatic theory, computing line profiles from the first few
orders of a density expansion of the autocorrelation function. The line shapes
in the emergent spectrum also depend on the distribution of absorbers as a
function of depth, which can be modelled with improved accuracy by new models
of dust condensation and settling.
The far red K I wings of the latest T dwarfs still show missing opacity in
these models, a phenomenon similar to what has been found for the Na I line
profiles observed in extremely cool, metal-rich white dwarfs. We show that the
line profile in both cases is strongly determined by multiple-perturber
interactions at short distances and can no longer be reproduced by a density
expansion, but requires calculation of the full profile in a unified theory.
Including such line profiles in stellar atmosphere codes will further improve
models for the coolest and densest dwarfs as well as for the deeper atmosphere
layers of substellar objects in general.Comment: VI Serbian Conference on Spectral Line Shapes in Astrophysics; to be
published by the American Institute of Physics, eds. Milan S. Dimitrijevic
and Luka C. Popovic; 6 pages, 6 figure
Quasi-molecular lines in Lyman wings of cool DA white dwarfs; Application to FUSE observations of G231-40
We present new theoretical calculations of the total line profiles of Lyman
alpha and Lyman beta which include perturbations by both neutral hydrogen AND
protons and all possible quasi-molecular states of H_2 and H_2^+. They are used
to improve theoretical modeling of synthetic spectra for cool DA white dwarfs.
We compare them with FUSE observation of G231-40. The appearance of the line
wings between Lyman alpha and Lyman beta is shown to be sensitive to the
relative abundance of hydrogen ions and neutral atoms, and thereby to provide a
temperature diagnostic for stellar atmospheres and laboratory plasmas.Comment: 6 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
The Effective temperature scale of M dwarfs from spectral synthesis
We present a comparison of low-resolution spectra of 60 stars covering the
whole M-dwarf sequence. Using the most recent PHOENIX BT-Settl stellar model
atmospheres (see paper by F. Allard, in this book) we do a first quantitative
compari- son to our observed spectra in the wavelength range 550-950 nm. We
perform a first confrontation between models and observations and we assign an
effective tempera- tures to the observed M-dwarfs. Teff-spectral type relations
are then compared with the published ones. This comparison also aims at
improving the models' opacities.Comment: To be published in the on-line version of the Proceedings of Cool
Stars 16 (ASP Conference Series) New version with bibliography correcte
Progress in Modeling Very Low Mass Stars, Brown Dwarfs, and Planetary Mass Objects
We review recent advancements in modeling the stellar to substellar
transition. The revised molecular opacities, solar oxygen abundances and cloud
models allow to reproduce the photometric and spectroscopic properties of this
transition to a degree never achieved before, but problems remain in the
important M-L transition characteristic of the effective temperature range of
characterizable exoplanets. We discuss of the validity of these classical
models. We also present new preliminary global Radiation HydroDynamical M
dwarfs simulations.Comment: Submitted to Mem. S. A. It. Supp
Steady-state MreB helices inside bacteria: dynamics without motors
Within individual bacteria, we combine force-dependent polymerization
dynamics of individual MreB protofilaments with an elastic model of
protofilament bundles buckled into helical configurations. We use variational
techniques and stochastic simulations to relate the pitch of the MreB helix,
the total abundance of MreB, and the number of protofilaments. By comparing our
simulations with mean-field calculations, we find that stochastic fluctuations
are significant. We examine the quasi-static evolution of the helical pitch
with cell growth, as well as timescales of helix turnover and denovo
establishment. We find that while the body of a polarized MreB helix treadmills
towards its slow-growing end, the fast-growing tips of laterally associated
protofilaments move towards the opposite fast-growing end of the MreB helix.
This offers a possible mechanism for targeted polar localization without
cytoplasmic motor proteins.Comment: 7 figures, 1 tabl
Photospheric properties and fundamental parameters of M dwarfs
M dwarfs are an important source of information when studying and probing the
lower end of the Hertzsprung-Russell (HR) diagram, down to the hydrogen-burning
limit. Being the most numerous and oldest stars in the galaxy, they carry
fundamental information on its chemical history. The presence of molecules in
their atmospheres, along with various condensed species, complicates our
understanding of their physical properties and thus makes the determination of
their fundamental stellar parameters more challenging and difficult. The aim of
this study is to perform a detailed spectroscopic analysis of the
high-resolution H-band spectra of M dwarfs in order to determine their
fundamental stellar parameters and to validate atmospheric models. The present
study will also help us to understand various processes, including dust
formation and depletion of metals onto dust grains in M dwarf atmospheres. The
high spectral resolution also provides a unique opportunity to constrain other
chemical and physical processes that occur in a cool atmosphere The
high-resolution APOGEE spectra of M dwarfs, covering the entire H-band, provide
a unique opportunity to measure their fundamental parameters. We have performed
a detailed spectral synthesis by comparing these high-resolution H-band spectra
to that of the most recent BT-settl model and have obtained fundamental
parameters such as effective temperature, surface gravity, and metallicity
(Teff, log g and [Fe/H]) respectively.Comment: 15 pages, 10 figures, accepted for publication in A&
Selective coupling of optical energy into the fundamental diffusion mode of a scattering medium
We demonstrate experimentally that optical wavefront shaping selectively
couples light into the fundamental diffusion mode of a scattering medium. The
total energy density inside a scattering medium of zinc oxide (ZnO)
nanoparticles was probed by measuring the emitted fluorescent power of spheres
that were randomly positioned inside the medium. The fluorescent power of an
optimized incident wave front is observed to be enhanced compared to a
non-optimized incident front. The observed enhancement increases with sample
thickness. Based on diffusion theory, we derive a model wherein the
distribution of energy density of wavefront-shaped light is described by the
fundamental diffusion mode. The agreement between our model and the data is
striking not in the least since there are no adjustable parameters. Enhanced
total energy density is crucial to increase the efficiency of white LEDs, solar
cells, and of random lasers, as well as to realize controlled illumination in
biomedical optics.Comment: 5 pages, 5 figure
Atmospheric Analysis of the M/L- and M/T-Dwarf Binary Systems LHS 102 and Gliese 229
We present 0.9-2.5um spectroscopy with R~800 and 1.12-1.22um spectroscopy
with R~5800 for the M dwarfs Gl 229A and LHS 102A, and for the L dwarf LHS
102B. We also report IZJHKL' photometry for both components of the LHS 102
system, and L' photometry for Gl 229A. The data are combined with previously
published spectroscopy and photometry to produce flux distributions for each
component of the kinematically old disk M/L-dwarf binary system LHS 102 and the
kinematically young disk M/T-dwarf binary system Gliese 229. The data are
analyzed using synthetic spectra generated by the latest "AMES-dusty" and
"AMES-cond" models by Allard & Hauschildt. Although the models are not able to
reproduce the overall slope of the infrared flux distribution of the L dwarf,
most likely due to the treatment of dust in the photosphere, the data for the M
dwarfs and the T dwarf are well matched. We find that the Gl 229 system is
metal-poor despite having kinematics of the young disk, and that the LHS 102
system has solar metallicity. The observed luminosities and derived
temperatures and gravities are consistent with evolutionary model predictions
if the Gl 229 system is very young (age ~30 Myr) with masses (A,B) of
(0.38,>0.007)M(sun), and the LHS 102 system is older, aged 1-10 Gyr with masses
(A,B) of (0.19,0.07)M(sun).Comment: 29 pages incl. 13 figures and 5 tables;; accepted for publication in
MNRA
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