2,547 research outputs found
Fingering convection and cloudless models for cool brown dwarf atmospheres
This work aims to improve the current understanding of the atmospheres of
brown dwarfs, especially cold ones with spectral type T and Y, whose modeling
is a current challenge. Silicate and iron clouds are believed to disappear at
the photosphere at the L/T transition, but cloudless models fail to reproduce
correctly the spectra of T dwarfs, advocating for the addition of more physics,
e.g. other types of clouds or internal energy transport mechanisms. We use a
one-dimensional (1D) radiative/convective equilibrium code ATMO to investigate
this issue. This code includes both equilibrium and out-of-equilibrium
chemistry and solves consistently the PT structure. Included opacity sources
are H2-H2, H2-He, H2O, CO, CO2, CH4, NH3, K, Na, and TiO, VO if they are
present in the atmosphere. We show that the spectra of Y dwarfs can be
accurately reproduced with a cloudless model if vertical mixing and NH3
quenching are taken into account. T dwarf spectra still have some reddening in
e.g. J - H compared to cloudless models. This reddening can be reproduced by
slightly reducing the temperature gradient in the atmosphere. We propose that
this reduction of the stabilizing temperature gradient in these layers, leading
to cooler structures, is due to the onset of fingering convection, triggered by
the destabilizing impact of condensation of very thin dust.Comment: Accepted in ApJ
Near-Infrared Spectroscopy of the Y0 WISEP J173835.52+273258.9 and the Y1 WISE J035000.32-565830.2: the Importance of Non-Equilibrium Chemistry
We present new near-infrared spectra, obtained at Gemini Observatory, for two
Y dwarfs: WISE J035000.32-565830.2 (W0350) and WISEP J173835.52+273258.9
(W1738). A FLAMINGOS-2 R=540 spectrum was obtained for W0350, covering 1.0 <
lambda um < 1.7, and a cross-dispersed GNIRS R=2800 spectrum was obtained for
W1738, covering 0.993-1.087 um, 1.191-1.305 um, 1.589-1.631 um, and 1.985-2.175
um, in four orders. We also present revised YJH photometry for W1738, using new
NIRI Y and J imaging, and a re-analysis of the previously published NIRI H band
images. We compare these data, together with previously published data for
late-T and Y dwarfs, to cloud-free models of solar metallicity, calculated both
in chemical equilibrium and with disequilibrium driven by vertical transport.
We find that for the Y dwarfs the non-equilibrium models reproduce the
near-infrared data better than the equilibrium models. The remaining
discrepancies suggest that fine-tuning the CH_4/CO and NH_3/N_2 balance is
needed. Improved trigonometric parallaxes would improve the analysis. Despite
the uncertainties and discrepancies, the models reproduce the observed
near-infrared spectra well. We find that for the Y0, W1738, T_eff = 425 +/- 25
K and log g = 4.0 +/- 0.25, and for the Y1, W0350, T_eff = 350 +/- 25 K and log
g = 4.0 +/- 0.25. W1738 may be metal-rich. Based on evolutionary models, these
temperatures and gravities correspond to a mass range for both Y dwarfs of 3-9
Jupiter masses, with W0350 being a cooler, slightly older, version of W1738;
the age of W0350 is 0.3-3 Gyr, and the age of W1738 is 0.15-1 Gyr.Comment: Accepted on March 30 2016 for publication in Ap
Superfluid turbulence and pulsar glitch statistics
Experimental evidence is reviewed for the existence of superfluid turbulence
in a differentially rotating, spherical shell at high Reynolds numbers
(\Rey\gsim 10^3), such as the outer core of a neutron star. It is shown that
torque variability increases with \Rey, suggesting that glitch activity in
radio pulsars may be a function of \Rey as well. The \Rey distribution of
the 67 glitching radio pulsars with characteristic ages {\rm
yr} is constructed from radio timing data and cooling curves and compared with
the \Rey distribution of all 348 known pulsars with {\rm
yr}. The two distributions are different, with a Kolmogorov-Smirnov probability
. The conclusion holds for (modified) Urca and
nonstandard cooling, and for Newtonian and superfluid viscosities
Radiation of Neutron Stars Produced by Superfluid Core
We find that neutron star interior is transparent for collisionless electron
sound, the same way as it is transparent for neutrinos. In the presence of
magnetic field the electron sound is coupled with electromagnetic radiation and
form the fast magnetosonic wave. We find that electron sound is generated by
superfluid vortices in the stellar core. Thermally excited helical vortex waves
produce fast magnetosonic waves in the stellar crust which propagate toward the
surface and transform into outgoing electromagnetic radiation. The vortex
radiation has the spectral index -0.45 and can explain nonthermal radiation of
middle-aged pulsars observed in the infrared, optical and hard X-ray bands. The
radiation is produced in the stellar interior which allows direct determination
of the core temperature. Comparing the theory with available spectra
observations we find that the core temperature of the Vela pulsar is T=8*10^8K,
while the core temperature of PSR B0656+14 and Geminga exceeds 2*10^8K. This is
the first measurement of the temperature of a neutron star core. The
temperature estimate rules out equation of states incorporating Bose
condensations of pions or kaons and quark matter in these objects. Based on the
temperature estimate and cooling models we determine the critical temperature
of triplet neutron superfluidity in the Vela core Tc=(7.5\pm 1.5)*10^9K which
agrees well with recent data on behavior of nucleon interactions at high
energies. Another finding is that in the middle aged neutron stars the vortex
radiation, rather then thermal conductivity, is the main mechanism of heat
transfer from the stellar core to the surface. Electron sound opens a
perspective of direct spectroscopic study of superdense matter in the neutron
star interiors.Comment: 43 pages, 7 figures, to appear in Astrophysical Journa
Treatment of overlapping gaseous absorption with the correlated-k method in hot Jupiter and brown dwarf atmosphere models
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.The correlated-k method is frequently used to speed up radiation calculations in both one-dimensional and three-dimensional atmosphere models. An inherent difficulty with this method is how to treat overlapping absorption, i.e. absorption by more than one gas in a given spectral region. We have evaluated the applicability of three different methods in hot Jupiter and brown dwarf atmosphere models, all of which have been previously applied within models in the literature: (i) Random overlap, both with and without resorting and rebinning, (ii) equivalent extinction and (iii) pre-mixing of opacities, where (i) and (ii) combine k-coefficients for different gases to obtain k-coefficients for a mixture of gases, while (iii) calculates k-coefficients for a given mixture from the corresponding mixed line-by-line opacities. We find that the random overlap method is the most accurate and flexible of these treatments, and is fast enough to be used in one-dimensional models with resorting and rebinning. In three-dimensional models such as GCMs it is too slow, however, and equivalent extinction can provide a speed-up of at least a factor of three with only a minor loss of accuracy while at
the same time retaining the flexibility gained by combining k-coefficients computed for each gas individually. Pre-mixed opacities are significantly less flexible, and we also find that particular care must be taken when using this method in order to to adequately resolve steep variations in composition at important chemical equilibrium boundaries. We use the random overlap method with resorting and rebinning in our one-dimensional atmosphere model and equivalent extinction in our GCM, which allows us to e.g. consistently treat the feedback of non-equilibrium chemistry on the total opacity and therefore the calculated P–T profiles in our modelsWe thank the referee, Mark Marley, for comments that significantly improved the paper. This work is partly supported by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013 Grant Agreement No. 247060-PEPS and grant No.
320478-TOFU). D.S.A. acknowledges support from the NASA Astrobiology Program through the Nexus for Exoplanet System Science. J.M. acknowledges the support of a Met Office Academic Partnership secondment. The calculations for this paper were performed on the DiRAC Complexity machine, jointly funded by STFC and the Large Facilities Capital Fund of BIS, and the University of
Exeter Super-computer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and the University of Exeter
A Hybrid Line List for CH4 and Hot Methane Continuum
Molecular line lists (a catalogue of transition frequencies and line strengths) are important for modelling absorption and emission processes in atmospheres of different astronomical objects, such as cool stars and exoplanets. In order to be applicable for high temperatures, line lists for molecules like methane must contain billions of transitions, which makes their direct (line-by-line) application in radiative transfer calculations impracticable. Here we suggest a new, hybrid line list format to mitigate this problem, based on the idea of temperature-dependent absorption continuum. Methods. The line list is partitioned into a large set of relatively weak lines and a small set of important, stronger lines. The weaker lines are then used either to construct a temperature-dependent (but pressure-independent) set of intensity cross sections or are blended into a greatly reduced set of super-lines. The strong lines are kept in the form of temperature independent Einstein A coefficients. Results. A line list for methane is constructed as a combination of 17 million strong absorption lines relative to the reference absorption spectra and a background methane continuum in two temperature-dependent forms, of cross sections and super-lines. This approach eases the use of large high temperature line lists significantly as the computationally expensive calculation of pressure dependent profiles only need to be performed for a relatively small number of lines. Both the line list and cross sections were generated using a new 34 billion methane line list (34 to10), which extends the 10to10 line list to higher temperatures (up to 2000 K). The new hybrid scheme can be applied to any large line lists containing billions of transitions. We recommend to use super-lines generated on a high resolution grid based on resolving power R = 1,000,000 to model the molecular continuum as a more flexible alternative to the temperature dependent cross sections
Implications of Hyperon Pairing for Cooling of Neutron Stars
The implications of hyperon pairing for the thermal evolution of neutron
stars containing hyperons are investigated. The outcome of cooling simulations
are compared for neutron star models composed only of nucleons and leptons,
models including hyperons, and models including pairing of hyperons. We show
that lambda and neutron pairing suppresses all possible fast neutrino emission
processes in not too massive neutron stars. The inclusion of lambda pairing
yields better agreement with X-ray observations of pulsars. Particularly, the
surface temperatures deduced from X-ray observations within the hydrogen
atmosphere model are more consistent with the thermal history of neutron stars
containing hyperons, if the critical temperature for the onset of lambda and
nucleon pairing is not too small.Comment: 7 pages, 3 figures. To be published in ApJL. The postscript and
additional tables can be found at
http://www.physik.uni-muenchen.de/sektion/suessmann/astro/cool/schaab.089
Progress toward ultra-stable lasers for use in space
This is a summary of a research project that has come to be known as SUNLITE, initially standing for Stanford University - NASA laser in space technology experiment. It involves scientists from the NASA Langley Research Center (LaRC), Stanford University, the National Institute of Standards and Technology (NIST), and the Joint Institute for Laboratory Astrophysics (JILA), and a growing number of other institutions. The long range objective of the SUNLITE effort is to examine the fundamental linewidth and frequency stability limits of an actively stabilized laser oscillator in the microgravity and vibration-free environment of space. The ground-based SUNLITE activities supporting that objective will develop a space-qualified, self-contained and completely automated terahertz oscillator stabilized to a linewidth of less than 3 Hz, along with a measurement system capable of determining laser linewidth to one part in 10(exp 16). The purpose of this paper is to discuss the critical technologies needed to place stabilized lasers in space and to describe the progress made by the SUNLITE project to develop these technologies
Non-equilibrium beta processes in superfluid neutron star cores
The influence of nucleons superfluidity on the beta relaxation time of
degenerate neutron star cores, composed of neutrons, protons and electrons, is
investigated. We numerically calculate the implied reduction factors for both
direct and modified Urca reactions, with isotropic pairing of protons or
anisotropic pairing of neutrons. We find that due to the non-zero value of the
temperature and/or to the vanishing of anisotropic gaps in some directions of
the phase-space, superfluidity does not always completely inhibit beta
relaxation, allowing for some reactions if the superfluid gap amplitude is not
too large in respect to both the typical thermal energy and the chemical
potential mismatch. We even observe that if the ratio between the critical
temperature and the actual temperature is very small, a suprathermal regime is
reached for which superfluidity is almost irrelevant. On the contrary, if the
gap is large enough, the composition of the nuclear matter can stay frozen for
very long durations, unless the departure from beta equilibrium is at least as
important as the gap amplitude. These results are crucial for precise
estimation of the superfluidity effect on the cooling/slowing-down of pulsars
and we provide online subroutines to be implemented in codes for simulating
such evolutions.Comment: 11 pages, 6 Figs., published, minor changes, subroutines can be found
on line at http://luth2.obspm.fr/~etu/villain/Micro/Resolution.htm
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