1,036 research outputs found
Theoretical Transmission Spectra During Extrasolar Giant Planet Transits
The recent transit observation of HD 209458 b - an extrasolar planet orbiting
a sun-like star - confirmed that it is a gas giant and determined that its
orbital inclination is 85 degrees. This inclination makes possible
investigations of the planet atmosphere. In this paper we discuss the planet
transmission spectra during a transit. The basic tenet of the method is that
the planet atmosphere absorption features will be superimposed on the stellar
flux as the stellar flux passes through the planet atmosphere above the limb.
The ratio of the planet's transparent atmosphere area to the star area is
small, approximately 10^{-3} to 10^{-4}; for this method to work very strong
planet spectral features are necessary. We use our models of close-in
extrasolar giant planets to estimate promising absorption signatures: the
alkali metal lines, in particular the Na I and K I resonance doublets, and the
He I - triplet line at 1083.0 nm. If successful, observations
will constrain the line-of-sight temperature, pressure, and density. The most
important point is that observations will constrain the cloud depth, which in
turn will distinguish between different atmosphere models. We also discuss the
potential of this method for EGPs at different orbital distances and orbiting
non-solar-type stars.Comment: revised to agree with accepted paper, ApJ, in press. 12 page
Lyalpha heating and its impact on early structure formation
In this paper we have calculated the effect of Lyalpha photons emitted by the
first stars on the evolution of the IGM temperature. We have considered both a
standard Salpeter IMF and a delta-function IMF for very massive stars with mass
300 M_sun. We find that the Lyalpha photons produced by the stellar populations
considered here are able to heat the IGM at z<25, although never above ~100 K.
Stars with a Salpeter IMF are more effective as, due to the contribution from
small-mass long-living stars, they produce a higher Lyalpha background. Lyalpha
heating can affect the subsequent formation of small mass objects by producing
an entropy floor that may limit the amount of gas able to collapse and reduce
the gas clumping.We find that the gas fraction in halos of mass below ~ 5 x
10^6 M_sun is less than 50% (for the smallest masses this fraction drops to 1%
or less) compared to a case without Lyalpha heating. Finally, Lyalpha photons
heat the IGM temperature above the CMB temperature and render the 21cm line
from neutral hydrogen visible in emission at z<15.Comment: 7 pages, 5 figures, to be printed in MNRA
Can early years professionals determine which preschoolers have comprehension delays? A comparison of two screening tools
Language comprehension delays in pre-schoolers are predictive of difficulties in a range of developmental domains. In England, early years setting staff are required to assess the language comprehension of two-year-olds in their care. Many use a format based on the Early Years Foundation Stage My Unique Child (EYFS:UCCS ) in which the childâs language comprehension is assigned to an age band based on written guidance. Seventy 2Âœ-3-year-olds were assessed on the comprehension component of the Preschool Language Scale (PLS) by psychology graduates. Early years practitioners assessed language comprehension in the same children using the EYFS:UCCS and the WellComm which involves some direct testing. The EYFS:UCCS had poor sensitivity and specificity and the understanding section did not correlate with the PLS. The WellComm had good-acceptable levels of sensitivity and specificity and significantly correlated with the PLS. Early years setting staff can accurately assess the language comprehension of two-year-olds if provided with a tool which gives specific instructions on administration, but current frequently used procedures (EYFS:UCCS) are not fit for this purpose
An Improved Method for Estimating the Masses of Stars with Transiting Planets
To determine the physical parameters of a transiting planet and its host star
from photometric and spectroscopic analysis, it is essential to independently
measure the stellar mass. This is often achieved by the use of evolutionary
tracks and isochrones, but the mass result is only as reliable as the models
used. The recent paper by Torres et al (2009) showed that accurate values for
stellar masses and radii could be obtained from a calibration using T_eff, log
g and [Fe/H]. We investigate whether a similarly good calibration can be
obtained by substituting log rho - the fundamental parameter measured for the
host star of a transiting planet - for log g, and apply this to star-exoplanet
systems. We perform a polynomial fit to stellar binary data provided in Torres
et al (2009) to obtain the stellar mass and radius as functions of T_eff, log
rho and [Fe/H], with uncertainties on the fit produced from a Monte Carlo
analysis. We apply the resulting equations to measurements for seventeen
SuperWASP host stars, and also demonstrate the application of the calibration
in a Markov Chain Monte Carlo analysis to obtain accurate system parameters
where spectroscopic estimates of effective stellar temperature and metallicity
are available. We show that the calibration using log rho produces accurate
values for the stellar masses and radii; we obtain masses and radii of the
SuperWASP stars in good agreement with isochrone analysis results. We ascertain
that the mass calibration is robust against uncertainties resulting from poor
photometry, although a good estimate of stellar radius requires good-quality
transit light curve to determine the duration of ingress and egress.Comment: 5 pages, 2 figures, accepted for publication in A&
Bayesian analysis of exoplanet and binary orbits
We introduce BASE (Bayesian astrometric and spectroscopic exoplanet detection
and characterisation tool), a novel program for the combined or separate
Bayesian analysis of astrometric and radial-velocity measurements of potential
exoplanet hosts and binary stars. The capabilities of BASE are demonstrated
using all publicly available data of the binary Mizar A.Comment: Accepted for publication in Astronomy & Astrophysic
Small-scale dynamo action during the formation of the first stars and galaxies. I. The ideal MHD limit
We explore the amplification of magnetic seed fields during the formation of
the first stars and galaxies. During gravitational collapse, turbulence is
created from accretion shocks, which may act to amplify weak magnetic fields in
the protostellar cloud. Numerical simulations showed that such turbulence is
sub-sonic in the first star-forming minihalos, and highly supersonic in the
first galaxies with virial temperatures larger than 10^4 K. We investigate the
magnetic field amplification during the collapse both for Kolmogorov and
Burgers-type turbulence with a semi-analytic model that incorporates the
effects of gravitational compression and small-scale dynamo amplification. We
find that the magnetic field may be substantially amplified before the
formation of a disk. On scales of 1/10 of the Jeans length, saturation occurs
after ~10^8 yr. Although the saturation behaviour of the small-scale dynamo is
still somewhat uncertain, we expect a saturation field strength of the order
~10^{-7} n^{0.5} G in the first star-forming halos, with n the number density
in cgs units. In the first galaxies with higher turbulent velocities, the
magnetic field strength may be increased by an order of magnitude, and
saturation may occur after 10^6 to 10^7 yr. In the Kolmogorov case, the
magnetic field strength on the integral scale (i.e. the scale with most
magnetic power) is higher due to the characteristic power-law indices, but the
difference is less than a factor of 2 in the saturated phase. Our results thus
indicate that the precise scaling of the turbulent velocity with length scale
is of minor importance. They further imply that magnetic fields will be
significantly enhanced before the formation of a protostellar disk, where they
may change the fragmentation properties of the gas and the accretion rate.Comment: 11 pages, 9 figures, accepted at A&
Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Power Spectra and WMAP-Derived Parameters
(Abridged) We present the angular power spectra derived from the 7-year maps
and discuss the cosmological conclusions that can be inferred from WMAP data
alone. The third acoustic peak in the TT spectrum is now well measured by WMAP.
In the context of a flat LambdaCDM model, this improvement allows us to place
tighter constraints on the matter density from WMAP data alone, and on the
epoch of matter-radiation equality, The temperature-polarization (TE) spectrum
is detected in the 7-year data with a significance of 20 sigma, compared to 13
sigma with the 5-year data. The low-l EE spectrum, a measure of the optical
depth due to reionization, is detected at 5.5 sigma significance when averaged
over l = 2-7. The BB spectrum, an important probe of gravitational waves from
inflation, remains consistent with zero. The upper limit on tensor modes from
polarization data alone is a factor of 2 lower with the 7-year data than it was
using the 5-year data (Komatsu et al. 2010). We test the parameter recovery
process for bias and find that the scalar spectral index, ns, is biased high,
but only by 0.09 sigma, while the remaining parameters are biased by < 0.15
sigma. The improvement in the third peak measurement leads to tighter lower
limits from WMAP on the number of relativistic degrees of freedom (e.g.,
neutrinos) in the early universe: Neff > 2.7 (95% CL). Also, using WMAP data
alone, the primordial helium mass fraction is found to be YHe = 0.28+0.14-0.15,
and with data from higher-resolution CMB experiments included, we now establish
the existence of pre-stellar helium at > 3 sigma (Komatsu et al. 2010).Comment: 22 pages, 14 figures, version accepted to Astrophysical Journal
Supplement Series, added high-l EE detection, consolidated parameter recovery
simulation
Lyalpha versus X-ray heating in the high-z IGM
In this paper we examine the effect of X-ray and Lyalpha photons on the
intergalactic medium temperature. We calculate the photon production from a
population of stars and micro-quasars in a set of cosmological hydrodynamic
simulations which self-consistently follow the dark matter dynamics, radiative
processes as well as star formation, black hole growth and associated feedback
processes. We find that, (i) IGM heating is always dominated by X-rays unless
the Lyalpha photon contribution from stars in objects with mass M<10^8 Msun
becomes significantly enhanced with respect to the X-ray contribution from BHs
in the same halo (which we do not directly model). (ii) Without overproducing
the unresolved X-ray background, the gas temperature becomes larger than the
CMB temperature, and thus an associated 21 cm signal should be expected in
emission, at z<11.5. We discuss how in such a scenario the transition redshift
between a 21 cm signal in absorption and in emission could be used to
constraint BHs accretion and associated feedback processes.Comment: 7 pages, 3 figures, accepted for publication in MNRA
The 21 Centimeter Forest
We examine the prospects for studying the pre-reionization intergalactic
medium (IGM) through the so-called 21 cm forest in spectra of bright
high-redshift radio sources. We first compute the evolution of the mean optical
depth for models that include X-ray heating of the IGM gas, Wouthuysen-Field
coupling, and reionization. Under most circumstances, the spin temperature T_S
grows large well before reionization begins in earnest. As a result, the
optical depth is less than 0.001 throughout most of reionization, and
background sources must sit well beyond the reionization surface in order to
experience measurable absorption. HII regions produce relatively large
"transmission gaps" and may therefore still be observable during the early
stages of reionization. Absorption from sheets and filaments in the cosmic web
fades once T_S becomes large and should be rare during reionization. Minihalos
can produce strong (albeit narrow) absorption features. Measuring their
abundance would yield useful limits on the strength of feedback processes in
the IGM as well as their effect on reionization.Comment: 9 pages, 5 figures, submitted to MNRA
Dust in Brown Dwarfs IV. Dust formation and driven turbulence on mesoscopic scales
Dust formation in brown dwarf atmospheres is studied by utilising a model for
driven turbulence in the mesoscopic scale regime. We apply a pseudo-spectral
method where waves are created and superimposed within a limited wavenumber
interval. The turbulent kinetic energy distribution follows the Kolmogoroff
spectrum which is assumed to be the most likely value. Such superimposed,
stochastic waves may occur in a convectively active environment. They cause
nucleation fronts and nucleation events and thereby initiate the dust formation
process which continues until all condensible material is consumed. Small
disturbances are found to have a large impact on the dust forming system. An
initially dust-hostile region, which may originally be optically thin, becomes
optically thick in a patchy way showing considerable variations in the dust
properties during the formation process. The dust appears in lanes and curls as
a result of the interaction with waves, i.e. turbulence, which form larger and
larger structures with time. Aiming on a physical understanding of the
variability of brown dwarfs, related to structure formation in substellar
atmospheres, we work out first necessary criteria for small-scale closure
models to be applied in macroscopic simulations of dust forming astrophysical
systems.Comment: A&A accepted, 20 page
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