205 research outputs found
Evidence for the disintegration of KIC 12557548 b
Context. The Kepler object KIC 12557548 b is peculiar. It exhibits
transit-like features every 15.7 hours that vary in depth between 0.2% and
1.2%. Rappaport et al. (2012) explain the observations in terms of a
disintegrating, rocky planet that has a trailing cloud of dust created and
constantly replenished by thermal surface erosion. The variability of the
transit depth is then a consequence of changes in the cloud optical depth.
Aims. We aim to validate the disintegrating-planet scenario by modeling the
detailed shape of the observed light curve, and thereby constrain the cloud
particle properties to better understand the nature of this intriguing object.
Methods. We analysed the six publicly-available quarters of raw Kepler data,
phase-folded the light curve and fitted it to a model for the trailing dust
cloud. Constraints on the particle properties were investigated with a
light-scattering code. Results. The light curve exhibits clear signatures of
light scattering and absorption by dust, including a brightening in flux just
before ingress correlated with the transit depth and explained by forward
scattering, and an asymmetry in the transit light curve shape, which is easily
reproduced by an exponentially decaying distribution of optically thin dust,
with a typical grain size of 0.1 micron. Conclusions. Our quantitative analysis
supports the hypothesis that the transit signal of KIC 12557548 b is due to a
variable cloud of dust, most likely originating from a disintegrating object.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Giant planet formation: episodic impacts vs. gradual core growth
We describe the growth of gas giant planets in the core accretion scenario.
The core growth is not modeled as a gradual accretion of planetesimals but as
episodic impacts of large mass ratios, i.e. we study impacts of 0.02 - 1 Earth
masses onto cores of 1-15 Earth masses. Such impacts could deliver the majority
of solid matter in the giant impact regime. We focus on the thermal response of
the envelope to the energy delivery. Previous studies have shown that sudden
shut off of core accretion can dramatically speed up gas accretion. We
therefore expect that giant impacts followed by periods of very low core
accretion will result in a net increase in gas accretion rate. This study aims
at modelling such a sequence of events and to understand the reaction of the
envelope to giant impacts in more detail.
To model this scenario, we spread the impact energy deposition over a time
that is long compared to the sound crossing time, but very short compared to
the Kelvin-Helmholtz time. The simulations are done in spherical symmetry and
assume quasi-hydrostatic equilibrium.
Results confirm what could be inferred from previous studies: gas can be
accreted faster onto the core for the same net core growth speed while at the
same time rapid gas accretion can occur for smaller cores -- significantly
smaller than the usual critical core mass. Furthermore our simulations show,
that significant mass fractions of the envelope can be ejected by such an
impact
Grids of Stellar Models and Frequencies with CLES + LOSC
We present a grid of stellar models, obtained with the CLES evolution code,
following the specification of ESTA-Task1, and the corresponfing seismic
properties, computed with the LOSC code. We provide a complete description of
the corresponding files that will be available on the ESTA web-pages.Comment: 8 pages, accepted for publication in Astrophys. Space Sci.
(CoRoT/ESTA Volume
Rotation and Convective Core Overshoot in theta Ophiuchi
(abridged) Recent work on several beta Cephei stars has succeeded in
constraining both their interior rotation profile and their convective core
overshoot. In particular, a recent study focusing on theta$ Oph has shown that
a convective core overshoot parameter of alpha = 0.44 is required to model the
observed pulsation frequencies, significantly higher than for other stars of
this type. We investigate the effects of rotation and overshoot in early type
main sequence pulsators, and attempt to use the low order pulsation frequencies
to constrain these parameters. This will be applied to a few test models and
theta Oph. We use a 2D stellar evolution code and a 2D linear adiabatic
pulsation code to calculate pulsation frequencies for 9.5 Msun models. We
calculate low order p-modes for models with a range of rotation rates and
convective core overshoot parameters. Using these models, we find that the
convective core overshoot has a larger effect on the pulsation frequencies than
the rotation, except in the most rapidly rotating models considered. When the
differences in radii are accounted for by scaling the frequencies, the effects
of rotation diminish, but are not entirely accounted for. We find that
increasing the convective core overshoot decreases the large separation, while
producing a slight increase in the small separations. We created a model
frequency grid which spanned several rotation rates and convective core
overshoot values. Using a modified chi^2 statistic, we are able to recover the
rotation velocity and core overshoot for a few test models. Finally, we discuss
the case of the beta Cephei star theta Oph. Using the observed frequencies and
a fixed mass and metallicity, we find a lower overshoot than previously
determined, with alpha = 0.28 +/- 0.05. Our determination of the rotation rate
agrees well with both previous work and observations, around 30 km/s.Comment: 10 pages, 14 figures. Accepted for publication in A&A
An asteroseismic study of the beta Cephei star beta Canis Majoris
We present the results of a detailed analysis of 452 ground-based
high-resolution high S/N spectroscopic measurements spread over 4.5 years for
beta Canis Majoris with the aim to determine the pulsational characteristics of
this star, and to use them to derive seismic constraints on the stellar
parameters. We determine pulsation frequencies in the SiIII 4553 Angstrom line
with Fourier methods. We identify the m-value of the modes by taking into
account the photometric identifications of the degrees l. To this end we use
the moment method together with the amplitude and phase variations across the
line profile. The frequencies of the identified modes are used for a seismic
interpretation of the structure of the star. We confirm the presence of the
three pulsation frequencies already detected in previous photometric datasets:
f_1 = 3.9793 c/d (46.057 microHz), f_2 = 3.9995 c/d (46.291 microHz) and f_3 =
4.1832 c/d (48.417 microHz). For the two modes with the highest amplitudes we
unambiguously identify (l_1,m_1) = (2,2) and (l_2,m_2) = (0,0). We cannot
conclude anything for the third mode identification, except that m_3 > 0. We
also deduce an equatorial rotational velocity of 31 +/- 5 Km/s for the star. We
show that the mode f_1 must be close to an avoided crossing. Constraints on the
mass (13.5 +/- 0.5 Msun), age (12.4 +/- 0.7 Myr) and core overshoot (0.20 +/-
0.05 H_P) of beta CMa are obtained from seismic modelling using f_1 and f_2.Comment: Accepted for publication in A&
ASTEC -- the Aarhus STellar Evolution Code
The Aarhus code is the result of a long development, starting in 1974, and
still ongoing. A novel feature is the integration of the computation of
adiabatic oscillations for specified models as part of the code. It offers
substantial flexibility in terms of microphysics and has been carefully tested
for the computation of solar models. However, considerable development is still
required in the treatment of nuclear reactions, diffusion and convective
mixing.Comment: Astrophys. Space Sci, in the pres
Comparison of shear measurements and mixing predictions with a direct observation of diapycnal mixing in the Atlantic thermocline
Detailed analysis of Balmer lines in cool dwarf stars
An analysis of H alpha and H beta spectra in a sample of 30 cool dwarf and
subgiant stars is presented using MARCS model atmospheres based on the most
recent calculations of the line opacities. A detailed quantitative comparison
of the solar flux spectra with model spectra shows that Balmer line profile
shapes, and therefore the temperature structure in the line formation region,
are best represented under the mixing length theory by any combination of a low
mixing-length parameter alpha and a low convective structure parameter y. A
slightly lower effective temperature is obtained for the sun than the accepted
value, which we attribute to errors in models and line opacities. The programme
stars span temperatures from 4800 to 7100 K and include a small number of
population II stars. Effective temperatures have been derived using a
quantitative fitting method with a detailed error analysis. Our temperatures
find good agreement with those from the Infrared Flux Method (IRFM) near solar
metallicity but show differences at low metallicity where the two available
IRFM determinations themselves are in disagreement. Comparison with recent
temperature determinations using Balmer lines by Fuhrmann (1998, 2000), who
employed a different description of the wing absorption due to self-broadening,
does not show the large differences predicted by Barklem et al. (2000). In
fact, perhaps fortuitously, reasonable agreement is found near solar
metallicity, while we find significantly cooler temperatures for low
metallicity stars of around solar temperature.Comment: 17 pages, 9 figures, to appear in A&
ADIPLS -- the Aarhus adiabatic oscillation package
Development of the Aarhus adiabatic pulsation code started around 1978.
Although the main features have been stable for more than a decade, development
of the code is continuing, concerning numerical properties and output. The code
has been provided as a generally available package and has seen substantial use
at a number of installations. Further development of the package, including
bringing the documentation closer to being up to date, is planned as part of
the HELAS Coordination Action.Comment: Astrophys. Space Sci., in the pres
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