144 research outputs found
Day-side z'-band emission and eccentricity of Wasp-12b
We report the detection of the eclipse of the very-hot Jupiter WASP-12b via
z'-band time-series photometry obtained with the 3.5-meter ARC telescope at
Apache Point Observatory. We measure a decrease in flux of 0.082+/-0.015%
during the passage of the planet behind the star. That planetary flux is
equally well reproduced by atmospheric models with and without extra absorbers,
and blackbody models with f > 0.585+/-0.080. It is therefore necessary to
measure the planet at other wavelengths to further constrain its atmospheric
properties. The eclipse appears centered at phase = 0.5100 (+0.0072,-0.0061),
consistent with an orbital eccentricity of |e cos w| = 0.016 (+0.011,-0.009)
(see note at end of Section 4). If the orbit of the planet is indeed eccentric,
the large radius of WASP-12b can be explained by tidal heating.Comment: One more author added. Version accepted for publication on ApJ
IRIS: A Generic Three-Dimensional Radiative Transfer Code
We present IRIS, a new generic three-dimensional (3D) spectral radiative
transfer code that generates synthetic spectra, or images. It can be used as a
diagnostic tool for comparison with astrophysical observations or laboratory
astrophysics experiments. We have developed a 3D short-characteristic solver
that works with a 3D nonuniform Cartesian grid. We have implemented a piecewise
cubic, locally monotonic, interpolation technique that dramatically reduces the
numerical diffusion effect. The code takes into account the velocity gradient
effect resulting in gradual Doppler shifts of photon frequencies and subsequent
alterations of spectral line profiles. It can also handle periodic boundary
conditions. This first version of the code assumes Local Thermodynamic
Equilibrium (LTE) and no scattering. The opacities and source functions are
specified by the user. In the near future, the capabilities of IRIS will be
extended to allow for non-LTE and scattering modeling. IRIS has been validated
through a number of tests. We provide the results for the most relevant ones,
in particular a searchlight beam test, a comparison with a 1D plane-parallel
model, and a test of the velocity gradient effect. IRIS is a generic code to
address a wide variety of astrophysical issues applied to different objects or
structures, such as accretion shocks, jets in young stellar objects, stellar
atmospheres, exoplanet atmospheres, accretion disks, rotating stellar winds,
cosmological structures. It can also be applied to model laboratory
astrophysics experiments, such as radiative shocks produced with high power
lasers.Comment: accepted for publication in A&A; 17 pages, 9 figures, 2 table
Optical Albedo Theory of Strongly-Irradiated Giant Planets: The Case of HD 209458b
We calculate a new suite of albedo models for close-in extrasolar giant
planets and compare with the recent stringent upper limit for HD 209458b of
Rowe et al. using MOST. We find that all models without scattering clouds are
consistent with this optical limit. We explore the dependence on wavelength and
waveband, metallicity, the degree of heat redistribution, and the possible
presence of thermal inversions and find a rich diversity of behaviors.
Measurements of transiting extrasolar giant planets (EGPs) at short wavelengths
by MOST, Kepler, and CoRoT, as well as by proposed dedicated multi-band
missions, can complement measurements in the near- and mid-IR using {\it
Spitzer} and JWST. Collectively, such measurements can help determine
metallicity, compositions, atmospheric temperatures, and the cause of thermal
inversions (when they arise) for EGPs with a broad range of radii, masses,
degrees of stellar insolation, and ages. With this paper, we reappraise and
highlight the diagnostic potential of albedo measurements of hot EGPs shortward
of 1.3 m.Comment: 6 pages, 1 table, 1 color figure; accepted to the Astrophysical
Journa
Origin of the wide-angle hot H2 in DG Tauri: New insight from SINFONI spectro-imaging
We wish to test the origins proposed for the extended hot H2 at 2000K around
the atomic jet from the T Tauri star DGTau, in order to constrain the
wide-angle wind structure and the possible presence of an MHD disk wind. We
present flux calibrated IFS observations in H2 1-0 S(1) obtained with
SINFONI/VLT. Thanks to spatial deconvolution by the PSF and to accurate
correction for uneven slit illumination, we performed a thorough analysis and
modeled the morphology, kinematics, and surface brightness. We also compared
our results with studies in [FeII], [OI], and FUV-pumped H2. The
limb-brightened H2 emission in the blue lobe is strikingly similar to
FUV-pumped H2 imaged 6yr later, confirming that they trace the same hot gas and
setting an upper limit of 12km/s on any expansion proper motion. The wide-angle
H2 rims are at lower blueshifts than probed by narrow long-slit spectra. We
confirm that they extend to larger angle and to lower speed the onion-like
velocity structure observed in optical atomic lines. The latter is shown to be
steady over more/equal than 4yr but undetected in [FeII] by SINFONI, probably
due to strong iron depletion. The H2 rim thickness less/equal than 14AU rules
out excitation by C-shocks, and J-shock speeds are constrained to 10km/s. We
find that explaining the H2 wide-angle emission with a shocked layer requires
either a recent outburst (15yr) into a pre-existing ambient outflow or an
excessive wind mass flux. A slow photoevaporative wind from the dense
irradiated disk surface and an MHD disk wind heated by ambipolar diffusion seem
to be more promising and need to be modeled in more detail
Tidal Heating Models for the Radii of the Inflated Transiting Giant Planets WASP-4b, WASP-6b, WASP-12b, and TrES-4
In order to explain the inflated radii of some transiting extrasolar giant
planets, we investigate a tidal heating scenario for the inflated planets
WASP-4b, WASP-6b, WASP-12b, WASP-15b, and TrES-4. To do so, we assume that they
retain a nonzero eccentricity, possibly by dint of continuing interaction with
a third body. We calculate the amount of extra heating in the envelope that is
then required to fit the radius of each planet, and we explore how this
additional power depends on the planetary atmospheric opacity and on the mass
of a heavy-element central core. There is a degeneracy between the core mass
and the heating . Therefore, in the case
of tidal heating, there is for each planet a range of the couple that can lead to the same radius, where is the tidal
dissipation factor and is the eccentricity. With this in mind, we also
investigate the case of the non-inflated planet HAT-P-12b, which can admit
solutions combining a heavy-element core and tidal heating. A substantial
improvement of the measured eccentricities of such planetary systems could
simplify this degeneracy by linking the two unknown parameters . Further independent constraints on either of these parameters
would, through our calculations, constrain the other.Comment: Accepted in ApJ; 17 pages, 3 figures, 6 tables (emulateapj format);
expanded explanatory tex
Explorations into the Viability of Coupled Radius-Orbit Evolutionary Models for Inflated Planets
The radii of some transiting extrasolar giant planets are larger than would
be expected by the standard theory. We address this puzzle with the model of
coupled radius-orbit tidal evolution developed by
\citet{Ibgui_and_Burrows_2009}. The planetary radius is evolved
self-consistently with orbital parameters, under the influence of tidal torques
and tidal dissipation in the interior of the planet. A general feature of this
model, which we have previously demonstrated in the generic case, is that a
possible transient inflation of the planetary radius can temporarily interrupt
its standard monotonic shrinking and can lead to the inflated radii that we
observe. In particular, a bloated planet with even a circular orbit may still
be inflated due to an earlier episode of tidal heating. We have modified our
model to include an orbital period dependence of the tidal dissipation factor
in the star, , .
With this model, we search, for a tidally heated planet, orbital and radius
evolutionary tracks that fall within the observational limits of the radius,
the semimajor axis, and the eccentricity of the planet in its current estimated
age range. We find that, for some inflated planets (WASP-6b and WASP-15b),
there are such tracks; for another (TrES-4), there are none; and for still
others (WASP-4b and WASP-12b), there are such tracks, but our model might imply
that we are observing the planets at a special time. Finally, we stress that
there is a two to three order-of-magnitude timescale uncertainty of the
inspiraling phase of the planet into its host star, arising from uncertainties
in the tidal dissipation factor in the star .Comment: Submitted to ApJ; 13 pages, 3 figures, 2 tables; (emulateapj format
Ohmic Dissipation in the Atmospheres of Hot Jupiters
Hot Jupiter atmospheres exhibit fast, weakly-ionized winds. The interaction
of these winds with the planetary magnetic field generates drag on the winds
and leads to ohmic dissipation of the induced electric currents. We study the
magnitude of ohmic dissipation in representative, three-dimensional atmospheric
circulation models of the hot Jupiter HD 209458b. We find that ohmic
dissipation can reach or exceed 1% of the stellar insolation power in the
deepest atmospheric layers, in models with and without dragged winds. Such
power, dissipated in the deep atmosphere, appears sufficient to slow down
planetary contraction and explain the typically inflated radii of hot Jupiters.
This atmospheric scenario does not require a top insulating layer or radial
currents that penetrate deep in the planetary interior. Circulation in the
deepest atmospheric layers may actually be driven by spatially non-uniform
ohmic dissipation. A consistent treatment of magnetic drag and ohmic
dissipation is required to further elucidate the consequences of magnetic
effects for the atmospheres and the contracting interiors of hot Jupiters.Comment: Accepted to the Astrophysical Journa
Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars
(abridged) AIMS. We investigate the dynamics and stability of post-shock
plasma streaming along nonuniform stellar magnetic fields at the impact region
of accretion columns. We study how the magnetic field configuration and
strength determine the structure, geometry, and location of the shock-heated
plasma. METHODS. We model the impact of an accretion stream onto the
chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our
model takes into account the gravity, the radiative cooling, and the
magnetic-field-oriented thermal conduction. RESULTS. The structure, stability,
and location of the shocked plasma strongly depend on the configuration and
strength of the magnetic field. For weak magnetic fields, a large component of
B may develop perpendicular to the stream at the base of the accretion column,
limiting the sinking of the shocked plasma into the chromosphere. An envelope
of dense and cold chromospheric material may also develop around the shocked
column. For strong magnetic fields, the field configuration determines the
position of the shock and its stand-off height. If the field is strongly
tapered close to the chromosphere, an oblique shock may form well above the
stellar surface. In general, a nonuniform magnetic field makes the distribution
of emission measure vs. temperature of the shocked plasma lower than in the
case of uniform magnetic field. CONCLUSIONS. The initial strength and
configuration of the magnetic field in the impact region of the stream are
expected to influence the chromospheric absorption and, therefore, the
observability of the shock-heated plasma in the X-ray band. The field strength
and configuration influence also the energy balance of the shocked plasma, its
emission measure at T > 1 MK being lower than expected for a uniform field. The
above effects contribute in underestimating the mass accretion rates derived in
the X-ray band.Comment: 11 pages, 11 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_accretion_shocks.pd
Escaping Particle fluxes in the atmospheres of close-in exoplanets: I. model of hydrogen
A multi-fluid model for an atomic hydrogen-proton mixture in the upper
atmosphere of extrosolar planet is presented when the continuity and momentum
equations of each component have been already solved with an energy equation.
The particle number density, the temperature distribution and the structure of
velocity can be found by means of the model. We chose two special objects, HD
209458b and HD 189733b, as discussion samples and the conclusion is that their
mass loss rates predicted by the model are in accordance with those of
observation. The most important physical process in coupling each component is
charge exchange which tightly couples atomic hydrogen with protons. Most of the
hydrogen escaping from hot Jupiters is protons, especially in young star-planet
system. We found that the single-fluid model can describe the escape of
particles when the mass loss rate is higher than a few times g/s while
below g/s the multi-fluid model is more suitable for it due to the
decoupling of particles. We found that the predicted mass loss rates of HD
189733b with the assumption of energy-limit are a factor of 10 larger than that
calculated by our models due to the high ionization degree. For the ionized
wind which is almost compose of protons, the assumption of energy-limit is no
longer effective. We fitted the mass loss rates of the ionized wind as a
function of by calculating the variation of the mass loss rates with
UV fluxes.Comment: 35 pages, 6 figures, submitted to Ap
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