1,294 research outputs found
On the penetration of meridional circulation below the solar convection zone
Meridional flows with velocities of a few meters per second are observed in
the uppermost regions of the solar convection zone. The amplitude and pattern
of the flows deeper in the solar interior, in particular near the top of the
radiative region, are of crucial importance to a wide range of solar
magnetohydrodynamical processes. In this paper, we provide a systematic study
of the penetration of large-scale meridional flows from the convection zone
into the radiative zone. In particular, we study the effects of the assumed
boundary conditions applied at the convective-radiative interface on the deeper
flows. Using simplified analytical models in conjunction with more complete
numerical methods, we show that penetration of the convectively-driven
meridional flows into the deeper interior is not necessarily limited to a
shallow Ekman depth but can penetrate much deeper, depending on how the
convective-radiative interface flows are modeled.Comment: 13 pages, 5 figures. Subitted to Ap
High Metallicity and Non-equilibrium Chemistry in the Dayside Atmosphere of hot-Neptune GJ 436b
We present a detailed analysis of the dayside atmosphere of the hot-Neptune
GJ~436b, based on recent Spitzer observations. We report statistical
constraints on the thermal and chemical properties of the planetary atmosphere,
study correlations between the various molecular species, and discuss scenarios
of equilibrium and non-equilibrium chemistry in GJ 436b. We model the
atmosphere with a one-dimensional line-by-line radiative transfer code with
parameterized molecular abundances and temperature structure. We explore the
model parameter space with 10^6 models, using a Markov chain Monte Carlo
scheme. Our results encompass previous findings, indicating a paucity of
methane, an overabundance of CO and CO2, and a slight underabundance of H2O, as
compared to equilibrium chemistry with solar metallicity. The concentrations of
the species are highly correlated. Our best-fit, and most plausible,
constraints require a CH4 mixing ratio of 1.0E-7 to 1.0E-6, with CO >= 1.0E-3,
CO2 ~ 1.0E-6 to 1.0E-4, and H2O <= 1.0E-4; higher CH4 would require much higher
CO and CO2. Based on calculations of equilibrium and non-equilibrium chemistry,
we find that the observed abundances can potentially be explained by a
combination of high metallicity (~ 10 x solar) and vertical mixing with Kzz ~
10^6 - 10^7 cm^2/s. The inferred metallicity is enhanced over that of the host
star which is known to be consistent with solar metallicity. Our constraints
rule out a dayside thermal inversion in GJ 436b. We emphasize that the
constraints reported in this work depend crucially on the observations in the
two Spitzer channels at 3.6 micron and 4.5 micron. Future observations with
warm Spitzer and with the James Webb Space Telescope will be extremely
important to improve upon the present constraints on the abundances of carbon
species in the dayside atmosphere of GJ 436b.Comment: Accepted version, 15 pages in emulateapj, 7 figures. Published in
ApJ, 2011, 729, 4
A Precise Estimate of the Radius of HD 149026b
We present Spitzer 8 micron transit observations of the extrasolar planet
system HD 149026. At this wavelength, transit light curves are weakly affected
by stellar limb-darkening, allowing for a simpler and more accurate
determination of planetary parameters. We measure a planet-star radius ratio of
R_p/R_s = 0.05158 +/- 0.00077, and in combination with ground-based data and
independent constraints on the stellar mass and radius, we derive an orbital
inclination of i = 85.4 +0.9/-0.8 deg. and a planet radius of 0.755 +/- 0.040
Jupiter radii. These measurements further support models in which the planet is
greatly enriched in heavy elements.Comment: To appear in the Proceedings of the 253rd IAU Symposium: "Transiting
Planets", May 2008, Cambridge, M
Resolving the Surfaces of Extrasolar Planets With Secondary Eclipse Light Curves
We present a method that employs the secondary eclipse light curves of
transiting extrasolar planets to probe the spatial variation of their thermal
emission. This technique permits an observer to resolve the surface of the
planet without the need to spatially resolve its central star. We evaluate the
feasibility of this technique for the HD 209458 system [..]. We consider two
representations of the planetary thermal emission; a simple model parameterized
by a sinusoidal dependence on longitude and latitude, as well as the results of
a three-dimensional dynamical simulation of the planetary atmosphere previously
published by Cooper & Showman. We find that observations of the secondary
eclipse light curve are most sensitive to a longitudinal offset in the
geometric and photometric centroids of the hemisphere of the planet visible
near opposition. To quantify this signal, we define a new parameter, the
``uniform time offset,'' which measures the time lag between the observed
secondary eclipse and that predicted by a planet with a uniform surface flux
distribution. We compare the predicted amplitude of this parameter for HD
209458 with the precision with which it could be measured with IRAC. We find
that IRAC observations at 3.6um a single secondary eclipse should permit
sufficient precision to confirm or reject the Cooper & Showman model of the
surface flux distribution for this planet. We quantify the signal-to-noise
ratio for this offset in the remaining IRAC bands (4.5um, 5.8um, and 8.0um),
and find that a modest improvement in photometric precision (as might be
realized through observations of several eclipse events) should permit a
similarly robust detection.Comment: AASTeX 5.2, 24 pages, 5 figures, accepted for publication in ApJ; v2:
clarifications, updated to version accepted by ApJ; v3: try to reduce spacin
The Broadband Infrared Emission Spectrum of the Exoplanet HD 189733b
We present Spitzer Space Telescope time series photometry of the exoplanet
system HD 189733 spanning two times of secondary eclipse, when the planet
passes out of view behind the parent star. We estimate the relative eclipse
depth in 5 distinct bands and find the planet-to-star flux ratio to be 0.256
+/- 0.014% (3.6 microns), 0.214 +/- 0.020% (4.5 microns), 0.310 +/- 0.034% (5.8
microns), 0.391 +/- 0.022% (8.0 microns), and 0.598 +/- 0.038% (24 microns).
For consistency, we re-analyze a previously published time series to deduce a
contrast ratio in an additional band, 0.519 +/- 0.020% (16 microns). Our data
are strongly inconsistent with a Planck spectrum, and we clearly detect
emission near 4 microns as predicted by published theoretical models in which
this feature arises from a corresponding opacity window. Unlike recent results
for the exoplanet HD 209458b, we find that the emergent spectrum from HD
189733b is best matched by models that do not include an atmospheric
temperature inversion. Taken together, these two studies provide initial
observational support for the idea that hot Jupiter atmospheres diverge into
two classes, in which a thermal inversion layer is present for the more
strongly irradiated objects.Comment: 20 pages, 3 figures, accepted to the Astrophysical Journal, minor
revision
An Upper Limit on the Reflected Light from the Planet Orbiting the Star tau Bootis
The planet orbiting tau Boo at a separation of 0.046 AU could produce a
reflected light flux as bright as 1e-4 relative to that of the star. A spectrum
of the system will contain a reflected light component which varies in
amplitude and Doppler-shift as the planet orbits the star. Assuming the
secondary spectrum is primarily the reflected stellar spectrum, we can limit
the relative reflected light flux to be less than 5e-5. This implies an upper
limit of 0.3 for the planetary geometric albedo near 480 nm, assuming a
planetary radius of 1.2 R_Jup. This albedo is significantly less than that of
any of the giant planets of the solar system, and is not consistent with
certain published theoretical predictions.Comment: 5 pages, 1 figure, accepted by ApJ Letter
Hubble Space Telescope times-series photometry of the planetary transit of HD189733: no moon, no rings, starspots
We monitored three transits of the giant gas planet around the nearby K dwarf
HD 189733 with the ACS camera on the Hubble Space Telescope. The resulting
very-high accuracy lightcurve (signal-to-noise ratio near 15000 on individual
measurements, 35000 on 10-minute averages) allows a direct geometric
measurement of the orbital inclination, radius ratio and scale of the system: i
= 85.68 +- 0.04, Rpl/R*=0.1572 +- 0.0004, a/R*=8.92 +- 0.09. We derive improved
values for the stellar and planetary radius, R*=0.755+- 0.011 Rsol, Rpl=1.154
+- 0.017 RJ, and the transit ephemerides, Ttr=2453931.12048 +- 0.00002 + n
2.218581 +- 0.000002$. The HST data also reveal clear evidence of the planet
occulting spots on the surface of the star. At least one large spot complex
(>80000 km) is required to explain the observed flux residuals and their colour
evolution. This feature is compatible in amplitude and phase with the
variability observed simultaneously from the ground. No evidence for satellites
or rings around HD 189733b is seen in the HST lightcurve. This allows us to
exlude with a high probability the presence of Earth-sized moons and
Saturn-type debris rings around this planet. The timing of the three transits
sampled is stable to the level of a few seconds, excluding a massive second
planet in outer 2:1 resonance.Comment: revised version. Significant updates and new figures; to appear in
Astronomy and Astrophysic
Imaging the Solar Tachocline by Time-Distance Helioseismology
The solar tachocline at the bottom of the convection zone is an important
region for the dynamics of the Sun and the solar dynamo. In this region, the
sound speed inferred by global helioseismology exhibits a bump of approximately
0.4% relative to the standard solar model. Global helioseismology does not
provide any information on possible latitudinal variations or asymmetries
between the Northern and Southern hemisphere. Here, we develop a time-distance
helioseismology technique, including surface- and deep-focusing measurement
schemes and a combination of both, for two-dimensional tomographic imaging of
the solar tachocline that infers radial and latitudinal variations in the sound
speed. We test the technique using artificial solar oscillation data obtained
from numerical simulations. The technique successfully recovers major features
of the simplified tachocline models. The technique is then applied to SOHO/MDI
medium-l data and provides for the first time a full two-dimensional
sound-speed perturbation image of the solar tachocline. The one-dimensional
radial profile obtained by latitudinal averaging of the image is in good
agreement with the previous global helioseismology result. It is found that the
amplitude of the sound-speed perturbation at the tachocline varies with
latitude, but it is not clear whether this is in part or fully an effect of
instrumental distortion. Our initial results demonstrate that time-distance
helioseismology can be used to probe the deep interior structure of the Sun,
including the solar tachocline.Comment: accepted for publication by Ap
A Time-Dependent Radiative Model of HD209458b
We present a time-dependent radiative model of the atmosphere of HD209458b
and investigate its thermal structure and chemical composition. In a first
step, the stellar heating profile and radiative timescales were calculated
under planet-averaged insolation conditions. We find that 99.99% of the
incoming stellar flux has been absorbed before reaching the 7 bar level.
Stellar photons cannot therefore penetrate deeply enough to explain the large
radius of the planet. We derive a radiative time constant which increases with
depth and reaches about 8 hr at 0.1 bar and 2.3 days at 1 bar. Time-dependent
temperature profiles were also calculated, in the limit of a zonal wind that is
independent on height (i.e. solid-body rotation) and constant absorption
coefficients. We predict day-night variations of the effective temperature of
\~600 K, for an equatorial rotation rate of 1 km/s, in good agreement with the
predictions by Showman &Guillot (2002). This rotation rate yields day-to-night
temperature variations in excess of 600 K above the 0.1-bar level. These
variations rapidly decrease with depth below the 1-bar level and become
negligible below the ~5--bar level for rotation rates of at least 0.5 km/s. At
high altitudes (mbar pressures or less), the night temperatures are low enough
to allow sodium to condense into Na2S. Synthetic transit spectra of the visible
Na doublet show a much weaker sodium absorption on the morning limb than on the
evening limb. The calculated dimming of the sodium feature during planetary
transites agrees with the value reported by Charbonneau et al. (2002).Comment: 9 pages, 8 figures, replaced with the revised versio
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