2,683 research outputs found
Identification of Absorption Features in an Extrasolar Planet Atmosphere
Water absorption is identified in the atmosphere of HD209458b by comparing
models for the planet's transmitted spectrum to recent, multi-wavelength,
eclipse-depth measurements (from 0.3 to 1 microns) published by Knutson et al.
(2007). A cloud-free model which includes solar abundances, rainout of
condensates, and photoionization of sodium and potassium is in good agreement
with the entire set of eclipse-depth measurements from the ultraviolet to
near-infrared. Constraints are placed on condensate removal by gravitational
settling, the bulk metallicity, and the redistribution of absorbed stellar
flux. Comparisons are also made to the Charbonneau et al. (2002) sodium
measurements.Comment: Accepted for publication in ApJL., in emulate ApJ forma
Characterization of the hot Neptune GJ 436b with Spitzer and ground-based observations
We present Spitzer Space Telescope infrared photometry of a secondary eclipse
of the hot Neptune GJ436b. The observations were obtained using the 8-micron
band of the InfraRed Array Camera (IRAC). The data spanning the predicted time
of secondary eclipse show a clear flux decrement with the expected shape and
duration. The observed eclipse depth of 0.58 mmag allows us to estimate a
blackbody brightness temperature of T_p = 717 +- 35 K at 8 microns. We compare
this infrared flux measurement to a model of the planetary thermal emission,
and show that this model reproduces properly the observed flux decrement. The
timing of the secondary eclipse confirms the non-zero orbital eccentricity of
the planet, while also increasing its precision (e = 0.14 +- 0.01). Additional
new spectroscopic and photometric observations allow us to estimate the
rotational period of the star and to assess the potential presence of another
planet.Comment: Accepted for publication in A&A on 11/09/2007; 7 pages, 6 figure
Gaseous Planets, Protostars And Young Brown Dwarfs : Birth And Fate
We review recent theoretical progress aimed at understanding the formation
and the early stages of evolution of giant planets, low-mass stars and brown
dwarfs. Calculations coupling giant planet formation, within a modern version
of the core accretion model, and subsequent evolution yield consistent
determinations of the planet structure and evolution. Because of the
uncertainties in the initial conditions, however, it is not possible to say
whether young planets are faint or bright compared with low-mass young brown
dwarfs. We review the effects of irradiation and evaporation on the evolution
of short period planets and argue that substantial mass loss may have occurred
for these objects. Concerning star formation, geometrical effects in protostar
core collapse are examined by comparing 1D and 3D calculations. Spherical
collapse is shown to overestimate the core inner density and temperature and
thus to yield incorrect initial conditions for PMS or young brown dwarf
evolution. Accretion is also shown to occur over a very limited fraction of the
protostar surface. Accretion affects the evolution of young brown dwarfs and
yields more compact structures for a given mass and age, thus fainter
luminosities. This can lead to severe misinterpretations of the mass and/or age
of young accreting objects from their location in the HR diagram. We argue that
newborn stars and brown dwarfs should appear rapidly over an extended area in
the HR diagram, depending on their accretion history, rather than on a well
defined birth line. Finally, we suggest that the distinction between planets
and brown dwarfs be based on an observational diagnostic, reflecting the
different formation mechanisms between these two distinct populations, rather
than on an arbitrary, confusing definition.Comment: Invited Review, Protostars and Planets V (Hawai, October 2005
The mass-radius relationship from solar-type stars to terrestrial planets: a review
In this review, we summarize our present knowledge of the behaviour of the
mass-radius relationship from solar-type stars down to terrestrial planets,
across the regime of substellar objects, brown dwarfs and giant planets.
Particular attention is paid to the identification of the main physical
properties or mechanisms responsible for this behaviour. Indeed, understanding
the mechanical structure of an object provides valuable information about its
internal structure, composition and heat content as well as its formation
history. Although the general description of these properties is reasonably
well mastered, disagreement between theory and observation in certain cases
points to some missing physics in our present modelling of at least some of
these objects. The mass-radius relationship in the overlaping domain between
giant planets and low-mass brown dwarfs is shown to represent a powerful
diagnostic to distinguish between these two different populations and shows
once again that the present IAU distinction between these two populations at a
given mass has no valid foundation.Comment: Cool Stars, Stellar Systems and the Sun 15, invited revie
Accurate Spitzer infrared radius measurement for the hot Neptune GJ 436b
We present Spitzer Space Telescope infrared photometry of a primary transit
of the hot Neptune GJ 436b. The observations were obtained using the 8 microns
band of the InfraRed Array Camera (IRAC). The high accuracy of the transit data
and the weak limb-darkening in the 8 microns IRAC band allow us to derive
(assuming M = 0.44 +- 0.04 Msun for the primary) a precise value for the
planetary radius (4.19 +0.21-0.16 Rearth), the stellar radius (0.463
+0.022-0.017 Rsun), the orbital inclination (85.90 +0.19-0.18 degrees) and
transit timing (2454280.78186 +0.00015-0.00008 HJD). Assuming current planet
models, an internal structure similar to that of Neptune with a small H/He
envelope is necessary to account for the measured radius of GJ 436b.Comment: Accepted for publication in A&A on 21/07/2007; 5 pages, 3 figure
Direct Imaging of Multiple Planets Orbiting the Star HR 8799
Direct imaging of exoplanetary systems is a powerful technique that can
reveal Jupiter-like planets in wide orbits, can enable detailed
characterization of planetary atmospheres, and is a key step towards imaging
Earth-like planets. Imaging detections are challenging due to the combined
effect of small angular separation and large luminosity contrast between a
planet and its host star. High-contrast observations with the Keck and Gemini
telescopes have revealed three planets orbiting the star HR 8799, with
projected separations of 24, 38, and 68 astronomical units. Multi-epoch data
show counter-clockwise orbital motion for all three imaged planets. The low
luminosity of the companions and the estimated age of the system imply
planetary masses between 5 and 13 times that of Jupiter. This system resembles
a scaled-up version of the outer portion of our Solar System.Comment: 30 pages, 5 figures, Research Article published online in Science
Express Nov 13th, 200
Astrometric Monitoring of the HR 8799 Planets: Orbit Constraints from Self-Consistent Measurements
We present new astrometric measurements from our ongoing monitoring campaign
of the HR 8799 directly imaged planetary system. These new data points were
obtained with NIRC2 on the W.M. Keck II 10 meter telescope between 2009 and
2014. In addition, we present updated astrometry from previously published
observations in 2007 and 2008. All data were reduced using the SOSIE algorithm,
which accounts for systematic biases present in previously published
observations. This allows us to construct a self-consistent data set derived
entirely from NIRC2 data alone. From this dataset, we detect acceleration for
two of the planets (HR 8799b and e) at 3. We also assess possible
orbital parameters for each of the four planets independently. We find no
statistically significant difference in the allowed inclinations of the
planets. Fitting the astrometry while forcing coplanarity also returns
consistent to within 1 of the best fit values, suggesting that if
inclination offsets of 20 are present, they are not detectable
with current data. Our orbital fits also favor low eccentricities, consistent
with predictions from dynamical modeling. We also find period distributions
consistent to within 1 with a 1:2:4:8 resonance between all planets.
This analysis demonstrates the importance of minimizing astrometric systematics
when fitting for solutions to highly undersampled orbits.Comment: 18 pages, 11 figures. Accepted for publication in A
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