1,528 research outputs found
Detection of an Extrasolar Planet Atmosphere
We report high precision spectrophotometric observations of four planetary
transits of HD 209458, in the region of the sodium resonance doublet at 589.3
nm. We find that the photometric dimming during transit in a bandpass centered
on the sodium feature is deeper by (2.32 +/- 0.57) x 10^{-4} relative to
simultaneous observations of the transit in adjacent bands. We interpret this
additional dimming as absorption from sodium in the planetary atmosphere, as
recently predicted from several theoretical modeling efforts. Our model for a
cloudless planetary atmosphere with a solar abundance of sodium in atomic form
predicts more sodium absorption than we observe. There are several
possibilities that may account for this reduced amplitude, including reaction
of atomic sodium into molecular gases and/or condensates, photoionization of
sodium by the stellar flux, a low primordial abundance of sodium, or the
presence of clouds high in the atmosphere.Comment: 26 pages, 8 figures, accepted by ApJ 2001 November 1
Astrometric detection of exoplanets from the ground
Astrometry is a powerful technique to study the populations of extrasolar
planets around nearby stars. It gives access to a unique parameter space and is
therefore required for obtaining a comprehensive picture of the properties,
abundances, and architectures of exoplanetary systems. In this review, we
discuss the scientific potential, present the available techniques and
instruments, and highlight a few results of astrometric planet searches, with
an emphasis on observations from the ground. In particular, we discuss
astrometric observations with the Very Large Telescope (VLT) Interferometer and
a programme employing optical imaging with a VLT camera, both aimed at the
astrometric detection of exoplanets. Finally, we set these efforts into the
context of Gaia, ESA's astrometry mission scheduled for launch in 2013, and
present an outlook on the future of astrometric exoplanet detection from the
ground.Comment: 9 pages, 3 figures. Invited contribution to the SPIE conference
"Techniques and Instrumentation for Detection of Exoplanets VI" held in San
Diego, CA, August 25-29, 201
A Dynamical Method for Measuring Masses of Stars with Transiting Planets
As a planet transits the face of a star, it accelerates along the
line-of-sight. The changing delay in the propagation of photons produces an
apparent deceleration of the planet across the sky throughout the transit. This
persistent transverse deceleration breaks the time-reversal symmetry in the
transit lightcurve of a spherical planet in a circular orbit around a perfectly
symmetric star. For `hot Jupiter' systems, ingress advances at a higher rate
than egress by a fraction of 10^{-4}-10^{-3}. Forthcoming space telescopes such
as Kepler or COROT will reach the sensitivity required to detect this
asymmetry. The scaling of the fractional asymmetry with stellar mass M and
planetary orbital radius R as M/R^2 is different from the scaling of the
orbital period as (M/R^3)^{-1/2}. Therefore, this effect constitutes a new
method for a purely dynamical determination of the mass of the star, which is
currently inferred indirectly with theoretical uncertainties based on spectral
modeling. Radial velocity data for the reflex motion of the star can then be
used to determine the planet's mass. Although orbital eccentricity could
introduce a larger asymmetry than the light propagation delay, the eccentricity
is expected to decay by tidal dissipation to negligible values for a close-in
planet with no perturbing third body. Future detection of the eclipse of a
planet's emission by its star could be used to measure the light propagation
delay across the orbital diameter, 46.7(R/7x10^{11}cm) seconds, and also
determine the stellar mass from the orbital period.Comment: 4 pages, 2 figures, submitted to ApJ
A photometric study of the hot exoplanet WASP-19b
Context: When the planet transits its host star, it is possible to measure
the planetary radius and (with radial velocity data) the planet mass. For the
study of planetary atmospheres, it is essential to obtain transit and
occultation measurements at multiple wavelengths.
Aims: We aim to characterize the transiting hot Jupiter WASP-19b by deriving
accurate and precise planetary parameters from a dedicated observing campaign
of transits and occultations.
Methods: We have obtained a total of 14 transit lightcurves in the r'-Gunn,
IC, z'-Gunn and I+z' filters and 10 occultation lightcurves in z'-Gunn using
EulerCam on the Euler-Swiss telescope and TRAPPIST. We have also obtained one
lightcurve through the narrow-band NB1190 filter of HAWK-I on the VLT measuring
an occultation at 1.19 micron. We have performed a global MCMC analysis of all
new data together with some archive data in order to refine the planetary
parameters and measure the occultation depths in z'-band and at 1.19 micron.
Results: We measure a planetary radius of R_p = 1.376 (+/-0.046) R_j, a
planetary mass of M_p = 1.165 (+/-0.068) M_j, and find a very low eccentricity
of e = 0.0077 (+/-0.0068), compatible with a circular orbit. We have detected
the z'-band occultation at 3 sigma significance and measure it to be dF_z'= 352
(+/-116) ppm, more than a factor of 2 smaller than previously published. The
occultation at 1.19 micron is only marginally constrained at dF_1190 = 1711
(+/-745) ppm.
Conclusions: We have shown that the detection of occultations in the visible
is within reach even for 1m class telescopes if a considerable number of
individual events are observed. Our results suggest an oxygen-dominated
atmosphere of WASP-19b, making the planet an interesting test case for
oxygen-rich planets without temperature inversion.Comment: Published in Astronomy & Astrophysics. 11 pages, 11 figures, 4 table
The CORALIE survey for southern extra-solar planets VIII. The very low-mass companions of HD141937, HD162020, HD168443, HD202206: brown dwarfs or superplanets?
Doppler CORALIE measurements of the solar-type stars HD141937, HD162020,
HD168443 and HD202206 show Keplerian radial-velocity variations revealing the
presence of 4 new companions with minimum masses close to the
planet/brown-dwarf transition, namely with m_2sin(i) = 9.7, 14.4, 16.9, and
17.5 M_Jup, respectively. The orbits present fairly large eccentricities
(0.22<e<0.43). Except for HD162020, the parent stars are metal rich compared to
the Sun, as are most of the detected extra-solar planet hosts. Considerations
of tidal dissipation in the short-period HD162020 system points towards a
brown-dwarf nature for the low-mass companion. HD168443 is a multiple system
with two low-mass companions being either brown dwarfs or formed simultaneously
in the protoplanetary disks as superplanets. For HD202206, the radial
velocities show an additional drift revealing a further outer companion, the
nature of which is still unknown. Finally, the stellar-host and orbital
properties of massive planets are examined in comparison to lighter exoplanets.
Observed trends include the need of metal-rich stars to form massive exoplanets
and the lack of short periods for massive planets. If confirmed with improved
statistics, these features may provide constraints for the migration scenario.Comment: 14 pages including figures, accepted for publication in A&
Pushing the precision limit of ground-based eclipse photometry
Until recently, it was considered by many that ground-based photometry could
not reach the high cadence sub-mmag regime because of the presence of the
atmosphere. Indeed, high frequency atmospheric noises (mainly scintillation)
limit the precision that high SNR photometry can reach within small time bins.
If one is ready to damage the sampling of his photometric time-series, binning
the data (or using longer exposures) allows to get better errors, but the
obtained precision will be finally limited by low frequency noises. To observe
several times the same planetary eclipse and to fold the photometry with the
orbital period is thus generally considered as the only option to get very well
sampled and precise eclipse light curve from the ground. Nevertheless, we show
here that reaching the sub-mmag sub-min regime for one eclipse is possible with
a ground-based instrument. This has important implications for transiting
planets characterization, secondary eclipses measurement and small planets
detection from the ground.Comment: Transiting Planets Proceeding IAU Symposium No.253, 2008. 7 pages, 4
figure
Astrometric orbit of a low-mass companion to an ultracool dwarf
Little is known about the existence of extrasolar planets around ultracool
dwarfs. Furthermore, binary stars with Sun-like primaries and very low-mass
binaries composed of ultracool dwarfs show differences in the distributions of
mass ratio and orbital separation that can be indicative of distinct formation
mechanisms. Using FORS2/VLT optical imaging for high precision astrometry we
are searching for planets and substellar objects around ultracool dwarfs to
investigate their multiplicity properties for very low companion masses. Here
we report astrometric measurements with an accuracy of two tenths of a
milli-arcsecond over two years that reveal orbital motion of the nearby L1.5
dwarf DENIS-P J082303.1-491201 located at 20.77 +/- 0.08 pc caused by an unseen
companion that revolves about its host on an eccentric orbit in 246.4 +/- 1.4
days. We estimate the L1.5 dwarf to have 7.5 +/- 0.7 % of the Sun's mass that
implies a companion mass of 28 +/- 2 Jupiter masses. This new system has the
smallest mass ratio (0.36 +/- 0.02) of known very low-mass binaries with
characterised orbits. With this discovery we demonstrate 200 micro-arcsecond
astrometry over an arc-minute field and over several years that is sufficient
to discover sub-Jupiter mass planets around ultracool dwarfs. We also show that
the achieved parallax accuracy of < 0.4 % makes it possible to remove distance
as a dominant source of uncertainty in the modelling of ultracool dwarfs.Comment: 9 pages, 8 figures, accepted for publication in Astronomy and
Astrophysics. The reduced astrometry data will be made publically available
through the CD
Astrometric planet search around southern ultracool dwarfs II: Astrometric reduction methods and a deep astrometric catalogue
We describe the astrometric reduction of images obtained with the FORS2/VLT
camera in the framework of an astrometric planet search around 20
M/L-transition dwarfs. We present the correction of systematic errors, the
achieved astrometric performance, and a new astrometric catalogue containing
the faint reference stars in 20 fields located close to the Galactic plane. We
detected three types of systematic errors in the FORS2 astrometry: the relative
motion of the camera's two CCD chips, errors that are correlated in space, and
an error contribution of yet unexplained origin. The relative CCD motion has
probably a thermal origin and usually is 0.001-0.010 px (~0.1-1 mas), but
sometimes amounts to 0.02-0.05 px (3-6 mas). This instability and
space-correlated errors are detected and mitigated using reference stars. The
third component of unknown origin has an amplitude of 0.03-0.14 mas and is
independent of the observing conditions. We find that a consecutive sequence of
32 images of a well-exposed star over 40 min at 0.6" seeing results in a median
r.m.s. of the epoch residuals of 0.126 mas. Overall, the epoch residuals are
distributed according to a normal law with a chi2~1. We compiled a catalogue of
12000 stars with I-band magnitudes of 16-22 located in 20 fields, each covering
~2x2'. It contains I-band magnitudes, ICRF positions with 40-70 mas precision,
and relative proper motions and absolute trigonometric parallaxes with a
precision of 0.1 mas/yr and 0.1 mas at the bright end, respectively.Comment: 17 pages, 19 figures, 4 tables, accepted for publication in A&A on
March 14, 201
Astrometric planet search around southern ultracool dwarfs III. Discovery of a brown dwarf in a 3-year orbit around DE0630-18
Using astrometric measurements obtained with the FORS2/VLT camera, we are
searching for low-mass companions around 20 nearby ultracool dwarfs. With a
single-measurement precision of 0.1 milli-arcseconds, our survey is sensitive
to a wide range of companion masses from planetary companions to binary
systems. Here, we report the discovery and orbit characterisation of a new
ultracool binary at a distance of 19.5 pc from Earth that is composed of the
M8.5-dwarf primary DE0630-18 and a substellar companion. The nearly edge-on
orbit is moderately eccentric (e=0.23) with an orbital period of 1120 d, which
corresponds to a relative separation in semimajor axis of approximately 1.1 AU.
We obtained a high-resolution optical spectrum with UVES/VLT and measured the
system's heliocentric radial velocity. The spectrum does not exhibit lithium
absorption at 670.8 nm, indicating that the system is not extremely young. A
preliminary estimate of the binary's physical parameters tells us that it is
composed of a primary at the stellar-substellar limit and a massive brown-dwarf
companion. DE0630-18 is a new very low-mass binary system with a
well-characterised orbit.Comment: 4 pages, 7 figures. Accepted for publication in A&
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