1,192 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
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&
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
An Investigation into the Radial Velocity Variations of CoRoT-7
CoRoT-7b, the first transiting ``superearth'' exoplanet, has a radius of 1.7
R_Earth and a mass of 4.8 M_Earth. Ground-based radial velocity measurements
also detected an additional companion with a period of 3.7 days (CoRoT-7c) and
a mass of 8.4 M_Earth. The mass of CoRoT-7b is a crucial parameter for planet
structure models, but is difficult to determine because CoRoT-7 is a modestly
active star and there is at least one additional companion. A Fourier analysis
was performed on spectral data for CoRoT-7 taken with the HARPS spectrograph.
These data include RV measurements, spectral line bisectors, the full width at
half maximum of the cross-correlation function, and Ca II emission. The latter
3 quantities vary due to stellar activity and were used to assess the nature of
the observed RV variations. An analysis of a sub-set of the RV measurements
where multiple observations were made per night was also used to estimate the
RV amplitude from CoRoT-7b that was less sensitive to activity variations. Our
analysis indicates that the 0.85-d and 3.7-d RV signals of CoRoT-7b and
CoRoT-7c are present in the spectral data with a high degree of statistical
significance. We also find evidence for another significant RV signal at 9
days. An analysis of the activity indicator data reveals that this 9-d signal
most likely does not arise from activity, but possibly from an additional
companion. If due to a planetary companion the mass is m = 19.5 M_Earth,
assuming co-planarity with CoRoT-7b. A dynamical study of the three planet
system shows that it is stable over several hundred millions of years. Our
analysis yields a RV amplitude of 5.04 +/- 1.09 m/s for CoRoT-7b which
corresponds to a planet mass of m = 6.9 +/- 1.4 M_Earth. This increased mass
would make the planet CoRoT-7b more Earth-like in its internal structure.Comment: 20 pages, 20 figure
The CORALIE survey for southern extra-solar planets. X. A Hot Jupiter orbiting HD73256
Recent radial-velocity measurements obtained with the CORALIE spectrograph on
the 1.2-m Euler Swiss telescope at La Silla unveil the presence of a new
Jovian-mass Hot Jupiter around HD 73256. The 1.85-M_Jup planet moves on an
extremely short-period (P=2.5486 d), quasi-circular orbit. The best Keplerian
orbital solution is presented together with an unsuccessful photometric
planetary-transit search performed with the SAT Danish telescope at La Silla.
Over the time span of the observations, the photometric follow-up of the
candidate has nevertheless revealed a P=14-d photometric periodicity
corresponding to the rotational period of the star. This variation as well as
the radial-velocity jitter around the Keplerian solution are shown to be
related to the fair activity level known for HD 73256.Comment: 7 pages, 7 figures. Accepted in A&
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 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 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&
- …