780 research outputs found
SOPHIE+: First results of an octagonal-section fiber for high-precision radial velocity measurements
High-precision spectrographs play a key role in exoplanet searches and
Doppler asteroseismology using the radial velocity technique. The 1 m/s level
of precision requires very high stability and uniformity of the illumination of
the spectrograph. In fiber-fed spectrographs such as SOPHIE, the fiber-link
scrambling properties are one of the main conditions for high precision. To
significantly improve the radial velocity precision of the SOPHIE spectrograph,
which was limited to 5-6 m/s, we implemented a piece of octagonal-section fiber
in the fiber link. We present here the scientific validation of the upgrade of
this instrument, demonstrating a real improvement. The upgraded instrument,
renamed SOPHIE+, reaches radial velocity precision in the range of 1-2 m/s. It
is now fully efficient for the detection of low-mass exoplanets down to 5-10
Earth mass and for the identification of acoustic modes down to a few tens of
cm/s.Comment: 12 pages, 11 figures, accepted in Astronomy and Astrophysic
The non-detection of oscillations in Procyon by MOST: is it really a surprise?
We argue that the non-detection of oscillations in Procyon by the MOST
satellite reported by Matthews et al. (2004) is fully consistent with published
ground-based velocity observations of this star. We also examine the claims
that the MOST observations represent the best photometric precision so far
reported in the literature by about an order of magnitude and are the most
sensitive data set for asteroseismology available for any star other than the
Sun. These statements are not correct, with the most notable exceptions being
observations of oscillations in alpha Cen A that are far superior. We further
disagree that the hump of excess power seen repeatedly from velocity
observations of Procyon can be explained as an artefact caused by gaps in the
data. The MOST observations failed to reveal oscillations clearly because their
noise level is too high, possibly from scattered Earthlight in the instrument.
We did find an excess of strong peaks in the MOST amplitude spectrum that is
inconsistent with a simple noise source such as granulation, and may perhaps
indicate oscillations at roughly the expected level.Comment: 6 pages, accepted for publication in A&A Letter
Interpreting the yield of transit surveys: Are there groups in the known transiting planets population?
Each transiting planet discovered is characterized by 7 measurable
quantities, that may or may not be linked together (planet mass, radius,
orbital period, and star mass, radius, effective temperature, and metallicity).
Correlations between planet mass and period, surface gravity and period, planet
radius and star temperature have been previously observed among the known
transiting giant planets. Two classes of planets have been previously
identified based on their Safronov number. We use the CoRoTlux code to compare
simulated events to the sample of discovered planets and test the statistical
significance of these correlations. We first generate a stellar field with
planetary companions based on radial velocity discoveries and a planetary
evolution model, then apply a detection criterion that includes both
statistical and red noise sources. We compare the yield of our simulated survey
with the ensemble of 31 well-characterized giant transiting planets, using a
multivariate logistic analysis to assess whether the simulated distribution
matches the known transiting planets. Our multivariate analysis shows that our
simulated sample and observations are consistent to 76%. The mass vs. period
correlation for giant planets first observed with radial velocity holds with
transiting planets. Our model naturally explains the correlation between planet
surface gravity and period and the one between planet radius and stellar
effective temperature. Finally, we are also able to reproduce the previously
observed apparent bimodal distribution of Safronov numbers in 10% of our
simulated cases, although our model predicts a continuous distribution. This
shows that the evidence for the existence of two groups of planets with
different intrinsic properties is not statistically significant.Comment: 17 page
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
CoRoT 101186644: A transiting low-mass dense M-dwarf on an eccentric 20.7-day period orbit around a late F-star
We present the study of the CoRoT transiting planet candidate 101186644, also
named LRc01_E1_4780. Analysis of the CoRoT lightcurve and the HARPS
spectroscopic follow-up observations of this faint (m_V = 16) candidate
revealed an eclipsing binary composed of a late F-type primary (T_eff = 6090
+/- 200 K) and a low-mass, dense late M-dwarf secondary on an eccentric (e =
0.4) orbit with a period of ~20.7 days. The M-dwarf has a mass of 0.096 +/-
0.011 M_Sun, and a radius of 0.104 +0.026/-0.006 R_Sun, which possibly makes it
the smallest and densest late M-dwarf reported so far. Unlike the claim that
theoretical models predict radii that are 5%-15% smaller than measured for
low-mass stars, this one seems to have a radius that is consistent and might
even be below the radius predicted by theoretical models.Comment: Accepted for publication in Astronomy & Astrophysics, 8 pages, 10
figure
Core properties of alpha Cen A using asteroseismology
A set of long and nearly continuous observations of alpha Centauri A should
allow us to derive an accurate set of asteroseismic constraints to compare to
models, and make inferences on the internal structure of our closest stellar
neighbour. We intend to improve the knowledge of the interior of alpha Centauri
A by determining the nature of its core. We combined the radial velocity time
series obtained in May 2001 with three spectrographs in Chile and Australia:
CORALIE, UVES, and UCLES. The resulting combined time series has a length of
12.45 days and contains over 10,000 data points and allows to greatly reduce
the daily alias peaks in the power spectral window. We detected 44 frequencies
that are in good overall agreement with previous studies, and found that 14 of
these show possible rotational splittings. New values for the large and small
separations have been derived. A comparison with stellar models indicates that
the asteroseismic constraints determined in this study allows us to set an
upper limit to the amount of convective-core overshooting needed to model stars
of mass and metallicity similar to those of alpha Cen A.Comment: 8 pages, 11 figures, A&A accepte
The HARPS search for southern extra-solar planets. VI. A Neptune-mass planet around the nearby M dwarf Gl 581
We report the discovery of a Neptune-mass planet around Gl 581 (M3V, M = 0.31
Msol), based on precise Doppler measurements with the HARPS spectrograph at La
Silla Observatory. The radial velocities reveal a circular orbit of period P =
5.366 days and semi-amplitude K1 = 13.2 m/s. The resulting minimum mass of the
planet (m2 sin i) is only 0.052 Mjup = 0.97 Mnep = 16.6 Mearth making Gl 581b
one of the lightest extra-solar planet known to date. The Gl 581 planetary
system is only the third centered on an M dwarf, joining the Gl 876
three-planet system and the lone planet around Gl 436. Its discovery reinforces
the emerging tendency of such planets to be of low mass, and found at short
orbital periods. The statistical properties of the planets orbiting M dwarfs do
not seem to match a simple mass scaling of their counterparts around solar-type
stars.Comment: letter submitted to A&
The SOPHIE search for northern extrasolar planets. II. A multi-planet system around HD9446
We report the discovery of a planetary system around HD9446, performed from
radial velocity measurements secured with the spectrograph SOPHIE at the 193-cm
telescope of the Haute-Provence Observatory during more than two years. At
least two planets orbit this G5V, active star: HD9446b has a minimum mass of
0.7 M_Jup and a slightly eccentric orbit with a period of 30 days, whereas
HD9446c has a minimum mass of 1.8 M_Jup and a circular orbit with a period of
193 days. As for most of the known multi-planet systems, the HD9446-system
presents a hierarchical disposition, with a massive outer planet and a lighter
inner planet.Comment: 7 pages, 6 figures, 3 tables, accepted for publication in A&
Tidal interactions of close-in extrasolar planets: the OGLE cases
Close-in extrasolar planets experience extreme tidal interactions with their
host stars. This may lead to a reduction of the planetary orbit and a spin-up
of stellar rotation. Tidal interactions have been computed for a number of
extrasolar planets in circular orbits within 0.06 AU, namely for OGLE-TR-56 b.
We compare our range of the tidal dissipation value with two dissipation models
from Sasselov (2003) and conclude that our choices are equivalent to these
models. However, applied to the planet OGLE-TR-56 b, we find in contrast to
Sasselov (2003) that this planet will spiral-in toward the host star in a few
billion years. We show that the average and maximum value of our range of
dissipation are equivalent to the linear and quadratic dissipation models of
Sasselov (2003). Due to limitations in the observational techniques, we do not
see a possibility to distinguish between the two dissipation models as outlined
by Sasselov (2003). OGLE-TR-56 b may therefore not serve as a test case for
dissipation models. The probable existence of OGLE-TR-3 b at 0.02 AU and the
discovery of OGLE-TR-113 b at 0.023 AU and OGLE-TR-132 b at 0.03 AU may also
counter Sasselovs (2003) assumption of a pile-up stopping boundary at 0.04 AU.Comment: 7 pages, 4 figure
The HARPS search for southern extra-solar planets. XXIV. Companions to HD 85390, HD 90156 and HD 103197: A Neptune analogue and two intermediate mass planets
We report the detection of three new extrasolar planets orbiting the solar
type stars HD 85390, HD 90156 and HD 103197 with the HARPS spectrograph mounted
on the ESO 3.6-m telescope at La Silla observatory. HD 85390 has a planetary
companion with a projected intermediate mass (42.0 Earth masses) on a 788-day
orbit (a=1.52 AU) with an eccentricity of 0.41, for which there is no analogue
in the solar system. A drift in the data indicates the presence of another
companion on a long period orbit, which is however not covered by our
measurements. HD 90156 is orbited by a warm Neptune analogue with a minimum
mass of 17.98 Earth masses (1.05 Neptune masses), a period of 49.8 days (a=0.25
AU) and an eccentricity of 0.31. HD 103197 has an intermediate mass planet on a
circular orbit (P=47.8 d, Msini=31.2 Earth masses). We discuss the formation of
planets of intermediate mass (about 30-100 Earth masses) which should be rare
inside a few AU according to core accretion formation models.Comment: 9 pages, 5 figures. Accepted to A&
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