678 research outputs found
Modeling the RV jitter of early M dwarfs using tomographic imaging
In this paper we show how tomographic imaging (Zeeman Doppler Imaging, ZDI)
can be used to characterize stellar activity and magnetic field topologies,
ultimately allowing to filter out the radial velocity (RV) activity jitter of
M-dwarf moderate rotators. This work is based on spectropolarimetric
observations of a sample of five weakly-active early M-dwarfs (GJ 205, GJ 358,
GJ 410, GJ479, GJ 846) with HARPS-Pol and NARVAL. These stars have v sin i and
RV jitters in the range 1-2 km/s and 2.7-10.0 m/s rms respectively. Using a
modified version of ZDI applied to sets of phase-resolved Least-Squares- Decon-
volved (LSD) profiles of unpolarized spectral lines, we are able to
characterize the distribution of active regions at the stellar surfaces. We
find that darks spots cover less than 2% of the total surface of the stars of
our sample. Our technique is e cient at modeling the rotationally mod- ulated
component of the activity jitter, and succeeds at decreasing the amplitude of
this com- ponent by typical factors of 2-3 and up to 6 in optimal cases. From
the rotationally modulated time-series of circularly polarized spectra and with
ZDI, we also reconstruct the large-scale magnetic field topology. These fields
suggest that bi-stability of dynamo processes observed in active M dwarfs may
also be at work for moderately active M dwarfs. Comparing spot distributions
with field topologies suggest that dark spots causing activity jitter
concentrate at the magnetic pole and/or equator, to be confirmed with future
data on a larger sample.Comment: 34 pages, accepted for publication in MNRA
Acaulon fontiquerianum Casas et SĂ©rgio (Musci, Pottiaceae) nouveau pour la bryoflore du Portugal, de France et de Corse
Découverte au Portugal (Cabo Espichel), dans le sud-est de la France (Bouches-du RhBne: Crau, Var: environs de Pierrefeu) et en Corse méridionale (Bocca di Curali), d'Acaulon fontiquerianurn Casas et Sérgio récemment décrit d'Espagne. Aprbs un rappel des caracteres morphologiques et anatomiques tres particuliers de cet Acaulon, les spores de spécimens récoltés au Portugal et dans la Crau son illustrées (microscopie électronique 21 balayage). L'écologie d'Acaulon fontiquerianum dans les 5 nouvelles stations est précisée (substrat, végétation, bryophytes associées).Discovery in Portugal (Cabo Espichel), south-eastern France (Bouches-du-RhBne: Crau, Var: vicinity of Pierrefeu) and Corsica (Bocca di Curali) of Acaulon fontiquerianurn Casas et Sérgio, recently described from Spain. After a recapitulation of the very particular morphological and anatomical features of Acaulon fontiquerianum, the spores of specimens collected in Portugal and Crau are illustrated using S.E.M. The ecology of the moss in the 5 new stations is stated (substrate, vegetation, associated bryophytes)
Improved parameters of seven Kepler giant companions characterized with SOPHIE and HARPS-N
Radial-velocity observations of Kepler candidates obtained with the SOPHIE
and HARPS-N spectrographs have permitted unveiling the nature of the five giant
planets Kepler-41b, Kepler-43b, Kepler-44b, Kepler-74b, and Kepler-75b, the
massive companion Kepler-39b, and the brown dwarf KOI-205b. These companions
were previously characterized with long-cadence (LC) Kepler data. Here we aim
at refining the parameters of these transiting systems by i) modelling the
published radial velocities (RV) and Kepler short-cadence (SC) data that
provide a much better sampling of the transits, ii) performing new spectral
analyses of the SOPHIE and ESPaDOnS spectra, and iii) improving stellar
rotation periods hence stellar age estimates through gyrochronology, when
possible. Posterior distributions of the system parameters were derived with a
differential evolution Markov chain Monte Carlo approach. Our main results are
as follows: a) Kepler-41b is significantly larger and less dense than
previously found because a lower orbital inclination is favoured by SC data.
This also affects the determination of the geometric albedo that is lower than
previously derived: Ag < 0.135; b) Kepler-44b is moderately smaller and denser
than reported in the discovery paper; c) good agreement was achieved with
published Kepler-43, Kepler-75, and KOI-205 system parameters, although the
host stars Kepler-75 and KOI-205 were found to be slightly richer in metals and
hotter, respectively; d) the previously reported non-zero eccentricities of
Kepler-39b and Kepler-74b might be spurious. If their orbits were circular, the
two companions would be smaller and denser than in the eccentric case. The
radius of Kepler-39b is still larger than predicted by theoretical isochrones.
Its parent star is hotter and richer in metals than previously determined.
[ABRIDGED]Comment: 17 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
PASTIS: Bayesian extrasolar planet validation II. Constraining exoplanet blend scenarios using spectroscopic diagnoses
The statistical validation of transiting exoplanets proved to be an efficient
technique to secure the nature of small exoplanet signals which cannot be
established by purely spectroscopic means. However, the spectroscopic diagnoses
are providing us with useful constraints on the presence of blended stellar
contaminants. In this paper, we present how a contaminating star affects the
measurements of the various spectroscopic diagnoses as function of the
parameters of the target and contaminating stars using the model implemented
into the PASTIS planet-validation software. We find particular cases for which
a blend might produce a large radial velocity signal but no bisector variation.
It might also produce a bisector variation anti-correlated with the radial
velocity one, as in the case of stellar spots. In those cases, the full width
half maximum variation provides complementary constraints. These results can be
used to constrain blend scenarios for transiting planet candidates or radial
velocity planets. We review all the spectroscopic diagnoses reported in the
literature so far, especially the ones to monitor the line asymmetry. We
estimate their uncertainty and compare their sensitivity to blends. Based on
that, we recommend the use of BiGauss which is the most sensitive diagnosis to
monitor line-profile asymmetry. In this paper, we also investigate the
sensitivity of the radial velocities to constrain blend scenarios and develop a
formalism to estimate the level of dilution of a blended signal. Finally, we
apply our blend model to re-analyse the spectroscopic diagnoses of HD16702, an
unresolved face-on binary which exhibits bisector variations.Comment: Accepted for publication in MNRA
Detection of oxygen and carbon in the hydrodynamically escaping atmosphere of the extrasolar planet HD209458b
Four transits of the planet orbiting the star HD209458 were observed with the
STIS spectrograph on board HST. The wavelength domain (1180-1710A) includes HI
as well as CI, CII, CIV, NV, OI, SI, SiII, SiIII and SiIV lines. During the
transits, absorptions are detected in HI, OI and CII (5+/-2%, 13+/-4.5% and
7.5+/-3.5%, respectively). No absorptions are detected for other lines. The 5%
mean absorption over the whole HI Lyman alpha line is consistent with the
previous detection at higher resolution (Vidal-Madjar et al. 2003). The
absorption depths in OI and CII show that oxygen and carbon are present in the
extended upper atmosphere of HD209458b. These species must be carried out up to
the Roche lobe and beyond, most likely in a state of hydrodynamic escape.Comment: 6 pages, 4 figures, 1 table, submitted to ApJ Letters, revised
version with slightly revisited absorption depth estimate
SOPHIE velocimetry of Kepler transit candidates VI. An additional companion in the KOI-13 system
We report the discovery of a new stellar companion in the KOI-13 system.
KOI-13 is composed by two fast-rotating A-type stars of similar magnitude. One
of these two stars hosts a transiting planet discovered by Kepler. We obtained
new radial velocity measurements using the SOPHIE spectrograph at the
Observatoire de Haute-Provence that revealed an additional companion in this
system. This companion has a mass between 0.4 and 1 Msun and orbits one of the
two main stars with a period of 65.831 \pm 0.029 days and an eccentricity of
0.52 \pm 0.02. The radial velocities of the two stars were derived using a
model of two fast-rotating line profiles. From the residuals, we found a hint
of the stellar variations seen in the Kepler light curve with an amplitude of
about 1.41 km/s and a period close to the rotational period. This signal
appears to be about three order of magnitude larger than expected for stellar
activity. From the analysis of the residuals, we also put a 3-sigma upper-limit
on the mass of the transiting planet KOI-13.01 of 14.8 Mjup and 9.4 Mjup,
depending on which star hosts the transit. We found that this new companion has
no significant impact on the photometric determination of the mass of KOI-13.01
but is expected to affect precise infrared photometry. Finally, using dynamical
simulations, we infer that the new companion is orbiting around KOI-13B while
the transiting planet candidate is expected to orbit KOI-13A. Thus, the
transiting planet candidate KOI-13.01 is orbiting the main component of a
hierarchical triple system.Comment: Accepted in A&A Letters. 4 pages including 4 figures and the RV tabl
FUSE observations of the HI interstellar gas of IZw18
We present the analysis of FUSE observations of the metal-deficient dwarf
galaxy IZw18. We measured column densities of HI, NI, OI, ArI, SiII, and FeII.
The OI/HI ratio (log(OI/HI)=-4.7^{+0.8}_{-0.6}) is consistent with the O/H
ratio observed in the HII regions (all uncertainties are 2-sigma). If the
oxygen is depleted in the HI region compared to the HII regions, the depletion
is at most 0.5dex. This is also consistent with the log(O/H) ratios ~-5
measured with FUSE in the HI regions of other blue compact dwarf galaxies. With
log(NI/OI)=-2.4^{+0.6}_{-0.8}, the measured NI/OI ratio is lower than expected
for primary nitrogen. The determination of the NII column density is needed to
discriminate between a large ionization of NI or a possible nitrogen
deficiency. The neutral argon is also apparently underabundant, indicating that
ionization into ArII is likely important. The column densities of the other
alpha-chain elements SiII and ArI favor the lower edge of the permitted range
of OI column density, log(N(OI))~16.3.Comment: Accepted for publication in A&
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