993 research outputs found
Metallicity of M dwarfs: the link to exoplanets
The determination of the stellar parameters of M dwarfs is of prime
importance in the fields of galactic, stellar and planetary astronomy. M stars
are the least studied galactic component regarding their fundamental
parameters. Yet, they are the most numerous stars in the galaxy and contribute
to most of its total (baryonic) mass. In particular, we are interested in their
metallicity in order to study the star-planet connection and to refine the
planetary parameters. As a preliminary result we present a test of the
metallicity calibrations of Bonfils et al. (2005), Johnson & Apps (2009), and
Schlaufman & Laughlin (2010) using a new sample of 17 binaries with precise V
band photometry.Comment: 2 pages, 1 figure, to appear in the Proceedings of the IAU Symposium
276,2011: The Astrophysics of Planetary Systems: Formation, Structure, and
Dynamical Evolution, ed. A. Sozzetti, M. Lattanzi, and A. Boss, Cambridge
University Pres
Violences à l'école. Normes et professionnalités en questions
International audienceLa question des violences Ă lâĂ©cole est plus que jamais mise Ă lâagenda â mĂ©diatique, scientifique et politique. Au-delĂ de lâactualitĂ© du sujet, cet ouvrage se propose dâen interroger les fondements normatifs tant dans son analyse que dans les rĂ©ponses apportĂ©es en soulevant un certain nombre de questions Ă©mergentes et peu Ă©tudiĂ©es.Un double niveau de questionnement est sollicitĂ© : dâune part, celui de lâĂ©tude de la construction du « problĂšme social » des violences Ă lâĂ©cole, de ses fondements et de ses implications ; dâautre part, celui dâune analyse fine et contextualisĂ©e de ce phĂ©nomĂšne, sâattachant au sens que donnent les acteurs Ă ce quâils vivent, analysant le vĂ©cu des sujets en lien avec leur activitĂ© de travail rendue parfois difficile.Cet ouvrage convoque une pluralitĂ© dâapproches et de disciplines, ce qui permet de rendre compte de la complexitĂ© du phĂ©nomĂšne et de ses enjeux. La violence, loin dâĂȘtre gratuite, surgissant du vide ou le remplissant suite Ă un Ă©chec des mots, apparaĂźt alors Ă©troitement liĂ©e Ă des situations didactiques, pĂ©dagogiques et socio-historiques. Câest finalement la question de la construction de professionnalitĂ©s qui apparaĂźt centrale. Cet ouvrage sâadresse aux praticiens, aux chercheurs et aux Ă©tudiants sâintĂ©ressant au monde Ă©ducati
SOAP-T: A tool to study the light-curve and radial velocity of a system with a transiting planet and a rotating spotted star
We present an improved version of SOAP (Boisse et al. 2012) named "SOAP-T",
which can generate the radial velocity variations and light-curves for systems
consisting of a rotating spotted star with a transiting planet. This tool can
be used to study the anomalies inside transit light-curves and the
Rossiter-McLaughlin effect, to better constrain the orbital configuration and
properties of planetary systems and active zones of their host stars. Tests of
the code are presented to illustrate its performance and to validate its
capability when compared with analytical models and real data. Finally, we
apply SOAP-T to the active star, HAT-P-11, observed by the NASA Kepler space
telescope and use this system to discuss the capability of this tool in
analyzing light-curves for the cases where the transiting planet overlaps with
the star's spots.Comment: 9 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
A Physically-Motivated Photometric Calibration of M Dwarf Metallicity
The location of M dwarfs in the V-K_s--M_Ks color-magnitude diagram (CMD) has
been shown to correlate with metallicity. We demonstrate that previous
empirical photometric calibrations of M dwarf metallicity exploiting this
correlation systematically underestimate or overestimate metallicity at the
extremes of their range. We improve upon previous calibrations in three ways.
We use both a volume-limited and kinematically-matched sample of F and G dwarfs
from the Geneva-Copehnagen Survey (GCS) to infer the mean metallicity of M
dwarfs in the Solar Neighborhood, we use theoretical models of M dwarf
interiors and atmospheres to determine the effect of metallicity on M dwarfs in
the V-K_s--M_Ks CMD, and we base our final calibration purely on
high-resolution spectroscopy of FGK primaries with M dwarf companions. As a
result, we explain an order of magnitude more of the variance in the
calibration sample than previous photometric calibrations. We
non-parametrically quantify the significance of the observation that M dwarfs
that host exoplanets are preferentially in a region of the V-K_s--M_Ks plane
populated by metal-rich M dwarfs. We find that the probability p that
planet-hosting M dwarfs are distributed across the V-K_s--M_Ks CMD in the same
way as field M dwarfs is p = 0.06 +/- 0.008. Interestingly, the subsample of M
dwarfs that host Neptune and sub-Neptune mass planets may also be
preferentially located in the region of the V-K_s--M_Ks plane populated by
high-metallicity M dwarfs. The probability of this occurrence by chance is p =
0.40 +/- 0.02, and this observation hints that low-mass planets may be more
likely to be found around metal-rich M dwarfs. An increased rate of low-mass
planet occurrence around metal-rich M dwarfs would be a natural consequence of
the core-accretion model of planet formation. (abridged)Comment: 10 pages, 4 figures, and 1 table in A&A format; accepted for
publication in A&
Metallicity of M dwarfs IV. A high-precision [Fe/H] and Teff technique from high-resolution optical spectra for M dwarfs
Aims. In this work we develop a technique to obtain high precision
determinations of both metallicity and effective temperature of M dwarfs in the
optical.
Methods. A new method is presented that makes use of the information of 4104
lines in the 530-690 nm spectral region. It consists in the measurement of
pseudo equivalent widths and their correlation with established scales of
[Fe/H] and .
Results. Our technique achieves a of 0.080.01 for [Fe/H],
9113 K for , and is valid in the (-0.85, 0.26 dex), (2800, 4100
K), and (M0.0, M5.0) intervals for [Fe/H], and spectral type
respectively. We also calculated the RMSE which estimates uncertainties
of the order of 0.12 dex for the metallicity and of 293 K for the effective
temperature. The technique has an activity limit and should only be used for
stars with . Our method is available
online at \url{http://www.astro.up.pt/resources/mcal}.Comment: Accepted in Astronomy and Astrophysics. Updated one important
reference in the introduction. Some typos correcte
The HARPS search for southern extra-solar planets XIX. Characterization and dynamics of the GJ876 planetary system
Precise radial-velocity measurements for data acquired with the HARPS
spectrograph infer that three planets orbit the M4 dwarf star GJ876. In
particular, we confirm the existence of planet "d", which orbits every 1.93785
days. We find that its orbit may have significant eccentricity (e=0.14), and
deduce a more accurate estimate of its minimum mass of 6.3 Earth masses.
Dynamical modeling of the HARPS measurements combined with literature
velocities from the Keck Observatory strongly constrain the orbital
inclinations of the "b" and "c" planets. We find that i_b = 48.9 degrees and
i_c = 48.1 degrees, which infers the true planet masses of M_b = 2.64 Jupiter
masses and M_c = 0.83 Jupiter masses, respectively. Radial velocities alone, in
this favorable case, can therefore fully determine the orbital architecture of
a multi-planet system, without the input from astrometry or transits.
The orbits of the two giant planets are nearly coplanar, and their 2:1 mean
motion resonance ensures stability over at least 5 Gyr. The libration amplitude
is smaller than 2 degrees, suggesting that it was damped by some dissipative
process during planet formation. The system has space for a stable fourth
planet in a 4:1 mean motion resonance with planet "b", with a period around 15
days. The radial velocity measurements constrain the mass of this possible
additional planet to be at most that of the Earth.Comment: 10 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
Metallicity of M dwarfs III. Planet-metallicity and planet-stellar mass correlations of the HARPS GTO M dwarf sample
Aims. The aim of this work is the study of the planet-metallicity and the
planet-stellar mass correlations for M dwarfs from the HARPS GTO M dwarf
subsample
Methods. We use a new method that takes advantage of the HARPS
high-resolution spectra to increase the precision of metallicity, using
previous photometric calibrations of [Fe/H] and effective temperature as
starting values.
Results. In this work we use our new calibration (rms = 0.08 dex) to study
the planet-metallicity relation of our sample. The well-known correlation for
Giant planet FGKM hosts with metallicity is present. Regarding Neptunians and
smaller hosts no correlation is found but there is a hint that an
anti-correlation with [Fe/H] may exist. We combined our sample with the
California Planet Survey late-K and M-type dwarf sample to increase our
statistics but found no new trends. We fitted a power law to the frequency
histogram of the Jovian hosts for our sample and for the combined sample, f_p =
C10^\alpha[Fe/H], using two different approaches: a direct bin fitting and a
bayesian fitting procedure. We obtained a value for C between 0.02 and 0.04 and
for \alpha between 1.26 and 2.94.
Regarding stellar mass, an hypothetical correlation with planets was
discovered, but was found to be the result of a detection bias.Comment: Accepted for publication in A&A. 18 pages, 11 Figures, 12 Table
The HARPS search for southern extra-solar planets XXXV. Super-Earths around the M-dwarf neighbors Gl433 and Gl667C
M dwarfs have been found to often have super-Earth planets with short orbital
periods. Such stars are thus preferential targets in searches for rocky or
ocean planets in the solar neighbourhood. In a recent paper (Bonfils et al.
2011), we announced the discovery of respectively 1 and 2 low mass planets
around the M1.5V stars Gl433 and Gl667C. We found those planets with the HARPS
spectrograph on the ESO~3.6-m telescope at La Silla Observatory, from
observations obtained during the Guaranteed Time Observing program of that
instrument. We have obtained additional HARPS observations of those two stars,
for a total of respectively 67 and 179 Radial Velocity measurements for Gl433
and Gl667C, and present here an orbital analysis of those extended data sets
and our main conclusion about both planetary systems. One of the three planets,
Gl667Cc, has a mass of only M2.sin(i)~4.25 M_earth and orbits in the central
habitable zone of its host star. It receives just 10% less stellar energy from
Gl667C than the Earth receives from the Sun. However planet evolution in
habitable zone can be very different if the host star is a M dwarf or a
solar-like star, without necessarily questioning the presence of water. The two
other planets, Gl433b and Gl667Cb, both have M2.sin(i) of ~5.5 M_earth and
periods of ~7 days. The Radial Velocity measurements of both stars contain
longer time scale signals, which we fit as longer period Keplerians. For Gl433
that signal probably originates in a Magnetic Cycle, while a longer time span
will be needed to conclude for Gl667C. The metallicity of Gl433 is close to
solar, while Gl667C is metal poor with [Fe/H] ~ -0.6. This reinforces the
recent conclusion that the occurence of Super-Earth planets does not strongly
correlate with stellar metallicity.Comment: 14 pages, 8 figures, submitted to A&
Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program II. Activity and radial velocity
Due to their low mass and luminosity, M dwarfs are ideal targets if one hopes
to find low-mass planets similar to Earth by using the radial velocity (RV)
method. However, stellar magnetic cycles could add noise or even mimic the RV
signal of a long-period companion. Following our previous work that studied the
correlation between activity cycles and long-term RV variations for K dwarfs we
now expand that research to the lower-end of the main sequence. Our objective
is to detect any correlations between long-term activity variations and the
observed RV of a sample of M dwarfs. We used a sample of 27 M-dwarfs with a
median observational timespan of 5.9 years. The cross-correlation function
(CCF) with its parameters RV, bisector inverse slope (BIS), full-width-at-half-
maximum (FWHM) and contrast have been computed from the HARPS spectrum. The
activity index have been derived using the Na I D doublet. These parameters
were compared with the activity level of the stars to search for correlations.
We detected RV variations up to ~5 m/s that we can attribute to activity cycle
effects. However, only 36% of the stars with long-term activity variability
appear to have their RV affected by magnetic cycles, on the typical timescale
of ~6 years. Therefore, we suggest a careful analysis of activity data when
searching for extrasolar planets using long-timespan RV data.Comment: 20 pages, 12 figures, 3 tables, accepted for publication in Astronomy
and Astophysic
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