113 research outputs found
Using the Sun to estimate Earth-like planets detection capabilities.I. Impact of cold spots
Stellar spots may in some cases produce radial velocity (RV) signatures
similar to those of exoplanets. To further investigate the impact of spots, we
aim at studying the detectability of Earth mass planets in the habitable zone
(HZ) of solar type stars, if covered by spots similar to the sunspots. We have
used the Sunspots properties recorded over one solar cycle between 1993 and
2003 to build the RV curve that a solar type star seen edge-on would show, if
covered by such spots with Tsun -Tspot = 550K. We also simulate the RV of such
a spotted star surrounded by an Earth mass planet located in the HZ. Under
present assumptions, the detection of a 1 M Earth planet located between 0.8
and 1.2 AU requires an intensive monitoring (weekly or better), during several
years of low activity phasis. The temporal sampling is more crucial than the
precision of the data (assuming precisions in the range [1-10] cm/s). Cooler
spots may become a problem for such detections. Also, we anticipate that
plages, not considered in this paper, could further complicate or even
compromise the detections
Extrasolar planets and brown dwarfs around A-F type stars V. A planetary system found with HARPS around the F6IV-V star HD 60532
Aims: In the frame of the search for extrasolar planets and brown dwarfs
around early-type stars, we present the results obtained for the F-type
main-sequence star HD 60532 (F6V) with HARPS.
Methods: Using 147 spectra obtained with HARPS at La Silla on a time baseline
of two years, we study the radial velocities of this star.
Results: HD 60532 radial velocities are periodically variable, and the
variations have a Keplerian origin. This star is surrounded by a planetary
system of two planets with minimum masses of 1 and 2.5 Mjup and orbital
separations of 0.76 and 1.58 AU respectively. We also detect high-frequency,
low-amplitude (10 m/s peak-to-peak) pulsations. Dynamical studies of the system
point toward a possible 3:1 mean-motion resonance which should be confirmed
within the next decade.Comment: 7 pages, 11 figures, accepted for publication in A&
HD60532, a planetary system in a 3:1 mean motion resonance
In a recent paper it was reported a planetary system around the star HD60532,
composed by two giant planets in a possible 3:1 mean motion resonance, that
should be confirmed within the next decade. Here we show that the analysis of
the global dynamics of the system allows to confirm this resonance. The present
best fit to data already corresponds to this resonant configuration and the
system is stable for at least 5Gry. The 3:1 resonance is so robust that
stability is still possible for a wide variety of orbital parameters around the
best fit solution and also if the inclination of the system orbital plane with
respect to the plane of the sky is as small as 15 deg. Moreover, if the
inclination is taken as a free parameter in the adjustment to the observations,
we find an inclination ~ 20 deg, which corresponds to M_b =3.1 M_Jup and M_c =
7.4 M_Jup for the planetary companions.Comment: 4 Pages, 4 Figures, accepted by A&
Extrasolar planets and brown dwarfs around A-F type stars VI. High precision RV survey of early type dwarfs with HARPS
(Abridged) Aims: Systematic surveys to search for exoplanets have been mostly
dedicated to solar-type stars sofar. We developed in 2004 a method to extend
such searches to earlier A-F type dwarfs and started spectroscopic surveys to
search for planets and quantify the detection limit achievable when taking into
account the stars properties and their actual levels of intrinsic variations.
We give here the first results of our southern survey with HARPS.
Results: 1) 64% of the 170 stars with enough data points are found to be
variable. 20 are found to be binaries or candidate binaries (with stars or
brown dwarfs). More than 80% or the latest type stars (once binaries are
removed) are intrinsically variable at a 2 m/s precision level. Stars with
earlier spectral type (B-V <= 0.2) are either variable or associated to levels
of uncertainties comparable to the RV rms observed on variable stars of same
B-V. 2) We have detected one long-period planetary system around an F6IV-V
star. 3) We have quantified the jitter due to stellar activity and we show that
taking into account this jitter in addition to the stellar parameters, it is
still possible to detect planets with HARPS with periods of 3 days (resp. 10
days and 100 days) on 91% (resp. 83%, 61%) of them. We show that even the
earliest spectral type stars are accessible to this type of search, provided
they have a low vsini and low levels of activity. 4) Taking into account the
present data, we compute the actually achieved detection limits for 107 targets
and discuss the limits as a function of B-V. Given the data at hand, our survey
is sensitive to short-period (few days) planets and to longer ones (100 days)
at a lower extent (latest type stars). We derive first constrains on the
presence of planets around A-F stars for these ranges of periods.Comment: 18 pages, 12 figures, 5 tables, A&A accepte
Comparison of different exoplanet mass detection limit methods using a sample of main-sequence intermediate-type stars
The radial velocity (RV) technique is a powerful tool for detecting
extrasolar planets and deriving mass detection limits that are useful for
constraining planet pulsations and formation models. Detection limit methods
must take into account the temporal distribution of power of various origins in
the stellar signal. These methods must also be able to be applied to large
samples of stellar RV time series We describe new methods for providing
detection limits. We compute the detection limits for a sample of ten main
sequence stars, which are of G-F-A type, in general active, and/or with
detected planets, and various properties. We use them to compare the
performances of these methods with those of two other methods used in the
litterature. We obtained detection limits in the 2-1000 day period range for
ten stars. Two of the proposed methods, based on the correlation between
periodograms and the power in the periodogram of the RV time series in specific
period ranges, are robust and represent a significant improvement compared to a
method based on the root mean square of the RV signal. We conclude that two of
the new methods (correlation-based method and local power analysis, i.e. LPA,
method) provide robust detection limits, which are better than those provided
by methods that do not take into account the temporal sampling.Comment: 18 pages, 15 figures Accepted in Astronomy & Astrophysic
Reconstructing the solar integrated radial velocity using MDI/SOHO
Searches for exoplanets with radial velocity techniques are increasingly
sensitive to stellar activity. It is therefore crucial to characterize how this
activity influences radial velocity measurements in their study of the
detectability of planets in these conditions. In a previous work we simulated
the impact of spots and plages on the radial velocity of the Sun. Our objective
is to compare this simulation with the observed radial velocity of the Sun for
the same period. We use Dopplergrams and magnetograms obtained by MDI/SOHO over
one solar cycle to reconstruct the solar integrated radial velocity in the Ni
line 6768 \AA. We also characterize the relation between the velocity and the
local magnetic field to interpret our results. We obtain a stronger redshift in
places where the local magnetic field is larger (and as a consequence for
larger magnetic structures): hence we find a higher attenuation of the
convective blueshift in plages than in the network. Our results are compatible
with an attenuation of this blueshift by about 50% when averaged over plages
and network. We obtain an integrated radial velocity with an amplitude over the
solar cycle of about 8 m/s, with small-scale variations similar to the results
of the simulation, once they are scaled to the Ni line. The observed solar
integrated radial velocity agrees with the result of the simulation made in our
previous work within 30%, which validates this simulation. The observed
amplitude confirms that the impact of the convective blueshift attenuation in
magnetic regions will be critical to detect Earth-mass planets in the habitable
zone around solar-like stars.Comment: 17 pages, 11 figures, accepted in Astronomy and Astrophysic
Extrasolar planets and brown dwarfs around A-F type stars - VII. Theta Cygni radial velocity variations: planets or stellar phenomenon?
(abridged) In the frame of the search for extrasolar planets and brown dwarfs
around early-type main-sequence stars, we present the results obtained on the
early F-type star Theta Cygni. Elodie and Sophie at OHP were used to obtain the
spectra. Our dedicated radial-velocity measurement method was used to monitor
the star's radial velocities over five years. We also use complementary, high
angular resolution and high-contrast images taken with PUEO at CFHT. We show
that Theta Cygni radial velocities are quasi-periodically variable, with a
~150-day period. These variations are not due to the ~0.35-Msun stellar
companion that we detected in imaging at more than 46 AU from the star. The
absence of correlation between the bisector velocity span variations and the
radial velocity variations for this 7 km/s vsini star, as well as other
criteria indicate that the observed radial velocity variations are not due to
stellar spots. The observed amplitude of the bisector velocity span variations
also seems to rule out stellar pulsations. However, we observe a peak in the
bisector velocity span periodogram at the same period as the one found in the
radial velocity periodogram, which indicates a probable link between these
radial velocity variations and the low amplitude lineshape variations which are
of stellar origin. Long-period variations are not expected from this type of
star to our knowledge. If a stellar origin (hence of new type) was to be
confirmed for these long-period radial velocity variations, this would have
several consequences on the search for planets around main-sequence stars, both
in terms of observational strategy and data analysis. An alternative
explanation for these variable radial velocities is the presence of at least
one planet of a few Jupiter masses orbiting at less than 1 AU. (abridged)Comment: 9 pages, accepted in A
Using the Sun to estimate Earth-like planets detection capabilities. III. Impact of spots and plages on astrometric detection
Stellar activity is a potential important limitation to the detection of low
mass extrasolar planets with indirect methods (RV, photometry, astrometry). In
previous papers, using the Sun as a proxy, we investigated the impact of
stellar activity (spots, plages, convection) on the detectability of an
Earth-mass planet in the habitable zone (HZ) of solar-type stars with RV
techniques. We extend here the detectability study to the case of astrometry.
We used the sunspot and plages properties recorded over one solar cycle to
infer the astrometric variations that a Sun-like star seen edge-on, 10 pc away,
would exhibit, if covered by such spots/bright structures. We compare the
signal to the one expected from the astrometric wobble (0.3 {\mu}as) of such a
star surrounded by a one Earth-mass planet in the HZ. We also briefly
investigate higher levels of activity. The activity-induced astrometric signal
along the equatorial plane has an amplitude of typ. less than 0.2 {\mu}as
(rms=0.07 {\mu}as), smaller than the one expected from an Earth-mass planet at
1 AU. Hence, for this level of activity, the detectability is governed by the
instrumental precision rather than the activity. We show that for instance a
one Earth-mass planet at 1 AU would be detected with a monthly visit during
less than 5 years and an instrumental precision of 0.8 {\mu}as. A level of
activity 5 times higher would still allow such a detection with a precision of
0.35 {\mu}as. We conclude that astrometry is an attractive approach to search
for such planets around solar type stars with most levels of stellar activity.Comment: Letter accepted by A&
Search for exoplanets with the radial-velocity technique: quantitative diagnostics of stellar activity
Aims: Stellar activity may complicate the analysis of high-precision
radial-velocity spectroscopic data when looking for exoplanets signatures. We
aim at quantifying the impact of stellar spots on stars with various spectral
types and rotational velocities and comparing the simulations with data
obtained with the HARPS spectrograph. Methods: We have developed detailed
simulations of stellar spots and estimated their effects on a number of
observables commonly used in the analysis of radial-velocity data when looking
for extrasolar planets, such as radial-velocity curves, cross-correlation
functions, bisector spans and photometric curves. The computed stellar spectra
are then analyzed in the same way as when searching for exoplanets. Results: 1)
A first grid of simulation results is built for F-K type stars, with different
stellar and spot properties. 2) It is shown quantitatively that star spots with
typical sizes of 1% can mimic both radial-velocity curves and the bisector
behavior of short-period giant planets around G-K type stars with a vsini lower
than the spectrograph resolution. For stars with intermediate vsini, smaller
spots may produce similar features. In these cases, additional observables
(e.g., photometry, spectroscopic diagnostics) are mandatory to confirm the
presence of short-period planets. We show that, in some cases, photometric
variations may not be enough to clearly rule out spots as explanations of the
observed radial-velocity variations. This is particularly important when
searching for super-Earth planets. 3) It is also stressed that quantitative
values obtained for radial-velocity and bisector span amplitudes depend
strongly on the detailed star properties, on the spectrograph used, on the set
of lines used, and on the way they are measured.Comment: 12 pages, 16 figures, accepted for publication in A&
The HARPS search for southern extra-solar planets. XX. Planets around the active star BD-08:2823
We report the detection of a planetary system around BD-08:2823, that
includes at least one Uranus-mass planet and one Saturn-mass planet. This
discovery serendipitously originates from a search for planetary transits in
the Hipparcos photometry database. This program preferentially selected active
stars and did not allow the detection of new transiting planets. It allowed
however the identification of the K3V star BD-08:2823 as a target harboring a
multiplanet system, that we secured and characterized thanks to an intensive
monitoring with the HARPS spectrograph at the 3.6-m ESO telescope in La Silla.
The stellar activity level of BD-08:2823 complicates the analysis but does not
prohibit the detection of two planets around this star. BD-08:2823b has a
minimum mass of 14.4+/-2.1 M_Earth and an orbital period of 5.60 days, whereas
BD-08:2823c has a minimum mass of 0.33+/-0.03 M_Jup and an orbital period of
237.6 days. This new system strengthens the fact that low-mass planets are
preferentially found in multiplanetary systems, but not around high-metallicity
stars as this is the case for massive planets. It also supports the belief that
active stars should not be neglected in exoplanet searches, even when searching
for low-mass planets.Comment: 10 pages, 8 figures, 3 tables, accepted for publication in A&
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