326 research outputs found
Using the Sun to estimate Earth-like planets detection capabilities. V. Parameterizing the impact of solar activity components on radial velocities
Stellar activity induced by active structures (eg, spots, faculae) is known
to strongly impact the radial velocity time series. It then limits the
detection of small planetary RV signals (eg, an Earth-mass planet in the
habitable zone of a solar-like star). In previous papers, we studied the
detectability of such planets around the Sun seen as an edge-on star. For that
purpose, we computed the RV and photometric variations induced by solar
magnetic activity, using all active structures observed over one entire cycle.
Our goal is to perform similar studies on stars with different physical and
geometrical properties. As a first step, we focus on Sun-like stars seen with
various inclinations, and on estimating detection capabilities with forthcoming
instruments. To do so, we first parameterize the solar active structures with
the most realistic pattern so as to obtain results consistent with the observed
ones. We simulate the growth, evolution and decay of solar spots, faculae and
network, using parameters and empiric laws derived from solar observations and
literature. We generate the corresponding structure lists over a full solar
cycle. We then build the resulting spectra and deduce the RV and photometric
variations for a `Sun' seen with various inclinations. The produced RV signal
takes into account the photometric contribution of structures as well as the
attenuation of the convective blueshift. The comparison between our simulated
activity pattern and the observed one validates our model. We show that the
inclination of the stellar rotation axis has a significant impact on the time
series. RV long-term amplitudes as well as short-term jitters are significantly
reduced when going from edge-on to pole-on configurations. Assuming spin-orbit
alignment, the optimal configuration for planet detection is an inclined star
(i~45{\deg}).Comment: Accepted to Astronomy and Astrophysics on May, 27th 2015. The
manuscript includes 22 pages, 20 figure
Variability in stellar granulation and convective blueshift with spectral type and magnetic activity. II. From young to old main-sequence K-G-F stars
The inhibition of small-scale convection in the Sun dominates the long-term
radial velocity (RV) variability: it therefore has a critical effect on light
exoplanet detectability using RV techniques. We here extend our previous
analysis of stellar convective blueshift and its dependence on magnetic
activity to a larger sample of stars in order to extend the Teff range, to
study the impact of other stellar properties, and finally to improve the
comparison between observed RV jitter and expected RV variations. We estimate a
differential velocity shift for Fe and Ti lines of different depths and derive
an absolute convective blueshift using the Sun as a reference for a sample of
360 F7-K4 stars with different properties (age, Teff, metallicity). We confirm
the strong variation in convective blueshift with Teff and its dependence on
(as shown in the line list in Paper I) activity level. Although we do not
observe a significant effect of age or cyclic activity, stars with a higher
metallicity tend to have a lower convective blueshift, with a larger effect
than expected from numerical simulations. Finally, we estimate that for 71% of
the stars in our sample the RV and LogR'HK variations are compatible with the
effect of activity on convection, as observed in the solar case, while for the
other stars, other sources (such as binarity or companions) must be invoked to
explain the large RV variations. We also confirm a relationship between LogR'HK
and metallicity, which may affect discussions of the possible relationship
between metallicity and exoplanets, as RV surveys are biased toward low LogR'HK
and possibly toward high-metallicity stars. We conclude that activity and
metallicity strongly affect the small-scale convection levels in stars in the
F7-K4 range, with a lower amplitude for the lower mass stars and a larger
amplitude for low-metallicity stars.Comment: 12 pages, 9 figures ; accepted in Astronomy and Astrophysic
Extrasolar planets and brown dwarfs around AF-type stars. IX. The HARPS southern sample
Massive, Main-Sequence AF-type stars have so far remained unexplored in past
radial velocity surveys, due to their small number of spectral lines and their
high rotational velocities that prevent the classic RV computation method. Our
aim was to search for giant planets around AF MS stars, to get first
statistical information on their occurrence rate and to compare the results
with evolved stars and lower-mass MS stars. We used the HARPS spectrograph
located on the 3.6m telescope at ESO La Silla Observatory to observe 108 AF MS
stars with B-V in the -0.04 to 0.58 range and masses in the range 1.1-3.6 Msun.
We used our SAFIR software specifically developed to compute the radial
velocities of these early-type stars. We report the new detection of a mpsini =
4.51 Mjup companion with a ~826-day period to the F6V dwarf HD111998. We
present new data on the 2-planet system around the F6IV-V dwarf HD60532. We
also report the detection of 14 binaries with long-term RV trends. 70% of our
targets show detection limits between 0.1 and 10 Mjup in the 1 to 10^3-day
range. We derive brown dwarf (13 < mpsini < 80 Mjup) occurrence rates in the 1
to 10^3-day range of % and % for stars with
masses in the ranges 1.1-1.5 and 1.5-3 Msun, respectively. As for Jupiter-mass
companions (1 < mpsini < 13 Mjup), we get occurrence rates in the 1 to 10^3-day
range of % and % respectively for the same
stellar mass ranges. When considering the same Jupiter-mass companions but
periods in the 1 to 100-day range only, we get occurrence rates of
% and %. Given the present error bars, these
results do not show a significant difference with companion frequencies derived
for solar-like stars.Comment: 23 pages (text), 15 figures, accepted in Astronomy and Astrophysic
Independent confirmation of {\beta} Pictoris b imaging with NICI
Context. {\beta} Pictoris b is one of the most studied objects nowadays since
it was identified with VLT/NaCo as a bona-fide exoplanet with a mass of about 9
times that of Jupiter at an orbital separation of 8-9 AU. The link between the
planet and the dusty disk is unambiguously attested and this system provides an
opportunity to study the disk/planet interactions and to constrain formation
and evolutionary models of gas giant planets. Still, {\beta} Pictoris b had
never been confirmed with other telescopes so far. Aims. We aimed at an
independent confirmation using a different instrument. Methods. We retrieved
archive images from Gemini South obtained with the instrument NICI, which is
designed for high contrast imaging. The observations combine coronagraphy and
angular differential imaging and were obtained at three epochs in Nov. 2008,
Dec. 2009 and Dec. 2010. Results. We report the detection with NICI of the
planet {\beta} Pictoris b in Dec. 2010 images at a separation of 404 \pm 10 mas
and P A = 212.1 \pm 0.7{\deg} . It is the first time this planet is observed
with a telescope different than the VLT.Comment: Letter accepted for publication in Astronomy and Astrophysics on Feb.
21, 2013. 4 pages, 2 figure
Exocomets in the circumstellar gas disk of HD 172555
The source HD172555 is a young A7V star surrounded by a debris disk with a
gaseous component. Here, we present the detection of variable absorption
features detected simultaneously in the Ca II K and H doublet lines (at 3,933
and 3,968 Angstrom). We identified the presence of these absorption signatures
at four different epochs in the 129 HARPS high-resolution spectra gathered
between 2004 and 2011. These transient absorption features are most likely due
to Falling Evaporating Bodies (FEBs, or exocomets) that produce absorbing gas
observed transiting in front of the central star. We also detect a stable Ca II
absorption component at the star's radial velocity. With no corresponding
detection in the Na I line, the resulting very low upper limit for the NaI/CaII
ratio suggests that this absorption is due to circumstellar gas.Comment: Accepted for publication in Astronomy&Astrophysics Letter
Activity time series of old stars from late F to early K VI. Exoplanet mass characterisation and detectability in radial velocity
Stellar variability impacts radial velocities at various timescales and
therefore the detectability of exoplanets and the mass determination based on
this technique. It is necessary to implement systematic studies, to delineate
the current limitations of RV techniques to detect Earth-like planets. This
paper aims are to investigate whether the targeted 10% mass uncertainty from RV
follow-up of transits detected by PLATO can be reached, and to analyse and
quantify Earth-like planet detectability for various spectral types. We
implemented blind tests based on a large data set of realistic synthetic time
series reproducing different phenomena leading to stellar variability such as
complex magnetic activity patterns as well as flows, covering F6-K4 stars and a
wide range of activity levels. The 10% mass uncertainty for a 1 MEarth in the
habitable zone of a G2 star cannot be reached, even with an improved version of
a usual correction of stellar activity and even for long-duration (ten years)
well-sampled observations. This level can be reached for masses above 3 MEarth
or for K4 stars alone. We quantify the maximum dispersion of the RV residuals
needed to reach this 10% level, assuming the correction method and models do
not affect the planetary signal. Several other methods were tested and do not
allow a significantly improvement of this limited performance. Similarly, such
low-mass planets in the habitable zone cannot be detected with a similar
correction: blind tests lead to very low detection rates for 1 MEarth and a
very high level of false positives. Very significant and new improvements with
respect to methods based on activity indicators to correct for stellar activity
must be devised at all timescales to reach the objective of 10% uncertainty on
the mass or to detect such planets in RV. Methods based on the correlation with
activity indicators are unlikely to be sufficient.Comment: Paper accepted in Astronomy and Astrophysic
The Arches Cluster - Evidence for a Truncated Mass Function?
We have analyzed high-resolution, adaptive optics (AO) HK observations of the
Arches cluster obtained with NAOS/CONICA. With a spatial resolution of 84 mas,
the cluster center is uniquely resolved. From these data the present-day mass
function (MF) of Arches is derived down to about 4 Msun. The integrated MF as
well as the core and 2nd annulus MFs are consistent with a turn-over at 6-7
Msun. This turn-over indicates severe depletion of intermediate and low-mass
stars in the Arches cluster, possibly caused by its evolution in the Galactic
Center environment. The Arches MF represents the first resolved observation of
a starburst cluster exhibiting a low-mass truncated MF. This finding has severe
implications for stellar population synthesis modelling of extragalactic
starbursts, the derivation of integrated properties such as the total mass of
star clusters in dense environments, the survival of low-mass remnants from
starburst populations, and chemical enrichment during starburst phases.Comment: aastex preprint, 14 pages, 4 figures, accepted by ApJ Letter
Near-Infrared Detection and Characterization of the Exoplanet HD 95086 b with the Gemini Planet Imager
HD 95086 is an intermediate-mass debris-disk-bearing star. VLT/NaCo observations revealed it hosts a companion (HD
95086 b) at AU. Follow-up observations at 1.66 and 2.18
yielded a null detection, suggesting extremely red colors for the planet and
the need for deeper direct-imaging data. In this Letter, we report H- () and - () band detections of HD 95086 b from
Gemini Planet Imager (GPI) commissioning observations taken by the GPI team.
The planet position in both spectral channels is consistent with the NaCo
measurements and we confirm it to be comoving. Our photometry yields colors of
H-L'= mag and K-L'= mag, consistent with
previously reported 5- upper limits in H and Ks. The photometry of HD
95086 b best matches that of 2M 1207 b and HR 8799 cde. Comparing its spectral
energy distribution with the BT-SETTL and LESIA planet atmospheric models
yields T600-1500 K and log g2.1-4.5. Hot-start
evolutionary models yield M= M. Warm-start models reproduce the
combined absolute fluxes of the object for M=4-14 M for a wide range of
plausible initial conditions (S=8-13 k/baryon). The
color-magnitude diagram location of HD 95086 b and its estimated
T and log g suggest that the planet is a peculiar L-T
transition object with an enhanced amount of photospheric dust.Comment: 4 pages, 4 figures, 3 tables, accepted on April, 15th, 201
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