643 research outputs found
Pushing the precision limit of ground-based eclipse photometry
Until recently, it was considered by many that ground-based photometry could
not reach the high cadence sub-mmag regime because of the presence of the
atmosphere. Indeed, high frequency atmospheric noises (mainly scintillation)
limit the precision that high SNR photometry can reach within small time bins.
If one is ready to damage the sampling of his photometric time-series, binning
the data (or using longer exposures) allows to get better errors, but the
obtained precision will be finally limited by low frequency noises. To observe
several times the same planetary eclipse and to fold the photometry with the
orbital period is thus generally considered as the only option to get very well
sampled and precise eclipse light curve from the ground. Nevertheless, we show
here that reaching the sub-mmag sub-min regime for one eclipse is possible with
a ground-based instrument. This has important implications for transiting
planets characterization, secondary eclipses measurement and small planets
detection from the ground.Comment: Transiting Planets Proceeding IAU Symposium No.253, 2008. 7 pages, 4
figure
Refraction in exoplanet atmospheres: Photometric signatures, implications for transmission spectroscopy, and search in Kepler data
Refraction deflects photons that pass through atmospheres, which affects
transit light curves. Refraction thus provides an avenue to probe physical
properties of exoplanet atmospheres and to constrain the presence of clouds and
hazes. In addition, an effective surface can be imposed by refraction, thereby
limiting the pressure levels probed by transmission spectroscopy. The main
objective of the paper is to model the effects of refraction on photometric
light curves for realistic planets and to explore the dependencies on
atmospheric physical parameters. We also explore under which circumstances
transmission spectra are significantly affected by refraction. Finally, we
search for refraction signatures in photometric residuals in Kepler data. We
use the model of Hui & Seager (2002) to compute deflection angles and
refraction transit light curves, allowing us to explore the parameter space of
atmospheric properties. The observational search is performed by stacking large
samples of transit light curves from Kepler. We find that out-of-transit
refraction shoulders are the most easily observable features, which can reach
peak amplitudes of ~10 parts per million (ppm) for planets around Sun-like
stars. More typical amplitudes are a few ppm or less for Jovians and at the
sub-ppm level for super-Earths. Interestingly, the signal-to-noise ratio of any
refraction residuals for planets orbiting Sun-like hosts are expected to be
similar for planets orbiting red dwarfs. We also find that the maximum depth
probed by transmission spectroscopy is not limited by refraction for weakly
lensing planets, but that the incidence of refraction can vary significantly
for strongly lensing planets. We find no signs of refraction features in the
stacked Kepler light curves, which is in agreement with our model predictions.Comment: Accepted for publication in A&
The Spitzer search for the transits of HARPS low-mass planets - II. Null results for 19 planets
Short-period super-Earths and Neptunes are now known to be very frequent
around solar-type stars. Improving our understanding of these mysterious
planets requires the detection of a significant sample of objects suitable for
detailed characterization. Searching for the transits of the low-mass planets
detected by Doppler surveys is a straightforward way to achieve this goal.
Indeed, Doppler surveys target the most nearby main-sequence stars, they
regularly detect close-in low-mass planets with significant transit
probability, and their radial velocity data constrain strongly the ephemeris of
possible transits. In this context, we initiated in 2010 an ambitious Spitzer
multi-Cycle transit search project that targeted 25 low-mass planets detected
by radial velocity, focusing mainly on the shortest-period planets detected by
the HARPS spectrograph. We report here null results for 19 targets of the
project. For 16 planets out of 19, a transiting configuration is strongly
disfavored or firmly rejected by our data for most planetary compositions. We
derive a posterior probability of 83% that none of the probed 19 planets
transits (for a prior probability of 22%), which still leaves a significant
probability of 17% that at least one of them does transit. Globally, our
Spitzer project revealed or confirmed transits for three of its 25 targeted
planets, and discarded or disfavored the transiting nature of 20 of them. Our
light curves demonstrate for Warm Spitzer excellent photometric precisions: for
14 targets out of 19, we were able to reach standard deviations that were
better than 50ppm per 30 min intervals. Combined with its Earth-trailing orbit,
which makes it capable of pointing any star in the sky and to monitor it
continuously for days, this work confirms Spitzer as an optimal instrument to
detect sub-mmag-deep transits on the bright nearby stars targeted by Doppler
surveys.Comment: Accepted for publication in Astronomy and Astrophysics. 23 pages, 21
figure
GJ 436c? The contribution of transit timings
From recent high-accuracy transit timings measurements, we discard the 5 M planet recently proposed by Ribas et al. (2008). Thanks to a combined radial-velocity and transit timings overview we also define a mass/period domain in which a secondary planet may be found in the system. We also show that timings obtained until now, although not sufficient to remove degeneracies on mass and period, can still restrict the parameter space of the potential secondary plane
Transit confirmation and improved stellar and planet parameters for the super-Earth HD 97658 b and its host star
Super-Earths transiting nearby bright stars are key objects that
simultaneously allow for accurate measurements of both their mass and radius,
providing essential constraints on their internal composition. We present here
the confirmation, based on Spitzer transit observations, that the super-Earth
HD 97658 b transits its host star. HD 97658 is a low-mass
() K1 dwarf, as determined from the Hipparcos
parallax and stellar evolution modeling. To constrain the planet parameters, we
carry out Bayesian global analyses of Keck-HIRES radial velocities, and MOST
and Spitzer photometry. HD 97658 b is a massive () and large ( at 4.5
m) super-Earth. We investigate the possible internal compositions for HD
97658 b. Our results indicate a large rocky component, by at least 60% by mass,
and very little H-He components, at most 2% by mass. We also discuss how future
asteroseismic observations can improve the knowledge of the HD 97658 system, in
particular by constraining its age. Orbiting a bright host star, HD 97658 b
will be a key target for coming space missions TESS, CHEOPS, PLATO, and also
JWST, to characterize thoroughly its structure and atmosphere.Comment: 8 figures, accepted to Ap
Adaptive Optics Images of Kepler Objects of Interest
All transiting planets are at risk of contamination by blends with nearby,
unresolved stars. Blends dilute the transit signal, causing the planet to
appear smaller than it really is, or produce a false positive detection when
the target star is blended with eclipsing binary stars. This paper reports on
high spatial-resolution adaptive optics images of 90 Kepler planetary
candidates. Companion stars are detected as close as 0.1 arcsec from the target
star. Images were taken in the near-infrared (J and Ks bands) with ARIES on the
MMT and PHARO on the Palomar Hale 200-inch. Most objects (60%) have at least
one star within 6 arcsec separation and a magnitude difference of 9. Eighteen
objects (20%) have at least one companion within 2 arcsec of the target star; 6
companions (7%) are closer than 0.5 arcsec. Most of these companions were
previously unknown, and the associated planetary candidates should receive
additional scrutiny. Limits are placed on the presence of additional companions
for every system observed, which can be used to validate planets statistically
using the BLENDER method. Validation is particularly critical for low-mass,
potentially Earth-like worlds, which are not detectable with current-generation
radial velocity techniques. High-resolution images are thus a crucial component
of any transit follow-up program.Comment: 9 pages, 4 figures, accepted to A
The Spitzer search for the transits of HARPS low-mass planets - I. No transit for the super-Earth HD 40307b
We have used Spitzer and its IRAC camera to search for the transit of the
super-Earth HD 40307b. The transiting nature of the planet could not be firmly
discarded from our first photometric monitoring of a transit window because of
the uncertainty coming from the modeling of the photometric baseline. To obtain
a firm result, two more transit windows were observed and a global Bayesian
analysis of the three IRAC time series and the HARPS radial velocities was
performed. Unfortunately, any transit of the planet during the observed phase
window is firmly discarded, while the probability that the planet transits but
that the eclipse was missed by our observations is nearly negligible (0.26%).Comment: Submitted to A&
The CORALIE survey for southern extrasolar planets. XVI. Discovery of a planetary system around HD 147018 and of two long period and massive planets orbiting HD 171238 and HD 204313
We report the detection of a double planetary system around HD 140718 as well
as the discovery of two long period and massive planets orbiting HD 171238 and
HD 204313. Those discoveries were made with the CORALIE Echelle spectrograph
mounted on the 1.2-m Euler Swiss telescope located at La Silla Observatory,
Chile. The planetary system orbiting the nearby G9 dwarf HD 147018 is composed
of an eccentric inner planet (e=0.47) with twice the mass of Jupiter (2.1 MJup
) and with an orbital period of 44.24 days. The outer planet is even more
massive (6.6 MJup) with a slightly eccentric orbit (e=0.13) and a period of
1008 days. The planet orbiting HD 171238 has a minimum mass of 2.6 MJup, a
period of 1523 days and an eccentricity of 0.40. It orbits a G8 dwarfs at 2.5
AU. The last planet, HD 204313 b, is a 4.0 MJup -planet with a period of 5.3
years and has a low eccentricity (e = 0.13). It orbits a G5 dwarfs at 3.1 AU.
The three parent stars are metal rich, which further strengthened the case that
massive planets tend to form around metal rich stars.Comment: 6 pages, 6 figures, accepted for publication in A&
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