675 research outputs found
Astrophysically robust systematics removal using variational inference: application to the first month of Kepler data
Space-based transit search missions such as Kepler are collecting large
numbers of stellar light curves of unprecedented photometric precision and time
coverage. However, before this scientific goldmine can be exploited fully, the
data must be cleaned of instrumental artefacts. We present a new method to
correct common-mode systematics in large ensembles of very high precision light
curves. It is based on a Bayesian linear basis model and uses shrinkage priors
for robustness, variational inference for speed, and a de-noising step based on
empirical mode decomposition to prevent the introduction of spurious noise into
the corrected light curves. After demonstrating the performance of our method
on a synthetic dataset, we apply it to the first month of Kepler data. We
compare the results, which are publicly available, to the output of the Kepler
pipeline's pre-search data conditioning, and show that the two generally give
similar results, but the light curves corrected using our approach have lower
scatter, on average, on both long and short timescales. We finish by discussing
some limitations of our method and outlining some avenues for further
development. The trend-corrected data produced by our approach are publicly
available.Comment: 15 pages, 13 figures, accepted for publication in MNRA
The photospheric abundances of active binaries I Detailed analysis of HD 113816 (IS Vir) and HD 119285 (V851 Cen)
The high-resolution optical spectra of the two X-ray active binaries RS CVn
stars HD 113816 (IS Vir) and HD 119285 (V851 Cen) are analysed and their Na,
Mg, Al, Si, Ca, Sc, Ti, Co and Ni contents determined. The analysis of IS Vir
and V851 Cen is performed with three different LTE methods. In the first one,
abundances are derived using measured equivalent widths and Kurucz LTE model
atmospheres as input for the MOOG software package. The input atmospheric
parameters and abundances are iteratively modified until (i) the Fe I
abundances exhibit no trend with excitation potential or equivalent width, (ii)
Fe I and Fe II average abundances are the same and (iii) Fe and Alpha elements
average abundances are consistent with the input values. The second method
follows a similar approach, but relies on the B-V and V-I colour indices to
determine the temperature. The third method relies on fitting the 6162 A Ca I
line wing profiles to derive the surface gravity. The reliability of these
methods is investigated in the context of single line RS CVn stars. It is shown
that the V-I photometric index gives, on a broader sample of stars,
significantly cooler estimates of the effective temperature than the B-V index.
All other approaches give results in good agreement with each other. The
analysis of IS Vir and V851 Cen results in both cases in their primaries being
giant stars of near-solar metallicity. Their parameters as derived with the
first method are respectively Teff = 4720 K, log g = 2.65, [Fe/H] = +0.04 and
Teff = 4700 K, log g = 3.0 and [Fe/H] = -0.13. In the case of V851 Cen the
derived iron content is significantly higher than a previous determination in
the literature. Both stars exhibit relative overabundances of several elements
(e.g. Ca) with respect to the solar mix.Comment: 12 Pages, 2 Figures, Accepted for publication in A&
Accuracy of stellar parameters of exoplanet-host stars determined from asteroseismology
In the context of the space-based mission CoRoT, devoted to asteroseismology
and search for planet transits, we analyse the accuracy of fundamental stellar
parameters (mass, radius, luminosity) that can be obtained from
asteroseismological data.} Our work is motivated by the large uncertainties on
planetary radius determination of transiting planets which are mainly due to
uncertainties on the stellar parameters. Our goal is to analyse uncertainties
of fundamental stellar parameters for a given accuracy of oscillation frequency
determination. We generate grids of equilibrium models of stars and compute
their pulsation spectra based on a linear nonadiabatic stability analysis.
Using differents methods of comparison of oscillation mode spectra, we derive
uncertainties on fundamental stellar parameters and analyse the effect of
varying the number of considered modes.} The limits obtained depend strongly on
the adapted method to compare spectra. We find a degeneracy in the stellar
parameter solutions, up to a few % in mass (from less than 1% to more than 7%
depending on the method used and the number of considered modes), luminosity
(from 2% to more than 10%) or radius (from less than 1% to 3%), for a given
pulsation spectrum.Comment: 7 pages, 9 figure
Quasi-periodic Gaussian processes for stellar activity: From physical to kernel parameters
In recent years, Gaussian Process (GP) regression has become widely used to analyse stellar and exoplanet time-series data sets. For spotted stars, the most popular GP covariance function is the quasi-periodic (QP) kernel, whose hyperparameters of the GP have a plausible interpretation in terms of physical properties of the star and spots. In this paper, we test the reliability of this interpretation by modelling data simulated using a spot model using a QP GP, and the recently proposed quasi-periodic plus cosine (QPC) GP, comparing the posterior distributions of the GP hyperparameters to the input parameters of the spot model. We find excellent agreement between the input stellar rotation period and the QP and QPC GP period, and very good agreement between the spot decay time-scale and the length scale of the squared exponential term. We also compare the hyperparameters derived from light and radial velocity (RV) curves for a given star, finding that the period and evolution time-scales are in good agreement. However, the harmonic complexity of the GP, while displaying no clear correlation with the spot properties in our simulations, is systematically higher for the RV than for the light-curve data. Finally, for the QP kernel, we investigate the impact of noise and time-sampling on the hyperparameters in the case of RVs. Our results indicate that good coverage of rotation period and spot evolution time-scales is more important than the total number of points, and noise characteristics govern the harmonic complexity
A Gaussian process framework for modelling instrumental systematics: application to transmission spectroscopy
Transmission spectroscopy, which consists of measuring the
wavelength-dependent absorption of starlight by a planet's atmosphere during a
transit, is a powerful probe of atmospheric composition. However, the expected
signal is typically orders of magnitude smaller than instrumental systematics,
and the results are crucially dependent on the treatment of the latter. In this
paper, we propose a new method to infer transit parameters in the presence of
systematic noise using Gaussian processes, a technique widely used in the
machine learning community for Bayesian regression and classification problems.
Our method makes use of auxiliary information about the state of the
instrument, but does so in a non-parametric manner, without imposing a specific
dependence of the systematics on the instrumental parameters, and naturally
allows for the correlated nature of the noise. We give an example application
of the method to archival NICMOS transmission spectroscopy of the hot Jupiter
HD 189733, which goes some way towards reconciling the controversy surrounding
this dataset in the literature. Finally, we provide an appendix giving a
general introduction to Gaussian processes for regression, in order to
encourage their application to a wider range of problems.Comment: 6 figures, 1 table, accepted for publication in MNRA
Modelling solar-like variability for the detection of Earth-like planetary transits. I. Performance of the three-spot modelling and harmonic function fitting
We present a comparison of two methods of fitting solar-like variability to
increase the efficiency of detection of Earth-like planetary transits across
the disk of a Sun-like star. One of them is the harmonic fitting method that
coupled with the BLS detection algorithm demonstrated the best performance
during the first CoRoT blind test. We apply a Monte Carlo approach by
simulating a large number of light curves of duration 150 days for different
values of planetary radius, orbital period, epoch of the first transit, and
standard deviation of the photon shot noise. Stellar variability is assumed in
all the cases to be given by the Total Solar Irradiance variations as observed
close to the maximum of solar cycle 23. After fitting solar variability,
transits are searched for by means of the BLS algorithm. We find that a model
based on three point-like active regions is better suited than a best fit with
a linear combination of 200 harmonic functions to reduce the impact of stellar
microvariability provided that the standard deviation of the noise is 2-4 times
larger than the central depth of the transits. On the other hand, the
200-harmonic fit is better when the standard deviation of the noise is
comparable to the transit depth. Our results show the advantage of a model
including a simple but physically motivated treatment of stellar
microvariability for the detection of planetary transits when the standard
deviation of the photon shot noise is greater than the transit depth and
stellar variability is analogous to solar irradiance variations.Comment: 8 pages, 6 figures, accepted by Astronomy & Astrophysic
Detecting planetary transits in the presence of stellar variability. Optimal filtering and the use of colour information
Copyright © The European Southern Observatory (ESO)Space-based photometric transit searches, such as the ESA mission Eddington (planned for launch in 2007), are expected to detect large numbers of terrestrial planets outside the solar system, including some Earth-like planets. Combining simplicity of concept with efficiency, the transit method consists in detecting the periodic luminosity drop in stellar light curves caused by the transit of a planet in front of its parent star. In a previous paper (Aigrain & Favata 2002, hereafter Paper I), we developed a Bayesian transit detection algorithm and evaluated its performance on simulated light curves dominated by photon noise. In this paper, we examine the impact of intrinsic stellar variability. Running the algorithm on light curves with added stellar variability (constructed using data from the VIRGO/PMO6 instrument on board SoHO) demonstrated the need for pre-processing to remove the stellar noise. We have developed an effective variability filter, based on an ad-hoc optimal approach, and run extensive simulations to test the filter and detection algorithm combination for a range of stellar magnitudes and activity levels. These show that activity levels up to solar maximum are not an obstacle to habitable planet detection. We also evaluated the benefits of using colour information in the detection process, and concluded that in the case of Eddington they are outweighed by the implied loss of photometric accuracy
The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations
The hot Jupiter HD189733b is the most extensively observed exoplanet. Its
atmosphere has been detected and characterised in transmission and eclipse
spectroscopy, and its phase curve measured at several wavelengths. This paper
brings together results of our campaign to obtain the complete transmission
spectrum of the atmosphere of this planet from UV to IR with HST, using STIS,
ACS and WFC3. We provide a new tabulation of the transmission spectrum across
the entire visible and IR range. The radius ratio in each wavelength band was
rederived to ensure a consistent treatment of the bulk transit parameters and
stellar limb-darkening. Special care was taken to correct for, and derive
realistic estimates of the uncertainties due to, both occulted and unocculted
star spots. The combined spectrum is very different from the predictions of
cloud-free models: it is dominated by Rayleigh scattering over the whole
visible and near infrared range, the only detected features being narrow Na and
K lines. We interpret this as the signature of a haze of condensate grains
extending over at least 5 scale heights. We show that a dust-dominated
atmosphere could also explain several puzzling features of the emission
spectrum and phase curves, including the large amplitude of the phase curve at
3.6um, the small hot-spot longitude shift and the hot mid-infrared emission
spectrum. We discuss possible compositions and derive some first-order
estimates for the properties of the putative condensate haze/clouds. We finish
by speculating that the dichotomy between the two observationally defined
classes of hot Jupiter atmospheres, of which HD189733b and HD209458b are the
prototypes, might not be whether they possess a temperature inversion, but
whether they are clear or dusty. We also consider the possibility of a
continuum of cloud properties between hot Jupiters, young Jupiters and L-type
brown dwarfs.Comment: Accepted for publication in MNRAS. 31 pages, 19 figures, 8 table
Exoplanets with JWST: degeneracy, systematics and how to avoid them
The high sensitivity and broad wavelength coverage of the James Webb Space Telescope will transform the field of exoplanet transit spectroscopy. Transit spectra are inferred from minute, wavelength-dependent variations in the depth of a transit or eclipse as the planet passes in front of or is obscured by its star, and the spectra contain information about the composition, structure and cloudiness of exoplanet atmospheres. Atmospheric retrieval is the preferred technique for extracting information from these spectra, but the process can be confused by astrophysical and instrumental systematic noise. We present results of retrieval tests based on synthetic, noisy JWST spectra, for clear and cloudy planets and active and inactive stars. We find that the ability to correct for stellar activity is likely to be a limiting factor for cloudy planets, as the effects of unocculted star spots may mimic the presence of a scattering slope due to clouds. We discuss the pros and cons of the available JWST instrument combinations for transit spectroscopy, and consider the effect of clouds and aerosols on the spectra. Aerosol high in a planet’s atmosphere obscures molecular absorption features in transmission, reducing the information content of spectra in wavelength regions where the cloud is optically thick. We discuss the usefulness of particular wavelength regions for identifying the presence of cloud, and suggest strategies for solving the highly-degenerate retrieval problem for these objects
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