M. Montalto

Comparing HARPS and Kepler surveys - The alignment of multiple-planet systems

Context. The recent results of the HARPS and Kepler surveys provided us with a bounty of extrasolar systems. While the two teams extensively analyzed each of their data-sets, little work has been done comparing the two. Aims. We study a subset of the planetary population whose characterization is simultaneously within reach of both instruments. We compare the statistical properties of planets in systems with m sini > 5−10 M⊕ and R > 2R⊕, as inferred from the HARPS and Kepler surveys, respectively. If we assume that the underlying population has the same characteristics, the different detection sensitivity to the orbital inclination relative to the line of sight allows us to probe the planets’ mutual inclination. Methods. We considered the frequency of systems with one, two, and three planets as dictated by HARPS data. We used Kepler’s planetary period and host mass and radius distributions (corrected from detection bias) to model planetary systems in a simple, yet physically plausible way. We then varied the mutual inclination between planets in a system according to different prescriptions (completely aligned, Rayleigh distributions, and isotropic) and compared the transit frequencies with one, two, or three planets with those measured by Kepler. Results. The results show that the two datasets are compatible, a remarkable result especially because there are no tunable knobs other than the assumed inclination distribution. For m sini cutoffs of 7–10 M⊕, which are those expected to correspond to the radius cutoff of 2R⊕, we conclude that the results are better described by a Rayleigh distribution with a mode of 1 ◦ or smaller. We show that the best-fit scenario only becomes a Rayleigh distribution with a mode of 5 ◦ if we assume a quite extreme mass-radius relationship for the planetary population. Conclusions. These results have important consequences for our understanding of the role of several proposed formation and evolution mechanisms. They confirm that planets are likely to have been formed in a disk and show that most planetary systems evolve quietly without strong angular momentum exchanges such as those produced by Kozai mechanism or planet scattering.

Effect of stellar spots on high-precision transit light-curve

Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii-Manoa, 2680 Woodlawn Drive, Honolulu, HI96822,USAReceived XXX; accepted XXXABSTRACTStellar-activity features such as spots can complicate the determination of planetary parameters through spectroscopic andphotometric observations. The overlap of a transiting planet and a stellar spot, for instance, can produce anomalies in thetransit light-curves that may lead to an inaccurate estimation of the transit duration, depth, and timing. These inaccuraciescan for instance affect the precise derivation of the planet radius. We present the results of a quantitative study on the effectsof stellar spots on high-precision transit light-curves. We show that spot anomalies can lead to an estimate of a planet radiusthat is 4% smaller than the real value. Likewise, the transit duration may be estimated about 4%, longer or shorter. Dependingon the size and distribution of spots, anomalies can also produce transit-timing variations (TTVs) with significant amplitudes.For instance, TTVs with signal amplitudes of 200 seconds can be produced when the spot is completely dark and has the sizeof the largest Sun spot. Our study also indicates that the smallest size of a stellar spot that still has detectable affects on ahigh-precision transit light-curve is around 0.03 time the stellar radius for typical Kepler telescope precision. We also show thatthe strategy of including more free parameters (such as transit depth and duration) in the fitting procedure to measure thetransit time of each individual transit will not produce accurate results for active stars.Key words. methods: numerical- planetary system- techniques: photometry, Stellar activity

Impact of occultations of stellar active regions on transmission spectra - Can occultation of a plage mimic the signature of a blue sky?

Transmission spectroscopy during planetary transits, which is based on the measurements of the variations of planet-to-star radius ratio as a function of wavelength, is a powerful technique to study the atmospheric properties of transiting planets. One of the main limitation of this technique is the effects of stellar activity, which up until now, have been taken into account only by assessing the effect of non-occulted stellar spots on the estimates of planet-to-star radius ratio. In this paper, we study, for the first time, the impact of the occultation of a stellar spot and plage on the transmission spectra of transiting exoplanets. We simulated this effect by generating a large number of transit light curves for different transiting planets, stellar spectral types, and for different wavelengths. Results of our simulations indicate that the anomalies inside the transit light curve can lead to a significant underestimation or overestimation of the planet-to-star radius ratio as a function of wavelength. At short wavelengths, the effect can reach to a difference of up to 10% in the planet-to-star radius ratio, mimicking the signature of light scattering in the planetary atmosphere. Atmospheric scattering has been proposed to interpret the increasing slopes of transmission spectra toward blue for exoplanets HD 189733b and GJ 3470b. Here we show that these signatures can be alternatively interpreted by the occultation of stellar plages. Results also suggest that the best strategy to identify and quantify the effects of stellar activities on the transmission spectrum of a planet is to perform several observations during the transit epoch at the same wavelength. This will allow for identifying the possible variations in transit depth as a function of time due to stellar activity variability.

New and updated stellar parameters for 90 transit hosts - The effect of the surface gravity

Context. Precise stellar parameters are crucial in exoplanet research for correctly determining the planetary parameters. For stars hosting a transiting planet, determining the planetary mass and radius depends on the stellar mass and radius, which in turn depend on the atmospheric stellar parameters. Different methods can provide different results, which leads to different planet characteristics. Aims. In this paper, we use a uniform method to spectroscopically derive stellar atmospheric parameters, chemical abundances, stellar masses, and stellar radii for a sample of 90 transit hosts. Surface gravities are also derived photometrically using the stellar density as derived from the light curve. We study the effect of using these different surface gravities on the determination of the chemical abundances and the stellar mass and radius. Methods. A spectroscopic analysis based on Kurucz models in local thermodynamical equilibrium was performed through the MOOG code to derive the atmospheric parameters and the chemical abundances. The photometric surface gravity was determined through isochrone fitting and the use of the stellar density, directly determined from the light curve. Stellar masses and radii are determined through calibration formulae. Results. Spectroscopic and photometric surface gravities differ, but this has very little effect on the precise determination of the stellar mass in our spectroscopic analysis. The stellar radius, and hence the planetary radius, is most affected by the surface gravity discrepancies. For the chemical abundances, the difference is, as expected, only noticable for the abundances derived from analyzing lines of ionized species.

Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b

Context. The detection of reflected light from an exoplanet is a difficult technical challenge at optical wavelengths. Even though this signal is expected to replicate the stellar signal, not only is it several orders of magnitude fainter, but it is also hidden among the stellar noise. Aims. We apply a variant of the cross-correlation technique to HARPS observations of 51 Peg to detect the reflected signal from planet 51 Peg b. Methods. Our method makes use of the cross-correlation function (CCF) of a binary mask with high-resolution spectra to amplify the minute planetary signal that is present in the spectra by a factor proportional to the number of spectral lines when performing the cross correlation. The resulting cross-correlation functions are then normalized by a stellar template to remove the stellar signal. Carefully selected sections of the resulting normalized CCFs are stacked to increase the planetary signal further. The recovered signal allows probing several of the planetary properties, including its real mass and albedo. Results. We detect evidence for the reflected signal from planet 51 Peg b at a significance of 3 sigma(noise). The detection of the signal permits us to infer a real mass of 0.46(-0.01)(+0.06) M-Jup (assuming a stellar mass of 1.04 M-Sun) for the planet and an orbital inclination of 80(-19)(+10) degrees. The analysis of the data also allows us to infer a tentative value for the (radius-dependent) geometric albedo of the planet. The results suggest that 51Peg b may be an inflated hot Jupiter with a high albedo (e.g., an albedo of 0.5 yields a radius of 1.9 +/- 0.3 R-Jup for a signal amplitude of 6.0 +/- 0.4 x 10(-5)). Conclusions. We confirm that the method we perfected can be used to retrieve an exoplanets reflected signal, even with current observing facilities. The advent of next generation of instruments (e.g. VLT-ESO /ESPRESSO) and observing facilities (e.g. a new generation of ELT telescopes) will yield new opportunities for this type of technique to probe deeper into exoplanets and their atmospheres.

New analytical expressions of the Rossiter-McLaughlin effect adapted to different observation techniques

The Rossiter-McLaughlin (hereafter RM) e ect is a key tool for measuring the projected spin-orbit angle between stellar spin axes and orbits of transiting planets. However, the measured radial velocity (RV) anomalies produced by this e ect are not intrinsic and depend on both instrumental resolution and data reduction routines. Using inappropriate formulas to model the RM e ect introduces biases, at least in the projected velocity V sini? compared to the spectroscopic value. Currently, only the iodine cell technique has been modeled, which corresponds to observations done by, e.g., the HIRES spectrograph of the Keck telescope. In this paper, we provide a simple expression of the RM e ect specially designed to model observations done by the Gaussian fit of a cross-correlation function (CCF) as in the routines performed by the HARPS team. We derived also a new analytical formulation of the RV anomaly associated to the iodine cell technique. For both formulas, we modeled the subplanet mean velocity vp and dispersion p accurately taking the rotational broadening on the subplanet profile into account. We compare our formulas adapted to the CCF technique with simulated data generated with the numerical software SOAP-T and find good agreement up to V sini? . 20 km.s 1 . In contrast, the analytical models simulating the two di erent observation techniques can disagree by about 10 in V sini? for large spin-orbit misalignments. It is thus important to apply the adapted model when fitting data.

Degeneracy in the characterization of non-transiting planets from transit timing variations

The transit timing variation (TTV) method allows the detection of non-transiting planets through their gravitational perturbations. Since TTVs are strongly enhanced in systems close to mean-motion resonances (MMRs), even a low-mass planet can produce an observable signal. This technique has thus been proposed to detect terrestrial planets. In this Letter, we analyse TTV signals for systems in or close to MMR in order to illustrate the difficulties arising in the determination of planetary parameters. TTVs are computed numerically with an N-body integrator for a variety of systems close to MMR. The main features of these TTVs are also derived analytically. Systems deeply inside of the MMR do not produce particularly strong TTVs, while those close to MMR generate quasi-periodic TTVs characterized by a dominant long-period term and a low-amplitude remainder. If the remainder is too weak to be detected, then the signal is strongly degenerate and this prevents the determination of the planetary parameters. Even though an Earth-mass planet can be detected by the TTV method if it is close to an MMR, it may not be possible to assert that this planet is actually an Earth-mass planet. On the other hand, if the system is right in the centre of an MMR, the high-amplitude oscillation of the TTV signal vanishes and the detection of the perturber becomes as difficult as it is far from the MMR.

A new analysis of the WASP-3 system: no evidence for an additional companion

In this work, we investigate the problem concerning the presence of additional bodies gravitationally bound with the WASP-3 system. We present eight new transits of this planet gathered between 2009 May and 2011 September by using the 30-cm telescope at the Crow Observatory-Portalegre, and analyse all the photometric and radial velocity data published so far. We did not observe significant periodicities in the Fourier spectrum of the observed minus calculated (O − C) transit timing and radial velocity diagrams (the highest peak having false-alarm probabilities of 56 and 31 per cent, respectively) or long-term trends. Combining all the available information, we conclude that the radial velocity and transit timing techniques exclude, at 99 per cent confidence limit, any perturber more massive than M ≳ 100 Mearth with periods up to 10 times the period of the inner planet. We also investigate the possible presence of an exomoon in this system and determine that considering the scatter of the O − C transit timing residuals a coplanar exomoon would likely produce detectable transits. This hypothesis is however apparently ruled out by observations conducted by other researchers. In the case where the orbit of the moon is not coplanar, the accuracy of our transit timing and transit duration measurements prevents any significant statement. Interestingly, on the basis of our reanalysis of SOPHIE data we noted that WASP-3 passed from a less active () to a more active () state during the 3 yr monitoring period spanned by the observations. Despite the fact that no clear spot crossing has been reported for this system, this analysis suggests a more intensive monitoring of the activity level of this star in order to understand its impact on photometric and radial velocity measurements.

Metallicities for six nearby open clusters from high-resolution spectra of giant stars - [Fe/H] values for a planet search sample

We present a study of the stellar parameters and iron abundances of 18 giant stars in six open clusters. The analysis was based on high-resolution and high-S/N spectra obtained with the UVES spectrograph (VLT-UT2). The results complement our previous study where 13 clusters were already analyzed. The total sample of 18 clusters is part of a program to search for planets around giant stars. The results show that the 18 clusters cover a metallicity range between −0.23 and +0.23 dex. Together with the derivation of the stellar masses, these metallicities will allow the metallicity and mass effects to be disentangled when analyzing the frequency of planets as a function of these stellar parameters.

Spectroscopic parameters for solar-type stars with moderate-to-high rotation - New parameters for ten planet hosts

Planetary studies demand precise and accurate stellar parameters as input to infer the planetary properties. Different methods often provide different results that could lead to biases in the planetary parameters. In this work, we present a refinement of the spectral synthesis technique designed to treat better more rapidly rotating FGK stars. This method is used to derive precise stellar parameters, namely effective temperature, surface gravity, metallicitity and rotational velocity. This procedure is tested for samples of low and moderate/fast rotating FGK stars. The spectroscopic analysis is based on the spectral synthesis package Spectroscopy Made Easy (SME), assuming Kurucz model atmospheres in LTE. The line list where the synthesis is conducted, is comprised of iron lines and the atomic data are derived after solar calibration. The comparison of our stellar parameters shows good agreement with literature values, both for low and for higher rotating stars. In addition, our results are on the same scale with the parameters derived from the iron ionization and excitation method presented in our previous works. We present new atmospheric parameters for 10 transiting planet-hosts as an update to the SWEET-Cat catalogue. We also re-analyse their transit light curves to derive new updated planetary properties.