Search

Context. Stellar activity is the most relevant types of astrophysical noise that affect the discovery and characterization of extrasolar planets. On the other hand, the amplitude of stellar activity could hint at an interaction between the star and a close-in giant planet. Progress has been made in recent years in understanding how to deal with stellar activity and search for observational evidence of star-planet interactions. Aims. The aim of this work is to characterize the chromospheric activity of stars hosting short-period exoplanets by studying the correlations between the chromospheric emission (CE) in the Ca II H&K and the planetary parameters. Methods. We measured CE in the Ca II H&K lines using more than 1900 high-resolution spectra of a sample composed of 76 targets, observed with the HARPS-N spectrograph between 2012 and 2020. We transformed the fluxes into bolometric- and photospheric-corrected chromospheric emission ratios, R′HK. Furthermore, we completed the sample of hosts digging for data in previous works. Stellar parameters Teff, B–V, and V were retrieved homogeneously from the Gaia DR3. Then, M★, R★, and ages were determined from isochrone fitting. We retrieved planetary data from the literature and catalogs. The search for correlations between the log(R′HK) and planetary parameters have been performed through both Spearman's rank and its statistics as well as the more sophisticated Gaussian mixture model method. Results. We found that the distribution of log(R′HK) for the transiting planet hosts is different from the distribution of field main-sequence and sub-giant stars. The log(R′HK) of planetary hosts is correlated with planetary parameters proportional to the planetary radius to the power of n (RPn, indicating a common origin for the correlations. The statistical analysis has also highlighted four clusters of host stars with different behavior in terms of their stellar activity with respect to the planetary surface gravity. Some of the host stars have a value of log(R′HK) that is lower than the basal level of activity for main sequence stars. The planets of these systems are very close to filling their Roche lobe, suggesting that they evaporate through hydrodynamic escape under the strong irradiation of the host star, creating shrouds that absorb the core of the chromospheric resonance lines. [ABSTRACT FROM AUTHOR]

The scope of the project 'A PSF-based Approach to TESS High Quality data Of Stellar clusters' (PATHOS) is the extraction and analysis of high-precision light curves of stars in stellar clusters and young associations for the identification of candidate exoplanets and variable stars. The cutting-edge tools used in this project allow us to measure the real flux of stars in dense fields, minimizing the effects due to contamination by neighbour sources. We extracted about 200 000 light curves of stars in 645 open clusters located in the Southern ecliptic hemisphere and observed by TESS during the first year of its mission. We searched for transiting signals and we found 33 objects of interest, 11 of which are strong candidate exoplanets. Because of the limited SNR, we did not find any Earth or super-Earth. We identified two Neptune-size planets orbiting stars with |$R_{\star }\lt 1.5\, \mathrm{\it R}_{\odot }$|⁠ , implying a frequency |$f_{\star }=1.34 \pm 0.95\, {{\ \rm per\ cent}}$|⁠ , consistent with the frequency around field stars. The seven Jupiter candidates around stars with |$R_{\star }\lt \, 1.5\, \mathrm{\it R}_{\odot }$| imply a frequency |$f_{\star }=0.19\pm 0.07\, {{\ \rm per\ cent}}$|⁠ , which is smaller than in the field. A more complete estimate of the survey completeness and false positive rate is needed to confirm these results. Light curves used in this work will be made available to the astronomical community on the Mikulski Archive for Space Telescope under the project PATHOS. [ABSTRACT FROM AUTHOR]

Aims. The aim of this work is to search for planets around intermediate-mass stars in open clusters using data from an extensive survey with more than 15 yr of observations. Methods. We obtain high-precision radial velocities (RV) with the HARPS spectrograph for a sample of 142 giant stars in 17 open clusters. We fit Keplerian orbits when a significant periodic signal is detected. We also study the variation of stellar activity indicators and line-profile variations to discard stellar-induced signals. Results. We present the discovery of a periodic RV signal compatible with the presence of a planet candidate in the 1.15 Gyr open cluster IC 4651 orbiting the 2.06 M⊙ star No. 9122. If confirmed, the planet candidate would have a minimum mass of 7.2 MJ and a period of 747 days. However, we also find that the full width at half maximum (FWHM) of the cross-correlation function (CCF) varies with a period close to the RV, casting doubts on the planetary nature of the signal. We also provide refined parameters for the previously discovered planet around NGC 2423 No. 3, but show evidence that the bisector inverse slope (BIS) of the CCF is correlated with the RV during some of the observing periods. We consider this fact as a warning that this might not be a real planet and that the RV variations could be caused by stellar activity and/or pulsations. Finally, we show that the previously reported signal by a brown dwarf around NGC 4349 No. 127 is presumably produced by stellar activity modulation. Conclusions. The long-term monitoring of several red giants in open clusters has allowed us to find periodic RV variations in several stars. However, we also show that the follow-up of this kind of stars should last more than one orbital period to detect long-term signals of stellar origin. This work highlights the fact that although it is possible to detect planets around red giants, large-amplitude, long-period RV modulations do exist in such stars that can mimic the presence of an orbiting planetary body. Therefore, we need to better understand how such RV modulations behave as stars evolve along the red giant branch and perform a detailed study of all the possible stellar-induced signals (e.g., spots, pulsations, granulation) to comprehend the origin of RV variations. [ABSTRACT FROM AUTHOR]

Aims. In this paper we explore the possibility that the recently detected reflected light signal of 51 Peg b could be caused by a ring system around the planet. Methods. We use a simple model to compare the observed signal with the expected signal from a short-period giant planet with rings. We also use simple dynamical arguments to understand the possible geometry of such a system. Results. We provide evidence that, to a good approximation, the observations are compatible with the signal expected from a ringed planet, assuming that the rings are non-coplanar with the orbital plane. However, based on dynamical arguments, we also show that this configuration is unlikely. In the case of coplanar rings we then demonstrate that the incident flux on the ring surface is about 2% the value received by the planet, a value that renders the ring explanation unlikely. Conclusions. The results suggest that the signal observed cannot in principle be explained by a planet+ring system. We discuss, however, the possibility of using reflected light spectra to detect and characterize the presence of rings around short-period planets. Finally, we show that ring systems could have already been detected by photometric transit campaigns, but their signal could have been easily misinterpreted by the expected light curve of an eclipsing binary. [ABSTRACT FROM AUTHOR]

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 3σnoise. The detection of the signal permits us to infer a real mass of 0:46+0:06 >-0:01 MJup (assuming a stellar mass of 1:04 MSun) for the planet and an orbital inclination of 80+10-19 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 RJup for a signal amplitude of 6:0 ± 0:4 × 10-5). Conclusions. We confirm that the method we perfected can be used to retrieve an exoplanet's 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. [ABSTRACT FROM AUTHOR]

Stellar-activity features such as spots can complicate the determination of planetary parameters through spectroscopic and photometric observations. The overlap of a transiting planet and a stellar spot, for instance, can produce anomalies in the transit light-curves that may lead to an inaccurate estimation of the transit duration, depth, and timing. These inaccuracies can for instance affect the precise derivation of the planet radius. We present the results of a quantitative study on the effects of stellar spots on high-precision transit light-curves. We show that spot anomalies can lead to an estimate of a planet radius that is 4% smaller than the real value. Likewise, the transit duration may be estimated about 4%, longer or shorter. Depending on 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 s can be produced when the spot is completely dark and has the size of the largest Sun spot. Our study also indicates that the smallest size of a stellar spot that still has detectable effects on a high-precision transit light-curve is around 0.03 time the stellar radius for typical Kepler Telescope precision. We also show that the strategy of including more free parameters (such as transit depth and duration) in the fitting procedure to measure the transit time of each individual transit will not produce accurate results for active stars. [ABSTRACT FROM AUTHOR]

The Rossiter-McLaughlin (hereafter RM) effect 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 effect are not intrinsic and depend on both instrumental resolution and data reduction routines. Using inappropriate formulas to model the RM eβect introduces biases, at least in the projected velocity V sin i⋆ 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 effect 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 a new analytical formulation of the RV anomaly associated to the iodine cell technique. For both formulas, we modeled the subplanet mean velocity vpand dispersion βp 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 sin i⋆ ≲β 20 km s−1. In contrast, the analytical models simulating the two diβerent observation techniques can disagree by about 10β in V sin i⋆ for large spin-orbit misalignments. It is thus important to apply the adapted model when fitting data. [ABSTRACT FROM AUTHOR]

We present an improved version of SOAP named "SOAP-T", which can generate the radial velocity variations and light curves for systems consisting of a rotating spotted star with a transiting planet. This tool can be used to study the anomalies inside transit light curves and the Rossiter-McLaughlin effect, to better constrain the orbital configuration and properties of planetary systems and the active zones of their host stars. Tests of the code are presented to illustrate its performance and to validate its capability when compared with analytical models and real data. Finally, we apply SOAP-T to the active star, HAT-P-11, observed by the NASA Kepler space telescope and use this system to discuss the capability of this tool in analyzing light curves for the cases where the transiting planet overlaps with the star's spots. [ABSTRACT FROM AUTHOR]

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 sin i > 5-10 M☉ and R > 2 R☉, 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 msin i cutoffs of 7-10 M☉, which are those expected to correspond to the radius cutoff of 2 R☉, 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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]