M. Oshagh

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

Orbital and physical properties of planets and their hosts: new insights on planet formation and evolution

Aims. We explore the relations between physical and orbital properties of planets and properties of their host stars to identify the main observable signatures of the formation and evolution processes of planetary systems. Methods. We used a large sample of FGK dwarf planet-hosting stars with stellar parameters derived in a homogeneous way from the SWEET-Cat database to study the relation between stellar metallicity and position of planets in the period-mass diagram. We then used all the radial-velocity-detected planets orbiting FGK stars to explore the role of planet-disk and planet-planet interaction on the evolution of orbital properties of planets with masses above 1 MJup. Results. Using a large sample of FGK dwarf hosts we show that planets orbiting metal-poor stars have longer periods than those in metal-rich systems. This trend is valid for masses at least from ≈10 M⊕ to ≈4 MJup. Earth-like planets orbiting metal-rich stars always show shorter periods (fewer than 20 days) than those orbiting metal-poor stars. However, in the short-period regime there are a similar number of planets orbiting metal-poor stars. We also found statistically significant evidence that very high mass giants (with a mass higher than 4 MJup) have on average more eccentric orbits than giant planets with lower mass. Finally, we show that the eccentricity of planets with masses higher than 4 MJup tends to be lower for planets with shorter periods. Conclusions. Our results suggest that the planets in the P −MP diagram are evolving differently because of a mechanism that operates over a wide range of planetary masses. This mechanism is stronger or weaker, depending on the metallicity of the respective system. One possibility is that planets in metal-poor disks form farther out from their central star and/or they form later and do not have time to migrate as far as the planets in metal-rich systems. The trends and dependencies obtained for very high mass planetary systems suggest that planet-disk interaction is a very important and orbit-shaping mechanism for planets in the high-mass domain.

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.

The HARPS search for southern extra-solar planets XXXV. The interesting case of HD 41248: stellar activity, no planets?

Context. The search for planets orbiting metal-poor stars is of utmost importance for our understanding of planet formation models. However, no dedicated searches have been conducted so far for very low mass planets orbiting such objects. Only a few cases of low-mass planets orbiting metal-poor stars are thus known. Amongst these, HD 41248 is a metal-poor, solar-type star on the orbit of which a resonant pair of super-Earth-like planets has been announced. This detection was based on 62 radial velocity measurements obtained with the HARPS spectrograph (public data). Aims. We present a new planet search program that is using the HARPS spectrograph to search for Neptunes and super-Earths that orbit a sample of metal-poor FGK dwarfs. We then present a detailed analysis of 162 additional radial velocity measurements of HD 41248, obtained within this program, with the goal of confirming the existence of the proposed planetary system. Methods. We analysed the precise radial velocities, obtained with the HARPS spectrograph, together with several stellar activity diagnostics and line profile indicators. Results. A careful analysis shows no evidence for the planetary system. One of the signals, with a period of similar to 25 days, is shown to be related to the rotational period of the star, and is clearly seen in some of the activity proxies. We were unable to convincingly retrieve the remaining signal (P similar to 18 days) in the new dataset. Conclusions. We discuss possible causes for the complex (evolving) signals observed in the data of HD 41248, proposing that they might be explained by the appearance and disappearance of active regions on the surface of a star with strong differential rotation, or by a combination of the sparse data sampling and active region evolution.

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.

Uncovering the planets and stellar activity of CoRoT-7 using only radial velocities

Stellar activity can induce signals in the radial velocities of stars, complicating the detection of orbiting low-mass planets. We present a method to determine the number of planetary signals present in radial-velocity datasets of active stars, using only radial-velocity observations. Instead of considering separate fits with different number of planets, we use a birth-death Markov chain Monte Carlo algorithm to infer the posterior distribution for the number of planets in a single run. In a natural way, the marginal distributions for the orbital parameters of all planets are also inferred. This method is applied to HARPS data of CoRoT-7. We confidently recover both CoRoT-7b and CoRoT-7c although the data show evidence for additional signals.

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.

Detection of titanium oxide in the atmosphere of a hot Jupiter

As an exoplanet transits its host star, some of the light from the star is absorbed by the atoms and molecules in the planet’s atmosphere, causing the planet to seem bigger; plotting the planet’s observed size as a function of the wavelength of the light produces a transmission spectrum. Measuring the tiny variations in the transmission spectrum, together with atmospheric modelling, then gives clues to the properties of the exoplanet’s atmosphere. Chemical species composed of light elements—such as hydrogen, oxygen, carbon, sodium and potassium—have in this way been detected in the atmospheres of several hot giant exoplanets, but molecules composed of heavier elements have thus far proved elusive. Nonetheless, it has been predicted that metal oxides such as titanium oxide (TiO) and vanadium oxide occur in the observable regions of the very hottest exoplanetary atmospheres, causing thermal inversions on the dayside. Here we report the detection of TiO in the atmosphere of the hot-Jupiter planet WASP-19b. Our combined spectrum, with its wide spectral coverage, reveals the presence of TiO (to a confidence level of 7.7σ), a strongly scattering haze (7.4σ) and sodium (3.4σ), and confirms the presence of water (7.9σ) in the atmosphere.

Modeling the Rossiter-McLaughlin effect: Impact of the convective center-to-limb variations in the stellar photosphere

Observations of the Rossiter–McLaughlin (RM) effect provide information on star–planet alignments, which can inform planetary migration and evolution theories. Here, we go beyond the classical RM modeling and explore the impact of a convective blueshift that varies across the stellar disk and non-Gaussian stellar photospheric profiles. We simulated an aligned hot Jupiter with a four-day orbit about a Sun-like star and injected center-to-limb velocity (and profile shape) variations based on radiative 3D magnetohydrodynamic simulations of solar surface convection. The residuals between our modeling and classical RM modeling were dependent on the intrinsic profile width and v sin i; the amplitude of the residuals increased with increasing v sin i and with decreasing intrinsic profile width. For slowly rotating stars the center-to-limb convective variation dominated the residuals (with amplitudes of 10 s of cm s−1 to ~1 m s−1); however, for faster rotating stars the dominant residual signature was due a non-Gaussian intrinsic profile (with amplitudes from 0.5 to 9 m s−1). When the impact factor was 0, neglecting to account for the convective center-to-limb variation led to an uncertainty in the obliquity of ~10°–20°, even though the true v sin i was known. Additionally, neglecting to properly model an asymmetric intrinsic profile had a greater impact for more rapidly rotating stars (e.g., v sin i = 6 km s−1) and caused systematic errors on the order of ~20° in the measured obliquities. Hence, neglecting the impact of stellar surface convection may bias star–planet alignment measurements and consequently theories on planetary migration and evolution.