The Experts below are selected from a list of 327 Experts worldwide ranked by ideXlab platform
Nuno C. Santos - One of the best experts on this subject based on the ideXlab platform.
-
Stellar Activity consequence on the retrieved transmission spectra through chromatic Rossiter-McLaughlin observations
Astronomy & Astrophysics, 2020Co-Authors: S. Boldt, Nuno C. Santos, Mahmoudreza Oshagh, Stefan Dreizler, Ansgar Reiners, M. Mallonn, Antonio Claret, E. SedaghatiAbstract:Mostly multiband photometric transit observations have been used so far to retrieve broadband transmission spectra of transiting exoplanets in order to study their atmosphere. An alternative method has been proposed and has only been used once to recover transmission spectra using chromatic Rossiter-McLaughlin observations. Stellar Activity has been shown to potentially imitate narrow and broadband features in the transmission spectra retrieved from multiband photometric observations; however, there has been no study regarding the influence of Stellar Activity on the retrieved transmission spectra through chromatic Rossiter-McLaughlin. In this study with the modified SOAP3.0 tool, we consider different types of Stellar Activity features (spots and plages), and we generated a large number of realistic chromatic Rossiter-McLaughlin curves for different types of planets and stars. We were then able to retrieve their transmission spectra to evaluate the impact of Stellar Activity on them. We find that chromatic Rossiter-McLaughlin observations are also not immune to Stellar Activity, which can mimic broadband features, such as Rayleigh scattering slope, in their retrieved transmission spectra. We also find that the influence is independent of the planet radius, orbital orientations, orbital period, and Stellar rotation rate. However, more general simulations demonstrate that the probability of mimicking strong broadband features is lower than 25% and that can be mitigated by combining several Rossiter-McLaughlin observations obtained during several transits.
-
Distinguishing the albedo of exoplanets from Stellar Activity
Astronomy & Astrophysics, 2018Co-Authors: L. M. Serrano, Nuno C. Santos, Susana C. C. Barros, Mahmoudreza Oshagh, J. P. Faria, Olivier Demangeon, S. G. Sousa, Monika LendlAbstract:Light curves show the flux variation from the target star and its orbiting planets as a function of time. In addition to the transit features created by the planets, the flux also includes the reflected light component of each planet, which depends on the planetary albedo. This signal is typically referred to as phase curve and could be easily identified if there were no additional noise. As well as instrumental noise, Stellar Activity, such as spots, can create a modulation in the data, which may be very difficult to distinguish from the planetary signal. We analyze the limitations imposed by the Stellar Activity on the detection of the planetary albedo, considering the limitations imposed by the predicted level of instrumental noise and the short duration of the observations planned in the context of the CHEOPS mission. As initial condition, we have assumed that each star is characterized by just one orbiting planet. We built mock light curves that included a realistic Stellar Activity pattern, the reflected light component of the planet and an instrumental noise level, which we have chosen to be at the same level as predicted for CHEOPS. We then fit these light curves to try to recover the reflected light component, assuming the Activity patterns can be modeled with a Gaussian process.We estimate that at least one full Stellar rotation is necessary to obtain a reliable detection of the planetary albedo. This result is independent of the level of noise, but it depends on the limitation of the Gaussian process to describe the Stellar Activity when the light curve time-span is shorter than the Stellar rotation. Finally, in presence of typical CHEOPS gaps in the simulations, we confirm that it is still possible to obtain a reliable albedo.
-
Understanding Stellar Activity-induced radial velocity jitter using simultaneous K2 photometry and HARPS RV measurements
Astronomy & Astrophysics, 2017Co-Authors: Mahmoudreza Oshagh, Nuno C. Santos, Susana C. C. Barros, J. P. Faria, Pedro Figueira, Christopher A Watson, V. Adibekyan, H. M. Cegla, Jean-francois Donati, Xavier DumusqueAbstract:One of the best ways to improve our understanding of the Stellar Activity-induced signal in radial velocity (RV) measurements is through simultaneous high-precision photometric and RV observations. This is of prime importance to mitigate the RV signal induced by Stellar Activity and therefore unveil the presence of low-mass exoplanets. The K2 Campaign 7 and 8 field-of-views were located in the southern hemisphere, and provided a unique opportunity to gather unprecedented simultaneous high precision photometric observation with K2 and high-precision RV measurements with the HARPS spectrograph to study the relationship between photometric variability and RV jitter. We observed nine stars with different levels of Activity; from quiet to very active. We probe the presence of any meaningful relation between measured RV jitter and the simultaneous photometric variation, and also other Activity indicators (e.g. BIS, FWHM, $logR'_{HK}$, and F8), by evaluating the strength and significance of the correlation between RVs and each indicator. We found that for the case of very active stars, strong and significant correlations exist between almost all the observables and measured RVs; however, for lower Activity levels the correlations become random. Except for the F8 which its strong correlation with RV jitter persists over a wide range of Stellar Activity level, and thus our result suggests that F8 might be a powerful proxy for Activity induced RV jitter. Moreover, we examine the capability of two state-of-the-art modeling techniques, namely the FF' method and SOAP2.0, in accurately predicting the RV jitter amplitude using the simultaneous photometric observation. We found that for the very active stars both techniques can reasonably well predict the amplitude of the RV jitter, however, at lower Activity levels the FF' method underpredicts the RV jitter amplitude.
-
Can Stellar Activity make a planet seem misaligned
Astronomy & Astrophysics, 2016Co-Authors: Mahmoudreza Oshagh, Nuno C. Santos, Stefan Dreizler, Pedro Figueira, Ansgar ReinersAbstract:Several studies have shown that the occultation of Stellar active regions by the transiting planet can generate anomalies in the high-precision transit light curves, and these anomalies may lead to an inaccurate estimate of the planetary parameters (e.g., the planet radius). Since the physics and geometry behind the transit light curve and the Rossiter- McLaughlin effect (spectroscopic transit) are the same, the Rossiter-McLaughlin observations are expected to be affected by the occultation of Stellar active regions in a similar way. In this paper we perform a fundamental test on the spin-orbit angles as derived by Rossiter-McLaughlin measurements, and we examine the impact of the occultation of Stellar active regions by the transiting planet on the spin-orbit angle estimations. Our results show that the inaccurate estimation on the spin-orbit angle due to Stellar Activity can be quite significant (up to 30 degrees), particularly for the edge-on, aligned, and small transiting planets. Therefore, our results suggest that the aligned transiting planets are the ones that can be easily misinterpreted as misaligned owing to the Stellar Activity. In other words, the biases introduced by ignoring Stellar Activity are unlikely to be the culprit for the highly misaligned systems.
-
Uncovering the planets and Stellar Activity of CoRoT-7 using only radial velocities
Astronomy & Astrophysics, 2016Co-Authors: J. P. Faria, Mahmoudreza Oshagh, Pedro Figueira, A Santerne, Raphaëlle D. Haywood, Brendon J. Brewer, Nuno C. SantosAbstract: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.
Xavier Bonfils - One of the best experts on this subject based on the ideXlab platform.
-
Disentangling between Stellar Activity and planetary signals
Astronomy & Astrophysics, 2011Co-Authors: Isabelle Boisse, François Bouchy, Guillaume Hébrard, Nuno C. Santos, Xavier Bonfils, S. VauclairAbstract:Photospheric Stellar Activity (i.e. dark spots or bright plages) might be an important source of noise and confusion in Stellar radial-velocity (RV) measurements. Radial-velocimetry planet search surveys as well as follow-up of photometric transit surveys require a deeper understanding and characterization of the effects of Stellar activities to differentiate them from planetary signals. We simulate dark spots on a rotating Stellar photosphere. The variations in the photometry, RV, and spectral line shapes are characterized and analyzed according to the Stellar inclination, the latitude, and the number of spots. We show that the anti-correlation between RV and bisector span, known to be a signature of Activity, requires a good sampling to be resolved when there are several spots on the photosphere. The Lomb-Scargle periodograms of the RV variations induced by Activity present power at the rotational period P rot of the star and its two first harmonics P rot /2 and P rot /3. Three adjusted sinusoids fixed at the fundamental period and its two-first harmonics allow us to remove about 90% of the RV jitter amplitude. We apply and validate our approach on four known active planet-host stars: HD 189733, GJ 674, CoRoT-7, and Hor. We succeed in fitting simultaneously Activity and planetary signals on GJ674 and CoRoT-7. This simultaneous modeling of the Activity and planetary parameters leads to slightly higher masses of CoRoT-7b and c of respectively, 5.7 ± 2.5 M Earth and 13.2 ± 4.1 M Earth . The larger uncertainties properly take into account the Stellar active jitter. We exclude short-period low-mass exoplanets around Hor. For data with realistic time-sampling and white Gaussian noise, we use simulations to show that our approach is effective in distinguishing reflex-motion due to a planetary companion and Stellar-Activity-induced RV variations provided that 1) the planetary orbital period is not close to that of the Stellar rotation or one of its two first harmonics; 2) the semi-amplitude of the planet exceeds ∼30% of the semi-amplitude of the active signal; 3) the rotational period of the star is accurately known, and 4) the data cover more than one Stellar rotational period.
-
Disentangling between Stellar Activity and planetary signals
Proceedings of the International Astronomical Union, 2010Co-Authors: Isabelle Boisse, François Bouchy, Guillaume Hébrard, Xavier Bonfils, Nuno Santos, S. VauclairAbstract:AbstractPhotospheric Stellar Activity (i.e. dark spots or bright plages) might be an important source of noise and confusion in the radial-velocity (RV) measurements. Radial-velocimetry planet search surveys as well as follow-up of photometric transit surveys require a deeper understanding and characterization of the effects of Stellar activities to disentangle it from planetary signals.We simulate dark spots on a rotating Stellar photosphere. The variations of the RV are characterized and analyzed according to the Stellar inclination, the latitude and the number of spots. The Lomb-Scargle periodograms of the RV variations induced by Activity present power at the rotational period Prot of the star and its two-first harmonics Prot/2 and Prot/3. Three adjusted sinusoids fixed at the fundamental period and its two-first harmonics allow to remove about 90% of the RV jitter amplitude. We apply and validate our approach on four known active planet-host stars: HD 189733, GJ 674, CoRoT-7 and ι Hor. We succeed in fitting simultaneously Activity and planetary signals on GJ674 and CoRoT-7. We excluded short-period low-mass exoplanets around ι Hor. Our approach is efficient to disentangle reflex-motion due to a planetary companion and Stellar-Activity induced-RV variations provided that 1) the planetary orbital period is not close to that of the Stellar rotation or one of its two-first harmonics, 2) the rotational period of the star is accurately known, 3) the data cover more than one Stellar rotational period.
-
Disentangling Stellar Activity and planetary signals
Proceedings of the International Astronomical Union, 2010Co-Authors: Isabelle Boisse, François Bouchy, Guillaume Hébrard, Nuno C. Santos, Xavier Bonfils, S. VauclairAbstract:Photospheric Stellar Activity might be an important source of noise and confusion in the radial-velocity measurements. RV planet search surveys as well as follow-up of photometric transit surveys require a deeper understanding and characterization of the effects of Stellar activities to disentangle it from planetary signals. We simulate dark spots on a rotating Stellar photosphere. The variations of the photometry, RV and spectral line shapes are characterized and analyzed according to the Stellar inclination, the latitude and the number of spots. The Lomb-Scargle periodograms of the RV variations induced by Activity present power at the rotational period Prot of the star and its two-first harmonics Prot/2 and Prot/3. Three adjusted sinusoids fixed at Prot and its two-first harmonics allow to remove about 90% of the RV jitter amplitude. We apply and validate our approach on four known active planet-host stars: HD189733, GJ674, CoRoT-7 and iHor. We succeed in fitting simultaneously Activity and planetary signals on GJ674 and CoRoT-7. This simultaneous modeling of the Activity and planetary parameters leads to slightly larger masses of CoRoT-7b and c: respectively, 5.7+/-2.5ME and 13.1+/-4.1ME. The larger uncertainties take into account properly for the Stellar active jitter. We excluded short-period low-mass exoplanets around iHor. For data with realistic time-sampling and white Gaussian noise, we use simulations to show that our approach is efficient to disentangle reflex-motion due to a planetary companion and Stellar-Activity induced-RV variations provided that 1) the planetary orbital period is not close to that of the Stellar rotation or one of its two-first harmonics 2) the semi-amplitude of the planet exceeds 30% of the semi-amplitude of the active signal 3) the rotational period of the star is accurately known 4) the data cover more than one Stellar rotational period.
-
Stellar Activity of planetary host star hd 189 733
Astronomy and Astrophysics, 2009Co-Authors: I Boisse, C. Moutou, Frederic Pont, Xavier Bonfils, F. Bouchy, G. Hebrard, Bryce Croll, A Vidalmadjar, Xavier DelfosseAbstract:Aims. Extra-solar planet search programs require high-precision velocity measurements. They need to determine how to differentiate between radial-velocity variations due to Doppler motion and the noise induced by Stellar Activity. Methods. We monitored the active K2V star HD 189733 and its transiting planetary companion, which has a 2.2-day orbital period. We used the high-resolution spectograph SOPHIE mounted on the 1.93-m telescope at the Observatoire de Haute-Provence to obtain 55 spectra of HD 189733 over nearly two months. We refined the HD 189733b orbit parameters and placed limits on both the eccentricity and long-term velocity gradient. After subtracting the orbital motion of the planet, we compared the variability in spectroscopic Activity indices with the evolution in the radial-velocity residuals and the shape of spectral lines. Results. The radial velocity, the spectral-line profile, and the acti vity indices measured in He I (5875.62 A), Hα (6562.81 A), and both of the Ca II H&K lines (3968.47 A and 3933.66 A, respectively) exhibit a periodicity close to the Stellar-rotation pe riod and the correlations between them are consistent with a spotted Stellar surface in rotation. We used these correlations to corr ect for the radialvelocity jitter due to Stellar Activity. This results in ach ieving high precision in measuring the orbital parameters, with a semi-amplitude ◦ .
-
Stellar Activity of planetary host star HD 189733
Astronomy & Astrophysics, 2009Co-Authors: Isabelle Boisse, C. Moutou, Alfred Vidal-madjar, Frederic Pont, Xavier Bonfils, Xavier Delfosse, F. Bouchy, G. Hebrard, Bryce Croll, M. DesortAbstract:Extra-solar planet search programs require high-precision velocity measurements. They need to study how to disentangle radial-velocity variations due to Doppler motion from the noise induced by Stellar Activity. We monitored the active K2V star HD 189733 and its transiting planetary companion that has a 2.2-day orbital period. We used the high-resolution spectograph SOPHIE mounted on the 1.93-m telescope at the Observatoire de Haute-Provence to obtain 55 spectra of HD 189733 over nearly two months. We refined the HD 189733b orbit parameters and put limits on the eccentricity and on a long-term velocity gradient. After subtracting the orbital motion of the planet, we compared the variability of spectroscopic Activity indices to the evolution of the radial-velocity residuals and the shape of spectral lines. The radial velocity, the spectral-line profile and the Activity indices measured in HeI (5875.62 \AA), Halpha (6562.81 \AA) and the CaII H&K lines (3968.47 \AA and 3933.66 \AA, respectively) show a periodicity around the Stellar rotation period and the correlations between them are consistent with a spotted Stellar surface in rotation. We used such correlations to correct for the radial-velocity jitter due to Stellar Activity. This results in achieving high precision on the orbit parameters, with a semi-amplitude K = 200.56 \pm 0.88 m.s-1 and a derived planet mass of M_{P}=1.13 \pm 0.03 M$_{Jup}$.
Xavier Dumusque - One of the best experts on this subject based on the ideXlab platform.
-
Long-term Stellar Activity variations and their effect on radial-velocity measurements
Monthly Notices of the Royal Astronomical Society, 2021Co-Authors: Jean C Costes, James S. Jenkins, Xavier Dumusque, Christopher A Watson, Ernst De Mooij, Steven H Saar, Andrew Collier Cameron, David F Phillips, Maximilian N Günther, Annelies MortierAbstract:Abstract Long-term Stellar Activity variations can affect the detectability of long-period and Earth-analogue extrasolar planets. We have, for 54 stars, analysed the long-term trend of five Activity indicators: log $R^{\prime }_\mathrm{{HK}}$, the cross-correlation function (CCF) bisector span, CCF full-width-at-half-maximum, CCF contrast, and the area of the Gaussian fit to the CCF; and studied their correlation with the RVs. The sign of the correlations appears to vary as a function of Stellar spectral type, and the transition in sign signals a noteworthy change in the Stellar Activity properties where earlier type stars appear more plage dominated. These transitions become more clearly defined when considered as a function of the convective zone depth. Therefore, it is the convective zone depth (which can be altered by Stellar metallicity) that appears to be the underlying fundamental parameter driving the observed Activity correlations. In addition, for most of the stars, we find that the RVs become increasingly red-shifted as Activity levels increase, which can be explained by the increase in the suppression of convective blue-shift. However, we also find a minority of stars where the RVs become increasingly blue-shifted as Activity levels increase. Finally, using the correlation found between Activity indicators and RVs, we removed RV signals generated by long-term changes in Stellar Activity. We find that performing simple cleaning of such long-term signals enables improved planet detection at longer orbital periods.
-
Improving Exoplanet Detection Power: Multivariate Gaussian Process Models for Stellar Activity
arXiv: Instrumentation and Methods for Astrophysics, 2017Co-Authors: David E. Jones, D. C. Stenning, Eric B. Ford, Robert L. Wolpert, Thomas J. Loredo, Christian Gilbertson, Xavier DumusqueAbstract:The radial velocity method is one of the most successful techniques for detecting exoplanets. It works by detecting the velocity of a host star induced by the gravitational effect of an orbiting planet, specifically the velocity along our line of sight, which is called the radial velocity of the star. Low-mass planets typically cause their host star to move with radial velocities of 1 m/s or less. By analyzing a time series of Stellar spectra from a host star, modern astronomical instruments can in theory detect such planets. However, in practice, intrinsic Stellar variability (e.g., star spots, convective motion, pulsations) affects the spectra and often mimics a radial velocity signal. This signal contamination makes it difficult to reliably detect low-mass planets. A principled approach to recovering planet radial velocity signals in the presence of Stellar Activity was proposed by Rajpaul et al. (2015). It uses a multivariate Gaussian process model to jointly capture time series of the apparent radial velocity and multiple indicators of Stellar Activity. We build on this work in two ways: (i) we propose using dimension reduction techniques to construct new high-information Stellar Activity indicators; and (ii) we extend the Rajpaul et al. (2015) model to a larger class of models and use a power-based model comparison procedure to select the best model. Despite significant interest in exoplanets, previous efforts have not performed large-scale Stellar Activity model selection or attempted to evaluate models based on planet detection power. In the case of main sequence G2V stars, we find that our method substantially improves planet detection power compared to previous state-of-the-art approaches.
-
Understanding Stellar Activity-induced radial velocity jitter using simultaneous K2 photometry and HARPS RV measurements
Astronomy & Astrophysics, 2017Co-Authors: Mahmoudreza Oshagh, Nuno C. Santos, Susana C. C. Barros, J. P. Faria, Pedro Figueira, Christopher A Watson, V. Adibekyan, H. M. Cegla, Jean-francois Donati, Xavier DumusqueAbstract:One of the best ways to improve our understanding of the Stellar Activity-induced signal in radial velocity (RV) measurements is through simultaneous high-precision photometric and RV observations. This is of prime importance to mitigate the RV signal induced by Stellar Activity and therefore unveil the presence of low-mass exoplanets. The K2 Campaign 7 and 8 field-of-views were located in the southern hemisphere, and provided a unique opportunity to gather unprecedented simultaneous high precision photometric observation with K2 and high-precision RV measurements with the HARPS spectrograph to study the relationship between photometric variability and RV jitter. We observed nine stars with different levels of Activity; from quiet to very active. We probe the presence of any meaningful relation between measured RV jitter and the simultaneous photometric variation, and also other Activity indicators (e.g. BIS, FWHM, $logR'_{HK}$, and F8), by evaluating the strength and significance of the correlation between RVs and each indicator. We found that for the case of very active stars, strong and significant correlations exist between almost all the observables and measured RVs; however, for lower Activity levels the correlations become random. Except for the F8 which its strong correlation with RV jitter persists over a wide range of Stellar Activity level, and thus our result suggests that F8 might be a powerful proxy for Activity induced RV jitter. Moreover, we examine the capability of two state-of-the-art modeling techniques, namely the FF' method and SOAP2.0, in accurately predicting the RV jitter amplitude using the simultaneous photometric observation. We found that for the very active stars both techniques can reasonably well predict the amplitude of the RV jitter, however, at lower Activity levels the FF' method underpredicts the RV jitter amplitude.
-
Insights on the Spectral Signatures of Stellar Activity and Planets from PCA
The Astrophysical Journal, 2017Co-Authors: Allen B. Davis, Xavier Dumusque, Jessi Cisewski, Debra A. Fischer, Eric B. FordAbstract:Photospheric velocities and Stellar Activity features such as spots and faculae produce measurable radial velocity signals that currently obscure the detection of sub-meter-per-second planetary signals. However, photospheric velocities are imprinted differently in a high-resolution spectrum than Keplerian Doppler shifts. Photospheric Activity produces subtle differences in the shapes of absorption lines due to differences in how temperature or pressure affects the atomic transitions. In contrast, Keplerian Doppler shifts affect every spectral line in the same way. With high enough S/N and high enough resolution, statistical techniques can exploit differences in spectra to disentangle the photospheric velocities and detect lower-amplitude exoplanet signals. We use simulated disk-integrated time-series spectra and principal component analysis (PCA) to show that photospheric signals introduce spectral line variability that is distinct from Doppler shifts. We quantify the impact of instrumental resolution and S/N for this work.
-
Deriving Stellar Inclination of Slow Rotators Using Stellar Activity
The Astrophysical Journal, 2014Co-Authors: Xavier DumusqueAbstract:Stellar inclination is an important parameter for many astrophysical studies. Although different techniques allow us to estimate Stellar inclinationt for fast rotators, it becomes much more difficult when stars are rotating slower than $\sim2$-2.5 \kms. By using the new Activity simulation SOAP 2.0 that can reproduce the photometric and spectroscopic variations induced by Stellar Activity, we are able to fit observations of solar-type stars and derive their inclination. For HD189733, we estimate the Stellar inclination to be $i=84^{+6}_{-20}$ degrees, which implies a star-planet obliquity of $\psi=4^{+18}_{-4}$ considering previous measurements of the spin-orbit angle. For $\alpha$ Cen B, we derive an inclination of $i=45^{+9}_{-19}$, which implies that the rotational spin of the star is not aligned with the orbital spin of the $\alpha$ Cen binary system. In addition, assuming that $\alpha$ Cen Bb is aligned with its host star, no transit would occur. The inclination of $\alpha$ Cen B can be measured using 40 radial-velocity measurements, which is remarkable given that the projected rotational velocity of the star is smaller than $1.15\,km\,s^{-1}$.
Mahmoudreza Oshagh - One of the best experts on this subject based on the ideXlab platform.
-
Stellar Activity consequence on the retrieved transmission spectra through chromatic Rossiter-McLaughlin observations
Astronomy & Astrophysics, 2020Co-Authors: S. Boldt, Nuno C. Santos, Mahmoudreza Oshagh, Stefan Dreizler, Ansgar Reiners, M. Mallonn, Antonio Claret, E. SedaghatiAbstract:Mostly multiband photometric transit observations have been used so far to retrieve broadband transmission spectra of transiting exoplanets in order to study their atmosphere. An alternative method has been proposed and has only been used once to recover transmission spectra using chromatic Rossiter-McLaughlin observations. Stellar Activity has been shown to potentially imitate narrow and broadband features in the transmission spectra retrieved from multiband photometric observations; however, there has been no study regarding the influence of Stellar Activity on the retrieved transmission spectra through chromatic Rossiter-McLaughlin. In this study with the modified SOAP3.0 tool, we consider different types of Stellar Activity features (spots and plages), and we generated a large number of realistic chromatic Rossiter-McLaughlin curves for different types of planets and stars. We were then able to retrieve their transmission spectra to evaluate the impact of Stellar Activity on them. We find that chromatic Rossiter-McLaughlin observations are also not immune to Stellar Activity, which can mimic broadband features, such as Rayleigh scattering slope, in their retrieved transmission spectra. We also find that the influence is independent of the planet radius, orbital orientations, orbital period, and Stellar rotation rate. However, more general simulations demonstrate that the probability of mimicking strong broadband features is lower than 25% and that can be mitigated by combining several Rossiter-McLaughlin observations obtained during several transits.
-
Distinguishing the albedo of exoplanets from Stellar Activity
Astronomy & Astrophysics, 2018Co-Authors: L. M. Serrano, Nuno C. Santos, Susana C. C. Barros, Mahmoudreza Oshagh, J. P. Faria, Olivier Demangeon, S. G. Sousa, Monika LendlAbstract:Light curves show the flux variation from the target star and its orbiting planets as a function of time. In addition to the transit features created by the planets, the flux also includes the reflected light component of each planet, which depends on the planetary albedo. This signal is typically referred to as phase curve and could be easily identified if there were no additional noise. As well as instrumental noise, Stellar Activity, such as spots, can create a modulation in the data, which may be very difficult to distinguish from the planetary signal. We analyze the limitations imposed by the Stellar Activity on the detection of the planetary albedo, considering the limitations imposed by the predicted level of instrumental noise and the short duration of the observations planned in the context of the CHEOPS mission. As initial condition, we have assumed that each star is characterized by just one orbiting planet. We built mock light curves that included a realistic Stellar Activity pattern, the reflected light component of the planet and an instrumental noise level, which we have chosen to be at the same level as predicted for CHEOPS. We then fit these light curves to try to recover the reflected light component, assuming the Activity patterns can be modeled with a Gaussian process.We estimate that at least one full Stellar rotation is necessary to obtain a reliable detection of the planetary albedo. This result is independent of the level of noise, but it depends on the limitation of the Gaussian process to describe the Stellar Activity when the light curve time-span is shorter than the Stellar rotation. Finally, in presence of typical CHEOPS gaps in the simulations, we confirm that it is still possible to obtain a reliable albedo.
-
Understanding Stellar Activity-induced radial velocity jitter using simultaneous K2 photometry and HARPS RV measurements
Astronomy & Astrophysics, 2017Co-Authors: Mahmoudreza Oshagh, Nuno C. Santos, Susana C. C. Barros, J. P. Faria, Pedro Figueira, Christopher A Watson, V. Adibekyan, H. M. Cegla, Jean-francois Donati, Xavier DumusqueAbstract:One of the best ways to improve our understanding of the Stellar Activity-induced signal in radial velocity (RV) measurements is through simultaneous high-precision photometric and RV observations. This is of prime importance to mitigate the RV signal induced by Stellar Activity and therefore unveil the presence of low-mass exoplanets. The K2 Campaign 7 and 8 field-of-views were located in the southern hemisphere, and provided a unique opportunity to gather unprecedented simultaneous high precision photometric observation with K2 and high-precision RV measurements with the HARPS spectrograph to study the relationship between photometric variability and RV jitter. We observed nine stars with different levels of Activity; from quiet to very active. We probe the presence of any meaningful relation between measured RV jitter and the simultaneous photometric variation, and also other Activity indicators (e.g. BIS, FWHM, $logR'_{HK}$, and F8), by evaluating the strength and significance of the correlation between RVs and each indicator. We found that for the case of very active stars, strong and significant correlations exist between almost all the observables and measured RVs; however, for lower Activity levels the correlations become random. Except for the F8 which its strong correlation with RV jitter persists over a wide range of Stellar Activity level, and thus our result suggests that F8 might be a powerful proxy for Activity induced RV jitter. Moreover, we examine the capability of two state-of-the-art modeling techniques, namely the FF' method and SOAP2.0, in accurately predicting the RV jitter amplitude using the simultaneous photometric observation. We found that for the very active stars both techniques can reasonably well predict the amplitude of the RV jitter, however, at lower Activity levels the FF' method underpredicts the RV jitter amplitude.
-
Can Stellar Activity make a planet seem misaligned
Astronomy & Astrophysics, 2016Co-Authors: Mahmoudreza Oshagh, Nuno C. Santos, Stefan Dreizler, Pedro Figueira, Ansgar ReinersAbstract:Several studies have shown that the occultation of Stellar active regions by the transiting planet can generate anomalies in the high-precision transit light curves, and these anomalies may lead to an inaccurate estimate of the planetary parameters (e.g., the planet radius). Since the physics and geometry behind the transit light curve and the Rossiter- McLaughlin effect (spectroscopic transit) are the same, the Rossiter-McLaughlin observations are expected to be affected by the occultation of Stellar active regions in a similar way. In this paper we perform a fundamental test on the spin-orbit angles as derived by Rossiter-McLaughlin measurements, and we examine the impact of the occultation of Stellar active regions by the transiting planet on the spin-orbit angle estimations. Our results show that the inaccurate estimation on the spin-orbit angle due to Stellar Activity can be quite significant (up to 30 degrees), particularly for the edge-on, aligned, and small transiting planets. Therefore, our results suggest that the aligned transiting planets are the ones that can be easily misinterpreted as misaligned owing to the Stellar Activity. In other words, the biases introduced by ignoring Stellar Activity are unlikely to be the culprit for the highly misaligned systems.
-
Uncovering the planets and Stellar Activity of CoRoT-7 using only radial velocities
Astronomy & Astrophysics, 2016Co-Authors: J. P. Faria, Mahmoudreza Oshagh, Pedro Figueira, A Santerne, Raphaëlle D. Haywood, Brendon J. Brewer, Nuno C. SantosAbstract: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.
Xavier Delfosse - One of the best experts on this subject based on the ideXlab platform.
-
Stellar Activity of planetary host star hd 189 733
Astronomy and Astrophysics, 2009Co-Authors: I Boisse, C. Moutou, Frederic Pont, Xavier Bonfils, F. Bouchy, G. Hebrard, Bryce Croll, A Vidalmadjar, Xavier DelfosseAbstract:Aims. Extra-solar planet search programs require high-precision velocity measurements. They need to determine how to differentiate between radial-velocity variations due to Doppler motion and the noise induced by Stellar Activity. Methods. We monitored the active K2V star HD 189733 and its transiting planetary companion, which has a 2.2-day orbital period. We used the high-resolution spectograph SOPHIE mounted on the 1.93-m telescope at the Observatoire de Haute-Provence to obtain 55 spectra of HD 189733 over nearly two months. We refined the HD 189733b orbit parameters and placed limits on both the eccentricity and long-term velocity gradient. After subtracting the orbital motion of the planet, we compared the variability in spectroscopic Activity indices with the evolution in the radial-velocity residuals and the shape of spectral lines. Results. The radial velocity, the spectral-line profile, and the acti vity indices measured in He I (5875.62 A), Hα (6562.81 A), and both of the Ca II H&K lines (3968.47 A and 3933.66 A, respectively) exhibit a periodicity close to the Stellar-rotation pe riod and the correlations between them are consistent with a spotted Stellar surface in rotation. We used these correlations to corr ect for the radialvelocity jitter due to Stellar Activity. This results in ach ieving high precision in measuring the orbital parameters, with a semi-amplitude ◦ .
-
Stellar Activity of planetary host star HD 189733
Astronomy & Astrophysics, 2009Co-Authors: Isabelle Boisse, C. Moutou, Alfred Vidal-madjar, Frederic Pont, Xavier Bonfils, Xavier Delfosse, F. Bouchy, G. Hebrard, Bryce Croll, M. DesortAbstract:Extra-solar planet search programs require high-precision velocity measurements. They need to study how to disentangle radial-velocity variations due to Doppler motion from the noise induced by Stellar Activity. We monitored the active K2V star HD 189733 and its transiting planetary companion that has a 2.2-day orbital period. We used the high-resolution spectograph SOPHIE mounted on the 1.93-m telescope at the Observatoire de Haute-Provence to obtain 55 spectra of HD 189733 over nearly two months. We refined the HD 189733b orbit parameters and put limits on the eccentricity and on a long-term velocity gradient. After subtracting the orbital motion of the planet, we compared the variability of spectroscopic Activity indices to the evolution of the radial-velocity residuals and the shape of spectral lines. The radial velocity, the spectral-line profile and the Activity indices measured in HeI (5875.62 \AA), Halpha (6562.81 \AA) and the CaII H&K lines (3968.47 \AA and 3933.66 \AA, respectively) show a periodicity around the Stellar rotation period and the correlations between them are consistent with a spotted Stellar surface in rotation. We used such correlations to correct for the radial-velocity jitter due to Stellar Activity. This results in achieving high precision on the orbit parameters, with a semi-amplitude K = 200.56 \pm 0.88 m.s-1 and a derived planet mass of M_{P}=1.13 \pm 0.03 M$_{Jup}$.
-
Stellar Activity of planetary host star HD 189 733
Astronomy and Astrophysics - A&A, 2009Co-Authors: I Boisse, C. Moutou, Alfred Vidal-madjar, Frederic Pont, Xavier Bonfils, Xavier Delfosse, F. Bouchy, G. Hebrard, Bryce Croll, M. DesortAbstract:Aims: Extra-solar planet search programs require high-precision velocity measurements. They need to determine how to differentiate between radial-velocity variations due to Doppler motion and the noise induced by Stellar Activity. Methods: We monitored the active K2V star HD 189 733 and its transiting planetary companion, which has a 2.2-day orbital period. We used the high-resolution spectograph SOPHIE mounted on the 1.93-m telescope at the Observatoire de Haute-Provence to obtain 55 spectra of HD 189 733 over nearly two months. We refined the HD 189 733b orbit parameters and placed limits on both the eccentricity and long-term velocity gradient. After subtracting the orbital motion of the planet, we compared the variability in spectroscopic Activity indices with the evolution in the radial-velocity residuals and the shape of spectral lines. Results: The radial velocity, the spectral-line profile, and the Activity indices measured in He I (5875.62 Å), Hα (6562.81 Å), and both of the Ca II H&K lines (3968.47 Å and 3933.66 Å, respectively) exhibit a periodicity close to the Stellar-rotation period and the correlations between them are consistent with a spotted Stellar surface in rotation. We used these correlations to correct for the radial-velocity jitter due to Stellar Activity. This results in achieving high precision in measuring the orbital parameters, with a semi-amplitude K = 200.56 ± 0.88 m s-1 and a derived planet mass of MP = 1.13 ± 0.03 M_Jup. Based on observations collected with the SOPHIE spectrograph on the 1.93-m telescope at Observatoire de Haute-Provence (CNRS), France, by the SOPHIE Consortium (program 07A.PNP.CONS).
-
Stellar Activity of planetary host star HD 189733
Proceedings of the International Astronomical Union, 2008Co-Authors: Isabelle Boisse, François Bouchy, Guillaume Hébrard, C. Moutou, Alfred Vidal-madjar, Frederic Pont, Xavier Bonfils, Xavier Delfosse, M. Desort, T. ForveilleAbstract:Exoplanet search programs need to study how to disentangle radial-velocity (RV) variations due to Doppler motion and the noise induced by Stellar Activity. We monitored the active K2V HD 189733 with the high-resolution SOPHIE spectrograph (OHP, France). We refined the orbital parameters of HD 189733b and put limitations on the eccentricity and on a long-term velocity gradient. We subtracted the orbital motion of the planet and compared the variability of Activity spectroscopic indices (HeI, Hα, Ca II H&K lines) to the evolution of the RV residuals and the shape of spectral lines. All are in agreement with an active Stellar surface in rotation. We used such correlations to correct for the RV jitter due to Stellar Activity. This results in achieving a high precision on the orbital parameters, with a semi-amplitude: K =200.56±0.88 m ⋅ s −1 and a derived planet mass of M P =1.13±0.03 M Jup .