Oblique rings from migrating exomoons: A possible origin for long-period exoplanets with enlarged radii

Context. The extremely low density of several long-period exoplanets in mature systems is still unexplained -- with HIP 41378 f being archetypical of this category. It has been proposed that such planets could actually have normal densities but be surrounded by a ring observed approximately face on, mimicking the transit depth of a puffy planet. This would imply that the equator of the planet is nearly perpendicular to its orbit plane, which is at odds with the formation process of gas giants. Yet, in the context of the Solar System planets, it has been shown that after gigayears of evolution, the tidal migration of a moon can naturally lead to a very tilted planet with a ring. Aims. As exomoons are expected to be ubiquitous around giant exoplanets, this mechanism may be responsible for the anomalous radii of some observed exoplanets. In preparation for the future discoveries of the PLATO mission, we present a simple method for checking the plausibility of this mechanism for a given exoplanet. Methods. Analytical formulas give the probability density function of the relevant precession harmonics of the planet. For each harmonic, simple criteria set the moon mass and other properties required for the mechanism to operate. Results. We applied this methodology to HIP 41378 f, and we show that in order to reproduce the observed configuration, a hypothetical former moon should have had a moon-to-planet mass ratio of a few times 1e-4 (i.e. roughly the mass of our Moon) and have migrated over a distance of a few planet's radii on a gigayear timescale. These orders of magnitude match the properties of moons expected to exist around gaseous exoplanets. Conclusions. We conclude that the migration of a former moon is a viable formation pathway for the proposed ring and tilt of HIP 41378 f. This example strengthens the ring hypothesis and motivates its application to other targets.Comment: Accepted for publication in Astronomy and Astrophysic

Activity time series of old stars from late F to early K VI. Exoplanet mass characterisation and detectability in radial velocity

Stellar variability impacts radial velocities at various timescales and therefore the detectability of exoplanets and the mass determination based on this technique. It is necessary to implement systematic studies, to delineate the current limitations of RV techniques to detect Earth-like planets. This paper aims are to investigate whether the targeted 10% mass uncertainty from RV follow-up of transits detected by PLATO can be reached, and to analyse and quantify Earth-like planet detectability for various spectral types. We implemented blind tests based on a large data set of realistic synthetic time series reproducing different phenomena leading to stellar variability such as complex magnetic activity patterns as well as flows, covering F6-K4 stars and a wide range of activity levels. The 10% mass uncertainty for a 1 MEarth in the habitable zone of a G2 star cannot be reached, even with an improved version of a usual correction of stellar activity and even for long-duration (ten years) well-sampled observations. This level can be reached for masses above 3 MEarth or for K4 stars alone. We quantify the maximum dispersion of the RV residuals needed to reach this 10% level, assuming the correction method and models do not affect the planetary signal. Several other methods were tested and do not allow a significantly improvement of this limited performance. Similarly, such low-mass planets in the habitable zone cannot be detected with a similar correction: blind tests lead to very low detection rates for 1 MEarth and a very high level of false positives. Very significant and new improvements with respect to methods based on activity indicators to correct for stellar activity must be devised at all timescales to reach the objective of 10% uncertainty on the mass or to detect such planets in RV. Methods based on the correlation with activity indicators are unlikely to be sufficient.Comment: Paper accepted in Astronomy and Astrophysic

Méthodes statistiques utilisant des simulations hydrodynamiques d'atmosphères stellaires pour détecter des exoplanètes en vitesse radiale

Considering a time series affected by a colored noise of unknown statistics, a difficulty for periodic signal detection is to control the true significance level at which the detection tests are conducted. The objective of this thesis is to develop a new method using training datasets of the noise to improve this control. For the case of regularly sampled observations, we analyze the performances of various detectors applied to periodograms standardized using the noise training datasets. The proposed standardization leads, in some cases, to powerful constant false alarm rate tests. Thanks to the development of the asymptotical distribution of the standardized periodogram, we derive analytical expressions for the false alarm and detection rates of several tests. In the case of irregular sampling, we show that it is possible to combine the proposed periodogram standardization and bootstrap techniques to consistently estimate the false alarm rate. We also show that the procedure can be improved by using generalized extreme value distributions. This study has been applied to the case of extrasolar planet detection in radial velocity (RV) data. The main barrier to detect Earth-mass planets comes from the host star activity, as the convection at the stellar surface. This work investigates the possibility of using hydrodynamic simulations of the stellar convection in the detection process to control exoplanet detection claims.Considérant une série temporelle affectée par un bruit coloré dont les propriétés statistiques sont inconnues, la difficulté pour la détection de signaux périodiques est de contrôler le degré de confiance avec lequel les tests de détection rejettent l'hypothèse nulle en faveur de l'hypothèse alternative. L'objectif de cette thèse est de développer une nouvelle méthode utilisant des séries temporelles simulées du bruit pour améliorer ce contrôle. Dans le cas d'un échantillonnage régulier, nous avons analysé les performances de différents tests de détection appliqués à un périodogramme standardisé par le spectre simulé du bruit. La standardisation proposée entraîne, dans la majorité des cas, des tests de détection puissants dont les taux de fausses alarmes sont constants. Grâce au développement des distributions asymptotiques de ce périodogramme, nous avons déterminé des expressions analytiques pour les probabilités de fausses alarmes (PFA) et de détections de différents tests. Dans le cas d'un échantillonnage irrégulier, nous montrons qu'il est possible de combiner la standardisation proposée du périodogramme avec des techniques de bootstrap pour contrôler la PFA de manière fiable. La procédure peut être optimisée en utilisant les valeurs extrêmes généralisées. Cette étude a été appliquée au cas de la détection de planètes extrasolaires par la méthode des vitesses radiales dont l'une des principales barrières pour détecter des planètes de masse terrestre vient de l'activité de l'étoile hôte, notamment la convection de surface. Le travail effectué dans cette thèse a porté sur la contribution de simulations hydrodynamiques du bruit convectif dans le processus de détection

Statistical methods using hydrodynamic simulations of stellar atmospheres for detecting exoplanets in radial velocity data

Considérant une série temporelle affectée par un bruit coloré dont les propriétés statistiques sont inconnues, la difficulté pour la détection de signaux périodiques est de contrôler le degré de confiance avec lequel les tests de détection rejettent l'hypothèse nulle en faveur de l'hypothèse alternative. L'objectif de cette thèse est de développer une nouvelle méthode utilisant des séries temporelles simulées du bruit pour améliorer ce contrôle. Dans le cas d'un échantillonnage régulier, nous avons analysé les performances de différents tests de détection appliqués à un périodogramme standardisé par le spectre simulé du bruit. La standardisation proposée entraîne, dans la majorité des cas, des tests de détection puissants dont les taux de fausses alarmes sont constants. Grâce au développement des distributions asymptotiques de ce périodogramme, nous avons déterminé des expressions analytiques pour les probabilités de fausses alarmes (PFA) et de détections de différents tests. Dans le cas d'un échantillonnage irrégulier, nous montrons qu'il est possible de combiner la standardisation proposée du périodogramme avec des techniques de bootstrap pour contrôler la PFA de manière fiable. La procédure peut être optimisée en utilisant les valeurs extrêmes généralisées. Cette étude a été appliquée au cas de la détection de planètes extrasolaires par la méthode des vitesses radiales dont l'une des principales barrières pour détecter des planètes de masse terrestre vient de l'activité de l'étoile hôte, notamment la convection de surface. Le travail effectué dans cette thèse a porté sur la contribution de simulations hydrodynamiques du bruit convectif dans le processus de détection.Considering a time series affected by a colored noise of unknown statistics, a difficulty for periodic signal detection is to control the true significance level at which the detection tests are conducted. The objective of this thesis is to develop a new method using training datasets of the noise to improve this control. For the case of regularly sampled observations, we analyze the performances of various detectors applied to periodograms standardized using the noise training datasets. The proposed standardization leads, in some cases, to powerful constant false alarm rate tests. Thanks to the development of the asymptotical distribution of the standardized periodogram, we derive analytical expressions for the false alarm and detection rates of several tests. In the case of irregular sampling, we show that it is possible to combine the proposed periodogram standardization and bootstrap techniques to consistently estimate the false alarm rate. We also show that the procedure can be improved by using generalized extreme value distributions. This study has been applied to the case of extrasolar planet detection in radial velocity (RV) data. The main barrier to detect Earth-mass planets comes from the host star activity, as the convection at the stellar surface. This work investigates the possibility of using hydrodynamic simulations of the stellar convection in the detection process to control exoplanet detection claims

Oblique rings from migrating exomoons: A possible origin for long-period exoplanets with enlarged radii

Accepted for publication in Astronomy and AstrophysicsInternational audienceContext. The extremely low density of several long-period exoplanets in mature systems is still unexplained -- with HIP 41378 f being archetypical of this category. It has been proposed that such planets could actually have normal densities but be surrounded by a ring observed approximately face on, mimicking the transit depth of a puffy planet. This would imply that the equator of the planet is nearly perpendicular to its orbit plane, which is at odds with the formation process of gas giants. Yet, in the context of the Solar System planets, it has been shown that after gigayears of evolution, the tidal migration of a moon can naturally lead to a very tilted planet with a ring.Aims. As exomoons are expected to be ubiquitous around giant exoplanets, this mechanism may be responsible for the anomalous radii of some observed exoplanets. In preparation for the future discoveries of the PLATO mission, we present a simple method for checking the plausibility of this mechanism for a given exoplanet.Methods. Analytical formulas give the probability density function of the relevant precession harmonics of the planet. For each harmonic, simple criteria set the moon mass and other properties required for the mechanism to operate.Results. We applied this methodology to HIP 41378 f, and we show that in order to reproduce the observed configuration, a hypothetical former moon should have had a moon-to-planet mass ratio of a few times 1e-4 (i.e. roughly the mass of our Moon) and have migrated over a distance of a few planet's radii on a gigayear timescale. These orders of magnitude match the properties of moons expected to exist around gaseous exoplanets.Conclusions. We conclude that the migration of a former moon is a viable formation pathway for the proposed ring and tilt of HIP 41378 f. This example strengthens the ring hypothesis and motivates its application to other targets

Observations of the nightside venusian hydrogen corona with SPICAV/VEX

International audienceObservations of the Venusian hydrogen corona on the nightside (8 PM local time) have been performed with SPICAV instrument on board Venus Express from 12 to 15 October 2011. These observations were associated with interplanetary observations obtained at apoapsis to subtract the interplanetary contribution. The observed brightness variation with altitude on the disk and above the limb shows that one part of the interplanetary emission is scattered out of the line of sight by the Venusian hydrogen atoms above the CO2 absorption limb. The emission of the H corona beyond the cylindrical shadow is also scattered in the same way. Taking into account the geometry of the line of sight and this scattering in a radiative transfer model allows us to retrieve for the first time the vertical profile of the hydrogen density on the nightside of Venus from Lyman-α measurements. The derived cold hydrogen density at the exobase is ∼ 40 times larger than the dayside hydrogen density derived by the same instrument, showing an evening bulge in agreement with in situ measurements of Pioneer Venus. The hot hydrogen density derived at the nightside is ∼ 5 times larger than the average dayside hot hydrogen density found previously with SPICAV/Venus Express. A decrease of the hot hydrogen content by a factor ∼ 2 is observed during these three days of observations. This decrease could be explained by a decrease of the hot hydrogen production rate on the nightside driven by a variation of the dayside ionosphere

Observation of the Venusian nightside hydrogen corona by SPICAV/VEX

Atomic hydrogen in the upper atmosphere of Venus is produced by chemical reactions involving hydrogen bearing molecules such as H2O. Due to its low mass, atomic hydrogen can reach high altitudes and become the dominant species in the Venusian exosphere. In the Venusian upper atmosphere, ions-neutral interactions could produce the non-thermal hydrogen population that has been observed by numerous past missions and confirmed recently by Venus Express missions at dayside. Observations of the nightside Venusian hydrogen corona have been performed in October 2011 by SPICAV-UV on Venus Express. Both disk and limb have been observed. With these observations,we also measured the hydrogen content at nightside by absorption of the interplanetary emission. This second method provides us an absolute measurement of the hydrogen content independent on the absolute calibration of the instrument. Results from both emission and absorption will be presented

Connecting photometric and spectroscopic granulation signals with CHEOPS and ESPRESSO

S. Sulis et al.[Context] Stellar granulation generates fluctuations in photometric and spectroscopic data whose properties depend on the stellar type, composition, and evolutionary state. Characterizing granulation is key for understanding stellar atmospheres and detecting planets.[Aims] We aim to detect the signatures of stellar granulation, link spectroscopic and photometric signatures of convection for main-sequence stars, and test predictions from 3D hydrodynamic models.[Methods] For the first time, we observed two bright stars (Teff = 5833 and 6205 K) with high-precision observations taken simultaneously with CHEOPS and ESPRESSO. We analyzed the properties of the stellar granulation signal in each individual dataset. We compared them to Kepler observations and 3D hydrodynamic models. While isolating the granulation-induced changes by attenuating and filtering the p-mode oscillation signals, we studied the relationship between photometric and spectroscopic observables.[Results] The signature of stellar granulation is detected and precisely characterized for the hotter F star in the CHEOPS and ESPRESSO observations. For the cooler G star, we obtain a clear detection in the CHEOPS dataset only. The TESS observations are blind to this stellar signal. Based on CHEOPS observations, we show that the inferred properties of stellar granulation are in agreement with both Kepler observations and hydrodynamic models. Comparing their periodograms, we observe a strong link between spectroscopic and photometric observables. Correlations of this stellar signal in the time domain (flux versus radial velocities, RV) and with specific spectroscopic observables (shape of the cross-correlation functions) are however difficult to isolate due to S/N dependent variations.[Conclusions] In the context of the upcoming PLATO mission and the extreme precision RV surveys, a thorough understanding of the properties of the stellar granulation signal is needed. The CHEOPS and ESPRESSO observations pave the way for detailed analyses of this stellar process.M.L. acknowledges support of the Swiss National Science Foundation under grant number PCEFP2_194576. The contributions of M.L. and A.K. have been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation. H.M.C. acknowledges support from the UKRI under a Future Leader Fellowship (grant number MR/S035214/1). Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (Grant agreement no.: DNRF106). L.F.R.D. acknowledges support from the Independent Research Fund Denmark (Research grant 7027-00096B). A.C.C. acknowledges support from STFC consolidated grant numbers ST/R000824/1 and ST/V000861/1, and UKSA grant number ST/R003203/1. P.M. acknowledges support from STFC research grant number ST/M001040/1. M.F. and C.M.P. gratefully acknowledge the support of the Swedish National Space Agency (DNR 65/19, 174/18). S.G.S. acknowledge support from FCT through FCT contract nr. CEECIND/00826/2018 and POPH/FSE (EC). A.C.C. and T.G.W. acknowledge support from STFC consolidated grant numbers ST/R000824/1 and ST/V000861/1, and UKSA grant number ST/R003203/1. S.H. gratefully acknowledges CNES funding through the grant 837319. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (project FOUR ACES. Y.A. and M.J.H. acknowledge the support of the Swiss National Fund under grant 200020_172746. We acknowledge support from the Spanish Ministry of Science and Innovation and the European Regional Development Fund through grants ESP2016-80435-C2-1-R, ESP2016-80435-C2-2-R, PGC2018-098153-B-C33, PGC2018-098153-B-C31, ESP2017-87676-C5-1-R, MDM-2017-0737 Unidad de Excelencia Maria de Maeztu-Centro de Astrobiologia (INTA-CSIC), as well as the support of the Generalitat de Catalunya/CERCA programme. The MOC activities have been supported by the ESA contract No. 4000124370. S.C.C.B. acknowledges support from FCT through FCT contracts nr. IF/01312/2014/CP1215/CT0004. X.B., S.C., D.G., M.F. and J.L. acknowledge their role as ESA-appointed CHEOPS science team members. ABr was supported by the SNSA. This project was supported by the CNES. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (project FOUR ACES, grant agreement No. 724427). This work was also partially supported by a grant from the Simons Foundation (PI: Queloz, grant number 327127). I.R.I. acknowledges support from the Spanish Ministry of Science and Innovation and the European Regional Development Fund through grant PGC2018-098153-B- C33, as well as the support of the Generalitat de Catalunya/CERCA programme. GyMSz acknowledges the support of the Hungarian National Research, Development and Innovation Office (NKFIH) grant K-125015, a a PRODEX Experiment Agreement No. 4000137122, the Lendület LP2018-7/2021 grant of the Hungarian Academy of Science and the support of the city of Szombathely. N.A.W. acknowledges UKSA grant ST/R004838/1.Peer reviewe

Scaling relations of convective granulation noise across the HR diagram from 3D stellar atmosphere models

International audienceHigh-precision photometric data from space missions have improved our understanding of stellar granulation. These observations have shown with precision the stochastic brightness fluctuations of stars across the HR diagram, allowing us to better understand how stellar surface convection reacts to a change in stellar parameters. These fluctuations need to be understood and quantified in order to improve the detection and characterization of exoplanets. In this work, we provide new scaling relations of two characteristic properties of the brightness fluctuations time series, the standard deviation (σ) and the auto-correlation time ($\tau \rm _{ACF}$). This was done by using long time series of 3D stellar atmosphere models at different metallicities and across the HR diagram, generated with a 3D radiative hydrodynamical code: the STAGGER code. We compared our synthetic granulation properties with the values of a large sample of Kepler stars, and analyzed selected stars with accurate stellar parameters from the Kepler LEGACY sample. Our 3D models showed that σ $\propto \nu \rm _{max}^{-0.567\pm 0.012}$ and $\tau \rm _{ACF}$$\propto \nu \rm _{max}^{-0.997\pm 0.018}$ for stars at solar metallicity. We showed that both σ and $\tau \rm _{ACF}$ decrease with metallicity, although the metallicity dependence is more significant on σ. Unlike previous studies, we found very good agreement between σ from Kepler targets and the 3D models at $\rm {\log }g$ ≤3.5, and a good correlation between the stars and models with $\rm {\log }g$ ≥3.5. For $\tau \rm _{ACF}$, we found that the 3D models reproduced well the Kepler LEGACY star values. Overall, this study shows that 3D stellar atmosphere models reproduce the granulation properties of stars across the HR diagram