Transits of exoplanets across cool stars contain blended information about structures on the stellar surface and about the planetary body and atmosphere. To advance understanding of how this information is entangled, a surface-flux transport code, based on observed properties of the Sun's magnetic field, is used to simulate the appearance of hypothetical stellar photospheres from the visible near 4000 Angstrom to the near-IR at 1.6 micron, by mapping intensities characteristic of faculae and spots onto stellar disks. Stellar appearances are computed for a Sun-like star of solar activity up to a star with mean magnetic flux density ~30 times higher. Simulated transit signals for a Jupiter-class planet are compared with observations. This (1) suggests that the solar paradigm is consistent with observations for stars throughout the activity range explored provided that infrequent large active regions with fluxes up to $\sim 3\times 10^{23}$ Mx are included in the emergence spectrum, (2) quantitatively confirms that for such a model, faculae brighten relatively inactive stars while starspots dim more active stars, (3) suggests that large starspots inferred from transits of active stars are consistent with clusters of more compact spots seen in the model runs, (4) that wavelength-dependent transit-depth effects caused by stellar magnetic activity for the range of activity and the planetary diameter studied here can introduce apparent changes in the inferred exoplanetary radii across wavelengths from a few hundred to a few thousand kilometers, increasing with activity, and (5) that activity-modulated distortions of broadband stellar radiance across the visible to near-IR spectrum can reach several percent.
Cyclic activity on the Sun and stars is primarily explained by generation of the magnetic field by a dynamo mechanism, which converts the energy of the poloidal field into the energy of the toroidal component due to differential rotation. There is, however, an alternative point of view, which explains the field generation by gravitational influence of the planetary system and, first of all, Jupiter. This hypothesis can be verified by comparing the characteristics of exoplanets with the activity variations on their associated stars. We have performed such a comparison and have drawn a negative …
Current radial velocity data from specialized instruments contain a large amount of information that may pass unnoticed if their analysis is not accurate. The joint use of Bayesian inference tools and frequency analysis has been shown effective to reveal exoplanets but they have been used less frequently to investigate stellar activity. We intend to use radial velocity data of the exoplanet host star GJ 3512 to investigate its magnetic activity. Our study includes the analysis of the photometric data available. The main objectives of our work are to constrain the orbital parameters of the exop…
It has been proposed that magnetic activity could be enhanced due to interactions between close-in massive planets and their host stars. In this article, I present a brief overview of the connection between stellar magnetic activity and exoplanets. Stellar activity can be probed in chromospheric lines, coronal emission, surface spot coverage, etc. Since these are manifestations of stellar magnetism, these measurements are often used as proxies for the magnetic field of stars. Here, instead of focusing on the magnetic proxies, I overview some recent results of magnetic field measurements using …
The architecture of many exoplanetary systems is different from the solar system, with exoplanets being in close orbits around their host stars and having orbital periods of only a few days. We can expect interactions between the star and the exoplanet for such systems that are similar to the tidal interactions observed in close stellar binary systems. For the exoplanet, tidal interaction can lead to circularization of its orbit and the synchronization of its rotational and orbital period. For the host star, it has long been speculated if significant angular momentum transfer can take place be…
Stellar magnetic activity is an important factor in the formation and evolution of exoplanets. Magnetic phenomena like stellar flares, coronal mass ejections, and high-energy emission affect the exoplanetary atmosphere and its mass loss over time. One major question is whether the magnetic evolution of exoplanet host stars is the same as for stars without planets; tidal and magnetic interactions of a star and its close-in planets may play a role in this. Stellar magnetic activity also shapes our ability to detect exoplanets with different methods in the first place, and therefore we need to un…
We embark on a detailed study of the lightcurves of Keplers most Earth-like exoplanet host stars using the full length of Kepler data. We derive rotation periods, photometric activity indices, flaring energies, mass loss rates, gyrochronological ages, X-ray luminosities and consider implications for the planetary magnetospheres and habitability. Furthermore, we present the detection of superflares in the lightcurve of Kepler-438, the exoplanet with the highest Earth Similarity Index to date. Kepler-438b orbits at a distance of 0.166AU to its host star, and hence may be susceptible to atmospher…
The magnetic activity of planet-hosting stars is an important factor to estimate the atmospheric stability of close-in exoplanets and the age of their host stars. It has long been speculated that close-in exoplanets can influence the stellar activity level. However, testing for tidal or magnetic interaction effects in samples of planet-hosting stars is difficult because stellar activity hinders exoplanet detection, so that stellar samples with detected exoplanets show a bias towards low activity for small exoplanets. We aim to test if exoplanets in close orbits influence the stellar rotation a…
Accurately modeling effects from stellar activity is a key step in detecting radial velocity signals of low-mass and long-period exoplanets. Radial velocities from stellar activity are dominated by magnetic active regions that move in and out of sight as the star rotates, producing signals with timescales related to the stellar rotation period. Methods to characterize radial velocity periodograms assume that peaks from magnetic active regions will typically occur at the stellar rotation period or a related harmonic. However, with surface features unevenly spaced and evolving over time, signals…
The photospheric spot activity of some of the stars with transiting planets discovered by the CoRoT space experiment is reviewed. Their out-of-transit light modulations are fitted by a spot model previously tested with the total solar irradiance variations. This approach allows us to study the longitude distribution of the spotted area and its variations versus time during the five months of a typical CoRoT time series. The migration of the spots in longitude provides a lower limit for the surface differential rotation, while the variation of the total spotted area can be used to search for sh…
We present an in-depth analysis of stellar activity and its effects on radial velocity (RV) for the M2 dwarf GJ 176 based on spectra taken over 10 years from the High Resolution Spectrograph on the Hobby-Eberly Telescope. These data are supplemented with spectra from previous observations with the HIRES and HARPS spectrographs, and V- and R-band photometry taken over 6 years at the Dyer and Fairborn observatories. Previous studies of GJ 176 revealed a super-Earth exoplanet in an 8.8-day orbit. However, the velocities of this star are also known to be contaminated by activity, particularly at t…
The Mount Wilson Ca HK survey revealed magnetic activity variations in a large sample of solar-type stars with timescales ranging from 2.5 to 25 years. This broad range of cycle periods is thought to reflect differences in the rotational properties and the depths of the surface convection zones for stars with various masses and ages. In 2007 we initiated a long-term monitoring campaign of Ca II H and K emission for a sample of 57 southern solar-type stars to measure their magnetic activity cycles and their rotational properties when possible. We report the discovery of a 1.6-year magnetic acti…
The ubiquity of M dwarf stars combined with their low masses and luminosities make them prime targets in the search for nearby, habitable exoplanets. We investigate the effects of starspot-induced radial velocity (RV) jitter on detection and characterization of planets orbiting M dwarfs. We create surface spot configurations with both random spot coverage and active regions. Synthetic stellar spectra are calculated from a given spot map, and RV measurements are obtained using cross-correlation technique. We add the RV signal of an orbiting planet to these jitter measurements, and reduce the da…
The detected exoplanet population displays a dearth of planets with sizes of about two Earth radii, the so-called radius gap. This is interpreted as an evolutionary effect driven by a variety of possible atmospheric mass loss processes of exoplanets. For mass loss driven by an exoplanet's irradiation by stellar X-ray and extreme-UV photons, the time evolution of the stellar magnetic activity is important. It is known from observations of open stellar clusters that stars of the same age and mass do not all follow the same time evolution of activity-induced X-ray and extreme-UV luminosities. Her…
We present a survey of far-ultraviolet (FUV; 1150 - 1450 Ang) emission line spectra from 71 planet-hosting and 33 non-planet-hosting F, G, K, and M dwarfs with the goals of characterizing their range of FUV activity levels, calibrating the FUV activity level to the 90 - 360 Ang extreme-ultraviolet (EUV) stellar flux, and investigating the potential for FUV emission lines to probe star-planet interactions (SPIs). We build this emission line sample from a combination of new and archival observations with the Hubble Space Telescope-COS and -STIS instruments, targeting the chromospheric and transi…
Magnetic activity in cool stars: manifestations and relevance to exoplanets
Max-Planck-Institut für Sonnensystemforschung（马克斯·普朗克太阳系统研究所）；Centro de Rádio Astronomia e Astrofísica Mackenzie, Universidade Presbiteriana Mackenzie（麦肯齐射电天文学与天体物理学中心，麦肯齐长老会大学）；Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM（巴黎萨克雷大学，巴黎西岱大学，法国原子能和替代能源委员会，法国国家科学研究中心，天文信息与管理实验室）；Leibniz-Institut für Astrophysik Potsdam (AIP)（波茨坦莱布尼茨天体物理研究所）；HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Observatory（匈牙利研究网络地球与天文学研究中心，孔科利天文台）；Instituto de Astronomía y Física del Espacio (IAFE, UBA-CONICET)（空间天体物理学与天体物理研究所）；The Catholic University of America（美国天主教大学）；The Hakubi Center for Advanced Research, Kyoto University（京都大学博雅教育研究中心）；Department of Physics, Kyoto University（京都大学物理学系）；University of Helsinki（赫尔辛基大学）
Many current and future exoplanet missions are pushing to infrared (IR) wavelengths where the flux contrast between the planet and star is more favorable (Deming et al. 2009), and the impact of stellar magnetic activity is decreased. Indeed, a recent analysis of starspots and faculae found these forms of stellar activity do not substantially impact the transit signatures or science potential for FGKM stars with JWST (Zellem et al. 2017). However, this is not true in the case of flares, which I demonstrate can be a hinderance to transit studies in this note.
The active K2 dwarf epsilon Eri has been extensively characterized, both as a young solar analog and more recently as an exoplanet host star. As one of the nearest and brightest stars in the sky, it provides an unparalleled opportunity to constrain stellar dynamo theory beyond the Sun. We confirm and document the 3 year magnetic activity cycle in epsilon Eri originally reported by Hatzes and coworkers, and we examine the archival data from previous observations spanning 45 years. The data show coexisting 3 year and 13 year periods leading into a broad activity minimum that resembles a Maunder …
Stellar activity poses one of the main obstacles for the detection and characterisation of small exoplanets around cool stars, as it can induce radial velocity (RV) signals that can hide or mimic the presence of planetary companions. Several indicators of stellar activity are routinely used to identify activity-related signals in RVs, but not all indicators trace exactly the same activity effects, nor are any of them always effective in all stars. We evaluate the performance of a set of spectroscopic activity indicators for M dwarf stars with different masses and activity levels with the aim o…
Past UV and optical observations of stars hosting hot Jupiters have shown that some of these stars present an anomalously low chromospheric activity, significantly below the basal level. For WASP-13, observations have shown that the apparent lack of activity is possibly caused by absorption from the intervening ISM. Inspired by this result, we study the effect of ISM absorption on activity measurements (S and logR'$_{\rm HK}$ indices) for main-sequence late-type stars. To this end, we employ synthetic stellar photospheric spectra combined with varying amounts of chromospheric emission and ISM …
In the search for planets orbiting distant stars the presence of stellar activity in the atmospheres of observed stars can obscure the radial velocity signal used to detect such planets. Furthermore, this stellar activity contamination is set by the star itself and cannot simply be avoided with better instrumentation. Various stellar activity indicators have been developed that may correlate with this contamination. We introduce a new stellar activity indicator called the Stellar Activity F-statistic for Exoplanet surveys (SAFE) that has higher statistical power (i.e., probability of detecting…
Stellar activity strongly affects and may prevent the detection of Earth-mass planets in the habitable zone of solar-type stars with radial velocity technics. Astrometry is in principle less sensitive to stellar activity because the situation is more favourable: the stellar astrometric signal is expected to be fainter than the planetary astrometric signal compared to radial velocities. We quantify the effect of stellar activity on high-precision astrometry when Earth-mass planets are searched for in the habitable zone around old main-sequence solar-type stars. We used a very large set of magne…
As long as astronomers have searched for exoplanets, the intrinsic variability of host stars has interfered with the ability to reliably detect and confirm exoplanets. One particular source of false positives is the presence of stellar magnetic or chromospheric activity that can mimic the radial-velocity reflex motion of a planet. Here we present the results of a photometric data analysis for the known planet hosting star, BD-06 1339, observed by the Transiting Exoplanet Survey Satellite (TESS) during Sector 6 at 2 minute cadence. We discuss evidence that suggests the observed 3.9 day periodic…
Current exoplanet surveys using the radial velocity (RV) technique are targeting M dwarfs because any habitable zone terrestrial-mass planets will induce a high RV and orbit on shorter periods than for more massive stars. One of the main caveats is that M dwarfs show a wide range of activity levels from inactive to very active, which can induce an asymmetry in the line profiles and, consequently, a spurious RV measurement. We aim to benchmark the impact of stellar activity on high-precision RV measurements using regular-cadence CARMENES visible and near-infrared observations of the active M3.5…
Aims. We investigate a method to increase the efficiency of a targeted exoplanet search with the transit technique by preselecting a subset of candidates from large catalogs of stars. Assuming spin-orbit alignment, this can be done by considering stars that have higher probability to be oriented nearly equator-on (inclination close to 90^{\circ}). Methods. We use activity-rotation velocity relations for low-mass stars with a convective envelope to study the dependence of the position in the activity-vsini diagram on the stellar axis inclination. We compose a catalog of G-, K-, M-type main sequ…
From an observational standpoint, stellar activity poses a critical challenge to exoplanet science, as it inhibits the detection of planets and the precise measurement of their parameters. Radial velocity and transit searches revealed a significant fraction of exoplanet hosts is active, and showed the need to fully understand the different facets of stellar activity and its impact on observables. Moreover, the activity correction is of prime importance for the detection and characterisation of Earth analogues. We present a review of the effects that stellar activity features such as starspots,…
Several "hot Jupiter" host stars appear to exhibit enhanced chromospheric activity, coronal flaring, and starspot development synchronized to their planetary orbits. These effects have been attributed to tidal and/or magnetic interactions between the star and exoplanet. The best studied among such systems is HD 189733, which has previously been observed from radio to X-ray wavelengths. Here, I present ~4.75 GHz Arecibo radio telescope observations of HD 189733 during a fraction of the exoplanet orbital phase range previously associated with enhanced coronal X-ray flaring. No radio flares from …
HWO Target Stars and Systems: Activity and Rotation Catalog (ARC) of Potential Target Stars for the Habitable Worlds Observatory
University of California, Riverside（加州大学河滨分校）；National Optical-Infrared Astronomy Research Laboratory（国家光学红外天文研究实验室）；Dartmouth College（达特茅斯学院）；Instituto de Astronomía y Física del Espacio (IAFE, CONICET-UBA)（太空天体物理学与天文学研究所 (IAFE, 阿根廷国家科学研究委员会-布宜诺斯艾利斯大学)）；Universidad de Buenos Aires（布宜诺斯艾利斯大学）；Jet Propulsion Laboratory, California Institute of Technology（加州理工学院喷气推进实验室）；Centro de Astrobiología, CSIC-INTA（西班牙国家航空航天技术研究所-西班牙国家研究委员会天体生物学中心）；Georgia State University（佐治亚州立大学）；RECONS Institute（RECONS研究所）；UNTREF（特立尼达德大学）
Chromospherically sensitive atomic lines display different spectra in stellar active regions, spots, and the photosphere, raising the possibility that exoplanet transmission spectra are contaminated by the contrast between various portions of the stellar disk. To explore this effect, we performed transit simulations of G and K-type stars for the spectral lines Ca II K at 3933 Å, Na I 5890 Å, H I 6563 Å (H$α$), and He I 10830 Å. We find that strong facular emission and large coverage fractions can contribute a non-negligible amount to transmission spectra, especially for H$α$, Ca II K, and Na I…
Insights into the Exoplanet Radius Valley from Host-Star Ages, Activity, Chemistry, and Birth Radius
School of Science, Hangzhou Dianzi University（杭州电子科技大学理学院）；National Astronomical Data Center Zhijiang Branch（国家天文台紫江分中心）；CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台光学天文重点实验室）；Department of Astronomy, The Ohio State University（俄亥俄州立大学天文学系）；Center for Cosmology and Astroparticle Physics (CCAPP), The Ohio State University（俄亥俄州立大学宇宙学与高能粒子物理中心）；School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）；School of Science, Xichang University（西昌学院理学院）
We determined new linear ephemerides of transiting exoplanets using long-cadence de-trended data from quarters Q1 to Q17 of Kepler mission. We analysed TTV diagrams of 2098 extrasolar planets. The TTVs of 121 objects were excluded (because of insufficient data-points, the influence of stellar activity, etc). Finally, new linear ephemerides of 1977 exoplanets from the Kepler archive are presented. The significant linear trend was observed on TTV diagrams of approximately 35% of studied exoplanets. Knowing correct linear ephemeris is principal for successful follow-up observations of transits. R…
Photometric observations of exoplanet transits can be used to derive the orbital and physical parameters of an exoplanet. We analyzed several transit light curves of exoplanets that are suitable for ground-based observations whose complete information is available on the Exoplanet Transit Database (ETD). We analyzed transit data of planets including HAT-P-8 b, HAT-P-16 b, HAT-P-21 b, HAT-P-22 b, HAT-P-28 b and HAT-P-30 b using the AstroImageJ (AIJ) software package. In this paper, we investigated 82 transit light curves from ETD, deriving their physical parameters as well as computing their mi…
Identifying Exoplanets with Deep Learning VI. Enhancing neural network mitigation of stellar activity RV signals with additional metrics
Imperial College London（帝国理工学院）；Massachusetts Institute of Technology（麻省理工学院）；University of Birmingham（伯明翰大学）；Technical University of Denmark（丹麦技术大学）；University of St Andrews（圣安德鲁斯大学）；Fundación Galileo Galilei—INAF（伽利略基金会-INAF）；Observatoire de Genève, Université de Genève（日内瓦天文台，日内瓦大学）
Stellar variability due to magnetic activity and flows at different spatial scales strongly impacts radial velocities. This variability is seen as oscillations, granulation, supergranulation, and meridional flows. The effect of this latter process is poorly known but could affect exoplanet detectability. We aim to quantify its amplitude when integrated over the disc and its temporal variability, first for the Sun, seen with different inclinations, and then for other solar-type stars. We used long time series of solar latitudinal meridional circulation to reconstruct its integrated contribution…
The effect of stellar activity on RV appears to be a limiting factor in detecting Earth-mass planets in the habitable zone of a star similar to the Sun in spectral type and activity level. It is crucial to estimate if this conclusion remain true for other stars with current correction methods. We built realistic time series in RV and chromospheric emission for old main-sequence F6-K4 stars. The stellar parameters are spectral type, activity level, rotation period, cycle period and amplitude, latitude coverage, and spot constrast, which we chose to be in ranges that are compatible with our curr…
We present photometric transit observations of the exoplanet WASP-2b using the MTM-500 telescope of the Crimean Astrophysical Observatory of RAS. A brief history of the discovery of the exoplanet WASP-2b and its main characteristics are considered. Observations and the processing procedure are described. The analysis of the obtained results was carried out and compared with information from the ETD and NASA Exoplanet Archive databases. The photometry data of the star WASP-2 and the comparison star GSC 0052201406 are given in the supplementary table.
Lightcurves have long been used to study stellar activity and have more recently become a major tool in the field of exoplanet research. We discuss the various ways in which stellar activity can influence transit lightcurves, and study the effects using the outstanding photometric data of the CoRoT-2 exoplanet system. We report a relation between the `global' lightcurve and the transit profiles which turn out to be shallower during high spot coverage on the stellar surface. Furthermore, our analysis reveals a color dependence of the transit lightcurve compatible with a wavelength-dependent lim…
We present the results of the analysis of the photometric data collected in long and short-cadence mode by the Transiting Exoplanet Survey Satellite (TESS) for GJ 504, a well studied planet-hosting solar-like star, whose fundamental parameters have been largely debated during the last decade. Several attempts have been made by the present authors to isolate the oscillatory properties expected on this main-sequence star, but we did not find any presence of solar-like pulsations. The suppression of the amplitude of the acoustic modes can be explained by the high level of magnetic activity reveal…
The CARMENES instrument is searching for periodic radial-velocity (RV) variations of M dwarfs, which are induced by orbiting planets. However, there are other potential sources of such variations, including rotational modulation caused by stellar activity. We aim to investigate four M dwarfs (Ross 318, YZ CMi, TYC 3529-1437-1, and EV Lac) with different activity levels and spectral sub-types. Our goal is to compare the periodicities seen in 22 activity indicators and the stellar RVs, and to examine their stability over time. For each star, we calculated GLS periodograms of pseudo-equivalent wi…
It has been asserted that the primary star in the HD 189733 system steadily accretes evaporated exospheric gases from its ``hot Jupiter'' companion, rather like a T Tauri star accreting from a disk. We conduct statistical and periodogram analyses of the photometric time series of the primary, as acquired by the automated photoelectric telescope (APT), Microvariability and Oscillations of Stars (MOST), and Wise Observatory, to investigate this claim with the goal of revealing the presence of accretion shocks or photospheric accretion hotspots as are found in T Tauri systems such as AA Tau. None…
Massachusetts Institute of Technology（麻省理工学院）；University of Texas at Austin（德克萨斯大学奥斯汀分校）；University of Wisconsin-Madison（威斯康星大学麦迪逊分校）；Université de Genève（日内瓦大学）；Observatoire de Genève（日内瓦天文台）；University of St Andrews（圣安德鲁斯大学）；Facebook（脸书）；Technical University of Denmark（丹麦技术大学）；DTU Space, National Space Institute（丹麦技术大学国家空间研究所）
Exoplanet detection with precise radial velocity (RV) observations is currently limited by spurious RV signals introduced by stellar activity. We show that machine learning techniques such as linear regression and neural networks can effectively remove the activity signals (due to starspots/faculae) from RV observations. Previous efforts focused on carefully filtering out activity signals in time using modeling techniques like Gaussian Process regression (e.g. Haywood et al. 2014). Instead, we systematically remove activity signals using only changes to the average shape of spectral lines, and…
We perform a series of time dependent Magnetohydrodynamic simulations of the HD 189733 star-planet system in order to predict radio transit modulations due to the interaction between the stellar wind and planetary magnetic field. The simulation combines a model for the stellar corona and wind with an exoplanet that is orbiting the star in a fully dynamic, time-dependent manner. Our simulations generate synthetic radio images that enable us to obtain synthetic radio lightcurves in different frequencies. We find a clear evidence for the planetary motion in the radio light curves. Moreover, we fi…
RedDots: Magnetic field of the nearby active M dwarf GJ 729, and a search for companions
Central Queensland University（中昆士兰大学）；Centre for Astrophysics, University of Southern Queensland（南昆士兰大学天体物理中心）；Thüringer Landessternwarte Tautenburg（图林根州陶滕堡天文台）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）；IRAP, Universite de Toulouse, CNRS/UMR 5277, UPS-OMP（图卢兹大学红外天文物理研究所 (IRAP)）；Department of Physical Sciences, The Open University（开放大学物理科学系）；Institut für Astrophysik und Geophysik (IAG), Universität Göttingen（哥廷根大学天体物理学与地球物理学研究所）
In this research, 14 light curves of 10 hot Jupiter exoplanets available on Exoplanet Transit Database (ETD) were analyzed. We extracted the transit parameters using EXOFAST software. Finally, we compared the planet's radius parameter calculated by the EXOFAST with the value at the NASA Exoplanet Archive (NEA) using the confidence interval method. According to the results obtained from this comparison, there is an acceptable match for the planet's radius with NEA values. Also, based on the average value of 350 mm optics in this study, it shows that the results obtained using small telescopes c…
We present a comparative analysis of observations of the selected exoplanet transits obtained at the Kyiv Comet station with the database of the TESS (Transiting Exoplanet Survey Satellite) and Kepler space telescopes. The light curves obtained by the TESS and Kepler orbital telescopes were processed using a program based on the Python package Lightkurve 2.3v which is freely available in the MUST archive (Barbara A. Mikulski Archive for Space Telescopes). The ground-based observations were carried out with the 70-cm telescope AZT-8 (Lisnyky). Photometric processing of the ground-based observat…
