319.01 — How Stellar Flares and Storms Regulate Atmospheric Losses from the TRAPPIST-1 Planets
Chuanfei Dong1; Meng Jin3; Manasvi Lingam4; Kevin
France2
1 Princeton University (Plainsboro, New Jersey, United States) 2 Astrophysical and Planetary Science, University of Colorado (Boulder, Colorado, United States)
3 SETI Institute (Mountain View, California, United States)
4 Harvard-Smithsonian Center for Astrophysics (Cambridge, Mas- sachusetts, United States)
Stellar flares have been observed to produce a burst of radiation over a wide range of wavelengths, among which X-rays and EUV constitute the major ionizing stellar radiation for planetary atmospheres at low and high altitudes, respectively. Stellar flares are considered an impediment to habitability, es- pecially in the case of close-in exoplanets around M-dwarfs since these stars are highly active. At the same time, there has been a growing awareness that coronal mass ejections (CMEs) — sometimes termed as stellar storms — associated with stellar flares pose severe threats to planetary atmospheric retention. It is evident that understanding atmo- spheric escape is vital from the standpoint of hab- itability since atmospheric evolution influences the climate and the fluxes of ionizing radiation reaching the surface, among other factors.
Until now, there have been no systematic studies of the impact of stellar flares and associated storms on exoplanetary atmospheric losses despite their in- dubitable occurrence and pertinence. Here, we carry out sophisticated 3D MHD simulations (that in- cludes important photochemistry) to assess how the atmospheric escape rates of the TRAPPIST-1 planets evolve during 1) a 1033erg flare (based on observa- tions) without a CME (where the CME may be sup- pressed or deviated from the planet) and, 2) a 1033 erg flare with a CME, where the CME is initialized and modeled according to the flare energy by using a stellar wind model. We found that the atmospheric escape rates are enhanced by 1-3 orders of magnitude compared to our previous study that used normal stellar wind conditions. For the outmost TRAPPIST- 1h, if such flares occur at a frequency of∼1 per day, a 1-bar atmosphere will be scavenged on the time scale of∼100 million years. This time scale reduces to∼1 million years for the innermost TRAPPIST-1b. This represents the first study where the roles of stel- lar flares and storms on exoplanetary atmospheric escape for the TRAPPIST-1 planets are clearly eluci- dated. The new results obtained herein would be of considerable interest to a wide audience and thus de- serving an oral presentation.
319.03 — Magnetospheres of the TRAPPIST-1 plan- ets
Adam Boldog1,2; Vera Dobos1,2; László L. Kiss1,3 1 Konkoly Observatory, Research Centre for Astronomy and Earth Sciences (Budapest, Hungary)
3 Sydney Institute for Astronomy (Sydney, New South Wales, Australia)
The number of exoplanets in the habitable zone of M dwarfs has increased in the last few years thanks to ground-based observatories and space telescopes. M dwarfs are among the most active stars produc- ing frequent and strong flares, strong stellar wind and high energy radiation. The habitability of these worlds strongly depends on their capability of retain- ing their atmospheres. Planetary magnetospheres play a crucial role in reducing atmospheric loss and thus providing a potentially habitable enviroment. The M dwarf star TRAPPIST-1 is of particular in- terest because it hosts three Earth-sized rocky plan- ets in its habitale zone. We calculated the magnetic properties of these planets, such as the suface dipolar field strength and magnetic dipole moment, using a method based on the example of the early Earth, which assumes a process involving the exsolution of MgO as the source of the planetary dynamo. In or- der to follow the evolution of these properties, we ap- plied a thermal evolution model for the TRAPPIST-1 planets. The sizes of the magnetospheres, described by the magnetoshperic standoff distance (i.e. the dis- tance from the planet to the point where the stellar wind is balanced by the planetary magnetic field), were derived using previously modeled stellar wind parameters. Additionally, we calculated the polar cap area, which indicates the fraction of a planet’s surface where magnetic field lines are open and at- mosperic escape is possible. Based on our results, we will estimate the atmospheric mass loss, which can significantly limit habitability on the planets.
319.04 — It’s raining hot Jupiters: 3D MHD simu- lations of planetary atmospheric escape
Simon Daley-Yates1; Ian Stevens1
1 University of Birmingham (Paris, France)
We present 3D MHD simulations of the wind-wind interactions that occur between a solar-type star and a short period hot Jupiter exoplanet. A planetary outflow results from atmospheric escape induced by the host stars incident radiation. Circumstellar and circumplanetary material which accretes onto the stellar surface in a form of coronal rain, we charac- terise this interaction for a representative hot Jupiter hosting system and predict the accretion point, size and extent. The nature of this accretion is variable in both location and rate, with the final accretion point occurring at 133 degrees west and 53 degrees east of the subplanetary point. The size of the accretion spot itself has been found to vary with a period of 67 ks
(approximately 1/5 of the orbital period). The results are highly dependent on the magnetic fields of both the star and the planet and on the atmospheric con- ditions of the hot Jupiter. We characterise this be- haviour as Star-Planet-Wind Interaction (SPWI).
319.05 — Interactions of tidal flows and convec- tion: frequency dependence and indications of anti-dissipation
Craig D. Duguid1; Adrian John Barker1; Chris A. Jones1 1 University of Leeds (Leeds, United Kingdom)
A key mechanism in the orbital and spin evolution of close proximity bodies is the dissipation caused by the interactions of tidal flows with convection. It is expected that the effective viscosity of this inter- action (νE) would depend on the tidal frequency (ω)
but to what extent is a matter of debate, particularly in the regime of fast tides (e.g. Zahn 1966; Goldre- ich and Nicholson 1977). It is essential to resolve this in order to correctly predict the tidal evolution of hot Jupiters. We have performed the most com- prehensive investigation to date of this mechanism by way of hydrodynamical simulations and exten- sions to existing theory, building upon prior work by Penev et al. 2009 and Ogilvie and Lesur 2012. Our re- sults provide a clear scaling law for the dependence of the effective viscosity on tidal frequency and also convincing evidence which suggests that this mecha- nism can operate as anti-dissipation (which could re- sult in outward migration or excitation of eccentric- ities, contrary to prior expectations). These results can help guide the correct implementation of tidal dissipation for planet-star interactions. We will also discuss the consequences of our results for the orbital decay of hot Jupiters.
319.06 — Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (Mega-MUSCLES)
David John Wilson1; Cynthia Froning1; Kevin France2; Allison Youngblood3; Girish M. Duvvuri2; Alexander Brown2; P. Christian Schneider4; Adam Kowalski2; R. O. Parke Loyd5; Zachory Berta- Berta-Thompson2; J. Sebastian Pineda2; Jeffrey Linsky2; Sarah Rugheimer6; Elizabeth Newton7; Yamila Miguel8; Aki Roberge3; An- drea P. Buccino9; Jonathan Irwin10; Lisa Kaltenegger11;
Mariela Vieytes12; Pablo Mauas9; Seth Redfield13;
Suzanne Hawley15; Feng Tian14
1 McDonald Observatory, University of Texas at Austin (Austin, Texas, United States)
2 Smithsonian Astrophysical Observatory (Cambridge, Mas- sachusetts, United States)
3 Cornell University (Ithaca, New York, United States) 4 I. Astronomia y Fis Espacio (Buenos Aires, Argentina) 5 Wesleyan University (Middletown, Connecticut, United States) 6 Chinese Academy of Sciences (Beijing, China)
7 University of Washington (Seattle, Washington, United States) 8 Astrophysical and Planetary Science, University of Colorado (Boulder, Colorado, United States)
9 Goddard Space Flight Center (Greenbelt, Maryland, United States)
10 Hamburger Sternwarte (Hamburg, Germany)
11 School of Earth and Space Exploration, Arizona State University (Tempe, Arizona, United States)
12 University of Oxford (Oxford, United Kingdom) 13 Massachusetts Institute of Technology (Cambridge, Mas- sachusetts, United States)
14 Leiden University (Leiden, Netherlands)
15 Universidad de Buenos Aires (Buenos Aires, Argentina) M dwarf stars have emerged as ideal targets for ex- oplanet observations. Their small radii aids plane- tary discovery, their close-in habitable zones allow short observing campaigns, and their red spectra provide opportunities for transit spectroscopy with JWST. The potential of M dwarfs has been under- lined by the discovery of remarkable systems such as the seven Earth-sized planets orbiting TRAPPIST- 1 and the habitable-zone planet around the closest star to the Sun.
However, to accurately assess the conditions in these systems requires a firm understanding of how M dwarfs differ from the Sun, beyond just their smaller size and mass. Of particular importance are the time-variable, high-energy ultraviolet and x-ray regions of the M dwarf spectral energy distribution (SED), which can influence the chemistry and life- time of exoplanet atmospheres, as well as their sur- face radiation environments.
The Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (Mega-MUSCLES) Treasury project, together with the precursor MUSCLES project, aims to produce full SEDs of a representative sample of M dwarfs, covering a wide range of stellar mass, age, and plan- etary system architecture. We have obtained x-ray and ultraviolet data for 13 stars using the Hubble, Chandra and XMM space telescopes, along with ground-based data in the optical and state-of-the-art DEM modelling to fill in the unobservable extreme ultraviolet regions. Our completed SEDs will be available as a community resource, with the aim that a close MUSCLES analogue should exist for most M dwarfs of interest.
In this presentation I will overview the Mega- MUSCLES project, describing our choice of targets,
observation strategy and SED production methodol- ogy. I will also discuss notable targets such as the TRAPPIST-1 host star, comparing our observations with previous data and model predictions. Finally, I will present an exciting by-product of the Mega- MUSCLES project: time-resolved ultraviolet spec- troscopy of stellar flares at multiple targets, spanning a range of stellar types, ages and flare energies.
319.07 — Eating Planets for Breakfast, Lunch, and Dinner: Signatures of Planetary Engulfment at all Phases of Stellar Evolution
Alexander Patrick Stephan1; Smadar Naoz1; B. Scott
Gaudi2; Jesus M. Salas1
1 Physics and Astronomy, University of California, Los Angeles (Los Angeles, California, United States)
2 Astronomy, Ohio State University (Columbus, Ohio, United States)
Most, if not all, TESS target stars can be expected to reside in binaries, as these stars are more mas- sive than the Sun. Gravitational perturbations from a companion can drive a planet closer to its host star, potentially plunging the planet all the way into the star. While it is challenging to observe a planet dur- ing its plunge, we have predicted that, prior to its demise, such a planet will appear as hot as a Hot Jupiter (Stephan et al. 2018). This new class of ’Tem- porary Hot Jupiters’ has recently been confirmed by TESS observations (e.g., HD 202772A b). As the planet is eventually eaten, it can impart distinct sig- natures onto the star. We follow the engulfment of planets by their host stars during different stellar life phases and calculate the changes in stellar param- eters, such as stellar spin or luminosity, caused by this process. Our predictions for the observable sig- natures of these engulfment events will enable future and current endeavors to find post-engulfment stars, thus, advancing our understanding of planetary sys- tem architectures and dynamical evolution.
319.08 — Impact of Stellar Magnetism on Star- planet Tidal Interactions
Aurélie Astoul1; Stéphane Mathis1; Clément Baruteau2;
Florian Gallet3; Antoine Strugarek1; Kyle Augustson1;
Allan Sacha Brun1; Emeline Bolmont4 1 DAP, CEA/Saclay (Bures sur Yvette, France) 2 IRAP (Toulouse, France)
3 IPAG (Grenoble, France)
4 Département d’Astronomie, Université de Genève (Genève, France)
Over the last two decades, about 4000 exoplanets have been discovered around low-mass stars. For the
shortest period exoplanets, star-planet tidal interac- tions are likely to have played a major role in the ul- timate orbital evolution and on the spin axis evolu- tion of the host stars. Although low-mass stars are magnetically active objects, the question of how the star’s magnetic field impacts the excitation, propaga- tion and dissipation of tidal waves remains open.
In this work, we have derived the magnetic con- tribution to the tidal force and estimated its am- plitude all along the structural and rotational evo- lutions of low-mass stars (from M to F-type). For this purpose, we have used detailed grids of rotat- ing stellar models computed with the stellar evolu- tion code STAREVOL. The amplitude of dynamo- generated magnetic fields is estimated via physical scaling laws at the base and the top of the convective envelope. We find that the star’s magnetic field has little influence on the excitation of tidal waves in near circular and coplanar Hot-Jupiter systems, but that it has a major impact on the waves dissipation. Our results therefore indicate that a full MHD treatment of the propagation and dissipation of tidal waves is needed to assess the impact of star-planet tidal inter- actions for all low-mass stars along their evolution.
319.09 — Magnetic fields of hot Jupiters calculated from star-planet interactions
Paul Wilson Cauley1; Evgenya L. Shkolnik4; Joe Llama2;
Antonino Lanza3
1 LASP, University of Colorado at Boulder (Boulder, Colorado, United States)
2 Lowell Observatory (Flagstaff, Arizona, United States) 3 Observatorio Astrofisico di Catania, Instituto Nazionale di As- troFisica (Catania, Italy)
4 School of Earth and Space Sciences, Arizona State University (Tempe, Arizona, United States)
Planetary magnetic fields have a critical impact on atmospheric physics, damping winds on hot, short- period planets and potentially creating the neces- sary conditions for habitability on temperate terres- trial worlds by deflecting stellar wind particles. De- spite their importance, exoplanet magnetic field de- tections remain elusive. For the first time, we re- port the derivation of the magnetic fields of a sample of hot Jupiters using flux-calibrated signals of mag- netic star-planet interactions (SPI). We find that the surface magnetic field values for the hot Jupiters in our sample range from 20 G to 120 G, 10 - 50 times larger than the values predicted by pure dynamo theories for planets with rotation periods of 2 to 4 days. Such large field strengths should have severe consequences for velocity flows in the planets’ at- mospheres and exhibit peak frequencies of electron-
cyclotron emission in the range of facilities such as LOFAR.
319.10 — Exploring the Stellar CME-flare Relation: from Historic Events’ Analysis to Stellar Activity Modeling
Sofia Moschou1; Jeremy J. Drake1; Ofer Cohen2; Ce-
cilia Garraffo1; Julian D. Alvarado-Gomez1; Federico
Fraschetti1
1 Center for Astrophysics | Harvard & Smithsonian (Cambridge, Massachusetts, United States)
2 University of Massachusetts Lowell (Lowell, Massachusetts, United States)
A crucial aspect of habitability is the space environ- ment of the planet, which can be extreme and violent for short-orbit planets or planets orbiting active M- dwarfs. CMEs, flares and energetic particles are the most dramatic stellar activity events. How the stored magnetic energy gets distributed between these dif- ferent energetic coronal phenomena remains a vital field of research. In the Sun, more energetic solar X- ray flares are associated with faster and more mas- sive CMEs (e.g. Drake et al 2013). While highly ener- getic flares are continuously observed in active stars, such as M-dwarfs, no stellar CME has been defini- tively observed yet (e.g. Crosley & Osten 2018). In this presentation we discuss our most recent unpub- lished results (paper accepted by ApJ) on the stellar CME-flare relation by examining the most probable historic CME candidates, all of which were observed on magnetically active stars. We use the CME cone model to infer masses and kinetic energies from ob- served quantities, and convert the associated emis- sion to the GOES 1–8 Å band. When the inferred CME masses, found in the range ∼1015 to 1022 g,
are presented as a function of associated flare en- ergy they lie on the solar extrapolated trend. The kinetic energies, found in the 1031to 1037erg, how- ever, lie below the extrapolated relation used on so- lar events. This is an indication that in the stellar regime there is an energy partition between flare X- ray and CME kinetic energies, contrary to the solar case where X-ray flares have 100 times less energy than their associated CMEs. A possible mechanism responsible for constraining the CME speeds more than their masses is the effect of strong large-scale overlying magnetic fields. Stellar CME with lower ki- netic energies present an optimistic scenario for the impact of mass ejecta on close in exoplanets relative to their stellar hosts.
319.11 — Precise characterisation of the 55 Cnc and HD219134 transiting exoplanetary systems
Roxanne Ligi1; Caroline Dorn2; Aurélien Crida3,4;
Francesco Borsa1
1 INAF - Osservatorio Astronomico di Brera (Merate, Italy) 2 University of Zurich (Zürich, Switzerland)
3 Observatoire de la Côte d’Azur (Nice, France) 4 Institut Universitaire de France (Paris, France)
The harvest of exoplanets detection has led to the quest of their characterisation. In particular, the de- termination of exoplanet masses, radii and densities is mandatory to derive their bulk composition, itself related to their formation and potential habitability. However, these parameters totally rely on their host star parameters. Indeed, the transit method (resp. radial velocity measurements) provide the ratio of planetary to stellar radius (resp. mass). When both methods can be applied to a system, then the plan- etary density can be obtained. Unfortunately, most of transiting exoplanets hosts are too faint for their mass and radius to be accurately determined, lead- ing to approximate or imprecise parameters. We will present two systems that host transiting exoplanets, 55 Cnc and HD219134. Since the stars are bright, we performed interferometric observations to measure their angular diameter, leading to accurate radii R. Using the transit light curves, we also directly derive their density. We thus obtain an independent mea- surement of their mass M. More precisely, we com- puted the joint probability density function of these parameters, and the correlation between R and M. This allows to derive the planetary parameters, inde- pendently from any stellar evolution model. Using the R-M correlation, the planetary density can be re- fined, and subsequently the internal composition of the transiting exoplanets. Contrary to what previous studies claim, the transiting exoplanet 55 Cnc e may only have a thin atmosphere and its interior struc- ture might not be dominated by carbon. The system of HD219134 hosts two transiting exoplanets of sim- ilar properties, but HD210134 b shows a higher den- sity while smaller radius and mass than HD219134 c. This could be explained by a molten interior possibly induced by tidal heating caused by a high eccentric- ity during its formation. Those systems are bench- marks to investigate both exoplanet and stellar prop- erties in detail thanks to our method that provides the best characterisation of these systems so far. It will be generalised with the coming bright TESS and PLATO targets.
319.12 — Evryscope & K2 Constraints on TRAPPIST-1 Superflare Occurrence and Plan- etary Habitability
Amy Louise Glazier1; Ward Howard1; Henry Corbett1; Nicholas Law1; Jeffrey Ratzloff1; Octavi Fors4; Daniel del Ser4; Anna Hughes2; Robert Quimby3
1 Physics & Astronomy, University of North Carolina, Chapel Hill (Chapel Hill, North Carolina, United States)
2 Physics & Astronomy, University of British Columbia, Vancouver