Publication Search Results

ID: 156892
Type: article
Authors: Alvarado-Gómez, Julián D.; Drake, Jeremy J.; Fraschetti, Federico; Garraffo, Cecilia; Cohen, Ofer; Vocks, Christian; Poppenhäger, Katja; Moschou, Sofia P.; Yadav, Rakesh K.; Manchester, Ward B.,IV
Abstract: Coronal mass ejections (CMEs) on stars other than the Sun have proven very difficult to detect. One promising pathway lies in the detection of type II radio bursts. Their appearance and distinctive properties are associated with the development of an outward propagating CME-driven shock. However, dedicated radio searches have not been able to identify these transient features in other stars. Large Alfvén speeds and the magnetic suppression of CMEs in active stars have been proposed to render stellar eruptions "radio-quiet." Employing 3D magnetohydrodynamic simulations, we study the distribution of the coronal Alfvén speed, focusing on two cases representative of a young Sun-like star and a mid- activity M-dwarf (Proxima Centauri). These results are compared with a standard solar simulation and used to characterize the shock-prone regions in the stellar corona and wind. Furthermore, using a flux-rope eruption model, we drive realistic CME events within our M-dwarf simulation. We consider eruptions with different energies to probe the regimes of weak and partial CME magnetic confinement. While these CMEs are able to generate shocks in the corona, those are pushed much farther out compared to their solar counterparts. This drastically reduces the resulting type II radio burst frequencies down to the ionospheric cutoff, which impedes their detection with ground-based instrumentation.

ID: 157612
Type: article
Authors: Alvarado-Gómez, Julián D.; Drake, Jeremy J.; Garraffo, Cecilia; Cohen, Ofer; Poppenhaeger, Katja; Yadav, Rakesh K.; Moschou, Sofia P.
Abstract: A new planet has been recently discovered around Proxima Centauri. With an orbital separation of ∼1.44 au and a minimum mass of about $7\,{M}_{\oplus }$ , Proxima c is a prime direct imaging target for atmospheric characterization. The latter can only be performed with a good understanding of the space environment of the planet, as multiple processes can have profound effects on the atmospheric structure and evolution. Here, we take one step in this direction by generating physically realistic numerical simulations of Proxima's stellar wind, coupled to a magnetosphere and ionosphere model around Proxima c. We evaluate their expected variation due to the magnetic cycle of the host star, as well as for plausible inclination angles for the exoplanet orbit. Our results indicate stellar wind dynamic pressures comparable to present-day Earth, with a slight increase (by a factor of 2) during high-activity periods of the star. A relatively weak interplanetary magnetic field at the distance of Proxima c leads to negligible stellar wind Joule heating of the upper atmosphere (about 10% of the solar wind contribution on Earth) for an Earth-like planetary magnetic field (0.3 G). Finally, we provide an assessment of the likely extreme conditions experienced by the exoplanet candidate Proxima d, tentatively located at 0.029 au with a minimum mass of 0.29 M_⊕.

ID: 157761
Type: article
Authors: Cohen, Ofer; Garraffo, Cecilia; Moschou, Sofia-Paraskevi; Drake, Jeremy J.; Alvarado-Gómez, J. D.; Glocer, Alex; Fraschetti, Federico
Abstract: We investigate the space environment conditions near the Earth-size planet TOI 700 d using a set of numerical models for the stellar corona and wind, the planetary magnetosphere, and the planetary ionosphere. We drive our simulations using a scaled-down stellar input and a scaled-up solar input in order to obtain two independent solutions. We find that for the particular parameters used in our study, the stellar wind conditions near the planet are not very extreme-slightly stronger than that near the Earth in terms of the stellar wind ram pressure and the intensity of the interplanetary magnetic field. Thus, the space environment near TOI 700 d may not be extremely harmful to the planetary atmosphere, assuming the planet resembles the Earth. Nevertheless, we stress that the stellar input parameters and the actual planetary parameters are unconstrained, and different parameters may result in a much greater effect on the atmosphere of TOI 700 d. Finally, we compare our results to solar wind measurements in the solar system and stress that modest stellar wind conditions may not guarantee atmospheric retention of exoplanets.

ID: 158876
Type: article
Authors: France, Kevin; Duvvuri, Girish; Egan, Hilary; Koskinen, Tommi; Wilson, David J.; Youngblood, Allison; Froning, Cynthia S.; Brown, Alexander; Alvarado-Gómez, Julián D.; Berta-Thompson, Zachory K.; Drake, Jeremy J.; Garraffo, Cecilia; Kaltenegger, Lisa; Kowalski, Adam F.; Linsky, Jeffrey L.; Loyd, R. O. Parke; Mauas, Pablo J. D.; Miguel, Yamila; Pineda, J. Sebastian; Rugheimer, Sarah; Schneider, P. Christian; Tian, Feng; Vieytes, Mariela
Abstract: Recent work has demonstrated that high levels of X-ray and UV activity on young M dwarfs may drive rapid atmospheric escape on temperate, terrestrial planets orbiting within the habitable zone. However, secondary atmospheres on planets orbiting older, less active M dwarfs may be stable and present more promising candidates for biomarker searches. In order to evaluate the potential habitability of Earth-like planets around old, inactive M dwarfs, we present new Hubble Space Telescope and Chandra X-ray Observatory observations of Barnard&'s Star (GJ 699), a 10 Gyr old M3.5 dwarf, acquired as part of the Mega-MUSCLES program. Despite the old age and long rotation period of Barnard&'s Star, we observe two FUV (d₁₃₀ ? 5000 s; E₁₃₀ ? 10^29.5 erg each) and one X-ray (E_X ? 10^29.2 erg) flares, and we estimate a high-energy flare duty cycle (defined here as the fraction of the time the star is in a flare state) of ˜25%. A publicly available 5 Å to 10 µm spectral energy distribution of GJ 699 is created and used to evaluate the atmospheric stability of a hypothetical, unmagnetized terrestrial planet in the habitable zone (r_HZ ˜ 0.1 au). Both thermal and nonthermal escape modeling indicate (1) the quiescent stellar XUV flux does not lead to strong atmospheric escape: atmospheric heating rates are comparable to periods of high solar activity on modern Earth, and (2) the flare environment could drive the atmosphere into a hydrodynamic loss regime at the observed flare duty cycle: sustained exposure to the flare environment of GJ 699 results in the loss of ?87 Earth atmospheres Gyr^-1 through thermal processes and ?3 Earth atmospheres Gyr^-1 through ion loss processes. These results suggest that if rocky planet atmospheres can survive the initial ˜5 Gyr of high stellar activity, or if a second-generation atmosphere can be formed or acquired, the flare duty cycle may be the controlling stellar parameter for the stability of Earth-like atmospheres around old M stars.

ID: 157762
Type: article
Authors: Gronoff, G.; Arras, P.; Baraka, S.; Bell, J. M.; Cessateur, G.; Cohen, O.; Curry, S. M.; Drake, Jeremy J.; Elrod, M.; Erwin, J.; Garcia-Sage, K.; Garraffo, Cecilia; Glocer, A.; Heavens, N. G.; Lovato, K.; Maggiolo, R.; Parkinson, C. D.; Simon Wedlund, C.; Weimer, D. R.; Moore, W. B.
Abstract: The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical, and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our solar system: It is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H-dominated thermosphere for astrophysicists or the Sun-like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so-called Xenon paradox.

ID: 154628
Type: article
Authors: Alvarado-Gómez, Julián D.; Drake, Jeremy J.; Moschou, Sofia P.; Garraffo, Cecilia; Cohen, Ofer; NASA LWS Focus Science Team: Solar-Stellar Connection; Yadav, Rakesh K.; Fraschetti, Federico
Abstract: We report the results of the first state-of-the-art numerical simulations of coronal mass ejections (CMEs) taking place in realistic magnetic field configurations of moderately active M-dwarf stars. Our analysis indicates that a clear, novel, and observable, coronal response is generated due to the collapse of the eruption and its eventual release into the stellar wind. Escaping CME events, weakly suppressed by the large-scale field, induce a flare-like signature in the emission from coronal material at different temperatures due to compression and associated heating. Such flare-like profiles display a distinctive temporal evolution in their Doppler shift signal (from red to blue), as the eruption first collapses toward the star and then perturbs the ambient magnetized plasma on its way outwards. For stellar fields providing partial confinement, CME fragmentation takes place, leading to rise and fall flow patterns which resemble the solar coronal rain cycle. In strongly suppressed events, the response is better described as a gradual brightening, in which the failed CME is deposited in the form of a coronal rain cloud leading to a much slower rise in the ambient high-energy flux by relatively small factors (̃2-3). In all the considered cases (escaping/confined) a fractional decrease in the emission from midrange coronal temperature plasma occurs, similar to the coronal dimming events observed on the Sun. Detection of the observational signatures of these CME-induced features requires a sensitive next generation X-ray space telescope.

ID: 155312
Type: article
Authors: Alvarado-Gómez, Julián D.; Garraffo, Cecilia; Drake, Jeremy J.; Brown, Benjamin P.; Oishi, Jeffrey S.; Moschou, Sofia P.; Cohen, Ofer
Abstract: A physically realistic stellar wind model based on Alfvén wave dissipation has been used to simulate the wind from Barnard's Star and to estimate the conditions at the location of its recently discovered planetary companion. Such models require knowledge of the stellar surface magnetic field that is currently unknown for Barnard's Star. We circumvent this by considering the observed field distributions of three different stars that constitute admissible magnetic proxies of this object. Under these considerations, Barnard's Star b experiences less intense wind pressure than the much more close-in planet Proxima b and the planets of the TRAPPIST-1 system. The milder wind conditions are more a result of its much greater orbital distance rather than in differences in the surface magnetic field strengths of the host stars. The dynamic pressure experienced by the planet is comparable to present- day Earth values, but it can undergo variations by factors of several during current sheet crossings in each orbit. The magnetospause standoff distance would be ∼20%-40% smaller than that of the Earth for an equivalent planetary magnetic field strength.

ID: 155410
Type: article
Authors: Fraschetti, Federico; Drake, Jeremy J.; Alvarado-Gómez, Julian D.; Moschou, Sofia-Paraskevi; Garraffo, Cecilia; Cohen, O.
Abstract: Planets in close proximity to their parent star, such as those in the habitable zones around M dwarfs, could be subject to particularly high doses of particle radiation. We have carried out test-particle simulations of ∼GeV protons to investigate the propagation of energetic particles accelerated by flares or traveling shock waves within the stellar wind and magnetic field of a TRAPPIST-1-like system. Turbulence was simulated with small-scale magnetostatic perturbations with an isotropic power spectrum. We find that only a few percent of particles injected within half a stellar radius from the stellar surface escape, and that the escaping fraction increases strongly with increasing injection radius. Escaping particles are increasingly deflected and focused by the ambient spiraling magnetic field as the superimposed turbulence amplitude is increased. In our TRAPPIST-1-like simulations, regardless of the angular region of injection, particles are strongly focused onto two caps within the fast wind regions and centered on the equatorial planetary orbital plane. Based on a scaling relation between far-UV emission and energetic protons for solar flares applied to M dwarfs, the innermost putative habitable planet, TRAPPIST-1e, is bombarded by a proton flux up to 6 orders of magnitude larger than experienced by the present-day Earth. We note two mechanisms that could strongly limit EP fluxes from active stars: EPs from flares are contained by the stellar magnetic field; and potential CMEs that might generate EPs at larger distances also fail to escape.

ID: 152917
Type: article
Authors: Moschou, Sofia-Paraskevi; Drake, Jeremy J.; Cohen, Ofer; Alvarado-Gómez, Julián D.; Garraffo, Cecilia; Fraschetti, Federico
Abstract: Solar coronal mass ejections (CMEs) and flares have a statistically well-defined relationship, with more energetic X-ray flares corresponding to faster and more massive CMEs. How this relationship extends to more magnetically active stars is a subject of open research. Here we study the most probable stellar CME candidates associated with flares captured in the literature to date, all of which were observed on magnetically active stars. We use a simple CME model to derive masses and kinetic energies from observed quantities and transform associated flare data to the Geostationary Operational Environmental Satellite 1–8 Å band. Derived CME masses range from ∼10¹⁵ to 10²² g. Associated flare X-ray energies range from 10³¹ to 10³⁷ erg. Stellar CME masses as a function of associated flare energy generally lie along or below the extrapolated mean for solar events. In contrast, CME kinetic energies lie below the analogous solar extrapolation by roughly 2 orders of magnitude, indicating approximate parity between flare X-ray and CME kinetic energies. These results suggest that the CMEs associated with very energetic flares on active stars are more limited in terms of the ejecta velocity than the ejecta mass, possibly because of the restraining influence of strong overlying magnetic fields and stellar wind drag. Lower CME kinetic energies and velocities present a more optimistic scenario for the effects of CME impacts on exoplanets in close proximity to active stellar hosts.

ID: 149000
Type: article
Authors: Alvarado-Gómez, Julián D.; Drake, Jeremy J.; Cohen, Ofer; Moschou, Sofia P.; Garraffo, Cecilia
Abstract: We present results from a set of numerical simulations aimed at exploring the mechanism of coronal mass ejection (CME) suppression in active stars by an overlying large-scale magnetic field. We use a state-of-the-art 3D magnetohydrodynamic code that considers a self-consistent coupling between an Alfvén wave-driven stellar wind solution, and a first-principles CME model based on the eruption of a flux rope anchored to a mixed-polarity region. By replicating the driving conditions used in simulations of strong solar CMEs, we show that a large-scale dipolar magnetic field of 75 G is able to fully confine eruptions within the stellar corona. Our simulations also consider CMEs exceeding the magnetic energy used in solar studies, which are able to escape the large-scale magnetic field confinement. The analysis includes a qualitative and quantitative description of the simulated CMEs and their dynamics, which reveals a drastic reduction of the radial speed caused by the overlying magnetic field. With the aid of recent observational studies, we place our numerical results in the context of solar and stellar flaring events. In this way, we find that this particular large-scale magnetic field configuration establishes a suppression threshold around ~3 × 10³² erg in the CME kinetic energy. Extending the solar flare-CME relations to other stars, such CME kinetic energies could be typically achieved during erupting flaring events with total energies larger than 6 × 10³² erg (GOES class ~X70).

ID: 145790
Type: article
Authors: Alvarado-Gómez, Julián D.; Hussain, Gaitee A. J.; Drake, Jeremy J.; Donati, Jean-François; Sanz-Forcada, Jorge; Stelzer, Beate; Cohen, Ofer; Amazo-Gómez, Eliana M.; Grunhut, Jason H.; Garraffo, Cecilia; Moschou, Sofia P.; Silvester, James; Oksala, Mary E.
Abstract: A former member of the Hyades cluster, ι Horologii (ι Hor) is a planet-hosting Sun-like star which displays the shortest coronal activity cycle known to date (P_cyc ∼ 1.6 yr). With an age of ∼625 Myr, ι Hor is also the youngest star with a detected activity cycle. The study of its magnetic properties holds the potential to provide fundamental information to understand the origin of cyclic activity and stellar magnetism in late-type stars. In this series of papers, we present the results of a comprehensive project aimed at studying the evolving magnetic field in this star and how this evolution influences its circumstellar environment. This paper summarizes the first stage of this investigation, with results from a long-term observing campaign of ι Hor using ground-based high-resolution spectropolarimetry. The analysis includes precise measurements of the magnetic activity and radial velocity of the star, and their multiple time-scales of variability. In combination with values reported in the literature, we show that the long-term chromospheric activity evolution of ι Hor follows a beating pattern, caused by the superposition of two periodic signals of similar amplitude at P₁ ≃ 1.97 ± 0.02 yr and P₂ ≃ 1.41 ± 0.01 yr. Additionally, using the most recent parameters for ι Hor b in combination with our activity and radial velocity measurements, we find that stellar activity dominates the radial velocity residuals, making the detection of additional planets in this system challenging. Finally, we report here the first measurements of the surface longitudinal magnetic field strength of ι Hor, which displays varying amplitudes within ±4 G and served to estimate the rotation period of the star (P_rot = 7.70^{+0.18}_{-0.67} d).

ID: 146143
Type: article
Authors: Cohen, Ofer; Glocer, Alex; Garraffo, Cecilia; Drake, Jeremy J.; Bell, Jared M.
Abstract: We present a method to quantify the upper limit of the energy transmitted from the intense stellar wind to the upper atmospheres of three of the TRAPPIST-1 planets (e, f, and g). We use a formalism that treats the system as two electromagnetic regions, where the efficiency of the energy transmission between one region (the stellar wind at the planetary orbits) to the other (the planetary ionospheres) depends on the relation between the conductances and impedances of the two regions. Since the energy flux of the stellar wind is very high at these planetary orbits, we find that for the case of high transmission efficiency (when the conductances and impedances are close in magnitude), the energy dissipation in the upper planetary atmospheres is also very large. On average, the Ohmic energy can reach 0.5–1 W m^‑2, about 1% of the stellar irradiance and 5–15 times the EUV irradiance. Here, using constant values for the ionospheric conductance, we demonstrate that the stellar wind energy could potentially drive large atmospheric heating in terrestrial planets, as well as in hot Jupiters. More detailed calculations are needed to assess the ionospheric conductance and to determine more accurately the amount of heating the stellar wind can drive in close-orbit planets.

ID: 150049
Type: article
Authors: Cohen, Ofer; Moschou, Sofia-Paraskevi; Glocer, Alex; Sokolov, Igor V.; Mazeh, Tsevi; Drake, Jeremy J.; Garraffo, Cecilia; Alvarado-Gómez, Julian D.
Abstract: The search for exoplanets in the radio bands has been focused on detecting radio emissions produced by the interaction between magnetized planets and the stellar wind (auroral emission). Here we introduce a new tool, which is part of our MHD stellar corona model, to predict the ambient coronal radio emission and its modulations induced by a close planet. For simplicity, the present work assumes that the exoplanet is stationary in the frame rotating with the stellar rotation. We explore the radio flux modulations using a limited parameter space of idealized cases by changing the magnitude of the planetary field, its polarity, the planetary orbital separation, and the strength of the stellar field. We find that the modulations induced by the planet could be significant and observable in the case of hot Jupiter planets— above 100% modulation with respect to the ambient flux in the 10–100 MHz range in some cases, and 2%–10% in the frequency bands above 250 MHz for some cases. Thus, our work indicates that radio signature of exoplanets might not be limited to low-frequency radio range. We find that the intensity modulations are sensitive to the planetary magnetic field polarity for short-orbit planets, and to the stellar magnetic field strength for all cases. The new radio tool, when applied to real systems, could provide predictions for the frequency range at which the modulations can be observed by current facilities.

ID: 150057
Type: article
Authors: Moschou, Sofia-Paraskevi; Sokolov, Igor; Cohen, Ofer; Drake, Jeremy J.; Borovikov, Dmitry; Kasper, Justin C.; Alvarado-Gomez, Julian D.; Garraffo, Cecilia
Abstract: Radio observations grant access to a wide range of physical processes through different emission mechanisms. These processes range from thermal and quiescent to eruptive phenomena, such as shock waves and particle beams. We present a new synthetic radio imaging tool that calculates and visualizes the bremsstrahlung radio emission. This tool works concurrently with state-of-the-art magnetohydrodynamic simulations of the solar corona using the code Block-Adaptive Tree Solarwind Roe Upwind Scheme (BATS-R-US). Our model produces results that are in good agreement with both high- and low-frequency observations of the solar disk. In this study, a ray-tracing algorithm is used, and the radio intensity is computed along the actual curved ray trajectories. We illustrate the importance of refraction in locating the radio-emitting source by comparison of the radio imaging illustrations when the line of sight is considered instead of the refracted paths. We are planning to incorporate nonthermal radio emission mechanisms in a future version of the radio imaging tool.

ID: 146147
Type: article
Authors: Pognan, Quentin; Garraffo, Cecilia; Cohen, Ofer; Drake, Jeremy J.
Abstract: We use magnetograms of eight solar analogs of ages 30 Myr–3.6 Gyr obtained from Zeeman Doppler Imaging and taken from the literature, together with two solar magnetograms, to drive magnetohydrodynamical wind simulations and construct an evolutionary scenario of the solar wind environment and its angular momentum loss rate. With observed magnetograms of the radial field strength as the only variant in the wind model, we find that a power-law model fitted to the derived angular momentum loss rate against time, t, results in a spin-down relation Ω ∝ t ^‑0.51, for angular speed Ω, which is remarkably consistent with the well-established Skumanich law Ω ∝ t ^‑0.5. We use the model wind conditions to estimate the magnetospheric standoff distances for an Earth-like test planet situated at 1 au for each of the stellar cases, and to obtain trends of minimum and maximum wind ram pressure and average ram pressure in the solar system through time. The wind ram pressure declines with time as \overline{{P}_ram}}\propto {t}^2/3, amounting to a factor of 50 or so over the present lifetime of the solar system.

ID: 142332
Type: article
Authors: Cohen, O.; Yadav, R.; Garraffo, Cecilia; Saar, S. H.; Wolk, S. J.; Kashyap, V. L.; Drake, Jeremy J.; Pillitteri, Ignazio
Abstract: Observations indicate that magnetic fields in rapidly rotating stars are very strong, on both small and large scales. What is the nature of the resulting corona? Here we seek to shed some light on this question. We use the results of an anelastic dynamo simulation of a rapidly rotating fully convective M star to drive a physics-based model for the stellar corona. We find that due to the several kilo Gauss large-scale magnetic fields at high latitudes, the corona, and its X-ray emission are dominated by star-size large hot loops, while the smaller, underlying colder loops are not visible much in the X-ray. Based on this result, we propose that, in rapidly rotating stars, emission from such coronal structures dominates the quiescent, cooler but saturated X-ray emission.

ID: 143807
Type: article
Authors: Garraffo, Cecilia; Drake, Jeremy J.; Cohen, Ofer; Alvarado-Gómez, Julian D.; Moschou, Sofia P.
Abstract: Recently, four additional Earth-mass planets were discovered orbiting the nearby ultracool M8 dwarf, TRAPPIST-1, making a remarkable total of seven planets with equilibrium temperatures compatible with the presence of liquid water on their surface. Temperate terrestrial planets around an M-dwarf orbit close to their parent star, rendering their atmospheres vulnerable to erosion by the stellar wind and energetic electromagnetic and particle radiation. Here, we use state-of-the-art 3D magnetohydrodynamic models to simulate the wind around TRAPPIST-1 and study the conditions at each planetary orbit. All planets experience a stellar wind pressure between 10³ and 10⁵ times the solar wind pressure on Earth. All orbits pass through wind pressure changes of an order of magnitude and most planets spend a large fraction of their orbital period in the sub-Alfvénic regime. For plausible planetary magnetic field strengths, all magnetospheres are greatly compressed and undergo much more dynamic change than that of the Earth. The planetary magnetic fields connect with the stellar radial field over much of the planetary surface, allowing the direct flow of stellar wind particles onto the planetary atmosphere. These conditions could result in strong atmospheric stripping and evaporation and should be taken into account for any realistic assessment of the evolution and habitability of the TRAPPIST-1 planets.

ID: 145658
Type: article
Authors: Moschou, Sofia-Paraskevi; Drake, Jeremy J.; Cohen, Ofer; Alvarado-Gomez, Julian D.; Garraffo, Cecilia
Abstract: We explore the scenario of a coronal mass ejection (CME) being the cause of the observed continuous X-ray absorption of the 1997 August 30 superflare on the eclipsing binary Algol (the Demon Star). The temporal decay of the absorption is consistent with absorption by a CME undergoing self-similar evolution with uniform expansion velocity. We investigate the kinematic and energetic properties of the CME using the ice cream cone model for its three-dimensional structure in combination with the observed profile of the hydrogen column density decline with time. Different physically justified length scales were used that allowed us to estimate lower and upper limits of the possible CME characteristics. Further consideration of the maximum available magnetic energy in starspots leads us to quantify its mass as likely lying in the range 2× {10}²¹ {--} 2× {10}²² g and kinetic energy in the range 7× {10}³⁵ {--} 3× {10}³⁸ erg. The results are in reasonable agreement with extrapolated relations between flare X-ray fluence and CME mass and kinetic energy derived for solar CMEs.

ID: 139649
Type: article
Authors: Alvarado-Gómez, J. D.; Hussain, G. A. J.; Cohen, O.; Drake, Jeremy J.; Garraffo, Cecilia; Grunhut, J.; Gombosi, T. I.
Abstract: We present the results of a detailed numerical simulation of the circumstellar environment around three exoplanet-hosting stars. A modern global magnetohydrodynamic model is considered that includes Alfvén wave dissipation as a self-consistent coronal heating mechanism. This paper contains the description of the numerical set-up, evaluation procedure, and the simulated coronal structure of each system (HD 1237, HD 22049, and HD 147513). The simulations are driven by surface magnetic field maps, recovered with the observational technique of Zeeman-Doppler imaging. A detailed comparison of the simulations is performed, where two different implementations of this mapping routine are used to generate the surface field distributions. Quantitative and qualitative descriptions of the coronae of these systems are presented, including synthetic high-energy emission maps in the extreme ultraviolet (EUV) and soft X-ray (SXR) ranges. Using the simulation results, we are able to recover similar trends as in previous observational studies, including the relation between the magnetic flux and the coronal X-ray emission. Furthermore, for HD 1237 we estimate the rotational modulation of the high-energy emission that is due to the various coronal features developed in the simulation. We obtain variations during a single stellar rotation cycle of up to 15% for the EUV and SXR ranges. The results presented here will be used in a follow-up paper to self-consistently simulate the stellar winds and inner astrospheres of these systems.

ID: 142025
Type: article
Authors: Alvarado-Gómez, J. D.; Hussain, G. A. J.; Cohen, O.; Drake, Jeremy J.; Garraffo, Cecilia; Grunhut, J.; Gombosi, T. I.
Abstract: We present the results of a comprehensive numerical simulation of the environment around three exoplanet-host stars (HD 1237, HD 22049, and HD 147513). Our simulations consider one of the latest models currently used for space weather studies in the Heliosphere, with turbulent Alfvén wave dissipation as the source of coronal heating and stellar wind acceleration. Large-scale magnetic field maps, recovered with two implementations of the tomographic technique of Zeeman-Doppler imaging, serve to drive steady-state solutions in each system. This paper contains the description of the stellar wind and inner astrosphere, while the coronal structure was discussed in a previous paper. The analysis includes the magneto-hydrodynamical properties of the stellar wind, the associated mass and angular momentum loss rates, as well as the topology of the astrospheric current sheet in each system. A systematic comparison among the considered cases is performed, including two reference solar simulations covering activity minimum and maximum. For HD 1237, we investigate the interactions between the structure of the developed stellar wind, and a possible magnetosphere around the Jupiter-mass planet in this system. We find that the process of particle injection into the planetary atmosphere is dominated by the density distribution rather than the velocity profile of the stellar wind. In this context, we predict a maximum exoplanetary radio emission of 12 mJy at 40 MHz in this system, assuming the crossing of a high-density streamer during periastron passage. Furthermore, in combination with the analysis performed in the first paper of this study, we obtain for the first time a fully simulated mass loss-activity relation. This relation is compared and discussed in the context of the previously proposed observational counterpart, derived from astrospheric detections. Finally, we provide a characterisation of the global 3D properties of the stellar wind of these systems, at the inner edges of their habitable zones.