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Showing 1-15 of about 15 results.
Tuning the Exospace Weather Radio for Stellar Coronal Mass EjectionsAlvarado-Gómez, Julián D.Drake, Jeremy J.Fraschetti, FedericoGarraffo, CeciliaCohen, OferVocks, ChristianPoppenhäger, KatjaMoschou, Sofia P.Yadav, Rakesh K.Manchester, Ward B.,IVDOI: info:10.3847/1538-4357/ab88a3v. 89547
Alvarado-Gómez, Julián D., Drake, Jeremy J., Fraschetti, Federico, Garraffo, Cecilia, Cohen, Ofer, Vocks, Christian, Poppenhäger, Katja, Moschou, Sofia P., Yadav, Rakesh K., and Manchester, Ward B.,IV. 2020. "Tuning the Exospace Weather Radio for Stellar Coronal Mass Ejections." The Astrophysical Journal 895:47. https://doi.org/10.3847/1538-4357/ab88a3
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.
An Earth-like Stellar Wind Environment for Proxima Centauri cAlvarado-Gómez, Julián D.Drake, Jeremy J.Garraffo, CeciliaCohen, OferPoppenhaeger, KatjaYadav, Rakesh K.Moschou, Sofia P.DOI: info:10.3847/2041-8213/abb885v. 902L9
Alvarado-Gómez, Julián D., Drake, Jeremy J., Garraffo, Cecilia, Cohen, Ofer, Poppenhaeger, Katja, Yadav, Rakesh K., and Moschou, Sofia P. 2020. "An Earth-like Stellar Wind Environment for Proxima Centauri c." The Astrophysical Journal 902:L9. https://doi.org/10.3847/2041-8213/abb885
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.
The Space Environment and Atmospheric Joule Heating of the Habitable Zone Exoplanet TOI 700 dCohen, OferGarraffo, CeciliaMoschou, Sofia-ParaskeviDrake, Jeremy J.Alvarado-Gómez, J. D.Glocer, AlexFraschetti, FedericoDOI: info:10.3847/1538-4357/ab9637v. 897101
Cohen, Ofer, Garraffo, Cecilia, Moschou, Sofia-Paraskevi, Drake, Jeremy J., Alvarado-Gómez, J. D., Glocer, Alex, and Fraschetti, Federico. 2020. "The Space Environment and Atmospheric Joule Heating of the Habitable Zone Exoplanet TOI 700 d." The Astrophysical Journal 897:101. https://doi.org/10.3847/1538-4357/ab9637
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.
X-Ray Observations of the Peculiar Cepheid V473 Lyr Identify A Low-mass CompanionEvans, Nancy RemagePillitteri, IgnazioMolnar, LaszloSzabados, LaszloPlachy, EmeseSzabo, RobertEngle, ScottGuinan, EdwardWolk, ScottGünther, H. MoritzNeilson, HildingMarengo, MassimoMatthews, Lynn D.Moschou, SofiaDrake, Jeremy J.Kashyap, VinayKervella, PierreTordai, TamasSomogyi, PeterBurki, GilbertDOI: info:10.3847/1538-3881/ab7121v. 159121
Evans, Nancy Remage, Pillitteri, Ignazio, Molnar, Laszlo, Szabados, Laszlo, Plachy, Emese, Szabo, Robert, Engle, Scott, Guinan, Edward, Wolk, Scott, Günther, H. Moritz, Neilson, Hilding, Marengo, Massimo, Matthews, Lynn D., Moschou, Sofia, Drake, Jeremy J., Kashyap, Vinay, Kervella, Pierre, Tordai, Tamas, Somogyi, Peter, and Burki, Gilbert. 2020. "X-Ray Observations of the Peculiar Cepheid V473 Lyr Identify A Low-mass Companion." The Astronomical Journal 159:121. https://doi.org/10.3847/1538-3881/ab7121
ID: 156367
Type: article
Authors: Evans, Nancy Remage; Pillitteri, Ignazio; Molnar, Laszlo; Szabados, Laszlo; Plachy, Emese; Szabo, Robert; Engle, Scott; Guinan, Edward; Wolk, Scott; Günther, H. Moritz; Neilson, Hilding; Marengo, Massimo; Matthews, Lynn D.; Moschou, Sofia; Drake, Jeremy J.; Kashyap, Vinay; Kervella, Pierre; Tordai, Tamas; Somogyi, Peter; Burki, Gilbert
Abstract: V473 Lyr is a classical Cepheid that is unique in having substantial amplitude variations with a period of approximately 3.3 yr, thought to be similar to the Blazhko variations in RR Lyrae stars. We obtained an XMM-Newton observation of this star to follow up a previous detection in X-rays. Rather than the X-ray burst and rapid decline near maximum radius seen in δ Cephei itself, the X-ray flux in V473 Lyr remained constant for a third of the pulsation cycle covered by the observation. Thus the X-rays are probably not produced by the changes around the pulsation cycle. The X-ray spectrum is soft (kT = 0.6 keV), with X-ray properties consistent with a young low-mass companion. Previously there was no evidence of a companion in radial velocities or in Gaia and Hipparcos proper motions. While this rules out companions that are very close or very distant, a binary companion at a separation between 30 and 300 au is possible. This is an example of an X-ray observation revealing evidence of a low-mass companion, which is important in completing the mass ratio statistics of binary Cepheids. Furthermore, the detection of a young X-ray bright companion is a further indication that the Cepheid (primary) is a Population I star, even though its pulsation behavior differs from other classical Cepheids.
Phase-modulated X-Ray Emission from Cepheids due to Pulsation-driven ShocksMoschou, Sofia-ParaskeviVlahakis, NektariosDrake, Jeremy J.Evans, Nancy RemageNeilson, Hilding R.Guzik, Joyce AnnZuhone, JohnDOI: info:10.3847/1538-4357/aba8fav. 900157
Moschou, Sofia-Paraskevi, Vlahakis, Nektarios, Drake, Jeremy J., Evans, Nancy Remage, Neilson, Hilding R., Guzik, Joyce Ann, and Zuhone, John. 2020. "Phase-modulated X-Ray Emission from Cepheids due to Pulsation-driven Shocks." The Astrophysical Journal 900:157. https://doi.org/10.3847/1538-4357/aba8fa
ID: 157760
Type: article
Authors: Moschou, Sofia-Paraskevi; Vlahakis, Nektarios; Drake, Jeremy J.; Evans, Nancy Remage; Neilson, Hilding R.; Guzik, Joyce Ann; Zuhone, John
Abstract: Cepheids are pulsating variable stars with a periodic chromospheric response at UV wavelengths close to their minimum radius phase. Recently, an X-ray variable signature was captured in observations during the maximum radius phase. This X-ray emission came as a surprise and is not understood. In this work, we use the modern astrophysical code PLUTO to investigate the effects of pulsations on Cepheid X-ray emission. We run a number of hydrodynamic numerical simulations with a variety of initial and boundary conditions in order to explore the capability of shocks to produce the observed phase-dependent X-ray behavior. Finally, we use the Simulated Observations of X-ray Sources (SOXS) package to create synthetic spectra for each simulation case and link our simulations to observables. We show that, for certain conditions, we can reproduce observed X-ray fluxes at phases 0.4-0.8 when the Cepheid is at maximum radius. Our results span a wide range of mass-loss rates, 2 × 10-13 M yr-1 to 3 × 10-8 M yr-1, and peak X-ray luminosities, 5 × 10-17 erg cm-2 s-1 to 1.4 × 10-12 erg cm-2 s-1. We conclude that Cepheids exhibit two-component emission with (a) shock waves being responsible for the phase-dependent variable emission (phases 0.2-0.6) and (b) a separate quiescent mechanism being the dominant emission mechanism for the remaining phases.
Coronal Response to Magnetically Suppressed CME Events in M-dwarf StarsAlvarado-Gómez, Julián D.Drake, Jeremy J.Moschou, Sofia P.Garraffo, CeciliaCohen, OferNASA LWS Focus Science Team: Solar-Stellar ConnectionYadav, Rakesh K.Fraschetti, FedericoDOI: info:10.3847/2041-8213/ab44d0v. 884L13
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., and Fraschetti, Federico. 2019. "Coronal Response to Magnetically Suppressed CME Events in M-dwarf Stars." The Astrophysical Journal 884:L13. https://doi.org/10.3847/2041-8213/ab44d0
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.
Breezing through the Space Environment of Barnard's Star bAlvarado-Gómez, Julián D.Garraffo, CeciliaDrake, Jeremy J.Brown, Benjamin P.Oishi, Jeffrey S.Moschou, Sofia P.Cohen, OferDOI: info:10.3847/2041-8213/ab1489v. 875L12
Alvarado-Gómez, Julián D., Garraffo, Cecilia, Drake, Jeremy J., Brown, Benjamin P., Oishi, Jeffrey S., Moschou, Sofia P., and Cohen, Ofer. 2019. "Breezing through the Space Environment of Barnard's Star b." The Astrophysical Journal 875:L12. https://doi.org/10.3847/2041-8213/ab1489
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.
Stellar Energetic Particles in the Magnetically Turbulent Habitable Zones of TRAPPIST-1-like Planetary SystemsFraschetti, FedericoDrake, Jeremy J.Alvarado-Gómez, Julian D.Moschou, Sofia-ParaskeviGarraffo, CeciliaCohen, O.DOI: info:10.3847/1538-4357/ab05e4v. 87421
Fraschetti, Federico, Drake, Jeremy J., Alvarado-Gómez, Julian D., Moschou, Sofia-Paraskevi, Garraffo, Cecilia, and Cohen, O. 2019. "Stellar Energetic Particles in the Magnetically Turbulent Habitable Zones of TRAPPIST-1-like Planetary Systems." The Astrophysical Journal 874:21. https://doi.org/10.3847/1538-4357/ab05e4
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.
The Stellar CME–Flare Relation: What Do Historic Observations Reveal?Moschou, Sofia-ParaskeviDrake, Jeremy J.Cohen, OferAlvarado-Gómez, Julián D.Garraffo, CeciliaFraschetti, FedericoDOI: info:10.3847/1538-4357/ab1b37v. 877105
Moschou, Sofia-Paraskevi, Drake, Jeremy J., Cohen, Ofer, Alvarado-Gómez, Julián D., Garraffo, Cecilia, and Fraschetti, Federico. 2019. "The Stellar CME–Flare Relation: What Do Historic Observations Reveal?." The Astrophysical Journal 877:105. https://doi.org/10.3847/1538-4357/ab1b37
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.
Suppression of Coronal Mass Ejections in Active Stars by an Overlying Large-scale Magnetic Field: A Numerical StudyAlvarado-Gómez, Julián D.Drake, Jeremy J.Cohen, OferMoschou, Sofia P.Garraffo, CeciliaDOI: info:10.3847/1538-4357/aacb7fv. 86293
Alvarado-Gómez, Julián D., Drake, Jeremy J., Cohen, Ofer, Moschou, Sofia P., and Garraffo, Cecilia. 2018. "Suppression of Coronal Mass Ejections in Active Stars by an Overlying Large-scale Magnetic Field: A Numerical Study." The Astrophysical Journal 862:93. https://doi.org/10.3847/1538-4357/aacb7f
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).
Far beyond the Sun - I. The beating magnetic heart in HorologiumAlvarado-Gómez, Julián D.Hussain, Gaitee A. J.Drake, Jeremy J.Donati, Jean-FrançoisSanz-Forcada, JorgeStelzer, BeateCohen, OferAmazo-Gómez, Eliana M.Grunhut, Jason H.Garraffo, CeciliaMoschou, Sofia P.Silvester, JamesOksala, Mary E.DOI: info:10.1093/mnras/stx2642v. 4734326–4338
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, and Oksala, Mary E. 2018. "Far beyond the Sun - I. The beating magnetic heart in Horologium." Monthly Notices of the Royal Astronomical Society 473:4326– 4338. https://doi.org/10.1093/mnras/stx2642
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 (Pcyc ∼ 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).
Exoplanet Modulation of Stellar Coronal Radio EmissionCohen, OferMoschou, Sofia-ParaskeviGlocer, AlexSokolov, Igor V.Mazeh, TseviDrake, Jeremy J.Garraffo, CeciliaAlvarado-Gómez, Julian D.DOI: info:10.3847/1538-3881/aae1f2v. 156202
Cohen, Ofer, Moschou, Sofia-Paraskevi, Glocer, Alex, Sokolov, Igor V., Mazeh, Tsevi, Drake, Jeremy J., Garraffo, Cecilia, and Alvarado-Gómez, Julian D. 2018. "Exoplanet Modulation of Stellar Coronal Radio Emission." The Astronomical Journal 156:202. https://doi.org/10.3847/1538-3881/aae1f2
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.
Synthetic Radio Imaging for Quiescent and CME-flare ScenariosMoschou, Sofia-ParaskeviSokolov, IgorCohen, OferDrake, Jeremy J.Borovikov, DmitryKasper, Justin C.Alvarado-Gomez, Julian D.Garraffo, CeciliaDOI: info:10.3847/1538-4357/aae58cv. 86751
Moschou, Sofia-Paraskevi, Sokolov, Igor, Cohen, Ofer, Drake, Jeremy J., Borovikov, Dmitry, Kasper, Justin C., Alvarado-Gomez, Julian D., and Garraffo, Cecilia. 2018. "Synthetic Radio Imaging for Quiescent and CME-flare Scenarios." The Astrophysical Journal 867:51. https://doi.org/10.3847/1538-4357/aae58c
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.
The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 PlanetsGarraffo, CeciliaDrake, Jeremy J.Cohen, OferAlvarado-Gómez, Julian D.Moschou, Sofia P.DOI: info:10.3847/2041-8213/aa79edv. 843L33
Garraffo, Cecilia, Drake, Jeremy J., Cohen, Ofer, Alvarado-Gómez, Julian D., and Moschou, Sofia P. 2017. "The Threatening Magnetic and Plasma Environment of the TRAPPIST-1 Planets." Astrophysical Journal Letters 843:L33. https://doi.org/10.3847/2041-8213/aa79ed
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.
A Monster CME Obscuring a Demon Star FlareMoschou, Sofia-ParaskeviDrake, Jeremy J.Cohen, OferAlvarado-Gomez, Julian D.Garraffo, CeciliaDOI: info:10.3847/1538-4357/aa9520v. 850191
Moschou, Sofia-Paraskevi, Drake, Jeremy J., Cohen, Ofer, Alvarado-Gomez, Julian D., and Garraffo, Cecilia. 2017. "A Monster CME Obscuring a Demon Star Flare." The Astrophysical Journal 850:191. https://doi.org/10.3847/1538-4357/aa9520
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.