Browsing by Author "Farrish, Alison O."

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    Exploring the Effects of Stellar Magnetism on the Potential Habitability of Exoplanets
    (IOP Publishing, 2024) Atkinson, Anthony S.; Alexander, David; Farrish, Alison O.
    Considerable interest has centered on Earth-like planets orbiting in the circumstellar habitable zone (CHZ) of its star. However, the potential habitability of an exoplanet depends upon a number of additional factors, including the presence and strength of any planetary magnetic field and the interaction of this field with that of the host star. Not only must the exoplanet have a strong enough magnetic field to shield against stellar activity, but it must also orbit far enough from the star to avoid direct magnetic connectivity. We characterize stellar activity by the star’s Rossby number, Ro, the ratio of stellar rotation rate to convective turnover time. We employ a scaled model of the solar magnetic field to determine the star’s Alfvén radius, the distance at which the stellar wind becomes super-Alfvénic. Planets residing within the Alfvén surface may have a direct magnetic connection to the star and therefore not be the most viable candidates for habitability. Here, we determine the Rossby number of a sample of 1053 exoplanet-hosting stars for which the rotation rates have been observed and for which a convective turnover time can be calculated. We find that 84 exoplanets in our sample have orbits which lie inside the CHZ and that also lie outside the star’s Alfvén surface: 34 of these have been classified as terran (11) or superterran (23) planets. Applying the Alfvén surface habitability criterion yields a subset of the confirmed exoplanets that may be optimal targets for future observations in the search for signatures of life.
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    Modeling of Solar-Stellar Connections and the Heliophysics of Exoplanet Systems
    (2021-04-28) Farrish, Alison O.; Alexander, David
    In the past two and a half decades, advances in the field of exoplanet detection have confirmed more than 4,000 known planets outside of our Solar System \cite{exoplanets}. With this wealth of data, the field is now poised to transition from a phase of detection to one of more in-depth characterization of planetary processes and evolution. Exoplanet systems are of interest not only for the potential for habitability, but also in the opportunity they provide for the study of comparative heliophysics - the similarities and differences in physical interactions between the central host star and any associated planets. In applying solar- and heliophysics-based knowledge and tools to the study of exoplanet systems, we can expand our understanding of the breadth of possible star-planet interactions and the influence of stellar behavior on planetary environments and processes such as atmospheric loss, planetary magnetosphere dynamics, ionospheric emission, and more. We present here a series of studies of solar-stellar connections and the heliophysics of exoplanet systems, employing a surface flux transport treatment of photospheric flux emergence, migration, and dispersal, and the application of this solar-based modeling framework to exoplanet host stars. In Chapter 1, we describe the state of the field of exoplanet characterization, relevant solar physics concepts, and the context for making solar-stellar comparisons. Chapter 2 comprises the methodology employed in modeling stellar photospheres, coronae, and asterospheres with relevance to exoplanet space weather environments. Chapter 3 expands upon the solar-stellar connection, demonstrating the application of magnetic flux transport modeling to the simulation of stellar activity across a broad population of cool stars. Chapter 4 details our modeling of magnetic and energetic environments driving star-planet interaction. In Chapter 5, we present examples of applications of this work to planetary response modeling, detailed investigations of stellar extreme ultraviolet (EUV) emission, and young Sun analogs. Future applications of our integrated modeling approach, particularly in comparisons with solar dynamo models and upcoming observing campaigns from Parker Solar Probe and James Webb Space Telescope, are discussed in Chapter 6.
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    Modeling Stellar Activity-rotation Relations in Unsaturated Cool Stars
    (IOP Publishing, 2021) Farrish, Alison O.; Alexander, David; Johns-Krull, Christopher M.; Li, Minjing
    We apply a surface flux transport model developed for the Sun to reconstruct the stellar activity-rotation relationship, LX/Lbol versus Ro, observed for unsaturated cool stars (Rossby numbers Ro ≳ 0.1). This empirical flux transport model incorporates modulations of magnetic flux strength consistent with observed solar activity cycles, as well as surface flux dynamics consistent with observed and modeled stellar relationships. We find that for stellar flux models corresponding to a range of 0.1 ≲ (Ro/RoSun) ≲ 1.2, the LX/Lbol versus Ro relation matches well the power-law behavior observed in the unsaturated regime of cool stars. Additionally, the magnetic activity cycles captured by the stellar simulations produce a spread about the power-law relation consistent with that observed in cool star populations, indicating that the observed spread may be caused by intrinsic variations resulting from cyclic stellar behavior. The success of our flux transport modeling in reproducing the observed activity relationship across a wide range of late-F, G, K, and M stars suggests that the photospheric magnetic fields of all unsaturated cool stars exhibit similar flux emergence and surface dynamic behavior, and may hint at possible similarities in stellar dynamo action across a broad range of stellar types.