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Coronal mass ejections (CMEs) on stars can change the stars' magnetic field configurations and mass loss rates during the eruption and propagation and therefore, may affect the stars' rotation properties on long time-scales. The dynamics of stellar CMEs and their influence on the stellar angular momentum loss rate are not yet well understood. In order to start investigating these CME-related aspects on other stars, we conducted a series of magnetohydrodynamic simulations of CMEs on a solar-type star of moderate activity levels. The propagation and evolution of the CMEs were traced in the three-dimensional outputs and the temporal evolution of their dynamic properties (such as masses, velocities, and kinetic energies) were determined. The simulated stellar CMEs are more massive and energetic than their solar analog, which is a result of the stronger magnetic field on the surface of the simulated star than that of the Sun. The simulated CMEs display masses ranging from ~10^16 g to ~10^18 g and kinetic energies from ~10^31 erg to ~10^33 erg. We also investigated the instantaneous influence of the CMEs to the star's angular momentum loss rate. Our results suggest that angular momentum can either be added to or be removed from the star during the evolution of CME events. We found a positive correlation between the amplitude of the angular momentum loss rate variation and the CME's kinetic energy as well as mass, suggesting that more energetic/massive CMEs have higher possibility to add angular momentum to the star., Comment: 16 pages, 10 figures; accepted by ApJ

PLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2 R_(Earth)) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5 %, 10 %, 10 % for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO's target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile at the beginning of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases.

In 2020, the Transiting Exoplanet Survey Satellite (TESS) observed a rapidly rotating M7 dwarf, TIC 277539431, produce a flare at 81{\deg} latitude, the highest latitude flare located to date. This is in stark contrast to solar flares that occur much closer to the equator, typically below 30{\deg}. The mechanisms that allow flares at high latitudes to occur are poorly understood. We studied five Sectors of TESS monitoring, and obtained 36 ks of XMM-Newton observations to investigate the coronal and flaring activity of TIC 277539431. From the observations, we infer the optical flare frequency distribution, flare loop sizes and magnetic field strengths, the soft X-ray flux, luminosity and coronal temperatures, as well as the energy, loop size and field strength of a large flare in the XMM-Newton observations. We find that TIC 277539431's corona does not differ significantly from other low mass stars on the canonical saturated activity branch with respect to coronal temperatures and flaring activity, but shows lower luminosity in soft X-ray emission by about an order of magnitude, consistent with other late M dwarfs. The lack of X-ray flux, the high latitude flare, the star's viewing geometry, and the otherwise typical stellar corona taken together can be explained by the migration of flux emergence to the poles in rapid rotators like TIC 277539431 that drain the star's equatorial regions of magnetic flux, but preserve its ability to produce powerful flares., Comment: 11 pages, 5 figues. Accepted to Astronomy & Astrophysics on May 6, 2024

The dynamical evolution of tight star-planet systems is influenced by tidal interactions between the star and the planet, as was shown recently. The rate at which spins and orbits in such a system evolve depends on the stellar and planetary tidal dissipation efficiency. Here, we present a method to constrain the modified tidal quality factor $Q'_*$ of a planet-hosting star whose rotational evolution has been altered by its planet through angular momentum transfer from the planetary orbital motion to the rotation of the stellar convective zone. The altered rotation is estimated from an observed discrepancy of magnetic activity of the planet-hosting star and a coeval companion star, i.e. this method is applicable to star-planet systems with wide stellar companions. We give an example of the planet-hosting wide binary system HD189733 and find that the planet host's modified tidal quality factor is constrained to be $Q'_* \leq 2.33 \times 10^7$., Comment: Accepted for publication in Astronomische Nachrichten (AN); reference list corrected

Betelgeuse, a red supergiant star of semi-regular variability, reached a historical minimum brightness in February 2020, known as the Great Dimming. Even though the brightness has returned to the values prior to the Great Dimming now, it continues to exhibit highly unusual behavior. Our goal is to study long-term dynamics of the photosphere, including during the Great Dimming. We applied the tomographic method, which allows different layers in the stellar atmosphere to be probed in order to reconstruct depth-dependent velocity fields. The method is based on the construction of spectral masks by grouping spectral lines from specific optical depths. These masks are cross-correlated with the observed spectra to recover the velocity field inside each atmospheric layer. We obtained about 2800 spectra over the past 15 years that were observed with the STELLA robotic telescope in Tenerife. We analyzed the variability of five different layers of Betelgeuse's photosphere. We found phase shift between the layers, as well as between the variability of velocity and photometry. The time variations of the widths of the cross-correlation function reveal propagation of two shockwaves during the Great Dimming. For about two years after the dimming, the timescale of variability was different between the inner and outer photospheric layers. By 2022, all the layers were pulsating with higher frequency corresponding with the first overtone. The combination of the extensive high-resolution spectroscopic data set with the tomographic method revealed the variable velocity fields in the photosphere of Betelgeuse, for the first time in such detail. Our results demonstrate that powerful shocks are the triggering mechanism for episodic mass-loss events, which may be the missing component to explain the mass-loss process in red supergiants., Comment: 14 pages, accepted for publication in Astronomy & Astrophysics

Planets orbiting young stars are thought to experience atmospheric evaporation as a result of the host stars' high magnetic activity. We study the evaporation history and expected future of the three known transiting exoplanets in the young multiplanet system K2-198. Based on spectroscopic and photometric measurements, we estimate an age of the K-dwarf host star between 200 and 500 Myr, and calculate the high-energy environment of these planets using eROSITA X-ray measurements. We find that the innermost planet K2-198c has likely lost its primordial envelope within the first few tens of Myr regardless of the age at which the star drops out of the saturated X-ray regime. For the two outer planets, a range of initial envelope mass fractions is possible, depending on the not-yet-measured planetary mass and the stars' spin-down history. Regarding the future of the system, we find that the outermost planet K2-198b is stable against photoevaporation for a wide range of planetary masses, while the middle planet K2-198d is only able to retain an atmosphere for a mass range between ~7 and 18 Earth-masses. Lower-mass planets are too susceptible to mass loss, and a very thin present-day envelope for higher-mass planets is easily lost with the estimated mass-loss rates. Our results support the idea that all three planets started out above the radius valley in the (sub-)Neptune regime and were then transformed into their current states by atmospheric evaporation, but also stress the importance of measuring planetary masses for (young) multiplanet systems before conducting more detailed photoevaporation simulations.

In many star-planet systems discovered so far, the innermost planet orbits within only a few stellar radii. In these systems, planets could become in-situ probes of the extended stellar magnetic field. Because they disturb the field as they move, they are expected to trigger flares in the corona. Potential differences to the energies and morphologies of intrinsic stellar flares are poorly constrained. However, as we expect planet-induced flares to correlate with the planet's orbital period, we can identify them from a clustering of flares in phase with the planet's orbit. We used the excellent phase coverage from Kepler and the Transiting Exoplanet Survey Satellite to find flaring star-planet systems, compile a catalog of all their flares, and measure how much they cluster in orbital phase. In the 1811 searched systems, we found 25 single stars with three or more flares each. We quantified the significance of the clustering in each system, and compared it against the theoretically expected power of magnetic interaction that leads to planet-induced flaring. Most systems do not show any clustering, consistent with low expected power. Those we expect to show clustering fall on two branches. An inactive one, without any signs of clustering, and a tentative active one, where the clustering becomes more pronounced as the expected power of interaction increases. The flares in HIP 67522 are prominently clustered (p<0.006). This young Hot Jupiter system is the most promising candidate for magnetic star-planet interaction in our sample., Comment: 32 pages, 12 figures, accepted to MNRAS on Oct 30, 2023. Reproducible paper made with showyourwork, see https://show-your.work/en/latest/

Most ultra hot Jupiters (UHJs) show evidence of temperature inversions, in which temperature increases with altitude over a range of pressures. Temperature inversions can occur when there is a species that absorbs the stellar irradiation at a relatively high level of the atmospheres. However, the species responsible for this absorption remains unidentified. In particular, the UHJ KELT-20b is known to have a temperature inversion. Using high resolution emission spectroscopy from LBT/PEPSI we investigate the atomic and molecular opacity sources that may cause the inversion in KELT-20b, as well as explore its atmospheric chemistry. We confirm the presence of Fe I with a significance of 17$\sigma$. We also report a tentative $4.3\sigma$ detection of Ni I. A nominally $4.5\sigma$ detection of Mg I emission in the PEPSI blue arm is likely in fact due to aliasing between the Mg I cross-correlation template and the Fe I lines present in the spectrum. We cannot reproduce a recent detection of Cr I, while we do not have the wavelength coverage to robustly test past detections of Fe II and Si I. Together with non-detections of molecular species like TiO, this suggests that Fe I is likely to be the dominant optical opacity source in the dayside atmosphere of KELT-20b and may be responsible for the temperature inversion. We explore ways to reconcile the differences between our results and those in literature and point to future paths to understand atmospheric variability., Comment: Revised version resubmitted to MNRAS. 15 pages, 8 figures

The magnetic activity of low-mass stars changes as they age. The primary process decreasing the stellar activity level is the angular momentum loss via magnetized stellar wind. However, processes like tidal interactions between stars and their close companions may slow down the braking effect and the subsequent decrease of the activity level. Until now, the tidal impact of substellar objects like brown dwarfs on the evolution of their central stars has not been quantified. Here, we analyse the X-ray properties of NLTT 41135, an M dwarf tightly orbited by a brown dwarf, to determine the impact of tidal interactions between them. We find that NLTT 41135 is more than an order of magnitude brighter in the X-ray regime than its stellar companion NLTT 41136, also an M dwarf star, with whom it forms a wide binary system. To characterize the typical intrinsic activity scatter between coeval M dwarf stars, we analyse a control sample of 25 M dwarf wide binary systems, observed with XMM-Newton and Chandra telescopes and the eROSITA instrument onboard the Spectrum R\"ontgen Gamma satellite. The activity difference in the NLTT 41135/41136 system is a $3.44 \sigma$ outlier compared to the intrinsic activity scatter of the control systems. Therefore, the most convincing explanation for the observed activity discrepancy is tidal interactions between the M dwarf and its brown dwarf. This shows that tidal interactions between a star and a substellar companion can moderately alter the expected angular-momentum evolution of the star, making standard observational proxies for its age, such as X-ray emission, unreliable., Comment: accepted for publication in MNRAS, 15 pages, 5 figures

As a cool star evolves, it loses mass and angular momentum due to magnetized stellar winds which affect its rotational evolution. This change has consequences that range from the alteration of its activity to influences over the atmosphere of any orbiting planet. Despite their importance, observations constraining the properties of stellar winds in cool stars are extremely limited. Therefore, numerical simulations provide a valuable way to understand the structure and properties of these winds. In this work, we simulate the magnetized winds of 21 cool main-sequence stars (F-type to M-dwarfs), using a state-of-the-art 3D MHD code driven by observed large-scale magnetic field distributions. We perform a qualitative and quantitative characterization of our solutions, analyzing the dependencies between the driving conditions (e.g., spectral type, rotation, magnetic field strength) and the resulting stellar wind parameters (e.g., Alfv\'en surface size, mass loss rate, angular momentum loss rate, stellar wind speeds). We compare our models with the current observational knowledge on stellar winds in cool stars and explore the behaviour of the mass loss rate as a function of the Rossby number. Furthermore, our 3D models encompass the entire classical Habitable Zones (HZ) of all the stars in our sample. This allows us to provide the stellar wind dynamic pressure at both edges of the HZ and analyze the variations of this parameter across spectral type and orbital inclination. The results here presented could serve to inform future studies of stellar wind-magnetosphere interactions and stellar wind erosion of planetary atmospheres via ion escape processe., Comment: 21 pages, 10 figures

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. Here we explore how a realistic spread of different stellar activity tracks influences the mass loss and radius evolution of a simulated population of small exoplanets and the observable properties of the radius gap. Our results show qualitatively that different saturation time scales, i.e. the young age at which stellar high-energy emission starts to decline, and different activity decay tracks over moderate stellar ages can cause changes in the population density of planets in the gap, as well as in the observable width of the gap. We also find that while the first 100 million years of mass loss are highly important to shape the radius gap, significant evolution of the gap properties is expected to take place for at least the first 500-600 million years, i.e. the age of the Hyades cluster. Observations of exoplanet populations with defined ages will be able to shed more light on the radius gap evolution., Comment: 26 pages. Accepted for publication in MNRAS

Recent observations have shown that the atmospheres of ultra hot Jupiters (UHJs) commonly possess temperature inversions, where the temperature increases with increasing altitude. Nonetheless, which opacity sources are responsible for the presence of these inversions remains largely observationally unconstrained. We used LBT/PEPSI to observe the atmosphere of the UHJ KELT-20 b in both transmission and emission in order to search for molecular agents which could be responsible for the temperature inversion. We validate our methodology by confirming previous detections of Fe I in emission at $16.9\sigma$. Our search for the inversion agents TiO, VO, FeH, and CaH results in non-detections. Using injection-recovery testing we set $4\sigma$ upper limits upon the volume mixing ratios for these constituents as low as $\sim1\times10^{-9}$ for TiO. For TiO, VO, and CaH, our limits are much lower than expectations from an equilibrium chemical model, while we cannot set constraining limits on FeH with our data. We thus rule out TiO and CaH as the source of the temperature inversion in KELT-20 b, and VO only if the line lists are sufficiently accurate., Comment: 18 pages, 11 figures. Accepted for publication in AJ

Planets that closely orbit magnetically active stars are thought to be able to interact with their magnetic fields in a way that modulates stellar activity. This modulation in phase with the planetary orbit, such as enhanced X-ray activity, chromospheric spots, radio emission, or flares, is considered the clearest sign of magnetic star-planet interaction (SPI). However, the magnitude of this interaction is poorly constrained, and the intermittent nature of the interaction is a challenge for observers. AU Mic is an early M dwarf, and the most actively flaring planet host detected to date. Its innermost companion, AU Mic b, is a promising target for magnetic SPI observations. We used optical light curves of AU Mic obtained by the Transiting Exoplanet Survey Satellite to search for signs of flaring SPI with AU Mic b using a customized Anderson-Darling test. In the about $50$ days of observations, the flare distributions with orbital, rotational, and synodic periods were generally consistent with intrinsic stellar flaring. We found the strongest deviation ($p=0.07,\;n=71$) from intrinsic flaring with the orbital period of AU Mic b, in the high energy half of our sample ($ED>1$ s). If it reflects the true SPI signal from AU Mic b, extending the observing time by a factor of $2-3$ will yield a $>3\sigma$ detection. Continued monitoring of AU Mic may therefore reveal flaring SPI with orbital phase, while rotational modulation will smear out due to the star's strong differential rotation., Comment: 9 pages, 4 figures. Accepted to MNRAS

The study of exoplanets and especially their atmospheres can reveal key insights on their evolution by identifying specific atmospheric species. For such atmospheric investigations, high-resolution transmission spectroscopy has shown great success, especially for Jupiter-type planets. Towards the atmospheric characterization of smaller planets, the super-Earth exoplanet 55 Cnc e is one of the most promising terrestrial exoplanets studied to date. Here, we present a high-resolution spectroscopic transit observation of this planet, acquired with the PEPSI instrument at the Large Binocular Telescope. Assuming the presence of Earth-like crust species on the surface of 55 Cnc e, from which a possible silicate-vapor atmosphere could have originated, we search in its transmission spectrum for absorption of various atomic and ionized species such as Fe , Fe+, Ca , Ca+, Mg and K , among others. Not finding absorption for any of the investigated species, we are able to set absorption limits with a median value of 1.9 x RP. In conclusion, we do not find evidence of a widely extended silicate envelope on this super-Earth reaching several planetary radii., Comment: MNRAS, in press

Tidal interaction between an exoplanet and its host star is a possible pathway to transfer angular momentum between the planetary orbit and the stellar spin. In cases where the planetary orbital period is shorter than the stellar rotation period, this may lead to angular momentum being transferred into the star's rotation, possibly counteracting the intrinsic stellar spin-down induced by magnetic braking. Observationally, detecting altered rotational states of single, cool field stars is challenging, as precise ages for such stars are rarely available. Here we present an empirical investigation of the rotation and magnetic activity of a sample of planet-hosting stars that are accompanied by wide stellar companions. Without needing knowledge about the absolute ages of the stars, we test for relative differences in activity and rotation of the planet hosts and their co-eval companions, using X-ray observations to measure the stellar activity levels. Employing three different tidal interaction models, we find that host stars with planets that are expected to tidally interact display elevated activity levels compared to their companion stars. We also find that those activity levels agree with the observed rotational periods for the host stars along the usual rotation-activity relationships, implying that the effect is indeed caused by a tidal interaction and not a purely magnetic interaction which would be expected to affect the stellar activity, but not necessarily the rotation. We conclude that massive, close-in planets have an impact on the stellar rotational evolution, while the smaller, more distant planets do not have a significant influence., Comment: 27 pages, 9 figures. Accepted on 2022 March 23 for publication in MNRAS

Two close-in planets have been recently found around the M-dwarf flare star AU Microscopii (AU Mic). These Neptune-sized planets (AU Mic b and c) seem to be located very close to the so-called "evaporation valley" in the exoplanet population, making this system an important target for studying atmospheric loss on exoplanets. This process, while mainly driven by the high-energy stellar radiation, will be strongly mediated by the space environment surrounding the planets. Here we present an investigation on this last area, performing 3D numerical modeling of the quiescent stellar wind from AU Mic, as well as time-dependent simulations describing the evolution of a highly energetic Coronal Mass Ejection (CME) event in this system. Observational constraints on the stellar magnetic field and properties of the eruption are incorporated in our models. We carry out qualitative and quantitative characterizations of the stellar wind, the emerging CMEs, as well as the expected steady and transient conditions along the orbit of both exoplanets. Our results predict an extreme space weather for AU Mic and its planets. This includes sub-Alfv\'enic regions for the large majority of the exoplanet orbits, very high dynamic and magnetic pressure values in quiescence (varying within $10^{2} - 10^{5}$ times the dynamic pressure experienced by the Earth), and an even harsher environment during the passage of any escaping CME associated with the frequent flaring observed in AU Mic. These space weather conditions alone pose an immense challenge for the survival of the exoplanetary atmospheres (if any) in this system., Comment: 15 Pages, 7 Figures, 2 Tables, Accepted for publication in The Astrophysical Journal

X-ray observations of star-planet systems are important to grow our understanding of exoplanets; these observation allow for studies of photoevaporation of the exoplanetary atmosphere, and in some cases even estimations of the size of the outer planetary atmosphere. The German-Russian eROSITA instrument onboard the SRG (Spectrum Roentgen Gamma) mission is performing the first all-sky X-ray survey since the 1990s, and provides X-ray fluxes and spectra of exoplanet host stars over a much larger volume than was accessible before. Using new eROSITA data as well as archival data from XMM-Newton, Chandra and ROSAT we estimate mass loss rates of exoplanets under an energy-limited escape scenario, and identify several exoplanets with strong X-ray irradiation and expected mass-loss that are amenable to follow-up observations at other wavelengths. We model sample spectra using a toy model of an exoplanetary atmosphere to predict what exoplanet transit observations with future X-ray missions such as Athena will look like, and estimate the observable X-ray transmission spectrum for a typical Hot Jupiter-type exoplanet., Comment: 6 pages, 6 figures, XMM-Newton 2021 Science Workshop proceedings

We develop PLATYPOS (PLAneTarY PhOtoevaporation Simulator), a python code to perform planetary photoevaporative mass-loss calculations for close-in planets with hydrogen-helium envelopes atop Earth-like rocky cores. With physical and model parameters as input, PLATYPOS calculates the atmospheric mass loss and with it the radius evolution of a planet over time, taking into account also the thermal cooling and subsequent radius evolution of the planet. In particular, we implement different stellar activity evolution tracks over time. Our setup allows for a prediction of whether a planet can hold on to a significant fraction of its atmosphere, or fully evaporates, leaving behind only the bare rocky core. The user supplies information about the star-planet system of interest, which includes planetary and host star parameters, as well as the star's rotational and thus activity evolution. In addition, several details for the evaporative mass-loss rate estimation can be chosen. This includes the effective absorption cross-section for high energy photons, the evaporation efficiency, and the hydrodynamic escape model., Comment: 8 pages, 4 figures, proceedings of the XMM-Newton Workshop 2021 "A High-Energy View of Exoplanets and their Environments". Original article can be found at https://doi.org/10.1002/asna.20210105

Stars are uniform spheres, but only to first order. The way in which stellar rotation and magnetism break this symmetry places important observational constraints on stellar magnetic fields, and factors in the assessment of the impact of stellar activity on exoplanet atmospheres. The spatial distribution of flares on the solar surface is well known to be non-uniform, but elusive on other stars. We briefly review the techniques available to recover the loci of stellar flares, and highlight a new method that enables systematic flare localization directly from optical light curves. We provide an estimate of the number of flares we may be able to localize with the Transiting Exoplanet Survey Satellite (TESS), and show that it is consistent with the results obtained from the first full sky scan of the mission. We suggest that non-uniform flare latitude distributions need to be taken into account in accurate assessments of exoplanet habitability., Comment: 6 pages, 2 figures. Proceedings of the 2021 XMM-Newton workshop "A high-energy view of exoplanets and their environments" accepted to Astronomische Nachrichten

The cumulative effect of the magnetized stellar winds on exoplanets dominates over other forms of star-planet interactions. When combined with photoevaporation, these winds will lead to atmospheric erosion. This is directly connected with the concept of Habitable Zone (HZ) planets around late-type stars. Our knowledge of these magnetized winds is limited, making numerical models useful tools to explore them. In this preliminary study, we focus on solar-like stars exploring how different stellar wind properties scale with one another. We used one of the most detailed physics-based models, the 3D Alfv\'en Wave Solar Model part of the Space Weather ModelingFramework, and applied it to the stellar winds domain. Our simulations showed that the magnetic field topology on the star surface plays a fundamental role in shaping the different stellar wind properties (wind speed, mass loss rate, angular momentum loss rate). We conclude that a characterization of the Alfv\'en surface is crucial when studying star-planet interaction as it can serve as an inner-boundary of the HZ, Comment: 7 pages, 6 pictures, proceeding

Coronal mass ejections (CMEs) are more energetic than any other class of solar phenomena. They arise from the rapid release of up to $10^{33}$ erg of magnetic energy mainly in the form of particle acceleration and bulk plasma motion. Their stellar counterparts, presumably involving much larger energies, are expected to play a fundamental role in shaping the environmental conditions around low-mass stars, in some cases perhaps with catastrophic consequences for planetary systems due to processes such as atmospheric erosion and depletion. Despite their importance, the direct observational evidence for stellar CMEs is almost non-existent. In this way, numerical simulations constitute extremely valuable tools to shed some light on eruptive behavior in the stellar regime. Here we review recent results obtained from realistic modeling of CMEs in active stars, highlighting their key role in the interpretation of currently available observational constraints. We include studies performed on M-dwarf stars, focusing on how emerging signatures in different wavelengths related to these events vary as a function of the magnetic properties of the star. Finally, the implications and relevance of these numerical results are discussed in the context of future characterization of host star-exoplanet systems., Comment: 8 pages, 3 figures, Accepted for publication in Astronomical Notes (Astronomische Nachrichten). Based on an invited review talk at the XMM-Newton Science Workshop 2021: "A High-Energy View of Exoplanets and Their Environments"

The 23 Myr system V1298 Tau hosts four transiting planets and is a valuable laboratory for exploring the early stages of planet evolution soon after formation of the star. We observe the innermost planet, V1298 Tau c, during transit using LBT PEPSI to obtain high spectral resolution characterization of escaping material near the H-alpha line. We find no strong evidence for atmospheric material escaping at the orbital velocity of the planet. Instead, we find a deep stellar feature that is variable on the few percent level, similar to a previous observation of the planet and can be explained by stellar activity. We attempted to monitor the broadband optical transit with LBT MODS but do not achieve the precision needed to characterize the atmosphere or improve the ephemeris., Comment: Published in the Research Notes of the AAS

White-light flares are magnetically driven localized brightenings on the surfaces of stars. Their temporal, spectral, and statistical properties present a treasury of physical information about stellar magnetic fields. The spatial distributions of magnetic spots and associated flaring regions help constrain dynamo theories. Moreover, flares are thought to crucially affect the habitability of exoplanets that orbit these stars. Measuring the location of flares on stars other than the Sun is challenging due to the lack of spatial resolution. Here we present four fully convective stars observed with the Transiting Exoplanet Survey Satellite (TESS) that displayed large, long-duration flares in white-light which were modulated in brightness by the stars' fast rotation. This allowed us to determine the loci of these flares directly from the light curves. All four flares occurred at latitudes between 55 deg and 81 deg, far higher than typical solar flare latitudes. Our findings are evidence that strong magnetic fields tend to emerge close to the stellar rotational poles for fully convective stars, and suggest that the impact of flares on the habitability of exoplanets around small stars could be weaker than previously thought., Comment: 23 pages, 18 figures, accepted to MNRAS, see conference poster https://doi.org/10.5281/zenodo.4558791 for short summary

Flares, energetic eruptions on the surfaces of stars, are an unmistakable manifestation of magnetically driven emission. Their occurrence rates and energy distributions trace stellar characteristics such as mass and age. But before flares can be used to constrain stellar properties, the flaring-age-mass relation requires proper calibration. This work sets out to quantify flaring activity of independently age-dated main sequence stars for a broad range of spectral types using optical light curves obtained by the Kepler satellite. Drawing from the complete K2 archive, we searched 3435 $\sim 80$ day long light curves of 2111 open cluster members for flares using the open-source software packages K2SC to remove instrumental and astrophysical variability from K2 light curves, and AltaiPony to search and characterize the flare candidates. We confirmed a total of 3844 flares on high probability open cluster members with ages from zero age main sequence (Pleiades) to 3.6 Gyr (M67). We extended the mass range probed in the first study of this series to span from Sun-like stars to mid-M dwarfs. We added the Hyades (690 Myr) to the sample as a comparison cluster to Praesepe (750 Myr), the 2.6 Gyr old Ruprecht 147, and several hundred light curves from the late K2 Campaigns in the remaining clusters. The flare energy distribution was similar in the entire parameter space, following a power law relation with exponent $\alpha\approx 1.84-2.39$. The flaring rates declined with age, and declined faster for higher mass stars. We found evidence that a rapid decline in flaring activity occurred in M1-M2 dwarfs around Hyades/Praesepe age, when these stars spun down to rotation periods of about 10 days, while higher mass stars had already transitioned to lower flaring rates, and lower mass stars still resided in the saturated activity regime. (abridged), Comment: 27 pages, 13 figures. Accepted to A&A

A new planet has been recently discovered around Proxima Centauri. With an orbital separation of $\sim$$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 $0.29$ $M_{\oplus}$., Comment: 9 Pages, 4 Figures, 1 Table. Accepted for publication in The Astrophysical Journal Letters (ApJL)

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 Alfv\'en 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 here the distribution of the coronal Alfv\'en 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., Comment: 13 Pages, 6 Figures, 2 Tables. Accepted for publication in The Astrophysical Journal

In this work, we investigate the potassium excess absorption around 7699A of the exoplanets HD189733b and HD209458b. For this purpose, we used high spectral resolution transit observations acquired with the 2 x 8.4m Large Binocular Telescope (LBT) and the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI). For a bandwidth of 0.8A, we present a detection > 7-sigma with an absorption level of 0.18% for HD189733b. Applying the same analysis to HD209458b, we can set 3-sigma upper limit of 0.09%, even though we do not detect a K- excess absorption. The investigation suggests that the K- feature is less present in the atmosphere of HD209458b than in the one of HD189733b. This comparison confirms previous claims that the atmospheres of these two planets must have fundamentally different properties., Comment: Accepted in M.N.R.A.S, https://doi.org/10.1093/mnrasl/slz123

We propose the development of X-ray interferometry (XRI), to reveal the universe at high energies with ultra-high spatial resolution. With baselines which can be accommodated on a single spacecraft, XRI can reach 100 $\mu$as resolution at 10 \AA (1.2 keV) and 20 $\mu$as at 2 \AA (6 keV), enabling imaging and imaging-spectroscopy of (for example) X-ray coronae of nearby accreting supermassive black holes (SMBH) and the SMBH `shadow'; SMBH accretion flows and outflows; X-ray binary winds and orbits; stellar coronae within ~100 pc and many exoplanets which transit across them. For sufficiently luminous sources XRI will resolve sub-pc scales across the entire observable universe, revealing accreting binary SMBHs and enabling trigonometric measurements of the Hubble constant with X-ray light echoes from quasars or explosive transients. A multi-spacecraft `constellation' interferometer would resolve well below 1 $\mu$as, enabling SMBH event horizons to be resolved in many active galaxies and the detailed study of the effects of strong field gravity on the dynamics and emission from accreting gas close to the black hole., Comment: White Paper submitted in response to ESA's Voyage 2050 call

We present the discovery of NGTS-7Ab, a high mass brown dwarf transiting an M dwarf with a period of 16.2 hours, discovered as part of the Next Generation Transit Survey (NGTS). This is the shortest period transiting brown dwarf around a main or pre-main sequence star to date. The M star host (NGTS-7A) has an age of roughly 55 Myr and is in a state of spin-orbit synchronisation, which we attribute to tidal interaction with the brown dwarf acting to spin up the star. The host star is magnetically active and shows multiple flares across the NGTS and follow up lightcurves, which we use to probe the flare-starspot phase relation. The host star also has an M star companion at a separation of 1.13 arcseconds with very similar proper motion and systemic velocity, suggesting the NGTS-7 system is a hierarchical triple. The combination of tidal synchronisation and magnetic braking is expected to drive ongoing decay of the brown dwarf orbit, with a remaining lifetime of only 5-10 Myr., Comment: 21 pages, 16 figures, accepted for publication in the Monthly Notices of the Royal Astronomical Society

Context: Planetary population analysis gives us insight into formation and evolution processes. For short-period planets, the subJovian desert has been discussed in recent years with regard to the planet population in the mass/period and radius/period parameter space without taking stellar parameters into account. The Next Generation Transit Survey (NGTS) is optimised for detecting planets in this regime, which allows for further analysis of the sub-Jovian desert. Aims: With high-precision photometric surveys (e.g. with NGTS and TESS), which aim to detect short period planets especially around M/K-type host stars, stellar parameters need to be accounted for when empirical data are compared to model predictions. Presenting a newly discovered planet at the boundary of the sub-Jovian desert, we analyse its bulk properties and use it to show the properties of exoplanets that border the sub-Jovian desert. Methods: Using NGTS light curve and spectroscopic follow-up observations, we confirm the planetary nature of planet NGTS-5b and determine its mass. Using exoplanet archives, we set the planet in context with other discoveries. Results: NGTS-5b is a short-period planet with an orbital period of 3.3569866 +- 0.0000026 days. With a mass of 0.229 +- 0.037 MJup and a radius of 1.136 +- 0.023 RJup, it is highly inflated. Its mass places it at the upper boundary of the sub-Jovian desert. Because the host is a K2 dwarf, we need to account for the stellar parameters when NGTS-5b is analysed with regard to planet populations. Conclusions: With red-sensitive surveys (e.g. with NGTS and TESS), we expect many more planets around late-type stars to be detected. An empirical analysis of the sub-Jovian desert should therefore take stellar parameters into account., Comment: 10 pages, 12 figures, accepted for publication in A&A

Over the last two decades, the discovery of exoplanets has fundamentally changed our perception of the universe and humanity's place within it. Recent work indicates that a solar system's X-ray and high energy particle environment is of fundamental importance to the formation and development of the atmospheres of close-in planets such as hot Jupiters, and Earth-like planets around M stars. X-ray imaging and spectroscopy provide powerful and unique windows into the high energy flux that an exoplanet experiences, and X-ray photons also serve as proxies for potentially transfigurative coronal mass ejections. Finally, if the host star is a bright enough X-ray source, transit measurements akin to those in the optical and infrared are possible and allow for direct characterization of the upper atmospheres of exoplanets. In this brief white paper, we discuss contributions to the study of exoplanets and their environs which can be made by X-ray data of increasingly high quality that are achievable in the next 10--15 years., Comment: 7 pages with 2 figures. A 2020 Decadal Survey White Paper

The architecture of exoplanetary systems is often different from the solar system, with some exoplanets being in close orbits around their host stars and having orbital periods of only a few days. In analogy to interactions between stars in close binary systems, one may expect interactions between the star and the exoplanet as well. From theoretical considerations, effects on the host star through tidal and magnetic interaction with the exoplanet are possible; for the exoplanet, some interesting implications are the evaporation of the planetary atmosphere and potential effects on the planetary magnetism. In this review, several possible interaction pathways and their observational prospects and existing evidence are discussed. A particular emphasis is put on observational opportunities for these kinds of effects in the high-energy regime., Comment: Accepted for publication in Astronomical Notes. Invited review, XMM-Newton 2018 Science Workshop on Time-Domain Astronomy: a High Energy View

It is now recognized that energetic stellar photon and particle radiation evaporates and erodes planetary atmospheres and controls upper atmospheric chemistry. Key exoplanet host stars will be too faint at X-ray wavelengths for accurate characterization using existing generation and future slated X-ray telescopes. Observation of stellar coronal mass ejections and winds are also beyond current instrumentation. In line with theCommittee on an Exoplanet Science Strategy recognition that holistic observational approaches are needed, we point out here that a full understanding of exoplanet atmospheres, their evolution and determination of habitability requires a powerful high-resolution X-ray imaging and spectroscopic observatory. This is the only capability that can: (1) characterize by proxy the crucial, difficult to observe, EUV stellar flux, its history and its variations for planet hosting stars; (2) observe the stellar wind; (3) detect the subtle Doppler signatures of coronal mass ejections., Comment: Astro2020 Decadal Survey Science White Paper

We present the detection of an energetic flare on the pre-main sequence M3 star NGTS J121939.5-355557, which we estimate as only 2 Myr old. The flare had an energy of $3.2\pm^{0.4}_{0.3}\times 10^{36}$erg and a fractional amplitude of $7.2\pm0.8$, making it one of the most energetic flares seen on an M star. The star is also X-ray active, in the saturated regime with $log L_{X}/L_{Bol} = -3.1$. In the flare peak we have identified multi-mode quasi-periodic pulsations formed of two statistically significant periods of approximately 320 and 660 seconds. This flare is one of the largest amplitude events to exhibit such pulsations. The shorter period mode is observed to start after a short-lived spike in flux lasting around 30 seconds, which would not have been resolved in Kepler or TESS short cadence modes. Our data shows how the high cadence of NGTS can be used to apply solar techniques to stellar flares and identify potential causes of the observed oscillations. We also discuss the implications of this flare for the habitability of planets around M star hosts and how NGTS can aid in our understanding of this., Comment: 15 pages, 10 figures, Accepted for Publication in Monthly Notices of the Royal Astronomical Society

We analyze the X-ray spectra of 19 main sequence stars observed by Chandra using its LETGS configuration. Emission measure (EM) distributions are computed based on emission line measurements, an analysis that also yields evaluations of coronal abundances. The use of newer atomic physics data results in significant changes compared to past published analyses. The stellar EM distributions correlate with surface X-ray flux (F_X) in a predictable way, regardless of spectral type. Thus, we provide EM distributions as a function of F_X, which can be used to estimate the EM distribution of any main sequence star with a measured broadband X-ray luminosity. Comparisons are made with solar EM distributions, both full-disk distributions and spatially resolved ones from active regions (ARs), flares, and the quiet Sun. For moderately active stars, the slopes and magnitudes of the EM distributions are in excellent agreement with those of solar ARs for log T<6.6, suggesting that such stars have surfaces completely filled with solar-like ARs. A stellar surface covered with solar X-class flares yields a reasonable approximation for the EM distributions of the most active stars. Unlike the EM distributions, coronal abundances are very spectral-type dependent, and we provide relations with surface temperature for both relative and absolute abundances. Finally, the coronal abundances of the exoplanet host star Tau Boo A (F7 V) are anomalous, and we propose that this is due to the presence of the exoplanet., Comment: 30 pages, 12 figures, to appear in The Astrophysical Journal

We report the discovery of NGTS-2b, an inflated hot-Jupiter transiting a bright F5V star (2MASS J14202949-3112074; $T_{\rm eff}$=$6478^{+94}_{-89}$ K), discovered as part of the Next Generation Transit Survey (NGTS). The planet is in a P=4.51 day orbit with mass $0.74^{+0.13}_{-0.12}$ M$_{J}$, radius $1.595^{+0.047}_{-0.045}$ R$_{J}$ and density $0.226^{+0.040}_{-0.038}$ g cm$^{-3}$; therefore one of the lowest density exoplanets currently known. With a relatively deep 1.0% transit around a bright V=10.96 host star, NGTS-2b is a prime target for probing giant planet composition via atmospheric transmission spectroscopy. The rapid rotation ($vsin$i=$15.2\pm0.8$ km s$^{-1}$) also makes this system an excellent candidate for Rossiter-McLaughlin follow-up observations, to measure the sky-projected stellar obliquity. NGTS-2b was confirmed without the need for follow-up photometry, due to the high precision of the NGTS photometry., Comment: 12 pages, 8 figures, accepted for publication in MNRAS

State of the art exoplanet transit surveys are producing ever increasing quantities of data. To make the best use of this resource, in detecting interesting planetary systems or in determining accurate planetary population statistics, requires new automated methods. Here we describe a machine learning algorithm that forms an integral part of the pipeline for the NGTS transit survey, demonstrating the efficacy of machine learning in selecting planetary candidates from multi-night ground based survey data. Our method uses a combination of random forests and self-organising-maps to rank planetary candidates, achieving an AUC score of 97.6\% in ranking 12368 injected planets against 27496 false positives in the NGTS data. We build on past examples by using injected transit signals to form a training set, a necessary development for applying similar methods to upcoming surveys. We also make the \texttt{autovet} code used to implement the algorithm publicly accessible. \texttt{autovet} is designed to perform machine learned vetting of planetary candidates, and can utilise a variety of methods. The apparent robustness of machine learning techniques, whether on space-based or the qualitatively different ground-based data, highlights their importance to future surveys such as TESS and PLATO and the need to better understand their advantages and pitfalls in an exoplanetary context., Comment: Accepted for publication in MNRAS, 15 pages

We present the discovery of NGTS-3Ab, a hot Jupiter found transiting the primary star of an unresolved binary system. We develop a joint analysis of multi-colour photometry, centroids, radial velocity (RV) cross-correlation function (CCF) profiles and their bisector inverse slopes (BIS) to disentangle this three-body system. Data from the Next Generation Transit Survey (NGTS), SPECULOOS and HARPS are analysed and modelled with our new blendfitter software. We find that the binary consists of NGTS-3A (G6V-dwarf) and NGTS-3B (K1V-dwarf) at <1 arcsec separation. NGTS-3Ab orbits every P = 1.675 days. The planet radius and mass are R_planet = 1.48+-0.37 R_J and M_planet = 2.38+-0.26 M_J, suggesting it is potentially inflated. We emphasise that only combining all the information from multi-colour photometry, centroids and RV CCF profiles can resolve systems like NGTS-3. Such systems cannot be disentangled from single-colour photometry and RV measurements alone. Importantly, the presence of a BIS correlation indicates a blend scenario, but is not sufficient to determine which star is orbited by the third body. Moreover, even if no BIS correlation is detected, a blend scenario cannot be ruled out without further information. The choice of methodology for calculating the BIS can influence the measured significance of its correlation. The presented findings are crucial to consider for wide-field transit surveys, which require wide CCD pixels (>5 arcsec) and are prone to contamination by blended objects. With TESS on the horizon, it is pivotal for the candidate vetting to incorporate all available follow-up information from multi-colour photometry and RV CCF profiles., Comment: Accepted for publication in MNRAS 2 May 2018. 20 pages, 11 figures, 9 tables. This is the authors' version of the manuscript

We describe the Next Generation Transit Survey (NGTS), which is a ground-based project searching for transiting exoplanets orbiting bright stars. NGTS builds on the legacy of previous surveys, most notably WASP, and is designed to achieve higher photometric precision and hence find smaller planets than have previously been detected from the ground. It also operates in red light, maximising sensitivity to late K and early M dwarf stars. The survey specifications call for photometric precision of 0.1 per cent in red light over an instantaneous field of view of 100 square degrees, enabling the detection of Neptune-sized exoplanets around Sun-like stars and super-Earths around M dwarfs. The survey is carried out with a purpose-built facility at Cerro Paranal, Chile, which is the premier site of the European Southern Observatory (ESO). An array of twelve 20cm f/2.8 telescopes fitted with back-illuminated deep-depletion CCD cameras are used to survey fields intensively at intermediate Galactic latitudes. The instrument is also ideally suited to ground-based photometric follow-up of exoplanet candidates from space telescopes such as TESS, Gaia and PLATO. We present observations that combine precise autoguiding and the superb observing conditions at Paranal to provide routine photometric precision of 0.1 per cent in 1 hour for stars with I-band magnitudes brighter than 13. We describe the instrument and data analysis methods as well as the status of the survey, which achieved first light in 2015 and began full survey operations in 2016. NGTS data will be made publicly available through the ESO archive., Comment: 20 pages, 17 Figures, Accepted for publication in MNRAS

We present the discovery of NGTS-1b, a hot-Jupiter transiting an early M-dwarf host ($T_{eff}=3916^{+71}_{-63}~K$) in a P=2.674d orbit discovered as part of the Next Generation Transit Survey (NGTS). The planet has a mass of $0.812^{+0.066}_{-0.075}~M_{J}$, making it the most massive planet ever discovered transiting an M-dwarf. The radius of the planet is $1.33^{+0.61}_{-0.33}~R_{J}$. Since the transit is grazing, we determine this radius by modelling the data and placing a prior on the density from the population of known gas giant planets. NGTS-1b is the third transiting giant planet found around an M-dwarf, reinforcing the notion that close-in gas giants can form and migrate similar to the known population of hot Jupiters around solar type stars. The host star shows no signs of activity, and the kinematics hint at the star being from the thick disk population. With a deep (2.5%) transit around a $K=11.9$ host, NGTS-1b will be a strong candidate to probe giant planet composition around M-dwarfs via JWST transmission spectroscopy., Comment: 10 pages, 7 figures, accepted for publication in MNRAS

The Next Generation Transit Survey (NGTS), operating in Paranal since 2016, is a wide-field survey to detect Neptunes and super-Earths transiting bright stars, which are suitable for precise radial velocity follow-up and characterisation. Thereby, its sub-mmag photometric precision and ability to identify false positives are crucial. Particularly, variable background objects blended in the photometric aperture frequently mimic Neptune-sized transits and are costly in follow-up time. These objects can best be identified with the centroiding technique: if the photometric flux is lost off-centre during an eclipse, the flux centroid shifts towards the centre of the target star. Although this method has successfully been employed by the Kepler mission, it has previously not been implemented from the ground. We present a fully-automated centroid vetting algorithm developed for NGTS, enabled by our high-precision auto-guiding. Our method allows detecting centroid shifts with an average precision of 0.75 milli-pixel, and down to 0.25 milli-pixel for specific targets, for a pixel size of 4.97 arcsec. The algorithm is now part of the NGTS candidate vetting pipeline and automatically employed for all detected signals. Further, we develop a joint Bayesian fitting model for all photometric and centroid data, allowing to disentangle which object (target or background) is causing the signal, and what its astrophysical parameters are. We demonstrate our method on two NGTS objects of interest. These achievements make NGTS the first ground-based wide-field transit survey ever to successfully apply the centroiding technique for automated candidate vetting, enabling the production of a robust candidate list before follow-up., Comment: Accepted for publication in MNRAS 25 Jul 2017. 15 pages, 15 figures, 3 tables. This is the authors' version of the manuscript

TYC 8241 2652 1 is a young star that showed a strong mid-infrared (mid-IR, 8-25 mu) excess in all observations before 2008 consistent with a dusty disk. Between 2008 and 2010 the mid-IR luminosity of this system dropped dramatically by at least a factor of 30 suggesting a loss of dust mass of an order of magnitude or more. We aim to constrain possible models including removal of disk material by stellar activity processes, the presence of a binary companion, or other explanations suggested in the literature. We present new X-ray observations, optical spectroscopy, near-IR interferometry, and mid-IR photometry of this system to constrain its parameters and further explore the cause of the dust mass loss. In X-rays TYC 8241 2652 1 has all properties expected from a young star: Its luminosity is in the saturation regime and the abundance pattern shows enhancement of O/Fe. The photospheric Ha line is filled with a weak emission feature, indicating chromospheric activity consistent with the observed level of coronal emission. Interferometry does not detect a companion and sets upper limits on the companion mass of 0.2, 0.35, 0.1 and 0.05 M_sun at projected physical separations of 0.1-4 AU,4-5 AU, 5-10 AU, and 10-30 AU, respectively (assuming a distance of 120.9 pc). Our mid-IR measurements, the first of the system since 2012, are consistent with the depleted dust level seen after 2009. The new data confirms that stellar activity is unlikely to destroy the dust in the disk and shows that scenarios where either TYC 8241 2652 1 heats the disk of a binary companion or a potential companion heats the disk of TYC 8241 2652 1 are unlikely., Comment: accepted by A&A

Earth sustains its magnetic field by a dynamo process driven by convection in the liquid outer core. Geodynamo simulations have been successful in reproducing many observed properties of the geomagnetic field. However, while theoretical considerations suggest that flow in the core is governed by a balance between Lorentz force, rotational force and buoyancy (called MAC balance for Magnetic, Archimedean, Coriolis) with only minute roles for viscous and inertial forces, dynamo simulations must employ viscosity values that are many orders of magnitude larger than in the core due to computational constraints. In typical geodynamo models viscous and inertial forces are not much smaller than the Coriolis force and the Lorentz force plays a sub-dominant role. This has led to conclusions that these simulations are viscously controlled and do not represent the physics of the geodynamo. Here we show by a direct analysis of the relevant forces that a MAC balance can be achieved when the viscosity is reduced to values close to the current practical limit. Lorentz force, buoyancy and the uncompensated (by pressure) part of the Coriolis force are of very similar strength, whereas viscous and inertia are smaller by a factor of at least 20 in the bulk of the fluid volume. Compared to non-magnetic convection at otherwise identical parameters, the dynamo flow is of larger scale, less invariant parallel to the rotation axis (less geostrophic) and convection transports twice as much heat, all of which is expected when the Lorentz force strongly influences the convection properties., Comment: Accepted for publication in the Proceedings of the National Academy of Sciences (PNAS)

The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period $\lesssim 20$ days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here we present an anelastic dynamo simulation designed to mimic some of the physical characteristics of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone which drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the `activity' cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age., Comment: 8 pages, 4 figures, double column; Accepted in ApJ Letters

Despite the lack of a shear-rich tachocline region low-mass fully convective stars are capable of generating strong magnetic fields, indicating that a dynamo mechanism fundamentally different from the solar dynamo is at work in these objects. We present a self-consistent three dimensional model of magnetic field generation in low-mass fully convective stars. The model utilizes the anelastic magnetohydrodynamic equations to simulate compressible convection in a rotating sphere. A distributed dynamo working in the model spontaneously produces a dipole-dominated surface magnetic field of the observed strength. The interaction of this field with the turbulent convection in outer layers shreds it, producing small-scale fields that carry most of the magnetic flux. The Zeeman-Doppler-Imaging technique applied to synthetic spectropolarimetric data based on our model recovers most of the large-scale field. Our model simultaneously reproduces the morphology and magnitude of the large-scale field as well as the magnitude of the small-scale field observed on low-mass fully convective stars., Comment: Double column format, 6 pages, 5 figures. To appear in ApJL

We present an IR-monitoring survey with the $Spitzer$ Space Telescope of the star forming region GGD 12-15. Over 1000 objects were monitored including about 350 objects within the central 5 arcminutes which is found to be especially dense in cluster members. The monitoring took place over 38 days and is part of the Young Stellar Object VARiability (YSOVAR) project. The region was also the subject of a contemporaneous 67ks $Chandra$ observation. The field includes 119 previously identified pre-main sequence star candidates. X-rays are detected from 164 objects, 90 of which are identified with cluster members. Overall, we find that about half the objects in the central 5 arcminutes are young stellar objects based on a combination of their spectral energy distribution, IR variability and X-ray emission. Most of the stars with IR excess relative to a photosphere show large amplitude (>0.1 mag) mid-IR variability. There are 39 periodic sources, all but one of these is found to be a cluster member. Almost half of the periodic sources do not show IR excesses. Overall, more than 85% of the Class I, flat spectrum, and Class II sources are found to vary. The amplitude of the variability is larger in more embedded young stellar objects. Most of the Class~I/II objects exhibit redder colors in a fainter state, compatible with time-variable extinction. A few become bluer when fainter, which can be explained with significant changes in the structure of the inner disk. A search for changes in the IR due to X-ray events is carried out, but the low number of flares prevented an analysis of the direct impact of X-ray flares on the IR lightcurves. However, we find that X-ray detected Class II sources have longer timescales for change in the mid-IR than a similar set of non-X-ray detected Class IIs., Comment: Accepted by AJ 47 (large print) pages including 20 figures and 4 tables