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The relationship between magnetic activity and Rossby number is one way through which stellar dynamos can be understood. Using measured rotation rates and X-ray to bolometric luminosity ratios of an ensemble of stars, we derive empirical convective turnover times based on recent observations and re-evaluate the X-ray activity-Rossby number relationship. In doing so, we find a sharp rise in the convective turnover time for stars in the mass range of $0.35-0.4\ \rm M_{\odot}$, associated with the onset of a fully convective internal stellar structure. Using $\texttt{MESA}$ stellar evolution models, we infer the location of dynamo action implied by the empirical convective turnover time. The empirical convective turnover time is found to be indicative of dynamo action deep within the convective envelope in stars with masses $0.1-1.2\ \rm M_{\odot}$, crossing the fully convective boundary. Our results corroborate past works suggesting that partially and fully convective stars follow the same activity-Rossby relation, possibly owing to similar dynamo mechanisms. Our stellar models also give insight into the dynamo mechanism. We find that empirically determined convective turnover times correlate with properties of the deep stellar interior. These findings are in agreement with global dynamo models that see a reservoir of magnetic flux accumulate deep in the convection zone before buoyantly rising to the surface., Comment: 9 Figures and 2 Tables

We undertake a comprehensive investigation into the distribution of insitu stars within Milky Way-like galaxies, leveraging TNG50 simulations and comparing their predictions with data from the H3 survey. Our analysis reveals that 28% of galaxies demonstrate reasonable agreement with H3, while only 12% exhibit excellent alignment in their profiles, regardless of the specific spatial cut employed to define insitu stars. To uncover the underlying factors contributing to deviations between TNG50 and H3 distributions, we scrutinize correlation coefficients among internal drivers(e.g., virial radius, star formation rate [SFR]) and merger-related parameters (such as the effective mass-ratio, mean distance, average redshift, total number of mergers, average spin-ratio and maximum spin alignment between merging galaxies). Notably, we identify significant correlations between deviations from observational data and key parameters such as the median slope of virial radius, mean SFR values, and the rate of SFR change across different redshift scans. Furthermore, positive correlations emerge between deviations from observational data and parameters related to galaxy mergers. We validate these correlations using the Random Forest Regression method. Our findings underscore the invaluable insights provided by the H3 survey in unraveling the cosmic history of galaxies akin to the Milky Way, thereby advancing our understanding of galactic evolution and shedding light on the formation and evolution of Milky Way-like galaxies in cosmological simulations., Comment: 20 pages, 13 figures

We report the serendipitous discovery of a large edge-on protoplanetary disk associated with the infrared source IRAS 23077+6707. The disk's apparent size in the Pan-STARRS (PS1) images is ~11", making this one of the largest known disks on the sky. It is likely a young system, still surrounded by the envelope which is very faint but still visible in the PS1 images in the northern part (alternatively this structure could be filaments from the disk itself). We use the PS1 magnitudes and other available photometric data to construct the spectral energy distribution (SED) of the disk. An optical spectrum indicates that the obscured star is hot, most likely late A. We adopt a distance of 300 pc for this object based on Gaia DR3 extinctions. We model the system using the HOCHUNK3D radiative transfer software and find that the system is consistent with a hot star of effective temperature 8000 K surrounded by a disk of size 1650 AU and mass 0.2 M_solar at inclination 82 degrees., Comment: 11 pages, 4 figures, accepted in ApJL

Exoplanets with short orbit period reside very close to their host stars. They transition very rapidly between different sectors of the circumstellar space environment along their orbit, leading to large variations of the magnetic field in the vicinity of the planet on short timescales. This rapid change of the magnetic flux through the conducting and resistive layer of the planetary upper atmosphere may drive currents that dissipate in the form of Joule Heating. Here, we estimate the amount of Joule Heating dissipation in the upper atmosphere of Trappist-1e, and two hypothetical planets orbiting the Sun in close-in orbits. We find that the rapid orbital motion could drive a significant amount of atmospheric heating and could significantly affect the planetary atmosphere escape rate. Thus, the process should be accounted for when studying the long-term evolution of exoplanetary atmospheres., Comment: 8 pages, 4 figures, accepted to ApJ

Shadow detection and removal is a challenging problem in the analysis of hyperspectral images. Yet, this step is crucial for analyzing data for remote sensing applications like methane detection. In this work, we develop a shadow detection and removal method only based on the spectrum of each pixel and the overall distribution of spectral values. We first introduce Iterative Logistic Regression (ILR) to learn a spectral basis in which shadows can be linearly classified. We then model the joint distribution of the mean radiance and the projection coefficients of the spectra onto the above basis as a parametric linear combination of Gaussians. We can then extract the maximum likelihood mixing parameter of the Gaussians to estimate the shadow coverage and to correct the shadowed spectra. Our correction scheme reduces correction artefacts at shadow borders. The shadow detection and removal method is applied to hyperspectral images from MethaneAIR, a precursor to the satellite MethaneSAT.

In this paper, we introduce a novel data augmentation methodology based on Conditional Progressive Generative Adversarial Networks (CPGAN) to generate diverse black hole (BH) images, accounting for variations in spin and electron temperature prescriptions. These generated images are valuable resources for training deep learning algorithms to accurately estimate black hole parameters from observational data. Our model can generate BH images for any spin value within the range of [-1, 1], given an electron temperature distribution. To validate the effectiveness of our approach, we employ a convolutional neural network to predict the BH spin using both the GRMHD images and the images generated by our proposed model. Our results demonstrate a significant performance improvement when training is conducted with the augmented dataset while testing is performed using GRMHD simulated data, as indicated by the high R2 score. Consequently, we propose that GANs can be employed as cost effective models for black hole image generation and reliably augment training datasets for other parameterization algorithms., Comment: 11 pages, 7 figures. Accepted by Monthly Notices of the Royal Astronomical Society Journal

In the last few years, there has been significant progress in the development of machine learning methods tailored to astrophysics and cosmology. We have recently applied one of these, namely, the neural network bundle method, to the cosmological scenario. Moreover, we showed that in some cases the computational times of the Bayesian inference process can be reduced. In this paper, we present an improvement to the neural network bundle method that results in a significant reduction of the computational times of the statistical analysis. The novel aspect consists of the use of the neural network bundle method to calculate the luminosity distance of type Ia supernovae, which is usually computed through an integral with numerical methods. In this work, we have applied this improvement to the Hu-Sawicki and Starobinsky $f(R)$ models. We also performed a statistical analysis with data from type Ia supernovae of the Pantheon+ compilation and cosmic chronometers. Another original aspect of this work is the different treatment we provide for the absolute magnitude of type Ia supernovae during the inference process, which results in different estimates of the distortion parameter than the ones obtained in the literature. We show that the statistical analyses carried out with our new method require lower computational times than the ones performed with both the numerical and the neural network method from our previous work. This reduction in time is more significant in the case of a difficult computational problem such as the ones addressed in this work., Comment: 14 pages, 5 figures, 3 tables

The high-energy radiation emitted by young stars can have a strong influence on their rotational evolution at later stages. This is because internal photoevaporation is one of the major drivers of the dispersal of circumstellar disks, which surround all newly born low-mass stars during the first few million years of their evolution. Employing an internal EUV/X-ray photoevaporation model, we have derived a simple recipe for calculating realistic inner disk lifetimes of protoplanetary disks. This prescription was implemented into a magnetic morphology-driven rotational evolution model and is used to investigate the impact of disk-locking on the spin evolution of low-mass stars. We find that the length of the disk-locking phase has a profound impact on the subsequent rotational evolution of a young star, and the implementation of realistic disk lifetimes leads to an improved agreement of model outcomes with observed rotation period distributions for open clusters of various ages. However, for both young star-forming regions tested in our model, the strong bimodality in rotation periods that is observed in hPer could not be recovered. hPer is only successfully recovered, if the model is started from a double-peaked distribution with an initial disk fraction of $65\,\%$. However, at an age of only $\sim 1\,\mathrm{Myr}$, such a low disk fraction can only be achieved if an additional disk dispersal process, such as external photoevaporation, is invoked. These results therefore highlight the importance of including realistic disk dispersal mechanisms in rotational evolution models of young stars., Comment: accepted for publication in ApJ

We outline some of the highlights of the scientific case for the advancement of stellar high energy physics using the Line Emission Mapper X-ray Probe ({\it LEM}). The key to advancements with LEM lie in its large effective area -- up to 100 times that of the {\it Chandra} MEG -- and 1~eV spectral resolution. The large effective area opens up for the first time the ability to study time-dependent phenomena on their natural timescales at high resolution, such as flares and coronal mass ejections, and also opens the sky to much fainter targets than available to {\it Chandra} or {\it XMM-Newton}., Comment: A Line Emission Mapper X-ray Probe White Paper

Black hole X-ray binaries (BHBs) offer insights into extreme gravitational environments and the testing of general relativity. The X-ray spectrum collected by NICER offers valuable information on the properties and behaviour of BHBs through spectral fitting. However, traditional spectral fitting methods are slow and scale poorly with model complexity. This paper presents a new semi-supervised autoencoder neural network for parameter prediction and spectral reconstruction of BHBs, showing an improvement of up to a factor of 2,700 in speed while maintaining comparable accuracy. The approach maps the spectral features from the numerous outbursts catalogued by NICER and generalises them to new systems for efficient and accurate spectral fitting. The effectiveness of this approach is demonstrated in the spectral fitting of BHBs and holds promise for use in other areas of astronomy and physics for categorising large datasets., Comment: 13 pages, 13 figures

Advancements in space telescopes have opened new avenues for gathering vast amounts of data on exoplanet atmosphere spectra. However, accurately extracting chemical and physical properties from these spectra poses significant challenges due to the non-linear nature of the underlying physics. This paper presents novel machine learning models developed by the AstroAI team for the Ariel Data Challenge 2023, where one of the models secured the top position among 293 competitors. Leveraging Normalizing Flows, our models predict the posterior probability distribution of atmospheric parameters under different atmospheric assumptions. Moreover, we introduce an alternative model that exhibits higher performance potential than the winning model, despite scoring lower in the challenge. These findings highlight the need to reevaluate the evaluation metric and prompt further exploration of more efficient and accurate approaches for exoplanet atmosphere spectra analysis. Finally, we present recommendations to enhance the challenge and models, providing valuable insights for future applications on real observational data. These advancements pave the way for more effective and timely analysis of exoplanet atmospheric properties, advancing our understanding of these distant worlds., Comment: Conference proceeding for the ECML PKDD 2023

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

Astronomy is presently experiencing profound growth in the deployment of machine learning to explore large datasets. However, transient quasi-periodic oscillations (QPOs) which appear in power density spectra of many X-ray binary system observations are an intriguing phenomena heretofore not explored with machine learning. In light of this, we propose and experiment with novel methodologies for predicting the presence and properties of QPOs to make the first ever detections and characterizations of QPOs with machine learning models. We base our findings on raw energy spectra and processed features derived from energy spectra using an abundance of data from the NICER and RXTE space telescope archives for two black hole low mass X-ray binary sources, GRS 1915+105 and MAXI J1535-571. We advance these non-traditional methods as a foundation for using machine learning to discover global inter-object generalizations between - and provide unique insights about - energy and timing phenomena to assist with the ongoing challenge of unambiguously understanding the nature and origin of QPOs. Additionally, we have developed a publicly available Python machine learning library, QPOML, to enable further Machine Learning aided investigations into QPOs., Comment: 18 pages, 12 figures, accepted by MNRAS

We present a three-dimensional, time-dependent, MHD simulation of the short-term interaction between a protoplanetary disk and the stellar corona in a T Tauri system. The simulation includes the stellar magnetic field, self-consistent coronal heating and stellar wind acceleration, and a disk rotating at sub-Keplerian velocity to induce accretion. We find that initially, as the system relaxes from the assumed initial conditions, the inner part of the disk winds around and moves inward and close to the star as expected. However, the self-consistent coronal heating and stellar wind acceleration build up the original state after some time, significantly pushing the disk out beyond $10R_\star$. After this initial relaxation period, we do not find clear evidence of a strong, steady accretion flow funneled along coronal field lines, but only weak, sporadic accretion. We produce synthetic coronal X-ray line emission light curves which show flare-like increases that are not correlated with accretion events nor with heating events. These variations in the line emission flux are the result of compression and expansion due to disk-corona pressure variations. Vertical disk evaporation evolves above and below the disk. However, the disk - stellar wind boundary stays quite stable, and any disk material that reaches the stellar wind region is advected out by the stellar wind., Comment: Accepted to ApJ, 12 pages, 11 figures

Black hole X-ray binary systems (BHBs) contain a close companion star accreting onto a stellar-mass black hole. A typical BHB undergoes transient outbursts during which it exhibits a sequence of long-lived spectral states, each of which is relatively stable. GRS 1915+105 is a unique BHB that exhibits an unequaled number and variety of distinct variability patterns in X-rays. Many of these patterns contain unusual behaviour not seen in other sources. These variability patterns have been sorted into different classes based on count rate and color characteristics by Belloni et al (2000). In order to remove human decision-making from the pattern-recognition process, we employ an unsupervised machine learning algorithm called an auto-encoder to learn what classifications are naturally distinct by allowing the algorithm to cluster observations. We focus on observations taken by the Rossi X-ray Timing Explorer's Proportional Counter Array. We find that the auto-encoder closely groups observations together that are classified as similar under the Belloni et al (2000) system, but that there is reasonable grounds for defining each class as made up of components from 3 groups of distinct behaviour., Comment: 20 pages, 27 figures. For associated projection code to view the interactive 3D plot, see https://github.com/bjricketts/grs1915-auto-encoder

Close-in planets orbiting around low-mass stars are exposed to intense energetic photon and particle radiation and harsh space weather. We have modeled such conditions for Proxima Centauri b (Garraffo et al. 2016b), a rocky planet orbiting in the habitable-zone of our closest neighboring star, finding a stellar wind pressure three orders of magnitude higher than the solar wind pressure on Earth. At that time, no Zeeman-Doppler observations of the surface magnetic field distribution of Proxima Cen were available and a proxy from a star with similar Rossby number to Proxima was used to drive the MHD model. Recently, the first ZDI observation of Proxima Cen became available (Klein et al. 2021). We have modeled Proxima b's space weather using this map and compared it with the results from the proxy magnetogram. We also computed models for a high-resolution synthetic magnetogram for Proxima b generated by a state-of-the-art dynamo model. The resulting space weather conditions for these three scenarios are similar with only small differences found between the models based on the ZDI observed magnetogram and the proxy. We conclude that our proxy magnetogram prescription based on Rossby number is valid, and provides a simple way to estimate stellar magnetic flux distributions when no direct observations are available. Comparison with models based on the synthetic magnetogram show that the exact magnetogram details are not important for predicting global space weather conditions of planets, reinforcing earlier conclusions that the large-scale (low-order) field dominates, and that the small-scale field does not have much influence on the ambient stellar wind., Comment: Accepted to ApJL, 11 pages, 3 figures

The field of machine learning has drawn increasing interest from various other fields due to the success of its methods at solving a plethora of different problems. An application of these has been to train artificial neural networks to solve differential equations without the need of a numerical solver. This particular application offers an alternative to conventional numerical methods, with advantages such as lower memory required to store solutions, parallelization, and, in some cases, a lower overall computational cost than its numerical counterparts. In this work, we train artificial neural networks to represent a bundle of solutions of the differential equations that govern the background dynamics of the Universe for four different models. The models we have chosen are $\Lambda \mathrm{CDM}$, the Chevallier-Polarski-Linder parametric dark energy model, a quintessence model with an exponential potential, and the Hu-Sawicki $f(R)$ model. We use the solutions that the networks provide to perform statistical analyses to estimate the values of each model's parameters with observational data; namely, estimates of the Hubble parameter from cosmic chronometers, type Ia supernovae data from the Pantheon compilation, and measurements from baryon acoustic oscillations. The results we obtain for all models match similar estimations done in the literature using numerical solvers. In addition, we estimate the error of the solutions that the trained networks provide by comparing them with the analytical solution when there is one, or to a high-precision numerical solution when there is not. Through these estimations we find that the error of the solutions is at most $\sim1\%$ in the region of the parameter space that concerns the $95\%$ confidence regions that we find using the data, for all models and all statistical analyses performed in this work., Comment: 28 pages, 10 figures, 10 tables, supplemental material available at https://github.com/at-chantada/Supplemental-Materials

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

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"

Characterizing the fundamental parameters of stars from observations is crucial for studying the stars themselves, their planets, and the galaxy as a whole. Stellar evolution theory predicting the properties of stars as a function of stellar age and mass enables translating observables into physical stellar parameters by fitting the observed data to synthetic isochrones. However, the complexity of overlapping evolutionary tracks often makes this task numerically challenging, and with a precision that can be highly variable, depending on the area of the parameter space the observation lies in. This work presents StelNet, a Deep Neural Network trained on stellar evolutionary tracks that quickly and accurately predicts mass and age from absolute luminosity and effective temperature for stars with close to solar metallicity. The underlying model makes no assumption on the evolutionary stage and includes the pre-main sequence phase. We use bootstrapping and train many models to quantify the uncertainty of the model. To break the model's intrinsic degeneracy resulting from overlapping evolutionary paths, we also built a hierarchical model that retrieves realistic posterior probability distributions of the stellar mass and age. We further test and train StelNet using a sample of stars with well-determined masses and ages from the literature., Comment: 17 pages, 15 figures, accepted to AJ

Stellar wind and photon radiation interactions with a planet can cause atmospheric depletion, which may have a potentially catastrophic impact on a planet's habitability. While the implications of photoevaporation on atmospheric erosion have been researched to some degree, studies of the influence of the stellar wind on atmospheric loss are in their infancy. Here, we use three-dimensional magnetohydrodynamic simulations to model the effect of the stellar wind on the magnetosphere and outflow of a hypothetical planet, modeled to have an H-rich evaporating envelope with a pre-defined mass loss rate, orbiting in the habitable zone close to a low-mass M dwarf. We take the TRAPPIST-1 system as a prototype, with our simulated planet situated at the orbit of TRAPPIST-1e. We show that the atmospheric outflow is dragged and accelerated upon interaction with the wind, resulting in a diverse range of planetary magnetosphere morphologies and plasma distributions as local stellar wind conditions change. We consider the implications of the wind-outflow interaction on potential hydrogen Lyman-alpha (Lya) observations of the planetary atmosphere during transits. The Lya observational signatures depend strongly on the local wind conditions at the time of the observation and can be subject to considerable variation on timescales as short as an hour. Our results indicate that observed variations in exoplanet Lya transit signatures could be explained by wind-outflow interaction., Comment: Submitted to ApJ. 18 pages, 7 figures

Two magnetic braking models are implemented in MESA for use in the MIST stellar model grids. Stars less than about 1.3 $M_{\odot}$ are observed to spin down over time through interaction with their magnetized stellar winds (i.e., magnetic braking). This is the basis for gyrochronology, and fundamental to the evolution of lower mass stars. The detailed physics behind magnetic braking are uncertain, as are 1D stellar evolution models. Thus, we calibrate our models and compare to data from open clusters. Each braking model tested here is capable of reproducing the data, albeit with some important distinctions. The Matt et al. (2015) prescription matches the slowly rotating stars observed in open clusters, but tends to overestimate the presence of rapidly rotating stars. The Garraffo et al. (2018) prescription often produces too much angular momentum loss to accurately match the observed slow sequence for lower mass stars, but reproduces the bimodal nature of slow and rapidly rotating stars observed in open clusters fairly well. We find additional evidence that some level of mass dependency may be missing in these braking models to match the rotation periods observed in clusters older than 1 Gyr better., Comment: 18 pages, 6 figures, submitted to ApJ (accepted)

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)

High levels of X-ray and UV activity on young M dwarfs may drive rapid atmospheric escape on temperate, terrestrial planets orbiting within the liquid water habitable zone. However, secondary atmospheres on planets orbiting older, less active M dwarfs may be stable and present more promising candidates for biomarker searches. We present new HST and Chandra observations of Barnard's Star (GJ 699), a 10 Gyr old M3.5 dwarf, acquired as part of the Mega-MUSCLES program. Despite the old age and long rotation period of Barnard's star, we observe two FUV ($\delta_{130}$ $\approx$ 5000s; $E_{130}$ $\approx$ 10$^{29.5}$ erg each) and one X-ray ($E_{X}$ $\approx$ 10$^{29.2}$ erg) flares, and estimate a high-energy flare duty cycle (defined here as the fraction of the time the star is in a flare state) of $\sim$ 25\%. A 5 A - 10 $\mu$m SED of GJ 699 is created and used to evaluate the atmospheric stability of a hypothetical, unmagnetized terrestrial planet in the habitable zone ($r_{HZ}$ $\sim$ 0.1 AU). Both thermal and non-thermal escape modeling indicate (1) the $quiescent$ stellar XUV flux does not lead to strong atmospheric escape: atmospheric heating rates are comparable to periods of high solar activity on modern Earth, and (2) the $flare$ environment could drive the atmosphere into a hydrodynamic loss regime at the observed flare duty cycle: sustained exposure to the flare environment of GJ 699 results in the loss of $\approx$ 87 Earth atmospheres Gyr$^{-1}$ through thermal processes and $\approx$ 3 Earth atmospheres Gyr$^{-1}$ through ion loss processes, respectively. These results suggest that if rocky planet atmospheres can survive the initial $\sim$ 5 Gyr of high stellar activity, or if a second generation atmosphere can be formed or acquired, the flare duty cycle may be the controlling stellar parameter for the stability of Earth-like atmospheres around old M stars., Comment: Accepted to AJ

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

The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, chemical and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our Solar system: it is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H-dominated thermosphere for astrophysicists, or the Sun-like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so-called Xenon paradox.

The stellar magnetic field completely dominates the environment around late-type stars. It is responsible for driving the coronal high-energy radiation (e.g. EUV/X-rays), the development of stellar winds, and the generation transient events such as flares and coronal mass ejections (CMEs). While progress has been made for the first two processes, our understanding of the eruptive behavior in late-type stars is still very limited. One example of this is the fact that despite the frequent and highly energetic flaring observed in active stars, direct evidence for stellar CMEs is almost non-existent. Here we discuss realistic 3D simulations of stellar CMEs, analyzing their resulting properties in contrast with solar eruptions, and use them to provide a common framework to interpret the available stellar observations. Additionally, we present results from the first 3D CME simulations in M-dwarf stars, with emphasis on possible observable signatures imprinted in the stellar corona., Comment: 7 pages, 2 figures, to appear in the proceedings of the IAU Symposium 354, Solar and Stellar Magnetic Fields: Origins and Manifestations. Eds: A. Kosovichev, K. Strassmeier, M. Jardine; based on an invited talk

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 towards 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 ($\sim2-3$). In all the considered cases (escaping/confined) a fractional decrease in the emission from mid-range 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., Comment: 10 Pages, 5 Figures, Accepted to The Astrophysical Journal Letters

Solar CMEs and flares have a statistically well defined relation, with more energetic X-ray flares corresponding to faster and more massive CMEs. How this relation 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 GOES 1--8~\AA\ band. Derived CME masses range from $\sim 10^{15}$ to $10^{22}$~g. Associated flare X-ray energies range from $10^{31}$ to $10^{37}$~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 two 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., Comment: 23 pages, 3 tables, 4 figures, accepted by ApJ

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

The EUV (100-912 {\AA}) is a spectral region notoriously difficult to observe due to attenuation by neutral hydrogen gas in the interstellar medium. Despite this, hundreds to thousands of nearby stars of different spectral types and magnetic activity levels are accessible in the EUV range. The EUV probes interesting and complicated regions in the stellar atmosphere like the lower corona and transition region that are inaccessible from other spectral regions. In this white paper we describe how direct EUV observations, which require a dedicated grazing-incidence observatory, cannot yet be accurately substituted with models and theory. Exploring EUV emission from cool dwarf stars in the time domain can make a major contribution to understanding stellar outer atmospheres and magnetism, and offers the clearest path toward detecting coronal mass ejections on stars other than the Sun., Comment: Science white paper submitted to the Astro2020 Decadal Survey. 8 pages, 2 figures

The planetary effective surface temperature alone is insufficient to characterize exoplanet atmospheres and their stability or evolution. Considering the star-planet system as a whole is necessary, and a critical component of the system is the photoionizing stellar extreme ultraviolet emission (EUV; 100-912 {\AA}). EUV photons drive atmospheric mass loss through thermal and nonthermal processes, and an accurate accounting of the EUV energy deposition in a planet's energy budget is essential, especially for terrestrial habitable zone planets and close-in gaseous planets. Direct EUV observations of exoplanet host stars would require a new, dedicated observatory. Archival observations from the $\textit{EUVE}$ satellite, models, and theory alone are insufficient to accurately characterize EUV spectra of the majority of exoplanet host stars, especially for low-mass stars., Comment: Science white paper submitted to the Astro2020 Decadal Survey. 7 pages, 2 figures

A physically realistic stellar wind model based on Alfv\'en 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 $\sim$\,$20 - 40$\,\% smaller than that of the Earth for an equivalent planetary magnetic field strength., Comment: J. D. Alvarado-G\'omez & C. Garraffo contributed equally to this work. 8 Pages, 4 Figures. Accepted to The Astrophysical Journal Letters

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 (MHD) simulations of the solar corona using the code 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 instead of the refracted paths are considered. We are planning to incorporate non-thermal radio emission mechanisms in a future version of the radio imaging tool., Comment: 15 pages, 8 figures, Accepted by ApJ (NOTE: The current version includes extra material compared to the accepted to ApJ version.)

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., Comment: Accepted to AJ

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 (MHD) code which considers a self-consistent coupling between an Alfv\'en 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 $\sim$$3 \times 10^{32}$~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 $6 \times 10^{32}$ erg (GOES class $\sim$X70)., Comment: 16 Pages, 8 figures, Accepted for publication in The Astrophysical Journal

We use magnetograms of 8 solar analogues of ages 30~Myr to 3.6~Gyr obtained from Zeeman Doppler Imaging (ZDI) and taken from the literature, together with two solar magnetograms, to drive magnetohydrodynamical (MHD) wind simulations and construct an evolutionary scenario of the solar wind environment and its angular momentum loss rate. With observed magnetograms of the radial field strength as the only variant in the wind model, we find that power law model fitted to the derived angular momentum loss rate against time, $t$, results in a spin down relation $\Omega\propto t^{-0.51}$, for angular speed $\Omega$, which is remarkably consistent with the well-established Skumanich law $\Omega\propto t^{-0.5}$. We use the model wind conditions to estimate the magnetospheric standoff distances for an Earth-like test planet situated at 1~AU for each of the stellar cases, and to obtain trends of minimum and maximum wind ram pressure and average ram pressure in the solar system through time. The wind ram pressure declines with time as $\overline{P_{ram}}\propto t^{2/3}$, amounting to a factor of 50 or so over the present lifetime of the solar system., Comment: 21 pages, 11 figures, accepted for publication in ApJ, 8th February 2018

We explore the scenario of a Coronal Mass Ejection (CME) being the cause of the observed continuous X-ray absorption of the August 30 1997 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 $2\times 10^{21}$-$2\times 10^{22}$ g and kinetic energy in the range $7\times 10^{35}$-$3\times 10^{38}$ 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., Comment: 10 pages, 5 figures, accepted for publication in ApJ

A former member of the Hyades cluster, $\iota$ Horologii ($\iota$ Hor) is a planet-hosting Sun-like star which displays the shortest coronal activity cycle known to date ($P_{\rm cyc}\sim$ 1.6 yr). With an age of $\sim$$625$ Myr, $\iota$ 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 articles, 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 $\iota$ 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 $\iota$ Hor follows a beating pattern, caused by the superposition of two periodic signals of similar amplitude at $P_{1} \simeq 1.97 \pm 0.02$ yr and $P_{2} \simeq 1.41 \pm 0.01$ yr. Additionally, using the most recent parameters for $\iota$ 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 $\iota$ Hor, which displays varying amplitudes within $\pm4$ G and served to estimate the rotation period of the star ($P_{\rm rot} =~7.70^{+0.18}_{-0.67}$ d)., Comment: 14 pages, 7 Figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society (MNRAS)

The evolution of protoplanetary disks is believed to be driven largely by angular momentum transport resulting from magnetized disk winds and turbulent viscosity. The ionization of the disk that is essential for these processes has been thought due to host star coronal X-rays but could also arise from energetic particles produced by coronal flares or by travelling shock waves and advected by the stellar wind. We have performed test-particle numerical simulations of energetic protons propagating into a realistic T~Tauri stellar wind, including a superposed small-scale magnetostatic turbulence. The isotropic (Kolmogorov power spectrum) turbulent component is synthesised along the individual particle trajectories. We have investigated the energy range $[0.1 - 10]$ GeV, consistent with expectations from {\it Chandra} X-ray observations of large flares on T~Tauri stars and with recent indications by the {\it Herschel} Space Observatory of a significant contribution of energetic particles to the disk ionization of young stars. In contrast with a previous theoretical study finding dominance of energetic particles over X-ray in the ionization throughout the disk, we find that the disk ionization is likely dominated by X-rays over much of its area except within narrow regions where particles are channeled onto the disk by the strongly-tangled and turbulent magnetic field. The radial thickness of such regions is $\sim 5$ stellar radii close to the star and broadens with increasing radial distance. This likely continues out to large distances from the star ($10$ AU or greater) where particles can be copiously advected and diffused by the turbulent wind., Comment: 18 pages, 14 figures, ApJ, in press

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 $10^3$ and $10^5$ 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-Alfv\'enic 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 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., Comment: 12 pages, 5 figures, to appear in ApJ Letters

Flares we observe on stars in white light, UV or soft X-rays are probably harbingers of coronal mass ejections (CMEs). If we use the Sun as a guide, large stellar flares will dissipate two orders of magnitude less X-ray radiative energy than the kinetic energy in the associated CME. Since coronal emission on active stars appears to be dominated by flare activity, CMEs pose a quandary for understanding the fraction of their energy budget stars can spend on magnetic activity. One answer is magnetic suppression of CMEs, in which the strong large-scale fields of active stars entrap and prevent CMEs unless their free energy exceeds a critical value. The CME-less flaring active region NOAA 2192 presents a possible solar analogue of this. Monster CMEs will still exist, and have the potential to ravage planetary atmospheres., Comment: 6 pages, to appear in Proceedings of IAU Symposium No. 320

A planet orbiting in the "habitable zone" of our closest neighboring star, Proxima Centauri, has recently been discovered, and the next natural question is whether or not Proxima b is "habitable". Stellar winds are likely a source of atmospheric erosion that could be particularly severe in the case of M dwarf habitable zone planets that reside close to their parent star. Here we study the stellar wind conditions that Proxima b experiences over its orbit. We construct 3-D MHD models of the wind and magnetic field around Proxima Centauri using a surface magnetic field map for a star of the same spectral type and scaled to match the observed ~ 600 G surface magnetic field strength of Proxima. We examine the wind conditions and dynamic pressure over different plausible orbits that sample the constrained parameters of the orbit of Proxima b. For all the parameter space explored, the planet is subject to stellar wind pressures of more than 2000 times those experienced by Earth from the solar wind. During an orbit, Proxima b is also subject to pressure changes of 1 to 3 orders of magnitude on timescales of a day. Its magnetopause standoff distance consequently undergoes sudden and periodic changes by a factor of 2 to 5. Proxima b will traverse the interplanetary current sheet twice each orbit, and likely crosses into regions of subsonic wind quite frequently. These effects should be taken into account in any physically realistic assessment or prediction of its atmospheric reservoir, characteristics and loss., Comment: 14 pages, 5 figures, ApJ Letters, 833, 1

This study examines the relationship between magnetic field complexity and mass and angular momentum losses. Observations of open clusters have revealed a bimodal distribution of the rotation periods of solar-like stars that has proven difficult to explain under the existing rubric of magnetic braking. Recent studies suggest that magnetic complexity can play an important role in controlling stellar spin-down rates. However, magnetic morphology is still neglected in most rotation evolution models due to the difficulty of properly accounting for its effects on wind driving and angular momentum loss. Using state-of-the-art magnetohydrodynamical magnetized wind simulations we study the effect that different distributions of the magnetic flux at different levels of geometrical complexity have on mass and angular momentum loss rates. Angular momentum loss rates depend strongly on the level of complexity of the field but are independent of the way this complexity is distributed. We deduce the analytical terms representing the magnetic field morphology dependence of mass and angular momentum loss rates. We also define a parameter that best represents complexity for real stars. As a test, we use these analytical methods to estimate mass and angular momentum loss rates for $8$ stars with observed magnetograms and compare them to the simulated results. Magnetic field complexity provides a natural physical basis for stellar rotation evolution models requiring a rapid transition between weak and strong spin-down modes., Comment: 13 pages, 6 figures, accepted for publication in A&A

Rotation evolution of late-type stars is dominated by magnetic braking and the underlying factors that control this angular momentum loss are important for the study of stellar spin-down. In this work, we study angular momentum loss as a function of two different aspects of magnetic activity using a calibrated Alfv\'en wave-driven magnetohydrodynamic wind model: the strengths of magnetic spots and their distribution in latitude. By driving the model using solar and modified solar surface magnetograms, we show that the topology of the field arising from the net interaction of both small-scale and large-scale field is important for spin-down rates and that angular momentum loss is not a simple function of large scale magnetic field strength. We find that changing the latitude of magnetic spots can modify mass and angular momentum loss rates by a factor of two. The general effect that causes these differences is the closing down of large-scale open field at mid- and high-latitudes by the addition of the small-scale field. These effects might give rise to modulation of mass and angular momentum loss through stellar cycles, and present a problem for ab initio attempts to predict stellar spin-down based on wind models. For all the magnetogram cases considered here, from dipoles to various spotted distributions, we find that angular momentum loss is dominated by the mass loss at mid-latitudes. The spin-down torque applied by magnetized winds therefore acts at specific latitudes and is not evenly distributed over the stellar surface, though this aspect is unlikely to be important for understanding spin-down and surface flows on stars., Comment: 13 pages, 7 figures, 1 table, accepted for publication in ApJ

Observations of young open clusters have revealed a bimodal distribution of fast and slower rotation rates that has proven difficult to explain with predictive models of spin down that depend on rotation rates alone. The Metastable Dynamo Model proposed recently by Brown, employing a stochastic transition probability from slow to more rapid spin down regimes, appears to be more successful but lacks a physical basis for such duality. Using detailed 3D MHD wind models computed for idealized multipole magnetic fields, we show that surface magnetic field complexity can provide this basis. Both mass and angular momentum losses decline sharply with increasing field complexity. Combined with observation evidence for complex field morphologies in magnetically active stars, our results support a picture in which young, rapid rotators lose angular momentum in an inefficient way because of field complexity. During this slow spin-down phase, magnetic complexity is eroded, precipitating a rapid transition from weak to strong wind coupling., Comment: 15 pages, 5 figures, accepted for publication by ApJ Letters