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Tidal disruption events (TDEs) can potentially probe low-mass black holes in host galaxies that might not adhere to bulge or stellar-dispersion relationships. At least initially, TDEs can also reveal super-Eddington accretion. X-ray spectroscopy can potentially constrain black hole masses, and reveal ionized outflows associated with super-Eddington accretion. Our analysis of XMM-Newton X-ray observations of the TDE AT2021ehb, around 300 days post-disruption, reveals a soft spectrum and can be fit with a combination of multi-color disk blackbody and power-law components. Using two independent disk models with properties suited to TDEs, we estimate a black hole mass at $M \simeq 10^{5.5}~M_{\odot}$, indicating AT2021ehb may expose the elusive low-mass end of the nuclear black hole population. These models offer simple yet robust characterization; more complicated models are not required, but provide important context and caveats in the limit of moderately sensitive data. If disk reflection is included, the disk flux is lower and inferred black hole masses are $\sim$ 0.35 dex higher. Simple wind formulations imply an extremely fast $v_{\mathrm{out}} = -0.2~c$ outflow and obviate a disk continuum component. Assuming a unity filling factor, such a wind implies an instantaneous mass outflow rate of $\dot{M} \simeq 5~M_{\odot}~{\rm yr}^{-1}$. Such a high rate suggests that the filling factor for the Ultra Fast Outflow (UFO) must be extremely low, and/or the UFO phase is ephemeral. We discuss the strengths and limitations of our analysis and avenues for future observations of TDEs., Comment: 19 pages, 4 figures

The origin of bright X-ray emission lines that appear late in a nova eruption remains largely a puzzle. We present two high-resolution X-ray grating spectra of the classical nova YZ Ret, observed 77 and 115 days post-eruption, using XMM-Newton and Chandra , respectively. Both spectra feature resolved emission lines blueshifted by $v = -1500$ km s$^{-1}$ and broadened by $\sigma_v=500$ km s$^{-1}$. The two spectra are well described by a collisionally ionized plasma of $kT\sim 70$ eV that dimmed by a factor of $\sim40$ between the two exposures. The spectra also show narrow radiative recombination continua (RRCs) of C$^{+4}$, C$^{+5}$, and N$^{+5}$, indicating the interaction of the hot ionized plasma with cold electrons of $kT\sim 2$ eV. The high-$n$ Rydberg series of C$^{+4}$ is anomalously bright, allowing us to measure the electron density through continuum lowering, which is in agreement with the He-like N$^{+5}$ density diagnostic of $n_e=(1.7\pm0.4)\times10^{11}$ cm$^{-3}$. The high population of these high-$n$ levels constitutes the best evidence to date of charge exchange (CX) with neutral H in an astrophysical ionized plasma. The remarkable fact that the velocity and plasma temperature are the same after 38 days, despite the high density and decreasing flux is evidence for ongoing heating. We suggest the heating is due to a reverse shock in the nova ejecta, which forms a thin X-ray shell. The narrow RRCs and CX are attributed to direct mixing with cold gas, which overtakes the hot plasma either from the shock front, or through the contact discontinuity., Comment: 19 pages, 13 figures

We present resolved images of IRAS 23077+6707 ("Dracula's Chivito") in 1.3 mm/225 GHz thermal dust and CO gas emission with the Submillimeter Array (SMA) and optical (0.5-$0.8\,\mu\mathrm{m}$) scattered light with the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The Pan-STARRS data show a bipolar distribution of optically scattering dust that is characteristic for disks observed at high inclinations. Its scattered light emission spans ${\sim}14''$, with two highly asymmetric filaments extending along the upper bounds of each nebula by ${\sim}9''$. The SMA data measure 1.3 mm continuum dust as well as $^{12}$CO, $^{13}$CO and C$^{18}$O $J$=2$-$1 line emission over $12''$-$14''$ extents, with the gas presenting the typical morphology of a disk in Keplerian rotation, in both position-velocity space and in each CO line spectrum. IRAS 23077+6707 has no reported distance estimate, but if it is located in the Cepheus star-forming region (180-800 pc), it would have a radius spanning thousands of au. Taken together, we infer IRAS 23077+6707 to be a giant and gas-rich edge-on protoplanetary disk, which to our knowledge is the largest in extent so far discovered., Comment: published in The Astrophysical Journal Letters 967:L2 (9pp), 2024 May 20

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

One of the key research themes identified by the Astro2020 decadal survey is Worlds and Suns in Context. The Advanced X-ray Imaging Satellite (AXIS) is a proposed NASA APEX mission that will become the prime high-energy instrument for studying star-planet connections from birth to death. This work explores the major advances in this broad domain of research that will be enabled by the AXIS mission, through X-ray observations of stars in clusters spanning a broad range of ages, flaring M-dwarf stars known to host exoplanets, and young stars exhibiting accretion interactions with their protoplanetary disks. In addition, we explore the ability of AXIS to use planetary nebulae, white dwarfs, and the Solar System to constrain important physical processes from the microscopic (e.g., charge exchange) to the macroscopic (e.g., stellar wind interactions with the surrounding interstellar medium)., Comment: This White Paper is part of a series commissioned for the AXIS Probe Concept Mission

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

We present some of the salient aspects of the scientific motivation for high resolution soft X-ray spectroscopy of early-type stars with the Line Emission Mapper X-ray Probe. The major strength of {\it LEM} for hot star physics is its large effective area, aided by the inherent energy resolution of its microcalorimeter that readily achieves resolving powers of 1000 and obviates the need for relatively inefficient dispersive optical elements. This increased sensitivity enables much fainter and more distant high mass stars to be observed than are accessible with present-day facilities, greatly increasing the pool of potential targets. For brighter sources, the sensitivity opens up time domain studies, wherein sufficient signal can be garnered in short order and exposure times, probing source variations on ks timescales. We argue that these capabilities of {\it LEM} will yield breakthroughs in all types of hot star systems, from understanding single OB and WR star winds and how they vary with metallicity, to probing the shocks of colliding wind systems and the magnetically channeled winds of magnetic OB stars. {\it LEM} will also study the energetics of WR star bubbles and feedback from their powerful pre-SN stellar winds., Comment: A Line Emission Mapper White Paper

Ultra-cool dwarf stars are abundant, long-lived, and uniquely suited to enable the atmospheric study of transiting terrestrial companions with JWST. Amongst them, the most prominent is the M8.5V star TRAPPIST-1 and its seven planets. While JWST Cycle 1 observations have started to yield preliminary insights into the planets, they have also revealed that their atmospheric exploration requires a better understanding of their host star. Here, we propose a roadmap to characterize the TRAPPIST-1 system -- and others like it -- in an efficient and robust manner. We notably recommend that -- although more challenging to schedule -- multi-transit windows be prioritized to mitigate the effects of stellar activity and gather up to twice more transits per JWST hour spent. We conclude that, for such systems, planets cannot be studied in isolation by small programs, but rather need large-scale, jointly space- and ground-based initiatives to fully exploit the capabilities of JWST for the exploration of terrestrial planets.

The Line Emission Mapper (LEM) is an X-ray Probe with with spectral resolution ~2 eV FWHM from 0.2 to 2.5 keV and effective area >2,500 cm$^2$ at 1 keV, covering a 33 arcmin diameter Field of View with 15 arcsec angular resolution, capable of performing efficient scanning observations of very large sky areas and enabling the first high spectral resolution survey of the full sky. The LEM-All-Sky Survey (LASS) is expected to follow the success of previous all sky surveys such as ROSAT and eROSITA, adding a third dimension provided by the high resolution microcalorimeter spectrometer, with each 15 arcsec pixel of the survey including a full 1-2 eV resolution energy spectrum that can be integrated over any area of the sky to provide statistical accuracy. Like its predecessors, LASS will provide both a long-lasting legacy and open the door to the unknown, enabling new discoveries and delivering the baseline for unique GO studies. No other current or planned mission has the combination of microcalorimeter energy resolution and large grasp to cover the whole sky while maintaining good angular resolution and imaging capabilities. LASS will be able to probe the physical conditions of the hot phases of the Milky Way at multiple scales, from emission in the Solar system due to Solar Wind Charge eXchange, to the interstellar and circumgalactic media, including the North Polar Spur and the Fermi/eROSITA bubbles. It will measure velocities of gas in the inner part of the Galaxy and extract the emissivity of the Local Hot Bubble. By maintaining the original angular resolution, LASS will also be able to study classes of point sources through stacking. For classes with ~$10^4$ objects, it will provide the equivalent of 1 Ms of high spectral resolution data. We describe the technical specifications of LASS and highlight the main scientific objectives that will be addressed. (Abridged), Comment: White Paper in support of a mission concept to be submitted for the 2023 NASA Astrophysics Probes opportunity. This White Paper will be updated when required. 30 pages, 25 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

The bright star $\lambda$ Ser hosts a hot Neptune with a minimum mass of 13.6 $M_\oplus$ and a 15.5 day orbit. It also appears to be a solar analog, with a mean rotation period of 25.8 days and surface differential rotation very similar to the Sun. We aim to characterize the fundamental properties of this system, and to constrain the evolutionary pathway that led to its present configuration. We detect solar-like oscillations in time series photometry from the Transiting Exoplanet Survey Satellite (TESS), and we derive precise asteroseismic properties from detailed modeling. We obtain new spectropolarimetric data, and we use them to reconstruct the large-scale magnetic field morphology. We reanalyze the complete time series of chromospheric activity measurements from the Mount Wilson Observatory, and we present new X-ray and ultraviolet observations from the Chandra and Hubble space telescopes. Finally, we use the updated observational constraints to assess the rotational history of the star and to estimate the wind braking torque. We conclude that the remaining uncertainty on stellar age currently prevents an unambiguous interpretation of the properties of $\lambda$ Ser, and that the rate of angular momentum loss appears to be higher than for other stars with similar Rossby number. Future asteroseismic observations may help to improve the precision of the stellar age., Comment: 19 pages including 9 figures and 6 tables. Astronomical Journal, accepted

We have devised a predominantly Naive Bayes method to classify the optical/IR matches to X-ray sources detected by Chandra in the Cygnus OB2 association into foreground, member, and background objects. We employ a variety of X-ray, optical, and infrared characteristics to construct likelihoods using training sets defined by well-measured sources. Combinations of optical photometry from SDSS (riz) and IPHAS (riHa), IR magnitudes from UKIDSS and 2MASS (JHK), X-ray quantiles and hardness ratios, and estimates of extinction Av are used to compute the relative probabilities that a given source belongs to one of the classes. We use Principal Component Analysis of photometric magnitude combinations to isolate the best axes for classification. We incorporate measurement errors into the classification. We evaluate the accuracy of the classification by inspection and reclassify a number of sources based on IR magnitudes, presence of disks, and X-ray spectral hardness. We also consider systematic errors due to extinction. We find that about 6100 objects are association members, 1400 are background, and 500 are foreground objects. The overall classification accuracy is 95%., Comment: 27 pages, 23 figures, 6 tables; accepted for publication in ApJS. Full Table 3 is in Zenodo at https://zenodo.org/record/8025756

Rapid X-ray phase-dependent flux enhancement in the archetype classical Cepheid star $\delta$~Cep was observed by XMM-Newton and Chandra. We jointly analyse thermal and non-thermal components of the time-resolved X-ray spectra prior to, during and after the enhancement. A comparison of the time scales of shock particle acceleration and energy losses is consistent with the scenario of a pulsation-driven shock wave traveling into the stellar corona and accelerating electrons to $\sim$ GeV energies and with Inverse Compton (IC) emission from the UV stellar background leading to the observed X-ray enhancement. The index of the non-thermal IC photon spectrum, assumed to be a simple power-law in the $[1-8]$ keV energy range, radially integrated within the shell $[3 - 10]$ stellar radii, is consistent with an enhanced X-ray spectrum powered by shock-accelerated electrons. An unlikely $\sim$100-fold amplification { via turbulent dynamo} of the magnetic field at the shock propagating through density inhomogeneities in the stellar corona is required for the synchrotron emission to dominate over the IC; the lack of time-correlation between radio synchrotron and stellar pulsation contributes to make synchrotron as an unlikely emission mechanism for the flux enhancement. Although current observations cannot rule out a high-flux two-temperature thermal spectrum with a negligible non-thermal component, this event might confirm for the first time the association of Cepheids pulsation with shock-accelerated GeV electrons., Comment: Accepted for publication in ApJ

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 1215.67A HI Lyman alpha emission line dominates the ultraviolet flux of low mass stars, including the majority of known exoplanet hosts. Unfortunately, strong attenuation by the interstellar medium (ISM) obscures the line core at most stars, requiring the intrinsic Lyman alpha flux to be reconstructed based on fits to the line wings. We present a test of the widely-used Lyman alpha emission line reconstruction code LYAPY using phase-resolved, medium-resolution STIS G140M observations of the close white dwarf-M dwarf binary EG UMa. The Doppler shifts induced by the binary orbital motion move the Lyman alpha emission line in and out of the region of strong ISM attenuation. Reconstructions to each spectrum should produce the same Lyman alpha profile regardless of phase, under the well-justified assumption that there is no intrinsic line variability between observations. Instead, we find that the reconstructions underestimate the Lyman alpha flux by almost a factor of two for the lowest-velocity, most attenuated spectrum, due to a degeneracy between the intrinsic Lyman alpha and ISM profiles. Our results imply that many stellar Lyman alpha fluxes derived from G140M spectra reported in the literature may be underestimated, with potential consequences for, for example, estimates of extreme-ultraviolet stellar spectra and ultraviolet inputs into simulations of exoplanet atmospheres., Comment: Accepted for publication in ApJ

X-ray observations have been made of a sample of 20 classical Cepheids, including two new observations (Polaris and {\it l} Car) reported here. The occurrence of X-ray flux around the pulsation cycle is discussed. Three Cepheids are detected ($\delta$ Cep, $\beta$ Dor, and Polaris). X-rays have also been detected from the low--mass F, G, and K companions of 4 Cepheids (V473 Lyr, R Cru, V659 Cen, and W Sgr), and one hot companion (S Mus). Upper limits on the X-ray flux of the remaining Cepheids provide an estimate that 28\% have low mass companions. This fraction of low--mass companions in intermediate mass Cepheids is significantly lower than expected from random pairing with the field IMF. Combining the companion fraction from X-rays with that from ultraviolet observations results in a binary/multiple fraction of 57\% $\pm$12\% for Cepheids with the ratios q $>$ 0.1 and separations a $>$ 1 au. This is a lower limit since M stars are not included. X-ray observations detect less massive companions than other existing studies of intermediate mass stars. Our measured occurrence rate of unresolved, low-mass companions to Cepheids suggests that intermediate-period binaries derive from a combination of disk and core fragmentation and accretion. This yields a hybrid mass-ratio distribution that is skewed toward small values compared to a uniform distribution but is still top-heavy compared to random pairings drawn from the IMF., Comment: Accepted by ApJ

X-ray observations of low-mass stars in open clusters are critical to understanding the dependence of magnetic activity on stellar properties and their evolution. Praesepe and the Hyades, two of the nearest, most-studied open clusters, are among the best available laboratories for examining the dependence of magnetic activity on rotation for stars with masses lower than $\approx 1\ M_{\odot}$. We present an updated study of the rotation X-ray activity relation in the two clusters. We updated membership catalogs that combine pre-Gaia catalogs with new catalogs based on Gaia Data Release 2. The resulting catalogs are the most inclusive ones for both clusters: 1739 Praesepe and 1315 Hyades stars. We collected X-ray detections for cluster members, for which we analyzed, re-analyzed, or collated data from ROSAT, the Chandra X-ray Observatory, the Neil Gehrels Swift Observatory, and XMM-Newton. We have detections for 326 Praesepe and 462 Hyades members, of which 273 and 164, respectively, have rotation periods, an increase of 6$\times$ relative to what was previously available. We find that at $\approx$700 Myr, only M dwarfs remain saturated in X-rays, with only tentative evidence for supersaturation. We also find a tight relation between the Rossby number and fractional X-ray luminosity $L_\mathrm{X}/L_\mathrm{bol}$ in unsaturated single members, suggesting a power-law index between $-3.2$ and $-3.9$. Lastly, we find no difference in the coronal parameters between binary and single members. These results provide essential insight into the relative efficiency of magnetic heating of the stars' atmospheres, thereby informing the development of robust age-rotation-activity relations., Comment: 37 pages, 14 figures and 2 figure sets. Accepted for publication in the AAS Journals

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"

Nova Her 2021 (V1674 Her), which erupted on 2021 June 12, reached naked-eye brightness and has been detected from radio to $\gamma$-rays. An extremely fast optical decline of 2 magnitudes in 1.2 days and strong Ne lines imply a high-mass white dwarf. The optical pre-outburst detection of a 501.42s oscillation suggests a magnetic white dwarf. This is the first time that an oscillation of this magnitude has been detected in a classical nova prior to outburst. We report X-ray outburst observations from {\it Swift} and {\it Chandra} which uniquely show: (1) a very strong modulation of super-soft X-rays at a different period from reported optical periods; (2) strong pulse profile variations and the possible presence of period variations of the order of 0.1-0.3s; and (3) rich grating spectra that vary with modulation phase and show P Cygni-type emission lines with two dominant blue-shifted absorption components at $\sim 3000$ and 9000 km s$^{-1}$ indicating expansion velocities up to 11000 km s$^{-1}$. X-ray oscillations most likely arise from inhomogeneous photospheric emission related to the magnetic field. Period differences between reported pre- and post-outburst optical observations, if not due to other period drift mechanisms, suggest a large ejected mass for such a fast nova, in the range $2\times 10^{-5}$-$2\times 10^{-4} M_\odot$. A difference between the period found in the {\it Chandra} data and a reported contemporaneous post-outburst optical period, as well as the presence of period drifts, could be due to weakly non-rigid photospheric rotation., Comment: 11 pages, 5 figures. Accepted for publication in Astrophysical Journal Letters

As main-sequence stars with C$>$O, dwarf carbon (dC) stars are never born alone but inherit carbon-enriched material from a former asymptotic giant branch (AGB) companion. In contrast to M dwarfs in post-mass transfer binaries, C$_2$ and/or CN molecular bands allow dCs to be identified with modest-resolution optical spectroscopy, even after the AGB remnant has cooled beyond detectability. Accretion of substantial material from the AGB stars should spin up the dCs, potentially causing a rejuvenation of activity detectable in X-rays. Indeed, a few dozen dCs have recently been found to have photometric variability with periods under a day. However, most of those are likely post-common-envelope binaries (PCEBs), spin-orbit locked by tidal forces, rather than solely spun-up by accretion. Here, we study the X-ray properties of a sample of the five nearest known dCs with $Chandra$. Two are detected in X-rays, the only two for which we also detected short-period photometric variability. We suggest that the coronal activity detected so far in dCs is attributable to rapid rotation due to tidal locking in short binary orbits after a common-envelope phase, late in the thermally pulsing (TP) phase of the former C-AGB primary (TP-AGB)., Comment: 18 pages, 9 figures, 4 tables, Accepted to ApJ, title changed based on referee recommendation

Young solar-type stars are known to be strong X-ray emitters and their X-ray spectra have been widely studied. X-rays from the central star may play a crucial role in the thermodynamics and chemistry of the circumstellar material as well as in the atmospheric evolution of young planets. In this paper we present model spectra based on spectral parameters derived from the observations of young stars in the Orion Nebula Cluster from the Chandra Orion Ultradeep Project (COUP). The spectra are then used to calculate new photoevaporation prescriptions that can be used in disc and planet population synthesis models. Our models clearly show that disc wind mass loss rates are controlled by the stellar luminosity in the soft (100 eV - 1 keV) X- ray band. New analytical relations are provided for the mass loss rates and profiles of photoevaporative winds as a function of the luminosity in the soft X-ray band. The agreement between observed and predicted transition disc statistics moderately improved using the new spectra, but the observed population of strongly accreting large cavity discs can still not be reproduced by these models. Furthermore, our models predict a population of non-accreting transition discs that are not observed. This highlights the importance of considering the depletion of millimeter-sized dust grains from the outer disc, which is a likely reason why such discs have not been detected yet., Comment: 12 pages, 7 tables, 9 figures, accepted for publication in MNRAS

We describe a process for cross-calibrating the effective areas of X-ray telescopes that observe common targets. The targets are not assumed to be "standard candles" in the classic sense, in that we assume that the source fluxes have well-defined, but {\it a priori} unknown values. Using a technique developed by Chen et al. (2019, arXiv:1711.09429) that involves a statistical method called {\em shrinkage estimation}, we determine effective area correction factors for each instrument that brings estimated fluxes into the best agreement, consistent with prior knowledge of their effective areas. We expand the technique to allow unique priors on systematic uncertainties in effective areas for each X-ray astronomy instrument and to allow correlations between effective areas in different energy bands. We demonstrate the method with several data sets from various X-ray telescopes., Comment: 37 pages, 9 figures, and 10 tables plus an appendix with 7 data tables. Accepted for publication in the Astronomical Journal

We present the discovery of a magnetic field on the white dwarf component in the detached post common envelope binary (PCEB) CC Cet. Magnetic white dwarfs in detached PCEBs are extremely rare, in contrast to the high incidence of magnetism in single white dwarfs and cataclysmic variables. We find Zeeman-split absorption lines in both ultraviolet Hubble Space Telescope (HST) spectra and archival optical spectra of CC Cet. Model fits to the lines return a mean magnetic field strength of approximately 600-700 kG. Differences in the best-fit magnetic field strength between two separate HST observations and the high v sin i of the lines indicate that the white dwarf is rotating with a period ~0.5 hours, and that the magnetic field is not axisymmetric about the spin axis. The magnetic field strength and rotation period are consistent with those observed among the intermediate polar class of cataclysmic variable, and we compute stellar evolution models that predict CC Cet will evolve into an intermediate polar in 7-17 Gyr. Among the small number of known PCEBs containing a confirmed magnetic white dwarf, CC Cet is the hottest (and thus youngest), with the weakest field strength, and cannot have formed via the recently proposed crystallisation/spin-up scenario. In addition to the magnetic field measurements, we update the atmospheric parameters of the CC Cet white dwarf via model spectra fits to the HST data and provide a refined orbital period and ephemeris from TESS photometry., Comment: Accepted for publication in MNRAS

During the first half of main-sequence lifetimes, the evolution of rotation and magnetic activity in solar-type stars appears to be strongly coupled. Recent observations suggest that rotation rates evolve much more slowly beyond middle-age, while stellar activity continues to decline. We aim to characterize this mid-life transition by combining archival stellar activity data from the Mount Wilson Observatory with asteroseismology from the Transiting Exoplanet Survey Satellite (TESS). For two stars on opposite sides of the transition (88 Leo and $\rho$ CrB), we independently assess the mean activity levels and rotation periods previously reported in the literature. For the less active star ($\rho$ CrB), we detect solar-like oscillations from TESS photometry, and we obtain precise stellar properties from asteroseismic modeling. We derive updated X-ray luminosities for both stars to estimate their mass-loss rates, and we use previously published constraints on magnetic morphology to model the evolutionary change in magnetic braking torque. We then attempt to match the observations with rotational evolution models, assuming either standard spin-down or weakened magnetic braking. We conclude that the asteroseismic age of $\rho$ CrB is consistent with the expected evolution of its mean activity level, and that weakened braking models can more readily explain its relatively fast rotation rate. Future spectropolarimetric observations across a range of spectral types promise to further characterize the shift in magnetic morphology that apparently drives this mid-life transition in solar-type stars., Comment: 11 pages of text including 6 figures and 2 tables. ApJ accepted

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

X-ray bursts have recently been discovered in the Cepheids $\delta$ Cep and $\beta$ Dor modulated by the pulsation cycle. We have obtained an observation of the Cepheid $\eta$ Aql with the XMM-Newton satellite at the phase of maximum radius, the phase at which there is a burst of X-rays in $\delta$ Cep. No X-rays were seen from the Cepheid $\eta$ Aql at this phase, and the implications for Cepheid upper atmospheres are discussed. We have also used the combination of X-ray sources and Gaia and 2MASS data to search for a possible grouping around the young intermediate mass Cepheid. No indication of such a group was found., Comment: accepted by AJ

High resolution UV spectra of stellar H I Lyman-alpha lines from the Hubble Space Telescope (HST) provide observational constraints on the winds of coronal main sequence stars, thanks to an astrospheric absorption signature created by the interaction between the stellar winds and the interstellar medium. We report the results of a new HST survey of M dwarf stars, yielding six new detections of astrospheric absorption. We estimate mass-loss rates for these detections, and upper limits for nondetections. These new constraints allow us to characterize the nature of M dwarf winds and their dependence on coronal activity for the first time. For a clear majority of the M dwarfs, we find winds that are weaker or comparable in strength to that of the Sun, i.e. Mdot<=1 Mdot_sun. However, two of the M dwarfs have much stronger winds: YZ CMi (M4 Ve; Mdot=30 Mdot_sun) and GJ 15AB (M2 V+M3.5 V; Mdot=10 Mdot_sun). Even these winds are much weaker than expectations if the solar relation between flare energy and coronal mass ejection (CME) mass extended to M dwarfs. Thus, the solar flare/CME relation does not appear to apply to M dwarfs, with important ramifications for the habitability of exoplanets around M dwarfs. There is evidence for some increase in Mdot with coronal activity as quantified by X-ray flux, but with much scatter. One or more other factors must be involved in determining wind strength besides spectral type and coronal activity, with magnetic topology being one clear possibility., Comment: 40 pages, 11 figures, to appear in The Astrophysical Journal

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

Cepheids are pulsating variable stars with a periodic chromospheric response at UV wavelengths close to their minimum radius phase. Recently, an X-ray variable signature was captured in observations during the \emph{maximum} radius phase. This X-ray emission came as a surprise and is not understood. In this work, we use the modern astrophysical code, PLUTO, to investigate the effects of pulsations on Cepheid X-ray emission. We run a number of hydrodynamic numerical simulations with a variety of initial and boundary conditions in order to explore the capability of shocks to produce the observed phase-dependent X-ray behavior. Finally we use the Simulated Observations of X-ray Sources (SOXS) package to create synthetic spectra for each simulation case and link our simulations to observables. We show that, for certain conditions, we can reproduce observed X-ray fluxes at phases 0.4--0.8 when the Cepheid is at maximum radius. Our results span a wide range of mass-loss rates, $2\times10^{-13}$--$3\times10^{-8}$ $M_\odot$ yr$^{-1}$, and peak X-ray luminosities, $5\times10^{-17}$--$1.4\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$. We conclude that Cepheids exhibit \textit{two component emission} with (a) shock waves being responsible for the phase dependent variable emission (phases 0.2 - 0.6), and (b) a separate quiescent mechanism being the dominant emission mechanism for the remaining phases.

The recurrent nova (RN) V3890 Sgr was observed during the 7th day after the onset of its most recent outburst, with the Chandra ACIS-S camera and High Energy Transmission Gratings (HETG). A rich emission line spectrum was detected, due to transitions of Fe-L and K-shell ions ranging from neon to iron. The measured absorbed flux is $\approx 10^{-10}$ erg cm$^{-2}$ s$^{-1}$ in the 1.4-15 Angstrom range (0.77-8.86 keV). The line profiles are asymmetric, blue-shifted and skewed towards the blue side, as if the ejecta moving towards us are less absorbed than the receding ones. The full width at half maximum of most emission lines is 1000-1200 km s$^{-1}$, with some extended blue wings. The spectrum is thermal and consistent with a plasma in collisional ionization equilibrium with column density 1.3 $\times 10^{22}$ cm$^{-2}$ and at least two components at temperatures of about 1 keV and 4 keV, possibly a forward and a reverse shock, or regions with differently mixed ejecta and red giant wind. The spectrum is remarkably similar to the symbiotic RNe V745 Sco and RS Oph, but we cannot distinguish whether the shocks occurred at a distance of few AU from the red giant, or near the giant's photosphere, in a high density medium containing only a small mass. The ratios of the flux in lines of aluminum, magnesium and neon relative to the flux in lines of silicon and iron probably indicate a carbon-oxygen white dwarf (CO WD)., Comment: In press on the ApJ

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.

Cepheid masses derived from pulsations or binary dynamics are generally lower than those derived from stellar evolution models. Recent efforts have been dedicated to investigating the effects of abundances, mass loss, rotation, convection and overshooting prescriptions for modifying the evolution tracks to reduce or remove this Cepheid mass discrepancy. While these approaches are promising, either alone or in combination, more work is required to distinguish between possible solutions. Here we investigate nuclear reaction rate and opacity modifications on Cepheid evolution using the MESA code. We discuss the effects of opacity increases at envelope temperatures of 200,000-400,000 K proposed to explain the pulsation properties of hybrid main-sequence beta Cep/Slowly Pulsating B (SPB) variables which will evolve into Cepheids. We make use of the RSP nonlinear radial pulsation modeling capability in MESA to calculate periods and radial velocity amplitudes of Galactic Cepheids V1334 Cyg, Polaris, and delta Cep., Comment: Revised version, 6 pages, 3 figures, enlarged Figure 2 and revised captions, submitted for Astronomical Society of the Pacific Conference Series "RRLyrae/Cepheid2019: Frontiers of Classical Pulsators: Theory and Observations", Cloudcroft, NM, USA October 13-18, 2019

V473 Lyr is a classical Cepheid which is unique in having substantial amplitude variations with a period of approximately 3.3 years, thought to be similar to the Blazhko variations in RR Lyrae stars. We obtained an {\it XMM-Newton} observation of this star to followup a previous detection in X-rays. Rather than the X-ray burst and rapid decline near maximum radius seen in $\delta$ Cephei itself, the X-ray flux in V473 Lyr remained constant for a third of the pulsation cycle covered by the observation. Thus the X-rays are most probably not produced by the changes around the pulsation cycle. The X-ray spectrum is soft (kT = 0.6 keV), with X-ray properties which are consistent with a young low mass companion. Previously there was no evidence of a companion in radial velocities or in {\it Gaia} and {\it Hipparcos} proper motions. While this rules out companions which are very close or very distant, a binary companion at a separation between 30 and 300 AU is possible. This is an example of an X-ray observation revealing evidence of a low mass companion, which is important in completing the mass ratio statistics of binary Cepheids. Furthermore, the detection of a young X-ray bright companion is a further indication that the Cepheid (primary) is a Population I star, even though its pulsation behavior differs from other classical Cepheids., Comment: Accepted by AJ

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

The Chandra X-ray Observatory has completed a remarkable twenty years in orbit. A large part of the science program of Chandra during this time has involved the study of stars and their planetary systems. This primer aims to give the reader a taste of the enormous range of stellar and planetary astrophysics that Chandra has enabled. Beginning with a tour of the X-ray solar system zoo, including the stunning pulsating X-ray aurorae of Jupiter, we then move on to the hot million-degree outer atmospheres of stars like our own Sun, whose X-ray emission is driven by an internal magnetic dynamo. The same emission processes are also vigorously present in the youngest stars, and we highlight some Chandra observations and results on nascent stellar and planetary systems. Chandra surveys and high-resolution spectroscopy of massive stars have provided a new window on the means by which they scavenge X-ray emission from their radiatively-driven winds, sometimes modulating this output by strong underlying stellar magnetic fields. We touch upon the evanescent X-radiation from intermediate-mass stars before arriving at the inevitable evolutionary endpoints of all but massive stars, first in energized X-ray emitting planetary nebulae, then in the slowly cooling, soft-X-ray emitting photospheres of white dwarfs. We conclude with white dwarfs in close binary systems, rejuvenated by interaction with a companion and where accretion gives play to a new range of energetic behavior even more spectacular and cataclysmic than the coruscant astrophysical road down which they have travelled., Comment: A primer on Chandra contributions to X-ray studies of stars and planetary systems. 96 pages

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

Carbon stars (with C/O> 1) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. The case is nearly iron-clad now that the formerly mysterious dwarf carbon (dC) stars are actually far more common than C giants, and have accreted carbon-rich material from a former AGB companion, yielding a white dwarf and a dC star that has gained both significant mass and angular momentum. Some such dC systems have undergone a planetary nebula phase, and some may evolve to become CH, CEMP, or Ba giants. Recent studies indicate that most dCs are likely from older, metal-poor kinematic populations. Given the well-known anti-correlation of age and activity, dCs would not be expected to show significant X-ray emission related to coronal activity. However, accretion spin-up might be expected to rejuvenate magnetic dynamos in these post mass-transfer binary systems. We describe our Chandra pilot study of six dCs selected from the SDSS for Halpha emission and/or a hot white dwarf companion, to test whether their X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo. We detect all 6 dCs in the sample, which have X-ray luminosities ranging from logLx= 28.5 - 29.7, preliminary evidence that dCs may be active at a level consistent with stars that have short rotation periods of several days or less. More definitive results require a sample of typical dCs with deeper X-ray observations to better constrain their plasma temperatures., Comment: 13 pages, 5 figures. Revised and resubmitted June 20, accepted June 21, 2019 to ApJ

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

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

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

X-ray observatories contribute fundamental advances in Solar System studies by probing Sun-object interactions, developing planet and satellite surface composition maps, probing global magnetospheric dynamics, and tracking astrochemical reactions. Despite these crucial results, the technological limitations of current X-ray instruments hinder the overall scope and impact for broader scientific application of X-ray observations both now and in the coming decade. Implementation of modern advances in X-ray optics will provide improvements in effective area, spatial resolution, and spectral resolution for future instruments. These improvements will usher in a truly transformative era of Solar System science through the study of X-ray emission., Comment: White paper submitted to Astro2020, the Astronomy and Astrophysics Decadal Survey

All cool stars show magnetic activity, and X-ray emission is the hallmark of this activity. Gaining an understanding of activity aids us in answering fundamental questions about stellar astrophysics and in determining the impact of activity on the exoplanets that orbit these stars. Stellar activity is driven by magnetic fields, which are ultimately powered by convection and stellar rotation. However, the resulting dynamo properties heavily depend on the stellar interior structure and are far from being understood. X-ray radiation can evaporate exoplanet atmospheres and damage organic materials on the planetary surface, reducing the probability that life can form or be sustained, but also provides an important source of energy for prebiotic chemical reactions. Over the next two years, the TESS mission will deliver a catalog of the closest exoplanets, along with rotation periods and activity diagnostics for millions of stars, whether or not they have a planet. We propose to include all cool stars that are TESS targets and bright enough for Chandra observations, as determined by their detection and flux in the ROSAT all-sky survey (RASS), to the list of Chandra Cool Attitude Targets (CATs). For each target, the signal will be sufficient to fit the coronal plasma with at least two temperature components, and compare abundances of groups of elements with low, medium, or high first ionization potential. Similar to the known relation between X-ray luminosity $L_X$ and rotation period, we can correlate stellar properties with coronal temperatures and abundances to constrain models for stellar activity, coronal heating, and stellar dynamos. Detailed X-ray characterization for even a subset of planet-hosting systems would dramatically advance our knowledge of what impact these emissions have on orbiting planets., Comment: White paper for Chandra cool attitude targets (4 pages)