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The Kepler mission revealed a plethora of stellar variability in the light curves of many stars, some associated with magnetic activity or stellar oscillations. In this work, we analyse the periodic signal in 162 intermediate-mass stars, interpreted as Rossby modes and rotational modulation - the so-called \textit{hump \& spike} feature. We investigate whether the rotational modulation (\textit{spike}) is due to stellar spots caused by magnetic fields or due to Overstable Convective (OsC) modes resonantly exciting g~modes, with frequencies corresponding to the convective core rotation rate. Assuming that the spikes are created by magnetic spots at the stellar surface, we recover the amplitudes of the magnetic fields, which are in good agreement with theoretical predictions. Our data show a clear anti-correlation between the spike amplitudes and stellar mass and possibly a correlation with stellar age, consistent with the dynamo-generated magnetic fields theory in (sub)-surface convective layers. Investigating the harmonic behaviour, we find that for 125 stars neither of the two possible explanations can be excluded. While our results suggest that the dynamo-generated magnetic field scenario is more likely to explain the \textit{spike} feature, we assess further work is needed to distinguish between the two scenarios. One method for ruling out one of the two explanations is to directly observe magnetic fields in \textit{hump \& spike} stars. Another would be to impose additional constraints through detailed modelling of our stars, regarding the rotation requirement in the OsC mode scenario or the presence of a convective-core (stellar age)., 18 pages, 28 figures

We're in a crisis We are in the midst of an unprecedented global crisis. Just weeks since its outbreak, the Coronavirus pandemic (COVID-19) has already affected, and will continue to affect, our daily lives, around the globe, for the foreseeable future. The answers and the solutions to this crisis will come from science. But the crisis affects science, too.It affects students, educators, and researchers; not just their day-to-day lives, social ties, and work routines, but also their ability to actively collaborate, convene in face-to-face meetings, attend academic conferences, teach and learn in an open university setting, pay a visit to the library, work overnight at the laboratory, and so on.But the thing is: science cannot stop. Scientific progress must go on. For each one of the challenges that scientists face in this time of crisis, there is, or there will be, a solution. We believe that the solution is not to be found in a single technological tool, product, framework, institution, funding agency, or company. It is the global cyber-infrastructure of scientific collaboration, built on scientific rigor, intellectual curiosity, and cooperation, that will enable science to advance in such difficult times. The power of scientific collaborationAs scientists, publishers, science communicators and technologists, we believe that: a. Science is the solution to the ongoing crisis. Now more than ever, reliance on the scientific method, rigor and clarity of scientific communication, transparency, reproducibility, and seamless sharing of all research data (including negative results), are fundamental to solving this health crisis and advancing human progress.b. Global collaboration and cooperation, beyond and above national and economic interests, is necessary not only at the scientific level, but also at the political and societal level. We're more interconnected and interdependent today than ever. And such interconnectedness extends to the ecological ecosystem in which we live. A crisis of such scale requires global solidarity, bipartisan political action, civic participation, and long-term thinking.

Subsurface convection zones are ubiquitous in early-type stars. Driven by narrow opacity peaks, these thin convective regions transport little heat but play an important role in setting the magnetic properties and surface variability of stars. Here we demonstrate that these convection zones are \emph{not} present in as wide a range of stars as previously believed. In particular, there are regions which 1D stellar evolution models report to be convectively unstable but which fall below the critical Rayleigh number for onset of convection. For sub-solar metallicity this opens up a \emph{stability window} in which there are no subsurface convection zones. For LMC metallicity this surface stability region extends roughly between $8M_\odot$ and $16M_\odot$, increasing to $8M_\odot$ -- $35M_\odot$ for SMC metallicity. Such windows are then an excellent target for probing the relative influence of subsurface convection and other sources of photometric variability in massive stars., 9 pages, 4 figures. Accepted in ApJ

Stars are born with magnetic fields, but the distribution of their initial field strengths remains uncertain. We combine observations with theoretical models of magnetic field evolution to infer the initial distribution of magnetic fields for AB stars in the mass range of 1.6 - 3.4 M$_{\odot}$. We tested a variety of distributions with different shapes and found that a distribution with a mean of $\sim$800 G and a full width of $\sim$600 G is most consistent with the observed fraction of strongly magnetized stars as a function of mass. Our most-favored distribution is a Gaussian with a mean of $\mu$ = 770 G and standard deviation of $\sigma$ = 146 G. Independent approaches to measure the typical field strength suggest values closer to 2 - 3 kG, a discrepancy that could suggest a mass-dependent and bimodal initial field distribution, or an alternative theoretical picture for the origin of these magnetic fields., Comment: Published in ApJ

Explaining the currently observed magnetic fields in galaxies requires relatively strong seeding in the early Universe. One theory proposes that magnetic fields of the order of $μ$G were expelled by supernova (SN) explosions after primordial, nG or weaker fields were amplified in stellar interiors. In this work, we calculate the maximum magnetic energy that can be injected in the interstellar medium by a stellar cluster of mass $M_{cl}$ based on what is currently known about stellar magnetism. We consider early-type stars and adopt either a Salpeter or a top-heavy IMF. For their magnetic fields, we adopt either a Gaussian or a bimodal distribution. The Gaussian model assumes that all massive stars are magnetized with $10^3 < \langle B_* \rangle < 10^4$ G, while the bimodal, consistent with observations of Milky Way stars, assumes only 5-10 per cent of OB stars have $10^3 < \langle B_* \rangle < 10^4$ G, while the rest have $10 < \langle B_* \rangle < 10^2$ G. We find that the maximum magnetic energy that can be injected by a stellar population is between $10^{-10}-10^{-7}$ times the total SN energy. The highest end of these estimates is about five orders of magnitude lower than what is usually employed in cosmological simulations, where about $10^{-2}$ of the SN energy is injected as magnetic. Pure advection of the stellar magnetic field by SN explosions is a good candidate for seeding a dynamo, but not enough to magnetize galaxies. Assuming SNe as main mechanism for galactic magnetization, the magnetic field cannot exceed an intensity of $10^{-7}$ G in the best-case scenario for a population of $10^{5}$ solar masses in a superbubble of 300 pc radius, while more typical values are between $10^{-10}-10^{-9}$~G. Therefore, other scenarios for galactic magnetization at high redshift need to be explored., Accepted for publication in A&A letters

Convection is ubiquitous in stars and occurs under many different conditions. Here we explore convection in main-sequence stars through two lenses: dimensionless parameters arising from stellar structure and parameters which emerge from the application of mixing length theory. We first define each quantity in terms familiar both to the 1D stellar evolution community and the hydrodynamics community. We then explore the variation of these quantities across different convection zones, different masses, and different stages of main-sequence evolution. We find immense diversity across stellar convection zones. Convection occurs in thin shells, deep envelopes, and nearly-spherical cores; it can be efficient of inefficient, rotationally constrained or not, transsonic or deeply subsonic. This atlas serves as a guide for future theoretical and observational investigations by indicating which regimes of convection are active in a given star, and by describing appropriate model assumptions for numerical simulations., Comment: 62 pages, 76 figures. All your favorite dimensionless numbers and then some. Submitted to ApJS

Early-type stars show a bimodal distribution of magnetic field strengths, with some showing very strong fields ($\gtrsim 1\,\mathrm{kG}$) and others very weak fields ($\lesssim 10\,\mathrm{G}$). Recently, we proposed that this reflects the processing or lackthereof of fossil fields by subsurface convection zones. Stars with weak fossil fields process these at the surface into even weaker dynamo-generated fields, while in stars with stronger fossil fields magnetism inhibits convection, allowing the fossil field to remain as-is. We now expand on this theory and explore the time-scales involved in the evolution of near-surface magnetic fields. We find that mass loss strips near-surface regions faster than magnetic fields can diffuse through them. As a result, observations of surface magnetism directly probe the frozen-in remains of the convective dynamo. This explains the slow evolution of magnetism in stars with very weak fields: these dynamo-\emph{generated} magnetic fields evolve on the time-scale of the mass loss, not that of the dynamo., 12 pages, 7 figures, accepted in ApJ

Understanding common envelope (CE) evolution is an outstanding problem in binary evolution. Although the CE phase is not driven by gravitational-wave (GW) emission, the in-spiraling binary emits GWs that passively trace the CE dynamics. Detecting this GW signal would provide direct insight into the gas-driven physics. Even a non-detection might offer invaluable constraints. We investigate the prospects of detection of a Galactic CE by LISA. While the dynamical phase of the CE is likely sufficiently loud for detection, it is short and thus rare. We focus instead on the self-regulated phase that proceeds on a thermal timescale. Based on population synthesis calculations and the (unknown) signal duration in the LISA band, we expect $\sim 0.1-100$ sources in the Galaxy during the mission duration. We map the GW observable parameter space of frequency $f_\mathrm{GW}$ and its derivative $\dot f_\mathrm{GW}$ remaining agnostic on the specifics of the inspiral, and find that signals with $\mathrm{SNR}>10$ are possible if the CE stalls at separations such that $f_\mathrm{GW}\gtrsim2\times10^{-3}\,\mathrm{Hz}$. We investigate the possibility of misidentifying the signal with other known sources. If the second derivative $\ddot f_\mathrm{GW}$ can also be measured, the signal can be distinguished from other sources using a GW braking-index. Alternatively, coupling LISA with electromagnetic observations of peculiar red giant stars and/or infrared and optical transients might allow for the disentangling of a Galactic CE from other Galactic and extra-galactic GW sources., Accepted by ApJ -- author's version. Code available at: https://github.com/tcallister/LISA-and-CE-Evolution

Recent photometric observations of massive stars show ubiquitous low-frequency "red-noise" variability, which has been interpreted as internal gravity waves (IGWs). Simulations of IGWs generated by convection show smooth surface wave spectra, qualitatively matching the observed red-noise. On the other hand, theoretical calculations by Shiode et al (2013) and Lecoanet et al (2019) predict IGWs should manifest at the surface as regularly-spaced peaks associated with standing g-modes. In this work, we compare these theoretical approaches to simplified 2D numerical simulations. The simulations show g-mode peaks at their surface, and are in good agreement with Lecoanet et al (2019). The amplitude estimates of Shiode et al (2013) did not take into account the finite width of the g-mode peaks; after correcting for this finite width, we find good agreement with simulations. However, simulations need to be run for hundreds of convection turnover times for the peaks to become visible; this is a long time to run a simulation, but a short time in the life of a star. The final spectrum can be predicted by calculating the wave energy flux spectrum in much shorter simulations, and then either applying the theory of Shiode et al (2013) or Lecoanet et al (2019)., submitted to MNRAS

The gravitational redshift induced by stellar surface gravity is notoriously difficult to measure for non-degenerate stars, since its amplitude is small in comparison with the typical Doppler shift induced by stellar radial velocity. In this study, we make use of the large observational data set of the Gaia mission to achieve a significant reduction of noise caused by these random stellar motions. By measuring the differences in velocities between the components of pairs of co-moving stars and wide binaries, we are able to statistically measure the combined effects of gravitational redshift and convective blueshifting of spectral lines, and nullify the effect of the peculiar motions of the stars. For the subset of stars considered in this study, we find a positive correlation between the observed differences in Gaia radial velocities and the differences in surface gravity and convective blueshift inferred from effective temperature and luminosity measurements. The results rule out a null signal at the $5\sigma$ level for our full data-set. Additionally, we study the sub-dominant effects of binary motion, and possible systematic errors in radial velocity measurements within Gaia. Results from the technique presented in this study are expected to improve significantly with data from the next Gaia data release. Such improvements could be used to constrain the mass-luminosity relation and stellar models which predict the magnitude of convective blueshift., Comment: 7 pages, 4 figures MNRAS accepted version

High-precision photometric observations have revealed ubiquitous stochastic low-frequency photometric variability in early type stars. It has been suggested that this variability arises due to either subsurface convection or internal gravity waves launched by the convective core. Here we show that relevant properties of convection in subsurface convective layers correlate very well with the timescale and amplitude of stochastic low-frequency photometric variability, as well as with the amplitude of macroturbulence. We suggest that low-frequency, stochastic photometric variability and surface turbulence in massive stars are caused by the the presence of subsurface convection. We show that an explanation for the observed surface photometric variability and macroturbulence relying on convective core driven internal gravity waves encounters a number of difficulties and seems unlikely to be able to explain the observed trends., Comment: 17 pages, 17 figures, accepted for publication in ApJ

Stars can either be formed in or captured by the accretion disks in Active Galactic Nuclei (AGN). These AGN stars are irradiated and subject to extreme levels of accretion, which can turn even low-mass stars into very massive ones ($M > 100 {\rm M}_\odot$) whose evolution may result in the formation of massive compact objects ($M > 10 {\rm M}_\odot$). Here we explore the spins of these AGN stars and the remnants they leave behind. We find that AGN stars rapidly spin up via accretion, eventually reaching near-critical rotation rates. They further maintain near-critical rotation even as they shed their envelopes, become compact, and undergo late stages of burning. This makes them good candidates to produce high-spin massive black holes, such as the ones seen by LIGO-Virgo in GW190521g, as well as long Gamma Ray Bursts (GRBs) and the associated chemical pollution of the AGN disk., 19 pages, 7 figures. Submitted to ApJ, comments welcome!

Light curves and periodograms of 160 B stars observed by the TESS space mission and 29 main-sequence B stars from Kepler and K2 were used to classify the variability type. There are 114 main-sequence B stars in the TESS sample, of which 45 are classified as possible rotational variables. This confirms previous findings that a large fraction (about 40 percent) of A and B stars may exhibit rotational modulation. Gaia DR2 parallaxes were used to estimate luminosities, from which the radii and equatorial rotational velocities can be deduced. It is shown that observed values of the projected rotational velocities are lower than the estimated equatorial velocities for nearly all the stars, as they should be if rotation is the cause of the light variation. We conclude that a large fraction of main-sequence B stars appear to contain surface features which cannot likely be attributed to abundance patches., 13 pages, 4 figures, 3 tables

Stars are likely embedded in the gas disks of Active Galactic Nuclei (AGN). Theoretical models predict that in the inner regions of the disk these stars accrete rapidly, with fresh gas replenishing hydrogen in their cores faster than it is burned into helium, effectively stalling their evolution at hydrogen burning. We produce order-of-magnitude estimates of the number of such stars in a fiducial AGN disk. We find numbers of order $10^{2-4}$, confined to the inner $r_{\rm cap} \sim 3000 r_s \sim 0.03\rm pc$. These stars can profoundly alter the chemistry of AGN disks, enriching them in helium and depleting them in hydrogen, both by order-unity amounts. We further consider mergers between these stars and other disk objects, suggesting that star-star mergers result in rapid mass loss from the remnant to restore an equilibrium mass, while star-compact object mergers may result in exotic outcomes and even host binary black hole mergers within themselves. Finally, we examine how these stars react as the disk dissipates towards the end of its life, and find that they may return mass to the disk fast enough to extend its lifetime by a factor of several and/or may drive powerful outflows from the disk. Post-AGN, these stars rapidly lose mass and form a population of stellar mass black holes around $10M_{\odot}$. Due to the complex and uncertain interactions between embedded stars and the disk, their plausible ubiquity, and their order unity impact on disk structure and evolution, they must be included in realistic disk models., 12 pages, 2 figures. Accepted in ApJ

Convection is efficient when advection matters much more than thermal diffusion. Despite this conceptually simple definition, there are several different measures of convective efficiency which are not quite equivalent. Here we recall the definitions of these measures and examine how they are related in different limits.

Planetary engulfment events have long been proposed as a lithium (Li) enrichment mechanism contributing to the population of Li-rich giants (A(Li) >= 1.5 dex). Using MESA stellar models and A(Li) abundance measurements obtained by the GALAH survey, we calculate the strength and observability of the surface Li enrichment signature produced by the engulfment of a hot Jupiter (HJ). We consider solar-metallicity stars in the mass range of 1-2 $\mathrm{M_\odot}$ and the Li supplied by a HJ of 1.0 $\mathrm{M_J}$. We explore engulfment events that occur near the main sequence turn-off (MSTO) and out to orbital separations of 0.1 AU. We map our results onto the Hertzsprung-Russell Diagram, revealing the statistical significance and survival time of lithium enrichment. We identify the parameter space of masses and evolutionary phases where the engulfment of a HJ can lead to Li enrichment signatures at a $5\sigma$ confidence level and with meteoritic abundance strengths. The most compelling strengths and survival times of engulfment-derived Li enrichment are found among host stars of 1.4 $\mathrm{M_\odot}$ near the MSTO. Our calculations indicate that planetary engulfment is not a viable enrichment pathway for stars that have evolved beyond the subgiant branch. For these sources, observed Li enhancements are likely to be produced by other mechanisms, such as the Cameron-Fowler process or the accretion of material from an asymptotic giant branch (AGB) companion. Our results do not account for second-order effects, such as extra mixing processes, which can further dilute Li enrichment signatures., Comment: 18 pages, 8 figures

The recent detection of GW190521 stimulated ideas on how to populate the predicted black hole pair-instability mass gap. One proposed scenario is the dynamical merger of two stars below the pair instability regime forming a star with a small core and an over-sized envelope. We explore this scenario with detailed stellar evolution calculations, starting with ad-hoc initial conditions enforcing no core growth during the merger. We outline the main challenges this scenario has to overcome, in particular the requirement to retain enough of its mass at merger time, in the subsequent evolution, and at core-collapse. We found that these massive merger products are likely helium-rich, and spend most of their remaining lifetime within regions of the Herzsprung-Russell diagram where envelope instabilities akin to luminous blue variable (LBV) eruptions are expected. An energetic estimate of the amount of mass loss neglecting the back-reaction of the star suggests that the total amount of mass that can be removed at low metallicity is . 1 M . This is small enough that at core-collapse our models are retaining sufficient mass to form black holes in the pair-instability gap similar to the recent ones detected by LIGO/Virgo. However, mass loss at the time of merger and the neutrino-driven mass loss at core collapse still need to be quantified for these models in order to confirm the viability of this scenario., Comment: accepted version to appear in ApJL

Active Galactic Nuclei are powered by geometrically-thin accretion disks surrounding a central supermassive black hole. Here we explore the evolution of stars embedded in these extreme astrophysical environments (AGN stars). Because AGN disks are much hotter and denser than the interstellar medium, AGN stars are subject to very different boundary conditions than normal stars. They are also strongly affected by both mass accretion, which can runaway given the vast mass of the disk, and mass loss due to super-Eddington winds. Moreover, chemical mixing plays a critical role in the evolution of these stars by allowing fresh hydrogen accreted from the disk to mix into their cores. We find that, depending on the local AGN density and sound speed and the duration of the AGN phase, AGN stars can rapidly become very massive (M > 100 M$_\odot$). These stars undergo core-collapse, leave behind compact remnants and contribute to polluting the disk with heavy elements. We show that the evolution of AGN stars can have a profound impact on the evolution of AGN metallicities, as well as the production of gravitational waves sources observed by LIGO-Virgo. We point to our galactic center as a region well-suited to test some of our predictions of this exotic stellar evolutionary channel., 26 pages, 16 figures. Accepted by ApJ

In this paper we introduce the MOBSTER collaboration and lay out its scientific goals. We present first results based on the analysis of nineteen previously known magnetic O, B and A stars observed in 2-minute cadence in sectors 1 and 2 of the Transiting Exoplanet Survey Satellite (TESS) mission. We derive precise rotational periods from the newly obtained light curves and compare them to previously published values. We also discuss the overall photometric phenomenology of the known magnetic massive and intermediate-mass stars and propose an observational strategy to augment this population by taking advantage of the high-quality observations produced by TESS., Comment: 15 pages, 3 figures, accepted for publication by MNRAS (2019 April 22)

We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). RSP is a new functionality in MESAstar that models the non-linear radial stellar pulsations that characterize RR Lyrae, Cepheids, and other classes of variable stars. We significantly enhance numerical energy conservation capabilities, including during mass changes. For example, this enables calculations through the He flash that conserve energy to better than 0.001 %. To improve the modeling of rotating stars in MESA, we introduce a new approach to modifying the pressure and temperature equations of stellar structure, and a formulation of the projection effects of gravity darkening. A new scheme for tracking convective boundaries yields reliable values of the convective-core mass, and allows the natural emergence of adiabatic semiconvection regions during both core hydrogen- and helium-burning phases. We quantify the parallel performance of MESA on current generation multicore architectures and demonstrate improvements in the computational efficiency of radiative levitation. We report updates to the equation of state and nuclear reaction physics modules. We briefly discuss the current treatment of fallback in core-collapse supernova models and the thermodynamic evolution of supernova explosions. We close by discussing the new MESA Testhub software infrastructure to enhance source-code development., Comment: 57 pages, 57 figures; Accepted to ApJS

Disks of gas accreting onto supermassive black holes are thought to power active galactic nuclei (AGN). Stars may form in gravitationally unstable regions of these disks, or may be captured from nuclear star clusters. Because of the dense gas environment, the evolution of such embedded stars can diverge dramatically from those in the interstellar medium. This work extends previous studies of stellar evolution in AGN disks by exploring a variety of ways that accretion onto stars in AGN disks may differ from Bondi accretion. We find that tidal effects from the supermassive black hole significantly alter the evolution of stars in AGN disks, and that our results do not depend critically on assumptions about radiative feedback on the accretion stream. Thus, in addition to depending on $\rho/c_s^3$, the fate of stars in AGN disks depends sensitively on the distance to and mass of the supermassive black hole. This affects where in the disk stellar explosions occur, where compact remnants form and potentially merge to produce gravitational waves, and where different types of chemical enrichment take place., Comment: 17 pages, 10 figures, accepted for publication in ApJ

Uncertainties in stellar structure and evolution theory are largest for stars undergoing core convection on the main sequence. A powerful way to calibrate the free parameters used in the theory of stellar interiors is asteroseismology, which provides direct measurements of angular momentum and element transport. We report the detection and classification of new variable O and B stars using high-precision short-cadence (2-min) photometric observations assembled by the Transiting Exoplanet Survey Satellite (TESS). In our sample of 154 O and B stars, we detect a high percentage (90%) of variability. Among these we find 23 multiperiodic pulsators, 6 eclipsing binaries, 21 rotational variables, and 25 stars with stochastic low-frequency variability. Several additional variables overlap between these categories. Our study of O and B stars not only demonstrates the high data quality achieved by TESS for optimal studies of the variability of the most massive stars in the Universe, but also represents the first step towards the selection and composition of a large sample of O and B pulsators with high potential for joint asteroseismic and spectroscopic modeling of their interior structure with unprecedented precision., 16 pages, 4 figures, 1 Table, Accepted for publication in ApJL

(Abridged) Stars more massive than $20-30M_{\odot}$ are so luminous that the radiation force on the cooler, more opaque outer layers can balance or exceed the force of gravity. These near or super-Eddington outer envelopes represent a long standing challenge for calculating the evolution of massive stars in one dimension, a situation that limits our understanding of the stellar progenitors of some of the most exciting and energetic explosions in the universe. In particular, the proximity to the Eddington limit has been the suspected cause for the variability, large mass loss rate and giant eruptions of an enigmatic class of massive stars: the luminous blue variables (LBVs). When in quiescence, LBVs are usually found on the hot ($T_{eff} \approx 2 - 4 \times 10^4$ K) S Dor instability strip. While in outburst, most LBVs stay on the cold S Dor instability strip with a $T_{eff} \approx 9000$ K. Here we show that physically realistic three dimensional global radiation hydrodynamic simulations of radiation dominated massive stars with the largest supercomputers in the world naturally reproduce many observed properties of LBVs, specifically their location in the Hertzsprung-Russell (HR) diagram and their episodic mass loss with rates of $10^{-7}-10^{-5} M_{\odot}/yr$. The helium opacity peak is found to play an important role to determine these properties, which is not realized in the traditional one dimensional models of massive stars. The simulations also predict that convection causes irregular envelope oscillations and 10-30% brightness variations on a typical timescale of a few days. The variability is more prominent in our models that are on the cool part of the S Dor instability. These calculations pave the way to a quantitative understanding of the structure, stability and the dominant mode of mass loss of massive stars.

It has been demonstrated that the time variability of a star's brightness at different frequencies can be used to infer its surface gravity, radius, mass, and age. With large samples of light curves now available from Kepler and K2, and upcoming surveys like TESS, we wish to quantify the overall information content of this data and identify where the information resides. As a first look into this question we ask which stellar parameters we can predict from the brightness variations in red-giant stars data and to what precision, using a data-driven model. We demonstrate that the long-cadence (30-minute) Kepler light curves for 2000 red-giant stars can be used to predict their $T_{\rm eff}$ and $\log g$. Our inference makes use of a data-driven model of a part of the autocorrelation function (ACF) of the light curve, where we posit a polynomial relationship between stellar parameters and the ACF pixel values. We find that this model, trained using 1000 stars, can be used to recover the temperature $T_{\rm eff}$ to $, Comment: 10 pages, 7 Figures, submitted to ApJ

We present the results of numerical simulations of the prompt emission of short-duration gamma-ray bursts. We consider emission from the relativistic jet, the mildly relativistic cocoon, and the non-relativistic shocked ambient material. We find that the cocoon material is confined between off-axis angles 15, Comment: 6 pages, 8 figures, accepted for publication in ApJL

Asteroseismology probes the interiors of stars by studying oscillation modes at a star's surface. Although pulsation spectra are well understood for solar-like oscillators, a substantial fraction of red giant stars observed by Kepler exhibit abnormally low-amplitude dipole oscillation modes. Fuller et al. (2015) suggests this effect is produced by strong core magnetic fields that scatter dipole internal gravity waves (IGWs) into higher multipole IGWs or magnetic waves. In this paper, we study the interaction of IGWs with a magnetic field to test this mechanism. We consider two background stellar structures: one with a uniform magnetic field, and another with a magnetic field that varies both horizontally and vertically. We derive analytic solutions to the wave propagation problem and validate them with numerical simulations. In both cases, we find perfect conversion from IGWs into magnetic waves when the IGWs propagate into a region exceeding a critical magnetic field strength. Downward propagating IGWs cannot reflect into upward propagating IGWs because their vertical wavenumber never approaches zero. Instead, they are converted into upward propagating slow (Alfvenic) waves, and we show they will likely dissipate as they propagate back into weakly magnetized regions. Therefore, strong internal magnetic fields can produce dipole mode suppression in red giants, and gravity modes will likely be totally absent from the pulsation spectra of sufficiently magnetized stars., Comment: 13 pages, 7 figures, accepted to MNRAS

Bowman et al. (2019) reported low-frequency photometric variability in 164 O- and B-type stars observed with K2 and TESS. They interpret these motions as internal gravity waves, which could be excited stochastically by convection in the cores of these stars. The detection of internal gravity waves in massive stars would help distinguish between massive stars with convective or radiative cores, determine core size, and would provide important constraints on massive star structure and evolution. In this work, we study the observational signature of internal gravity waves generated by core convection. We calculate the \textit{wave transfer function}, which links the internal gravity wave amplitude at the base of the radiative zone to the surface luminosity variation. This transfer function varies by many orders of magnitude for frequencies $\lesssim 1 \, {\rm d}^{-1}$, and has regularly-spaced peaks near $1 \, {\rm d}^{-1}$ due to standing modes. This is inconsistent with the observed spectra which have smooth ``red noise'' profiles, without the predicted regularly-spaced peaks. The wave transfer function is only meaningful if the waves stay predominately linear. We next show that this is the case: low frequency traveling waves do not break unless their luminosity exceeds the radiative luminosity of the star; and, the observed luminosity fluctuations at high frequencies are so small that standing modes would be stable to nonlinear instability. These simple calculations suggest that the observed low-frequency photometric variability in massive stars is not due to internal gravity waves generated in the core of these stars. We finish with a discussion of (sub)surface convection, which produces low-frequency variability in low-mass stars, very similar to that observed in Bowman et al. (2019) in higher mass stars., Comment: Accepted to ApJL. Supplementary information available at: http://www.princeton.edu/~lecoanet/data/SupplementaryInfo_Lecoanet2019.pdf

Weak magnetic fields have recently been detected in a number of A-type stars, including Vega and Sirius. At the same time, space photometry observations of A- and late B-type stars from Kepler and TESS have highlighted the existence of rotational modulation of surface features akin to stellar spots. Here we explore the possibility that surface magnetic spots might be caused by the presence of small envelope convective layers at or just below the stellar surface, caused by recombination of H and He. Using 1D stellar evolution calculations and assuming an equipartition dynamo, we make simple estimates of field strength at the photosphere. For most models the largest effects are caused by a convective layer driven by second helium ionization. While it is difficult to predict the geometry of the magnetic field, we conclude that the majority of intermediate-mass stars should have dynamo-generated magnetic fields of order a few gauss at the surface. These magnetic fields can appear at the surface as bright spots, and cause photometric variability via rotational modulation, which could also be wide-spread in A-stars. The amplitude of surface magnetic fields and their associated photometric variability is expected to decrease with increasing stellar mass and surface temperature, so that magnetic spots and their observational effects should be much harder to detect in late B-type stars., Submitted to ApJ