M15 is a globular cluster with a known spread in neutron-capture elements. This paper presents abundances of neutron-capture elements for 62 stars in M15. Spectra were obtained with the Michigan/Magellan Fiber System spectrograph, covering a wavelength range from ∼4430 to 4630 Å. Spectral lines from Fe i, Fe ii, Sr i, Zr ii, Ba ii, La ii, Ce ii, Nd ii, Sm ii, Eu ii, and Dy ii were measured, enabling classifications and neutron-capture abundance patterns for the stars. Of the 62 targets, 44 are found to be highly Eu-enhanced -II stars, another 17 are moderately Eu-enhanced -I stars, and one star is found to have an -process signature. The neutron-capture patterns indicate that the majority of the stars are consistent with enrichment by the -process. The 62 target stars are found to show significant star-to-star spreads in Sr, Zr, Ba, La, Ce, Nd, Sm, Eu, and Dy, but no significant spread in Fe. The neutron-capture abundances are further found to have slight correlations with sodium abundances from the literature, unlike what has been previously found; follow-up studies are needed to verify this result. The findings in this paper suggest that the Eu-enhanced stars in M15 were enhanced by the same process, that the nucleosynthetic source of this Eu pollution was the -process, and that the -process source occurred as the first generation of cluster stars was forming.

It is well known that the power spectrum is not able to fully characterize the statistical properties of non-Gaussian density fields. Recently, many different statistics have been proposed to extract information from non-Gaussian cosmological fields that perform better than the power spectrum. The Fisher matrix formalism is commonly used to quantify the accuracy with which a given statistic can constrain the value of the cosmological parameters. However, these calculations typically rely on the assumption that the sampling distribution of the considered statistic follows a multivariate Gaussian distribution. In this work, we follow Sellentin & Heavens and use two different statistical tests to identify non-Gaussianities in different statistics such as the power spectrum, bispectrum, marked power spectrum, and wavelet scattering transform (WST). We remove the non-Gaussian components of the different statistics and perform Fisher matrix calculations with the statistics using Quijote simulations. We show that constraints on the parameters can change by a factor of ∼2 in some cases. We show with simple examples how statistics that do not follow a multivariate Gaussian distribution can achieve artificially tight bounds on the cosmological parameters when using the Fisher matrix formalism. We think that the non-Gaussian tests used in this work represent a powerful tool to quantify the robustness of Fisher matrix calculations and their underlying assumptions. We release the code used to compute the power spectra, bispectra, and WST that can be run on both CPUs and GPUs.

Overshooting of turbulent motions from convective regions into adjacent stably stratified zones plays a significant role in stellar interior dynamics, as this process may lead to mixing of chemical species and contribute to the transport of angular momentum and magnetic fields. We present a series of fully nonlinear, three-dimensional (3D) anelastic simulations of overshooting convection in a spherical shell that are focused on the dependence of the overshooting dynamics on the density stratification and the rotation, both key ingredients in stars that however have not been studied systematically together via global simulations. We demonstrate that the overshoot lengthscale is not simply a monotonic function of the density stratification in the convective region, but instead it depends on the ratio of the density stratifications in the two zones. Additionally, we find that the overshoot lengthscale decreases with decreasing Rossby number Ro and scales as Ro while it also depends on latitude with higher Rossby cases leading to a weaker latitudinal variation. We examine the mean flows arising due to rotation and find that they extend beyond the base of the convection zone into the stable region. Our findings may provide a better understanding of the dynamical interaction between stellar convective and radiative regions, and motivate future studies particularly related to the solar tachocline and the implications of its overlapping with the overshoot region.

Cosmic-ray (CR) sources temporarily enhance the relativistic particle density in their vicinity over the background distribution accumulated from the Galaxy-wide past injection activity and propagation. If individual sources are close enough to the solar system, their localized enhancements may present as features in the measured spectra of the CRs and in the associated secondary electromagnetic emissions. Large-scale loop-like structures visible in the radio sky are possible signatures of such nearby CR sources. If so, these loops may also have counterparts in the high-latitude -ray sky. Using ~10 yr of data from the Fermi Large Area Telescope, applying Bayesian analysis including Gaussian Processes, we search for extended enhanced emission associated with putative nearby CR sources in the energy range from 1 GeV to 1 TeV for the sky region || > 30°. We carefully control the systematic uncertainty due to imperfect knowledge of the interstellar gas distribution. Radio Loop IV is identified for the first time as a -ray emitter, and we also find significant emission from Loop I. Strong evidence is found for asymmetric features about the Galactic = 0° meridian that may be associated with parts of the so-called "Fermi Bubbles," and some evidence is also found for -ray emission from other radio loops. Implications for the CRs producing the features and possible locations of the sources of the emissions are discussed.

Herein we present a laboratory rotational study of cyanoacetic acid (CH(CN)C(O)OH), an organic acid as well as a -CN bearing molecule, that is a candidate molecular system to be detected in the interstellar medium (ISM). Our investigation aims to provide direct experimental frequencies of cyanoacetic acid to guide its eventual astronomical search in low-frequency surveys. Using different jet-cooled rotational spectroscopic techniques in the time domain, we have determined a precise set of the relevant rotational spectroscopic constants, including the N nuclear quadrupole coupling constants for the two distinct structures, - and - cyanoacetic acid. We believe this work will potentially allow the detection of cyanoacetic acid in the interstellar medium, whose rotational features have remained unknown until now.

Hot stars with hot Jupiters have a wide range of obliquities, while cool stars with hot Jupiters tend to have low obliquities. An enticing explanation for this pattern is tidal realignment of the cool host stars, although this explanation assumes that obliquity damping occurs faster than orbital decay, an assumption that needs further exploration. Here we revisit this tidal realignment problem, building on previous work identifying a low-frequency component of the time-variable tidal potential that affects the obliquity but not the orbital separation. We adopt a recent empirically based model for the stellar tidal quality factor and its sharp increase with forcing frequency. This leads to enhanced dissipation at low frequencies, and efficient obliquity damping. We model the tidal evolution of 46 observed hot Jupiters orbiting cool stars. A key parameter is the stellar age, which we determine in a homogeneous manner for the sample, taking advantage of Gaia DR2 data. We explore a variety of tidal histories and futures for each system, finding in most cases that the stellar obliquity is successfully damped before the planet is destroyed. A testable prediction of our model is that hot Jupiter hosts with orbital periods shorter than 2-3 days should have obliquities much smaller than 1°. With the possible exception of WASP-19b, the predicted future lifetimes of the planets range from 10 yr to more than 10 yr. Thus, our model implies that these hot Jupiters are probably not in immediate danger of being devoured by their host stars while they are on the main sequence.

Since its launch, the Alpha Magnetic Spectrometer-02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species ( , , and nuclei, H-O, Ne, Mg, Si) which resulted in a number of breakthroughs. One of the latest long-awaited surprises is the spectrum of Fe just published by AMS-02. Because of the large fragmentation cross section and large ionization energy losses, most of CR iron at low energies is local and may harbor some features associated with relatively recent supernova (SN) activity in the solar neighborhood. Our analysis of the new AMS-02 results, together with Voyager 1 and ACE-CRIS data, reveals an unexpected bump in the iron spectrum and in the Fe/He, Fe/O, and Fe/Si ratios at 1-2 GV, while a similar feature in the spectra of He, O, and Si and in their ratios is absent, hinting at a local source of low-energy CRs. The found excess extends the recent discoveries of radioactive Fe deposits in terrestrial and lunar samples and in CRs. We provide an updated local interstellar spectrum (LIS) of iron in the energy range from 1 MeV nucleon to ~10 TeV nucleon. Our calculations employ the GALPROP-HELMOD framework, which has proved to be a reliable tool in deriving the LIS of CR , , and nuclei ⩽ 28.

We study the nature of energy release and transfer for two sub-A class solar microflares observed during the second Focusing Optics X-ray Solar Imager (FOXSI-2) sounding rocket flight on 2014 December 11. FOXSI is the first solar-dedicated instrument to utilize focusing optics to image the Sun in the hard X-ray (HXR) regime, sensitive to energies of 4-20 keV. Through spectral analysis of the microflares using an optically thin isothermal plasma model, we find evidence for plasma heated to ~10 MK and emission measures down to ~10 cm. Though nonthermal emission was not detected for the FOXSI-2 microflares, a study of the parameter space for possible hidden nonthermal components shows that there could be enough energy in nonthermal electrons to account for the thermal energy in microflare 1, indicating that this flare is plausibly consistent with the standard thick-target model. With a solar-optimized design and improvements in HXR focusing optics, FOXSI-2 offers approximately five times greater sensitivity at 10 keV than the Nuclear Spectroscopic Telescope Array for typical microflare observations and allows for the first direct imaging spectroscopy of solar HXRs with an angular resolution at scales relevant for microflares. Harnessing these improved capabilities to study small-scale events, we find evidence for spatial and temporal complexity during a sub-A class flare. This analysis, combined with contemporaneous observations by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, indicates that these microflares are more similar to large flares in their evolution than to the single burst of energy expected for a nanoflare.

A recently observed bump in the cosmic-ray (CR) spectrum from 0.3 to 30 TV is likely caused by a stellar bow shock that reaccelerates preexisting CRs, which further propagate to the Sun along the magnetic field lines. Along their way, these particles generate an Iroshnikov-Kraichnan (I-K) turbulence that controls their propagation and sustains the bump. Ad hoc fitting of the bump shape requires six adjustable parameters. Our model requires none, merely depending on three physical unknowns that we constrain using the fit. These are the shock Mach number, , its size, , and the distance to it, . Altogether, they define the bump rigidity . With ≈ 1.5-1.6 and ≈ 4.4 TV, the model fits the data with ≈0.08% accuracy. The fit critically requires the I-K spectrum predicted by the model and rules out the alternatives. These attributes of the fit make an accidental agreement highly unlikely. In turn, the and derived from the fit impose the distance-size relation on the shock: . For sufficiently large bow shocks, = 10-10 pc, we find the distance of = 3-10 pc. Three promising stars in this range are the Scholz's Star at 6.8 pc, Epsilon Indi at 3.6 pc, and Epsilon Eridani at 3.2 pc. Based on their current positions and velocities, we propose that Epsilon Indi and Epsilon Eridani can produce the observed spectral bump. Moreover, Epsilon Eridani's position is only ~6°.7 off of the magnetic field direction in the solar neighborhood, which also changes the CR arrival direction distribution. Given the proximity of these stars, the bump appearance may change in a relatively short time.

We apply the helioseismic methodology of Fourier Legendre decomposition to 88 months of Dopplergrams obtained by the Helioseismic and Magnetic Imager (HMI) as the basis of inferring the depth variation of the mean meridional flow, as averaged between 20° and 60° latitude and in time, in both the northern and southern hemispheres. We develop and apply control procedures designed to assess and remove center-to-limb artifacts using measurements obtained by performing the analysis with respect to artificial poles at the east and west limbs. Forward modeling is carried out using sensitivity functions proportional to the mode kinetic energy density to evaluate the consistency of the corrected frequency shifts with models of the depth variation of the meridional circulation in the top half of the convection zone. The results, taken at face value, imply substantial differences between the meridional circulation in the northern and southern hemispheres. The inferred presence of a return (equatorward propagating) flow at a depth of approximately 40 Mm below the photosphere in the northern hemisphere is surprising and appears to be inconsistent with many other helioseismic analyses. This discrepancy may be the result of the inadequacy of our methodology to remove systematic errors in HMI data. Our results appear to be at least qualitatively similar to those by Gizon et al., which point to an anomaly in HMI data that is not present in MDI or GONG data.

Solar flares are explosive releases of magnetic energy. Hard X-ray (HXR) flare emission originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, giving insight into flare energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR) utilizes direct-focusing optics to attain much higher sensitivity in the HXR range than that of previous indirect imagers. This paper presents 11 NuSTAR microflares from two active regions (AR 12671 on 2017 August 21 and AR 12712 on 2018 May 29). The temporal, spatial, and energetic properties of each are discussed in context with previously published HXR brightenings. They are seen to display several "large flare" properties, such as impulsive time profiles and earlier peak times in higher-energy HXRs. For two events where the active region background could be removed, microflare emission did not display spatial complexity; differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high-energy excess over a single thermal model in all events. This excess was consistent with additional higher-temperature plasma volumes in 10/11 microflares and only with an accelerated particle distribution in the last. Previous NuSTAR studies focused on one or a few microflares at a time, making this the first to collectively examine a sizable number of events. Additionally, this paper introduces an observed variation in the NuSTAR gain unique to the extremely low livetime (<1%) regime and establishes a correction method to be used in future NuSTAR solar spectral analysis.

We present elemental abundance data of C, N, O, Na, Mg, Al, Ca, and Cr in Genesis silicon targets. For Na, Mg, Al, and Ca, data from three different SW regimes are also presented. Data were obtained by backside depth profiling using Secondary Ion Mass Spectrometry. The accuracy of these measurements exceeds those obtained by in-situ observations; therefore the Genesis data provide new insights into elemental fractionation between Sun and solar wind, including differences between solar wind regimes. We integrate previously published noble gas and hydrogen elemental abundances from Genesis targets, as well as preliminary values for K and Fe. The abundances of the solar wind elements measured display the well-known fractionation pattern that correlates with each elements' First Ionization Potential (FIP). When normalized either to spectroscopic photospheric solar abundances or to those derived from CI-chondritic meteorites, the fractionation factors of low-FIP elements (K, Na, Al, Ca, Cr, Mg, Fe) are essentially identical within uncertainties, but the data are equally consistent with an increasing fractionation with decreasing FIP. The elements with higher FIPs between ~11 and ~16 eV (C, N, O, H, Ar, Kr, Xe) display a relatively well-defined trend of increasing fractionation with decreasing FIP, if normalized to modern 3D photospheric model abundances. Among the three Genesis regimes, the Fast SW displays the least elemental fractionation for almost all elements (including the noble gases) but differences are modest: for low-FIP elements the precisely measured Fast-Slow SW variations are less than 3%.

Silicon is present in interstellar dust grains, meteorites and asteroids, and to date thirteen silicon-bearing molecules have been detected in the gas-phase towards late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those at the atmosphere of evolved stars. We have found that the chemistry of Si, H and H efficiently produces silane (SiH), disilane (SiH) and amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely detected towards the carbon-rich star IRC+10216, while disilane has not been detected in space yet. Thus, based on our results, we propose that gas-phase reactions of atomic Si with H and H are a plausible source of silane in C-rich AGBs, although its contribution to the total SiH abundance may be low in comparison with the suggested formation route by catalytic reactions on the surface of dust grains. In addition, the produced a-Si:H dust analogs decompose into SiH and SiH at temperatures above 500 K, suggesting an additional mechanism of formation of these species in envelopes around evolved stars. We have also found that the exposure of these dust analogs to water vapor leads to the incorporation of oxygen into Si-O-Si and Si-OH groups at the expense of SiH moieties, which implies that, if this type of grains are present in the interstellar medium, they will be probably processed into silicates through the interaction with water ices covering the surface of dust grains.

The scaling laws which relate the peak temperature and volumetric heating rate to the pressure and length for static coronal loops were established over 40 years ago; they have proved to be of immense value in a wide range of studies. Here we extend these scaling laws to loops, where enthalpy flux becomes important to the energy balance, and study impulsive heating/filling characterized by upward enthalpy flows. We show that for collision-dominated thermal conduction, the functional dependencies of the scaling laws are the same as for the static case, when the radiative losses scale as , but with a different constant of proportionality that depends on the Mach number of the flow. The dependence on the Mach number is such that the scaling laws for low to moderate Mach number flows are almost indistinguishable from the static case. When thermal conduction is limited by turbulent processes, however, the much weaker dependence of the scattering mean free path (and hence thermal conduction coefficient) on temperature leads to a limiting Mach number for return enthalpy fluxes driven by thermal conduction between the the corona and chromosphere.

Solar flares are known to release a large amount of energy into accelerating electrons. Studying small timescale (⩽2s) fluctuations in nonthermal X-ray flux offers the opportunity to probe the nature of those acceleration mechanisms. By comparing the durations, differences in timing between energy bands, and the periodicity of these spikes against the relevant timescales called for by various acceleration mechanisms, a test for each mechanism's validity can be made. This work details the analysis of fast fluctuations in Fermi Gamma-ray Burst Monitor (Fermi GBM) data from two M9.3 class solar flares that occurred on SOL2011-07-30 and SOL2011-08-04. This study shows the usefulness of Fermi GBM data as a means of examining these small timescale spikes and presents a rigorous method of identifying, counting, and measuring the temporal properties of these subsecond X-ray spikes. In the two flares examined we found spikes to primarily occur in spans of 60-100 s in the impulsive phase. The relative spike intensity averaged between 6% and 28% when compared to the slowly varying component of the X-ray flux. The average spike durations were 0.49 and 0.38 s for the 2 flares. The spike duration distribution for the SOL2011-08-04 flare was found to follow a power law with a -1.2 ± 0.3 index. Of the three spiking intervals identified, only one was found to have a periodicity, showing significant power at the 1.7 ± 0.1 Hz frequency.

Core-collapse supernovae (SNe) expand into a medium created by winds from the pre-SN progenitor. The SN explosion and resulting shock wave(s) heat up the surrounding plasma, giving rise to thermal X-ray emission, which depends on the density of the emitting material. Tracking the variation of the X-ray luminosity over long periods of time thus allows for investigation of the kinematics of the SN shock waves, the structure of the surrounding medium, and the nature of the progenitor star. In this paper, X-ray observations of five of the oldest known X-ray SNe-SN 1970G, SN 1968D, SN 1959D, SN 1957D, and SN 1941C-are analyzed, with the aim of reconstructing their light curves over several decades. For those SNe for which we can extract multiepoch data, the X-ray luminosity appears to decline with time, although with large error bars. No increase in the X-ray emission from SN 1970G is found at later epochs, contrary to previous reports. All five SNe show X-ray luminosities that are of comparable magnitude. We compare the late-time X-ray luminosities of these SNe to those of supernova remnants (SNRs) in the Galaxy, which are a few hundred years old, and find that when the tentative decline is taken into account, the luminosity of the old SNe studied herein could fall below the luminosity of some of the younger SNRs within a few hundred years. However, the X-ray luminosity should begin to increase as the SNe expand in the Sedov phase, thus reaching that of the observed SNRs.

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H and CO. In a previous study, we addressed the chemistry of carbon (C and C) with H showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C) with CH. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp and sp hydrocarbons with amorphous morphology.

NGC 4151 is the brightest Seyfert 1 nucleus in X-rays. It was the first object to show short time delays in the Fe K band, which were attributed to relativistic reverberation, providing a new tool for probing regions at the black hole scale. Here, we report the results of a large XMM-Newton campaign in 2015 to study these short delays further. Analyzing high quality data that span time scales between hours and decades, we find that neutral and ionized absorption contribute significantly to the spectral shape. Accounting for their effects, we find no evidence for a relativistic reflection component, contrary to early work. Energy-dependent lags are significantly measured in the new data, but with an energy profile that does not resemble a broad iron line, in contrast to the old data. The complex lag-energy spectra, along with the lack of strong evidence for a relativistic spectral component, suggest that the energy-dependent lags are produced by absorption effects. The long term spectral variations provide new details on the variability of the narrow Fe K line. We find that its variations are correlated with, and delayed with respect to, the primary X-ray continuum. We measure a delay of days, implying an origin in the inner broad line region (BLR). The delay is half the H line delay, suggesting a geometry that differs slightly from the optical BLR.

The velocity distribution function (VDF) of ions in the solar wind, as observed by spacecraft at 1 AU and elsewhere in the heliosphere, exhibits a consistent trend: at low energies in the solar wind frame, the distribution is largely Maxwellian-the core; at higher but still modest energies in the solar wind frame, the distribution follows a power law ( ∝ , where is the VDF, is the speed in the solar wind frame, and is an arbitrary spectral index parameter)-the tail-with a spectral index of ≈ 5 being extremely common. Several theories have been proposed to explain this common index. Among these theories is that the tail is a natural consequence of an ensemble of particles obeying Coulomb's law (Randol & Christian 2014, 2016). In this study, we derive a general analytical formula for the distribution of electric fields, and find that it always exhibits a power law tail with a spectral index of exactly 9/2, or 4.5, due to the spatial power law index of Coulomb's law. We then show how the VDF is a convolution of the distribution of electric fields with a pre-existing VDF, and that for small values of time after being created, the ion VDF always exhibits a = 9/2 power law, wherein the probability of the tail relative to the core depends on particle density, , and inversely on the pre-existing VDF thermal speed, . Finally, we compare our results with previous works, and find good agreement but with important distinctions.

The potential habitability of known exoplanets is often categorized by a nominal equilibrium temperature assuming a Bond albedo of either ∼0.3, similar to Earth, or 0. As an indicator of habitability, this leaves much to be desired, because albedos of other planets can be very different, and because surface temperature exceeds equilibrium temperature due to the atmospheric greenhouse effect. We use an ensemble of general circulation model simulations to show that for a range of habitable planets, much of the variability of Bond albedo, equilibrium temperature and even surface temperature can be predicted with useful accuracy from incident stellar flux and stellar temperature, two known parameters for every confirmed exoplanet. Earth's Bond albedo is near the minimum possible for habitable planets orbiting G stars, because of increasing contributions from clouds and sea ice/snow at higher and lower instellations, respectively. For habitable M star planets, Bond albedo is usually lower than Earth's because of near-IR HO absorption, except at high instellation where clouds are important. We apply relationships derived from this behavior to several known exoplanets to derive zeroth-order estimates of their potential habitability. More expansive multivariate statistical models that include currently non-observable parameters show that greenhouse gas variations produce significant variance in albedo and surface temperature, while increasing length of day and land fraction decrease surface temperature; insights for other parameters are limited by our sampling. We discuss how emerging information from global climate models might resolve some degeneracies and help focus scarce observing resources on the most promising planets.

We report a comprehensive list of accurate Ritz wavelengths and calculated transition probabilities for parity-forbidden [Mn II] lines. Ritz wavelengths have been derived from experimentally established energy level values resulting from an extensive analysis of a high-resolution Fourier-transform emission spectrum of singly ionized manganese. Our analysis includes transitions between all known metastable and other long-lived levels of Mn II giving a total of 1130 [Mn II] Ritz wavelengths. Our entire list of derived Ritz wavelengths for [Mn II] lines ranges between 237 nm and 170 m (42,125-58 cm). The accurate Ritz wavelengths and calculated transition probabilities for forbidden lines in this paper are useful in the study and diagnostics of nebulae and other low-density astrophysical plasmas.