adsabs/SciX_UAT_keywords huggingface dataset:
Pre-acceleration in the Electron Foreshock. II. Oblique Whistler Waves

Thermal electrons have gyroradii many orders of magnitude smaller than the finite width of a shock, thus need to be pre-accelerated before they can cross it and be accelerated by diffusive shock acceleration. One region where pre-acceleration may occur is the inner foreshock, which upstream electrons must pass through before any potential downstream crossing. In this paper, we perform a large-scale particle-in-cell simulation that generates a single shock with parameters motivated from supernova remnants. Within the foreshock, reflected electrons excite the oblique whistler instability and produce electromagnetic whistler waves, which comove with the upstream flow and as nonlinear structures eventually reach radii of up to 5 ion-gyroradii. We show that the inner electromagnetic configuration of the whistlers evolves into complex nonlinear structures bound by a strong magnetic field around four times the upstream value. Although these nonlinear structures do not in general interact with cospatial upstream electrons, they resonate with electrons that have been reflected at the shock. We show that they can scatter, or even trap, reflected electrons, confining around 0.8% of the total upstream electron population to the region close to the shock where they can undergo substantial pre-acceleration. This acceleration process is similar to, yet approximately three times more efficient than, stochastic shock drift acceleration.

Keywords: supernova remnants; shocks
Narrowband Spikes Observed during the 2013 November 7 Flare

Narrowband spikes have been observed in solar flares for several decades. However, their exact origin is still discussed. To contribute to understanding of these spikes, we analyze the narrowband spikes observed in the 800-2000 MHz range during the impulsive phase of the 2013 November 7 flare. In the radio spectrum, the spikes started with typical broadband clouds of spikes, and then their distribution in frequencies changed into unique, very narrow bands having noninteger frequency ratios. We successfully fitted frequencies of these narrow spike bands by those, calculating dispersion branches and growth rates of the Bernstein modes. For comparison, we also analyzed the model where the narrow bands of spikes are generated at the upper-hybrid frequencies. Using both models, we estimated the plasma density and magnetic field in spike sources. Then, the models are discussed, and arguments in favor of the model with the Bernstein modes are presented. Analyzing frequency profiles of this spike event by the Fourier method, we found the power-law spectra with the power-law indices varying in the -0.8 to -2.75 interval. Because at some times this power-law index was close to the Kolmogorov spectral index (-5/3), we propose that the spikes are generated through the Bernstein modes in turbulent plasma reconnection outflows or directly in the turbulent magnetic reconnection of solar flares.

Keywords: radio bursts; space plasmas; solar radio flares
What It Takes to Measure Reionization with Fast Radio Bursts

Fast Radio Bursts (FRBs) are extragalactic radio transients that exhibit a distance-dependent dispersion of their signal, and thus can be used as cosmological probes. In this article we, for the first time, apply a model-independent approach to measure reionization from synthetic FRB data assuming these signals are detected beyond redshift 5. This method allows us to constrain the full shape of the reionization history as well as the CMB optical depth τ while avoiding the problems of commonly used model-based techniques. A total of 100 localized FRBs, originating from redshifts 5-15, could constrain (at 68% confidence level) the CMB optical depth to within 11%, and the midpoint of reionization to 4%, surpassing current state-of-the-art CMB bounds and quasar limits. Owing to the higher numbers of expected FRBs at lower redshifts, the τ constraints are asymmetric (+14%, -7%), providing a much stronger lower limit. Finally, we show that the independent constraints on reionization from FRBs will improve limits on other cosmological parameters, such as the amplitude of the power spectrum of primordial fluctuations.

Keywords: reionization; radio transient sources; cosmology
High-resolution Observations of the Low Atmospheric Response to Small Coronal Heating Events in an Active Region Core

High-resolution spectral observations of the lower solar atmosphere (chromosphere and transition region) during coronal heating events, in combination with predictions from models of impulsively heated loops, provide powerful diagnostics of the properties of the heating in active region cores. Here, we analyze the first coordinated observations of such events with the Interface Region Imaging Spectrograph (IRIS) and the CHROMospheric Imaging Spectrometer (CHROMIS), at the Swedish 1 m Solar Telescope (SST), which provided extremely high spatial resolution and revealed chromospheric brightenings with spatial dimensions down to ~150 km. We use machine-learning methods (k-means clustering) and find significant coherence in the spatial and temporal properties of the chromospheric spectra, suggesting, in turn, coherence in the spatial and temporal distribution of the coronal heating. The comparison of IRIS and CHROMIS spectra with simulations suggests that both nonthermal electrons with low energy (low-energy cutoff ~5 keV) and direct heating in the corona transported by thermal conduction contribute to the heating of the low atmosphere. This is consistent with growing evidence that nonthermal electrons are not uncommon in small heating events (nanoflare to microflares), and that their properties can be constrained by chromospheric and transition region spectral observations.

Keywords: solar physics; active sun; active solar corona; solar active regions; solar atmosphere; solar chromosphere; solar chromospheric heating; solar coronal heating; solar coronal loops; solar ultraviolet emission; solar transition region; solar extreme ultraviolet emission
Improving the Lomb-Scargle Periodogram with the Thomson Multitaper

A common approach for characterizing the properties of time-series data that are evenly sampled in time is to estimate the power spectrum of the data using the periodogram. The periodogram as an estimator of the spectrum is (1) statistically inconsistent (i.e., its variance does not go to zero as infinite data are collected), (2) biased for finite samples, and (3) suffers from spectral leakage. In astronomy, time-series data are often unevenly sampled in time, and it is popular to use the Lomb-Scargle (LS) periodogram to estimate the spectrum. Unfortunately, from a statistical standpoint, the LS periodogram suffers from the same issues as the classical periodogram and has even worse spectral leakage. Here, we present an improvement on the LS periodogram by combining it with the Thomson multitaper approach. The multitaper spectral estimator is well established in the statistics and engineering literature for evenly sampled time series. It is attractive because it directly trades off bias and variance for frequency resolution, and is fast to compute: compared to an untapered spectral estimator, the multitaper adds no more than a couple of seconds for a time series with a million data points on a current desktop computer. Here, we describe an estimator that combines the multitaper with the LS periodogram. We show examples in which this new approach has improved properties compared to traditional approaches in the case of unevenly sampled time series. Finally, we demonstrate an application of the method to astronomy with an application to Kepler data.

Keywords: time series analysis; interdisciplinary astronomy; astrostatistics; algorithms; astrostatistics techniques; astrostatistics strategies; irregular cadence; asteroseismology; stellar oscillations
Minkowski Tensors in Redshift Space-Beyond the Plane-parallel Approximation

The Minkowski tensors (MTs) can be used to probe anisotropic signals in a field, and are well suited for measuring the redshift-space distortion (RSD) signal in large-scale structure catalogs. We consider how the linear RSD signal can be extracted from a field without resorting to the plane-parallel approximation. A spherically redshift-space distorted field is both anisotropic and inhomogeneous. We derive expressions for the two-point correlation functions that elucidate the inhomogeneity, and then explain how the breakdown of homogeneity impacts the volume and ensemble averages of the tensor Minkowski functionals. We construct the ensemble average of these quantities in curvilinear coordinates and show that the ensemble and volume averages can be approximately equated, but this depends on our choice of definition of the volume average of a tensor and the radial distance between the observer and field. We then extract the tensor Minkowski functionals from spherically redshift-space distorted, Gaussian random fields and gravitationally evolved dark matter density fields at z = 0 to test if we can successfully measure the Kaiser RSD signal. For the dark matter field, we find a significant, ~10% anomalous signal in the MT component parallel to the line of sight that is present even on large scales R G ≳ 15 Mpc, in addition to the Kaiser effect. This is due to the line-of-sight component of the MT being significantly contaminated by the Finger of God effect, which can be approximately modeled by an additional damping term in the cumulants.

Keywords: cosmology
Reading the CARDs: The Imprint of Accretion History in the Chemical Abundances of the Milky Way's Stellar Halo

In the era of large-scale spectroscopic surveys in the Local Group, we can explore using chemical abundances of halo stars to study the star formation and chemical enrichment histories of the dwarf galaxy progenitors of the Milky Way (MW) and M31 stellar halos. In this paper, we investigate using the chemical abundance ratio distributions (CARDs) of seven stellar halos from the Latte suite of FIRE-2 simulations. We attempt to infer galaxies' assembly histories by modeling the CARDs of the stellar halos of the Latte galaxies as a linear combination of template CARDs from disrupted dwarfs, with different stellar masses M ⋆ and quenching times t 100. We present a method for constructing these templates using present-day dwarf galaxies. For four of the seven Latte halos studied in this work, we recover the mass spectrum of accreted dwarfs to a precision of <10%. For the fraction of mass accreted as a function of t 100, we find the residuals of 20%-30% for five of the seven simulations. We discuss the failure modes of this method, which arise from the diversity of star formation and chemical enrichment histories that dwarf galaxies can take. These failure cases can be robustly identified by the high model residuals. Although the CARDs modeling method does not successfully infer the assembly histories in these cases, the CARDs of these disrupted dwarfs contain signatures of their unusual formation histories. Our results are promising for using CARDs to learn more about the histories of the progenitors of the MW and M31 stellar halos.

Keywords: milky way stellar halo; chemical abundances; galaxy chemical evolution
Spatial Distribution of Dark Matter in and Around Galaxy Clusters Traced by Galaxies, Gas, and Intracluster Stars in a Simulated Universe

To understand how well galaxies, gas, and intracluster stars trace dark matter in and around galaxy clusters, we use the IllustrisTNG cosmological hydrodynamical simulation and compare the spatial distribution of dark matter with that of baryonic components in clusters. To quantify the global morphology of the density distribution of each component in clusters, we fit an ellipse to the density contour of each component and derive shape parameters at different radii. We find that the ellipticity of dark matter is better correlated with that of galaxy mass-weighted number density, rather than with that of galaxy number density or galaxy velocity dispersion. We thus use the galaxy mass-weighted number density map as representative of the galaxy maps. Among three different density maps from galaxies, gas, and intracluster stars, the ellipticity of dark matter is best reproduced by that of the galaxy map over the entire radii. The virialized galaxy clusters show a better correlation of spatial distribution between dark matter and other components than the unvirialized clusters, suggesting that it requires some time for each component to follow the spatial distribution of dark matter after merging events. Our results demonstrate that galaxies are still good tracers of dark matter distribution even in the nonlinear regime corresponding to the scales in and around galaxy clusters, being consistent with the case where galaxies trace well the matter distribution on cosmologically large scales.

Keywords: galaxy clusters; intracluster medium; dark matter distribution; hydrodynamical simulations
A Generalized Fisk-type HMF: Implications of Spatially Dependent Photospheric Differential Rotation

The existence of a Fisk-type heliospheric magnetic field (HMF) has been debated ever since Fisk proposed an alternative to the traditional view of the HMF first proposed by Parker. Several modifications of the original Fisk field model have been published in the past, for example, the Schwadron field and the Fisk-Parker hybrid HMF model. This study presents a new generalized Fisk HMF model that implements for the first time a spatially dependent differential rotation rate of the photosphere, by mapping magnetic field lines from the solar wind source surface to the photosphere. Data analysis methods of Forsyth et al. are used to search for a signature of a Fisk-type field during solar minimum conditions, using the magnetic field data from the first solar orbit of the Ulysses spacecraft. The new generalized Fisk field agrees better with the observed magnetic field winding angle than a standard Parker field during the majority of intervals scanned by Ulysses.

Keywords: heliosphere; solar magnetic fields; solar wind
Ejection Patterns in the DG Tau Jet over the Last 40 yr: Insights into Mass Accretion Variability

We aim to clarify the link between mass accretion and ejection by analyzing DG Tau's jet observations from optical and near-infrared data spanning 1984–2019, alongside photometric variations between 1983 and 2015. We classified 12 moving knot groups among 17 total knot groups based on their constant proper motions and comparable radial velocities. A strong correlation emerges between the deprojected flow velocities of the knots and the photometric magnitudes of DG Tau. From 1983 to 1995, as the deprojected ejection velocities surged from ∼273 ± 15 to ∼427 ± 16 km s‑1, the photometric magnitudes (V) concurrently brightened from 12.3 to 11.4. Notably, when DG Tau became brighter than 12.2 in the V band, its (B ‑ V) color shifted bluer than its intrinsic color range of K5–M0. During this period, the launching point of the jet in the protoplanetary disk moved closer to 0.06 au from the star in 1995. Following a V magnitude drop from 11.7 to 13.4 in 1998, the star may have experienced significant extinction due to a dust wall created by the disk wind during the ejection of the high-velocity knot in 1999. Since then, the magnitude became fainter than 12.2, the (B ‑ V) and (V ‑ R) colors became redder, and the deprojected velocities consistently remained below 200 km s‑1. The launching point of the jet then moved away to ∼0.45 au by 2008. The prevailing factor influencing photometric magnitude appears to be the active mass accretion causing the variable mass ejection velocities. * Based on observations obtained at the international Gemini Observatory, a program of NSF's NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation on behalf of the Gemini Observatory partnership.

Keywords: star formation; star forming regions; young stellar objects; herbig-haro objects
Rotation Reduces Convective Mixing in Jupiter and Other Gas Giants

Recent measurements of Jupiter's gravitational moments by the Juno spacecraft and seismology of Saturn's rings suggest that the primordial composition gradients in the deep interior of these planets have persisted since their formation. One possible explanation is the presence of a double-diffusive staircase below the planet's outer convection zone, which inhibits mixing across the deeper layers. However, hydrodynamic simulations have shown that these staircases are not long-lasting and can be disrupted by overshooting convection. In this Letter, we suggests that planetary rotation could be another factor for the longevity of primordial composition gradients. Using rotational mixing-length theory and 3D hydrodynamic simulations, we demonstrate that rotation significantly reduces both the convective velocity and the mixing of primordial composition gradients. In particular, for Jovian conditions at t ~ 108 yr after formation, rotation reduces the convective velocity by a factor of 6, and in turn, the kinetic energy flux available for mixing gets reduced by a factor of 63 ~ 200. This leads to an entrainment timescale that is more than 2 orders of magnitude longer than without rotation. We encourage future hydrodynamic models of Jupiter and other gas giants to include rapid rotation because the decrease in the mixing efficiency could explain why Jupiter and Saturn are not fully mixed.

Keywords: planetary interior; solar system gas giant planets; jupiter; stellar convective zones; saturn
AGEL: Is the Conflict Real? Investigating Galaxy Evolution Models Using Strong Lensing at 0.3 < z < 0.9

Observed evolution of the total mass distribution with redshift is crucial to testing galaxy evolution theories. To measure the total mass distribution, strong gravitational lenses complement the resolved dynamical observations that are currently limited to z ≲ 0.5. Here we present the lens models for a pilot sample of seven galaxy-scale lenses from the ASTRO3D Galaxy Evolution with Lenses (AGEL) survey. The AGEL lenses, modeled using HST/WFC3-F140W images with Gravitational Lens Efficient Explorer (GLEE) software, have deflector redshifts in the range 0.3 < z defl < 0.9. Assuming a power-law density profile with slope γ, we measure the total density profile for the deflector galaxies via lens modeling. We also measure the stellar velocity dispersions (σ obs) for four lenses and obtain σ obs from SDSS-BOSS for the remaining lenses to test our lens models by comparing observed and model-predicted velocity dispersions. For the seven AGEL lenses, we measure an average density profile slope of ‑1.95 ± 0.09 and a γ–z relation that does not evolve with redshift at z < 1. Although our result is consistent with some observations and simulations, it differs from other studies at z < 1 that suggest the γ–z relation evolves with redshift. The apparent conflicts among observations and simulations may be due to a combination of (1) systematics in the lensing and dynamical modeling; (2) challenges in comparing observations with simulations; and (3) assuming a simple power law for the total mass distribution. By providing more lenses at z defl > 0.5, the AGEL survey will provide stronger constraints on whether the mass profiles evolve with redshift as predicted by current theoretical models.

Keywords: strong gravitational lensing; galaxy evolution; galaxy mergers; dark matter; early-type galaxies
Modeling of the Variable Circumstellar Absorption Features of WD 1145+017

We present an eccentric precessing gas disk model designed to study the variable circumstellar absorption features detected for WD 1145+017, a metal polluted white dwarf with an actively disintegrating asteroid around it. This model, inspired by one recently proposed by Cauley et al., calculates explicitly the gas opacity for any predetermined physical conditions in the disk, predicting the strength and shape of all absorption features, from the UV to the optical, at any given phase of the precession cycle. The successes and failures of this simple model provide valuable insight on the physical characteristics of the gas surrounding the star, notably its composition, temperature, and density. This eccentric disk model also highlights the need for supplementary components, most likely circular rings, in order to explain the presence of zero velocity absorption as well as highly ionized Si IV lines. We find that a precession period of 4.6 ± 0.3 yr can successfully reproduce the shape of the velocity profile observed at most epochs from 2015 April to 2018 January, although minor discrepancies at certain times indicate that the assumed geometric configuration may not be optimal yet. Finally, we show that our model can quantitatively explain the change in morphology of the circumstellar features during transiting events.

Keywords: debris disks; circumstellar gas; circumstellar matter; chemical abundances
The Power Spectrum of Episodic Stellar Mass Accretion

The process by which stars accumulate their mass is episodic, characterized by long-duration low-accretion phases punctuated with intense accretion bursts. The bursts are several orders of magnitude greater in accretion rate and contribute a significant fraction of the total mass. In this article we present results of performing a time series analysis using the Fourier transform on a computer simulation of the burst mode of mass accretion. We find that the most prominent peaks in the power spectrum correspond to characteristic timescales of several thousand years.

Keywords: star formation; young stellar objects; fu orionis stars; t tauri stars; circumstellar disks
Population Synthesis of Black Hole Binaries with Compact Star Companions

We perform a systematic study of merging black hole (BH) binaries with compact star (CS) companions, including black hole-white dwarf (BH-WD), black hole-neutron star (BH-NS), and black hole-black hole (BH-BH) systems. Previous studies have shown that mass transfer stability and common envelope evolution can significantly affect the formation of merging BH-CS binaries through isolated binary evolution. With detailed binary evolution simulations, we obtain easy-to-use criteria for the occurrence of the common envelope phase in mass-transferring BH binaries with a nondegenerate donor, and incorporate the criteria into population synthesis calculations. To explore the impact of a possible mass gap between NSs and BHs on the properties of merging BH-CS binary population, we adopt different supernova mechanisms involving the rapid, delayed, and stochastic prescriptions to deal with the compact remnant masses and the natal kicks. Our calculations show that there are ~105-106 BH-CS binaries in the Milky Way, among which dozens are observable by future space-based gravitational wave detectors. We estimate that the local merger rate density of all BH-CS systems is ~60-200 Gpc-3 yr-1. While there are no low-mass BHs formed via rapid supernovae, both delayed and stochastic prescriptions predict that ~100%/~70%/~30% of merging BH-WD/BH-NS/BH-BH binaries are likely to have BH components within the mass gap.

Keywords: gravitational waves; compact binary stars; black holes; stellar evolution; supernovae