adsabs/SciX_UAT_keywords huggingface dataset:
Timing the Pulsations of the Accreting Millisecond Pulsar SAX J1808.4-3658 during Its 2019 Outburst

In this paper we present a coherent timing analysis of the 401 Hz pulsations of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its 2019 outburst. Using observations collected with the Neutron Star Interior Composition Explorer (NICER), we establish the pulsar spin frequency and orbital phase during its latest epoch. We find that the 2019 outburst shows a pronounced evolution in pulse phase over the course of the outburst. These phase shifts are found to correlate with the source flux and are interpreted in terms of hot-spot drift on the stellar surface, driven by changes in the mass accretion rate. Additionally, we find that the long-term evolution of the pulsar spin frequency shows evidence for a modulation at the Earth's orbital period, allowing for pulsar timing based astrometry of this accreting millisecond pulsar.

Keywords: millisecond pulsars; low-mass x-ray binary stars
Solving Grain Size Inconsistency between ALMA Polarization and VLA Continuum in the Ophiuchus IRS 48 Protoplanetary Disk

The protoplanetary disk around Ophiuchus IRS 48 shows an azimuthally asymmetric dust distribution in (sub)millimeter observations, which is interpreted as a vortex, where millimeter/centimeter-sized particles are trapped at the location of the continuum peak. In this paper, we present 860 μm ALMA observations of polarized dust emission from this disk. The polarized emission was detected toward a part of the disk. The polarization vectors are parallel to the disk minor axis, and the polarization fraction was derived to be 1%-2%. These characteristics are consistent with models of self-scattering of submillimeter-wave emission, which indicate a maximum grain size of ∼100 μm. However, this is inconsistent with the previous interpretation of millimeter/centimeter dust particles being trapped by a vortex. To explain both ALMA polarization and previous ALMA and Very Large Array observations, we suggest that the thermal emission at 860 μm wavelength is optically thick (τabs ∼ 7.3) at the dust trap with a maximum observable grain size of ∼100 μm rather than an optically thin case with centimeter-sized dust grains. We note that we cannot rule out that larger dust grains are accumulated near the midplane if the 860 μm thermal emission is optically thick.

Keywords: protoplanetary disks; polarimetry; planet formation
Understanding the Level of Turbulence by Asymmetric Distributions: A Motivation for Measurements in Space Plasmas

In this article, on the basis of the Langevin equation applied to velocity fluctuations, we numerically model the partial variance of increments, which is a useful tool to measure time and spatial correlations in space plasmas. We consider a coupled map lattice model to relate the spatial scale of fluctuations, k, to some macro parameters of the systems, such as the Reynolds number, R λ , the κ parameter of Kappa distributions, and a skewness parameter, δ. To do so, we compute the velocity probability density function (pdf) for each spatial scale and different values of Reynolds number in the simulations. We fit the pdf with a Skew-Kappa distribution, and we obtain a numerical relationship between the level of turbulence of the plasma and the skewness of obtained distributions, namely, $\langle \delta \rangle \sim {R}_{\lambda }^{-1/2}$ . We expect the results exposed in this paper to be useful as a tool to characterize the turbulence in the context of space plasma and other environments.

Keywords: space plasmas; interplanetary turbulence; non-gaussianity
A Multiscale Picture of the Magnetic Field and Gravity from a Large-scale Filamentary Envelope to Core-accreting Dust Lanes in the High-mass Star-forming Region W51

We present 230 GHz continuum polarization observations with the Atacama Large Millimeter/Submillimeter Array at a resolution of 0.″1 (~540 au) in the high-mass star-forming regions W51 e2 and e8. These observations resolve a network of core-connecting dust lanes, marking a departure from earlier coarser, more spherical continuum structures. At the same time, the cores do not appear to fragment further. Polarized dust emission is clearly detected. The inferred magnetic field orientations are prevailingly parallel to dust lanes. This key structural feature is analyzed together with the local gravitational vector field. The direction of local gravity is found to typically align with dust lanes. With these findings, we derive a stability criterion that defines a maximum magnetic field strength that can be overcome by an observed magnetic field-gravity configuration. Equivalently, this defines a minimum field strength that can stabilize dust lanes against a radial collapse. We find that the detected dust lanes in W51 e2 and e8 are stable, hence possibly making them a fundamental component in the accretion onto central sources, providing support for massive star formation models without the need of large accretion disks. When comparing to coarser resolutions, covering the scales of envelope, global, and local collapse, we find recurring similarities in the magnetic field structures and their corresponding gravitational vector fields. These self-similar structures point at a multiscale collapse-within-collapse scenario until finally the scale of core-accreting dust lanes is reached where gravity is entraining the magnetic field and aligning it with the dust lanes.

Keywords: interstellar magnetic fields; star forming regions; interstellar filaments; high angular resolution; radio observatories; dust continuum emission
Investigating the Asymmetry of Magnetic Field Profiles of "Simple" Magnetic Ejecta through an Expansion-modified Flux Rope Model

Magnetic clouds (MCs) are most often fitted with flux rope models that are static and have symmetric magnetic field profiles. However, spacecraft measurements near 1 au show that MCs usually expand when propagating away from the Sun and that their magnetic field profiles are asymmetric. Both effects are expected to be related, since expansion has been shown to result in a shift of the peak of the magnetic field toward the front of the MC. In this study, we investigate the effects of expansion on the asymmetry of the total magnetic field strength profile of MCs. We restrict our study to the simplest events, i.e., those that are crossed close to the nose of the MC. From a list of 25 such "simple" events, we compare the fitting results of a specific expanding Lundquist model with those of a classical force-free circular cross-sectional static Lundquist model. We quantify the goodness of the fits by the χ 2 of the total magnetic field and identify three types of MCs: (i) those with little expansion, which are well fitted by both models; (ii) those with moderate expansion, which are well fitted by the expanding model, but not by the static model; and (iii) those with expansion, whose asymmetry of the magnetic field cannot be explained. We find that the assumption of self-similar expansion cannot explain the measured asymmetry in the magnetic field profiles of some of these magnetic ejecta (MEs). We discuss our results in terms of our understanding of the magnetic fields of the MEs and their evolution from the Sun to Earth.

Keywords: interplanetary magnetic fields; solar wind
Survey of Multiple Populations in Globular Clusters among Very-low-mass Stars

Recent work has shown that near-infrared (NIR) Hubble Space Telescope (HST) photometry allows us to disentangle multiple populations (MPs) among M dwarfs of globular clusters (GCs) and to investigate this phenomenon in very-low-mass (VLM) stars. Here, we present the color-magnitude diagrams of nine GCs and the open cluster NGC 6791 in the F110W and F160W bands of HST, showing that the main sequences (MSs) below the knee are either broadened or split, thus providing evidence of MPs among VLM stars. In contrast, the MS of NGC 6791 is consistent with a single population. The color distribution of M dwarfs dramatically changes between different GCs, and the color width correlates with the cluster mass. We conclude that the MP ubiquity, variety, and dependence on GC mass are properties common to VLM and more-massive stars. We combined UV, optical, and NIR observations of NGC 2808 and NGC 6121 (M4) to identify MPs along with a wide range of stellar masses (~0.2-0.8 ${{ \mathcal M }}_{\odot }$ ), from the MS turnoff to the VLM regime, and measured, for the first time, their mass functions (MFs). We find that the fraction of MPs does not depend on the stellar mass and that their MFs have similar slopes. These findings indicate that the properties of MPs do not depend on stellar mass. In a scenario where the second generations formed in higher-density environments than the first generations, the possibility that the MPs formed with the same initial MF would suggest that it does not depend on the environment.

Keywords: globular star clusters; hst photometry
Gravity-driven Magnetic Field at 1000 au Scales in High-mass Star Formation

A full understanding of high-mass star formation requires the study of one of the most elusive components of the energy balance in the interstellar medium: magnetic fields. We report Atacama Large Millimeter/submillimeter Array (ALMA) 1.2 mm, high-resolution (700 au) dust polarization and molecular line observations of the rotating hot molecular core embedded in the high-mass star-forming region IRAS 18089-1732. The dust continuum emission and magnetic field morphology present spiral-like features resembling a whirlpool. The velocity field traced by the H13CO+ (J = 3-2) transition line reveals a complex structure with spiral filaments that are likely infalling and rotating, dragging the field with them. We have modeled the magnetic field and find that the best model corresponds to a weakly magnetized core with a mass-to-magnetic-flux ratio (λ) of 8.38. The modeled magnetic field is dominated by a poloidal component, but with an important contribution from the toroidal component that has a magnitude of 30% of the poloidal component. Using the Davis-Chandrasekhar-Fermi method, we estimate a magnetic field strength of 3.5 mG. At the spatial scales accessible to ALMA, an analysis of the energy balance of the system indicates that gravity overwhelms turbulence, rotation, and the magnetic field. We show that high-mass star formation can occur in weakly magnetized environments, with gravity taking the dominant role.

Keywords: young stellar objects; magnetic fields; massive stars; star formation; polarimetry; dust continuum emission
Harmonic Electron Cyclotron Maser Emission Excited by Energetic Electrons Traveling inside a Coronal Loop

A complete understanding of solar radio bursts requires developing numerical techniques that can connect large-scale activities with kinetic plasma processes. As a starting point, this study presents a numerical scheme combining three different techniques: (1) extrapolation of the magnetic field overlying a specific active region in order to derive the background field, (2) guiding-center simulation of the dynamics of millions of particles within a selected loop to reveal the integral velocity distribution function (VDF) around certain sections of the loop, and (3) particle-in-cell simulation of kinetic instabilities driven by energetic electrons initiated by the obtained distributions. Scattering effects at various levels (weak, moderate, and strong) due to wave turbulence-particle interaction are considered using prescribed timescales of scattering. It was found that the obtained VDFs contain strip-like and loss-cone features with positive gradient, and both features are capable of driving electron cyclotron maser emission, which is a viable radiation mechanism for some solar radio bursts, in particular, solar radio spikes. The strip-like feature is important in driving the harmonic X mode, while the loss-cone feature can be important in driving the fundamental X mode. In the weak-scattering case, the rate of energy conversion from energetic electrons to X2 can reach up to $\sim 2.9\times {10}^{-3}\,{E}_{{k}_{0}}$ , where ${E}_{{k}_{0}}$ is the initial kinetic energy of energetic electrons. The study demonstrates a novel way of exciting the X2 mode in the corona during solar flares and provides new sight into how escaping radiation can be generated within a coronal loop.

Keywords: solar corona; solar activity; radio bursts; solar coronal radio emission; plasma astrophysics
Quantitative Comparisons between WSA Implementations

The Wang–Sheeley–Arge (WSA) model has been in use for decades and remains a popular, economical approach to modeling the solar coronal magnetic field and forecasting conditions in the inner heliosphere. Given its usefulness, it is unsurprising that a number of WSA implementations have been developed by various groups with different computational approaches. While the WSA magnetic field model has traditionally been calculated using a spherical harmonic expansion of the solar magnetic field, finite-difference potential field solutions can offer speed and/or accuracy advantages. However, the creation of new versions of WSA requires that we ensure the solutions from these new models are consistent with established versions and that we quantify for the user community to what degree and in what ways they differ. In this paper, we present side-by-side comparisons of WSA models produced using the traditional, spherical harmonic–based implementation developed by Wang, Sheeley, and Arge with WSA models produced using a recently open-sourced finite-difference code from the CORHEL modeling suite called POT3D. We present comparisons of the terminal solar wind speed and magnetic field at the outer boundaries of the models, weighing these against the variation of the WSA model in the presence of small perturbations in the computational procedure, parameters, and inputs. We also compare the footpoints of magnetic field lines traced from the outer boundaries and the locations of open field in the models. We find that the traced field-line footpoints show remarkable agreement, with the greatest differences near the magnetic neutral line and in the polar regions.

Keywords: solar corona; solar coronal lines; space weather; solar wind; interplanetary magnetic fields; model selection
The Archetypal Ultra-diffuse Galaxy, Dragonfly 44, Is not a Dark Milky Way

Due to the peculiar properties of ultra-diffuse galaxies (UDGs), understanding their origin presents a major challenge. Previous X-ray studies demonstrated that the bulk of UDGs lack substantial X-ray emission, implying that they reside in low-mass dark matter halos. This result, in concert with other observational and theoretical studies, pointed out that most UDGs belong to the class of dwarf galaxies. However, a subset of UDGs is believed to host a large population of globular clusters (GCs), which is indicative of massive dark matter halos. This, in turn, hints that some UDGs may be failed L⋆ galaxies. In this work, I present Chandra and XMM-Newton observations of two archetypal UDGs, Dragonfly 44 and DF X1, and I constrain their dark matter halo mass based on the X-ray emission originating from hot gaseous emission and from the population of low-mass X-ray binaries residing in GCs. Both Dragonfly 44 and DF X1 remain undetected in X-rays. The upper limits on the X-ray emission exclude the possibility that these galaxies reside in massive (Mvir ≳ 5 × 1011 M⊙) dark matter halos, suggesting that they are not failed L⋆ galaxies. These results demonstrate that even these iconic UDGs resemble to dwarf galaxies with Mvir ≲ 1011 M⊙, implying that UDGs represent a single galaxy population.

Keywords: active galactic nuclei; x-ray astronomy; x-ray sources; dwarf galaxies; galaxies; galaxy dark matter halos; galaxy formation; supermassive black holes
The Binary Broadening Function

We propose an extended formalism for the spectral broadening function (BF) based on the multiplication rule of block matrices. The formalism, which we named the binary BF (BBF), can produce decomposed BFs for individual components of a binary star system by using two spectral templates. The decomposed BFs can be used to derive precise rotational profiles and radial velocities for individual components. We test the BBF on simulated spectra and actual observational spectra to show that the method is feasible on spectroscopic binaries, even when the spectral lines of two stellar components are heavily blended. To demonstrate the capability of the method, we conduct a simulation of "sketching" (imaging) a transiting circumbinary exoplanet using the BBF. We also discuss issues of implementation such as the variation of BBF with biased templates, the pros and cons of BBF, and cases when the method is not applicable.

Keywords: spectroscopic binary stars; radial velocity; close binary stars; stellar spectral lines; computational methods
Three-dimensional Hydrodynamic Simulations of Convective Nuclear Burning in Massive Stars Near Iron Core Collapse

Nonspherical structure in massive stars at the point of iron core collapse can have a qualitative impact on the properties of the ensuing core-collapse supernova explosions and the multimessenger signals they produce. Strong perturbations can aid successful explosions by strengthening turbulence in the postshock region. Here we report on a set of 4π 3D hydrodynamic simulations of O- and Si-shell burning in massive star models of varied initial masses using MESA and the FLASH simulation framework. We evolve four separate 3D models for roughly the final 10 minutes prior to and including iron core collapse. We consider initial 1D MESA models with masses of 14, 20, and 25 M⊙ to survey a range of O/Si-shell density and compositional configurations. We characterize the convective shells in our 3D models and compare them to the corresponding 1D models. In general, we find that the angle-average convective speeds in our 3D simulations near collapse are three to four times larger than the convective speeds predicted by MESA at the same epoch for our chosen mixing length parameter of αMLT = 1.5. In three of our simulations, we observe significant power in the spherical harmonic decomposition of the radial velocity field at harmonic indices of ℓ = 1-3 near collapse. Our results suggest that large-scale modes are common in massive stars near collapse and should be considered a key aspect of presupernova progenitor models.

Keywords: stellar convective zones; hydrodynamics; late stellar evolution; massive stars; supernovae
System Architecture and Planetary Obliquity: Implications for Long-term Habitability

In the search for life beyond our solar system, attention should be focused on those planets that have the potential to maintain habitable conditions over the prolonged periods of time needed for the emergence and expansion of life as we know it. The observable planetary architecture is one of the determinants for long-term habitability as it controls the orbital evolution and ultimately the stellar fluxes received by the planet. With an ensemble of n-body simulations and obliquity models of hypothetical planetary systems, we demonstrate that the amplitude and period of the eccentricity, obliquity, and precession cycles of an Earth-like planet are sensitive to the orbital characteristics of a giant companion planet. A series of transient, ocean-coupled climate simulations show how these characteristics of astronomical cycles are decisive for the evolving surface conditions and long-term fractional habitability relative to the modern Earth. The habitability of Earth-like planets increases with the eccentricity of a Jupiter-like companion, provided that the mean obliquity is sufficiently low to maintain temperate temperatures over large parts of its surface throughout the orbital year. A giant companion closer in results in shorter eccentricity cycles of an Earth-like planet but longer, high-amplitude, obliquity cycles. The period and amplitude of obliquity cycles can be estimated to first order from the orbital pathways calculated by the n-body simulations. In the majority of simulations, the obliquity amplitude relates directly to the orbital inclination whereas the period of the obliquity cycle is a function of the nodal precession and the proximity of the giant companion.

Keywords: exoplanet dynamics; planetary climates; orbital evolution; astrobiology; habitable planets
Coronal X-Ray Emission from Nearby, Low-mass, Exoplanet Host Stars Observed by the MUSCLES and Mega-MUSCLES HST Treasury Survey Projects

The high-energy X-ray and ultraviolet (UV) radiation fields of exoplanet host stars play a crucial role in controlling the atmospheric conditions and the potential habitability of exoplanets. Major surveys of the X-ray/UV emissions from late-type (K and M spectral types) exoplanet hosts have been conducted by the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary systems (MUSCLES) and Mega-MUSCLES Hubble Space Telescope Treasury programs. These samples primarily consist of relatively old, "inactive," low-mass stars. In this paper we present results from X-ray observations of the coronal emission from these stars obtained using the Chandra X-ray Observatory, the XMM-Newton Observatory, and the Neil Gehrels Swift Observatory. The stars effectively sample the coronal activity of low-mass stars over a wide range of masses and ages. The vast majority (21 of 23) of the stars are detected and their X-ray luminosities measured. Short-term flaring variability is detected for most of the fully convective (M ≤ 0.35 M ⊙) stars but not for the more massive M dwarfs during these observations. Despite this difference, the mean X-ray luminosities for these two sets of M dwarfs are similar, with more massive (0.35 M ⊙ ≤ M ≤ 0.6 M ⊙) M dwarfs at ~5 × 1026 erg s-1 compared to ~2 × 1026 erg s-1 for fully convective stars older than 1 Gyr. Younger, fully convective M dwarfs have X-ray luminosities between 3 and 6 × 1027 erg s-1. The coronal X-ray spectra have been characterized and provide important information that is vital for the modeling of the stellar EUV spectra.

Keywords: m dwarf stars; k dwarf stars; stellar x-ray flares; planet hosting stars
Velocity Dispersions of Quiescent Galaxies in IllustrisTNG

We examine the central stellar velocity dispersion of subhalos based on IllustrisTNG cosmological hydrodynamic simulations. The central velocity dispersion is a fundamental observable that links galaxies with their dark matter subhalos. We carefully explore simulated stellar velocity dispersions derived with different definitions to assess possible systematics. We explore the impact of variation in the identification of member stellar particles, the viewing axes, the velocity dispersion computation technique, and simulation resolution. None of these issues impact the velocity dispersion significantly; any systematic uncertainties are smaller than the random error. We examine the stellar mass–velocity dispersion relation as an observational test of the simulations. At fixed stellar mass, the observed velocity dispersions significantly exceed the simulation results. This discrepancy is an interesting benchmark for the IllustrisTNG simulations because the simulations are not explicitly tuned to match this relation. We demonstrate that the stellar velocity dispersion provides measures of the dark matter velocity dispersion and the dark matter subhalo mass.

Keywords: elliptical galaxies; magnetohydrodynamical simulations; galaxy dark matter halos; galaxy properties