Sazuna/UAT_keywords huggingface dataset:
Stability Constrained Characterization of the 23 Myr Old V1298 Tau System: Do Young Planets Form in Mean Motion Resonance Chains?. A leading theoretical expectation for the final stages of planet formation is that disk migration should naturally drive orbits into chains of mean motion resonances (MMRs). In order to explain the dearth of MMR chains observed at Gyr ages (&lt;1%), this picture requires such configurations to destabilize and scramble period ratios following disk dispersal. Strikingly, the only two known stars with three or more planets younger than ≲100 Myr, HR 8799 and V1298 Tau, have been suggested to be in such MMR chains, given the orbits' near-integer period ratios. We incorporate recent transit and radial velocity (RV) observations of the V1298 Tau system, and investigate constraints on the system's orbital architecture imposed by requiring dynamical stability on timescales much shorter than the system's age. We show that the recent RV mass measurement of V1298 Tau b places it within a factor of 2 of the instability limit, and that this allows us to set significantly lower limits on the eccentricity (e  b  ≤0.17 at 99.7% confidence). Additionally, we rule out a resonant chain configuration for V1298 Tau at ≳99% confidence. Thus, if the ~23 Myr old V1298 Tau system did form as a resonant chain, it must have undergone instability and rearrangement shortly after disk dispersal. We expect that similar stability constrained characterization of future young multiplanet systems will be valuable in informing planet formation models.

Keywords: Exoplanet dynamics, Exoplanet evolution, Exoplanet formation, Orbital resonances
Features of Magnetic Field Switchbacks in Relation to the Local-field Geometry of Large-amplitude Alfvénic Oscillations: Wind and PSP Observations. In this Letter, we report observations of magnetic switchback (SB) features near 1 au using data from the Wind spacecraft. These features appear to be strikingly similar to the ones observed by the Parker Solar Probe mission closer to the Sun: namely, one-sided spikes (or enhancements) in the solar-wind bulk speed V that correlate/anticorrelate with the spikes seen in the radial-field component B  R . In the solar-wind streams that we analyzed, these specific SB features near 1 au are associated with large-amplitude Alfvénic oscillations that propagate outward from the Sun along a local background (prevalent) magnetic field B 0 that is nearly radial. We also show that, when B 0 is nearly perpendicular to the radial direction, the large-amplitude Alfvénic oscillations display variations in V that are two sided (i.e., V alternately increases and decreases depending on the vector Δ B = B - B 0). As a consequence, SBs may not always appear as one-sided spikes in V, especially at larger heliocentric distances where the local background field statistically departs from the radial direction. We suggest that SBs can be well described by large-amplitude Alfvénic fluctuations if the field rotation is computed with respect to a well-determined local background field that, in some cases, may deviate from the large-scale Parker field.

Keywords: Solar wind, Heliosphere
Revisiting a Disky Origin for the Faint Branch of the Sagittarius Stellar Stream. We investigate ways to produce the bifurcation observed in the stellar stream of the Sagittarius dwarf galaxy (Sgr). Our method consists of running N-body simulations of Sgr falling into the Milky Way for the last 3 Gyr, with added test particles on disk orbits that span a wide range of initial positions, energies, and angular momenta. We find that particles that end up in the faint branch are predominantly high-angular-momentum particles that can all originate from a single plane within the progenitor, nearly perpendicular both to the orbital plane of the progenitor and to the Milky Way stellar disk. Their original configuration at the start of the simulation corresponds to spiral features already present 3 Gyr ago, which could be, e.g., the result of a disk-like component being tidally perturbed, or the tidal tails of a satellite being disrupted within Sgr. We then run a simulation including the self-gravity of this disky component. Despite the remaining ambiguity of its origin, this disk component of the Sgr dwarf with spiral overdensities provides a first step toward a working model to reproduce the observed faint branch of the bifurcated Sgr stream.

Keywords: Galaxy dynamics, Sagittarius dwarf spheroidal galaxy, Dwarf galaxies, Stellar streams
Onset of Particle Acceleration during the Prompt Phase in Gamma-Ray Bursts as Revealed by Synchrotron Emission in GRB 160821A. The physical processes of gamma-ray emission and particle acceleration during the prompt phase in gamma-ray bursts (GRBs) are still unsettled. In order to perform unambiguous physical modeling of observations, a clear identification of the emission mechanism is needed. An instance of a clear identification is the synchrotron emission during the very strong flare in GRB 160821A, which occurred during the prompt phase at 135 s. Here we show that the distribution of the radiating electrons in this flare is initially very narrow but later develops a power-law tail of accelerated electrons. We thus identify for the first time the onset of particle acceleration in a GRB jet. The flare is consistent with a late energy release from the central engine causing an external shock as it encounters a preexisting ring nebula of a progenitor Wolf-Rayet star. Relativistic forward and reverse shocks develop, leading to two distinct emission zones with similar properties. The particle acceleration only occurs in the forward shock, moving into the dense nebula matter. Here, the magnetization also decreases below the critical value, which allows for Fermi acceleration to operate. Using this fact, we find a bulk Lorentz factor of 420 ≲ Γ ≲ 770 and an emission radius of R ~ 1018 cm, indicating a tenuous gas of the immediate circumburst surroundings. The observation of the onset of particle acceleration thus gives new and independent constraints on the properties of the flow as well as on theories of particle acceleration in collisionless astrophysical shocks.

Keywords: Gamma-ray bursts
The Local Stellar Halo is Not Dominated by a Single Radial Merger Event. We use halo dwarf stars with photometrically determined metallicities that are located within 2 kpc of the Sun to identify local halo substructure. The kinematic properties of these stars do not indicate a single, dominant radial merger event (RME). The retrograde Virgo Radial Merger (VRM) component has [Fe/H] = ‑1.7. A second, nonrotating RME component we name Nereus is identified with [Fe/H] = ‑2.1 and has similar energy to the VRM. We identify a possible third RME, which we name Cronus, that is corotating with the disk, has lower energy than the VRM, and has [Fe/H] = ‑1.2. We identify the Nyx Stream in the data. In addition to these substructures, we observe metal-poor halo stars ([Fe/H] ∼ ‑2.0 and σ  v  ∼ 180 km s‑1) and a disk/Splash component with lower rotational velocity than the disk and lower metallicity than typically associated with the Splash. An additional excess of halo stars with low velocity and metallicity of [Fe/H] = ‑1.5 could be associated with the shell of a lower-energy RME or indicate that lower-energy halo stars have higher metallicity. Stars that comprise the "Gaia Sausage" velocity structure are a combination of the components identified in this work.

Keywords: Galaxy mergers, Milky Way dynamics, Milky Way Galaxy, Galaxy accretion
Implication of Spin Constraints by the Event Horizon Telescope on Stellar Orbits in the Galactic Center. The center of the Milky Way hosts the closest supermassive black hole, Sgr A*. Decades of near-infrared observations of our Galactic Center have shown the presence of a small population of stars (the so-called S-star cluster) orbiting Sgr A*, which were recently reported to be arranged into two orthogonal disks. In this case, the timescale for the Lense-Thirring precession of S stars should be longer than their age, implying a low spin for Sgr A*. In contrast, the recent results by the Event Horizon Telescope favor a highly spinning Sgr A*, which seems to suggest that the S stars could not be arranged in disks. Alternatively, the spin of Sgr A* must be small, suggesting that the models for its observed image are incomplete.

Keywords: Galactic center, Black holes, Kerr black holes
Triggering Mechanism for Eruption of Two Filaments Observed by the Solar Dynamics Observatory, Nobeyama Radioheliograph, and RHESSI. We investigate the eruptive process of two filaments, which is associated with an M-class flare that occurred in 2011 August 4. The filaments are partly overlapped, one in the active region and the other just beside it, and erupt together as a halo coronal mass ejection. For this study, we used the Atmospheric Imaging Assembly and the Heliospheric Magnetic Imager on board the Solar Dynamics Observatory, the Nobeyama Radioheliograph 17 GHz, and the RHESSI Hard X-ray satellite. We found three distinct phases in the microwave flux profile and in the rising pattern of the filaments during the event. In the first phase, there was weak nonthermal emission at 17 GHz and hard X-rays. Those nonthermal sources appeared on one edge of the western filament (F2) in the active region. The F2 began to be bright and rose upward rapidly, while the eastern filament (F1), which was extended to the quiet region, started to brighten from the peak time of the 17 GHz flux. In the second phase, the nonthermal emission weakened and the F2 rose up slowly, while the F1 began to rise up. In the third phase, two filaments erupted together. Since the F1 was stable for a long time in the quiet region, breaking the equilibrium state of the F1 would be decisive for the successful eruption of two filaments and it seems clear that the evolution of the F2 provoked the unstable F1. We suggest that tether-cutting reconnection between two overlapped filaments triggers the eruption of the two filaments as a tangled identity.

Keywords: Solar filament eruptions, Solar flares, Solar corona, Solar filaments
The Binary Black Hole Spin Distribution Likely Broadens with Redshift. The population-level distributions of the masses, spins, and redshifts of binary black holes (BBHs) observed using gravitational waves can shed light on how these systems form and evolve. Because of the complex astrophysical processes shaping the inferred BBH population, models allowing for correlations among these parameters will be necessary to fully characterize these sources. We hierarchically analyze the BBH population detected by LIGO and Virgo with a model allowing for correlations between the effective aligned spin and the primary mass and redshift. We find that the width of the effective spin distribution grows with redshift at 98.6% credibility. We determine this trend to be robust under the application of several alternative models and additionally verify that such a correlation is unlikely to be spuriously introduced using a simulated population. We discuss the possibility that this correlation could be due to a change in the natal black hole spin distribution with redshift.

Keywords: Gravitational wave astronomy, Gravitational wave sources, Astrophysical black holes
Electromagnetic Fireworks: Fast Radio Bursts from Rapid Reconnection in the Compressed Magnetar Wind. One scenario for the generation of fast radio bursts (FRBs) is magnetic reconnection in a current sheet of the magnetar wind. Compressed by a strong magnetic pulse induced by a magnetar flare, the current sheet fragments into a self-similar chain of magnetic islands. Time-dependent plasma currents at their interfaces produce coherent radiation during their hierarchical coalescence. We investigate this scenario using 2D radiative relativistic particle-in-cell simulations to compute the efficiency of the coherent emission and to obtain frequency scalings. Consistent with expectations, a fraction of the reconnected magnetic field energy, f ~ 0.002, is converted to packets of high-frequency fast magnetosonic waves, which can escape from the magnetar wind as radio emission. In agreement with analytical estimates, we find that magnetic pulses of 1047 erg s-1 can trigger relatively narrowband GHz emission with luminosities of approximately 1042 erg s-1, sufficient to explain bright extragalactic FRBs. The mechanism provides a natural explanation for a downward frequency drift of burst signals, as well as the ~100 ns substructure recently detected in FRB 20200120E .

Keywords: Magnetars, Magnetic fields, Plasma astrophysics, Radio transient sources
Remarkable Correspondence of the Sagittarius A* Submillimeter Variability with a Stellar-wind-fed Accretion Flow Model. We compare the 230 GHz near-horizon emission from Sagittarius A* to simulations representing three classes of accretion flows. Using the structure function to capture the variability statistics of the light curve, we find a noticeable discrepancy between the observations and models based on torus-fed accretion disks, whether those disks bring in a small or large amount of net magnetic flux. On the other hand, the simulations that are fed more realistically by stellar winds match the observed structure function very well. We describe the differences between models, arguing that feeding by stellar winds may be a critical component in constructing theoretical models for accretion in the Galactic Center.

Keywords: Accretion, Supermassive black holes, Magnetohydrodynamics, Galactic center, Stellar winds, Low-luminosity active galactic nuclei, General relativity, Astrophysical fluid dynamics, Astrophysical black holes, Magnetohydrodynamical simulations
Consistency of the Parkes Pulsar Timing Array Signal with a Nanohertz Gravitational-wave Background. Pulsar timing array experiments have recently reported strong evidence for a common-spectrum stochastic process with a strain spectral index consistent with that expected of a nanohertz-frequency gravitational-wave background, but with negligible yet non-zero evidence for spatial correlations required for a definitive detection. However, it was pointed out by the Parkes Pulsar Timing Array (PPTA) collaboration that the same models used in recent analyses resulted in strong evidence for a common-spectrum process in simulations where none is present. In this work, we introduce a methodology to distinguish pulsar power spectra with the same amplitude from noise power spectra of similar but distinct amplitudes. The former is the signature of a spatially uncorrelated pulsar term of a nanohertz gravitational-wave background, whereas the latter could represent ensemble pulsar noise properties. We test the methodology on simulated data sets. We find that the reported common process in PPTA pulsars is indeed consistent with the spectral feature of a pulsar term. We recommend this methodology as one of the validity tests that the real astrophysical and cosmological backgrounds should pass, as well as for inferences about the spatially uncorrelated component of the background.

Keywords: Supermassive black holes, Bayesian statistics, Pulsar timing method, Millisecond pulsars, High energy astrophysics, Importance sampling, Astronomy data analysis, Gravitational wave astronomy, Gravitational waves, Astronomical methods, Hierarchical models
No Evolution in the Half-mass Radius of Milky Way-type Galaxies over the Last 10 Gyr. The Milky Way (MW) galaxy is in focus, thanks to new observational data. Here we shed new light on the MW's past by studying the structural evolution of MW progenitors, which we identify from extragalactic surveys. Specifically, we constrain the stellar-mass growth history (SMGH) of the MW with two methods: (i) direct measurement of the MW's star formation history, and (ii) assuming the MW is a typical star-forming galaxy that remains on the star-forming main sequence. We select MW progenitors based on these two SMGHs at z = 0.2-2.0 from the CANDELS/3D-HST data. We estimate the structural parameters (including half-mass radius r 50 and Sérsic index) from the stellar-mass profiles. Our key finding is that the progenitors of the MW galaxy grow self-similarly on spatially resolved scales with roughly a constant half-mass radius (~2-3 kpc) over the past 10 Gyr, while their stellar masses increase by about 1 dex, implying little-to-no inside-out growth. We discover that the radius containing 20% of the stellar mass (r 20) decreases by 60% between redshifts of z = 2.0 and z = 0.7, while the central stellar-mass density (Σ1) increases by a factor of 1.3 dex over the same time and the Sérsic index changes as $n\propto {\left(1+z\right)}^{-1.41\pm 0.19}$ . This is consistent with an early (z &gt; 1) formation of a thick disk, followed by the formation of a bar that led to an increase in the mass in the core. The formation and evolution of the thin disk had only little impact on the overall half-mass size. We also show that the constant-size evolution of the MW progenitors challenges semiempirical approaches and numerical simulations.

Keywords: Galaxy formation, Milky Way formation, Galaxy structure, Galaxy evolution, Milky Way evolution, Milky Way Galaxy
3D Hydrodynamical Simulations of Helium-ignited Double-degenerate White Dwarf Mergers. The origins of Type Ia supernovae (SNe Ia) are still debated. Some of the leading scenarios involve a double detonation in double white dwarf (WD) systems. In these scenarios, helium shell detonation occurs on top of a carbon-oxygen (CO) WD, which then drives the detonation of the CO core, producing an SN Ia. Extensive studies have been done on the possibility of a double helium detonation, following a dynamical helium mass-transfer phase onto a CO-WD. However, 3D self-consistent modeling of the double-WD system, the mass transfer, and the helium shell detonation have been little studied. Here we use 3D hydrodynamical simulations to explore this case in which a helium detonation occurs near the point of Roche lobe overflow of the donor WD and may lead to an SN Ia through the dynamically driven double-degenerate double-detonation (D6) mechanism. We find that the helium layer of the accreting primary WD does undergo a detonation, while the underlying CO core does not, leading to an extremely rapid and faint nova-like transient instead of a luminous SN Ia event. This failed core detonation suggests that D6 SNe Ia may be restricted to the most massive CO primary WDs. We highlight the nucleosynthesis of the long-lived radioisotope 44Ti during explosive helium burning, which may serve as a hallmark both of successful as well as failed D6 events, which subsequently detonate as classical double-degenerate mergers.

Keywords: Hydrodynamics, Stellar mergers, White dwarf stars, Nuclear astrophysics, Type Ia supernovae
GB6 J2113+1121: A Multiwavelength Flaring γ-Ray Blazar Temporally and Spatially Coincident with the Neutrino Event IceCube-191001A. A radio-emitting tidal disruption event (AT2019dsg) is proposed as a likely counterpart to the IceCube neutrino event IC 191001A. In this work, we have revisited the Fermi-LAT data in the direction of the neutrino and confirmed no signal at the site of AT2019dsg. Instead, at the edge of the 90% confidence level error region of this neutrino, there is a γ-ray transient source associated with the blazar GB6 J2113+1121. In 2019 May, GB6 J2113+1121 was undergoing a γ-ray flare that is unprecedented since the start of the Fermi-LAT operation, with a variability amplitude of about 20 fold. Similar violent flares of GB6 J2113+1121, unobserved before, have also been detected in the optical bands. Moreover, the blazar remained in a high-flux state in the infrared bands when IC 191001A arrived, though the blazar 's γ-ray and optical activities have temporarily ceased. Motivated by this spatial and temporal coincidence, we suggest that GB6 J2113+1121 is a candidate to be the counterpart to IC 191001A. The jet properties of GB6 J2113+1121 are investigated, which are found to be comparable with that of neutrino-emitting blazars (candidates). A specific analysis of archival IceCube data in this direction and future observations would put further constraints on the origin of the neutrino.

Keywords: Radio loud quasars, Gamma-ray sources, Galaxy jets
Discrepancy between the Low-frequency Cutoffs of Type III Radio Bursts Based on Simultaneous Observations by WIND and PSP. The cutoff frequency is an important characteristic parameter of type III radio bursts. Employing the radio data of the Parker Solar Probe (PSP) in the encounter phases of its first five orbits, our previous work revealed that the maximum probability distribution of the cutoff frequency f  lo  (~680 kHz) is remarkably higher than that based on Ulysses and WIND (~100 kHz) investigated by Leblanc et al. and Dulk et al. However, the main influencing factor of the discrepancy is still unknown though the possible reasons are discussed. In this study, we utilize the simultaneous observation by WIND and PSP to analyze statistically the distribution of the cutoff frequency of type III radio bursts, which had not been done before. Based on the automatic Canny edge detection and manual selection, we obtain the f  lo  of 491 (WIND) and 1194 (PSP) type III bursts from their simultaneous observations in the same solar activity period (from 2019 January 1 to 2020 July 31). The statistical results show that the dominant cutoff frequency measured by PSP (i.e., ~700 kHz) is still significantly higher than that by WIND (i.e., ~100 kHz). This implies that radiation attenuation is the main influencing factor for the difference in the statistical results of the cutoff frequency.

Keywords: Galaxy dynamics, Interplanetary physics
Possible Implications of Relatively High Levels of Initial <SUP>60</SUP>Fe in Iron Meteorites for the Noncarbonaceous-Carbonaceous Meteorite Dichotomy and Solar Nebula Formation. Cook et al. found that iron meteorites have an initial abundance ratio of the short-lived isotope 60Fe to the stable isotope 56Fe of 60Fe/56Fe ~ (6.4 ± 2.0) × 10-7. This appears to require the injection of live 60Fe from a Type II supernova (SN II) into the presolar molecular cloud core, as the observed ratio is over a factor of 10 times higher than would be expected to be found in the ambient interstellar medium (ISM) as a result of galactic chemical evolution. The supernova triggering and injection scenario offers a ready explanation for an elevated initial 60Fe level, and in addition provides a physical mechanism for explaining the noncarbonaceous-carbonaceous (NC-CC) dichotomy of meteorites. The NC-CC scenario hypothesizes the solar nebula first accreted material that was enriched in supernova-derived nuclides, and then later accreted material depleted in supernova-derived nuclides. While the NC-CC dichotomy refers to stable nuclides, not short-lived isotopes like 60Fe, the SN II triggering hypothesis provides an explanation for the otherwise unexplained change in nuclides being accreted by the solar nebula. Three-dimensional hydrodynamical models of SN II shock-triggered collapse show that after triggering collapse of the presolar cloud core, the shock front sweeps away the local ISM while accelerating the resulting protostar/disk to a speed of several kilometers per second, sufficient for the protostar/disk system to encounter within ~1 Myr the more distant regions of a giant molecular cloud complex that might be expected to have a depleted inventory of supernova-derived nuclides.

Keywords: Hydrodynamics, Supernova remnants, Star formation, Protoplanetary disks, Planet formation