Sazuna/UAT_keywords huggingface dataset:
Supermassive Black Holes as Possible Sources of Ultrahigh-energy Cosmic Rays. The production and acceleration mechanisms of ultrahigh-energy cosmic rays (UHECRs) of energy &gt;1020 eV, clearly beyond the GZK cutoff limit, remain unclear, which points to the exotic nature of the phenomena. Recent observations of extragalactic neutrinos may indicate that the source of UHECRs is an extragalactic supermassive black hole (SMBH). We demonstrate that ultraefficient energy extraction from a rotating SMBH driven by the magnetic Penrose process (MPP) could indeed fit the bill. We envision ionization of neutral particles, such as neutron beta decay, skirting close to the black hole horizon that energizes protons to over 1020 eV for an SMBH of mass 109M⊙ and magnetic field 104 G. Applied to the Galactic center SMBH, we have a proton energy of order ≈1015.6 eV that coincides with the knee of the cosmic-ray spectra. We show that large γz factors of high-energy particles along the escaping directions occur only in the presence of an induced charge of the black hole, which is known as the Wald charge in the case of a uniform magnetic field. It is remarkable that the process requires neither an extended acceleration zone nor fine-tuning of accreting-matter parameters. Further, this leads to certain verifiable constraints on the SMBH's mass and magnetic field strength as the source of UHECRs. This clearly makes the ultraefficient regime of the MPP one of the most promising mechanisms for fueling the UHECR powerhouse.

Keywords: Black hole physics, High energy astrophysics
A Kinematic View of NGC 1261: Structural Parameters, Internal Dispersion, Absolute Proper Motion, and Blue Straggler Stars. We constructed a Hubble Space Telescope (HST) astrophotometric catalog of the central region of the Galactic globular cluster NGC 1261. This catalog, complemented with Gaia DR2 data sampling the external regions, has been used to estimate the structural parameters of the system (I.e., core, half-mass, tidal radii, and concentration) from its resolved star density profile. We computed high-precision proper motions thanks to multi-epoch HST data and derived the cluster velocity dispersion profile in the plane of the sky for the innermost region, finding that the system is isotropic. The combination with the line-of-sight information collected from spectroscopy in the external regions provided us with the cluster velocity dispersion profile along the entire radial extension. We also measured the absolute proper motion of NGC 1261 using a few background galaxies as a reference. The radial distribution of the Blue Straggler Star population shows that the cluster is in a low/intermediate phase of dynamical evolution.

Keywords: Globular star clusters, Stellar photometry, Stellar kinematics, Stellar dynamics, Stellar astronomy, Proper motions, Photometry, Star clusters
Gamma-Ray Burst Afterglows: Time-varying Extinction, Polarization, and Colors due to Rotational Disruption of Dust Grains. Prompt optical emission of gamma-ray bursts (GRBs) is known to have important effects on the surrounding environment. In this paper, we study rotational disruption and alignment of dust grains by radiative torques (RATs) induced by GRB afterglows and predict their signatures on the observational properties. We first show that large grains (size &gt;0.1 μm) within a distance d &lt; 40 pc from the source can be disrupted into smaller grains by the RAdiative Torque Disruption (RATD) mechanism. We then model the extinction curve of GRB afterglows and find that optical-near-infrared extinction decreases, and ultraviolet (UV) extinction increases due to the enhancement of small grains. The total-to-selective visual extinction ratio, RV, is found to decrease from the standard value of ∼3.1 to ∼1.5 after disruption time tdisr ≲ 104 s. Next, we study grain alignment by RATs induced by GRB afterglows and model the wavelength-dependence polarization produced by grains aligned with magnetic fields. We find that optical-NIR polarization degree first increases due to enhanced alignment of small grains and then decreases when RATD begins. The maximum polarization wavelength, λmax, decreases rapidly from the standard value of ∼0.55 μm to ∼0.15 μm over alignment time of talign ≲ 30 s due to enhanced alignment of small grains. Our theoretical predictions can explain various observational properties of GRB afterglows, including steep extinction curves, time-variability of colors, and optical rebrightening of GRB afterglows.

Keywords: Interstellar dust, Gamma-ray bursts, Light curves, Interstellar dust extinction, Starlight polarization, Supernovae
The Angular Momentum of the Circumgalactic Medium in the TNG100 Simulation. We present an analysis of the angular momentum content of the circumgalactic medium (CGM) using TNG100, one of the flagship runs of the IllustrisTNG project. We focus on Milky Way-mass halos (∼1012 M⊙) at z = 0 but also analyze other masses and redshifts up to z = 5. We find that the CGM angular momentum properties are strongly correlated with the stellar angular momentum of the corresponding galaxy: the CGM surrounding high-angular momentum galaxies has a systematically higher angular momentum and is better aligned to the rotational axis of the galaxy itself than the CGM surrounding low-angular momentum galaxies. Both the hot and cold phases of the CGM show this dichotomy, though it is stronger for colder gas. The CGM of high-angular momentum galaxies is characterized by a large wedge of cold gas with rotational velocities at least ∼1/2 of the halo's virial velocity, extending out to ∼1/2 of the virial radius, and by biconical polar regions dominated by radial velocities suggestive of galactic fountains; both of these features are absent from the CGM of low-angular momentum galaxies. These conclusions are general to halo masses ≲1012 M⊙ and for z ≲ 2, but they do not apply for more massive halos or at the highest redshift studied. By comparing simulations run with alterations to the fiducial feedback model, we identify the better alignment of the CGM to high-angular momentum galaxies as a feedback-independent effect and the galactic winds as a dominant influence on the CGM's angular momentum.

Keywords: Hydrodynamical simulations, Galaxy formation, Galaxy structure, Galaxy dynamics, Galaxy kinematics, Circumgalactic medium
Electric Current Evolution at the Footpoints of Solar Eruptions. Electric currents play a critical role in the triggering of solar flares and their evolution. The aim of the present paper is to test whether the surface electric current has a surface or subsurface fixed source as predicted by the circuit approach of flare physics, or is the response of the surface magnetic field to the evolution of the coronal magnetic field as the MHD approach proposes? Out of all 19 X-class flares observed by SDO from 2011 to 2016 near the disk center, we analyzed the only nine eruptive flares for which clear ribbon hooks were identifiable. Flare ribbons with hooks are considered to be the footprints of eruptive flux ropes in MHD flare models. For the first time, fine measurements of the time evolution of electric currents inside the hooks in the observations as well as in the OHM 3D MHD simulation are performed. Our analysis shows a decrease of the electric current in the area surrounded by the ribbon hooks during and after the eruption. We interpret the decrease of the electric currents as due to the expansion of the flux rope in the corona during the eruption. Our analysis brings a new contribution to the standard flare model in 3D.

Keywords: Observational astronomy, Magnetohydrodynamics, Solar flares, Computational methods, Solar coronal mass ejections, Solar magnetic fields
Oceanographic Considerations for Exoplanet Life Detection. Liquid water oceans are at the center of our search for life on exoplanets because water is a strict requirement for life as we know it. However, oceans are dynamic habitats—and some oceans may be better hosts for life than others. In Earth's ocean, circulation transports essential nutrients such as phosphate and is a first-order control on the distribution and productivity of life. Of particular importance is upward flow from the dark depths of the ocean in response to wind-driven divergence in surface layers. This "upwelling" returns essential nutrients that tend to accumulate at depth via sinking of organic particulates back to the sunlit regions where photosynthetic life thrives. Ocean dynamics are likely to impose constraints on the activity and atmospheric expression of photosynthetic life in exo-oceans as well, but we lack an understanding of how ocean dynamics may differ on other planets. We address this issue by exploring the sensitivity of ocean dynamics to a suite of planetary parameters using ROCKE-3D, a fully coupled ocean-atmosphere general circulation model. Our results suggest that planets that rotate slower and have higher surface pressure than Earth may be the most attractive targets for remote life detection because upwelling is enhanced under these conditions, resulting in greater nutrient supply to the surface biosphere. Seasonal deepening of the mixed layer on high-obliquity planets may also enhance nutrient replenishment from depth into the surface mixed layer. Efficient nutrient recycling favors greater biological activity, more biosignature production, and thus more detectable life. More generally, our results demonstrate the importance of considering oceanographic phenomena for exoplanet life detection and motivate future interdisciplinary contributions to the emerging field of exo-oceanography.

Keywords: Ocean-atmosphere interactions, Astrobiology, Exoplanets
Studying the Local Magnetic Field and Anisotropy of Magnetic Turbulence by Synchrotron Polarization Derivative. Due to the inevitable accumulation of observational information in the direction of the line of sight, it is difficult to measure the local magnetic field of magnetohydrodynamic (MHD) turbulence. However, a correct understanding of the local magnetic field is a prerequisite for reconstructing the Galactic 3D magnetic field. We study how to reveal the local magnetic field direction and the eddy anisotropy on the basis of the statistics of synchrotron polarization derivative with respect to the squared wavelength dP/dλ2. In the low-frequency and strong Faraday rotation regime, we implement numerical simulations in the combination of multiple statistic techniques, such as structure function, quadrupole ratio modulus, spectral correlation function, correlation function anisotropy, and spatial gradient techniques. We find that (1) statistic analysis of dP/dλ2 indeed reveals the anisotropy of underlying MHD turbulence, the degree of which increases with the increase of the radiation frequency; and (2) the synergy of both correlation function anisotropy and gradient calculation of dP/dλ2 enables the measurement of the local magnetic field direction.

Keywords: Magnetohydrodynamics, Interplanetary shocks, Interstellar plasma, Alfven waves, Interstellar synchrotron emission
ALMA Observations of the Molecular Clouds in NGC 625. We present the highest-resolution (1″) 12CO observations of molecular gas in the dwarf starburst galaxy NGC 625 to date, obtained with Atacama Large Millimeter/submillimeter Array. The molecular gas, which is distributed in discrete clouds within an area of 0.4 kpc2, does not have well-ordered large-scale motions. We measure a molecular mass in NGC 625 of 5.3 × 106 ${M}_{\odot }$ , assuming a Milky Way CO-to- ${{\rm{H}}}_{2}$ conversion factor. We use the CPROPS package to identify molecular clouds and measure their properties. The 19 resolved CO clouds have a median radius of 20 pc, a median linewidth 2.5 $\mathrm{km}\,{{\rm{s}}}^{-1}$ , and a median surface density of 169 ${M}_{\odot }\,{{\rm{pc}}}^{-2}$ . Larson scaling relations suggest that molecular clouds in NGC 625 are mostly in virial equilibrium. Comparison of our high-resolution CO observations with a star formation rate map, inferred from ancillary optical observations, suggests that about 40% of the molecular clouds coincide with the brightest H II regions. These bright H II regions have a range of molecular gas depletion timescales, all within a factor of ∼3 of the global depletion time in NGC 625 of 106-134 Myr. The highest surface density molecular clouds toward the southwest of the galaxy, in a region we call the Butterfly, do not show strong star formation activity and suggest a depletion timescale longer than 5 Gyr.

Keywords: Interstellar molecules, Molecular gas, Starburst galaxies, Dwarf galaxies, Molecular clouds, Giant molecular clouds, Star formation, Interstellar medium, Submillimeter astronomy, Metallicity, Galaxies, Interferometry
Observational Features of Exoplanetary Synchrotron Radio Bursts. The magnetic fields of exoplanets shield the planets from cosmic rays and interplanetary plasma. Due to the interaction with the electrons from their host stars, the exoplanetary magnetospheres are predicted to have both cyclotron and synchrotron radio emissions, neither of which have been definitively identified in observations. As the coherent cyclotron emission has been extensively studied in the literature, here we focus on planetary synchrotron radiation with bursty behaviors (I.e., radio flares) caused by the outbreaks of energetic electron ejections from the host star. Two key parameters of the bursty synchrotron emissions, namely the flux density and burst rate, and two key features, namely the burst light curve and frequency shift, are predicted for star-hot Jupiter systems. The planetary orbital phase-burst rate relation is also considered as the signature of star-planet interactions. As examples, previous X-ray and radio observations of two well-studied candidate systems, HD 189733 and V830 τ, are adopted to predict their specific burst rates and fluxes of bursty synchrotron emissions for further observational confirmations. The detectability of such emissions by current and upcoming radio telescopes shows that we are at the dawn of discoveries.

Keywords: Magnetospheric radio emissions, Exoplanets, Radio bursts, Stellar flares, Magnetic fields
Two Substructures in the nearby Stellar Halo Found in Gaia and RAVE. We use the second Gaia data release (Gaia DR2), combined with Radial Velocity Experiment spectroscopic surveys, to identify the substructures in the nearby stellar halo. We select 3845 halo stars kinematically and chemically and determine their density distribution in energy and angular momentum space. To select the substructures from overdensities, we reshuffle the velocities and estimate their significance. Two statistically significant substructures, GR-1 and GR-2, are identified. GR-1 has a high binding energy and small z-angular momentum. GR-2 is metal-rich but retrograde. They are both new substructures, and may be accretion debris of dwarf galaxies.

Keywords: Galaxy formation, Galaxy dynamics, Solar neighborhood, Galaxy stellar halos, Galaxy abundances, Galaxy kinematics
Dynamical Properties of Molecular-forming Gas Clumps in Galaxies at the Epoch of Reionization. We study the properties of molecular-forming gas clumps (MGCs) at the epoch of reionization using cosmological zoom-in simulations. We identify MGCs in a $z\simeq 6$ prototypical galaxy ("Althæa") using an H2 density-based clump finder. We compare their mass, size, velocity dispersion, gas surface density, and virial parameter ( ${\alpha }_{\mathrm{vir}}$ ) to observations. In Althæa, the typical MGC mass and size are ${M}_{\mathrm{gas}}\simeq {10}^{6.5}$ M⊙ and $R\simeq 45\mbox{--}100$ pc, which are comparable to those found in nearby spirals and starburst galaxies. MGCs are highly supersonic and supported by turbulence, with rms velocity dispersions of ${\sigma }_{\mathrm{gas}}\,\simeq $ 20-100 km s-1 and pressure of $P/{{\rm{K}}}_{B}\simeq {10}^{7.6}\,{\rm{K}}\,$ ${\mathrm{cm}}^{-3}$ (I.e., $\gt 1000\times $ with respect to the Milky Way), similar to those found in nearby and z ∼ 2 gas-rich starburst galaxies. In addition, we perform stability analysis to understand the origin and dynamical properties of MGCs. We find that MGCs are globally stable in the main disk of Althæa. Densest regions where star formation is expected to take place in clouds and cores on even smaller scales instead have lower ${\alpha }_{\mathrm{vir}}$ and Toomre Q values. Detailed studies of the star-forming gas dynamics at the epoch of reionization thus require a spatial resolution of ≲40 pc (≃ $0\buildrel{\prime\prime}\over{.} 01$ ), which is within reach with the Atacama Large (sub-)Millimeter Array and the Next Generation Very Large Array.

Keywords: Galaxy formation, Galaxy properties, Molecular clouds, Interstellar medium, Lyman-break galaxies, High-redshift galaxies
A Magnetic White Dwarf Accretion Model for the Anomalous X-Ray Pulsar 4U 0142+61. The quiescent emission of the anomalous X-ray pulsar (AXP) 4U 0142+61 extends over a broad range of energy, from radio up to hard X-rays. In particular, this object is unique among soft gamma-ray repeaters (SGRs) and AXPs in presenting simultaneously mid-infrared emission and pulsed optical emission. In spite of the many propositions to explain this wide range of emission, it still lacks one that reproduces all of the observations. Filling this gap, we present a model to reproduce the quiescent spectral energy distribution of 4U 0142+61 from mid-infrared up to hard X-rays using plausible physical components and parameters. We propose that the persistent emission comes from a magnetic accreting white dwarf (WD) surrounded by a debris disk. This model assumes that (I) the hard X-rays are due to the bremsstrahlung emission from the postshock region of the accretion column, (II) the soft X-rays are originated by hot spots on the WD surface, and (III) the optical and infrared emissions are caused by an optically thick dusty disk, the WD photosphere, and the tail of the postshock region emission. In this scenario, the fitted model parameters indicate that 4U 0142+61 harbors a fast-rotator magnetic near-Chandrasekhar WD, which is very hot and hence young. Such a WD can be the recent outcome of a merger of two less massive WDs. In this case, 4U 0142+61 can evolve into a supernova Ia and hence give hints of the origin of these important astrophysical events. Additionally, we also present a new estimate of 4U 0142+61's distance, ${3.78}_{-0.18}^{+0.12}$ kpc, based on the measured hydrogen column density and new interstellar extinction 3D maps.

Keywords: Magnetic stars, White dwarf stars, Magnetic fields, Pulsars, Chandrasekhar limit, Soft gamma-ray repeaters, Young disk Cepheid variable stars, Stellar accretion disks, Magnetars, Optical pulsars, Debris disks
Radiative Transfer Modeling of EC 53: An Episodically Accreting Class I Young Stellar Object. In the episodic accretion scenario, a large fraction of the protostellar mass accretes during repeated and large bursts of accretion. Since outbursts on protostars are typically identified at specific wavelengths, interpreting these outbursts requires converting this change in flux to a change in total luminosity. The Class I young stellar object EC 53 in the Serpens Main cloud has undergone repeated increases in brightness at 850 μm that are likely caused by bursts of accretion. In this study, we perform two- and three-dimensional continuum radiative transfer modeling to quantify the internal luminosity rise in EC 53 that corresponds to the factor of ∼1.5 enhancement in flux at 850 μm. We model the spectral energy distribution and radial intensity profile in both the quiescent and outburst phases. The internal luminosity in the outburst phase is ∼3.3 times brighter than the luminosity in the quiescent phase. The radial intensity profile analysis demonstrates that the detected submillimeter flux variation of EC 53 comes from the heated envelope by the accretion burst. We also find that the role of external heating of the EC 53 envelope by the interstellar radiation field is insignificant.

Keywords: Infrared astronomy, Astronomy data modeling, Star formation, Radiative transfer
Tidal Deformations of Hybrid Stars with Sharp Phase Transitions and Elastic Crusts. Gravitational wave astronomy is expected to provide independent constraints on neutron-star properties, such as their equation of state. This is possible with the measurements of binary components' tidal deformability, which alter the point-particle gravitational waveforms of neutron-star binaries. Here, we provide a first study of the tidal deformability effects due to the elasticity/solidity of the crust (hadronic phase) in a hybrid neutron star, as well as the influence of a quark-hadronic phase density jump on tidal deformations. We employ the framework of non-radial perturbations with zero frequency and study hadronic phases presenting elastic aspects when perturbed (with the shear modulus approximately 1% of the pressure). We find that the relative tidal deformation change in a hybrid star with a perfect-fluid quark phase and a hadronic phase presenting an elastic part is never larger than about 2%-4% (with respect to a perfect-fluid counterpart). These maximum changes occur when the elastic region of a hybrid star is larger than approximately 60% of the star's radius, which may happen when its quark phase is small and the density jump is large enough, or even when a hybrid star has an elastic mixed phase. For other cases, tidal deformation changes due to an elastic crust are negligible ( ${10}^{-5}\mbox{--}{10}^{-1} \% $ ) and, therefore, unlikely to be measured even with third generation detectors. Thus, only when the size of the elastic hadronic region of a hybrid star is over half of its radius, could the effects of elasticity have a noticeable impact on tidal deformations.

Keywords: Neutron stars, General relativity, Gravitational waves
Pre-common-envelope Mass Loss from Coalescing Binary Systems. Binary systems undergoing unstable Roche Lobe overflow spill gas into their circumbinary environment as their orbits decay toward coalescence. In this paper, we use a suite of hydrodynamic models of coalescing binaries involving an extended donor and a more compact accretor. We focus on the period of unstable Roche Lobe overflow that ends as the accretor plunges within the envelope of the donor at the onset of a common envelope phase. During this stage, mass is removed from the donor and flung into the circumbinary environment. Across a wide range of binary mass ratios, we find that the mass expelled as the separation decreases from the Roche limit to the donor's original radius is of the order of 25% of the accretor's mass. We study the kinematics of this ejecta and its dependencies on binary properties and find that it assembles into a toroidal circumbinary distribution. These circumbinary tori have approximately constant specific angular momenta due to momentum transport by spiral shocks launched from the orbiting binary. We show that an analytic model with these torus properties captures many of the main features of the azimuthally averaged profiles of our hydrodynamic simulations. Our results, in particular the simple relationship between accretor mass and expelled mass and its spatial distribution, may be useful for interpreting stellar coalescence transients like luminous red novae and initializing hydrodynamic simulations of the subsequent common envelope phase.

Keywords: Hydrodynamical simulations, Common envelope evolution, Interacting binary stars