Sazuna/UAT_keywords huggingface dataset:
Archival VLBA Observations of the Cygnus A Nuclear Radio Transient (Cyg A-2) Strengthen the Tidal Disruption Event Interpretation. We have analyzed archival Very Long Baseline Array (VLBA) data for Cygnus A between 2002 and 2013, to search for radio emission from the transient discovered in 2015 by Perley et al. approximately 0"4 from the nucleus of Cygnus A (Cyg A-2). Perley et al. use Very Large Array and VLBA archival data (between 1989 and 1997) to show that the transient rises in flux density by a factor of at least 5 in less than approximately 20 yr. With the additional data presented here, we revise the rise time to between approximately 4 and 6 yr, based on a new detection of the source at 15.4 GHz from 2011 October. Our results strengthen the interpretation of Cyg A-2 as the result of a tidal disruption event (TDE), as we can identify the location of the compact object responsible for the TDE and can estimate the angular expansion speed of the resulting radio-emitting structures, equivalent to an apparent expansion speed of &lt;0.9c. While our results are consistent with recent X-ray analyses, we can rule out a previously suggested date of early 2013 for the timing of the TDE. We favor a timing between early 2009 and late 2011. Applying the model of Nakar &amp; Piran, we suggest a TDE causing a mildly relativistic outflow with a (density-dependent) total energy &gt;1049 erg. Due to the improved temporal coverage of our archival measurements, we find that it is unlikely that Cyg A-2 has previously been in a high luminosity radio state over the last 30 yr.

Keywords: Extragalactic radio sources, Active galaxies, Experimental techniques
A Helium-flash-induced Mixing Event Can Explain the Lithium Abundances of Red Clump Stars. Observations demonstrate that the surface abundance of ${}^{7}\mathrm{Li}$ in low-mass stars changes dramatically between the tip of the red giant branch and the red clump. This naturally suggests an association with the helium core flash, which occurs between these two stages. Using stellar evolution models and a simple, ad hoc mixing prescription, we demonstrate that the ${}^{7}\mathrm{Li}$ enhancement can be explained by a brief chemical mixing event that occurs at the time of the first, strongest He subflash. The amount of ${}^{7}\mathrm{Be}$ already present above the H-burning shell just before the flash, once it mixes into the cooler envelope and undergoes an electron capture converting it to ${}^{7}\mathrm{Li}$ , is sufficient to explain the observed abundance at the red clump. We suggest that the excitation of internal gravity waves by the vigorous turbulent convection during the flash may provide a physical mechanism that can induce such mixing.

Keywords: Red giant clump, Stellar abundances, Stellar evolution
Limits on Precursor and Afterglow Radio Emission from a Fast Radio Burst in a Star-forming Galaxy. We present a new fast radio burst (FRB) at 920 MHz discovered during commensal observations conducted with the Australian Square Kilometre Array Pathfinder (ASKAP) as part of the Commensal Real-time ASKAP Fast Transients (CRAFT) survey. FRB 191001 was detected at a dispersion measure (DM) of 506.92(4) pc cm-3 and its measured fluence of 143(15) Jy ms is the highest of the bursts localized to host galaxies by ASKAP to date. The subarcsecond localization of the FRB provided by ASKAP reveals that the burst originated in the outskirts of a highly star-forming spiral in a galaxy pair at redshift z = 0.2340(1). Radio observations show no evidence for a compact persistent radio source associated with the FRB 191001 above a flux density of 15 μJy. However, we detect diffuse synchrotron radio emission from the disk of the host galaxy that we ascribe to ongoing star formation. FRB 191001 was also detected as an image-plane transient in a single 10 s snapshot with a flux density of 19.3 mJy in the low-time-resolution visibilities obtained simultaneously with CRAFT data. The commensal observation facilitated a search for repeating and slowly varying radio emissions 8 hr before and 1 hr after the burst. We found no variable radio emission on timescales ranging from 1 ms to 1.4 hr. We report our upper limits and briefly review FRB progenitor theories in the literature that predict radio afterglows. Our data are still only weakly constraining of any afterglows at the redshift of the FRB. Future commensal observations of more nearby and bright FRBs will potentially provide stronger constraints.

Keywords: Radio continuum emission, Starburst galaxies, Interferometers, Polarimetry
Do All Interplanetary Coronal Mass Ejections Have a Magnetic Flux Rope Structure Near 1 au?. Interplanetary coronal mass ejections (ICMEs) often consist of a shock wave, sheath region, and ejecta region. The ejecta regions are divided into two broad classes: magnetic clouds (MCs) that exhibit the characteristics of magnetic flux ropes, and non-magnetic clouds (NMCs) that do not. As CMEs result from eruption of magnetic flux ropes, it is important to answer why NMCs do not have the flux rope features. One claims that NMCs lose their original flux rope features due to the interactions between ICMEs or ICMEs and other large-scale structures during their transit in the heliosphere. The other attributes this phenomenon to the geometric selection effect; i.e., when an ICME has its nose (flank, including leg and non-leg flanks) pass through the observing spacecraft, the MC (NMC) features will be detected along the spacecraft trajectory within the ejecta. In this Letter, we examine which explanation is more reasonable through the geometric properties of ICMEs. If the selection effect leads to different ejecta types, MCs should have narrower sheath region compared to NMCs from the statistical point of view, which is confirmed by our statistics. In addition, we find that NMCs have similar sizes in solar cycles 23 and 24, and NMCs are smaller than MCs in cycle 23 but larger than MCs in cycle 24. This suggests that most NMCs have their leg flank pass through the spacecraft. Our geometric analyses support that all ICMEs should have a magnetic flux rope structure near 1 au.

Keywords: Solar coronal mass ejection shocks, Solar storm, Solar coronal mass ejections
Fast Magnetic Reconnection with Turbulence in High Lundquist Number Limit. We use extensive 3D resistive MHD simulations to study how large-scale current sheets will undergo fast reconnection in the high Lundquist number S limit (above ∼104), when the system is subject to different externally driven turbulence levels and the self-generated turbulence produced by 3D reconnection dynamics. We find that the normalized global reconnection rate ∼0.01-0.13 is weakly dependent on S. Global reconnection with the classic inflow/outflow configurations is observed, and 3D flux ropes are hierarchically formed and ejected from reconnection regions. A statistical separation of the reconnected magnetic field lines follows a superdiffusive behavior, from which the rate is measured to be very similar to that obtained from the mixing of tracer populations. We find that the reconnection rate scales roughly linearly with the turbulence level during the peak of reconnection. This scaling is consistent with the turbulence properties produced by both the externally driven and self-generation processes. These results imply that large-scale thin current sheets tend to undergo rigorous reconnection.

Keywords: Interplanetary turbulence, Solar corona, Computational methods, Solar magnetic reconnection
The Downwind Solar Wind: Model Comparison with Pioneer 10 Observations. The solar wind in the upwind region has been well modeled using a pickup ion (PUI) mediated MHD model (Zank et al.). It suggests that PUIs have an important role in heating the solar wind in the outer heliosphere. However, the solar wind in the downwind region is not as well understood. Here, we compare the Zank et al. model with Pioneer 10 observations, which allows us to investigate the downwind solar wind out to 60 au. We use a model in which the hydrogen temperature is finite to obtain a proper hydrogen number density distribution in the downwind region and incorporate it into the model. Our results explain Pioneer 10 observations well and indicate that the heating due to PUIs is less effective than in the upwind region since the density of PUIs in the downwind region is less than the upwind PUIs density. We also derive parameters at several possible locations of the downwind termination shock.

Keywords: Pickup ions, Solar physics, Solar wind, Magnetohydrodynamics
On the Maximum Stellar Rotation to form a Black Hole without an Accompanying Luminous Transient. The collapse of a massive star with low angular momentum content is commonly thought to result in the formation of a black hole without an accompanying bright transient. Our goal in this Letter is to understand the flow in and around a newly formed black hole, involving accretion and rotation, via general relativistic hydrodynamics simulations aimed at studying the conditions under which infalling material can accrete without forming a centrifugally supported structure and, as a result, generate no effective feedback. On the other hand, if the feedback from the black hole is significant, the collapse would be halted and we suggest that the event is likely to be followed by a bright transient. We find that feedback is only efficient if the specific angular momentum of the infalling material at the innermost stable circular orbit exceeds that of geodesic circular flow at that radius by at least ≈20%. We use the results of our simulations to constrain the maximal stellar rotation rates of the disappearing massive progenitors PHL293B-LBV and N6946-BH1, and to provide an estimate of the overall rate of disappearing massive stars. We find that about a few percent of single O-type stars with measured rotational velocities are expected to spin below the critical value before collapse and are thus predicted to vanish without a trace.

Keywords: Accretion, Hydrodynamical simulations, Massive stars, General relativity, Relativistic disks
OGLE-GAL-ACEP-091: The First Known Multi-mode Anomalous Cepheid. Anomalous Cepheids (ACs) are metal-deficient, core-helium-burning pulsating stars with masses in the range 1.2-2.2 ${M}_{\odot }$ . Until recently, all known ACs were pure single-mode pulsators. The first candidate for an AC pulsating in more than one radial mode—OGLE-GAL-ACEP-091—was recently identified in the Milky Way based on the photometric database of the Optical Gravitational Lensing Experiment (OGLE) survey. We analyze this object showing that it is actually a triple-mode pulsator. Its position in the Petersen diagram, the light-curve morphology quantified by Fourier coefficients, and absolute magnitudes derived from the Gaia parallax are consistent with the assumption that OGLE-GAL-ACEP-091 is an AC. Our grid of linear pulsation models indicates that OGLE-GAL-ACEP-091 is a 1.8 ${M}_{\odot }$ star with a metallicity of about [Fe/H] = -0.5 dex.

Keywords: Multi-periodic pulsation, Pulsating variable stars, Anomalous Cepheid variable stars
Constraining the Jet Launching Time of GRB 170817A by Utilizing the Baryon Loading. The observed delay of GRB 170817A relative to GW170817 provides significant information about gamma-ray burst (GRB) physics, and is subject to intense debate. In this Letter, we present an approach for discussing the major source of this time delay. First, we use the structured jet model to fit the X-ray/optical/radio afterglows of GRB 170817A together with superluminal motion measured using Very Long Baseline Interferometry. Our structured jet is modeled with angle-dependent energy and baryon loading. It is found that our model well fits the afterglows of GRB 170817A. Then, the baryon loading in the jet is inferred based on our fitting results. By comparing the baryon loading to the mass outflow in different stages, we infer that the time lag of the jet launch relative to the merger is less than hundreds or tens of milliseconds. It suggests that the time delay of GRB 170817A relative to GW170817 is defined mostly by the spreading time of the jet propagating to its dissipation radius.

Keywords: Gamma-ray bursts, Relativistic jets, Gravitational waves, High energy astrophysics
Physical Conditions and Particle Acceleration in the Kiloparsec Jet of Centaurus A. The nonthermal emission from the kiloparsec-scale jet of Centaurus A exhibits two notable features, bright diffuse emission and many compact knots, which have been intensively studied in X-ray and radio observations. H.E.S.S. recently reported that the very-high-energy gamma-ray emission from this object is extended along the jet direction beyond a kiloparsec from the core. Here, we combine these observations to constrain the physical conditions of the kiloparsec jet and study the origin of the nonthermal emission. We show that the diffuse jet is weakly magnetized ( ${\eta }_{{\rm{B}}}\sim {10}^{-2}$ ) and energetically dominated by thermal particles. We also show that knots are the sites of both amplified magnetic field and particle (re)acceleration. To keep sufficient energy in thermal particles, the magnetic and nonthermal particle energy in the knot regions is tightly constrained. The most plausible condition is an energy equipartition between them, ${\eta }_{{\rm{B}}}\sim {\eta }_{e}\sim 0.1$ . Such weak magnetic energy implies that particles in the knots are in the slow cooling regime. We suggest that the entire kiloparsec-scale diffuse emission could be powered by particles that are accelerated at and escaped from knots.

Keywords: Jets, High energy astrophysics, Active galactic nuclei, Gamma-rays, Cosmic rays
Revealing the Formation of the Milky Way Nuclear Star Cluster via Chemo-dynamical Modeling. The Milky Way nuclear star cluster (MW NSC) has been used as a template to understand the origin and evolution of galactic nuclei and the interaction of nuclear star clusters with supermassive black holes. It is the only nuclear star cluster with a supermassive black hole where we can resolve individual stars to measure their kinematics and metal abundance to reconstruct its formation history. Here, we present results of the first chemo-dynamical model of the inner 1 pc of the MW NSC using metallicity and radial velocity data from the KMOS spectrograph on the Very Large Telescope. We find evidence for two kinematically and chemically distinct components in this region. The majority of the stars belong to a previously known supersolar metallicity component with a rotation axis perpendicular to the Galactic plane. However, we identify a new kinematically distinct subsolar metallicity component that contains about 7% of the stars and appears to be rotating faster than the main component with a rotation axis that may be misaligned. This second component may be evidence for an infalling star cluster or remnants of a dwarf galaxy, merging with the MW NSC. These measurements show that the combination of chemical abundances with kinematics is a promising method to directly study the MW NSC's origin and evolution.

Keywords: Supermassive black holes, Galactic center, Interstellar dynamics, Evolved stars, Galaxy structure, Stellar dynamics, Galaxy nuclei, Milky Way Galaxy physics, Active galactic nuclei, Metallicity, Star clusters
On the Origin of a Rotating Metal-poor Stellar Population in the Milky Way Nuclear Cluster. We explore the origin of a population of stars recently detected in the inner parsec of the Milky Way (MW) nuclear cluster, which exhibits subsolar metallicity and a higher rotation compared to the dominant population. Using state-of-the-art N-body simulations, we model the infall of massive stellar systems into the Galactic center, both of Galactic and extragalactic origin. We show that the newly discovered population can either be the remnant of a massive star cluster formed a few kiloparsecs away from the Galactic center (Galactic scenario) or be accreted from a dwarf galaxy originally located at 10-100 kpc (extragalactic scenario) and that reached the Galactic center 3-5 Gyr ago. A comparison between our models and characteristic Galactocentric distance and metallicity distributions of MW satellites and globular clusters favors the Galactic scenario. A comparison with clusters associated with the Enceladus-Sausage, Sequoia, Sagittarius, and Canis Major structures suggests that the progenitor of the observed metal-poor substructure formed in situ rather than being accreted.

Keywords: Globular star clusters, Supermassive black holes, Very Large Telescope, Galactic center, Stellar populations, Dwarf galaxies, Galaxy dynamics, Milky Way Galaxy physics
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: X-ray astronomy, Galaxy dark matter halos, Galaxy formation, Supermassive black holes, Dwarf galaxies, Active galactic nuclei, X-ray sources, Galaxies
Atmospheric Erosion by Giant Impacts onto Terrestrial Planets: A Scaling Law for any Speed, Angle, Mass, and Density. We present a new scaling law to predict the loss of atmosphere from planetary collisions for any speed, angle, impactor mass, target mass, and body composition, in the regime of giant impacts onto broadly terrestrial planets with relatively thin atmospheres. To this end, we examine the erosion caused by a wide range of impacts, using 3D smoothed particle hydrodynamics simulations with sufficiently high resolution to directly model the fate of low-mass atmospheres around 1% of the target's mass. Different collision scenarios lead to extremely different behaviors and consequences for the planets. In spite of this complexity, the fraction of lost atmosphere is fitted well by a power law. Scaling is independent of the system mass for a constant impactor mass ratio. Slow atmosphere-hosting impactors can also deliver a significant mass of atmosphere, but always accompanied by larger proportions of their mantle and core. Different Moon-forming impact hypotheses suggest that around 10%-60% of a primordial atmosphere could have been removed directly, depending on the scenario. We find no evident departure from the scaling trends at the extremes of the parameters explored. The scaling law can be incorporated readily into models of planet formation.

Keywords: Impact phenomena, Hydrodynamical simulations, Planetary atmospheres, Earth atmosphere
An Upper Limit on the Spin of SgrA* Based on Stellar Orbits in Its Vicinity. The spin of the massive black hole (BH) at the center of the Milky Way, SgrA*, has been poorly constrained so far. We place an upper limit on the spin of SgrA* based on the spatial distribution of the S-stars, which are arranged in two almost edge-on disks that are located at a position angle of approximately ±45° with respect to the Galactic plane, on a milliparsec scale around the Galactic center. Requiring that the frame-dragging precession has not had enough time to make the S-star orbital angular momentum precess, the spin of the massive BH at the center of the Milky Way can be constrained to χ ≲ 0.1.

Keywords: Black hole physics, Supermassive black holes, Galactic center, Kerr black holes, S stars, Astrophysical black holes
The Phase-curve Signature of Condensible Water-rich Atmospheres on Slowly Rotating Tidally Locked Exoplanets. We use an idealized three-dimensional general circulation model to study condensible-rich atmospheres with an ineffective cold trap on slowly rotating tidally locked terrestrial planets. In particular, we show the climate dynamics in a thin and temperate atmosphere with condensible water vapor. The similarities between our thin and temperate atmosphere and the warm and thick atmosphere approaching the water vapor runaway greenhouse in previous works are discussed, including the reversal of the thermal emission between the day and night hemispheres. Different from the transit spectroscopy of water vapor that depends on the absolute amount of atmospheric water vapor, the contrast between the dayside and nightside thermal emission provides information regarding the relative ratio of water vapor to the background atmosphere as well as the atmospheric pressure near the substellar tropopause and the emission level on the nightside on potentially habitable worlds.

Keywords: Extrasolar rocky planets, Exoplanet atmospheric composition , Water vapor, Atmospheric circulation, Computational methods, Exoplanet atmospheres
Black Hole Formation in the Lower Mass Gap through Mergers and Accretion in AGN Disks. The heaviest neutron stars and lightest black holes expected to be produced by stellar evolution leave the mass range 2.2 M ${}_{\odot }\lesssim m\lesssim 5$ ${M}_{\odot }$ largely unpopulated. Objects found in this so-called lower mass gap likely originate from a distinct astrophysical process. Such an object, with mass 2.6 M ${}_{\odot }$ was recently detected in the binary merger GW190814 through gravitational waves by LIGO/Virgo. Here we show that black holes in the mass gap are naturally assembled through mergers and accretion in active galactic nucleus (AGN) disks, and can subsequently participate in additional mergers. We compute the properties of AGN-assisted mergers involving neutron stars and black holes, accounting for accretion. We find that mergers in which one of the objects is in the lower mass gap represent up to 4% of AGN-assisted mergers detectable by LIGO/Virgo. The lighter object of GW190814, with mass 2.6 M ${}_{\odot }$ , could have grown in an AGN disk through accretion. We find that the unexpectedly high total mass of 3.4 M ${}_{\odot }$ observed in the neutron star merger GW190425 may also be due to accretion in an AGN disk.

Keywords: Neutron stars, Gravitational waves, Astrophysical black holes
Dynamo Saturation through the Latitudinal Variation of Bipolar Magnetic Regions in the Sun. Observations of the solar magnetic cycle showed that the amplitude of the cycle did not grow all the time in the past. Thus, there must be a mechanism to halt the growth of the magnetic field in the Sun. We demonstrate a recently proposed mechanism for this under the Babcock-Leighton dynamo framework, which is believed to be the most promising paradigm for the generation of the solar magnetic field at present. This mechanism is based on the observational fact that the stronger solar cycles produce bipolar magnetic regions (BMRs) at higher latitudes and thus have higher mean latitudes than the weaker ones. We capture this effect in our three-dimensional Babcock-Leighton solar dynamo model and show that when the toroidal magnetic field tries to grow, it produce BMRs at higher latitudes. The BMRs at higher latitudes generate a less poloidal field, which consequently limits the overall growth of the magnetic field in our model. Thus, our study suggests that the latitudinal variation of BMRs is a potential mechanism for limiting the magnetic field growth in the Sun.

Keywords: Solar magnetic fields, Sunspot cycle, Solar dynamo