adsabs/SciX_UAT_keywords huggingface dataset:
First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole

In this paper we provide a first physical interpretation for the Event Horizon Telescope's (EHT) 2017 observations of Sgr A*. Our main approach is to compare resolved EHT data at 230 GHz and unresolved non-EHT observations from radio to X-ray wavelengths to predictions from a library of models based on time-dependent general relativistic magnetohydrodynamics simulations, including aligned, tilted, and stellar-wind-fed simulations; radiative transfer is performed assuming both thermal and nonthermal electron distribution functions. We test the models against 11 constraints drawn from EHT 230 GHz data and observations at 86 GHz, 2.2 μm, and in the X-ray. All models fail at least one constraint. Light-curve variability provides a particularly severe constraint, failing nearly all strongly magnetized (magnetically arrested disk (MAD)) models and a large fraction of weakly magnetized models. A number of models fail only the variability constraints. We identify a promising cluster of these models, which are MAD and have inclination i ≤ 30°. They have accretion rate (5.2-9.5) × 10-9 M ⊙ yr-1, bolometric luminosity (6.8-9.2) × 1035 erg s-1, and outflow power (1.3-4.8) × 1038 erg s-1. We also find that all models with i ≥ 70° fail at least two constraints, as do all models with equal ion and electron temperature; exploratory, nonthermal model sets tend to have higher 2.2 μm flux density; and the population of cold electrons is limited by X-ray constraints due to the risk of bremsstrahlung overproduction. Finally, we discuss physical and numerical limitations of the models, highlighting the possible importance of kinetic effects and duration of the simulations.

Keywords: black hole physics; galactic center
Thermonuclear X-Ray Bursts with Late Secondary Peaks Observed from 4U 1608-52

We report the temporal and spectral analysis of three thermonuclear X-ray bursts from 4U 1608-52, observed by the Neutron Star Interior Composition Explorer (NICER) during and just after the outburst observed from the source in 2020. In two of the X-ray bursts, we detect secondary peaks 30 and 18 s after the initial peaks. The secondary peaks show a fast rise exponential decay-like shape resembling a thermonuclear X-ray burst. Time-resolved X-ray spectral analysis reveals that the peak flux, blackbody temperature, and apparent emitting radius values of the initial peaks are in agreement with X-ray bursts previously observed from 4U 1608-52, while the same values for the secondary peaks tend toward the lower end of the distribution of bursts observed from this source. The third X-ray burst, which happened during much lower accretion rates, did not show any evidence for a deviation from an exponential decay and was significantly brighter than the previous bursts. We present the properties of the secondary peaks and discuss the events within the framework of short recurrence time bursts or bursts with secondary peaks. We find that the current observations do not fit in standard scenarios and challenge our understanding of flame spreading.

Keywords: x-ray bursters
COCONUT, a Novel Fast-converging MHD Model for Solar Corona Simulations. II. Assessing the Impact of the Input Magnetic Map on Space-weather Forecasting at Minimum of Activity

This paper is dedicated to the new implicit unstructured coronal code COCONUT, which aims at providing fast and accurate inputs for space-weather forecasting as an alternative to empirical models. We use all 20 available magnetic maps of the solar photosphere covering the date of 2019 July 2, which corresponds to a solar eclipse on Earth. We use the same standard preprocessing on all maps, then perform coronal MHD simulations with the same numerical and physical parameters. We conclude by quantifying the performance of each map using three indicators from remote-sensing observations: white-light total solar eclipse images for the streamers' edges, EUV synoptic maps for coronal holes, and white-light coronagraph images for the heliospheric current sheet. We discuss the performance of space-weather forecasting and show that the choice of the input magnetic map has a strong impact. We find performances between 24% and 85% for the streamers' edges, 24%-88% for the coronal hole boundaries, and a mean deviation between 4° and 12° for the heliospheric current sheet position. We find that the HMI runs perform better on all indicators, with GONG-ADAPT being the second-best choice. HMI runs perform better for the streamers' edges, and GONG-ADAPT for polar coronal holes, HMI synchronic for equatorial coronal holes, and the streamer belt. We especially illustrate the importance of the filling of the poles. This demonstrates that the solar poles have to be taken into account even for ecliptic plane previsions.

Keywords: solar corona; solar wind; magnetohydrodynamics; solar magnetic fields; space weather
The Origin of Star Formation in Early-type Galaxies Inferred from Spatially Resolved Spectroscopy

We investigate the origin of star formation activity in early-type galaxies with current star formation using spatially resolved spectroscopic data from the Mapping Nearby Galaxies at Apache Point Observatory in the Sloan Digital Sky Survey (SDSS). We first identify star-forming early-type galaxies from the SDSS sample, which are morphologically early-type but show current star formation activity in their optical spectra. We then construct comparison samples with different combinations of star formation activity and morphology, which include star-forming late-type galaxies, quiescent early-type galaxies, and quiescent late-type galaxies. Our analysis of the optical spectra reveals that the star-forming early-type galaxies have two distinctive episodes of star formation, which is similar to late-type galaxies but different from quiescent early-type galaxies with a single star formation episode. Star-forming early-type galaxies have properties in common with star-forming late-type galaxies, which include stellar population, gas and dust content, mass, and environment. However, the physical properties of star-forming early-type galaxies derived from spatially resolved spectroscopy differ from those of star-forming late-type galaxies in the sense that the gas in star-forming early-type galaxies is more concentrated than their stars, and is often kinematically misaligned with stars. The age gradient of star-forming early-type galaxies also differs from those of star-forming late-type galaxies. Our findings suggest that the current star formation in star-forming early-type galaxies has an external origin including galaxy mergers or accretion gas from the cosmic web.

Keywords: galaxy formation; galaxy evolution; elliptical galaxies; star formation; galaxy structure
Evidence for Emergent Dark Energy

We introduce a generalized form of an emergent dark energy model with one degree of freedom for the dark energy sector that has the flexibility to include both the ΛCDM model as well as the Phenomenologically Emergent Dark Energy (PEDE) model proposed by Li & Shafieloo as two of its special limits. The free parameter for the dark energy sector, namely Δ, has a value of 0 for the Λ case and a value of 1 for the PEDE case. Fitting the introduced parametric form to Planck cosmic microwave background (CMB) data and recent H0 results from local observations of cepheids and supernovae, we show that the Δ = 0 associated with the ΛCDM model would fall out of the 4&sgr; confidence limits of the derived posterior of the Δ parameter. Moreover, H0 tensions with the emergent dark energy model will be alleviated and this model can satisfy the combination of Planck CMB data and local H0 observations with ΔDIC = -2.88 compared with the ΛCDM model.

Keywords: dark energy; hubble constant; cosmological constant
A Graphical Interpretation of Circumgalactic Precipitation

Both observations and recent numerical simulations of the circumgalactic medium (CGM) support the hypothesis that a self-regulating feedback loop suspends the gas density of the ambient CGM close to the galaxy in a state with a ratio of cooling time to freefall time ≳10. This limiting ratio is thought to arise because circumgalactic gas becomes increasingly susceptible to multiphase condensation as the ratio declines. If the timescale ratio gets too small, then cold clouds precipitate out of the CGM, rain into the galaxy, and fuel energetic feedback that raises the ambient cooling time. The astrophysical origin of this so-called precipitation limit is not simple but is critical to understanding the CGM and its role in galaxy evolution. This paper therefore attempts to interpret its origin as simply as possible, relying mainly on conceptual reasoning and schematic diagrams. It illustrates how the precipitation limit can depend on both the global configuration of a galactic atmosphere and the degree to which dynamical disturbances drive CGM perturbations. It also frames some tests of the precipitation hypothesis that can be applied to both CGM observations and numerical simulations of galaxy evolution.

Keywords: circumgalactic medium; stellar feedback; intracluster medium; cooling flows; active galactic nuclei
The Formation of the Stripped-envelope Type IIb Supernova Progenitors: Rotation, Metallicity, and Overshooting

Type IIb supernovae (SNe) are believed to originate from core-collapse progenitors having kept only a very thin hydrogen envelope. We aim to explore how some physical factors, such as rotation, metallicity, overshooting, and the initial orbital period in binaries significantly affect the Roche lobe overflow and the formation of Type IIb SNe (also denoted SNe IIb). It is found that binaries are the main channel that are capable of producing Type IIb SNe progenitors in the mass range for initial masses below 20 M ⊙. The formation of Type IIb SNe progenitors is extremely sensitive to the initial orbital period. A less massive hydrogen envelope mass indicates a smaller radius and higher effective temperatures, and vice versa. Binary systems with initial periods between 300 and 720 days produce Type IIb progenitors that are a red supergiant. Those with an initial period between 50 and 300 days produce yellow supergiant progenitors, and those with initial periods shorter than 50 days blue supergiant progenitors. Both rapid rotation and larger overshooting can enlarge the carbon-oxygen core mass and lead to higher core temperature and lower central density at the precollapse phase. They are also beneficial to surface nitrogen enrichment but restrict the efficiency of the first dredge-up. SN IIb progenitors with low metallicity have smaller hydrogen envelope masses and radii than their high-metallicity counterparts. Ultrastripped binary models have systematically higher core mass fraction 12C left, which has an important influence on the compactness of Type IIb progenitors.

Keywords: close binary stars; stellar rotation; massive stars; stellar structures; stellar properties
Code Comparison in Galaxy-scale Simulations with Resolved Supernova Feedback: Lagrangian versus Eulerian Methods

We present a suite of high-resolution simulations of an isolated dwarf galaxy using four different hydrodynamical codes: GIZMO, AREPO, GADGET, and RAMSES. All codes adopt the same physical model, which includes radiative cooling, photoelectric heating, star formation, and supernova (SN) feedback. Individual SN explosions are directly resolved without resorting to subgrid models, eliminating one of the major uncertainties in cosmological simulations. We find reasonable agreement on the time-averaged star formation rates as well as the joint density-temperature distributions between all codes. However, the Lagrangian codes show significantly burstier star formation, larger SN-driven bubbles, and stronger galactic outflows compared to the Eulerian code. This is caused by the behavior in the dense, collapsing gas clouds when the Jeans length becomes unresolved: Gas in Lagrangian codes collapses to much higher densities than that in Eulerian codes, as the latter is stabilized by the minimal cell size. Therefore, more of the gas cloud is converted to stars and SNe are much more clustered in the Lagrangian models, amplifying their dynamical impact. The differences between Lagrangian and Eulerian codes can be reduced by adopting a higher star formation efficiency in Eulerian codes, which significantly enhances SN clustering in the latter. Adopting a zero SN delay time reduces burstiness in all codes, resulting in vanishing outflows as SN clustering is suppressed.

Keywords: galaxy formation; stellar feedback; hydrodynamical simulations
Explosive Ejections Generated by Gravitational Interactions

During the fragmentation and collapse of a molecular cloud, it is expected that it will have close encounters with (proto)stellar objects that can lead to the ejection of a fraction of them as runaway objects. However, the duration and consequences of such encounters are perhaps small enough for there to be no direct evidence of their occurrence. As a first approximation, herein we analytically study the interaction of a massive object that moves at high velocity into a cluster of negligible-mass particles with an initial number density distribution ∝ R-α. We have found that the runaway conditions of the distribution after the encounter are related to the mass and the velocity of the star and the impact parameter of each particle to the stellar object. Then, the cluster particles are gravitationally accelerated by the external approaching star, destroying the cluster, and the dispersion and velocities of the particles have explosive characteristics. We compare this analytical model with several numerical simulations and, finally, apply our results to the Orion fingers in the Orion BN/KL region, which show an explosive outflow that could be triggered by the gravitational interaction of several (proto)stellar objects.

Keywords: interstellar dynamics; interstellar medium
Flare-forecasting Algorithms Based on High-gradient Polarity Inversion Lines in Active Regions

Solar flares emanate from solar active regions hosting complex and strong bipolar magnetic fluxes. Estimating the probability of an active region to flare and defining reliable precursors of intense flares are extremely challenging tasks in the space weather field. In this work, we focus on two metrics as flare precursors, the unsigned flux R, tested on Michelson Doppler Imager/Solar and Heliospheric Observatory data, one of the most used parameters for flare-forecasting applications, and a novel topological parameter D, representing the complexity of a solar active region. In greater detail, we propose an algorithm for the computation of the R value, which exploits the higher spatial resolution of Helioseismic Magnetic Imager maps. This algorithm leads to a differently computed R value, whose functionality is tested on a set of solar cycle 24 flares. Furthermore, we introduce a topological parameter based on the automatic recognition of magnetic polarity inversion lines in identified active regions and are able to evaluate its magnetic topological complexity. We use both a heuristic approach and a supervised machine-learning method to validate the effectiveness of these two descriptors to predict the occurrence of X- or M-class flares in a given solar active region during the following 24 hr period. Our feature ranking analysis shows that both parameters play a significant role in prediction performances. Moreover, the analysis demonstrates that the new topological parameter D is the only one, among 173 overall predictors, that is always present for all test subsets and is systematically ranked within the top 10 positions in all tests concerning the computation of the weights with which each predictor impacts the flare forecasting.

Keywords: solar flares; solar activity; solar physics; solar active regions; solar active region magnetic fields
Binary Planetesimal Formation from Gravitationally Collapsing Pebble Clouds

Planetesimals are compact astrophysical objects roughly 1-1000 km in size, massive enough to be held together by gravity. They can grow by accreting material to become full-size planets. Planetesimals themselves are thought to form by complex physical processes from small grains in protoplanetary disks. The streaming instability (SI) model states that millimeter/centimeter-sized particles (pebbles) are aerodynamically collected into self-gravitating clouds that then directly collapse into planetesimals. Here we analyze ATHENA simulations of the SI to characterize the initial properties (e.g., rotation) of pebble clouds. Their gravitational collapse is followed with the PKDGRAV N-body code, which has been modified to realistically account for pebble collisions. We find that pebble clouds rapidly collapse into short-lived disk structures from which planetesimals form. The planetesimal properties depend on the cloud's scaled angular momentum, $l=L/({{MR}}_{{\rm{H}}}^{2}{\rm{\Omega }})$ , where L and M are the angular momentum and mass, RH is the Hill radius, and Ω is the orbital frequency. Low-l pebble clouds produce tight (or contact) binaries and single planetesimals. Compact high-l clouds give birth to binary planetesimals with attributes that closely resemble the equally sized binaries found in the Kuiper Belt. Significantly, the SI-triggered gravitational collapse can explain the angular momentum distribution of known equally sized binaries—a result pending verification from studies with improved resolution. About 10% of collapse simulations produce hierarchical systems with two or more large moons. These systems should be found in the Kuiper Belt when observations reach the threshold sensitivity.

Keywords: planetesimals; kuiper belt; planet formation; protoplanetary disks; gravitational collapse
The Molecular Interstellar Medium in the Super Star Clusters of the Starburst NGC 253

We present submillimeter spectra of the (proto-)super star cluster (SSC) candidates in the starbursting center of the nearby galaxy NGC 253 identified by Leroy et al. The 2.5 pc resolution of our Atacama Large Millimeter/submillimeter Array cycle 3 observations approaches the size of the SSCs and allows for the study of physical and chemical properties of the molecular gas in these sources. In the 14 SSC sources and in the frequency ranges 342.0-345.8 GHz and 353.9-357.7 GHz, we detect 55 lines belonging to 19 different chemical species. The SSCs differ significantly in chemical complexity, with the richest clusters showing 19 species and the least complex showing four species. We detect HCN isotopologues and isomers (H13CN, HC15N, H15NC), abundant HC3N, SO and S18O, SO2, and H2CS. The gas ratios CO/HCN, CO/HCO+ are low, ∼1-10, implying high dense gas fractions in the SSCs. Line ratio analyses suggests chemistry consistent with photon-dominated regions and mechanical heating. None of the SSCs near the galaxy center show line ratios that imply an X-ray-dominated region, suggesting that heating by any (still unknown) active galactic nucleus does not play a major role. The gas temperatures are high in most sources, with an average rotational temperature of ∼130 K in SO2. The widespread existence of vibrationally excited HCN and HC3N transitions implies strong infrared radiation fields, potentially trapped by a greenhouse effect due to high continuum opacities.

Keywords: starburst galaxies; interstellar medium; star formation; astrochemistry; dense interstellar clouds; star clusters
The Powers of Relativistic Jets Depend on the Spin of Accreting Supermassive Black Holes

Theoretical models show that the power of relativistic jets of active galactic nuclei depends on the spin and mass of the central supermassive black holes, as well as the accretion. Here we report an analysis of archival observations of a sample of blazars. We find a significant correlation between jet kinetic power and the spin of supermassive black holes. At the same time, we use multiple linear regression to analyze the relationship between jet kinetic power and accretion, spin, and black hole mass. We find that the spin of supermassive black holes and accretion are the most important contributions to the jet kinetic power. The contribution rates of both the spin of supermassive black holes and accretion are more than 95%. These results suggest that the spin energy of supermassive black holes powers the relativistic jets. The jet production efficiency of almost all Fermi blazars can be explained by moderately thin, magnetically arrested accretion disks around rapidly spinning black holes.

Keywords: active galactic nuclei; blazars
A Transient "Changing-look" Active Galactic Nucleus Resolved on Month Timescales from First-year Sloan Digital Sky Survey V Data

We report the discovery of a new "changing-look" active galactic nucleus (CLAGN) event, in the quasar SDSS J162829.17+432948.5 at z = 0.2603, identified through repeat spectroscopy from the fifth Sloan Digital Sky Survey (SDSS-V). Optical photometry taken during 2020-2021 shows a dramatic dimming of Δg ≈ 1 mag, followed by a rapid recovery on a timescale of several months, with the ≲2 month period of rebrightening captured in new SDSS-V and Las Cumbres Observatory spectroscopy. This is one of the fastest CLAGN transitions observed to date. Archival observations suggest that the object experienced a much more gradual dimming over the period of 2011-2013. Our spectroscopy shows that the photometric changes were accompanied by dramatic variations in the quasar-like continuum and broad-line emission. The excellent agreement between the pre- and postdip photometric and spectroscopic appearances of the source, as well as the fact that the dimmest spectra can be reproduced by applying a single extinction law to the brighter spectral states, favor a variable line-of-sight obscuration as the driver of the observed transitions. Such an interpretation faces several theoretical challenges, and thus an alternative accretion-driven scenario cannot be excluded. The recent events observed in this quasar highlight the importance of spectroscopic monitoring of large active galactic nucleus samples on weeks-to-months timescales, which the SDSS-V is designed to achieve.

Keywords: supermassive black holes; quasars; active galactic nuclei; transient sources
Jupiter as an Exoplanet: Insights from Cassini Phase Curves

Due to its proximity to Earth, Jupiter of the solar system serves as a unique case study for gas-giant exoplanets. In the current Letter, we perform fits of ab initio, reflective, semi-infinite, homogeneous model atmospheres to 61 phase curves from 0.40 to 1.00 μm, obtained from the Cassini spacecraft, within a Bayesian framework. We reproduce the previous finding that atmospheric models using classic reflection laws (Lambertian, Rayleigh, single Henyey-Greenstein) provide poor fits to the data. Using the double Henyey-Greenstein reflection law, we extract posterior distributions of the single-scattering albedo and scattering asymmetry factors and tabulate their median values and uncertainties. We infer that the aerosols in the Jovian atmosphere are large, irregular, polydisperse particles that produce strong forward scattering together with a narrow backscattering lobe. The near-unity values of the single-scattering albedos imply that multiple scattering of radiation is an important effect. We speculate that the observed narrow backscattering lobe is caused by coherent backscattering of radiation, which is usually associated with solar system bodies with solid surfaces and regolith. Our findings demonstrate that precise, multiwavelength phase curves encode valuable information on the fundamental properties of cloud/haze particles. The method described in this Letter enables single-scattering albedos and scattering asymmetry factors to be retrieved from James Webb Space Telescope phase curves of exoplanets.

Keywords: exoplanet atmospheres; exoplanet atmospheric composition; exoplanet astronomy; solar system astronomy; solar system gas giant planets; jupiter; atmospheric clouds; atmospheric composition