Sazuna/UAT_keywords huggingface dataset:
Thermodynamic Characterization of Solar Wind Interaction Regions Using PSP Observations: Evidence of Subadiabatic and Superadiabatic States. This study provides a detailed analysis of the thermodynamic behavior within stream interaction regions (SIRs), focusing on an effective polytropic index assessment of 25 distinct events. To improve the accuracy of the effective polytropic index estimation, we applied a multistep filtering method based on the Bernoulli integral to remove outliers caused by streamline mixing and correlation coefficients to improve the applicability of the polytropic model. Considering the effective degree of freedom (f = 3), 6 out of the 25 analyzed events show superadiabatic behavior, and the remaining events span subadiabatic, adiabatic, and superadiabatic states. The distribution of the determined polytropic indices for all the events reveals two distinct thermodynamic regimes, with peaks at γ = 1.22 (subadiabatic) and γ = 2.34 (superadiabatic). This analysis offers valuable insights into the complex thermodynamic processes occurring within solar wind interactions. The effective polytropic index of the solar wind does not have a single characteristic value throughout the heliosphere. Instead, it reflects varying heating and cooling processes and/or degrees of freedom within these structures, contributing to a deeper understanding of SIR dynamics.

Keywords: Magnetohydrodynamics, Solar wind, Heliosphere
Early and Extensive Ultraviolet through Near Infrared Observations of the Intermediate-luminosity Type Iax Supernovae 2024pxl. We present ultraviolet (UV) through near-infrared (NIR) photometric and spectroscopic observations of the nearby SN 2024pxl, the third Type Ia supernova (SN Ia) in NGC 6384. SN 2024pxl is a Type Iax supernova (SN Iax) with an intermediate luminosity (Mr = −16.99 ± 0.32 mag) and an average SN Iax light curve decline rate. SN 2024pxl was discovered ∼3 days after first light, and the rising light curve follows a single power law that is inconsistent with significant interaction with a companion star or circumstellar material. Our extensive NIR photometric coverage is comparable to that of the well-observed SNe Iax 2005hk and 2012Z, and we demonstrate that the J − H colors of SNe Iax differ from normal SNe Ia and appear to be more homogeneous as a class. Spectroscopically, we report the earliest-ever NIR spectrum of an SN Iax as measured from maximum light (t ≍ −9 days): a featureless continuum with similarities to a ∼9000 K blackbody, and the line velocities are consistent with a mixed-ejecta structure, with C, Si, and Fe having similar velocities and velocity evolutions. We find a tentative correlation between the H-band break Co II velocity ∼20 days post-peak and absolute magnitude, with more luminous SNe Iax showing faster Co II velocities. Our observations suggest that SN 2024pxl resulted from the thermonuclear disruption of a CO white dwarf star that undergoes deflagration burning.

Keywords: Type Ia supernovae, Near infrared astronomy
The Effect of Plasma-β on the Heating Mechanisms during Magnetic Reconnection in Partially Ionized Low Solar Atmosphere. We performed numerical simulations of magnetic reconnection with different strengths of magnetic fields from the solar photosphere to the upper chromosphere. The main emphasis is to identify dominant mechanisms for heating plasmas in the reconnection region under different plasma-β conditions in the partially ionized low solar atmosphere. The numerical results show that more plasmoids are generated in a lower β reconnection event. The frequent coalescence of these plasmoids leads to a significant enhancement of turbulence and compression heating, which becomes the dominant mechanism for heating plasma in a lower plasma-β reconnection process. The average power density of the compression heating (Qcomp) decreases with increasing initial plasma-β as a power function:  Qcomp∼β0‑a , where the value a is 1.9 in the photosphere and decreases to about 1.29 in the upper chromosphere. In the photosphere and lower chromosphere, the Joule heating contributed by electron-neutral collisions Qen = ηenJ2 eventually dominates over the compression heating when the initial plasma-β is larger than the critical value β0‑critical = 8. In the upper chromosphere, the ambipolar diffusion heating and the viscous heating will become equally important as the compression heating when the initial plasma-β is larger than the critical value β0‑critical = 0.5. These results indicate that the compression heating caused by turbulent reconnection mediated with plasmoids is likely the major heating mechanism for the small-scale reconnection events with stronger magnetic fields, such as active region Ellerman bombs (EBs) and UV bursts. However, the heating caused by the partial ionization effects can not be ignored for those reconnection events with weaker magnetic fields, such as quiet Sun EBs and cold surges.

Keywords: Solar photosphere, Solar chromosphere, Solar magnetic reconnection, Magnetohydrodynamical simulations
Suppression of Shock X-Ray Emission in Novae from Turbulent Mixing with Cool Gas. Shock interaction in classical novae occurs when a fast outflow from the white dwarf ≳1000 km s‑1 collides with a slower, cooler shell of gas released earlier in the outburst. The shocks radiate across the electromagnetic spectrum, from radio synchrotron to GeV gamma rays. The hot shocked gas also emits ≳keV thermal X-rays, typically peaking weeks after the eruption, once the ejecta become transparent to photoelectric absorption. However, the observed hard X-ray luminosities are typically ≳4 orders of magnitude smaller than would be naively expected given the powerful shocks implied by the gamma rays. We argue that a key missing piece to this puzzle is turbulence behind the shock, driven, e.g., by thin-shell and/or thermal instabilities. Turbulence efficiently mixes the hot X-ray emitting gas with cooler gas, sapping the hot gas of energy faster than it can directly radiate. Using analytic arguments motivated by numerical simulations, we show that energy losses due to turbulent mixing can easily balance shock heating, greatly reducing the volume of the hot gas and suppressing the X-ray luminosity. Equating the characteristic thickness of the X-ray emitting region to the minimum outer length scale of the turbulence capable of cooling the hot gas through mixing, we obtain X-ray luminosities consistent with nova observations if only ∼1% of the shock's kinetic power goes into turbulent motions. A similar process may act to suppress thermal X-rays from other shock powered transients, such as interacting supernovae.

Keywords: Shocks, Classical novae
The Sun's Dark Core: Helioseismic and Neutrino Flux Constraints on a Compact Solar Center. As dark matter appears to comprise most of the Galactic mass, some of it may accumulate in the cores of stars, thereby making the Sun a laboratory for constraining various dark matter theories. We consider the effects on the solar structure arising from a general class of macroscopic dark matter candidates that include strange quark matter, compact dark objects, and others. We calibrate standard solar evolution models (i.e., models that reproduce the mass, luminosity, radius, and metallicity of the Sun at its present age) with variable compact dark core masses ranging from 10‑8 to 10‑2 M⊙ and assess their properties. We find that the weakest constraints come from solar neutrino flux measurements, which only rule out the most massive dark core comprising at least ∼1% of the total solar mass. The Sun's acoustic oscillations impose stronger constraints, probing masses down to ∼10‑5 M⊙. We find that a model with a 10‑3 M⊙ dark core appears to improve the agreement with helioseismic observations. We nevertheless caution against interpreting this as evidence for dark matter in the solar interior, and suggest plausible effects that the dark core may instead be emulating. Finally, we show that future measurements of solar g-modes may constrain dark core masses down to 10‑7 M⊙.

Keywords: Helioseismology, Solar core, Solar neutrinos, Dark matter
A Possible Source of Multiple Polarity Reversals of the Polar Magnetic Field during the Maximum Phase of Solar Cycle. During the maximum phase of solar activity, it is quite common for the polar magnetic field to undergo multiple polarity reversals. In this study, we aim to provide the likely source of this phenomenon. By analyzing synoptic magnetograms from the Michelson Doppler Imagers on the Solar and Helioseismic Observatory and the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory throughout Solar Cycle 24, we have identified magnetic structures with densely packed fields by setting area and magnetic field thresholds. These structures are then coupled as bipolar magnetic regions (BMRs) by visual recognition, representing the active regions (ARs) composed of positive and negative polarities. In total, we extracted over 700 BMRs during Solar Cycle 24. Interestingly, we discovered that approximately one-third of these BMRs do not conform to Hale's polarity rule or Joy's law and are thus classified as irregular ARs or abnormal ARs. These irregular ARs distribute in various phases of Solar Cycle 24 and play a negative role in the polar field according to the Babcock–Leighton solar dynamo model. Further, our study shifts to the evolution of these irregular ARs with the obvious poleward flux diffusion and reveals that these ARs indeed produce poleward surges of magnetic flux with opposite polarity in the hemisphere. These findings suggest that these irregular ARs result in the fluctuation of the polar field and even the multiple polarity reversals of the polar field when they emerge during solar maximum.

Keywords: Solar cycle, Solar photosphere, Solar dynamo, Stellar activity
Modeling Cosmic-ray Electron Spectra and Synchrotron Emission in the Multiphase Interstellar Medium. We model the transport and spectral evolution of 1–100 GeV cosmic-ray electrons (CREs) in TIGRESS magnetohydrodynamic simulations of the magnetized, multiphase interstellar medium. We postprocess a kpc-sized galactic disk patch representative of the solar neighborhood using a two-moment method for cosmic ray (CR) transport that includes advection, streaming, and diffusion. The diffusion coefficient is set by balancing wave growth via the CR streaming instability against wave damping (nonlinear Landau and ion–neutral collisions), depending on local gas and CR properties. Implemented energy loss mechanisms include synchrotron, inverse Compton, ionization, and bremsstrahlung. We evaluate CRE losses by different mechanisms as a function of energy and distance from the midplane, and compare loss timescales to transport and diffusion timescales. This comparison shows that CRE spectral steepening above p = 1 GeV c‑1 is due to a combination of energy-dependent transport and losses. Our evolved CRE spectra are consistent with direct observations in the solar neighborhood, with a spectral index that steepens from an injected value of ‑2.3 to an energy-dependent value between ‑2.7 and ‑3.3. We also show that the steepening is independent of the injection spectrum. Finally, we present potential applications of our models, including to the production of synthetic synchrotron emission. Our simulations demonstrate that the CRE spectral slope can be accurately recovered from pairs of radio observations in the range 1.5–45 GHz.

Keywords: Magnetohydrodynamics, Interstellar medium, Cosmic rays
Timescales of Polycyclic Aromatic Hydrocarbon and Dust Continuum Emission from Gas Clouds Compared to Molecular Gas Cloud Lifetimes in PHANGS-JWST Galaxies. Recent JWST mid-infrared (mid-IR) images, tracing polycyclic aromatic hydrocarbons (PAHs) and dust continuum emission, provide detailed views of the interstellar medium (ISM) in nearby galaxies. Leveraging PHANGS-JWST Cycle 1 and PHANGS-MUSE data, we measure the PAH and dust continuum emission lifetimes of gas clouds across 17 nearby star-forming galaxies by analyzing the relative spatial distributions of mid-IR (7.7─11.3 μm) and Hα emission at various scales. We find that the mid-IR emitting timescale of gas clouds in galactic disks (excluding centers) ranges from 10─30 Myr. After star formation is detected in Hα, mid-IR emission persists for 3─7 Myr during the stellar feedback phase, covering 70%─80% of the Hα emission. This significant overlap is due to intense radiation from star-forming regions, illuminating the surrounding PAHs and dust grains. In most galaxies, the mid-IR time-scale closely matches the molecular cloud lifetime measured with CO. Although mid-IR emission is complex, as influenced by ISM distribution, radiation, and abundances of dust and PAHs, the similarity between the two timescales suggests that once gas clouds form with compact mid-IR emission, they quickly provide sufficient shielding for stable CO formation. This is likely due to our focus on molecular gas-rich regions of galaxies with near-solar metallicity. Finally, we find that the mid-IR emitting timescale is longer in galaxies with well-defined H II regions and less structured backgrounds, allowing photons to more efficiently heat the ambient ISM surrounding the H II regions, rather than contributing to diffuse emission. This suggests that the shape of the ISM also influences mid-IR emission.

Keywords: Disk galaxies, Interstellar clouds, Star formation, Interstellar medium, Extragalactic astronomy
The Star Formation in Radio Survey: Adding 90 GHz Data to 3─33 GHz Observations of Star-forming Regions in Nearby Galaxies. We present 90 GHz continuum imaging of 119 star-forming regions in 30 nearby galaxies observed with MUSTANG-2 on the Robert C. Byrd Green Bank Telescope as part of the Star Formation in Radio Survey. The 90 GHz data were combined with 3, 15, and 33 GHz data taken previously by the Karl G. Jansky Very Large Array to decompose radio spectra on ≍0.8 kpc scales into their synchrotron, free─free, and thermal dust emission components. This was done using three scenarios: (i) a power-law fit from 3 to 33 GHz, (ii) Markov Chain Monte Carlo (MCMC) fitting from 3 to 90 GHz with a thermal dust component, and (iii) MCMC fitting from 3 to 33 GHz without a thermal dust component. For these cases, we find a median thermal (free─free) emission fraction at 33 GHz of (i) 88% ± 2% with a scatter of 17%, (ii) 76% ± 3% with a scatter of 25%, and (iii) 84% ± 2% with a scatter of 18%. From this we conclude that, on average, free─free emission, not thermal dust, remains the dominant emission component at 33 GHz. While scenario (ii) yields a thermal fraction that is ≍10% larger than scenario (iii), this difference decreases to ≍5% after active galactic nuclei are removed. Consequently, star formation rates measured with thermal fractions at 33 GHz are only mildly biased high without 90 GHz data for the spectral decomposition. Furthermore, a power-law fit of data from 3 to 33 GHz still provides a reliable estimate of the free─free emission at 33 GHz.

Keywords: Radio astronomy, Extragalactic radio sources, Star forming regions, Star formation, Galaxy evolution, Radio continuum emission, Radio interferometry, H II regions, Galaxies
A Map of the Outer Gas Disk of the Galaxy with Direct Distances from Young Stars. For more than 50 yr, astronomers have mapped the neutral hydrogen gas in the Galaxy assuming kinematically derived distances. We employ the distances of nearby young stars, which trace the gas from which they formed, in longitude─latitude─velocity space to map this gas without using kinematic distances. We denote this new method "pattern matching." Analysis of simulated spiral galaxies indicates that our pattern matching distances are 24% more accurate than kinematic distances for gas within 15 kpc of the Sun. The two methods provide similar agreement with parallaxes toward these masers, although the kinematic method shows a small systematic offset in distance that is not present in the pattern matching distances. Using parallaxes and velocities for masers, we show that this novel method, when matched with nearby Cepheids, performs well compared to kinematics. This analysis is restricted to sources that have a reasonably good match with a member of our Cepheid sample. The distances derived here, and the associated map, have broad utility—from improving our understanding of star formation and the dynamical structure of the Galaxy, to informing 3D dust maps.

Keywords: Milky Way disk, Galaxy structure, Interstellar medium
Proper Motion of the Neutron Star in the Supernova Remnant G18.9–1.1. In this paper, we present the first direct measurement of the proper motion of the neutron star (NS) in the supernova remnant (SNR) G18.9‑1.1 using a 15 yr Chandra baseline. After correcting the observations' astrometric solutions using reference Gaia stars' positions, we measure a total proper motion of 24.7 ± 6.8 mas yr‑1 at an angle of 336° ± 16° east of north. Using the distance estimates from the literature of 2.1 and 3.8 kpc, this proper motion corresponds to Galactic rotation-corrected transverse velocities of 264d2.1 ± 79 km s‑1 and 474d3.8 ± 129 km s‑1, respectively. Our power ratio method analysis of SNR ejecta slightly favors the higher velocity, as multipole moments calculated from the back-evolved center using the farther distance are more consistent with values from other CCSNRs. The NS's motion is directly opposite the motion of bulk ejecta in G18.9‑1.1, providing yet more evidence that NS kicks are generated via a conservation of momentum-like process between the NS and the ejecta, as has been observed in other SNRs.

Keywords: Neutron stars, Core-collapse supernovae, Stellar kinematics, X-ray point sources, Supernova remnants
High-resolution Solar Image Registration Using Unsupervised Deep Learning. High-resolution solar images captured at different wavelengths are essential for understanding solar activity. However, such images often exhibit geometric discrepancies due to varying instrument resolutions and observation conditions, making image registration a critical preprocessing step. In this study, we propose an unsupervised deep learning–based framework named hourGlass and haRdnEt (GRE) for accurate solar image registration. The method detects keypoints in both the reference and moving images, extracts local feature descriptors, performs bidirectional matching to establish reliable correspondences, and estimates affine transformation parameters to align the images. The proposed framework was evaluated on quiet Sun images from the New Vacuum Solar Telescope and active region (AR) images from the Goode Solar Telescope, covering both photospheric and chromospheric features. A synthetic data set with known transformations was also used to assess registration accuracy under controlled conditions. Registration performance was quantitatively measured using mutual information (MI) and structural similarity index (SSIM) methods, and results were compared with those obtained using the scale-invariant feature transform and intensity-based two-step methods. The experimental results demonstrate that the proposed method achieves accurate registration across different solar features and imaging scenarios. The method maintains structural consistency in both AR and quiet Sun observations, including time-series data, with MI and SSIM improvements over baseline methods. The approach provides a validated tool for solar image alignment, suitable for further solar physics studies.

Keywords: Astronomical techniques, Computational methods, Astronomy data analysis
Follow the Mass—A Concordance Picture of Tidal Disruption Events. Three recent global simulations of tidal disruption events (TDEs) have produced, using different numerical techniques and parameters, very similar pictures of their dynamics. In typical TDEs, after the star is disrupted by a supermassive black hole, the bound portion of the stellar debris follows highly eccentric trajectories, reaching apocenters of several thousand gravitational radii. Only a very small fraction is captured upon returning to the vicinity of the supermassive black hole. Nearly all of the debris returns to the apocenter, where shocks produce a thick irregular cloud on this radial scale and power the optical/UV flare. These simulation results imply that over a few years, the thick cloud settles into an accretion flow responsible for the long-term emission. Despite not being designed to match observations, and without any free parameters, the dynamical picture given by the three simulations aligns well with observations of typical events, correctly predicting the flares' typical total radiated energy, luminosity, temperature, and emission-line width. On the basis of these predictions, we provide an updated method (TDEMASS) to infer the stellar and black hole masses from a flare's peak luminosity and temperature. This picture also correctly predicts that the luminosity observed years after the flare should be nearly constant. In addition, we show that in a magnitude-limited survey, if the intrinsic rate of TDEs is independent of black hole mass, the detected events will preferentially have black hole masses ∼106.3±0.3M⊙ and stellar masses ∼1M⊙.

Keywords: X-ray transient sources, Tidal disruption, Ultraviolet transient sources
The Importance of the Population III Initial Mass Function in Determining the Characteristics of the Earliest Galaxies. We use large-scale cosmological simulations to study the prospect of observing Population III (Pop III) bright galaxies with the James Webb Space Telescope (JWST). To quantify the impact of radiative transfer (RT), we compare a simulation that includes moment-based RT with one in which RT is handled approximately. Both simulations include a subgrid model of turbulent mixing, which is essential in tracking the formation of Pop III stars. We find that RT has a minor impact on our results, and that the overall star formation rate densities for both simulations are in fair agreement with observations and other simulations. While our overall galaxy luminosity functions are consistent with current high-redshift observations, we predict a drop of a factor of at least 6 in detectable galaxy counts at z = 14 as compared to z = 12 at MAB ≤ ‑16. Modeling Pop III stars according to a log-normal, top-heavy initial mass function (IMF), we find that these stars contribute no more than ≈1% of the flux of potentially detectable lensed galaxies at z = 12–14 with MAB ≤ ‑15. This is because a top-heavy Pop III IMF results in 99% of Pop III stellar mass being recycled within 10 Myr, well before the ≈30 Myr timescale on which galaxies recover from supernova feedback and heating. These effects conspire to quickly extinguish Pop III star formation, making their detection difficult even for JWST.

Keywords: Population III stars, Star formation, High-redshift galaxies, Galaxy luminosities