Sazuna/UAT_keywords huggingface dataset:
Galaxy Spectroscopy without Spectra: Galaxy Properties from Photometric Images with Conditional Diffusion Models. Modern spectroscopic surveys can only target a small fraction of the vast amount of photometrically cataloged sources in wide-field surveys. Here, we report the development of a generative artificial intelligence (AI) method capable of predicting optical galaxy spectra from photometric broadband images alone. This method draws from the latest advances in diffusion models in combination with contrastive networks. We pass multiband galaxy images into the architecture to obtain optical spectra. From these, robust values for galaxy properties can be derived with any methods in the spectroscopic toolbox, such as standard population synthesis techniques and Lick indices. When trained and tested on 64 × 64 pixel images from the Sloan Digital Sky Survey, the global bimodality of star-forming and quiescent galaxies in photometric space is recovered, as well as a mass–metallicity relation of star-forming galaxies. The comparison between the observed and the artificially created spectra shows good agreement in overall metallicity, age, Dn4000, stellar velocity dispersion, and E(B ‑ V) values. Photometric redshift estimates of our generative algorithm can compete with other current, specialized deep learning techniques. Moreover, this work is the first attempt in the literature to infer velocity dispersion from photometric images. Additionally, we can predict the presence of an active galactic nucleus up to an accuracy of 82%. With our method, scientifically interesting galaxy properties, normally requiring spectroscopic inputs, can be obtained in future data sets from large-scale photometric surveys alone. The spectra prediction via AI can further assist in creating realistic mock catalogs.

Keywords: Sky surveys, Galaxy properties, Neural networks, Convolutional neural networks, Galaxy photometry, Galaxy spectroscopy
First Comparison of FLARIX Simulations with MSDP Observation of the C1.6 Solar Flare in the Hα Line of Hydrogen. The primary objective of this study is to develop a time-dependent model of the flaring atmosphere based on observational data. Here, we present, for the first time, a comparison between numerical simulations of the flaring emission, specifically focusing on the hydrogen Hα line, utilizing the FLARIX code and spectral observations of a compact C1.6 GOES-class flare observed on 2012 September 10, by the MSDP imaging spectrograph installed at the Białków Observatory. The Multichannel Subtractive Double Pass imaging spectrograph spectral data, collected with a temporal resolution as fine as 50 ms, enabled a comprehensive analysis of Hα line profiles and light curves measured within an area of the flare's emission. An initial atmospheric model close to VAL-C, with a modified temperature in the upper chromosphere, was employed in simulations. To enhance temporal resolution, modulations of the nonthermal electron (NTE) beam's parameters were introduced based on variations in observed hard X-ray (HXR) flux (using RHESSI data). The synthesized Hα line profiles were compared with the observed spectra. During the impulsive phase of the flare, the evolution of the observed and synthetic Hα line intensity agrees, but discrepancies were found in intensities at specific wavelengths of the Hα line profile. Fluctuations in the energy flux of NTEs exhibited a strong correlation with the Hα emission during the HXR pulse. After considering various effects (such as the filling factor FF = 0.20) that could influence observed emissions, relatively good agreement between theoretical and observed lines was achieved.

Keywords: Astronomy data modeling, Solar flares, Solar coronal loops, Astronomy data analysis, Solar flare spectra, Solar x-ray emission, Solar x-ray flares, Solar chromospheric heating, Active solar chromosphere, Solar chromosphere
The Star-forming Main Sequence in JADES and CEERS at z &gt; 1.4: Investigating the Burstiness of Star Formation. We have used public JWST/NIRSpec and JWST/NIRCam observations from the CEERS and JADES surveys in order to analyze the star-forming main sequence (SFMS) over the redshift range 1.4 ≤ z &lt; 7. We calculate the star formation rates (SFRs) of the galaxy sample using three approaches: Balmer line luminosity, spectral energy distribution (SED) fitting, and UV luminosity. We find a larger degree of scatter about the SFMS using the Balmer-based SFRs compared to the UV-based SFRs. Because these SFR indicators are sensitive to star formation on different timescales, the difference in scatter may be evidence of bursty star formation histories in the early Universe. We additionally compare the Hα-to-UV luminosity ratio (L(Hα)/ν L  ν,1600) for individual galaxies in the sample and find that 29%–52% of the ratios across the sample are poorly described by predictions from a smooth star formation history. Measuring the burstiness of star formation in the early Universe has multiple significant implications, such as deriving accurate physical parameters from SED fitting, explaining the evolution of the UV luminosity function, and providing constraints for subgrid models of feedback in simulations of galaxy formation and evolution.

Keywords: Galaxy spectroscopy, High-redshift galaxies, Galaxy evolution, Galaxies
The Peak Frequency and Luminosity of Synchrotron Emitting Shocks: From Nonrelativistic to Ultrarelativistic Explosions. Synchrotron emission is ubiquitous in explosive astrophysical events—it is a natural byproduct of shocks formed when matter expelled by the explosion collides with ambient material. This emission is well observed in various classes of transients, and is often interpreted within a canonical "equipartition" framework that allows physical properties of the shock to be inferred from the frequency and luminosity at which the observed spectral energy distribution (SED) peaks. This framework has been remarkably successful in explaining observations of radio supernovae. It has also been used for transrelativistic explosions, where the shock velocities approach the speed of light. However, the conventional framework does not incorporate relativistic effects. Neither does it account for thermal electrons, which have been shown to be important for high-velocity shocks. In this paper we describe a revised framework that accounts for these two effects, and is applicable to nonrelativistic, transrelativistic, and ultrarelativistic explosions. We show that accounting for these effects can dramatically change the inferred parameters of high-velocity shocks, and, in particular, that the shock velocity, ambient density, and total energy are overestimated by the conventional nonrelativistic framework. We delineate the phase-space where such modifications are important in terms of observationally measurable parameters. We also find a novel upper limit on the peak synchrotron luminosity of shock-powered transients, which is remarkably consistent with existing observations. Finally, we discuss a prediction of the model—that the SED will qualitatively change as a function of shock velocity—and show that this is broadly consistent with data for representative events (e.g., SN1998bw, AT2018cow, CSS161010, AT2020xnd).

Keywords: Gamma-ray bursts, High energy astrophysics, Time domain astronomy, Shocks, Supernovae, Radio transient sources
Filament Accretion and Fragmentation in the Perseus Molecular Cloud. Observations suggest that filaments in molecular clouds can grow by mass accretion while forming cores via fragmentation. Here, we present one of the first large-sample studies of filament accretion using velocity gradient measurements of star-forming filaments on the ∼0.05 pc scale with NH3 observations of the Perseus Molecular Cloud, primarily obtained as a part of the Green Bank Ammonia Survey. In this study, we find significant correlations between the velocity gradient, velocity dispersion, mass per unit length, and number of cores per unit length of the Perseus filaments. Our results suggest a scenario in which filaments not only grow through mass accretion, but also form new cores continuously in the process, well into the thermally supercritical regime. Such behavior is contrary to that expected from isolated filament models but consistent with how filaments form within a more realistic cloud environment, suggesting that the cloud environment plays a crucial role in shaping core formation and evolution in filaments. Furthermore, even though velocity gradients within filaments are not oriented randomly, we find no correlation between velocity gradient orientation and the filament properties we analyzed. This result suggests that gravity is unlikely to be the dominant mechanism imposing order on the ∼0.05 pc scale for dense star-forming gas.

Keywords: Molecular gas, Magnetohydrodynamics, Interstellar filaments, Hydrodynamics, Star forming regions, Molecular spectroscopy, Molecular clouds, Dense interstellar clouds, Star formation, Interstellar clouds
X-Ray Spectroscopy of the Dwarf Nova Z Chamaeleontis in Quiescence and Outburst Using the XMM-Newton Observatory. We present X-ray spectroscopy of the SU UMa─type dwarf nova Z Chamaeleontis using the European Photon Imaging Camera and reflection grating spectrometer (RGS) instruments on board the XMM-Newton observatory. The quiescent system can be modeled by collisional equilibrium (CIE) or nonequilibrium plasma models, yielding a kT of 8.2─13.0 keV at a luminosity of (5.0─6.0) × 1030 erg s−1. The spectra yield better   using partially covering absorbers of cold and/or photoionized nature. The ionized absorber has an equivalent N H = (3.4─5.9) × 1022 cm−2 and a log(ξ) = 3.5─3.7 with a (50─60)% covering fraction when the VNEI model (XSPEC) is used. The line diagnosis in quiescence shows no resonance lines, with only the forbidden lines of Ne, Mg, and Si detected. The H-like C, O, Ne, and Mg are detected. The strongest line is O VIII, with (2.7─4.6) × 10−14 erg s−1 cm−2. The quiescent X-ray-emitting plasma is not collisional and not in ionization equilibrium, which is consistent with hot ADAF-like accretion flows. The line diagnosis in the outburst shows He-like O and Ne, with intercombination lines being the strongest, along with weaker resonance lines. This indicates the plasma is more collisional and denser, but not yet in CIE, revealing ionization timescales of (0.97─1.4) × 1011 s cm−3. The R ratios in the outburst yield electron densities of (7─90) × 1011 cm−3 and the G ratios yield electron temperatures of (2─3) × 106 K. The outburst luminosity is (1.4─2.5) × 1030 erg s−1. The flow is inhomogeneous in density. All detected lines are narrow, with widths limited by the resolution of RGS, yielding Keplerian rotational velocities &lt;1000 km s−1. This is too low for boundary layers, consistent with the nature of ADAF-like hot flows.

Keywords: Accretion, Space plasmas, Close binary stars, Dwarf novae, Atomic spectroscopy, X-ray transient sources, Astrophysical processes, Stellar accretion disks, Cataclysmic variable stars, Line intensities
Revisiting the Activity–Rotation Relation for Evolved Stars. The magnetic dynamo mechanism of giant stars remains an open question, which can be explored by investigating their activity–rotation relations with multiple proxies. By using the data from the LAMOST and Galaxy Evolution Explorer surveys, we carried out a comprehensive study of activity–rotation relations of evolved stars based on Ca II H and K lines, Hα lines, and near-ultraviolet (NUV) emissions. Our results show that evolved stars and dwarfs obey a similar power law in the unsaturated region of the activity–rotation relation, indicating a common dynamo mechanism in both giants and dwarfs. There is no clear difference in the activity levels between red giant branch stars and red clump stars, nor between single giants and those in binaries. Additionally, our results show that the NUV activity levels of giants are comparable to those of G- and K-type dwarfs and are higher than those of M dwarfs.

Keywords: Red giant clump, Stellar rotation, Red giant stars, Stellar activity
Free–Free Gaunt Factors for Atmospheres of Accreting Pulsars Observable with X-Ray Space Missions. Free–free Gaunt factors for X-ray absorption by hot plasma in the presence of a strong magnetic field are reported. Modified formulae are used for application to the nonlocal thermodynamic equilibrium conditions found in accreting pulsar atmospheres. Given upcoming global X-ray polarimetric space missions, these can be used for the construction of an absorption matrix in discrete-ordinate polarized radiative transfer.

Keywords: Plasma astrophysics
A Compact, Coherent Representation of Stellar Surface Variation in the Spectral Domain. Time-varying inhomogeneities on stellar surfaces constitute one of the largest sources of radial velocity (RV) error for planet detection and characterization. We show that stellar variations, because they manifest on coherent, rotating surfaces, give rise to changes that are complex but usably compact and coherent in the spectral domain. Methods for disentangling stellar signals in RV measurements benefit from modeling the full domain of spectral pixels. We simulate spectra of spotted stars using starry and construct a simple spectrum projection space that is sensitive to the orientation and size of stellar surface features. Regressing measured RVs in this projection space reduces RV scatter by 60%–80% while preserving planet shifts. We note that stellar surface variability signals do not manifest in spectral changes that are purely orthogonal to a Doppler shift or exclusively asymmetric in line profiles; enforcing orthogonality or focusing exclusively on asymmetric features will not make use of all the information present in the spectra. We conclude with a discussion of existing and possible implementations on real data based on the presented compact, coherent framework for stellar signal mitigation.

Keywords: Radial velocity, Astronomy data analysis, Spectroscopy, Stellar activity
Center-to-limb Variations in Solar Plage Using IRIS Observations. The center-to-limb variations (CLV) of transition region line Gaussian fit parameters in solar plage are reported for the first time. The Si IV 1402.77 Å line observed by the Interface Region Imaging Spectrograph is used. The spectral intensity increases linearly from the disk center to the solar limb. Similarly, the nonthermal velocity also increases linearly from 23.6 (at the disk center) to 30.9 km s‑1 (at the solar limb). On the other hand, the Doppler velocity decreases from 8.9 ± 1.0 at the disk center to 0.0 km s‑1 at the limb. This CLV pattern in solar plages is consistent with the CLV pattern reported in the quiet Sun (QS). However, the average values of the parameters in the solar plage are significantly higher than in the QS. The intensity and nonthermal velocity increase linearly with the magnetic field at the disk center, while the Doppler velocity does not depend on the magnetic field. Due to the line-of-sight effect, the plasma column depth increases toward the solar limb, which leads to a linear increase in the spectral intensity. Further, the increasing plasma column depth toward the solar limb adds more and more unresolved motions, and as a result, the nonthermal velocity increases from the disk center to the solar limb. In the solar plages, the higher plasma density due to the strong magnetic field leads to higher intensity and nonthermal velocity compared to the QS and coronal hole.

Keywords: Center to limb observations, Ultraviolet spectroscopy, Solar transition region
Plasma Waves Can Gain Energy from Neutrals in Partially Ionized Plasmas. Partially ionized plasmas are widely present in astrophysical and space plasmas, and ion-neutral collisions have an important impact on the plasma dynamics, such as electrostatic wave propagation. In this work, the excitation and propagation characteristics of electrostatic ion cyclotron (EIC) waves were experimentally studied in a partially ionized plasma. The EIC waves were launched via a grid antenna, and two types of EIC waves were observed to be excited and propagated. In partially ionized plasmas with weak ion–neutral collisions, it was found that the EIC wave propagation is dominated by ion motion, resembling the classical EIC wave. However, in plasmas with strong ion–neutral collisions, experiments revealed for the first time that EIC wave propagation can be dominated by neutral motion, which leads to an increase in the collisional EIC wave's phase velocity. Additionally, the EIC wave dispersion relationships under collisions were determined to reveal the underlying processes responsible for the observations. This work verified that collisional EIC waves can gain energy from neutrals in partially ionized plasmas, which suggests that ion‒neutral collisions in the Earth and planetary ionosphere and solar atmosphere have a more complex impact on plasma wave propagation than previously thought.

Keywords: Solar atmosphere, Planetary ionospheres, Space plasmas, Earth ionosphere
[X/Fe] Marks the Spot: Mapping Chemical Azimuthal Variations in the Galactic Disk with APOGEE. Chemical cartography of the Galactic disk provides insights into its structure and assembly history over cosmic time. In this work, we use chemical cartography to explore chemical gradients and azimuthal substructure in the Milky Way disk with giant stars from Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR17. We confirm the existence of a radial metallicity gradient in the disk of Δ[Fe/H]/ΔR ∼ –0.0678 ± 0.0004 dex kpc‑1 and a vertical metallicity gradient of Δ[Fe/H]/ΔZ ∼ ‑0.164 ± 0.001. We find azimuthal variations (±0.1 dex) on top of the radial metallicity gradient that have been previously established with other surveys. The APOGEE giants show strong correlations with stellar age and the intensity of azimuthal variations in [Fe/H]; young populations and intermediate-aged populations both show significant deviations from the radial metallicity gradient, while older stellar populations show the largest deviations from the radial metallicity gradient. Beyond iron, we show that other elements (e.g., Mg, O) display azimuthal variations at the ±0.05 dex level across the Galactic disk. We illustrate that moving into the orbit-space could help constrain the mechanisms producing these azimuthal metallicity variations in the future. These results suggest that dynamical processes play an important role in the formation of azimuthal metallicity variations.

Keywords: Stellar abundances, Galactic archaeology, Milky Way disk
X-Ray, UV, and Optical Observations of Proxima Centauri's Stellar Cycle. Proxima Cen (GJ 551; dM5.5e) is one of only about a dozen fully convective stars known to have a stellar cycle, and the only one to have long-term X-ray monitoring. A previous analysis found that X-ray and mid-UV observations, particularly two epochs of data from Swift, were consistent with a well-sampled ∼7 yr optical cycle seen in All Sky Automated Survey project (ASAS) data, but not convincing by themselves. The present work incorporates several years of new ASAS-SN optical data and an additional 5 yr of Swift XRT and UVOT observations, with Swift observations now spanning 2009─2021 and optical coverage from late 2000. X-ray observations by XMM-Newton and Chandra are also included. Analysis of the combined data, which includes modeling and adjustments for stellar contamination in the optical and UV, now reveals clear cyclic behavior in all three wavebands with a period of 8.0 yr. We also show that UV and X-ray intensities are anticorrelated with optical brightness variations caused by the cycle and by rotational modulation, discuss possible indications of two coronal mass ejections, and provide updated results for the previous finding of a simple correlation between X-ray cycle amplitude and Rossby number over a wide range of stellar types and ages.

Keywords: M dwarf stars, Late-type dwarf stars, Stellar activity
Formation of Sub-Chandrasekhar-mass Black Holes and Red Stragglers via Hawking Stars in Ultrafaint Dwarf Galaxies. Primordial black holes (PBHs) in the asteroid mass window from 10‑16 M ⊙ to 10‑10 M ⊙ are currently a popular dark matter candidate. If they exist, some stars would capture them upon formation, and they would slowly accrete the star over gigayears. Such Hawking stars—stars with a central PBH—provide a novel channel for the formation of both sub-Chandrasekhar-mass black holes and red straggler stars. Here we report on stellar evolution models that extend our previous work to Hawking stars with masses between 0.5 and 1.4 M ⊙. We explore three accretion schemes, and find that a wide range of PBHs in the asteroid mass window can robustly accrete stars as small as 1 M ⊙ within the age of the Universe. This mechanism of producing subsolar-mass black holes is highly dependent on the assumed accretion physics and stellar metallicity. Lower-metallicity stars are generally accreted more rapidly, suggesting that it may be more likely for sub-Chandrasekhar-mass Hawking stars formed in the early Universe, such as those in ultrafaint dwarf (UFD) galaxies, to transmute their star into a sub-Chandrasekhar-mass black hole within a Hubble time. We present a stellar population synthesis of a Draco II–like UFD galaxy containing Hawking stars and show that the number of red stragglers they produce can qualitatively match the observed population for black hole seed masses around 10‑11 M ⊙ and under the assumption that they accrete with high radiative efficiency.

Keywords: Dwarf galaxies, Stellar evolutionary models, Bondi accretion, Primordial black holes
Proton Acceleration in Low-β Magnetic Reconnection with Energetic Particle Feedback. Magnetic reconnection regions in space and astrophysics are known as active particle acceleration sites. There is ample evidence showing that energetic particles can take a substantial amount of converted energy during magnetic reconnection. However, there has been a lack of studies understanding the backreaction of energetic particles at magnetohydrodynamical scales in magnetic reconnection. To address this, we have developed a new computational method to explore the feedback by nonthermal energetic particles. This approach considers the backreaction from these energetic particles by incorporating their pressure into magnetohydrodynamics (MHD) equations. The pressure of the energetic particles is evaluated from their distribution evolved through Parker's transport equation, solved using stochastic differential equations (SDEs), so we coin the name MHD-SDE. Applying this method to low-β magnetic reconnection simulations, we find that reconnection is capable of accelerating a large fraction of energetic particles that contain a substantial amount of energy. When the feedback from these particles is included, their pressure suppresses the compression structures generated by magnetic reconnection, thereby mediating particle energization. Consequently, the feedback from energetic particles results in a steeper power-law energy spectrum. These findings suggest that feedback from nonthermal energetic particles plays a crucial role in magnetic reconnection and particle acceleration.

Keywords: Magnetohydrodynamics, Solar magnetic reconnection