adsabs/SciX_UAT_keywords huggingface dataset:
Lunar Surface and Buried Rock Abundance Retrieved from Chang'E-2 Microwave and Diviner Data

Microwave emission of the Moon, measured by the Chang'E-2 Microwave Radiometer (MRM), provides an effective way to understand the physical properties of lunar near-surface materials. The observed microwave brightness temperature is affected by near-surface temperatures, which are controlled by the surface albedo, roughness, regolith thermophysical properties, and the high thermal inertia and permittivity of both surface and buried rocks. In this study, we propose a rock model using thermal infrared measurements from the Lunar Reconnaissance Orbiter's (LRO) Diviner as surface temperature constraints. We then retrieve the volumetric rock abundance (RA) from nighttime MRM data at several rocky areas. Although our retrieved MRM RA cannot be compared to the rock concentration measured with LRO Camera images directly, there is a good agreement with Diviner-derived RA and radar observations. The extent of several geological units, including rocky craters, hummocky regions, and impact melts, agree well with the distribution of elevated rock concentration. Based on seven large craters with published model ages, we present an inverse correlation between rock concentration and crater age. The result shows that the rock concentration decreases with crater age rapidly within 1 Ga but declines slowly after that. These data are consistent with a short survival time for exposed rocks and a long lifetime for buried rocks that are shielded from lunar surface processes.

Keywords: lunar surface; lunar geochronology; lunar craters; lunar evolution; lunar science; the moon
First Star Formation in the Presence of Primordial Magnetic Fields

It has been recently claimed that primordial magnetic fields could relieve the cosmological Hubble tension. Fields of sufficient strength to relieve this tension would result in a magnetic field whose Alfvén velocity, va, is comparable to the speed of sound, cs, at the start of structure formation. We consider the impact of such fields on the formation of the first cosmological objects, minihalos (<106 M⊙), forming stars with zoom-in cosmological simulations tracking a single such minihalo. We seed each simulation with present-day field strengths of 2 × 10-12-2 × 10-10 G corresponding to initial ratios of Alfvén velocity to the speed of sound of va/cs ≈ 0.03-3. We find that when va/cs≪1, the effects are modest. However, when va ∼ cs, the starting time of the gravitational collapse is delayed and the duration extended as much as by Δz = 2.5 in redshift. When va > cs, the collapse is completely suppressed and the minihalos continue to grow and are unlikely to collapse until reaching the atomic cooling limit. Employing current observational limits on primordial magnetic fields we conclude that inflationary-produced primordial magnetic fields could have a significant impact on first star formation.

Keywords: primordial magnetic fields; population iii stars; magnetic fields; star formation; magnetohydrodynamical simulations
The Application of Machine Learning to Quasar and Seyfert Classification

Machine learning can be utilized to classify spectra flagged as Active Galactic Nuclei (AGNs) belonging to Seyferts or Quasars, expediting data collection and aiding in analyzing the AGN types. While many properties of Seyferts and Quasars can be used as feature points in training a machine learning model, one relatively available property with high information density is the spectra of the AGN types. This paper aims to describe the training and results of a K-Nearest Neighbors and a Dense Neural Network machine learning model built to classify AGNs as Seyfert type 1s, Seyfert type 2s, or Quasars.

Keywords: seyfert galaxies; galaxy classification systems; neural networks; active galactic nuclei; active galaxies; quasars
Observations and Chemical Modeling of the Isotopologues of Formaldehyde and the Cations of Formyl and Protonated Formaldehyde in the Hot Molecular Core G331.512-0.103

In the interstellar cold gas, the chemistry of formaldehyde (H2CO) can be essential to explain the formation of complex organic molecules. On this matter, the massive and energetic protostellar object G331 is still unexplored; hence, we carried out a comprehensive study of the isotopologues of H2CO, the formyl cation (HCO+), and protonated formaldehyde (H2COH+) through the APEX observations in a spectral window of ~159-356 GHz. We employed observational and theoretical methods to derive the physical properties of the molecular gas combining LTE and non-LTE analyses. Formaldehyde was characterized via 35 lines of H2CO, ${{\rm{H}}}_{2}^{13}$ CO, HDCO, and H2C18O. The formyl cation was detected via eight lines of HCO+, H13CO+, HC18O+, and HC17O+. Deuterium was clearly detected via HDCO, whereas DCO+ remained undetected. The H2COH+ was detected through three clean lines. According to the radiative analysis, formaldehyde appears to be embedded in a bulk gas with a wide range of temperatures (T ~ 20-90 K), while HCO+ and H2COH+ are primarily associated with colder gas (T ≲ 30 K). The reaction H2CO+HCO+ → H2COH+ + CO is crucial for the balance of the three species. We used the Nautilus gas-grain code to predict the evolution of their molecular abundances relative to H2; their values at timescales of ~103 yr matched the observations in G331: [H2CO] = (0.2-2) × 10-8, [HCO+] = (0.5-4) × 10-9, and [H2COH+] = (0.2-2) × 10-10. Based on the molecular evolution of H2CO, HCO+, and H2COH+, we hypothesized about the young lifetime of G331, which is consistent with the active gas-grain chemistry of massive protostellar objects.

Keywords: interstellar molecules; astrochemistry; molecular data; astronomy data analysis; spectral line identification; star forming regions; molecular reactions
New Horizons Observations of an Ultraviolet Stellar Occultation and Appulse by Pluto’s Atmosphere

A few hours after its encounter with Pluto, the Alice UV instrument on the New Horizons spacecraft observed the simultaneous stellar occultation and appulse of two UV-bright stars, HD 43153 and HD 42545, respectively. As during the solar occultation that occurred immediately prior, the transmission of starlight through Pluto’s atmosphere was sensitive to absorption by N2, CH4, C2H6, C2H2, C2H4, and haze. The line of sight to each star passed over different areas of Pluto from those probed by the solar occultation, providing insight into the degree of spatial and diurnal variability of atmospheric composition on Pluto. In this work, we report the profile of CH4 derived from both the stellar occultation and appulse observation, as well as the profiles of C2H6, C2H2, C2H4, and haze for the stellar occultation only. We compare these profiles to the solar occultation results and discuss implications for atmospheric variability on Pluto.

Keywords: occultation; ultraviolet extinction; pluto
Spatial Distribution of Glycine and Aspartic Acid in Rapidly Frozen Brines Relevant to Enceladus

Saturn's moon Enceladus harbors a global, subsurface liquid ocean beneath an icy crust that actively erupts water jets from fissures in its south pole. Data returned from the Cassini mission have identified salts and organic matter within these ejected plume particles. Such combinations of water, organics, and salts present rather complex chemical environments that may hold direct implications for habitability. The fundamental behavior of organics in frozen brine systems upon exposure to relevant Enceladus surface conditions is an important aspect that has not been explored to date (e.g., how they organize and partition relative to the salt minerals within the ice matrix). The present work investigates this topic by characterizing the spatial distribution of two amino acids with different side chains (glycine and aspartic acid) in a putative frozen Enceladus brine, containing sodium, chloride, and carbonate ions, via micro-Raman imaging. The results show that both organic-salt solutions likely undergo some degree of vitrification upon flash freezing, especially for the chloride-bearing species. The subsequent annealing of the vitreous samples reveals a preferential association of the amino acids with crystalline salt hydrates, while minimal negative to no correlation is found between them and water ice. In particular, both amino acids exhibit stronger affinities for natron (Na2CO3•10H2O) than hydrohalite (NaCl•2H2O). This suggests that solute-solute interaction likely dominates in these frozen systems, especially when charged species are present. The results imply that salt-rich ice particles can serve as concentrators of organic biomarkers, enhancing their potential for detection from remote sensing or in situ analysis.

Keywords: enceladus; ice spectroscopy; experimental techniques; astrochemistry; planetary science
Evidence That Shocks Destroy Small PAH Molecules in Low-luminosity Active Galactic Nuclei

We combined mapping-mode mid-infrared Spitzer spectra with complementary infrared imaging to perform a spatially resolved study of polycyclic aromatic hydrocarbon (PAH) emission from the central regions of 66 nearby galaxies, roughly evenly divided into star-forming systems and low-luminosity active galactic nuclei (AGNs). In conjunction with similar measurements available for quasars, we aim to understand the physical properties of PAHs across a broad range of black hole accretion power, with the goal of identifying observational diagnostics that can be used to probe the effect of AGNs on the host galaxy. Whereas the PAH emission correlates tightly with far-ultraviolet luminosity in star-forming regions, the spatially resolved regions of AGNs tend to be PAH deficient. Moreover, AGN regions exhibit on average smaller PAH 6.2 μm/7.7 μm and larger PAH 11.3 μm/7.7 μm band ratios. Although the current data are highly restrictive, they suggest that these anomalous PAH band ratios cannot be explained by the effects of the AGN radiation field alone. Instead, they hint that small grains may be destroyed by the combined effects of radiative processes and shocks, which are plausibly linked to jets and outflows preferentially associated with highly sub-Eddington, radiatively inefficient AGNs. While quasars also present a PAH deficit and unusual PAH band ratios, their characteristics differ in detail compared to those observed in more weakly accreting AGNs, a possible indicator of fundamental differences in their modes of energy feedback.

Keywords: interstellar medium; interstellar molecules; interstellar emissions; low-luminosity active galactic nuclei; active galactic nuclei; star formation; polycyclic aromatic hydrocarbons; shocks
Sun-Heliosphere Observation-based Ionization Rates Model

The solar wind (SW) and the extreme ultraviolet (EUV) radiation modulate fluxes of interstellar and heliospheric particles inside the heliosphere both in time and in space. Understanding this modulation is necessary to correctly interpret measurements of particles of interstellar origin inside the heliosphere. We present a revision of heliospheric ionization rates and provide the Sun-Heliosphere Observation-based Ionization Rates model based on the currently available data. We calculate the total ionization rates using revised SW and solar EUV data. We study the in-ecliptic variation of the SW parameters, the latitudinal structure of the SW speed and density, and the reconstruction of the photoionization rates. The revision most affects the SW out of the ecliptic plane during solar maximum and the estimation of the photoionization rates, the latter due to a change of the reference data. The revised polar SW is slower and denser during the solar maximum of solar cycle (SC) 24. The current estimated total ionization rates are higher than the previous ones for H, O, and Ne, and lower for He. The changes for the in-ecliptic total ionization rates are less than 10% for H and He, up to 20% for O, and up to 35% for Ne. Additionally, the changes are not constant in time and vary as a function of time and latitude.

Keywords: photoionization; interstellar medium; solar activity; solar cycle; ionization; heliosphere; charge exchange ionization; solar wind
2.5D Magnetohydrodynamic Simulation of the Formation and Evolution of Plasmoids in Coronal Current Sheets

In the present paper, using MPI-AMRVAC, we perform a 2.5D numerical magnetohydrodynamic simulation of the dynamics and associated thermodynamical evolution of an initially force-free Harris current sheet subjected to an external velocity perturbation under the condition of uniform resistivity. The amplitude of the magnetic field is taken to be 10 G, typical of the solar corona. We impose a Gaussian velocity pulse across this current sheet that mimics the interaction of fast magnetoacoustic waves with a current sheet in the corona. This leads to a variety of dynamics and plasma processes in the current sheet, which is initially quasi-static. The initial pulse interacts with the current sheet and splits into a pair of counterpropagating wavefronts, which form a rarefied region that leads to an inflow and a thinning of the current sheet. The thinning results in Petschek-type magnetic reconnection followed by a tearing instability and plasmoid formation. The reconnection outflows containing outward-moving plasmoids have accelerated motions with velocities ranging from 105 to 303 km s‑1. The average temperature and density of the plasmoids are found to be 8 MK and twice the background density of the solar corona, respectively. These estimates of the velocity, temperature, and density of the plasmoids are similar to values reported from various solar coronal observations. Therefore, we infer that the external triggering of a quasi-static current sheet by a single-velocity pulse is capable of initiating magnetic reconnection and plasmoid formation in the absence of a localized enhancement of resistivity in the solar corona.

Keywords: solar atmosphere; solar corona; magnetohydrodynamics; magnetic fields; solar magnetic reconnection
Connecting MeerKAT Radio Continuum Properties to GAMA Optical Emission-line and WISE Mid-infrared Activity

The identification of active galactic nuclei (AGNs) in large surveys has been hampered by seemingly discordant classifications arising from differing diagnostic methods, usually tracing distinct processes specific to a particular wavelength regime. However, as shown in Yao et al., the combination of optical emission-line measurements and mid-infrared photometry can be used to optimize the discrimination capability between AGN and star formation activity. In this paper we test our new classification scheme by combining the existing GAMA-WISE data with high-quality MeerKAT radio continuum data covering 8 deg2 of the GAMA G23 region. Using this sample of 1841 galaxies (z < 0.25), we investigate the total infrared (derived from 12 μm) to radio luminosity ratio, q (TIR), and its relationship to optical-infrared AGN and star-forming (SF) classifications. We find that while q (TIR) is efficient at detecting AGN activity in massive galaxies generally appearing quiescent in the infrared, it becomes less reliable for cases where the emission from star formation in the host galaxy is dominant. However, we find that the q (TIR) can identify up to 70% more AGNs not discernible at optical and/or infrared wavelengths. The median q (TIR) of our SF sample is 2.57 ± 0.23, consistent with previous local universe estimates.

Keywords: radio active galactic nuclei; active galactic nuclei
On the Source Contribution to the Galactic Diffuse Gamma Rays above 398 TeV Detected by the Tibet ASγ Experiment

Potential contribution from gamma-ray sources to the Galactic diffuse gamma rays observed above 100 TeV (sub-PeV energy range) by the Tibet ASγ experiment is an important key to interpreting recent multimessenger observations. This paper reveals a surprising fact: none of the 23 Tibet ASγ diffuse gamma-ray events above 398 TeV within the Galactic latitudinal range of ∣b∣ < 10° come from the 43 sub-PeV gamma-ray sources reported in the 1LHAASO catalog, which proves that these sources are not the origins of the Tibet ASγ diffuse gamma-ray events. No positional overlap between the Tibet ASγ diffuse gamma-ray events and the sub-PeV LHAASO sources currently supports the diffusive nature of the Tibet ASγ diffuse gamma-ray events, although their potential origin in the gamma-ray sources yet unresolved in the sub-PeV energy range cannot be ruled out.

Keywords: galactic cosmic rays; gamma-ray astronomy
High-velocity Type Ia Supernova Has a Unique Host Environment

Ejecta velocity of Type Ia supernovae (SNe Ia) is one powerful tool to differentiate between progenitor scenarios and explosion mechanisms. Here we revisit the relation between photospheric Si II λ6355 velocities (VSi II) and host-galaxy properties with ∼280 SNe Ia. A more stringent criterion on the phase of SN spectra is adopted to classify SNe Ia in terms of their photospheric velocities. We find a significant trend that SNe Ia with faster Si II λ6355 (high-VSi II SNe Ia) tend to explode in massive environments, whereas their slower counterparts can be found in both lower-mass and massive environments. This trend is further supported by the direct measurements on host gas-phase metallicities. We suggest this relation is likely caused by at least two populations of SNe Ia. Since stars of higher metallicity (at a given mass) generally form less massive white dwarfs, our results support some theoretical models that high-VSi II SNe Ia may originate from sub-Chandrasekhar class of explosions. Previous observations also showed some evidence that high-VSi II SNe Ia could be related to the single degenerate systems. However, we find high-VSi II SNe Ia do not come from particularly young populations. We conclude metallicity is likely the dominant factor in forming high-VSi II SNe Ia. This also implies their potential evolution with redshift and impact on the precision of SN Ia cosmology.

Keywords: type ia supernovae; galaxy environments
Rocklines as Cradles for Refractory Solids in the Protosolar Nebula

In our solar system, terrestrial planets and meteoritical matter exhibit various bulk compositions. To understand this variety of compositions, formation mechanisms of meteorites are usually investigated via a thermodynamic approach that neglects the processes of transport throughout the protosolar nebula. Here, we investigate the role played by rocklines (condensation/sublimation lines of refractory materials) in the innermost regions of the protosolar nebula to compute the composition of particles migrating inward toward the disk as a function of time. To do so, we utilize a one-dimensional accretion disk model with a prescription for dust and vapor transport, sublimation, and recondensation of refractory materials (ferrosilite, enstatite, fayalite, forsterite, iron sulfide, metal iron, and nickel). We find that the diversity of the bulk composition of cosmic spherules, chondrules, and chondrites can be explained by their formation close to rocklines, suggesting that solid matter is concentrated in the vicinity of these sublimation/condensation fronts. Although our model relies a lot on the number of considered species and the availability of thermodynamic data governing state changes, it suggests that rocklines played a major role in the formation of small and large bodies in the innermost regions of the protosolar nebula. Our model gives insights on the mechanisms that might have contributed to the formation of Mercury's large core.

Keywords: chondrules; chemical abundances; meteorite composition; micrometeorites; protoplanetary disks; protoplanetary nebulae; planet formation; solar system terrestrial planets; mercury (planet); chondrites
Spatially Resolved Ultraviolet Spectroscopy of the Great Dimming of Betelgeuse

The bright supergiant Betelgeuse (Alpha Orionis, HD 39801) experienced a visual dimming during 2019 December and the first quarter of 2020, reaching an historic minimum 2020 February 7-13. During 2019 September-November, prior to the optical dimming event, the photosphere was expanding. At the same time, spatially resolved ultraviolet spectra using the Hubble Space Telescope/Space Telescope Imaging Spectrograph revealed a substantial increase in the ultraviolet spectrum and Mg II line emission from the chromosphere over the southern hemisphere of the star. Moreover, the temperature and electron density inferred from the spectrum and C II diagnostics also increased in this hemisphere. These changes happened prior to the Great Dimming Event. Variations in the Mg II k-line profiles suggest material moved outwards in response to the passage of a pulse or acoustic shock from 2019 September through November. It appears that this extraordinary outflow of material from the star, likely initiated by convective photospheric elements, was enhanced by the coincidence with the outward motions in this phase of the ∼400 day pulsation cycle. These ultraviolet observations appear to provide the connecting link between the known large convective cells in the photosphere and the mass ejection event that cooled to form the dust cloud in the southern hemisphere imaged in 2019 December, and led to the exceptional optical dimming of Betelgeuse in 2020 February.

Keywords: m supergiant stars; stellar chromospheres; stellar mass loss; stellar atmospheres
Statistical Analysis of the Relation between Coronal Mass Ejections and Solar Energetic Particles

To improve the forecasting capability of impactful solar energetic particle (SEP) events, the relation between coronal mass ejections (CMEs) and SEP events needs to be better understood. Here we present a statistical study of SEP occurrences and timescales with respect to the CME source locations and speeds, considering all 257 fast (vCME ≥ 900 km s-1) and wide (angular width ≥60°) CMEs that occurred between 2006 December and 2017 October. We associate them with SEP events at energies above 10 MeV. Examination of the source region of each CME reveals that CMEs more often accompany a SEP event if they originate from the longitude of E20-W100 relative to the observer. However, an SEP event could still be absent if the CME is <2000 km s-1. For the associated CME-SEP pairs, we compute three timescales for each of the SEP events, namely the timescale of the onset (TO), the rise time (TR), and the duration (TD). They are correlated with the longitude of the CME source region relative to the footpoint of the Parker spiral (ΔΦ) and vCME. The TO tends to be short for $| {\rm{\Delta }}{\rm{\Phi }}| \ \lt $ 60°. This trend is weaker for TR and TD. The SEP timescales are only weakly correlated with vCME. Positive correlations of both TR and TD with vCME are seen in poorly connected (large $| {\rm{\Delta }}{\rm{\Phi }}| $ ) events. Additionally, TO appears to be negatively correlated with vCME for events with small $| {\rm{\Delta }}{\rm{\Phi }}| $ .

Keywords: solar energetic particles; solar coronal mass ejections; solar coronal mass ejection shocks; space weather