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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Performance and Stability Characterization of the ICARUS Light Detection System

The ICARUS detector, a key component of the Short Baseline Neutrino (SBN) Program at Fermilab, consists of two identical T300 modules filled with liquid argon. It is equipped with a Light Detection System (LDS) based on 360 8-inch Hamamatsu R5912-MOD photomultiplier tubes (PMTs) arranged behind the wire planes to collect Vacuum Ultraviolet (VUV, $\sim$ 128 nm) scintillation light. Operating under cryogenic conditions ( $\sim$ 87 K), the LDS is essential for determining the event start time (t0) with nanosecond precision for beam spill synchronization, improving longitudinal spatial resolution, and contributing to the event trigger and cosmic-ray mitigation. The performance of the LDS was investigated addressing both hardware and data analysis aspects. Following a progressive degradation in PMT gain observed during operations at FNAL, systematic gain measurements were first carried out from room temperature down to low temperatures. The results show stable performance at room temperature but a significant, irreversible reduction in gain at low temperatures. Based on these findings, a series of mitigation strategies were implemented in the ICARUS detector to preserve PMT performance and ensure reliable cryogenic operation. Currently, ongoing waveform analysis of the PMT signals is being performed to characterize signal shape, charge integration, and timing properties, aiming to refine and improve the agreement between experimental data and Monte Carlo simulations.

Saia, Clara [U. Catania (main); INAF, Catania; Cat↗
DOE OSTI2021

Studying Interfacial Dark Reactions of Glyoxal and Hydrogen Peroxide Using Vacuum Ultraviolet Single Photon Ionization Mass Spectrometry

Aqueous secondary organic aerosol (aqSOA) formation from volatile and semivolatile organic compounds at the air–liquid interface is considered as an important source of fine particles in the atmosphere. However, due to the lack of in situ detecting techniques, the detailed interfacial reaction mechanism and dynamics still remain uncertain. In this study, synchrotron-based vacuum ultraviolet single-photon ionization mass spectrometry (VUV SPI-MS) was coupled with the System for Analysis at the Liquid Vacuum Interface (SALVI) to investigate glyoxal dark oxidation products at the aqueous surface. Mass spectral analysis and determination of appearance energies (AEs) suggest that the main products of glyoxal dark interfacial aging are carboxylic acid related oligomers. Furthermore, the VUV SPI-MS results were compared and validated against those of in situ liquid time-of-flight secondary ion mass spectrometry (ToF-SIMS). The reaction mechanisms of the dark glyoxal interfacial oxidation, obtained using two different approaches, indicate that differences in ionization and instrument operation principles could contribute to their abilities to detect different oligomers. Therefore, the mechanistic differences revealed between the VUV SPI-MS and ToF-SIMS indicate that more in situ and real-time techniques are needed to investigate the contribution of the air–liquid interfacial reactions leading to aqSOA formation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Using Macro- and Microscale Preservation in Vertebrate Fossils as Predictors for Molecular Preservation in Fluvial Environments

Exceptionally preserved fossils retain soft tissues and often the biomolecules that were present in an animal during its life. The majority of terrestrial vertebrate fossils are not traditionally considered exceptionally preserved, with fossils falling on a spectrum ranging from very well-preserved to poorly preserved when considering completeness, morphology and the presence of microstructures. Within this variability of anatomical preservation, high-quality macro-scale preservation (e.g., articulated skeletons) may not be reflected in molecular-scale preservation (i.e., biomolecules). Excavation of the Hayden Quarry (HQ; Chinle Formation, Ghost Ranch, NM, USA) has resulted in the recovery of thousands of fossilized vertebrate specimens. This has contributed greatly to our knowledge of early dinosaur evolution and paleoenvironmental conditions during the Late Triassic Period (~212 Ma). The number of specimens, completeness of skeletons and fidelity of osteohistological microstructures preserved in the bone all demonstrate the remarkable quality of the fossils preserved at this locality. Because the Hayden Quarry is an excellent example of good preservation in a fluvial environment, we have tested different fossil types (i.e., bone, tooth, coprolite) to examine the molecular preservation and overall taphonomy of the HQ to determine how different scales of preservation vary within a single locality. We used multiple high-resolution mass spectrometry techniques (TOF-SIMS, GC-MS, FT-ICR MS) to compare the fossils to unaltered bone from extant vertebrates, experimentally matured bone, and younger dinosaurian skeletal material from other fluvial environments. FT-ICR MS provides detailed molecular information about complex mixtures, and TOF-SIMS has high elemental spatial sensitivity. Using these techniques, we did not find convincing evidence of a molecular signal that can be confidently interpreted as endogenous, indicating that very good macro- and microscale preservation are not necessarily good predictors of molecular preservation.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Theoretical Analysis of Experimental Data of Sodium Diffusion in Oxidized Molybdenum Thin Films

In this work, the diffusion process of sodium (Na) in molybdenum (Mo) thin films while it was deposited on soda lime glass (SLG) was studied. A small amount of oxygen was present in the chamber while the direct-current (DC) magnetron sputtering was used for the deposition. The substrate temperatures were varied to observe its effect. Such molybdenum films, with or without oxidations, are often used in thin film solar cells, either as back contact or as hole transport layers. Secondary ion mass spectrometry (SIMS) was used to quantify the concentration of the species. A grain diffusion mechanistic model incorporating the effect of grain and grain boundary geometrical shape and size was developed. The model was used to provide an in-depth theoretical analysis of the sodium diffusion in molybdenum thin films that lead to the measured SIMS data. It was observed that not only diffusion coefficients should be considered when analyzing diffusion processes in thin films but also the ratio of grain boundary size to grain size. Both depend on substrate temperature and directly affect the amount of diffused species in the film. The data were analyzed under the light of the film growth speed versus diffusion front speed, the effect of oxygen content, and the effect of substrate temperature on the overall diffusion process. The temperature inversely affects the ratio of grain boundary size and grain size and directly affects the diffusion coefficient, which leads to a preferable temperature at which the highest amount of alkali can be found in the film.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Numerical Modeling and Dynamic Analysis of a Wave-Powered Reverse-Osmosis System

A wave energy converter (WEC) system has the potential to convert the wave energy resource directly into the high-pressure flow that is needed by the desalination system to pump saltwater to the reverse-osmosis membrane and provide the required pressure level to generate freshwater. In this study, a wave-to-water numerical model was developed to investigate the potential use of a wave-powered desalination system (WPDS) for water production. The model was developed by coupling a time-domain radiation-and-diffraction method-based numerical tool (WEC-Sim) for predicting the hydrodynamic performance of WECs with a solution-diffusion model that was used to simulate the reverse-osmosis (RO) process. The objective of this research is to evaluate the WPDS dynamics and the overall efficiency of the system. To evaluate the feasibility of the WPDS, the wave-to-water numerical model was applied to simulate a desalination system that used an oscillating surge WEC device to pump seawater through the system. The hydrodynamics WEC-Sim simulation results for the oscillating surge WEC device were validated against existing experimental data. The RO simulation was verified by comparing the results to those from the Dow Chemical Company’s reverse osmosis system analysis (ROSA) model, which has been widely used to design and simulate RO systems. The wave-to-water model was then used to analyze the WPDS under a range of wave conditions and for a two-WECs-coupled RO system to evaluate the influence of pressure and flow rate fluctuation on the WPDS performance. The results show that the instantaneous energy fluctuation from waves has a significant influence on the responding hydraulic pressure and flow rate, as well as the recovery ratio and, ultimately, the water-production quality. Nevertheless, it is possible to reduce the hydraulic fluctuation for different sea states while maintaining a certain level of freshwater production, and a WEC array that produces water can be a viable, near-term solution to the nation’s water supply. A discussion on the dynamic impact of hydraulic fluctuation on the WPDS performance and potential options to reduce the fluctuation and their trade-offs is also presented.

16 TIDAL AND WAVE POWER↗
DOE OSTI2022

Fabricating Graphene Oxide/h-BN Metal Insulator Semiconductor Diodes by Nanosecond Laser Irradiation

To employ graphene’s rapid conduction in 2D devices, a heterostructure with a broad bandgap dielectric that is free of traps is required. Within this paradigm, h-BN is a good candidate because of its graphene-like structure and ultrawide bandgap. We show how to make such a heterostructure by irradiating alternating layers of a-C and a-BN film with a nanosecond excimer laser, melting and zone-refining constituent layers in the process. With Raman spectroscopy and ToF-SIMS analyses, we demonstrate this localized zone-refining into phase-pure h-BN and rGO films with distinct Raman vibrational modes and SIMS profile flattening after laser irradiation. Furthermore, in comparing laser-irradiated rGO-Si MS and rGO/h-BN/Si MIS diodes, the MIS diodes exhibit an increased turn-on voltage (4.4 V) and low leakage current. The MIS diode I-V characteristics reveal direct tunneling conduction under low bias and Fowler-Nordheim tunneling in the high-voltage regime, turning the MIS diode ON with improved rectification and current flow. This study sheds light on the nonequilibrium approaches to engineering h-BN and graphene heterostructures for ultrathin field effect transistor device development.

2D materials↗
DOE OSTI2016

Nonuniversal star formation efficiency in turbulent ISM

We present a study of a star formation prescription in which star formation efficiency (SFE) depends on local gas density and turbulent velocity dispersion, as suggested by direct simulations of SF in turbulent giant molecular clouds (GMCs). We test the model using a simulation of an isolated Milky-Way-sized galaxy with a self-consistent treatment of turbulence on unresolved scales. We show that this prescription predicts a wide variation of local SFE per free-fall time, ϵ{sub ff} ∼ 0.1%–10%, and gas depletion time, t{sub dep} ∼ 0.1–10 Gyr. In addition, it predicts an effective density threshold for star formation due to suppression of ϵ{sub ff} in warm diffuse gas stabilized by thermal pressure. We show that the model predicts star formation rates (SFRs) in agreement with observations from the scales of individual star-forming regions to the kiloparsec scales. This agreement is nontrivial, as the model was not tuned in any way and the predicted SFRs on all scales are determined by the distribution of the GMC-scale densities and turbulent velocities σ in the cold gas within the galaxy, which is shaped by galactic dynamics. The broad agreement of the star formation prescription calibrated in the GMC-scale simulations with observations both gives credence to such simulations and promises to put star formation modeling in galaxy formation simulations on a much firmer theoretical footing.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

( Fermi Fermi)/LAT study of gamma-ray emission in the direction of the Monoceros Loop supernova remnant

We present an analysis of the gamma-ray measurements by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope in the region of the supernova remnant (SNR) Monoceros Loop (G205.5+0.5). The brightest gamma-ray peak is spatially correlated with the Rosette Nebula, which is a molecular cloud complex adjacent to the southeast edge of the SNR. After subtraction of this emission by spatial modeling, the gamma-ray emission from the SNR emerges, which is extended and fit by a Gaussian spatial template. The gamma-ray spectra are significantly better reproduced by a curved shape than a simple power law. The luminosities between 0.2 and 300 GeV are ∼4×10{sup 34} erg s{sup −1} for the SNR and ∼3×10{sup 34} erg s{sup −1} for the Rosette Nebula, respectively. We argue that the gamma-rays likely originate from the interactions of particles accelerated in the SNR. The decay of neutral pions produced in nucleon–nucleon interactions of accelerated hadrons with interstellar gas provides a reasonable explanation for the gamma-ray emission of both the Rosette Nebula and the Monoceros SNR.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The NuSTAR extragalactic surveys: the number counts of active galactic nuclei and the resolved fraction of the cosmic X-ray background

We present the 3–8 keV and 8–24 keV number counts of active galactic nuclei (AGNs) identified in the Nuclear Spectroscopic Telescope Array (NuSTAR) extragalactic surveys. NuSTAR has now resolved 33%–39% of the X-ray background in the 8–24 keV band, directly identifying AGNs with obscuring columns up to ∼10{sup 25} cm{sup −2}. In the softer 3–8 keV band the number counts are in general agreement with those measured by XMM-Newton and Chandra over the flux range 5×10{sup −15} ≲ S(3–8 keV)/erg s{sup −1} cm{sup −2} ≲ 10{sup −12} probed by NuSTAR. In the hard 8–24 keV band NuSTAR probes fluxes over the range 2×10{sup −14} ≲ S(8–24 keV)/erg s{sup −1} cm{sup −2} ≲ 10{sup −12}, a factor ∼100 fainter than previous measurements. The 8–24 keV number counts match predictions from AGN population synthesis models, directly confirming the existence of a population of obscured and/or hard X-ray sources inferred from the shape of the integrated cosmic X-ray background. The measured NuSTAR counts lie significantly above simple extrapolation with a Euclidian slope to low flux of the Swift/BAT 15–55 keV number counts measured at higher fluxes (S(15–55 keV) ≳ 10{sup −11} erg s{sup −1} cm{sup −2}), reflecting the evolution of the AGN population between the Swift/BAT local (z<0.1) sample and NuSTAR’s z∼1 sample. CXB synthesis models, which account for AGN evolution, lie above the Swift/BAT measurements, suggesting that they do not fully capture the evolution of obscured AGNs at low redshifts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The Redmapper galaxy cluster catalog from DES science verification data

We describe updates to the redMaPPer algorithm, a photometric red-sequence cluster finder specifically designed for large photometric surveys. The updated algorithm is applied to 150 deg{sup 2} of Science Verification (SV) data from the Dark Energy Survey (DES), and to the Sloan Digital Sky Survey (SDSS) DR8 photometric data set. The DES SV catalog is locally volume limited and contains 786 clusters with richness λ>20 (roughly equivalent to M{sub 500c}≳10{sup 14} h{sub 70}{sup −1} M{sub ⊙}) and 0.2<z<0.9. The DR8 catalog consists of 26,311 clusters with 0.08<z<0.6, with a sharply increasing richness threshold as a function of redshift for z≳0.35. The photometric redshift performance of both catalogs is shown to be excellent, with photometric redshift uncertainties controlled at the σ{sub z}/(1+z)∼0.01 level for z≲0.7, rising to ∼0.02 at z∼0.9 in DES SV. We make use of Chandra and XMM X-ray and South Pole Telescope Sunyaev–Zeldovich data to show that the centering performance and mass–richness scatter are consistent with expectations based on prior runs of redMaPPer on SDSS data. We also show how the redMaPPer photo-z and richness estimates are relatively insensitive to imperfect star/galaxy separation and small-scale star masks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe

We describe the Sloan Digital Sky Survey IV (SDSS-IV), a project encompassing three major spectroscopic programs. The Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) is observing hundreds of thousands of Milky Way stars at high resolution and high signal-to-noise ratios in the near-infrared. The Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey is obtaining spatially resolved spectroscopy for thousands of nearby galaxies (median z∼0.03). The extended Baryon Oscillation Spectroscopic Survey (eBOSS) is mapping the galaxy, quasar, and neutral gas distributions between z∼0.6 and 3.5 to constrain cosmology using baryon acoustic oscillations, redshift space distortions, and the shape of the power spectrum. Within eBOSS, we are conducting two major subprograms: the SPectroscopic IDentification of eROSITA Sources (SPIDERS), investigating X-ray AGNs and galaxies in X-ray clusters, and the Time Domain Spectroscopic Survey (TDSS), obtaining spectra of variable sources. All programs use the 2.5 m Sloan Foundation Telescope at the Apache Point Observatory; observations there began in Summer 2014. APOGEE-2 also operates a second near-infrared spectrograph at the 2.5 m du Pont Telescope at Las Campanas Observatory, with observations beginning in early 2017. Observations at both facilities are scheduled to continue through 2020. In keeping with previous SDSS policy, SDSS-IV provides regularly scheduled public data releases; the first one, Data Release 13, was made available in 2016 July.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

A Simple Analytical Model for Rocky Planet Interiors

This work aims to explore the scaling relations among rocky exoplanets. The assumption that the internal gravity increases linearly in the core and stays constant in the mantle is tested against numerical simulations, and a simple model is constructed, applicable to rocky exoplanets of CMF (core mass fraction) = 0.2–0.35 and mass = 0.1–30 M ⊕. Various scaling relations are derived: (1) CMF ≈ CRF 2 (core radius fraction squared), (2) ${P}_{\mathrm{typical}}\sim {g}_{s}^{2}$ (typical interior pressure scales as surface gravity squared), (3) energy released in core formation is $\sim \tfrac{1}{10}$ the total gravitational energy, (4) effective heat capacity of the mantle $\approx \left(\tfrac{{M}_{p}}{{M}_{\oplus }}\right)\times 7.5\times {10}^{27}$ J K -1 , (5) moment of inertia $\approx \tfrac{1}{3}{M}_{p}{R}_{p}^{2}$. These approximate relations are handy for quick use owing to their simplicity and lucidity, and provide insights into the interior structures of those exoplanets.

planets and satellites: interiors↗
DOE OSTI2017

Testing for X-Ray–SZ Differences and Redshift Evolution in the X-Ray Morphology of Galaxy Clusters

We present a quantitative study of the X-ray morphology of galaxy clusters, as a function of their detection method and redshift. We analyze two separate samples of galaxy clusters: a sample of 36 clusters at 0.35<z<0.9 selected in the X-ray with the ROSAT PSPC 400 deg{sup 2} survey, and a sample of 90 clusters at 0.25<z<1.2 selected via the Sunyaev–Zel’dovich (SZ) effect with the South Pole Telescope. Clusters from both samples have similar-quality Chandra observations, which allow us to quantify their X-ray morphologies via two distinct methods: centroid shifts (w) and photon asymmetry (A{sub phot}). The latter technique provides nearly unbiased morphology estimates for clusters spanning a broad range of redshift and data quality. We further compare the X-ray morphologies of X-ray- and SZ-selected clusters with those of simulated clusters. We do not find a statistically significant difference in the measured X-ray morphology of X-ray and SZ-selected clusters over the redshift range probed by these samples, suggesting that the two are probing similar populations of clusters. We find that the X-ray morphologies of simulated clusters are statistically indistinguishable from those of X-ray- or SZ-selected clusters, implying that the most important physics for dictating the large-scale gas morphology (outside of the core) is well-approximated in these simulations. Finally, we find no statistically significant redshift evolution in the X-ray morphology (both for observed and simulated clusters), over the range of z∼0.3 to z∼1, seemingly in contradiction with the redshift-dependent halo merger rate predicted by simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Constraining Reionization with the z ~ 5–6 Ly α Forest Power Spectrum: The Outlook after Planck

The latest measurements of cosmic microwave background electron-scattering optical depth reported by Planck significantly reduces the allowed space of ${\rm{H}}\,{\rm{I}}$ reionization models, pointing toward a later ending and/or less extended phase transition than previously believed. Reionization impulsively heats the intergalactic medium (IGM) to $\sim {10}^{4}\,{\rm{K}}$, and owing to long cooling and dynamical times in the diffuse gas that are comparable to the Hubble time, memory of reionization heating is retained. Therefore, a late-ending reionization has significant implications for the structure of the $z\sim 5\mbox{-}6$ Lyα forest. Using state-of-the-art hydrodynamical simulations that allow us to vary the timing of reionization and its associated heat injection, we argue that extant thermal signatures from reionization can be detected via the Lyα forest power spectrum at $5\lt z\lt 6$. This arises because the small-scale cutoff in the power depends not only the the IGM temperature at these epochs, but is also particularly sensitive to the pressure-smoothing scale set by the IGM full thermal history. Comparing our different reionization models with existing measurements of the Lyα forest flux power spectrum at $z=5.0\mbox{-}5.4$, we find that models satisfying Planck's ${\tau }_{e}$ constraint favor a moderate amount of heat injection consistent with galaxies driving reionization, but disfavoring quasar-driven scenarios. We study the feasibility of measuring the flux power spectrum at $z\simeq 6$ using mock quasar spectra and conclude that a sample of ~10 high-resolution spectra with an attainable signal-to-noise ratio will allow distinguishing between different reionization scenarios.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Determination of Dark Matter Halo Mass from Dynamics of Satellite Galaxies

We propose that the mass of a dark matter halo can be inferred from the dynamical status of its satellite galaxies. Using nine dark matter simulations of halos like the Milky Way (MW), we find that the present-day substructures in each halo follow a characteristic distribution in the phase space of orbital binding energy and angular momentum, and that this distribution is similar from halo to halo, but has an intrinsic dependence on the halo formation history. We build this distribution directly from the simulations for a specific halo and extend the result to halos of similar formation history but different masses by scaling. The mass of an observed halo can then be estimated by maximizing the likelihood in comparing the measured kinematic parameters of its satellite galaxies with these distributions. We test the validity and accuracy of this method with mock samples taken from the simulations. Using the positions, radial velocities, and proper motions of nine tracers and assuming observational uncertainties comparable to those of MW satellite galaxies, we find that the halo mass can be recovered to within $\sim 40 \% $. The accuracy can be improved to within ~25% if 30 tracers are used. Yet, the dependence of the phase-space distribution on the halo formation history sets a minimum uncertainty of $\sim 20 \% $ that cannot be reduced by using more tracers. We believe that this minimum uncertainty also applies to any mass determination for a halo when the phase-space information of other kinematic tracers is used.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina

Here, we present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the Carina II (Car II) and Carina III (Car III) dwarf galaxy candidates, recently discovered in the Magellanic Satellites Survey (MagLiteS). We identify 18 member stars in Car II, including two binaries with variable radial velocities and two RR Lyrae stars. The other 14 members have a mean heliocentric velocity ${v}_{\mathrm{hel}}=477.2\pm 1.2$ $\mathrm{km}\,{{\rm{s}}}^{-1}$ and a velocity dispersion of ${\sigma }_{v}={3.4}_{-0.8}^{+1.2}$ $\mathrm{km}\,{{\rm{s}}}^{-1}$. Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of ${1.0}_{-0.4}^{+0.8}\times {10}^{6}$ ${M}_{\odot }$, indicating a mass-to-light ratio of ${369}_{-161}^{+309}$ ${M}_{\odot }$/${L}_{\odot }$. From equivalent width measurements of the calcium triplet lines of nine red giant branch (RGB) stars, we derive a mean metallicity of ${\rm{[Fe/H]}}=-2.44\pm 0.09$ with dispersion ${\sigma }_{{\rm{[Fe/H]}}}={0.22}_{-0.07}^{+0.10}$. Considering both the kinematic and chemical properties, we conclude that Car II is a dark-matter-dominated dwarf galaxy. For Car III, we identify four member stars, from which we calculate a systemic velocity of ${v}_{\mathrm{hel}}={284.6}_{-3.1}^{+3.4}$ $\mathrm{km}\,{{\rm{s}}}^{-1}$. The brightest RGB member of Car III has a metallicity of ${\rm{[Fe/H]}}\,=-1.97\pm 0.12$. Due to the small size of the Car III spectroscopic sample, we cannot conclusively determine its nature. Although these two systems have the smallest known physical separation (${\rm{\Delta }}d\sim 10\,\mathrm{kpc}$) among Local Group satellites, the large difference in their systemic velocities, $\sim 200\,\mathrm{km}\,{{\rm{s}}}^{-1}$, indicates that they are unlikely to be a bound pair. One or both systems are likely associated with the Large Magellanic Cloud (LMC), and may remain LMC satellites today. No statistically significant excess of γ-ray emission is found at the locations of Car II and Car III in eight years of Fermi-LAT data.

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

The Origin of $r$-process Enhanced Metal-poor Halo Stars In Now-destroyed Ultra-faint Dwarf Galaxies

The highly $r$-process-enhanced (r-II) metal-poor halo stars we observe today could play a key role in understanding early ultra-faint dwarf galaxies (UFDs), the smallest building blocks of the Milky Way. If a significant fraction of metal-poor r-II halo stars originated in the UFDs that merged to help form the Milky Way, observations of r-II stars could help us study these now-destroyed systems and probe the formation history of our Galaxy. To conduct our initial investigation into this possible connection, we use high-resolution cosmological simulations of Milky Way-mass galaxies from the Caterpillar suite in combination with a simple, empirically motivated treatment of $r$-process enrichment. Further, we determine the fraction of metal-poor halo stars that could have formed from highly $r$-process-enhanced gas in now-destroyed low-mass UFDs, the simulated r-II fraction, and compare it to the “as observed” r-II fraction. We find that the simulated fraction, $f_{r-II,sim}$ ~1%–2%, can account for around half of the “as observed” fraction, $f_{r-II,obs}$ ~ 2%–4%. The “as observed” fraction likely overrepresents the fraction of r-II stars due to incomplete sampling, though, meaning $f_{r-II,sim}$ likely accounts for more than half of the true fr-II,obs. Further considering some parameter variations and scatter between individual simulations, the simulated fraction can account for around 20%–80% of the “as observed” fraction.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Electron and Proton Heating in Transrelativistic Guide Field Reconnection

The plasma in low-luminosity accretion flows, such as the one around the black hole at the center of M87 or Sgr A* at our Galactic Center, is expected to be collisioness and of two temperatures, with protons hotter than electrons. Here, particle heating is expected to be controlled by magnetic reconnection in the transrelativistic regime ${\sigma }_{w}\sim 0.1\mbox{--}1$, where the magnetization ${\sigma }_{w}$ is the ratio of magnetic energy density to plasma enthalpy density. Using large-scale 2D particle-in-cell simulations, we explore for a fiducial ${\sigma }_{w}=0.1$ how the dissipated magnetic energy is partitioned between electrons and protons as a function of ${\beta }_{{\rm{i}}}$ (the ratio of proton thermal pressure to magnetic pressure) and of the strength of a guide field ${B}_{{\rm{g}}}$ perpendicular to the reversing field B 0. At low ${\beta }_{{\rm{i}}}\ (\lesssim 0.1)$, we find that the fraction of initial magnetic energy per particle converted into electron irreversible heat is nearly independent of ${B}_{{\rm{g}}}/{B}_{0}$, whereas protons are heated much less with increasing ${B}_{{\rm{g}}}/{B}_{0}$. As a result, for large ${B}_{{\rm{g}}}/{B}_{0}$, electrons receive the overwhelming majority of irreversible particle heating (~93% for ${B}_{{\rm{g}}}/{B}_{0}=6$). This is significantly different than the antiparallel case ${B}_{{\rm{g}}}/{B}_{0}=0$, in which irreversible electron heating accounts for only ~18% of the total particle heating (Rowan et al. 2017). At ${\beta }_{{\rm{i}}}\sim 2$, when both species start already relativistically hot (for our fiducial ${\sigma }_{w}=0.1$), electrons and protons each receive ~50% of the irreversible particle heating, regardless of the guide field strength. Our results provide important insights into the plasma physics of electron and proton heating in hot accretion flows around supermassive black holes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗