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<jats:title>Abstract</jats:title> <jats:p>The density and temperature properties of the intergalactic medium (IGM) reflect the heating and ionization history during cosmological structure formation, and are primarily probed by the Ly<jats:italic>α</jats:italic> forest of neutral hydrogen absorption features in the observed spectra of background sources. We present the methodology and initial results from the <jats:italic>Cholla</jats:italic> IGM Photoheating Simulation (CHIPS) suite performed with the graphics process unit–accelerated <jats:italic>Cholla</jats:italic> code to study the IGM at high, uniform spatial resolution maintained over large volumes. In this first paper, we examine the IGM structure in CHIPS cosmological simulations that include IGM uniform photoheating and photoionization models where hydrogen reionization is completed early or by redshift <jats:italic>z</jats:italic> ∼ 6. Comparing with observations of the large- and small-scale Ly<jats:italic>α</jats:italic> transmitted flux power spectra <jats:italic>P</jats:italic>(<jats:italic>k</jats:italic>) at redshifts 2 ≲ <jats:italic>z</jats:italic> ≲ 5.5, the relative agreement of the models depends on scale, with the self-consistent Puchwein et al. IGM photoheating and photoionization model in good agreement with the flux <jats:italic>P</jats:italic>(<jats:italic>k</jats:italic>) at <jats:italic>k</jats:italic> ≳ 0.01 s km<jats:sup>−1</jats:sup> at redshifts 2 ≲ <jats:italic>z</jats:italic> ≲ 3.5. On larger scales, the <jats:italic>P</jats:italic>(<jats:italic>k</jats:italic>) measurements increase in amplitude from <jats:italic>z</jats:italic> ∼ 4.6 to <jats:italic>z</jats:italic> ∼ 2.2, faster than the models, and lie in between the model predictions at 2.2 ≲ <jats:italic>z</jats:italic> ≲ 4.6 for <jats:italic>k</jats:italic> ≈ 0.002–0.01 s km<jats:sup>−1</jats:sup>. We argue that the models could improve by changing the He <jats:sc>ii</jats:sc> photoheating rate associated with active galactic nuclei to reduce the IGM temperature at <jats:italic>z</jats:italic> ∼ 3. At higher redshifts, <jats:italic>z</jats:italic> ≳ 4.5, the observed flux <jats:italic>P</jats:italic>(<jats:italic>k</jats:italic>) amplitude increases at a rate intermediate between the models, and we argue that for models where hydrogen reionization is completed late (<jats:italic>z</jats:italic> ∼ 5.5–6), resolving this disagreement will require inhomogeneous or “patchy” reionization. We then use an additional set of simulations to demonstrate that our results have numerically converged and are not strongly affected by varying cosmological parameters.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>NASA’s Parker Solar Probe mission continues to travel closer to the Sun than any prior human-made object, with an expected closest approach of &lt;10 solar radii (&lt;0.046 au) by 2024. On board, the Integrated Science Investigation of the Sun instrument suite makes unprecedented in situ measurements of energetic particles in the near-Sun environment. The current low level of solar activity offers a prime opportunity to measure cosmic rays closer to the Sun than ever before. We present the first observations of anomalous cosmic rays in to 36 solar radii (0.166 au), focusing specifically on helium. Our results indicate a strong radial intensity gradient of ∼25 ± 5%/au over energies of ∼4 to ∼45 MeV/nuc. These values are larger than prior observations, further out in the heliosphere, and come at a unique time in our understanding and modeling of particle transport and acceleration, particularly as both Voyagers have crossed the heliopause and IBEX has accumulated a full solar cycle of observations. Thus, continued measurements of cosmic rays by Parker Solar Probe will play a critical role in linking past observations with our present knowledge and significantly advancing our understanding of cosmic ray transport in the heliosphere.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The delay-time distribution (DTD) is the occurrence rate of a class of objects as a function of time after a hypothetical burst of star formation. DTDs are mainly used as a statistical test of stellar evolution scenarios for supernova progenitors, but they can be applied to many other classes of astronomical objects. We calculate the first DTD for RR Lyrae variables using 29,810 RR Lyrae from the OGLE-IV survey and a map of the stellar age distribution (SAD) in the Large Magellanic Cloud (LMC). We find that ∼46% of the OGLE-IV RR Lyrae are associated with delay times greater than 8 Gyr (main-sequence progenitor masses less than 1 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>), and consistent with existing constraints on their ages, but surprisingly about 51% of RR Lyrae appear to have delay times of 1.2–8 Gyr (main-sequence masses between 1 and 2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> at LMC metallicity). This intermediate-age signal also persists outside the Bar region, where crowding is less of a concern, and we verified that without this signal the spatial distribution of the OGLE-IV RR Lyrae is inconsistent with the SAD map of the LMC. Since an intermediate-age RR Lyrae channel is in tension with the lack of RR Lyrae in intermediate-age clusters (noting issues with small-number statistics), and noting the age–metallicity constraints on LMC stars, our DTD result possibly indicates that systematic uncertainties may still exist in SAD measurements of old stellar populations, perhaps stemming from the construction methodology or the stellar evolution models used. We describe tests to further investigate this issue.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In ultraviolet (UV) spectropolarimetric observations of the solar corona, the existence of a magnetic field, solar wind velocity, and temperature anisotropies modify the linear polarization associated with resonant scattering. Unlike previous empirical models or global models, which present blended results of the above physical effects, in this work, we forward-model expected signals in the H <jats:sc>i</jats:sc> Ly<jats:italic>α</jats:italic> line (121.6 nm) by adopting an analytic model that can be adjusted to test the roles of different effects separately. We find that the impact of all three effects is most evident in the rotation of the linear polarization direction. In particular, (1) for magnetic fields between ∼10 and ∼100 G, the Hanle effect modifies the linear polarization at low coronal heights, rotating the linear polarization direction clockwise (counterclockwise) when the angle between the magnetic field and the local vertical is greater (less) than the van Vleck angle, which is consistent with the result of Zhao et al.; (2) solar wind velocity, which increases with height, has a significant effect through the Doppler dimming effect at higher coronal heights, rotating the linear polarization direction in an opposite fashion to the Hanle effect; and (3) kinetic temperature anisotropies are most significant at lower heights in open nonradial magnetic field regions, producing tilt opposite to isotropic Doppler dimming. The fact that the three effects operate differently in distinct spatial regimes opens up the possibility for using linear polarization measurements in UV lines to diagnose these important physical characteristics of the solar corona.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Radial diffusion in planetary radiation belts is a dominant transport mechanism resulting in the energization and losses of charged particles by large-scale electromagnetic fluctuations. In this study, we revisit the radial diffusion formalism by relaxing the assumption of zero correlation time in the spectrum of fluctuations responsible for the transport of charged particles. We derive a diffusion coefficient by assuming fluctuations that (1) are time homogeneous, (2) too small to trap the particles, and (3) can decorrelate on timescales comparable to the transit time of the particles. We demonstrate through self-similar solutions of the Fokker–Planck equation that autocorrelation time <jats:italic>τ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> much larger than the linear transit time/particle drift period <jats:inline-formula> <jats:tex-math> <?CDATA ${\tau }_{L}={{\rm{\Omega }}}_{D}^{-1}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">Ω</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>D</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabf04bieqn1.gif" xlink:type="simple" /> </jats:inline-formula> results in characteristic time for transport independent of the drift frequency and faster than for short correlation time. In both instances, that is for short (<jats:italic>τ</jats:italic> <jats:sub> <jats:italic>L</jats:italic> </jats:sub> ≫ <jats:italic>τ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub>) and long (<jats:italic>τ</jats:italic> <jats:sub> <jats:italic>L</jats:italic> </jats:sub> ≪ <jats:italic>τ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub>) autocorrelation time, the diffusion of particles is subdiffusive since the variance of increments along the magnetic drift shells <jats:italic>L</jats:italic>*, scales as 〈Δ<jats:italic>L</jats:italic>*<jats:sup>2</jats:sup>〉 ∼ <jats:italic>t</jats:italic> <jats:sup> <jats:italic>s</jats:italic> </jats:sup>, with <jats:italic>s</jats:italic> &lt; 1. However, in the absence of sources and sinks, particle transport for both short and long autocorrelation times result in equilibrium distribution along <jats:italic>L</jats:italic>* with differences of less than 10% across lower magnetic drift shells. The main consequence of incorporating finite correlation time appears in intermediate times much longer than the drift period but before the distribution function reaches equilibrium and indicates the importance of quantifying observationally the spectral properties of fluctuations for the modeling of planetary radiation belts.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Heating of neutral gas by energetic sources is crucial for the prediction of the 21 cm signal during the epoch of reionization. To investigate differences induced on the statistics of the 21 cm signal by various source types, we use five radiative transfer simulations that have the same stellar UV emission model and varying combinations of more energetic sources, such as X-ray binaries (XRBs), accreting nuclear black holes (BHs), and hot interstellar medium emission (ISM). We find that the efficient heating from the ISM increases the average global 21 cm signal while reducing its fluctuations and thus power spectrum. A clear impact is also observed in the bispectrum in terms of scale and timing of the transition between a positive and a negative value. The impact of XRBs is similar to that of the ISM, although it is delayed in time and reduced in intensity because of the less efficient heating. Due to the paucity of nuclear BHs, the behavior of the 21 cm statistics in their presence is very similar to that of a case when only stars are considered, with the exception of the latest stages of reionization, when the effect of BHs is clearly visible. We find that differences between the source scenarios investigated here are larger than the instrumental noise of SKA1-low at <jats:italic>z</jats:italic> ≳ 7–8, suggesting that in the future it might be possible to constrain the spectral energy distribution of the sources contributing to the reionization process.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present newly calibrated period–<jats:italic>ϕ</jats:italic> <jats:sub>31</jats:sub>–[Fe/H] relations for fundamental-mode RR Lyrae stars in the optical and, for the first time, mid-infrared. This work’s calibration data set provides the largest and most comprehensive span of parameter space to date, with homogeneous metallicities from −3 ≲ [Fe/H] ≲ 0.4 and accurate Fourier parameters derived from 1980 ASAS-SN (<jats:italic>V</jats:italic> band) and 1083 WISE (NEOWISE extension, <jats:italic>W</jats:italic>1 and <jats:italic>W</jats:italic>2 bands) RR Lyrae stars with well-sampled light curves. We compare our optical period–<jats:italic>ϕ</jats:italic> <jats:sub>31</jats:sub>–[Fe/H] relation with those available in the literature and demonstrate that our relation minimizes systematic trends in the lower and higher metallicity range. Moreover, a direct comparison shows that our optical photometric metallicities are consistent with both those from high-resolution spectroscopy and globular clusters, supporting the good performance of our relation. We found an intrinsic scatter in the photometric metallicities (0.41 dex in the <jats:italic>V</jats:italic> band and 0.50 dex in the infrared) by utilizing large calibration data sets covering a broad metallicity range. This scatter becomes smaller when optical and infrared bands are used together (0.37 dex). Overall, the relations derived in this work have many potential applications, including large-area photometric surveys with James Webb Space Telescope in the infrared and LSST in the optical.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The accurate measurement of stellar masses over a wide range of galaxy properties is essential for better constraining models of galaxy evolution. Emission-line galaxies (ELGs) tend to have better redshift estimates than continuum-selected objects, and have been shown to span a large range of physical properties, including stellar mass. Using data from the 3D-HST Treasury program, in this work, we construct a carefully vetted sample of 4350 ELGs at redshifts of 1.16 &lt; <jats:italic>z</jats:italic> &lt; 1.90. We combine the 3D-HST emission-line fluxes with far-UV through near-IR photometry, and use the <jats:monospace>MCSED</jats:monospace> spectral energy distribution fitting code to constrain the galaxies’ physical parameters, such as their star-formation rate and stellar masses. Our sample is consistent with the <jats:italic>z</jats:italic> ∼ 2 mass–metallicity relation. More importantly, we show that there is a simple, but tight correlation between stellar mass and absolute magnitude in a near-IR filter, which should prove particularly useful in terms of the rapid calculation of accurate stellar masses for millions of galaxies in upcoming missions such as Euclid, and the Nancy Grace Roman Space Telescope.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Although stellar-mass black holes (BHs) are likely to be abundant in the Milky Way (<jats:italic>N</jats:italic> = 10<jats:sup>8</jats:sup>–10<jats:sup>9</jats:sup>), only 20 have been detected to date, all in accreting binary systems. Gravitational microlensing is a proposed technique to search for <jats:italic>isolated</jats:italic> BHs, which have not yet been detected. Two specific microlensing events, MACHO-1996-BLG-5 (M96-B5) and MACHO-1998-BLG-6 (M98-B6), initially observed near the lens-source minimum angular separation in 1996 and 1998, respectively, have long Einstein crossing times (&gt;300 days), identifying the lenses as candidate black holes. Twenty years have elapsed since the time of lens-source closest approach for each of these events, indicating that if the lens and source are both luminous, and if their relative proper motion is sufficiently large, the two components should be spatially resolvable. In this work, we attempt to eliminate the possibility of a stellar lens for these events by: (1) using Keck near-infrared adaptive optics images to search for a potentially now-resolved, luminous lens, and (2) examining multi-band photometry of the source to search for flux contributions from a potentially unresolved, luminous lens. We combine detection limits from NIRC2 images with light-curve data to eliminate all non-BH lenses for relative lens-source proper motions above 0.81 mas yr<jats:sup>−1</jats:sup> for M96-B5, and 2.48 mas yr<jats:sup>−1</jats:sup> for M98-B6. Furthermore, we use WFPC2 broad-band images to eliminate the possibility of stellar lenses at any proper motion. We present the narrow range of non-BH possibilities permitted by our varied analyses. Finally, we suggest future observations to constrain the remaining parameter space via the methods developed in this work.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report photometric estimates of effective temperature, <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub>, metallicity, [Fe/H], carbonicity, [C/Fe], and absolute carbon abundances, <jats:italic>A</jats:italic>(C), for over 700,000 stars from the Southern Photometric Local Universe Survey (S-PLUS) Data Release 2, covering a substantial fraction of the equatorial Sloan Digital Sky Survey Stripe 82. We present an analysis for two stellar populations: (1) halo main-sequence turnoff stars and (2) K-dwarf stars of mass 0.58 &lt; <jats:italic>M</jats:italic>/<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> &lt; 0.75 in the Solar Neighborhood. Application of the Stellar Photometric Index Network Explorer (<jats:monospace>SPHINX</jats:monospace>) to the mixed-bandwidth (narrow- plus wide-band) filter photometry from S-PLUS produces robust estimates of the metallicities and carbon abundances in stellar atmospheres over a wide range of temperatures, 4250 &lt; <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub>(K) &lt; 7000. The use of multiple narrow-band S-PLUS filters enables <jats:monospace>SPHINX</jats:monospace> to achieve substantially lower levels of “catastrophic failures” (i.e., large offsets in metallicity estimates relative to spectroscopic determinations) than previous efforts using a single metallicity-sensitive narrow-band filter. We constrain the exponential slope of the Milky Way’s K-dwarf halo metallicity distribution function (MDF), <jats:italic>λ</jats:italic> <jats:sub>10,[Fe/H]</jats:sub> = 0.85 ± 0.21, over the metallicity range −2.5 &lt; [Fe/H] &lt; −1.0; the MDF of our local-volume K-dwarf sample is well-represented by a gamma distribution with parameters <jats:italic>α</jats:italic> = 2.8 and <jats:italic>β</jats:italic> = 4.2. S-PLUS photometry obtains absolute carbon abundances with a precision of ∼0.35 dex for stars with <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> &lt; 6500 K. We identify 364 candidate carbon-enhanced metal-poor stars, obtain assignments of these stars into the Yoon–Beers morphological groups in the <jats:italic>A</jats:italic>(C)-[Fe/H] space, and we derive the CEMP frequencies.</jats:p>