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<jats:title>Abstract</jats:title> <jats:p>The paper presents a new method for deriving turbulent heating of the solar wind using plasma moments and magnetic field data. We develop the method and then apply it to compute the turbulent heating of the solar wind proton plasma at 1 au. The method employs two physical properties of the expanding solar wind plasma, the wave-particle thermodynamic equilibrium, and the transport of entropic rate. We analyze plasma moments and field data taken from Wind S/C, in order to compute (i) the fluctuating magnetic energy, (ii) the corresponding correlation length, and (iii) the turbulent heating rate. We identify their relationships with the solar wind speed, as well as the variation of these relationships relative to solar wind and interplanetary coronal mass ejection plasma.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>NGC 7538 IRS 1 is a very young embedded O star driving an ionized jet and accreting mass with an accretion rate &gt;10<jats:sup>−4</jats:sup> <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf5bieqn1.gif" xlink:type="simple" /> </jats:inline-formula> yr<jats:sup>−1</jats:sup>, which is quenching the hypercompact H <jats:sc>ii</jats:sc> region. We use SOFIA GREAT data, Herschel PACS and SPIRE archive data, SOFIA FORCAST archive data, Onsala 20 m and CARMA data, and JCMT archive data to determine the properties of the O star and its outflow. IRS 1 appears to be a single O star with a bolometric luminosity &gt;1 × 10<jats:sup>5</jats:sup> <jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf5bieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, i.e., spectral type O7 or earlier. We find that IRS 1 drives a large molecular outflow with the blueshifted northern outflow lobe extending to ∼280″ or 3.6 pc from IRS 1. Near IRS 1 the outflow is well aligned with the ionized jet. The dynamical timescale of the outflow is ∼1.3 × 10<jats:sup>5</jats:sup> yr. The total outflow mass is ∼130 <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf5bieqn3.gif" xlink:type="simple" /> </jats:inline-formula>. We determine a mass outflow rate of 1.0 × 10<jats:sup>−3</jats:sup> <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf5bieqn4.gif" xlink:type="simple" /> </jats:inline-formula> yr<jats:sup>−1</jats:sup>, roughly consistent with the observed mass accretion rate. We observe strong high-velocity [C <jats:sc>ii</jats:sc>] emission in the outflow, confirming that strong UV radiation from IRS 1 escapes into the outflow lobes and is ionizing the gas. Many O stars may form like low-mass stars, but with a higher accretion rate and in a denser environment. As long as the accretion stays high enough to quench the H <jats:sc>ii</jats:sc> region, the star will continue to grow. When the accretion rate drops, the H <jats:sc>ii</jats:sc> region will rapidly start to expand.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>This report presents a three-dimensional (3D) numerical magnetohydrodynamics (MHD) model of the white-light coronagraph observational phenomena known as coronal inflows and in/out pairs. Coronal inflows in the Large Angle and Spectrometric Coronagraph/C2 field of view (approximately <jats:inline-formula> <jats:tex-math> <?CDATA $2\mbox{--}6\,{R}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabc5b3ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) were thought to arise from the dynamic and intermittent release of solar wind plasma associated with the helmet streamer belt as the counterpart to outward-propagating streamer blobs, formed by magnetic reconnection. This interpretation was essentially confirmed with the subsequent identification of in/out pairs and the multispacecraft observations of their 3D structure. The MHD simulation results show relatively narrow lanes of density depletion form high in the corona and propagate inward with sinuous motion that has been characterized as “tadpole-like” in coronagraph imagery. The height–time evolution and velocity profiles of the simulation inflows and in/out pairs are compared to their corresponding observations and a detailed analysis of the underlying magnetic field structure associated with the synthetic white-light and mass density evolution is presented. Understanding the physical origin of this structured component of the slow solar wind’s intrinsic variability could make a significant contribution to solar wind modeling and the interpretation of remote and in situ observations from Parker Solar Probe and Solar Orbiter.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent measurements of rotation periods (<jats:inline-formula> <jats:tex-math> <?CDATA ${P}_{\mathrm{rot}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf58ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) in the benchmark open clusters Praesepe (670 Myr), NGC 6811 (1 Gyr), and NGC 752 (1.4 Gyr) demonstrate that, after converging onto a tight sequence of slowly rotating stars in mass–period space, stars temporarily stop spinning down. These data also show that the duration of this epoch of stalled spin-down increases toward lower masses. To determine when stalled stars resume spinning down, we use data from the K2 mission and the Palomar Transient Factory to measure <jats:inline-formula> <jats:tex-math> <?CDATA ${P}_{\mathrm{rot}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf58ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> for 58 dwarf members of the 2.7 Gyr old cluster Ruprecht 147, 39 of which satisfy our criteria designed to remove short-period or near-equal-mass binaries. Combined with the Kepler <jats:inline-formula> <jats:tex-math> <?CDATA ${P}_{\mathrm{rot}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf58ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> data for the approximately coeval cluster NGC 6819 (30 stars with <jats:italic>M</jats:italic> <jats:sub>⋆</jats:sub> &gt; 0.85 <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf58ieqn4.gif" xlink:type="simple" /> </jats:inline-formula>), our new measurements more than double the number of ≈2.5 Gyr benchmark rotators and extend this sample down to ≈0.55 <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf58ieqn5.gif" xlink:type="simple" /> </jats:inline-formula>. The slowly rotating sequence for this joint sample appears relatively flat (22 ± 2 days) compared to sequences for younger clusters. This sequence also intersects the Kepler intermediate-period gap, demonstrating that this gap was not created by a lull in star formation. We calculate the time at which stars resume spinning down and find that 0.55 <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbf58ieqn6.gif" xlink:type="simple" /> </jats:inline-formula> stars remain stalled for at least 1.3 Gyr. To accurately age-date low-mass stars in the field, gyrochronology formulae must be modified to account for this stalling timescale. Empirically tuning a core–envelope coupling model with open cluster data can account for most of the apparent stalling effect. However, alternative explanations, e.g., a temporary reduction in the magnetic braking torque, cannot yet be ruled out.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The scaling laws that relate the peak temperature <jats:italic>T</jats:italic> <jats:sub> <jats:italic>M</jats:italic> </jats:sub> and volumetric heating rate <jats:italic>E</jats:italic> <jats:sub> <jats:italic>H</jats:italic> </jats:sub> to the pressure <jats:italic>P</jats:italic> and length <jats:italic>L</jats:italic> for static coronal loops were established over 40 yr ago; they have proved to be of immense value in a wide range of studies. Here we extend these scaling laws to dynamic loops, where enthalpy flux becomes important to the energy balance, and study impulsive heating/filling characterized by upward enthalpy flows. We show that for collision-dominated thermal conduction, the functional dependencies of the scaling laws are the same as for the static case, when the radiative losses scale as <jats:italic>T</jats:italic> <jats:sup>−1/2</jats:sup>, but with a different constant of proportionality that depends on the Mach number <jats:italic>M</jats:italic> of the flow. The dependence on the Mach number is such that the scaling laws for low to moderate Mach number flows are almost indistinguishable from the static case. When thermal conduction is limited by turbulent processes, however, the much weaker dependence of the scattering mean free path (and hence thermal conduction coefficient) on temperature leads to a limiting Mach number for return enthalpy fluxes driven by thermal conduction between the corona and chromosphere.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The X-ray binary 4U 1954+31 has been classified as a low-mass X-ray binary containing an M giant and a neutron star (NS). It has also been included in the rare class of X-ray symbiotic binaries. The Gaia parallax, infrared colors, spectral type, abundances, and orbital properties of the M star demonstrate that the cool star in this system is not a low-mass giant but a high-mass M supergiant. Thus, 4U 1954+31 is a high-mass X-ray binary (HMXB) containing a late-type supergiant. It is the only known binary system of this type. The mass of the M I is <jats:inline-formula> <jats:tex-math> <?CDATA ${9}_{-2}^{+6}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbe01ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> giving an age of this system in the range 12–50 Myr with the NS no more than 43 Myr old. The spin period of the NS is one of the longest known, 5 hr. The existence of M I plus NS binary systems is in accord with stellar evolution theory, with this system a more evolved member of the HMXB population.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo’s third observing run (O3). We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook follow-up with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization area of 4480 deg<jats:sup>2</jats:sup>, a median distance of 267 Mpc, and false-alarm rates ranging from 1.5 to 10<jats:sup>−25</jats:sup> yr<jats:sup>−1</jats:sup>. The ZTF coverage in the <jats:italic>g</jats:italic> and <jats:italic>r</jats:italic> bands had a median enclosed probability of 39%, median depth of 20.8 mag, and median time lag between merger and the start of observations of 1.5 hr. The O3 follow-up by the GROWTH team comprised 340 UltraViolet/Optical/InfraRed (UVOIR) photometric points, 64 OIR spectra, and three radio images using 17 different telescopes. We find no promising kilonovae (radioactivity-powered counterparts), and we show how to convert the upper limits to constrain the underlying kilonova luminosity function. Initially, we assume that all GW triggers are bona fide astrophysical events regardless of false-alarm rate and that kilonovae accompanying BNS and NSBH mergers are drawn from a common population; later, we relax these assumptions. Assuming that all kilonovae are at least as luminous as the discovery magnitude of GW170817 (−16.1 mag), we calculate that our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than −16.6 mag (the extrapolated peak magnitude of GW170817) and fade at a rate of 1 mag day<jats:sup>−1</jats:sup> (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations based on the online classifications, the joint probability of zero detections, assuming all kilonovae are brighter than −16.6 mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, no more than &lt;57% (&lt;89%) of putative kilonovae could be brighter than −16.6 mag assuming flat evolution (fading by 1 mag day<jats:sup>−1</jats:sup>) at the 90% confidence level. If we further take into account the online terrestrial probability for each GW trigger, we find that no more than &lt;68% of putative kilonovae could be brighter than −16.6 mag. Comparing to model grids, we find that some kilonovae must have <jats:italic>M</jats:italic> <jats:sub>ej</jats:sub> &lt; 0.03 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, <jats:italic>X</jats:italic> <jats:sub>lan</jats:sub> &gt; 10<jats:sup>−4</jats:sup>, or <jats:italic>ϕ</jats:italic> &gt; 30° to be consistent with our limits. We look forward to searches in the fourth GW observing run; even 17 neutron star mergers with only 50% coverage to a depth of −16 mag would constrain the maximum fraction of bright kilonovae to &lt;25%.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a spectroscopic survey of over 3400 potential members in the North America and Pelican Nebulae (NAP) using several low-resolution (<jats:italic>R</jats:italic> ≈ 1300–2000) spectrographs: Palomar/Norris, WIYN/Hydra, Keck/DEep Imaging Multi-Object Spectrograph (DEIMOS), and the Multiple Mirror Telescope (MMT)/Hectospec. We identify 580 young stars as likely members of the NAP region based on criteria involving infrared excess, Li <jats:sc>I</jats:sc> 6708 Å absorption, X-ray emission, parallax, and proper motions. The spectral types of individual spectra are derived by fitting them with templates that are either empirical spectra of pre-main-sequence stars or model atmospheres. The templates are artificially veiled, and a best-fit combination of spectral type and veiling parameter is derived for each star. We use the spectral types with archival photometry to derive <jats:italic>V</jats:italic>-band extinction and stellar luminosity. From the Hertzsprung–Russell diagram, the median age of the young stars is about 1 Myr, with a luminosity dispersion of ∼0.3–0.4 dex. We investigate the photometric variability of the spectroscopic member sample using Zwicky Transient Facility data and conclude that photometric variability, while present, does not significantly contribute to the luminosity dispersion. While larger than the formal errors, the luminosity dispersion is smaller than if veiling were not taken into account in our spectral typing process. The measured ages of the stellar kinematic groups, combined with the inferred ages for embedded stellar populations revealed by Spitzer, suggest a sequential history of star formation in the NAP region.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a comprehensive search for the 3.5 keV line using ∼51 Ms of archival Chandra observations peering through the Milky Way’s dark matter halo from across the entirety of the sky, gathered via the Chandra Source Catalog Release 2.0. We consider the data’s radial distribution, organizing observations into four data subsets based on angular distance from the Galactic center. All data are modeled using both background-subtracted and background-modeled approaches to account for the particle instrument background, demonstrating the statistical limitations of the currently available ∼1 Ms of particle background data. A nondetection is reported in the total data set, allowing us to set an upper limit on 3.5 keV line flux and constrain the sterile neutrino dark matter mixing angle. The upper limit on sin<jats:sup>2</jats:sup>(2<jats:italic>θ</jats:italic>) is <jats:inline-formula> <jats:tex-math> <?CDATA $2.58\times {10}^{-11}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbee9ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> (though systematic uncertainty may increase this by a factor of ∼2), corresponding to an upper limit on 3.5 keV line flux of <jats:inline-formula> <jats:tex-math> <?CDATA $2.34\times {10}^{-7}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabbee9ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> photons s<jats:sup>−1</jats:sup> cm<jats:sup>−2</jats:sup>. These limits show consistency with recent constraints and several prior detections. Nondetections are reported in all radial data subsets, allowing us to constrain the spatial profile of 3.5 keV line intensity, which does not conclusively differ from Navarro–Frenk–White predictions. Thus, while offering heavy constraints, we do not entirely rule out the sterile neutrino dark matter scenario or the more general decaying dark matter hypothesis for the 3.5 keV line. We have also used the nondetection of any unidentified emission lines across our continuum to further constrain the sterile neutrino parameter space.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Observations with the Atacama Large Millimeter/Submillimeter Array (ALMA) have dramatically improved our understanding of the site of exoplanet formation: protoplanetary disks. However, many basic properties of these disks are not well understood. The most fundamental of these is the total disk mass, which sets the mass budget for planet formation. Disks with sufficiently high masses can excite gravitational instability and drive spiral arms that are detectable with ALMA. Although spirals have been detected in ALMA observations of the dust, their association with gravitational instability, and high disk masses, is far from clear. Here we report a prediction for kinematic evidence of gravitational instability. Using hydrodynamics simulations coupled with radiative transfer calculations, we show that a disk undergoing such instability has clear kinematic signatures in molecular line observations across the entire disk azimuth and radius, which are independent of viewing angle. If these signatures are detected, it will provide the clearest evidence for the occurrence of gravitational instability in planet-forming disks, and provide a crucial way to measure disk masses.</jats:p>