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<jats:title>Abstract</jats:title> <jats:p>We present a search for quasi-monochromatic gravitational-wave signals from the young, energetic X-ray pulsar PSR J0537−6910 using data from the second and third observing runs of LIGO and Virgo. The search is enabled by a contemporaneous timing ephemeris obtained using Neutron star Interior Composition Explorer (NICER) data. The NICER ephemeris has also been extended through 2020 October and includes three new glitches. PSR J0537−6910 has the largest spin-down luminosity of any pulsar and exhibits fRequent and strong glitches. Analyses of its long-term and interglitch braking indices provide intriguing evidence that its spin-down energy budget may include gravitational-wave emission from a time-varying mass quadrupole moment. Its 62 Hz rotation frequency also puts its possible gravitational-wave emission in the most sensitive band of the LIGO/Virgo detectors. Motivated by these considerations, we search for gravitational-wave emission at both once and twice the rotation frequency from PSR J0537−6910. We find no signal, however, and report upper limits. Assuming a rigidly rotating triaxial star, our constraints reach below the gravitational-wave spin-down limit for this star for the first time by more than a factor of 2 and limit gravitational waves from the <jats:italic>l</jats:italic> = <jats:italic>m</jats:italic> = 2 mode to account for less than 14% of the spin-down energy budget. The fiducial equatorial ellipticity is constrained to less than about 3 ×10<jats:sup>−5</jats:sup>, which is the third best constraint for any young pulsar.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We identified eight additional stars as members of the Helmi stream (HStr) in the combined GALAH+ DR3 and Gaia EDR3 catalog. By consistently reevaluating claimed members from the literature, we consolidate a sample of 22 HStr stars with parameters determined from high-resolution spectroscopy and spanning a considerably wider (by ∼0.5 dex) metallicity interval (− 2.5 ≲ [Fe/H] &lt; − 1.0) than previously reported. Our study focuses on <jats:italic>α</jats:italic> (Mg and Ca) and neutron-capture (Ba and Eu) elements. We find that the chemistry of HStr is typical of dwarf spheroidal (dSph) galaxies, in good agreement with previous <jats:italic>N</jats:italic>-body simulations of this merging event. Stars of HStr constitute a clear declining sequence in [<jats:italic>α</jats:italic>/Fe] for increasing metallicity up to [Fe/H] ∼ −1.0. Moreover, stars of HStr show a median value of +0.5 dex for [Eu/Fe] with a small dispersion (±0.1 dex). Every star analyzed with [Fe/H] &lt; −1.2 belongs to the <jats:italic>r</jats:italic>-process enhanced ([Eu/Fe] &gt; +0.3 and [Ba/Eu] &lt; 0.0) metal-poor category, providing remarkable evidence that, at such a low-metallicity regime, stars of HStr experienced enrichment in neutron-capture elements predominantly via <jats:italic>r</jats:italic>-process nucleosynthesis. Finally, the extended metallicity range also suggests an increase in [Ba/Eu] for higher [Fe/H], in conformity with other surviving dwarf satellite galaxies of the Milky Way.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Several astrophysical scenarios have been proposed to explain the origin of the population of binary black hole (BBH) mergers detected in gravitational waves by the LIGO/Virgo Collaboration. Among them, BBH mergers assembled dynamically in young massive and open clusters have been shown to produce merger rate densities consistent with LIGO/Virgo estimated rates. We use the results of a suite of direct, high-precision <jats:italic>N</jats:italic>-body evolutionary models of young massive and open clusters and build the population of BBH mergers, by accounting for both a cosmologically motivated model for the formation of young massive and open clusters and the detection probability of LIGO/Virgo. We show that our models produce dynamically paired BBH mergers that are well consistent with the observed masses, mass ratios, effective spin parameters, and final spins of the second Gravitational Wave Transient Catalog (GWTC-2).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The abundances of <jats:italic>r</jats:italic>-process elements of very metal-poor stars capture the history of the <jats:italic>r</jats:italic>-process enrichment in the early stage of star formation in a galaxy. Currently, various types of astrophysical sites including neutron star mergers (NSMs), magneto-rotational supernovae, and collapsars, are suggested as the origin of <jats:italic>r</jats:italic>-process elements. The time delay between the star formation and the production of <jats:italic>r</jats:italic>-process elements is the key to distinguish these scenarios, with the caveat that the diffusion of <jats:italic>r</jats:italic>-process elements in the interstellar medium may induce the delay in <jats:italic>r</jats:italic>-process enrichment because <jats:italic>r</jats:italic>-process events are rare. Here we study the observed Ba abundance data of very metal-poor stars as the tracer of the early enrichment history of <jats:italic>r</jats:italic>-process elements. We find that the gradual increase of [Ba/Mg] with [Fe/H], which is remarkably similar among the Milky Way and classical dwarfs, Requires a significant time delay (100 Myr–1 Gyr) of <jats:italic>r</jats:italic>-process events from star formation rather than the diffusion-induced delay. We stress that this conclusion is robust to the assumption regarding <jats:italic>s</jats:italic>-process contamination in the Ba abundances because the sources with no delay would overproduce Ba at very low metallicities, even without the contribution from the <jats:italic>s</jats:italic>-process. Therefore, we conclude that sources with a delay, possibly NSMs, are the origins of <jats:italic>r</jats:italic>-process elements.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Although numerous white dwarf stars host dusty debris disks, the temperature distribution of these stars differs significantly from the white dwarf population as a whole. Dusty debris disks exist exclusively around white dwarfs cooler than 27,000 K. This is all the more enigmatic given that the formation processes of dusty debris disks should favor younger, hotter white dwarfs, which likely host more dynamically unstable planetary systems. Here we apply a sophisticated material sublimation model to white dwarf systems to show that these statistics are actually a natural result of the interplay of thermal and tidal forces and how they define the circumstellar regions where dusty debris disks can form. We demonstrate that these processes tend to prevent stability against both sublimative destruction and reaccretion into planetesimals for rocky materials until white dwarfs cool to below ∼25,000–32,000 K, in agreement with the observed limit of ∼27,000 K. For pure water ice, this critical temperature is less than 2700 K (requiring a cooling age older the universe); this precludes pure water ice–rich debris disks forming through the accepted two-step mechanism. The critical temperature is size-dependent; more massive white dwarfs could potentially host dusty debris disks at warmer temperatures. Our model suggests that the location of the disks within the PG 0010+280, GD 56, GD 362, and PG 1541+651 systems are consistent with a forsterite-dominated olivine composition. We also find that very cool white dwarfs may simultaneously host multiple, independently formed dusty debris disks, consistent with observations of the LSPM J0207+3331 system.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Observations of hot-Jupiter atmospheres show large variations in the location of the “hot spot” and the amplitude of spectral features. Atmospheric flow simulations using the commonly employed forcing and initialization have generally produced a large, monolithic patch of nearly stationary hot area located eastward of the substellar point at high altitude. Here we perform high-resolution (T682) pseudospectral simulations that accurately capture small-scale eddies and waves inherent in hot-Jupiter atmospheres due to ageostrophy. The atmospheres contain a large number of intense storms over a wide range of scales, including the planetary scale. The latter sized storms dictate the large-scale spatial distribution and temporal variability of hot, as well as cold, regions over the planet. In addition, they exhibit quasi-periodic life cycles within multiple equilibrium states—all identifiable in the disk-integrated time series of the temperature flux.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The unidentified TeV source MGRO J1908+06, with emission extending from hundreds of GeV to beyond 100 TeV, is one of the most intriguing sources in the Galactic plane. MGRO J1908+06 spatially associates with an IceCube hotspot of neutrino emission. Although the hotspot is not significant yet, this suggests a possible hadronic origin of the observed gamma-ray radiation. Here we describe a multiwavelength analysis on MGRO J1908+06 to determine its nature. We identify, for the first time, an extended GeV source as the counterpart of MGRO J1908 + 06, discovering possibly associated molecular clouds (MCs). The GeV spectrum shows two well-differentiated components: a soft spectral component below ∼10 GeV, and a hard one (Γ ∼ 1.6) above these energies. The lower-energy part is likely associated with the dense MCs surrounding the supernova remnant (SNR) G40.5−0.5, whereas the higher-energy component, which connects smoothly with the spectrum observed in TeV range, resembles the inverse Compton emission observed in relic pulsar wind nebulae. This simple scenario seems to describe the data satisfactorily, but raises questions about the interpretation of the emission at hundreds of TeV. In this scenario, no detectable neutrino flux would be expected.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The supernova remnant (SNR) 3C 397 is thought to originate from a Type Ia supernova (SN Ia) explosion of a near-Chandrasekhar-mass (<jats:italic>M</jats:italic> <jats:sub>Ch</jats:sub>) progenitor, based on the enhanced abundances of Mn and Ni revealed by previous X-ray study with Suzaku. Here we report follow-up XMM-Newton observations of this SNR, conducted with the aim of investigating the detailed spatial distribution of the Fe-peak elements. We have discovered an ejecta clump with extremely high abundances of Ti and Cr, in addition to Mn, Fe, and Ni, in the southern part of the SNR. The Fe mass of this ejecta clump is estimated to be ∼0.06 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, under the assumption of a typical Fe yield for SNe Ia (i.e., ∼0.8 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>). The observed mass ratios among the Fe-peak elements and Ti require substantial neutronization that is achieved only in the innermost regions of a near-<jats:italic>M</jats:italic> <jats:sub>Ch</jats:sub> SN Ia with a central density of <jats:italic>ρ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> ∼ 5 × 10<jats:sup>9</jats:sup> g cm<jats:sup>−3</jats:sup>, significantly higher than typically assumed for standard near-<jats:italic>M</jats:italic> <jats:sub>Ch</jats:sub> SNe Ia (<jats:italic>ρ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> ∼ 2 × 10<jats:sup>9</jats:sup> g cm<jats:sup>−3</jats:sup>). The overproduction of the neutron-rich isotopes (e.g., <jats:sup>50</jats:sup>Ti and <jats:sup>54</jats:sup>Cr) is significant in such high-<jats:italic>ρ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> SNe Ia, with respect to the solar composition. Therefore, if 3C 397 is a typical high-<jats:italic>ρ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> near-<jats:italic>M</jats:italic> <jats:sub>Ch</jats:sub> SN Ia remnant, the solar abundances of these isotopes could be reproduced by the mixture of the high- and low-<jats:italic>ρ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> near-<jats:italic>M</jats:italic> <jats:sub>Ch</jats:sub> and sub-<jats:italic>M</jats:italic> <jats:sub>Ch</jats:sub> Type Ia events, with ≲20% being high-<jats:italic>ρ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> near-<jats:italic>M</jats:italic> <jats:sub>Ch</jats:sub>.</jats:p>