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<jats:title>Abstract</jats:title> <jats:p>We report the APOGEE-2S+ discovery of a unique collection of nitrogen-enhanced mildly metal-poor giant stars, peaking at [Fe/H] ∼ −0.89 with no carbon enrichment, toward the Small and Large Magellanic Clouds (SMC and LMC), with abundances of light- (C, N), odd-Z (Al, K), and <jats:italic>α</jats:italic>-elements (O, Mg, Si) that are typically found in Galactic globular clusters (GCs). Here we present 44 stars in the SMC and LMC that exhibit significantly enhanced [N/Fe] abundance ratios, well above ([N/Fe] ≳ +0.6) typical Galactic levels at similar metallicity, and a star that is very nitrogen-enhanced ([N/Fe] &gt; +2.45). Our sample consists of luminous evolved stars on the asymptotic giant branch (AGB), eight of which are classified as bona fide semi-regular (SR) variables, as well as low-luminosity stars similar to those of stars on the tip of the red giant branch of stellar clusters in the SMC and LMC. It seems likely that whatever nucleosynthetic process is responsible for these anomalous SMC and LMC stars it is similar to that which caused the common stellar populations in GCs. We interpret these distinctive C–N patterns as observational evidence of the result of tidally shredded GCs in the SMC and LMC. These findings might explain some previous conflicting results over bulge N-rich stars, and broadly help to understand GC formation and evolution. Furthermore, the discovery of such a large population of N-rich AGB stars in the SMC and LMC suggests that multiple stellar populations might not only be exotic events from the past, but can also form at lower redshift.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In many astrophysical environments the plasma is only partially ionized, and therefore the interaction of charged and neutral particles may alter both the triggering of reconnection and its subsequent dynamical evolution. We derive the tearing mode maximum growth rate for partially ionized plasmas in the cases of weak and strong coupling between the plasma and the neutrals. In addition, critical scalings for current sheet aspect ratios are presented in terms of Lundquist number and ion–neutral collision frequencies for which the tearing mode becomes fast, or ideal. In the decoupled regime the standard tearing mode is recovered with a small correction that depends on the ion–neutral collision frequency; in the intermediate regime collisions with neutrals are shown to stabilize current sheets, resulting in larger critical aspect ratios for ideal tearing to occur. In the coupled regime, the growth rate depends on the density ratio between ions and neutrals through the collision frequency between these two species.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The recent gravitational-wave merger event, GW190521, has challenged our understanding of stellar-mass black hole (BH) formation. The primary and secondary BHs are both inferred to fall inside the pair-instability (PI) mass gap. Here we propose that the formation of such binaries is possible through gas accretion onto the BH remnants of Population III stars born in high-redshift (<jats:italic>z</jats:italic> &gt; 10) minihalos. Once the parent halo has grown to the atomic-cooling limit, even brief episodes of gas accretion in the dense central regions of the halo can increase the masses of Population III remnant BHs above the PI limit. Starting with a binary black hole (BBH) with an initial mass of O(100) <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> we find that it would only need to spend about 100 Myr in the inner few parsecs of an atomic-cooling halo to accrete about 50 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> of material and resemble a system similar to GW190521. The dynamical friction timescale for the binary to sink to the dense inner region of its parent halo is comparable or shorter than the accretion timescale required to increase their mass above the PI limit. Once in the core of the halo, the binary can enter a phase of hyper-Eddington accretion, where it would only take a few thousand years to exceed the PI limit through accretion. Even more massive BBHs could form through this channel, and be detectable by detectors with improved low-frequency sensitivity. Single Population III BH remnants would also grow through accretion and could later form binaries dynamically. As little as a few percent of Population III BH remnants may be sufficient to match the rate of massive BBH mergers inferred from GW190521 of <jats:inline-formula> <jats:tex-math> <?CDATA ${0.13}_{-0.11}^{+0.3}\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabc253ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present results of a two-dimensional fully kinetic particle-in-cell simulation in order to shed light on the role of whistler waves in the scattering of strahl electrons and in the heat-flux regulation in the solar wind. We model the electron velocity distribution function as initially composed of core and strahl populations as typically encountered in the near-Sun solar wind as observed by Parker Solar Probe. We demonstrate that, as a consequence of the evolution of the electron velocity distribution function (VDF), two branches of the whistler heat-flux instability can be excited, which can drive whistler waves propagating in the direction oblique or parallel to the background magnetic field. First, oblique whistler waves induce pitch-angle scattering of strahl electrons, toward higher perpendicular velocities. This leads to the broadening of the strahl pitch-angle distribution and hence to the formation of a halo-like population at the expense of the strahl. Later on, the electron VDF experiences the effect of parallel whistler waves, which contributes to the redistribution of the particles scattered in the perpendicular direction into a more symmetric halo, in agreement with observations. Simulation results show a remarkable agreement with the linear theory of the oblique whistler heat-flux instability. The process is accompanied by a significant decrease of the heat flux carried by the strahl population.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>HuBi 1 has been proposed to be member of the rare class of born-again planetary nebulae (PNe), i.e., its central star experienced a very late thermal pulse and ejected highly processed material at high speeds inside the old hydrogen-rich PN. In this Letter we present GTC MEGARA integral field spectroscopic observations of the innermost regions of HuBi 1 at high spectral resolution ≃16 km s<jats:sup>−1</jats:sup> and multi-epoch subarcsecond images obtained ≃12 yr apart. The analysis of these data indicates that the inner regions of HuBi 1 were ejected ≃200 yr ago and expand at velocities ≃300 km s<jats:sup>−1</jats:sup>, in excellent agreement with the born-again scenario. The unprecedented tomographic capabilities of the GTC MEGARA high-dispersion observations used here reveal that the ejecta in HuBi 1 has a shell-like structure, in contrast to the disrupted disk and jet morphology of the ejecta in other born-again PNe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Non-rocky sub-Jovian exoplanets in high-irradiation environments are rare. LTT 9779b, also known as Transiting Exoplanet Survey Satellite (TESS) object of interest (TOI) 193.01, is one of the few such planets discovered to date, and the first example of an ultrahot Neptune. The planet’s bulk density indicates that it has a substantial atmosphere, so to investigate its atmospheric composition and shed further light on its origin, we obtained Spitzer InfraRed Array Camera secondary eclipse observations of LTT 9779b at 3.6 and 4.5 <jats:italic>μ</jats:italic>m. We combined the Spitzer observations with a measurement of the secondary eclipse in the TESS bandpass. The resulting secondary eclipse spectrum strongly prefers a model that includes CO absorption over a blackbody spectrum, incidentally making LTT 9779b the first TESS exoplanet (and the first ultrahot Neptune) with evidence of a spectral feature in its atmosphere. We did not find evidence of a thermal inversion, at odds with expectations based on the atmospheres of similarly irradiated hot Jupiters. We also report a nominal dayside brightness temperature of 2305 ± 141 K (based on the 3.6 <jats:italic>μ</jats:italic>m secondary eclipse measurement), and we constrained the planet’s orbital eccentricity to <jats:italic>e</jats:italic> &lt; 0.01 at the 99.7% confidence level. Together with our analysis of LTT 9779b’s thermal phase curves reported in a companion paper, our results set the stage for similar investigations of a larger sample of exoplanets discovered in the hot-Neptune desert, investigations that are key to uncovering the origin of this population.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Both the long-duration gamma-ray bursts (LGRBs) and the Type I superluminous supernovae (SLSNe I) have been proposed to be primarily powered by central magnetars. A correlation, proposed between the initial spin period (<jats:italic>P</jats:italic> <jats:sub>0</jats:sub>) and the surface magnetic field (<jats:italic>B</jats:italic>) of the magnetars powering the X-ray plateaus in LGRB afterglows, indicates a possibility that the magnetars have reached an equilibrium spin period due to the fallback accretion. The corresponding accretion rates are inferred as <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}\approx {10}^{-4}\mbox{--}{10}^{-1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabc254ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> s<jats:sup>−1</jats:sup>, and this result holds for the cases of both isotropic and collimated magnetar wind. For the SLSNe I and a fraction of engine-powered normal Type Ic supernovae (SNe Ic) and the broad-lined subclass (SNe Ic-BL), the magnetars could also reach an accretion-induced spin equilibrium, but the corresponding <jats:inline-formula> <jats:tex-math> <?CDATA $B\mbox{--}{P}_{0}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabc254ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> distribution suggests a different accretion rate range, i.e., <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}\approx {10}^{-7}\mbox{--}{10}^{-3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabc254ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> s<jats:sup>−1</jats:sup>. Considering the effect of fallback accretion, magnetars with relatively weak fields are responsible for the SLSNe I, while those with stronger magnetic fields could power SNe Ic/Ic-BL. Some SLSNe I in our sample could arise from compact progenitor stars, while others that require longer-term accretion may originate from the progenitor stars with more extended envelopes or circumstellar medium.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We use a sample of star-forming field and protocluster galaxies at <jats:italic>z</jats:italic> = 2.0–2.5 with Keck/MOSFIRE <jats:italic>K</jats:italic>-band spectra, a wealth of rest-frame ultraviolet (UV) photometry, and Spitzer/MIPS and Herschel/PACS observations, to dissect the relation between the ratio of infrared (IR) to UV luminosity (IRX) versus UV slope (<jats:italic>β</jats:italic>) as a function of gas-phase metallicity (<jats:inline-formula> <jats:tex-math> <?CDATA $12+\mathrm{log}({\rm{O}}/{\rm{H}})$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabc1efieqn1.gif" xlink:type="simple" /> </jats:inline-formula> ∼ 8.2–8.7). We find no significant dependence of the IRX-<jats:italic>β</jats:italic> trend on environment. However, we find that at a given <jats:italic>β</jats:italic>, IRX is highly correlated with metallicity, and less correlated with mass, age, and specific star formation rate (sSFR). We conclude that, of the physical properties tested here, metallicity is the primary physical cause of the IRX-<jats:italic>β</jats:italic> scatter, and the IRX correlation with mass is presumably due to the mass dependence on metallicity. Our results indicate that the UV attenuation curve steepens with decreasing metallicity, and spans the full range of slope possibilities from a shallow Calzetti-type curve for galaxies with the highest metallicity in our sample (<jats:inline-formula> <jats:tex-math> <?CDATA $12+\mathrm{log}({\rm{O}}/{\rm{H}})$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabc1efieqn2.gif" xlink:type="simple" /> </jats:inline-formula> ∼ 8.6) to a steep Small Magellanic Cloud (SMC)-like curve for those with <jats:inline-formula> <jats:tex-math> <?CDATA $12+\mathrm{log}({\rm{O}}/{\rm{H}})$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlabc1efieqn3.gif" xlink:type="simple" /> </jats:inline-formula> ∼ 8.3. Using a Calzetti (SMC) curve for the low (high) metallicity galaxies can lead to up to a factor of 3 overestimation (underestimation) of the UV attenuation and obscured star formation rate. We speculate that this change is due to different properties of dust grains present in the interstellar medium of low- and high-metallicity galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Quasi-periodic oscillations inferred during rare magnetar giant flare tails were initially interpreted as torsional oscillations of the neutron star (NS) crust, and have been more recently described as global core+crust perturbations. Similar frequencies are also present in high-signal-to-noise magnetar short bursts. In magnetars, disturbances of the field are strongly coupled to the NS crust regardless of the triggering mechanism of short bursts. For low-altitude magnetospheric magnetar models of fast radio bursts (FRBs) associated with magnetar short bursts, such as the low-twist model, crustal oscillations may be associated with additional radio bursts in the encompassing short burst event (as recently suggested for SGR 1935+2154). Given the large extragalactic volume probed by wide-field radio transient facilities, this offers the prospect of studying NS crusts leveraging samples far more numerous than galactic high-energy magnetar bursts by studying statistics of subburst structure or clustered trains of FRBs. We explore the prospects for distinguishing NS equation of state models with increasingly larger future sets of FRB observations. Lower <jats:italic>l</jats:italic>-number eigenmodes (corresponding to FRB time intervals of ∼5–50 ms) are likely less susceptible than high-<jats:italic>l</jats:italic> modes to confusion by systematic effects associated with the NS crust physics, magnetic field, and damping. They may be more promising in their utility, and also may corroborate models where FRBs arise from mature magnetars. Future observational characterization of such signals can also determine whether they can be employed as cosmological “standard oscillators” to constrain redshift, or can be used to constrain the mass of FRB-producing magnetars when reliable redshifts are available.</jats:p>