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<jats:title>Abstract</jats:title> <jats:p>Magnetars are one of the potential power sources for some energetic supernova explosions such as type I superluminous supernovae (SLSNe I) and broad-lined type Ic supernovae (SNe Ic-BL). In order to explore the possible link between these two subclasses of supernovae (SNe), we study the effect of fallback accretion disk on magnetar evolution and magnetar-powered SNe. In this scenario, the interaction between a magnetar and a fallback accretion disk would accelerate the spin of the magnetar in the accretion regime but could result in substantial spin-down of the magnetars in the propeller regime. Thus, the initial rotation of the magnetar plays a less significant role in the spin evolution. Such a magnetar–disk interaction scenario can explain well the light curves of both SNe Ic-BL and SLSNe I, for which the observed differences are sensitive to the initial magnetic field of the magnetar and the fallback mass and timescale for the disk. Compared to the magnetars powering the SNe Ic-BL, those accounting for more luminous SNe usually maintain faster rotation and have relatively lower effective magnetic fields around peak time. In addition, the association between SLSNe I and long gamma-ray bursts, if observed in the future, could be explained in the context of a magnetar–disk system.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present new radio observations of the binary neutron star merger GW170817 carried out with the Karl G. Jansky Very large Array (VLA) more than 3 yr after the merger. Our combined data set is derived by coadding more than ≈32 hr of VLA time on-source, and as such provides the deepest combined observation (rms sensitivity ≈0.99 <jats:italic>μ</jats:italic>Jy) of the GW170817 field obtained to date at 3 GHz. We find no evidence for a late-time radio rebrightening at a mean epoch of <jats:italic>t</jats:italic> ≈ 1200 days since merger, in contrast to a ≈2.1<jats:italic>σ</jats:italic> excess observed at X-ray wavelengths at the same mean epoch. Our measurements agree with expectations from the post-peak decay of the radio afterglow of the GW170817 structured jet. Using these results, we constrain the parameter space of models that predict a late-time radio rebrightening possibly arising from the high-velocity tail of the GW170817 kilonova ejecta, which would dominate the radio and X-ray emission years after the merger (once the structured jet afterglow fades below detection level). Our results point to a steep energy-speed distribution of the kilonova ejecta (with energy-velocity power-law index <jats:italic>α</jats:italic> ≳ 5). We suggest possible implications of our radio analysis, when combined with the recent tentative evidence for a late-time rebrightening in the X-rays, and highlight the need for continued radio-to-X-ray monitoring to test different scenarios.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Cowan et al. review how roughly half the elements heavier than iron found in the Sun are produced by rapid neutron capture and half by slow neutron capture, the <jats:italic>r</jats:italic>- and <jats:italic>s</jats:italic>-processes. In the Sun, their relative contribution to individual elemental abundances is well understood, except for the lightest and heaviest elements beyond iron. Their contributions are especially uncertain for the heaviest nonradioactive element, lead (Pb, <jats:italic>Z</jats:italic> = 82). This is constrained by deriving lead abundances in metal-poor stars. For in the most metal-poor halo stars, strontium and heavier elements are found in the solar <jats:italic>r</jats:italic>-process proportion; <jats:italic>s</jats:italic>-process elements appear only at metallicities above one-thirtieth solar. In unevolved metal-poor stars of roughly solar heavy-element content, only two UV Pb lines are detectable. Four such stars have high-resolution spectra of the strongest line, Pb <jats:sc>ii</jats:sc> at 2203.53 Å. Roederer et al. analyzed this line in one star, deriving a lead-to-iron abundance ratio 10 times solar. This and its blueshifted profile suggested strong <jats:italic>s</jats:italic>-process production. This work analyzes the UV spectra of all four stars. Calculations including a predicted Fe <jats:sc>i</jats:sc> line blueward of the Pb <jats:sc>ii</jats:sc> line, and assuming the lead abundance scales with <jats:italic>r</jats:italic>-process abundances, match all four profiles extremely well. A scaled <jats:italic>s</jats:italic>-process contribution might improve the match to the much lower lead abundance found in the unevolved star analyzed previously, but its <jats:italic>s</jats:italic>-process excess is modest. An Fe <jats:sc>ii</jats:sc> line blends the other lead line, Pb <jats:sc>i</jats:sc> at 2833.05 Å, which constrains the lead abundance only in the coolest star.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the detection of 8.914 hr variability in both optical and ultraviolet light curves of LP 40−365 (also known as GD 492), the prototype for a class of partly burnt runaway stars that have been ejected from a binary due to a thermonuclear supernova event. We first detected this 1.0% amplitude variation in optical photometry collected by the Transiting Exoplanet Survey Satellite (TESS). Reanalysis of observations from the Hubble Space Telescope at the TESS period and ephemeris reveal a 5.8% variation in the ultraviolet of this 9800 K stellar remnant. We propose that this 8.914 hr photometric variation reveals the current surface rotation rate of LP 40−365, and is caused by some kind of surface inhomogeneity rotating in and out of view, though a lack of observed Zeeman splitting puts an upper limit on the magnetic field of &lt;20 kG. We explore ways in which the present rotation period can constrain progenitor scenarios if angular momentum was mostly conserved, which suggests that the survivor LP 40−365 was not the donor star but was most likely the bound remnant of a mostly disrupted white dwarf that underwent advanced burning from an underluminous (Type Iax) supernova.</jats:p>