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<jats:title>Abstract</jats:title> <jats:p>The microlensing effect has developed into a powerful technique for a diverse range of applications including exoplanet discoveries, structure of the Milky Way, constraints on MAssive Compact Halo Objects, and measurements of the size and profile of quasar accretion disks. In this paper, we consider a special type of microlensing events where the sources are fast radio bursts (FRBs) with ∼milliseconds (ms) durations for which the relative motion between the lens and source is negligible. In this scenario, it is possible to temporally resolve the individual microimages. As a result, a method beyond the inverse ray shooting method, which only evaluates the total magnification of all microimages, is needed. We therefore implement an algorithm for identifying individual microimages and computing their magnifications and relative time delays. We validate our algorithm by comparing to analytical predictions for a single microlens case and find excellent agreement. We show that the superposition of pulses from individual microimages produces a light curve that appears as multipeaked FRBs. The relative time delays between pulses can reach 0.1–1 ms for stellar-mass lenses and hence can already be resolved temporally by current facilities. Although not yet discovered, microlensing of FRBs will become regular events and surpass the number of quasar microlensing events in the near future when 10<jats:sup>4−5</jats:sup> FRBs are expected to be discovered on a daily basis. Our algorithm provides a way of generating the microlensing light curve that can be used for constraining stellar-mass distribution in distant galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The presence of current sheet instabilities, such as the tearing mode instability, are needed to account for the observed rate of energy release in solar flares. Insights into these current sheet dynamics can be revealed by the behavior of flare ribbon substructure, as magnetic reconnection accelerates particles down newly reconnected field lines into the chromosphere to mark the flare footpoints. Behavior in the ribbons can therefore be used to probe processes occurring in the current sheet. In this study, we use high-cadence (1.7 s) IRIS Slit Jaw Imager observations to probe for the growth and evolution of key spatial scales along the flare ribbons—resulting from dynamics across the current sheet of a small solar flare on 2016 December 6. Combining analyses of spatial scale growth with Si <jats:sc>iv</jats:sc> nonthermal velocities, we piece together a timeline of flare onset for this confined event, and provide evidence of the tearing mode instability triggering a cascade and inverse cascade toward a power spectrum consistent with plasma turbulence.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Multiplanetary systems are prevalent in our Galaxy. The long-term stability of such systems may be disrupted if a distant inclined companion excites the eccentricity and inclination of the inner planets via the eccentric Kozai–Lidov mechanism. However, the star–planet and the planet–planet interactions can help stabilize the system. In this work, we extend the previous stability criterion that only considered the companion–planet and planet–planet interactions by also accounting for short-range forces or effects, specifically, relativistic precession induced by the host star. A general analytical stability criterion is developed for planetary systems with <jats:italic>N</jats:italic> inner planets and a relatively distant inclined perturber by comparing precession rates of relevant dynamical effects. Furthermore, we demonstrate as examples that in systems with two and three inner planets, the analytical criterion is consistent with numerical simulations using a combination of Gauss’s averaging method and direct <jats:italic>N</jats:italic>-body integration. Finally, the criterion is applied to observed systems, constraining the orbital parameter space of a possible undiscovered companion. This new stability criterion extends the parameter space in which an inclined companion of multiplanet systems can inhabit.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We model the behavior of a torsional Alfvén pulse, assumed to propagate through the chromosphere. Building on our existing model, we utilize the zero-beta approximation appropriate for plasma in an intense magnetic flux tube, e.g., a magnetic bright point. The model is adapted to investigate the connection between these features and chromospheric spicules. A pulse is introduced at the lower, photospheric boundary of the tube as a magnetic shear perturbation, and the resulting propagating Alfvén waves are reflected from an upper boundary, representing the change in density found at the transition region. The induced upward mass flux is followed by the reversal of the flux that may be identified with the rising and falling behavior of certain lower solar atmospheric jets. The ratio of the transmitted and reflected mass flux is estimated and compared with the relative total mass of spicules and the solar wind. An example is used to study the properties of the pulse. We also find that the interaction between the initial and reflected waves may create a localized flow that persists independently from the pulse itself.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this work we present the results of a direct imaging survey for brown dwarf companions around the nearest stars at the mid-infrared 10 micron range (<jats:italic>λ</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> = 8.7 <jats:italic>μ</jats:italic>m, Δ<jats:italic>λ</jats:italic> = 1.1 <jats:italic>μ</jats:italic>m) using the CanariCam instrument on the 10.4 m Gran Telescopio Canarias (GTC). We imaged the 25 nearest stellar systems within 5 pc of the Sun at declinations <jats:italic>δ</jats:italic> &gt; −25° (at least half have planets from radial-velocity studies), reaching a mean detection limit of 11.3 ± 0.2 mag (1.5 mJy) in the Si-2 8.7 <jats:italic>μ</jats:italic>m band over a range of angular separations from 1″ to 10″. This would have allowed us to uncover substellar companions at projected orbital separations between ∼2 and 50 au, with effective temperatures down to 600 K and masses greater than 30 <jats:italic>M</jats:italic> <jats:sub>Jup</jats:sub> assuming an average age of 5 Gyr and masses down to the deuterium-burning mass limit for objects with ages &lt;1 Gyr. From the nondetection of such companions, we determined upper limits on their occurrence rate at depths and orbital separations yet unexplored by deep imaging programs. For the M dwarfs, the main component of our sample, we found with a 90% confidence level that fewer than 20% of these low-mass stars have L- and T-type brown dwarf companions with <jats:italic>m</jats:italic> ≳ 30 <jats:italic>M</jats:italic> <jats:sub>Jup</jats:sub> and <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> ≳ 600 K at ∼3.5–35 au projected orbital separations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We studied the harmonics of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron star low-mass X-ray binary 4U 1636–53 using the Rossi X-ray Timing Explorer observations. We detected the harmonics of the mHz QPOs in 73 data intervals, with most of them in the transitional spectra state. We found that the ratio between the rms amplitude of the harmonic and that of the fundamental remains constant in a wide range of the fundamental frequency. More importantly, we studied, for the first time, the rms amplitude of the harmonics versus energy in 4U 1636–53 in the 2–5 keV range. We found that the rms amplitude of both the harmonic and the fundamental shows a decreasing trend as the energy increases, which is different from the behaviors reported in QPOs in certain black hole systems. Furthermore, our results suggest that not all observations with mHz QPOs have the harmonic component, although the reason behind this is still unclear.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>As hosts of living high-mass stars, Wolf-Rayet (WR) regions or WR galaxies are ideal objects for constraining the high-mass end of the stellar initial mass function (IMF). We construct a large sample of 910 WR galaxies/regions that cover a wide range of stellar metallicity (from <jats:italic>Z</jats:italic> ∼ 0.001 to 0.03) by combining three catalogs of WR galaxies/regions previously selected from the SDSS and SDSS-IV/MaNGA surveys. We measure the equivalent widths of the WR blue bump at ∼4650 Å for each spectrum. They are compared with predictions from stellar evolutionary models <jats:monospace>Starburst99</jats:monospace> and <jats:monospace>BPASS</jats:monospace>, with different IMF assumptions (high-mass slope <jats:italic>α</jats:italic> of the IMF ranging from 1.0 to 3.3). Both singular evolution and binary evolution are considered. We also use a Bayesian inference code to perform full spectral fitting to WR spectra with stellar population spectra from <jats:monospace>BPASS</jats:monospace> as fitting templates. We then make a model selection among different <jats:italic>α</jats:italic> assumptions based on Bayesian evidence. These analyses have consistently led to a positive correlation of the IMF high-mass slope <jats:italic>α</jats:italic> with stellar metallicity <jats:italic>Z</jats:italic>, i.e., with a steeper IMF (more bottom-heavy) at higher metallicities. Specifically, an IMF with <jats:italic>α</jats:italic> = 1.00 is preferred at the lowest metallicity (<jats:italic>Z</jats:italic> ∼ 0.001), and an Salpeter or even steeper IMF is preferred at the highest metallicity (<jats:italic>Z</jats:italic> ∼ 0.03). These conclusions hold even when binary population models are adopted.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We examine the properties of spiral shocks from a steady, adiabatic, non-axisymmetric accretion disk around a compact star in a binary. We first incorporate all possible influences from a binary through adopting the Roche potential and Coriolis forces in the basic conservation equations. In this paper, we assume spiral shocks to be point-wise and self-similar, and that the flow is in vertical hydrostatic equilibrium to simplify the study. We also investigate mass outflow due to shock compression and apply it to an accreting white dwarf in a binary. We find that our model will be beneficial for overcoming the ad hoc assumption of an optically thick wind generally used in studies of the progenitors of supernovae Ia.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Based on Atacama Large Millimeter/submillimeter Array (ALMA) observations, compact protoplanetary disks with dust radii of <jats:italic>r</jats:italic> ≲ 20–40 au were found to be dominant in nearby low-mass star formation regions. However, their substructures have not been investigated because of the limited spatial resolution achieved so far. We apply a newly developed super-resolution imaging technique utilizing sparse modeling (SpM) to explore several au-scale structures in such compact disks. SpM imaging can directly solve for the incomplete sampling of visibilities in the spatial frequency and potentially improve the fidelity and effective spatial resolution of ALMA images. Here we present the results of the application to the T Tau system. We use the ALMA 1.3 mm continuum data and achieve an effective spatial resolution of ∼30% (5 au) compared with the conventional CLEAN beam size at a resolution of 17 au. The reconstructed image reveals a new annular gap structure at <jats:italic>r</jats:italic> = 12 au in the T Tau N compact disk, with a dust radius of 24 au, and resolves the T Tau Sa and Sb binary into two sources. If the observed gap structure in the T Tau N disk is caused by an embedded planet, we estimate a Saturn-mass planet when the viscous parameter of the disk is 10<jats:sup>−3</jats:sup>. Ultimately, ALMA observations with enough angular resolution and sensitivity should be able to verify the consistency of the super-resolution imaging and definitely confirm the existence of this disk substructure.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The classical definition of the virial temperature of a galaxy halo excludes a fundamental contribution to the energy partition of the halo: the kinetic energy of nonthermal gas motions. Using simulations of low-redshift, ∼<jats:italic>L</jats:italic>* galaxies from the Figuring Out Gas &amp; Galaxies In Enzo (FOGGIE) project that are optimized to resolve low-density gas, we show that the kinetic energy of nonthermal motions is roughly equal to the energy of thermal motions. The simulated FOGGIE halos have ∼2× lower bulk temperatures than expected from a classical virial equilibrium, owing to significant nonthermal kinetic energy that is formally excluded from the definition of <jats:italic>T</jats:italic> <jats:sub>vir</jats:sub>. We explicitly derive a modified virial temperature including nonthermal gas motions that provides a more accurate description of gas temperatures for simulated halos in virial equilibrium. Strong bursts of stellar feedback drive the simulated FOGGIE halos out of virial equilibrium, but the halo gas cannot be accurately described by the standard virial temperature even when in virial equilibrium. Compared to the standard virial temperature, the cooler modified virial temperature implies other effects on halo gas: (i) the thermal gas pressure is lower, (ii) radiative cooling is more efficient, (iii) O <jats:sc>vi</jats:sc> absorbing gas that traces the virial temperature may be prevalent in halos of a higher mass than expected, (iv) gas mass estimates from X-ray surface brightness profiles may be incorrect, and (v) turbulent motions make an important contribution to the energy balance of a galaxy halo.</jats:p>