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<jats:title>Abstract</jats:title> <jats:p>The distribution of angular momentum of planets and their host stars provides important information on the formation and evolution of the planetary system. However, mysteries still remain, partly due to bias and uncertainty of the current observational data sets and partly due to the fact that theoretical models for the formation and evolution of planetary systems are still underdeveloped. In this study, we calculate the spin angular momentum of host stars and the orbital angular momentum of their planets using data from the NASA Exoplanet Archive along with detailed analysis of observation dependent biases and uncertainty ranges. We also analyze the angular momentum of the planetary system as a function of star age to understand their variation in different evolutionary stages. In addition, we use a population of planets from theoretical model simulations to reexamine the observed patterns and compare the simulated population with the observed samples to assess variations and differences. We found the majority of exoplanets discovered thus far do not have the angular momentum distribution similar to that of planets in our solar system, though this could be due to the observation bias. When filtered by the observational biases, the model simulated angular momentum distributions are comparable to the observed pattern in general. However, the differences between the observation and model simulation in the parameter (angular momentum) space provide more rigorous constraints and insights on the issues that needed future improvement.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>G306.3–0.9 is an asymmetric Galactic supernova remnant (SNR), whose progenitor has been thought to be a Type Ia supernova (SN), but its high Ca abundance appears inconsistent with the Type Ia origin. Hoping to uncover the reason for its asymmetry and the origin of this SNR, we performed a spatially resolved X-ray spectroscopic analysis of XMM-Newton and Chandra observation data. We divided the SNR into 13 regions and analyzed the spectra using two-temperature models (0.2 keV + 1 keV). Compared to the southwestern regions, the northeastern regions have higher metal abundances and a lower gas density. This suggests that the asymmetric morphology results from the nonuniform ambient environment. We found that neither Type Ia nor core-collapse SN models can account for the abnormally high abundance ratios of Ar/Si, Ca/Si, or the shape of the abundance curve. A comparison with the Ca-rich transient models suggests that G306.3–0.9 is likely to be the first identified Galactic <jats:italic>Ca-rich transient</jats:italic> remnant, although the theoretical production of element S is lower. We also note that the conclusion for the SNR’s origin relies on the measured abundance ratios and existing nucleosynthesis models. Between two groups of Ca-rich transient explosion models, we prefer the He shell detonation for an accreting white dwarf, rather than the merger of a white dwarf and a neutron star.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>It was recently shown that, owing to the turbulent nature of the solar wind, the interplanetary magnetic field lines can be well described by stochastic Parker spirals. These are realizations of Brownian diffusion on a sphere of increasing radius, superimposed on the angular drift due to the solar rotation. In this work, we present a model for the transport of solar energetic particles along stochastic Parker spirals in the inner heliosphere. The transport model is governed by a set of four stochastic differential equations for the heliographic position <jats:inline-formula> <jats:tex-math> <?CDATA $(r,\alpha =\cos \theta ,\phi )$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:mi>r</mml:mi> <mml:mo>,</mml:mo> <mml:mi>α</mml:mi> <mml:mo>=</mml:mo> <mml:mi>cos</mml:mi> <mml:mi>θ</mml:mi> <mml:mo>,</mml:mo> <mml:mi>ϕ</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac2fabieqn1.gif" xlink:type="simple" /> </jats:inline-formula> of the guiding centers and the cosine of the pitch angle between the velocity vector and the Parker field. The model accounts for the role played by the combination of pitch angle scattering and magnetic focusing in the interplanetary medium. The effects of the dynamical evolution of the turbulence are included in the model by taking the field line angular diffusivity to be a function of the radial distance from the Sun. The heliolongitudinal distribution of particles propagating along stochastic Parker spirals is given by the wrapped Gaussian distribution. This angular distribution can also well be represented by the von Mises distribution that interpolates between the Gaussian distribution at small angular spread and the uniform distribution at large distances from the acceleration region of energetic particles in the aftermath of a solar eruption.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the correlation between Ly<jats:italic>α</jats:italic> absorbers and H <jats:sc>i</jats:sc> galaxies in the nearby universe (0.01 ≤ <jats:italic>z</jats:italic> ≤ 0.057). We find that Ly<jats:italic>α</jats:italic> absorbers are strongly correlated to H <jats:sc>i</jats:sc> galaxies at a projected separation of ≤0.5 Mpc and velocity separation of ≤50 km s<jats:sup>−1</jats:sup>. Ly<jats:italic>α</jats:italic> absorbers are 7.6 times more likely to be found near H <jats:sc>i</jats:sc> galaxies compared to a random distribution. The strength of correlations drops as the projected and/or velocity separation increases. We also find the correlation between Ly<jats:italic>α</jats:italic> absorbers and H <jats:sc>i</jats:sc> galaxies to be stronger than those observed between Ly<jats:italic>α</jats:italic> absorbers and optically selected galaxies. There is an enhancement in the number of absorbers at velocity separations of ≤30 km s<jats:sup>−1</jats:sup> from galaxies at distances larger than their viral radius. Combined with the fact that most of our galaxies are not driving strong outflows, we conclude that the absorbers at low-velocity separations are tracing reservoirs of cooler gas around galaxies. This conclusion is consistent with the predictions from cosmological simulations where faint gas from the IGM flows into the disks of galaxies leading to galaxy growth.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The supermassive black holes in most galaxies in the universe are powered by hot accretion flows. Both theoretical analysis and numerical simulations have indicated that, depending on the degree of magnetization, black hole hot accretion flow is divided into two modes, namely SANE (standard and normal evolution) and MAD (magnetically arrested disk). It has been an important question which mode the hot accretion flows in individual sources should belong to in reality, SANE or MAD. This issue has been investigated in some previous works but they all suffer from various uncertainties. By using the measured rotation measure (RM) values in the prototype low-luminosity active galactic nuclei in M87 at 2, 5, and 8 GHz along the jet at various distances from the black hole, combined with three-dimensional general relativity magnetohydrodynamical numerical simulations of SANE and MAD, we show in this paper that the RM values predicted by MAD are well consistent with observations, while the SANE model overestimates the RM by over two orders of magnitude and thus is ruled out.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a reanalysis of the Event Horizon Telescope 228 GHz observations of M87. We apply traditional hybrid mapping techniques to the publicly available <jats:italic>network-calibrated</jats:italic> data. We explore the impact on the final image of different starting models, including the following: a point source, a disk, an annulus, a Gaussian, and an asymmetric double Gaussian. The images converge to an extended source with a size ∼44 <jats:italic>μ</jats:italic>as. Starting with the annulus and disk models leads to images with the lowest noise, smallest off-source artifacts, and better closure residuals. The source appears as a ring, or edge-brightened disk, with higher surface brightness in the southern half, consistent with previous results. Starting with the other models leads to a surface brightness distribution with a similar size, and an internal depression, but not as clearly ringlike. A consideration of visibility amplitudes versus UV-distance argues for a roughly circularly symmetric structure of ∼50 <jats:italic>μ</jats:italic>as scale, with a sharp edge, based on a prominent minimum in the UV-distribution, and the amplitude of the secondary peak in the UV-plot is more consistent with an annular model than a flat disk model. With further processing, we find a possible modest extension from the ring toward the southwest, in a direction consistent with the southern limb of the jet seen on 3mm VLBI images on a factor of few larger scales. However, this extension appears along the direction of one of the principle sidelobes of the synthesized beam, and hence requires testing with better UV-coverage.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The aim of the present work is to study the propagation of the Alfvén wave around a 3D proper magnetic null point and its accompanying perturbations. In this line, the shock-capturing Godunov-type PLUTO code is used to solve the magnetohydrodynamic (MHD) equations. It is found that the Alfvén wave propagates toward the null point at the fan plane and the wave−wave interaction could be the main reason for the Alfvén wave energy dissipation, ehile, at two other planes including the spine axis, the Alfvén wave spreads toward the spine axis and accumulates along it. Furthermore, the fast magnetoacoustic wave moves toward the null point at the fan plane and also at two other planes including the spine axis. The fast magnetoacoustic wave also refracts around the null point without any significant accumulation along the spine axis. Finally, the slow mode moves toward the null point at the fan plane. It is illustrated that, at the <jats:italic>x,z</jats:italic> plane, in addition to the refraction of the slow wave around the null point, there is an accumulation of the slow mode along the spine axis, while, at the other plane including the spine axis, the slow magnetoacoustic wave refracts around the null point. Moreover, it is found that the 3D structure results in the high amplitude of MHD wave energy in comparison with the 2.5D structure. Finally, it is found that the Alfvén wave gives its energy to the induced fast and slow magnetoacoustic waves and they have more time to heat the plasma.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Merging compact black hole (BH) binaries are likely to exist in the nuclear star clusters around supermassive BHs (SMBHs), such as Sgr A*. They may also form in the accretion disks of active galactic nuclei. Such compact binaries can emit gravitational waves (GWs) in the low-frequency band (0.001–1 Hz) that are detectable by several planned space-borne GW observatories. We show that the angular momentum vector of the compact binary (<jats:bold> <jats:italic>L</jats:italic> </jats:bold> <jats:sub>in</jats:sub>) may experience significant variation due to the frame-dragging effect associated with the spin of the SMBH. The dynamical behavior of <jats:bold> <jats:italic>L</jats:italic> </jats:bold> <jats:sub>in</jats:sub> can be understood analytically as a resonance phenomenon. We show that the rate of change of <jats:bold> <jats:italic>L</jats:italic> </jats:bold> <jats:sub>in</jats:sub> encodes the information on the spin of the SMBH. Therefore, detecting GWs from compact binaries around SMBHs, particularly the modulation of the waveform associated with the variation of <jats:bold> <jats:italic>L</jats:italic> </jats:bold> <jats:sub>in</jats:sub>, can provide a new probe of the spins of SMBHs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Based on recent Hubble Space Telescope (HST) observations in the far-UV and ground-based observations in optical bands, Pavlov and colleagues have revealed a thermal component in the spectrum of the old pulsar B0950+08 (spin-down age 17.5 Myr) and estimated a neutron star (NS) surface temperature of (1–3) × 10<jats:sup>5</jats:sup> K. Our new HST observations in the optical have allowed us to resolve the pulsar from a close-by galaxy and measure the optical fluxes more accurately. Using the newly measured fluxes and a new calibration of the HST’s far-UV detector, we fit the optical-UV pulsar’s spectrum with a model that consists of a nonthermal power law (<jats:italic>f</jats:italic> <jats:sub> <jats:italic>ν</jats:italic> </jats:sub> ∝ <jats:italic>ν</jats:italic> <jats:sup> <jats:italic>α</jats:italic> </jats:sup>) and thermal blackbody components. We obtained the spectral slope <jats:italic>α</jats:italic> = −0.3 ± 0.3, considerably flatter than found from ground-based observations, and the best-fit temperature in the range of (6–12) × 10<jats:sup>4</jats:sup> K (as seen by a distant observer), depending on interstellar extinction and NS radius. The temperature is lower than reported previously, but still much higher than predicted by NS passive cooling scenarios for such an old pulsar. This means that some heating mechanisms operate in NSs, e.g., caused by the interaction of the faster-rotating neutron superfluid with the slower-rotating normal matter in the inner crust of the NS.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The LIGO Scientific Collaboration and Virgo Collaboration (LIGO/Virgo) have recently reported in GWTC-2.1 eight additional candidate events with a probability of astrophysical origin greater than 0.5 in the LIGO/Virgo’s deeper search on O3a running. In GWTC-2.1, the majority of the effective inspiral spins (<jats:italic>χ</jats:italic> <jats:sub>eff</jats:sub>) show magnitudes consistent with zero, while two (GW190403<jats:sub>-</jats:sub>051519 and GW190805<jats:sub>-</jats:sub>211137) of the eight new events have <jats:italic>χ</jats:italic> <jats:sub>eff</jats:sub> &gt; 0 (at 90% credibility). We note that GW190403<jats:sub>-</jats:sub>051519 was reported with <jats:italic>χ</jats:italic> <jats:sub>eff</jats:sub> = <jats:inline-formula> <jats:tex-math> <?CDATA ${0.70}_{-0.27}^{+0.15}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.70</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.27</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.15</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3982ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and mass ratio <jats:italic>q</jats:italic> = <jats:inline-formula> <jats:tex-math> <?CDATA ${0.25}_{-0.11}^{+0.54}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.25</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.54</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3982ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. Assuming a uniform prior probability between 0 and 1 for each black hole’s dimensionless spin magnitude, GW190403<jats:sub>-</jats:sub>051519 was reported with the dimensionless spin of the more massive black hole, <jats:italic>χ</jats:italic> <jats:sub>1</jats:sub> = <jats:inline-formula> <jats:tex-math> <?CDATA ${0.92}_{-0.22}^{+0.07}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.92</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.22</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.07</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac3982ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>. This is the fastest black hole ever measured in all current gravitational-wave events. If the immediate progenitor of GW190403<jats:sub>-</jats:sub>051519 is a close binary system composed of a black hole and a helium star, which can be the natural outcome of the classical isolated binary evolution through the common envelope phase, this extremely high spin challenges, at least in that case, the existence of an efficient angular momentum transport mechanism between the stellar core and the radiative envelope of massive stars, as for instance predicted by the Tayler–Spruit dynamo or its revised version by Fuller et al.</jats:p>