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<jats:title>Abstract</jats:title> <jats:p>Using the integral field unit data from the Mapping Nearby Galaxies at Apache Point Observatory survey, we select a sample of 101 galaxies with counterrotating stellar disks and regularly rotating ionized gas disks. We classify the 101 galaxies into four types based on the features of their stellar velocity fields. The relative fractions and stellar population age radial gradients of the four types are different in the blue cloud, green valley, and red sequence populations. We suggest different formation scenarios for counterrotating stellar disks; the key factors in the formation of counterrotating stellar disks include (1) the abundance of preexisting gas in the progenitor and (2) the efficiency in angular momentum consumption.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>With the aim of investigating how the magnetic field in solar active regions (ARs) controls flare activity, i.e., whether a confined or eruptive flare occurs, we analyze 106 flares of Geostationary Operational Environmental Satellite class ≥M1.0 during 2010–2019. We calculate mean characteristic twist parameters <jats:italic>α</jats:italic> <jats:sub>FPIL</jats:sub> within the “flaring polarity inversion line” region and <jats:italic>α</jats:italic> <jats:sub>HFED</jats:sub> within the area of high photospheric magnetic free energy density, which both provide measures of the nonpotentiality of the AR core region. Magnetic twist is thought to be related to the driving force of electric current-driven instabilities, such as the helical kink instability. We also calculate total unsigned magnetic flux (Φ<jats:sub>AR</jats:sub>) of ARs producing the flare, which describes the strength of the background field confinement. By considering both the constraining effect of background magnetic fields and the magnetic nonpotentiality of ARs, we propose a new parameter <jats:italic>α</jats:italic>/Φ<jats:sub>AR</jats:sub> to measure the probability for a large flare to be associated with a coronal mass ejection (CME). We find that in about 90% of eruptive flares, <jats:italic>α</jats:italic> <jats:sub>FPIL</jats:sub>/Φ<jats:sub>AR</jats:sub> and <jats:italic>α</jats:italic> <jats:sub>HFED</jats:sub>/Φ<jats:sub>AR</jats:sub> are beyond critical values (2.2 × 10<jats:sup>−24</jats:sup> and 3.2 × 10<jats:sup>−24</jats:sup> Mm<jats:sup>−1</jats:sup> Mx<jats:sup>−1</jats:sup>), whereas they are less than critical values in ∼80% of confined flares. This indicates that the new parameter <jats:italic>α</jats:italic>/Φ<jats:sub>AR</jats:sub> is well able to distinguish eruptive flares from confined flares. Our investigation suggests that the relative measure of magnetic nonpotentiality within the AR core over the restriction of the background field largely controls the capability of ARs to produce eruptive flares.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The first interstellar object to be observed in our solar system, 1I/2017 U1 ’Oumuamua, combines the lack of observable cometary activity with an extra-gravitational acceleration. This has given rise to several mutually exclusive explanations based on different assumptions in the material composition of ’Oumuamua. We show how a combination of observations in the infrared and optical spectra may serve to distinguish between these explanations once another object with ’Omuamua-like properties comes close enough to Earth. This possibility is linked to the widely different thermal properties of the different material models that have been proposed. Developing a model for the thermal conduction and infrared signal from a fractal model, we compare predictions of the infrared signal with that from standard thermal models that assume ’Oumuamua to be either a solid piece of rock/ice or a thin sheet.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The two sources AGC 226178 and NGVS 3543, an extremely faint, clumpy, blue stellar system and a low surface brightness dwarf spheroidal, are adjacent systems in the direction of the Virgo cluster. Both have been studied in detail previously, with it being suggested that they are unrelated normal dwarf galaxies or that NGVS 3543 recently lost its gas through ram pressure stripping and AGC 226178 formed from this stripped gas. However, with Hubble Space Telescope Advanced Camera for Surveys imaging, we demonstrate that the stellar population of NGVS 3543 is inconsistent with being at the distance of the Virgo cluster and that it is likely a foreground object at approximately 10 Mpc, whereas the stellar population of AGC 226178 is consistent with it being a very young (10–100 Myr) object in the Virgo cluster. Through a reanalysis of the original ALFALFA H <jats:sc>i</jats:sc> detection, we show that AGC 226178 likely formed from gas stripped from the nearby dwarf galaxy VCC 2034, a hypothesis strengthened by the high metallicity measured with MUSE VLT observations. However, it is unclear whether ram pressure or a tidal interaction is responsible for stripping the gas. Object AGC 226178 is one of at least five similar objects now known toward Virgo. These objects are all young and unlikely to remain visible for over ∼500 Myr, suggesting that they are continually produced in the cluster.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the discovery of a new double-detonation progenitor system consisting of a hot subdwarf B (sdB) binary with a white dwarf companion with a <jats:italic>P</jats:italic> <jats:sub>orb</jats:sub> = 76.34179(2) minutes orbital period. Spectroscopic observations are consistent with an sdB star during helium core burning residing on the extreme horizontal branch. Chimera light curves are dominated by ellipsoidal deformation of the sdB star and a weak eclipse of the companion white dwarf. Combining spectroscopic and light curve fits, we find a low-mass sdB star, <jats:italic>M</jats:italic> <jats:sub>sdB</jats:sub> = 0.383 ± 0.028 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> with a massive white dwarf companion, <jats:italic>M</jats:italic> <jats:sub>WD</jats:sub> = 0.725 ± 0.026 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. From the eclipses we find a blackbody temperature for the white dwarf of 26,800 K resulting in a cooling age of ≈25 Myr whereas our <jats:monospace>MESA</jats:monospace> model predicts an sdB age of ≈170 Myr. We conclude that the sdB formed first through stable mass transfer followed by a common envelope which led to the formation of the white dwarf companion ≈25 Myr ago. Using the <jats:monospace>MESA</jats:monospace> stellar evolutionary code we find that the sdB star will start mass transfer in ≈6 Myr and in ≈60 Myr the white dwarf will reach a total mass of 0.92 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> with a thick helium layer of 0.17 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. This will lead to a detonation that will likely destroy the white dwarf in a peculiar thermonuclear supernova. PTF1 J2238+7430 is only the second confirmed candidate for a double-detonation thermonuclear supernova. Using both systems we estimate that at least ≈1% of white dwarf thermonuclear supernovae originate from sdB+WD binaries with thick helium layers, consistent with the small number of observed peculiar thermonuclear explosions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Parker Solar Probe (PSP) entered a region of sub-Alfvénic solar wind during encounter 8, and we present the first detailed analysis of low-frequency turbulence properties in this novel region. The magnetic field and flow velocity vectors were highly aligned during this interval. By constructing spectrograms of the normalized magnetic helicity, cross-helicity, and residual energy, we find that PSP observed primarily Alfvénic fluctuations, a consequence of the highly field-aligned flow that renders quasi-2D fluctuations unobservable to PSP. We extend Taylor’s hypothesis to sub- and super-Alfvénic flows. Spectra for the fluctuating forward and backward Elsässer variables (<jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>±</jats:sup>, respectively) are presented, showing that <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>+</jats:sup> modes dominate <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>−</jats:sup> by an order of magnitude or more, and the <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>+</jats:sup> spectrum is a power law in frequency (parallel wavenumber) <jats:italic>f</jats:italic> <jats:sup>−3/2</jats:sup> (<jats:inline-formula> <jats:tex-math> <?CDATA ${k}_{\parallel }^{-3/2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>k</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">∥</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac51daieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) compared to the convex <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>−</jats:sup> spectrum with <jats:italic>f</jats:italic> <jats:sup>−3/2</jats:sup> (<jats:inline-formula> <jats:tex-math> <?CDATA ${k}_{\parallel }^{-3/2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>k</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">∥</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac51daieqn2.gif" xlink:type="simple" /> </jats:inline-formula>) at low frequencies, flattening around a transition frequency (at which the nonlinear and Alfvén timescales are balanced) to <jats:italic>f</jats:italic> <jats:sup>−1.25</jats:sup> at higher frequencies. The observed spectra are well fitted using a spectral theory for nearly incompressible magnetohydrodynamics assuming a wavenumber anisotropy <jats:inline-formula> <jats:tex-math> <?CDATA ${k}_{\perp }\sim {k}_{\parallel }^{3/4}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>k</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊥</mml:mo> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>k</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">∥</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac51daieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, that the <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>+</jats:sup> fluctuations experience primarily nonlinear interactions, and that the minority <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>−</jats:sup> fluctuations experience both nonlinear and Alfvénic interactions with <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>+</jats:sup> fluctuations. The density spectrum is a power law that resembles neither the <jats:bold> <jats:italic>z</jats:italic> </jats:bold> <jats:sup>±</jats:sup> spectra nor the compressible magnetic field spectrum, suggesting that these are advected entropic rather than magnetosonic modes and not due to the parametric decay instability. Spectra in the neighboring modestly super-Alfvénic intervals are similar.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent observations have shown that in many large solar energetic particle (SEP) events the event-integrated differential spectra resemble double power laws. We perform numerical modeling of particle acceleration at coronal shocks propagating through a streamer-like magnetic field by solving the Parker transport equation, including protons and heavier ions. We find that for all ion species the energy spectra integrated over the simulation domain can be described by a double power law, and the break energy depends on the ion charge-to-mass ratio as <jats:italic>E</jats:italic> <jats:sub> <jats:italic>B</jats:italic> </jats:sub> ∼ (<jats:italic>Q</jats:italic>/<jats:italic>A</jats:italic>)<jats:sup> <jats:italic>α</jats:italic> </jats:sup>, with <jats:italic>α</jats:italic> varying from 0.16 to 1.2 by considering different turbulence spectral indices. We suggest that the double-power-law distribution may emerge as a result of the superposition of energetic particles from different source regions where the acceleration rates differ significantly due to particle diffusion. The diffusion and mixing of energetic particles could also provide an explanation for the increase of Fe/O at high energies as observed in some SEP events. Although further mixing processes may occur, our simulations indicate that either a power-law break or rollover can occur near the Sun and predict that the spectral forms vary significantly along the shock front, which may be examined by upcoming near-Sun SEP measurements from the Parker Solar Probe and Solar Orbiter.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The obliquity of a star, or the angle between its spin axis and the average orbit normal of its companion planets, provides a unique constraint on that system’s evolutionary history. Unlike the solar system, where the Sun’s equator is nearly aligned with its companion planets, many hot-Jupiter systems have been discovered with large spin–orbit misalignments, hosting planets on polar or retrograde orbits. We demonstrate that, in contrast to stars harboring hot Jupiters on circular orbits, those with eccentric companions follow no population-wide obliquity trend with stellar temperature. This finding can be naturally explained through a combination of high-eccentricity migration and tidal damping. Furthermore, we show that the joint obliquity and eccentricity distributions observed today are consistent with the outcomes of high-eccentricity migration, with no strict requirement to invoke the other hot-Jupiter formation mechanisms of disk migration or in situ formation. At a population-wide level, high-eccentricity migration can consistently shape the dynamical evolution of hot-Jupiter systems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The final stage of gas giant formation involves accreting gas from the parent protoplanetary disk. In general, the infalling gas likely approaches a freefall velocity, creating an accretion shock, leading to strong shock heating and radiation. We investigate the kinematics and energetics of such accretion shocks using 1D radiation hydrodynamic simulations. Our simulations feature the first self-consistent treatment of hydrogen dissociation and ionization, radiation transport, and realistic gray opacity. By exploring a broad range of giant-planet masses (0.1–3<jats:italic>M</jats:italic> <jats:sub>J</jats:sub>) and accretion rates (10<jats:sup>−3</jats:sup>–10<jats:sup>−2</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊕</jats:sub> yr<jats:sup>−1</jats:sup>), we focus on global shock efficiency and the final entropy of the accreted gas. We find that radiation from the accretion shock can fully disassociate the molecular hydrogen of the incoming gas when the shock luminosity is above a critical luminosity. Meanwhile, the post-shock entropy generally falls into “cold” (≲12<jats:italic>k</jats:italic> <jats:sub>B</jats:sub>/<jats:italic>m</jats:italic> <jats:sub>H</jats:sub>) and “hot” (≳16<jats:italic>k</jats:italic> <jats:sub>B</jats:sub>/<jats:italic>m</jats:italic> <jats:sub>H</jats:sub>) groups which depend on the extent of the endothermic process of H<jats:sub>2</jats:sub> dissociation. While 2D or 3D simulations are needed for more realistic understandings of the accretion process, this distinction likely carries over and sheds light on the interpretation of young direct imaging planets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Saturn has a dynamically rich satellite system, which includes at least three orbital resonances between three pairs of moons: Mimas–Tethys 4:2, Enceladus–Dione 2:1, and Titan–Hyperion 4:3 mean-motion resonances. Studies of the orbital history of Saturn’s moons usually assume that their past dynamics was also dominated solely by two-body resonances. Using direct numerical integrations, we find that three-body resonances among Saturnian satellites were quite common in the past, and could result in a relatively long-term, but finite capture time (10 Myr or longer). We find that these three-body resonances are invariably of the eccentricity type and do not appear to affect the moons’ inclinations. While some three-body resonances are located close to two-body resonances (but involve the orbital precession of the third body), others are isolated, with no two-body arguments being near resonance. We conclude that future studies of the system’s past must take full account of three-body resonances, which have been overlooked in the past work.</jats:p>