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<jats:title>Abstract</jats:title> <jats:p>We report on the discovery in the Gaia DR3 astrometric and spectroscopic catalog of a new polar stream that is found as an overdensity in action space. This structure is unique as it has an extremely large apocenter distance, reaching beyond 100 kpc, and yet is detected as a coherent moving structure in the solar neighborhood with a width of ∼4 kpc. A subsample of these stars that was fortuitously observed by LAMOST has a mean spectroscopic metallicity of <jats:inline-formula> <jats:tex-math> <?CDATA $\langle [\mathrm{Fe}/{\rm{H}}]\rangle =-{1.60}_{-0.16}^{+0.15}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">〈</mml:mo> <mml:mo stretchy="false">[</mml:mo> <mml:mi>Fe</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">]</mml:mo> <mml:mo stretchy="false">〉</mml:mo> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1.60</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.16</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="apjlac874fieqn1.gif" xlink:type="simple" /> </jats:inline-formula> dex and possesses a resolved metallicity dispersion of <jats:inline-formula> <jats:tex-math> <?CDATA $\sigma ([\mathrm{Fe}/{\rm{H}}])={0.32}_{-0.06}^{+0.17}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>σ</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mo stretchy="false">[</mml:mo> <mml:mi>Fe</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">]</mml:mo> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0.32</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.06</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.17</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac874fieqn2.gif" xlink:type="simple" /> </jats:inline-formula> dex. The physical width of the stream, the metallicity dispersion, and the vertical action spread indicate that the progenitor was a dwarf galaxy. The existence of such a coherent and highly radial structure at their pericenters in the vicinity of the Sun suggests that many other dwarf galaxy fragments may be lurking in the outer halo.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the first unambiguous detection of an axial merger shock in the early-stage merging cluster Abell 98 using deep (227 ks) Chandra observations. The shock is about 420 kpc south from the northern subcluster of Abell 98, in between the northern and central subclusters, with a Mach number of <jats:inline-formula> <jats:tex-math> <?CDATA ${ \mathcal M }$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic"></mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac86d4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> ≈ 2.3 ± 0.3. Our discovery of the axial merger shock front unveils a critical epoch in the formation of a massive galaxy cluster, when two subclusters are caught in the early phase of the merging process. We find that the electron temperature in the postshock region favors the instant collisionless model, where electrons are strongly heated at the shock front, by interactions with the magnetic field. We also report on the detection of an intercluster gas filament, with a temperature of <jats:italic>kT</jats:italic> = 1.07 ± 0.29 keV, along the merger axis of Abell 98. The measured properties of the gas in the filament are consistent with previous observations and numerical simulations of the hottest, densest parts of the warm–hot intergalactic medium (WHIM), where WHIM filaments interface with the virialization regions of galaxy clusters.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Galactic outflows from local starburst galaxies typically exhibit a layered geometry, with cool 10<jats:sup>4</jats:sup> K flow sheathing a hotter 10<jats:sup>7</jats:sup> K, cylindrically collimated, X-ray-emitting plasma. Here we argue that winds driven by energy injection in a ring-like geometry can produce this distinctive large-scale multiphase morphology. The ring configuration is motivated by the observation that massive young star clusters are often distributed in a ring at the host galaxy’s inner Lindblad resonance, where larger-scale spiral arm structure terminates. We present parameterized three-dimensional radiative hydrodynamical simulations that follow the emergence and dynamics of energy-driven hot winds from starburst rings. In this letter, we show that the flow shocks on itself within the inner ring hole, maintaining high 10<jats:sup>7</jats:sup> K temperatures, while flows that emerge from the wind-driving ring unobstructed can undergo rapid bulk cooling down to 10<jats:sup>4</jats:sup> K, producing a fast hot biconical outflow enclosed by a sheath of cooler nearly comoving material without ram pressure acceleration. The hot flow is collimated along the ring axis, even in the absence of pressure confinement from a galactic disk or magnetic fields. In the early stages of expansion, the emerging wind forms a bubble-like shape reminiscent of the Milky Way’s eROSITA and Fermi bubbles and can reach velocities usually associated with active-galactic-nucleus-driven winds. We discuss the physics of the ring configuration, the conditions for radiative bulk cooling, and the implications for future X-ray observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>High-contrast imaging presents us with the opportunity to study circumstellar disks and the planets still embedded within them, providing key insights into the formation and evolution of planetary systems. However, the postprocessing techniques that are often needed to suppress stellar halo light typically result in significant and variable loss of circumstellar light, even when using relatively conservative approaches like reference star differential imaging (RDI). We introduce “constrained reference star differential imaging” (constrained RDI), a new class of RDI point-spread-function (PSF) subtraction techniques for systems with circumstellar disks. Constrained RDI utilizes either high-resolution polarized-intensity (PI) images or disk models to severely limit or even eliminate the signal loss due to oversubtraction that is common to RDI. We demonstrate the ability of constrained RDI utilizing polarimetric data to yield an oversubtraction-free detection of the AB Aurigae protoplanetary disk in total intensity. PI-constrained RDI allows us to decisively recover the spectral signature of the confirmed, recently discovered protoplanet, AB Aurigae b. We further demonstrate that constrained RDI can be a powerful analysis tool for soon-to-be-acquired James Webb Space Telescope coronagraphic imaging of disks. In both cases, constrained RDI provides analysis-ready products that enable more detailed studies of disks and more robust verification of embedded exoplanets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>It has been proposed that some black holes (BHs) in binary black hole (BBH) systems are born from “hierarchical mergers” (HMs), i.e., earlier mergers of smaller BHs. These HM products have spin magnitudes <jats:italic>χ</jats:italic> ∼ 0.7, and, if they are dynamically assembled into BBH systems, their spin orientations will sometimes be antialigned with the binary orbital angular momentum. In fact, as Baibhav et al. showed, ∼16% of BBH systems that include HM products will have an effective inspiral spin parameter, <jats:italic>χ</jats:italic> <jats:sub>eff</jats:sub> &lt; −0.3. Nevertheless, the LIGO–Virgo–KAGRA (LVK) gravitational-wave (GW) detectors have yet to observe a BBH system with <jats:italic>χ</jats:italic> <jats:sub>eff</jats:sub> ≲ −0.2, leading to upper limits on the fraction of HM products in the population. We fit the astrophysical mass and spin distribution of BBH systems and measure the fraction of BBH systems with <jats:italic>χ</jats:italic> <jats:sub>eff</jats:sub> &lt; −0.3, which implies an upper limit on the HM fraction. We find that fewer than 26% of systems in the underlying BBH population include HM products (90% credibility). Even among BBH systems with primary masses <jats:italic>m</jats:italic> <jats:sub>1</jats:sub> = 60 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, the HM fraction is less than 69%, which may constrain the location of the pair-instability mass gap. With 300 GW events (to be expected in the LVK’s next observing run), if we fail to observe a BBH with <jats:italic>χ</jats:italic> <jats:sub>eff</jats:sub> &lt; −0.3, we can conclude that the HM fraction is smaller than <jats:inline-formula> <jats:tex-math> <?CDATA ${2.5}_{-2.2}^{+9.1} \% $?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>2.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2.2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>9.1</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>%</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac86c4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent studies suggest that the magnetic switchbacks (SBs) detected by the Parker Solar Probe carry information on the scales of solar supergranulation (large scale) and granulation (medium scale). We test this claim using high-resolution H<jats:italic>α</jats:italic> images obtained with the visible spectropolarimeters of the Goode Solar Telescope in Big Bear Solar Observatory. As possible solar sources, we count all the spicule-like features standing along the chromospheric networks near the coronal hole boundary visible in the H<jats:italic>α</jats:italic> blue-wing but absent in the red-wing images and measure the geometric parameters of dense sections of individual flux tubes. Intervals between adjacent spicules located along the chromospheric networks are found in the range of 0.4–1.5 Mm (0.°03–0.°12) tending to be smaller than the medium scale of SBs. Interdistances between all pairs of the flux tubes are also counted and they appear in a single peak distribution around 0.7 Mm (0.°06) unlike the waiting-time distribution of SBs in a scale-free single power-law form. The length-to-diameter ratio of the dense section of flux tubes is as high as 6–40, similar to the aspect ratio of SBs. The number of spicules along a network can be as high as 40–100, consistent with numerous SBs within a patch. With these numbers, it is argued that the medium scale of SBs can be understood as an equilibrium distance resulting from a random walk within each diverging magnetic field funnel connected to the chromospheric networks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>If very low mass primordial black holes (PBH) within the asteroid/moon-mass range indeed reside in galactic dark matter halos, they must necessarily collide with galactic neutron stars (NSs). These collisions must, again necessarily, form light black holes (LBHs) with masses of typical NSs, <jats:italic>M</jats:italic> <jats:sub>LBH</jats:sub> ≈ 1–2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. LBHs may be behind events already detected by ground-based gravitational-wave detectors (GW170817, GW190425, and others such as a mixed stellar black hole–NS-mass event GW191219_163120), and most recently by microlensing (OGLE-BLG-2011-0462). Although the status of these observations as containing LBHs is not confirmed, there is no question that gravitational-wave detectors and microlensing are in principle and in practice capable of detecting LBHs. We have calculated the creation rate of LBHs resulting from these light primordial black hole (PBH) collisions with NSs. On this basis, we claim that if improved gravitational-wave detectors and microlensing statistics of the LBH events would indicate that the number of LBHs is significantly lower that what follows from the calculated creation rate, then this would be an unambiguous proof that there is no significant light PBH contribution to the galactic dark matter halos. Otherwise, if observed and calculated numbers of LBHs roughly agree, then the hypothesis of primordial black hole existence gets strong observational support, and in addition their collisions with NSs may be considered a natural creation channel for the LBHs, solving the problem of their origin, as it is known that they cannot be a product of standard stellar evolution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The solar wind undergoes significant heating as it propagates away from the Sun; the exact mechanisms responsible for this heating remain unclear. Using data from the first perihelion of the Parker Solar Probe mission, we examine the properties of proton and electron heating occurring within magnetic coherent structures identified by means of the Partial Variance of Increments (PVI) method. Statistically, regions of space with strong gradients in the magnetic field, PVI ≥ 1, are associated with strongly enhanced proton but only slightly elevated electron temperatures. Our analysis indicates a heating mechanism in the nascent solar wind environment facilitated by a nonlinear turbulent cascade that preferentially heats protons over electrons.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The abundances of mixing-sensitive elements including lithium, [C/N], and <jats:sup>12</jats:sup>C/<jats:sup>13</jats:sup>C are known to change near the red giant branch bump. The explanation most often offered for these alterations is double diffusive thermohaline mixing in the stellar interior. In this analysis, we investigate the ability of thermohaline mixing to explain the observed timing of these chemical depletion events. Recent observational measurements of lithium and [C/N] show that the abundance of lithium decreases before the abundance of [C/N], whereas numerical simulations of the propagation of the thermohaline-mixing region computed with MESA show that the synthetic abundances drop simultaneously. We therefore conclude that thermohaline mixing alone cannot explain the distinct events of lithium depletion and [C/N] depletion, as the simultaneity predicted by simulations is not consistent with the observation of separate drops. We thus invite more sophisticated theoretical explanations for the observed temporal separation of these chemical depletion episodes as well as more extensive observational explorations across a range of masses and metallicities.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Gaseous circumbinary disks (CBDs) that are highly inclined to the binary orbit are commonly observed in nature. These disks harbor particles that can reach large mutual inclinations as a result of nodal precession once the gas disk has dissipated. With <jats:italic>n</jats:italic>-body simulations that include fragmentation we demonstrate that misaligned disks of particles can be efficient progenitors of interstellar asteroids (ISAs). Collisions that take place between particles with large mutual inclinations have large impact velocities, which can result in mass ejection, with a wide range of fragment sizes and ejection velocities. We explore the binary parameters for which the majority of the terrestrial planet-forming material is ejected rather than accreted into planets. The misalignment required to eject significant material decreases with binary eccentricity. If the distribution of binary eccentricity is uniform and the initial particle CBD orientation relative to the binary orbit is isotropic, about 59% of binaries are more likely to eject the majority of their CBD terrestrial planet disk mass through high-velocity body–body collisions than to retain this material and build terrestrial planets. However, binary–disk interactions during the gas disk phase with nonzero disk viscosity will reduce this fraction. The composition, small size, highly elongated shape, and tumbling motion of ‘Oumuamua are consistent with ISAs generated by misaligned CBDs.</jats:p>