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<jats:title>Abstract</jats:title> <jats:p>The Japanese Hayabusa2 mission has revealed in detail the physical characteristics of the C-type asteroid 162173 Ryugu, in particular, its spinning top-shaped rubble-pile structure and potentially high organic content. A widely accepted formation scenario for Ryugu is catastrophic collision between larger asteroids and the subsequent slow gravitational accumulation of collisional debris. An alternative scenario is that Ryugu is an extinct comet that lost its icy components. Here, we numerically simulated the sublimation of water ice from a porous cometary nucleus until the refractory components, such as silicate rocks and organic matter, were left behind as evaporative residues. Such a process represents the transformation from a comet to an asteroid. The spin-up related to the shrinking nucleus, associated with water ice sublimation, was also calculated. The result of the calculation indicates that the cometary origin scenario can account for all the features of Ryugu discussed above. We conclude that organic-rich spinning top-shaped rubble-pile asteroids, such as Ryugu, are comet–asteroid transition objects or extinct comets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present analysis of 17,043 proton kinetic-scale current sheets (CSs) collected over 124 days of Wind spacecraft measurements in the solar wind at 11 samples s<jats:sup>−1</jats:sup> magnetic field resolution. The CSs have thickness, <jats:italic>λ,</jats:italic> from a few tens to one thousand kilometers with typical values around 100 km, or within about 0.1–10<jats:italic>λ</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub> in terms of local proton inertial length, <jats:italic>λ</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub>. We found that the current density is larger for smaller-scale CSs, <jats:italic>J</jats:italic> <jats:sub>0</jats:sub> ≈ 6 nAm<jats:sup>−2</jats:sup> · (<jats:italic>λ</jats:italic>/100 km)<jats:sup>−0.56</jats:sup>, but does not statistically exceed a critical value, <jats:italic>J</jats:italic> <jats:sub> <jats:italic>A</jats:italic> </jats:sub> <jats:italic>,</jats:italic> corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units: <jats:inline-formula> <jats:tex-math> <?CDATA ${J}_{0}/{J}_{A}\approx 0.17\cdot {(\lambda /{\lambda }_{p})}^{-0.51}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>J</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>J</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>A</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>0.17</mml:mn> <mml:mo>·</mml:mo> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi>λ</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.51</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac4fc4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. The CSs are statistically force-free with magnetic shear angle correlated with CS spatial scale: <jats:inline-formula> <jats:tex-math> <?CDATA ${\rm{\Delta }}\theta \approx 19^\circ \cdot {(\lambda /{\lambda }_{p})}^{0.5}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>θ</mml:mi> <mml:mo>≈</mml:mo> <mml:mn>19</mml:mn> <mml:mo>°</mml:mo> <mml:mo>·</mml:mo> <mml:msup> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi>λ</mml:mi> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> <mml:mrow> <mml:mn>0.5</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac4fc4ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. The observed correlations are consistent with local turbulence being the source of proton kinetic-scale CSs in the solar wind, while the mechanisms limiting the current density remain to be understood.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In 2021 September the magnetar SGR J1935+2154 entered a stage of burst/flaring activity in the hard X-ray band. On 2021 September 10, we observed SGR J1935+2154 with the fiber-fed fast optical photon counter IFI+Iqueye, mounted at the 1.22 m Galileo telescope in Asiago. During one of the IFI+Iqueye observing windows, a hard X-ray burst was detected with the Fermi Gamma-ray Burst Monitor. We performed a search for any significant increase in the count rate on the 1 s, 10 ms, and 1 ms binned IFI+Iqueye light curves around the time of the Fermi burst. No significant peak was detected with a significance above 3<jats:italic>σ</jats:italic> in an interval of ±90 s around the burst. Correcting for interstellar extinction (<jats:italic>A</jats:italic> <jats:sub> <jats:italic>V</jats:italic> </jats:sub> ≃ 5.8 mag), the IFI+Iqueye upper limits to any possible optical burst from SGR J1935+2154 are <jats:italic>V</jats:italic> = 10.1 mag, <jats:italic>V</jats:italic> = 7.2 mag, and <jats:italic>V</jats:italic> = 5.8 mag for the 1 s, 10 ms, and 1 ms binned light curves, respectively. The corresponding extinction-corrected upper limits to the fluence (specific fluence) are 3.1 × 10<jats:sup>−10</jats:sup> erg cm<jats:sup>−2</jats:sup> (0.35 Jy s), 4.2 × 10<jats:sup>−11</jats:sup> erg cm<jats:sup>−2</jats:sup> (4.8 Jy ·10 ms), and 1.6 × 10<jats:sup>−11</jats:sup> erg cm<jats:sup>−2</jats:sup> (17.9 Jy ms), orders of magnitude deeper than any previous simultaneous optical limit on a magnetar burst. The IFI+Iqueye measurement can also place a more stringent constraint on the spectral index of the optical to hard X-ray fluence of SGR J1935+2154, implying a spectrum steeper than <jats:italic>ν</jats:italic> <jats:sup>0.64</jats:sup>. Fast optical timing observations of bursts associated with radio emission then have the potential to yield a detection.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Asteroid impacts with Earth may have played an essential role in the emergence of life on Earth through their creation of favorable niches for life, changes to the atmosphere, and delivery of water. Consequently, we suggest two potential requirements for life in an exoplanetary system: first, that the system has an asteroid belt, and second, that there is a mechanism to drive asteroids to impact the terrestrial habitable planet. Since in the solar system the <jats:italic>ν</jats:italic> <jats:sub>6</jats:sub> secular resonance has been shown to have been important in driving these impacts, we explore how the masses and locations of two giant planets determine the location and strength of this secular resonance. Examining observed exoplanetary systems with two giant planets, we find that a secular resonance within the asteroid belt region may not be uncommon. Hence, the solar system is somewhat special, but the degree of fine-tuning that may be necessary for the emergence of life is not excessive. Finally, with <jats:italic>n</jats:italic>-body simulations, we show that when the two giant planets are close to the 2:1 mean motion resonance, the asteroid belt is unstable, but this does not lead to increased asteroid delivery.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first detection of pulsations from a neutron star in the submillimeter range. The source is the magnetar XTE J1810−197, observed with the James Clerk Maxwell Telescope on 2020 February 27, 2020 July 9, and 2021 May 15. XTE J1810−197 is detected at 353 GHz (<jats:italic>λ</jats:italic> = 0.85 mm) in the three epochs, but not detected in the simultaneously observed band at 666 GHz (<jats:italic>λ</jats:italic> = 0.45 mm). We measure an averaged flux density at 353 GHz of 6.7 ± 1.0, 4.0 ± 0.6, and 1.3 ± 0.3 mJy, and set 3<jats:italic>σ</jats:italic> flux density upper limits at 666 GHz of 11.3, 4.7, and 4.3 mJy, at each of the three observing epochs, respectively. Combining close-in-time observations with the Effelsberg 100 m and IRAM 30 m telescopes covering noncontiguously from 6 to 225 GHz (5.0 cm &gt; <jats:italic>λ</jats:italic> &gt; 1.33 mm), we investigate the spectral shape and frequency range of a potential spectral turn-up predicted by some pulsar radio emission models. The results demonstrate that the beamed radio emission from neutron stars can extend into the submillimeter regime, but are inconclusive on the existence and location of a potential spectral turn-up within the covered frequency range. The observed properties of the submillimeter emission resemble those of longer wavelengths and support a coherent mechanism for the production of pulsations at 353 GHz.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present Keck Cosmic Web Imager observations of giant Ly<jats:italic>α</jats:italic> halos surrounding nine galaxy groups and clusters at 2 &lt; <jats:italic>z</jats:italic> &lt; 3.3, including five new detections and one upper limit. We find observational evidence for the cold-stream to hot-accretion transition predicted by theory by measuring a decrease in the ratio between the spatially extended Ly<jats:italic>α</jats:italic> luminosity and the expected baryonic accretion rate (BAR), with increasing elongation above the transition mass (<jats:italic>M</jats:italic> <jats:sub>stream</jats:sub>). This implies a modulation of the share of BAR that remains cold, diminishing quasi-linearly (logarithmic slope of 0.97 ± 0.19, 5<jats:italic>σ</jats:italic> significance) with the halo to <jats:italic>M</jats:italic> <jats:sub>stream</jats:sub> mass ratio. The integrated star formation rates (SFRs) and active galactic nucleus (AGN) bolometric luminosities display a potentially consistent decrease, albeit significant only at 2.6<jats:italic>σ</jats:italic> and 1.3<jats:italic>σ</jats:italic>, respectively. The higher scatter in these tracers suggests the Ly<jats:italic>α</jats:italic> emission might be mostly a direct product of cold accretion in these structures rather than indirect, mediated by outflows and photoionization from SFR and AGNs; this is also supported by energetics considerations. Below <jats:italic>M</jats:italic> <jats:sub>stream</jats:sub> (cold-stream regime), we measure <jats:italic>L</jats:italic> <jats:sub>Ly<jats:italic>α</jats:italic> </jats:sub>/BAR = 10<jats:sup>40.51±0.16</jats:sup> erg s<jats:sup>−1</jats:sup> <jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{\odot }^{-1}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac531fieqn1.gif" xlink:type="simple" /> </jats:inline-formula> yr, consistent with predictions, and SFR/BAR = 10<jats:sup>−0.54±0.23</jats:sup>: on average, <jats:inline-formula> <jats:tex-math> <?CDATA ${30}_{-10}^{+20}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>30</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>20</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac531fieqn2.gif" xlink:type="simple" /> </jats:inline-formula>% of the cold streams go into stars. Above <jats:italic>M</jats:italic> <jats:sub>stream</jats:sub> (hot-accretion regime), <jats:italic>L</jats:italic> <jats:sub>Ly<jats:italic>α</jats:italic> </jats:sub> is set by <jats:italic>M</jats:italic> <jats:sub>stream</jats:sub> (within 0.2 dex scatter in our sample), independent of the halo mass but rising 10-fold from <jats:italic>z</jats:italic> = 2 to 3.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present high-pass-filtered continuum images of the inner 3.°5 × 2.°5 of the Galactic center at 20 cm with 6.″4 resolution. These mosaic images are taken with MeerKAT and reveal a large number of narrow filaments, roughly an order of magnitude increase in their numbers compared to past measurements. For the first time, we carry out population studies of the spectral index and magnetic field of the entire region. The mean spectral indices of the filaments are steeper than supernova remnants (SNRs) (−0.62) with a value of <jats:italic>α</jats:italic> ∼ −0.83. The variation in <jats:italic>α</jats:italic> is much larger than for the SNRs, suggesting that these characteristics have a different origin. A large-scale cosmic-ray-driven wind has recently been proposed to explain the origin of filaments and the large-scale 430 pc bipolar radio and X-ray structure. This favors the possibility that the large-scale bipolar radio/X-ray structure is produced by past activity of Sgr A* rather than a coordinated burst of supernovae. A trend of steeper indices is also noted with increasing distance from the Galactic plane. This could be explained either by synchrotron cooling or weak shocks accelerating cosmic-ray particles in the context of the cosmic-ray-driven wind. The mean magnetic field strengths along the filaments range from ∼100 to 400 <jats:italic>μ</jats:italic>G depending on the assumed ratio of cosmic-ray protons to electrons. Given that there is a high cosmic-ray pressure in the Galactic center, the large equipartition magnetic field implies that the magnetic filed is weak in most of the interstellar volume of the Galactic center.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Central Molecular Zone (CMZ) contains most of the mass of our Galaxy but its star formation rate is one order of magnitude lower than in the Galactic disk. This is likely related to the fact that the bulk of the gas in the CMZ is in a warm (&gt;100 K) and turbulent phase with little material in the prestellar phase. We present in this Letter observations of deuterium fractionation (D/H ratios) of HCN, HNC, HCO<jats:sup>+</jats:sup>, and N<jats:sub>2</jats:sub>H<jats:sup>+</jats:sup> toward the CMZ molecular cloud G+0.693–0.027. These observations clearly show, for the first time, the presence of a colder, denser, and less turbulent narrow component, with a line width of ∼9 km s<jats:sup>−1</jats:sup>, in addition to the warm, less dense, and turbulent broad component with a line width of ∼20 km s<jats:sup>−1</jats:sup>. The very low D/H ratio ≤6 × 10<jats:sup>−5</jats:sup> for HCO<jats:sup>+</jats:sup> and N<jats:sub>2</jats:sub>H<jats:sup>+</jats:sup>, close to the cosmic value (∼2.5 × 10<jats:sup>−5</jats:sup>), and the high D/H ratios, &gt;4 × 10<jats:sup>−4</jats:sup> for HCN and HNC, derived for the broad component confirm the presence of high-temperature deuteration routes for nitriles. For the narrow component we have derived D/H ratios &gt;10<jats:sup>−4</jats:sup> and excitation temperatures of 7 K for all molecules, suggesting kinetic temperatures ≤30 K and H<jats:sub>2</jats:sub> densities ≥5 × 10<jats:sup>4</jats:sup> cm<jats:sup>−3</jats:sup>, at least one order of magnitude larger than that for the broad component. The method presented in this Letter allows us to identify clouds on the verge of star formation, i.e., under prestellar conditions, toward the CMZ. This method can also be used for the identification of such clouds in external galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A hard TeV <jats:italic>γ</jats:italic>-ray component excess over the single-zone leptonic model prediction (TeV excess) is observed in the spectral energy distributions (SEDs) of some BL Lacs. Its origin is uncertain. We revisit this issue with four BL Lacs (1ES 0229+200, 1ES 0347–121, 1ES 1101–232, and H2356–309), in which the TeV excess is detected in their intrinsic SEDs. We represent their SEDs with a single-zone leptohadronic model, where radiations of the electrons and protons as well as the cascade electrons produced by the <jats:italic>γγ</jats:italic> and p<jats:italic>γ</jats:italic> interactions within their jets are considered. We show that the observed SEDs below the GeV gamma-ray band are attributed to the synchrotron radiations and self-Compton process of the primary electrons, and the TeV excess is explained with the <jats:italic>γ</jats:italic>-ray emission from the p<jats:italic>γ</jats:italic> process via the <jats:italic>π</jats:italic> <jats:sup>0</jats:sup> decay. The cascade emission of the electrons produced via the <jats:italic>γγ</jats:italic> and p<jats:italic>γ</jats:italic> interactions results in a keV–MeV excess in the SEDs, illustrated as a bump or plateau. This extra photon field enhances the production of TeV photons from the <jats:italic>p</jats:italic> <jats:italic>γ</jats:italic> process, resulting in a reduction of the proton power by about one order of magnitude. However, the derived powers are still 3–4 orders of magnitude larger than the Eddington limit, being challenged by the current black hole accretion physics. Applying our model to Mrk 421, we propose that synergic observations with current and upcoming TeV and keV–MeV telescopes for its tentative TeV and MeV excesses can give insights to the hadronic process in its jet.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Understanding the origin of accretion and dispersal of protoplanetary disks is fundamental for investigating planet formation. Recent numerical simulations show that launching winds are unavoidable when disks undergo magnetically driven accretion and/or are exposed to external UV radiation. Observations also hint that disk winds are common. We explore how the resulting wind mass loss rate can be used as a probe of both disk accretion and dispersal. As a proof-of-concept study, we focus on magnetocentrifugal winds, magnetorotational instability turbulence, and external photoevapotaion. By developing a simple yet physically motivated disk model and coupling it with simulation results available in the literature, we compute the wind mass loss rate as a function of external UV flux for each mechanism. We find that different mechanisms lead to different levels of mass loss rate, indicating that the origin of disk accretion and dispersal can be determined, by observing the wind mass loss rate resulting from each mechanism. This determination provides important implications for planet formation. This work thus shows that the ongoing and future observations of the wind mass loss rate for protoplanetary disks are paramount to reliably constrain how protoplanetary disks evolve with time and how planet formation takes place in the disks.</jats:p>