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<jats:title>Abstract</jats:title> <jats:p>Linearization of the vector field, as a common multispacecraft data analysis technique, has been widely used in (1) reconstruction of three-dimensional magnetic and velocity fields, (2) predictions of the possible topologies of linear fields, especially for the magnetic null-point classification, and (3) other data analysis techniques, such as the curlometer technique. However, the length scale of validity of the linear approximation in space plasmas is still an open question. In this study, we utilize the frozen-in condition as the criterion to estimate the accuracy of the linear method. We derive the linearization error theoretically, and find that the frozen-in condition cannot be satisfied everywhere in the linearly reconstructed fields as long as the fields have nonzero spatial gradients. This indicates that the use of the linear method must be treated with caution. We further investigate the length scale of validity of the linear method in space plasmas by utilizing the Magnetospheric Multiscale Mission data. Through two case studies and statistical analysis, we demonstrate that the linear approximation is acceptable at a length scale of, on average, 1.1 ion inertial lengths in the solar wind/magnetosheath, while in the magnetosphere the linear method exhibits great uncertainties. This study provides the theoretical basis for the application of the linear method in space plasmas.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Vamana is a mixture model framework that infers the astrophysical distribution of chirp mass, mass ratio, and spin component aligned with the orbital angular momentum for the binary black holes (BBH) population. We extend the mixing components in this framework to also model the redshift evolution of merger rate and report all the major one- and two-dimensional features in the BBH population using the 69 gravitational-wave signals detected with a false alarm rate &lt;1 yr<jats:sup>−1</jats:sup> in the third Gravitational-Wave Transient Catalog (GWTC-3). Endorsing our previous report and a recent corroborating report from LIGO Scientific, Virgo, and KAGRA Collaborations, we observe the chirp mass distribution has multiple peaks and a lack of mergers with chirp masses 10–12 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. In addition, we observe that aligned spins show mass dependence with heavier binaries exhibiting larger spins, the mass ratio shows a dependence on the chirp mass but not on the aligned spin, and the redshift evolution of the merger rate for the peaks in the mass distribution is disparate. These features possibly reflect the astrophysics associated with the BBH formation channels. However, additional observations are needed to improve our limited confidence in them.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We explore the three-dimensional properties of convective, luminous (<jats:italic>L</jats:italic> ≈ 10<jats:sup>4.5</jats:sup>–10<jats:sup>5</jats:sup> <jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub>), hydrogen-rich envelopes of red supergiants (RSGs) based on radiation hydrodynamic simulations in spherical geometry using <jats:monospace>Athena</jats:monospace>++. These computations comprise ≈30% of the stellar volume, include gas and radiation pressure, and self-consistently track the gravitational potential for the outer ≈3<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> of the simulated <jats:italic>M</jats:italic> ≈ 15<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> stars. This work reveals a radius, <jats:italic>R</jats:italic> <jats:sub>corr</jats:sub>, around which the nature of the convection changes. For <jats:italic>r</jats:italic> &gt; <jats:italic>R</jats:italic> <jats:sub>corr</jats:sub>, though still optically thick, diffusion of photons dominates the energy transport. Such a regime is well studied in less luminous stars, but in RSGs, the near- (or above-)Eddington luminosity (due to opacity enhancements at ionization transitions) leads to the unusual outcome of denser regions moving outward rather than inward. This region of the star also has a large amount of turbulent pressure, yielding a density structure much more extended than 1D stellar evolution predicts. This “halo” of material will impact predictions for both shock breakout and early lightcurves of Type IIP supernovae. Inside of <jats:italic>R</jats:italic> <jats:sub>corr</jats:sub>, we find a nearly flat entropy profile as expected in the efficient regime of mixing-length theory (MLT). Radiation pressure provides ≈1/3 of the support against gravity in this region. Our comparisons to MLT suggest a mixing length of <jats:italic>α</jats:italic> = 3–4, consistent with the sizes of convective plumes seen in the simulations. The temporal variability of these 3D models is mostly on the timescale of the convective plume lifetimes (≈300 days), with amplitudes consistent with those observed photometrically.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The abundance and distribution of solids inside the Hill sphere are central to our understanding of the giant planet dichotomy. Here, we present a 3D characterization of the dust density, mass flux, and mean opacities in the envelope of subthermal and superthermal-mass planets. We simulate the dynamics of multiple dust species in a global protoplanetary disk model accounting for dust feedback. We find that the meridional flows do not effectively stir dust grains at scales of the Bondi sphere. Thus the dust settling driven by the stellar gravitational potential sets the latitudinal dust density gradient within the planet envelope. Not only does the planet’s potential enhance this gradient, but also the spiral wakes serve as another source of asymmetry. These asymmetries substantially alter the inferred mean Rosseland and Planck opacities. In cases with moderate-to-strong dust settling, the opacity gradient can range from a few percent to more than two orders of magnitude between the midplane and the polar regions of the Bondi sphere. Finally, we show that this strong latitudinal opacity gradient can introduce a transition between optically thick and thin regimes at the scales of the planet envelope. We suggest that this transition is likely to occur when the equilibrium scale height of hundred-micron-sized particles is smaller than the Hill radius of the forming planet. This work calls into question the adoption of a constant opacity derived from well-mixed distributions and demonstrates the need for global radiation hydrodynamics models of giant planet formation that account for dust dynamics.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Centaurs are an ephemeral class of objects that can evolve into Jupiter-family comets (or JFCs) due to gravitational perturbations from the giant planets. In this work, we use numerical integration of massless test particles in the six-body problem (the Sun, four giant planets, and a test particle) to study the transformation of Centaurs into JFCs. We find that Centaurs can transform into JFCs via a rapid, continuous drop in perihelion or aphelion distance to a value below 5.2 au or 7 au, respectively, typically within 5 yr from the start of the drop. We call these JFC perihelion drops and JFC aphelion drops, respectively. These drops are correlated with close approaches to Jupiter. For such perihelion and aphelion drops, the maximum possible fractional change in the associated perihelion or aphelion distance increases with decreasing close-approach distance to Jupiter. A perihelion barrier may exist at 6.3 au. If so, then it must be crossed from above in order for a Centaur to transform into a JFC due to a single close approach to Jupiter. Currently, 93 (or 11%) of the known Centaurs have a perihelion distance, <jats:italic>q</jats:italic>, below 6.3 au. If the inclination of the orbit of the Centaur to the ecliptic plane is above 10.°2, then orbits with <jats:italic>q</jats:italic> &gt; 6.3 au and 2 &lt; <jats:italic>T</jats:italic> <jats:sub> <jats:italic>J</jats:italic> </jats:sub> &lt; 3 can exist, where <jats:italic>T</jats:italic> <jats:sub> <jats:italic>J</jats:italic> </jats:sub> is the Tisserand parameter with respect to Jupiter. Small bodies in such orbits could be classified as either Centaurs or JFCs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Galactic cosmic rays (GCRs), the highly energetic particles that may raise critical health issues for astronauts in space, are modulated by solar activity, with their intensity lagging behind the variation in sunspot number (SSN) by about one year. Previously, this lag has been attributed to the combined effect of outward convecting solar wind and inward propagating GCRs. However, the lag’s amplitude and its solar-cycle dependence are still not fully understood. By investigating the solar surface magnetic field, we find that the source of heliospheric magnetic field—the open magnetic flux on the Sun—already lags behind SSN before it convects into the heliosphere along with the solar wind. The delay during odd cycles is longer than that during sequential even cycles. Thus, we propose that the GCR lag is primarily due to the very late opening of the solar magnetic field with respect to SSN, though solar wind convection and particle transport in the heliosphere also matter. We further investigate the origin of the open flux from different latitudes of the Sun and find that the total open flux is significantly contributed by that from low latitudes, where coronal mass ejections frequently occur and also show an odd–even cyclic pattern. Our findings challenge existing theories, and may serve as the physical basis of long-term forecasts of radiation dose estimates for manned deep-space exploration missions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present precise abundance determinations of two near-pristine damped Ly<jats:italic>α</jats:italic> systems (DLAs) to assess the nature of the [O/Fe] ratio at [Fe/H] &lt; −3.0 (i.e., &lt;1/1000 of the solar metallicity). Prior observations indicate that the [O/Fe] ratio is consistent with a constant value, [O/Fe] ≃ +0.4, when −3 &lt; [Fe/H] &lt; −2, but this ratio may increase when [Fe/H] ≲ −3. In this paper, we test this picture by reporting new, high-precision [O/Fe] abundances in two of the most metal-poor DLAs currently known. We derive values of [O/Fe] = +0.50 ± 0.10 and [O/Fe] = +0.62 ± 0.05 for these two <jats:italic>z</jats:italic> ≃ 3 near-pristine gas clouds. These results strengthen the idea that the [O/Fe] abundances of the most metal-poor DLAs are elevated compared to DLAs with [Fe/H] ≳ −3. We compare the observed abundance pattern of the latter system to the nucleosynthetic yields of Population III supernovae (SNe), and find that the enrichment can be described by a (19–25) M<jats:sub>⊙</jats:sub> Population III SN that underwent a (0.9–2.4) × 10<jats:sup>51</jats:sup> erg explosion. These high-precision measurements showcase the behavior of [O/Fe] in the most metal-poor environments. Future high-precision measurements in new systems will contribute to a firm detection of the relationship between [O/Fe] and [Fe/H]. These data will reveal whether we are witnessing a chemical signature of enrichment from Population III stars and allow us to rule out contamination from Population II stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The SPitzer InfraRed Intensive Transients Survey (SPIRITS) searched for luminous infrared (IR) transients and variables in nearly 200 nearby galaxies from 2014 to 2019, using the warm Spitzer telescope at 3.6 and 4.5 <jats:italic>μ</jats:italic>m. Among the SPIRITS variables are IR-bright objects that are undetected in ground-based optical surveys. We classify them as (1) transients, (2) periodic variables, and (3) irregular variables. The transients include eSPecially Red Intermediate-luminosity Transient Events (SPRITEs), having maximum luminosities fainter than supernovae, red IR colors, and a wide range of outburst durations (days to years). Here we report deep optical and near-IR imaging with the Hubble Space Telescope (HST) of 21 SPIRITS variables. They were initially considered SPRITE transients, but many eventually proved instead to be periodic or irregular variables as more data were collected. HST images show most of these cool and dusty variables are associated with star-forming regions in late-type galaxies, implying an origin in massive stars. Two SPRITEs lacked optical progenitors in deep preoutburst HST images; however, one was detected during eruption at <jats:italic>J</jats:italic> and <jats:italic>H</jats:italic>, indicating a dusty object with an effective temperature of ∼1050 K. One faint SPRITE turned out to be a dusty classical nova. About half the HST targets proved to be periodic variables, with pulsation periods of 670–2160 days; they are likely dusty asymptotic-giant-branch (AGB) stars with masses of ∼5–10 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. A few of them were warm enough to be detected in deep HST frames, but most are too cool. Out of six irregular variables, two were red supergiants with optical counterparts in HST images; four were too enshrouded for HST detection.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We introduce an ALMA band 3 spectroscopic survey targeting the brightest submillimeter galaxies (SMGs) in the COSMOS field. Here we present the first results based on the 18 primary SMGs that have 870 <jats:italic>μ</jats:italic>m flux densities of <jats:italic>S</jats:italic> <jats:sub>870</jats:sub> = 12.4–19.3 mJy and are drawn from a parent sample of 260 ALMA-detected SMGs from the AS2COSMOS survey. We detect emission lines in 17 and determine their redshifts to be in the range of <jats:italic>z</jats:italic> = 2–5 with a median of 3.3 ± 0.3. We confirm that SMGs with brighter <jats:italic>S</jats:italic> <jats:sub>870</jats:sub> are located at higher redshifts. The data additionally cover five fainter companion SMGs, and we obtain line detection in one. Together with previous studies, our results indicate that for SMGs that satisfy our selection, their brightest companion SMGs are physically associated with their corresponding primary SMGs ≥40% of the time, suggesting that mergers play a role in the triggering of star formation. By modeling the foreground gravitational fields, &lt;10% of the primary SMGs can be strongly lensed with a magnification <jats:italic>μ</jats:italic> &gt; 2. We determine that about 90% of the primary SMGs have lines that are better described by double Gaussian profiles, and the median separation of the two Gaussian peaks is 430 ± 40 km s<jats:sup>−1</jats:sup>. This allows estimates of an average baryon mass, which, together with the line dispersion measurements, puts our primary SMGs on the similar mass–<jats:italic>σ</jats:italic> correlation found on local early-type galaxies. Finally, the number density of our <jats:italic>z</jats:italic> &gt; 4 primary SMGs is found to be <jats:inline-formula> <jats:tex-math> <?CDATA ${1}_{-0.6}^{+0.9}\times {10}^{-6}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.9</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac61dfieqn1.gif" xlink:type="simple" /> </jats:inline-formula> cMpc<jats:sup>−3</jats:sup>, suggesting that they can be the progenitors of <jats:italic>z</jats:italic> ∼ 3−4 massive quiescent galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze recent high-quality Atacama Large Millimeter Array (ALMA) molecular line mapping observations of the northeast jet of the young stellar object (YSO) OMC 2/FIR 6b (HOPS-60) and find that these ALMA observations are much more likely to indicate a twin-jet structure than jet rotation, as previously hypothesized. The interpretation of the line-of-sight velocity gradient across (perpendicular to its axis) the northeast jet of Fir 6b in terms of jet rotation leads to jet-launching radii of ≃2–3 au. However, the velocities of the jets ≃100–400 km s<jats:sup>−1</jats:sup> are much larger than the escape speed from these radii. We argue that the northeast jet of FIR 6b is instead compatible with a twin-jet structure, as observed in some planetary nebulae. Specifically, we find that the main, redshifted jet emanating from the central YSO is composed of two, very closely aligned, narrower jets that were launched by the central YSO at about the same time but at different inclinations with respect to the plain of the sky. This twin-jet structure removes the extreme requirement that jets with velocities similar to the escape velocity from the YSO be launched from very large radii. The YSO FIR 6b and certain planetary nebulae also share the characteristics of unequal structures and intensities of their two opposing bipolar jets. We propose that such opposing lobe asymmetries can result from a substellar binary companion on an eccentric orbit that is inclined to the accretion disk plane.</jats:p>