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<jats:title>Abstract</jats:title> <jats:p>Atacama Large Millimeter/submillimeter Array (ALMA) continuum observations of thermal emission from the dust component of protoplanetary disks have revealed an abundance of substructures that may be interpreted as evidence for embedded planets, but planet-driven spiral arms—perhaps one of the most compelling lines of evidence—have proven comparatively elusive. In this work, we test the capabilities of ALMA to detect the planet-driven spiral signal in continuum emission. Carrying out hydrodynamic simulations and radiative transfer calculations, we present synthetic Band 7 continuum images for a wide range of disk and observing conditions. We show that thermal mass planets at tens of astronomical units typically drive spirals detectable within a few hours of integration time, and the detectable planet mass may be as low as ∼Neptune mass (0.3 <jats:italic>M</jats:italic> <jats:sub>th</jats:sub>). The grains probed by ALMA form spirals morphologically identical to the underlying gas spiral. The temperature of the dust spiral is crucial in determining its contrast, and spirals are easier to detect in disks with an adiabatic equation of state and longer cooling times. Resolving the spiral is not necessary for its detection; with the help of residual maps, the optimal beam size is a few times the spiral width at a constant noise level. Finally, we show how the presence of gaps and rings can impair our ability to recognize colocated spirals. Our work demonstrates the planet-finding potential of the current design specification of ALMA, and suggests that observing capability is not the bottleneck in searching for spirals induced by thermal mass planets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Current estimates of the normalized accretion rates of quasars (<jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub>) rely on measuring the velocity widths of broad optical-UV emission lines (e.g., H<jats:italic>β</jats:italic> and Mg <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>2800). However, such lines tend to be weak or inaccessible in the most distant quasars, leading to increasing uncertainty in <jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> estimates at <jats:italic>z</jats:italic> &gt; 6. Utilizing a carefully selected sample of 53 radio-quiet quasars that have H<jats:italic>β</jats:italic> and C <jats:sc>iv</jats:sc> <jats:italic>λ</jats:italic>1549 spectroscopy as well as Chandra coverage, we searched for a robust accretion-rate indicator for quasars, particularly at the highest-accessible redshifts (<jats:italic>z</jats:italic> ∼ 6–7). Our analysis explored relationships between the H<jats:italic>β</jats:italic>-based <jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub>, the equivalent width (EW) of C <jats:sc>iv</jats:sc>, and the optical-to-X-ray spectral slope (<jats:italic>α</jats:italic> <jats:sub>ox</jats:sub>). Our results show that EW(C <jats:sc>iv</jats:sc>) is the strongest indicator of the H<jats:italic>β</jats:italic>-based <jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> parameter, consistent with previous studies, although significant scatter persists particularly for sources with weak C <jats:sc>iv</jats:sc> lines. We do not find evidence for the <jats:italic>α</jats:italic> <jats:sub>ox</jats:sub> parameter improving this relation, and we do not find a significant correlation between <jats:italic>α</jats:italic> <jats:sub>ox</jats:sub> and H<jats:italic>β</jats:italic>-based <jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub>. This absence of an improved relationship may reveal a limitation of our sample. X-ray observations of additional luminous sources, found at <jats:italic>z</jats:italic> ≳ 1, may allow us to mitigate the biases inherent in our archival sample and test whether X-ray data could improve <jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> estimates. Furthermore, deeper X-ray observations of our sources may provide accurate measurements of the hard-X-ray power-law photon index (Γ), which is considered an unbiased <jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> indicator. Correlations between EW(C <jats:sc>iv</jats:sc>) and <jats:italic>α</jats:italic> <jats:sub>ox</jats:sub> with a Γ-based <jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> may yield a more robust prediction of a quasar normalized accretion rate.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We analyze the formation mechanism of three homologous broad coronal mass ejections (CMEs) resulting from a series of solar blowout-eruption flares with successively increasing intensities (M2.0, M2.6, and X1.0). The flares originated from NOAA Active Region 12017 during 2014 March 28–29 within an interval of ≈24 hr. Coronal magnetic field modeling based on nonlinear force-free field extrapolation helps to identify low-lying closed bipolar loops within the flaring region enclosing magnetic flux ropes. We obtain a double flux rope system under closed bipolar fields for all the events. The sequential eruption of the flux ropes led to homologous flares, each followed by a CME. Each of the three CMEs formed from the eruptions gradually attained a large angular width, after expanding from the compact eruption-source site. We find these eruptions and CMEs to be consistent with the “magnetic-arch-blowout” scenario: each compact-flare blowout eruption was seated in one foot of a far-reaching magnetic arch, exploded up the encasing leg of the arch, and blew out the arch to make a broad CME.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent advances in submillimeter observations of young circumstellar nebulae have opened an unprecedented window into the structure of protoplanetary disks that has revealed the surprising ubiquity of broken and misaligned disks. In this work, we demonstrate that such disks are capable of torquing the spin axis of their host star, representing a hitherto unexplored pathway by which stellar obliquities may be generated. The basis of this mechanism is a crossing of the stellar spin precession and inner disk regression frequencies, resulting in adiabatic excitation of the stellar obliquity. We derive analytical expressions for the characteristic frequencies of the inner disk and star as a function of the disk gap boundaries and place an approximate limit on the disk architectures for which frequency crossing and the resulting obliquity excitation are expected, thereby illustrating the efficacy of this model. Cumulatively, our results support the emerging consensus that significant spin–orbit misalignments are an expected outcome of planet formation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>To reveal the effect of secondary electron emission on the charging properties of a surface covered by micron-sized insulating dust particles and the migration characteristics of these particles, for the first time we used a laser Doppler method to measure the diameters and velocities of micron-sized anorthite particles under electron beam irradiation with an incident energy of 350 eV. Here, anorthite particles are being treated as a proxy for lunar regolith. We experimentally confirm that the vertical transport of anorthite particles is always dominant, although horizontal transport occurs. In our experiments, some anorthite particles were observed to have large vertical velocities up to 9.74 m s<jats:sup>−1</jats:sup> at the measurement point. The upper boundary of the vertical velocities <jats:italic>V</jats:italic> <jats:sub>z</jats:sub> of these high-speed anorthite particles are well constrained by its diameter <jats:italic>D</jats:italic>, that is, <jats:inline-formula> <jats:tex-math> <?CDATA ${V}_{{\rm{z}}}^{2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>V</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">z</mml:mi> </mml:mrow> <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="apjac5d48ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> linearly depends on <jats:italic>D</jats:italic> <jats:sup>−2</jats:sup>. These velocity–diameter data provide strong constraints on the dust charging and transportation mechanisms. The shared charge model could not explain the observed velocity–diameter data. Both the isolated charge model and patched charge model appear to require a large dust charging potential of −350 to −78 V to reproduce the observed data. The microstructures of the dusty surface may play an important role in producing this charging potential and in understanding the pulse migration phenomenon observed in our experiment. The presented results and analysis in this paper are helpful for understanding the dust charging and electrostatic transport mechanisms in airless celestial bodies such as the Moon and asteroids in various plasma conditions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Young and magnetically active low-mass stars often exhibit nonthermal coronal radio emission owing to the gyration of electrons in their magnetized chromospheres. This emission is easily detectable at centimeter wavelengths with the current sensitivity of large radio interferometers like the Very Large Array (VLA). With the aim of identifying nearby stars adequate for future accurate radio astrometric monitoring using very long baseline interferometry (VLBI), we have used the VLA in its B configuration to search for radio emission at <jats:italic>ν</jats:italic> ≃ 6 GHz (<jats:italic>λ</jats:italic> ≃ 5 cm) toward a sample of 170 nearby (&lt;130 pc), mostly young (5–500 Myr) stars of spectral types between F4 and M2. At our mean 3<jats:italic>σ</jats:italic> detection limit of ≃50 <jats:italic>μ</jats:italic>Jy, we identify 31 young stars with coronal radio emission (an 18% system detection rate) and more than 600 background (most likely extragalactic) sources. Among the targeted stars, we find a significant decline of the detection rate with age from 56% ± 20% for stars with ages ≤10 Myr to 10% ± 3% for stars with ages 100–200 Myr. No star older than 200 Myr was detected. The detection rate also declines with <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> from 36% ± 10% for stars with <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> &lt; 4000 K to 13% ± 3% for earlier spectral types with <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> &gt; 5000 K. The binarity fraction among the radio-bright stars is at least twice as high as among the radio-quiet stars. The radio-bright nearby young stars identified here provide an interesting sample for future astrometric studies using VLBI arrays aimed at searching for hitherto-unknown tight binary components or even exoplanets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present results from MUSE spatially resolved spectroscopy of 21 post-starburst galaxies in the centers of eight clusters from <jats:italic>z</jats:italic> ∼ 0.3 to <jats:italic>z</jats:italic> ∼ 0.4. We measure spatially resolved star formation histories (SFHs), the time since quenching (<jats:italic>t</jats:italic> <jats:sub> <jats:italic>Q</jats:italic> </jats:sub>), and the fraction of stellar mass assembled in the past 1.5 Gyr (<jats:italic>μ</jats:italic> <jats:sub>1.5</jats:sub>). The SFHs display a clear enhancement of star formation prior to quenching for 16 out of 21 objects, with at least 10% (and up to &gt;50%) of the stellar mass being assembled in the past 1.5 Gyr and <jats:italic>t</jats:italic> <jats:sub> <jats:italic>Q</jats:italic> </jats:sub> ranging from less than 100 to ∼800 Myr. By mapping <jats:italic>t</jats:italic> <jats:sub> <jats:italic>Q</jats:italic> </jats:sub> and <jats:italic>μ</jats:italic> <jats:sub>1.5</jats:sub>, we analyze the quenching patterns of the galaxies. Most galaxies in our sample have quenched their star formation from the outside in or show a side-to-side/irregular pattern, both consistent with quenching by ram pressure stripping. Only three objects show an inside-out quenching pattern, all of which are at the high-mass end of our sample. At least two of them currently host an active galactic nucleus. In two post-starbursts, we identify tails of ionized gas indicating that these objects had their gas stripped by ram pressure very recently. Post-starburst features are also found in the stripped regions of galaxies undergoing ram pressure stripping in the same clusters, confirming the link between these classes of objects. Our results point to ram pressure stripping as the main driver of fast quenching in these environments, with active galactic nuclei playing a role at high stellar masses.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Existing star-forming vs. active galactic nucleus (AGN) classification schemes using optical emission-line diagnostics mostly fail for low-metallicity and/or highly star-forming galaxies, missing AGN in typical <jats:italic>z</jats:italic> ∼ 0 dwarfs. To recover AGN in dwarfs with strong emission lines (SELs), we present a classification scheme optimizing the use of existing optical diagnostics. We use Sloan Digital Sky Survey emission-line catalogs overlapping the volume- and mass-limited REsolved Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntex (ECO) surveys to determine the AGN percentage in SEL dwarfs. Our photoionization grids show that the [O <jats:sc>iii</jats:sc>]/H<jats:italic>β</jats:italic> versus [S <jats:sc>ii</jats:sc>]/H<jats:italic>α</jats:italic> diagram (S <jats:sc>ii</jats:sc> plot) and [O <jats:sc>iii</jats:sc>]/H<jats:italic>β</jats:italic> versus [O <jats:sc>i</jats:sc>]/H<jats:italic>α</jats:italic> diagram (O <jats:sc>i</jats:sc> plot) are less metallicity sensitive and more successful in identifying dwarf AGN than the popular [O <jats:sc>iii</jats:sc>]/H<jats:italic>β</jats:italic> versus [N <jats:sc>ii</jats:sc>]/H<jats:italic>α</jats:italic> diagnostic (N <jats:sc>ii</jats:sc> plot or “BPT diagram”). We identify a new category of “star-forming AGN” (SF-AGN) classified as star-forming by the N <jats:sc>ii</jats:sc> plot but as AGN by the S <jats:sc>ii</jats:sc> and/or O <jats:sc>i</jats:sc> plots. Including SF-AGN, we find the <jats:italic>z</jats:italic> ∼ 0 AGN percentage in dwarfs with SELs to be ∼3%–16%, far exceeding most previous optical estimates (∼1%). The large range in our dwarf AGN percentage reflects differences in spectral fitting methodologies between catalogs. The highly complete nature of RESOLVE and ECO allows us to normalize strong emission-line galaxy statistics to the full galaxy population, reducing the dwarf AGN percentage to ∼0.6%–3.0%. The newly identified SF-AGN are mostly gas-rich dwarfs with halo mass &lt;10<jats:sup>11.5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, where highly efficient cosmic gas accretion is expected. Almost all SF-AGN also have low metallicities (<jats:italic>Z</jats:italic> ≲ 0.4 <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>), demonstrating the advantage of our method.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We propose a mixture model of open clusters (OCs) in color–magnitude diagrams (CMDs) to measure the OC properties, including isochrone parameters (age, distance, metallicity, and dust extinction), stellar mass function (MF), and binary parameters (binary fraction and mass-ratio distribution), with high precision and reliability. The model treats an OC in the CMD as a mixture of single and binary member stars and field stars in the same region. The cluster members are modeled using a theoretical stellar model, mass function, and binary properties. The field component is modeled nonparametrically using a separate field-star sample in the vicinity of the cluster. Unlike conventional methods that rely on stringent member selection, ours allows us to use a sample of more complete cluster members and attendant field stars. The larger star sample reduces the statistical error and diminishes the potential bias by retaining more stars that are crucial for age estimation and MF measurement. After validating the method with 1000 mock clusters, we measured the parameters of 10 real OCs using Gaia EDR3 data. The best-fit isochrones are consistent with previous measurements in general but with more precise age estimates for several OCs. The inferred MF slope is −2.7 to −1.6 for clusters younger than 2 Gyr, while older clusters appear to have significantly flatter MFs. The binary fraction is 30%–50%. The photometric and astrometric distances agree well.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>X-ray observations of low-mass stars in open clusters are critical to understanding the dependence of magnetic activity on stellar properties and their evolution. Praesepe and the Hyades, two of the nearest, most-studied open clusters, are among the best available laboratories for examining the dependence of magnetic activity on rotation for stars with masses ≲1 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We present an updated study of the rotation–X-ray activity relation in the two clusters. We updated membership catalogs that combine pre-Gaia catalogs with new catalogs based on Gaia Data Release 2. The resulting catalogs are the most inclusive ones for both clusters: 1739 Praesepe and 1315 Hyades stars. We collected X-ray detections for cluster members, for which we analyzed, re-analyzed, or collated data from ROSAT, the Chandra X-ray Observatory, the Neil Gehrels Swift Observatory, and XMM-Newton. We have detections for 326 Praesepe and 462 Hyades members, of which 273 and 164, respectively, have rotation periods—an increase of 6× relative to what was previously available. We find that at ≈700 Myr, only M dwarfs remain saturated in X-rays, with only tentative evidence for supersaturation. We also find a tight relation between the Rossby number and fractional X-ray luminosity <jats:italic>L</jats:italic> <jats:sub>X</jats:sub>/<jats:italic>L</jats:italic> <jats:sub>bol</jats:sub> in unsaturated single members, suggesting a power-law index between −3.2 and −3.9. Lastly, we find no difference in the coronal parameters between binary and single members. These results provide essential insight into the relative efficiency of magnetic heating of the stars’ atmospheres, thereby informing the development of robust age-rotation-activity relations.</jats:p>