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<jats:title>Abstract</jats:title> <jats:p>A recent study demonstrated that freedom of convection and strength of magnetic field in the photospheric feet of active-region (AR) coronal loops, together, can engender or quench heating in them. Other studies stress that magnetic flux cancellation at the loop-feet potentially drives heating in loops. We follow 24 hr movies of a bipolar AR, using extreme ultraviolet images from the Atmospheric Imaging Assembly/Solar Dynamics Observatory (SDO) and line-of-sight (LOS) magnetograms from the Helioseismic and Magnetic Imager (HMI)/SDO, to examine magnetic polarities at the feet of 23 of the brightest coronal loops. We derived Fe <jats:sc>xviii</jats:sc> emission (hot-94) images (using the Warren et al. method) to select the hottest/brightest loops, and confirm their footpoint locations via non-force-free field extrapolations. From 6″ × 6″ boxes centered at each loop foot in LOS magnetograms we find that ∼40% of the loops have both feet in unipolar flux, and ∼60% of the loops have at least one foot in mixed-polarity flux. The loops with both feet unipolar are ∼15% shorter lived on average than the loops having mixed-polarity foot-point flux, but their peak-intensity averages are equal. The presence of mixed-polarity magnetic flux in at least one foot in the majority of the loops suggests that flux cancellation at the footpoints may drive most of the heating. But the absence of mixed-polarity magnetic flux (to the detection limit of HMI) in ∼40% of the loops suggests that flux cancellation may not be necessary to drive heating in coronal loops—magnetoconvection and field strength at both loop feet possibly drive much of the heating, even in the cases where a loop foot presents mixed-polarity magnetic flux.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The first neutron star binary merger detected in gravitational waves, GW170817, and the subsequent detection of its emission across the electromagnetic spectrum showed that these systems are viable progenitors of short <jats:italic>γ</jats:italic>-ray bursts (sGRB). The afterglow signal of GW170817 has been found to be consistent with a structured GRB jet seen off-axis, requiring significant amounts of relativistic material at large angles. This trait can be attributed to the interaction of the relativistic jet with the external wind medium. Here we perform numerical simulations of relativistic jets interacting with realistic wind environments in order to explore how the properties of the wind and central engine affect the structure of successful jets. We find that the angular energy distribution of the jet depends primarily on the ratio between the lifetime of the jet and the time it takes the merger remnant to collapse. We make use of these simulations to constrain the time it took for the merger remnant in GW170817 to collapse into a black hole based on the angular structure of the jet as inferred from afterglow observations. We conclude that the lifetime of the merger remnant in GW170817 was ≈1–1.7 s, which, after collapse, triggered the formation of the jet.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>For the extremely bright lensed galaxy SDSS J1723+3411 at <jats:italic>z</jats:italic> = 1.3293, we analyze spatially integrated MMT, Keck, and Hubble Space Telescope spectra that fully cover the rest-frame wavelength range of 1400–7200 Å. We also analyze near-IR spectra from Gemini that cover H<jats:italic>α</jats:italic> for a portion of the lensed arc. We report fluxes for 42 detected emission lines, and upper limits for an additional 22. This galaxy has extreme emission-line ratios and high equivalent widths that are characteristic of extreme emission-line galaxies. We compute strong emission-line diagnostics from both the rest-frame optical and rest-frame ultraviolet (UV), to constrain physical conditions and test the spectral diagnostics themselves. We tightly determine the nebular physical conditions using the most reliable diagnostics, and then compare to results from other diagnostics. We find disappointing performance from the UV-only diagnostics: they either are unable to measure the metallicity or dramatically underestimate it; they overestimate the pressure; and the UV diagnostic of ionization parameter has a strong metallicity dependence in this regime. Based on these results, we suggest that upcoming James Webb Space Telescope (JWST) spectroscopic surveys of galaxies in the reionization epoch should invest the additional integration time to capture the optical [O <jats:sc>ii</jats:sc>] and [O <jats:sc>iii</jats:sc>] emission lines, and not rely solely on the rest-frame UV emission lines. We make available the spectra; they represent one of the highest-quality emission-line spectral atlases of star-forming galaxies available beyond the local universe, and will aid the planning of observations with JWST.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Complex organic molecules (COMs) are believed to form in the ice mantle of dust grains and are released to the gas by thermal sublimation when grain mantles are heated to temperatures of <jats:inline-formula> <jats:tex-math> <?CDATA ${T}_{{\rm{d}}}\gtrsim 100\,{\rm{K}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">d</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≳</mml:mo> <mml:mn>100</mml:mn> <mml:mspace width="0.25em" /> <mml:mi mathvariant="normal">K</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabccbeieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. However, some COMs are detected in regions with temperatures below 100 K. Recently, a new mechanism of rotational desorption due to centrifugal stress induced by radiative torques (RATs) was proposed by Hoang &amp; Tram (2020) that can desorb COMs at low temperatures. In this paper, we report observational evidence for rotational desorption of COMs toward the nearest massive star-forming region, Orion BN/KL. We compare the abundance of three representative COMs that have very high binding energy computed by the rotational desorption mechanism with observations by ALMA, and demonstrate that the rotational desorption mechanism can explain the existence of such COMs. We also analyze the polarization data from SOFIA/HAWC+ and JCMT/SCUBA-2 and find that the polarization degree at far-infrared/submillimeter decreases with increasing the grain temperature for <jats:inline-formula> <jats:tex-math> <?CDATA ${T}_{{\rm{d}}}\gtrsim 71\,{\rm{K}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">d</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≳</mml:mo> <mml:mn>71</mml:mn> <mml:mspace width="0.25em" /> <mml:mi mathvariant="normal">K</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabccbeieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. This is consistent with the theoretical prediction using the RAT alignment theory and Radiative Torque Disruption mechanism. Such an anticorrelation between dust polarization and dust temperature supports the rotational disruption as well as rotational desorption mechanism of COMs induced by RATs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We have extended the work of Papers I and II of this series to determine at a higher spatial resolution the properties of the embedded Orion-S Molecular Cloud that lies within the ionized cavity of the Orion Nebula and of the thin ionized layer that lies between the Cloud and the observer. This was done using existing and new [N <jats:sc>ii</jats:sc>] (658.3 nm) and [O <jats:sc>iii</jats:sc>] (500.7 nm) spectra that map the central region of the Orion Nebula (the Huygens region). Although our observations show that the surface brightness of the ionized layer on the Orion-S Molecular Cloud and that of the nearer foreground ionized layer are linked, the process by which this is done is unclear. It is shown that the Cloud modifies the outer parts of the Huygens region in the direction of the extended hot X-ray gas.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Physical processes that redistribute or remove angular momentum from protoplanetary disks can drive mass accretion onto the star and affect the outcome of planet formation. Despite ubiquitous evidence that protoplanetary disks are engaged in accretion, the process(es) responsible remain unclear. Here we present evidence for redshifted molecular absorption in the spectrum of a Class I source that indicates rapid inflow at the disk surface. High-resolution mid-infrared spectroscopy of GV Tau N reveals a rich absorption spectrum of individual lines of C<jats:sub>2</jats:sub>H<jats:sub>2</jats:sub>, HCN, NH<jats:sub>3</jats:sub>, and H<jats:sub>2</jats:sub>O. From the properties of the molecular absorption, we can infer that it carries a significant accretion rate <jats:inline-formula> <jats:tex-math> <?CDATA ${\dot{M}}_{\mathrm{abs}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabcfc6ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> ∼ 10<jats:sup>−8</jats:sup>–10<jats:sup>−7</jats:sup> <jats:italic> M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>, comparable to the stellar accretion rates of active T Tauri stars. Thus, we may be observing disk accretion in action. The results may provide observational evidence for supersonic “surface accretion flows,” which have been found in MHD simulations of magnetized disks. The observed spectra also represent the first detection of NH<jats:sub>3</jats:sub> in the planet formation region of a protoplanetary disk. With NH<jats:sub>3</jats:sub> only comparable in abundance to HCN, it cannot be a major missing reservoir of nitrogen. If, as expected, the dominant nitrogen reservoir in inner disks is instead N<jats:sub>2</jats:sub>, its high volatility would make it difficult to incorporate into forming planets, which may help to explain the low nitrogen content of the bulk Earth.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present an updated global model of the solar corona, including the transition region. We simulate the realistic three-dimensional (3D) magnetic field using the data from the photospheric magnetic field measurements and assume the magnetohydrodynamic (MHD) Alfvén wave turbulence and its nonlinear dissipation to be the only source for heating the coronal plasma and driving the solar wind. In closed-field regions, the dissipation efficiency in a balanced turbulence is enhanced. In the coronal holes, we account for a reflection of the outward-propagating waves, which is accompanied by the generation of weaker counterpropagating waves. The nonlinear cascade rate degrades in strongly imbalanced turbulence, thus resulting in colder coronal holes. The distinctive feature of the presented model is the description of the low corona as almost-steady-state low-beta plasma motion and heat flux transfer along the magnetic field lines. We trace the magnetic field lines through each grid point of the lower boundary of the global corona model, chosen at some heliocentric distance, <jats:italic>R</jats:italic> = <jats:italic>R</jats:italic> <jats:sub> <jats:italic>b</jats:italic> </jats:sub> ∼ 1.1<jats:italic>R</jats:italic> <jats:sub>⊙</jats:sub>, well above the transition region. One can readily solve the plasma parameters along the magnetic field line from 1D equations for the plasma motion and heat transport together with the Alfvén wave propagation, which adequately describe the physics within the heliocentric distance range<jats:italic> R</jats:italic> <jats:sub>⊙</jats:sub> &lt; <jats:italic>R</jats:italic> &lt; <jats:italic>R</jats:italic> <jats:sub> <jats:italic>b</jats:italic> </jats:sub>, in the low solar corona. By interfacing this threaded-field-line model with the full MHD global corona model at <jats:italic>r</jats:italic> = <jats:italic>R</jats:italic> <jats:sub> <jats:italic>b</jats:italic> </jats:sub>, we find the global solution and achieve a faster-than-real-time performance of the model on ∼200 cores.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The random walk of magnetic field lines is an important ingredient in understanding how the connectivity of the magnetic field affects the spatial transport and diffusion of charged particles. As solar energetic particles propagate away from near-solar sources, they interact with the fluctuating magnetic field, which modifies their distributions. We develop a formalism in which the differential equation describing the field line random walk contains both effects due to localized magnetic displacements and a non-stochastic contribution from the large-scale expansion. We use this formalism together with a global magnetohydrodynamic simulation of the inner-heliospheric solar wind, which includes a turbulence transport model, to estimate the diffusive spreading of magnetic field lines that originate in different regions of the solar atmosphere. We first use this model to quantify field line spreading at 1 au, starting from a localized solar source region, and find rms angular spreads of about 20°–60°. In the second instance, we use the model to estimate the size of the source regions from which field lines observed at 1 au may have originated, thus quantifying the uncertainty in calculations of magnetic connectivity; the angular uncertainty is estimated to be about 20°. Finally, we estimate the filamentation distance, i.e., the heliocentric distance up to which field lines originating in magnetic islands can remain strongly trapped in filamentary structures. We emphasize the key role of slab-like fluctuations in the transition from filamentary to more diffusive transport at greater heliocentric distances.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We utilize machine-learning methods to distinguish BL Lacertae (BL Lac) objects from flat spectrum radio quasars (FSRQs) within a sample of likely X-ray blazar counterparts to Fermi 3FGL unassociated gamma-ray sources. From our previous work, we have extracted 84 sources that were classified as ≥99% likely to be blazars. We then utilize Swift-XRT, Fermi, and The Wide-field Infrared Survey Explorer (WISE) data together to distinguish the specific type of blazar, FSRQs, or BL Lac objects. Various X-ray and gamma-ray parameters can be used to differentiate between these subclasses. These are also known to occupy different parameter space on the WISE color–color diagram. Using all these data together would provide more robust results for the classified sources. We utilized a random forest classifier to calculate the probability for each blazar to be associated with a BL Lac object or an FSRQ. Based on <jats:italic>P</jats:italic> <jats:sub>bll</jats:sub>, which is the probability for each source to be a BL Lac object, we placed our sources into five different categories based on this value as follows: <jats:italic>P</jats:italic> <jats:sub>bll</jats:sub> ≥ 99%: highly likely BL Lac object, <jats:italic>P</jats:italic> <jats:sub>bll</jats:sub> ≥ 90%: likely BL Lac object, <jats:italic>P</jats:italic> <jats:sub>bll</jats:sub> ≤ 1%: highly likely FSRQ, <jats:italic>P</jats:italic> <jats:sub>bll</jats:sub> ≤ 10%: likely FSRQ, and 90% &lt; <jats:italic>P</jats:italic> <jats:sub>bll</jats:sub> &lt; 10%: ambiguous. Our results categorize the 84 blazar candidates as 50 likely BL Lac objects and the other 34 as being ambiguous. A small subset of these sources have been listed as associated sources in the most recent Fermi catalog, 4FGL, and in these cases our results are in agreement on the classification.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this work, we report a radio galaxy with precessing jets, double-peaked emission lines and the presence of two compact radio components with a projected separation of ∼6 parsec in very long baseline interferometry (VLBI) observations. The emission-line peak separations could be suggesting the presence of a supermassive binary black hole (BBH) with a separation of 6.3 parsec, matching the VLBI results. The kinematic jet precession model applied to the jets of J1328+2752 indicates that if it is a BBH, the accretion disk of the primary black hole is not coplanar with the binary system orbit, making its jet precess under the effect of the torque produced by the secondary black hole. However, we find that the Bardeen–Petterson effect can also provide precession timescales compatible with the jet precession period inferred in this source. This source has previously been identified as a restarted double–double radio galaxy (DDRG). Our findings therefore have important ramifications for the nature of DDRGs in general.</jats:p>