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<jats:title>Abstract</jats:title> <jats:p>We present high-resolution Magellan/MIKE spectra of 22 bright (9 &lt; <jats:italic>V</jats:italic> &lt; 13.5) metal-poor stars (−3.18 &lt; [Fe/H] &lt; −1.37) in three different stellar streams, the Helmi debris stream, the Helmi trail stream, and the <jats:italic>ω</jats:italic> Centauri progenitor stream. We augment our Helmi debris sample with results for 10 stars by Roederer et al. for a total of 32 stars. Detailed chemical abundances of light elements as well as heavy neutron-capture elements have been determined for our 22 stars. All three streams contain carbon-enhanced stars. For 13 stars, neutron-capture element lines were detectable, and they all show signatures in agreement with the scaled solar <jats:italic>r</jats:italic>-process pattern, albeit with a large spread of −0.5 &lt; [Eu/Fe] &lt; +1.3. Eight of these stars show an additional small <jats:italic>s</jats:italic>-process contribution superposed onto their <jats:italic>r</jats:italic>-process pattern. This could be discerned because of the relatively high signal-to-noise ratio of the spectra given that the stars are close by in the halo. Our results suggest that the progenitors of these streams experienced one or more <jats:italic>r</jats:italic>-process events early on, such as a neutron star merger or another prolific <jats:italic>r</jats:italic>-process source. This widely enriched these host systems before their accretion by the Milky Way. The small <jats:italic>s</jats:italic>-process contribution suggests the presence of asymptotic giant branch stars and associated local (inhomogeneous) enrichment as part of the ongoing chemical evolution by low-mass stars. Stars in stellar streams may thus be a promising avenue for studying the detailed history of large dwarf galaxies and their role in halo assembly with easily accessible targets for high-quality spectra of many stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The gas content of the complete compilation of Local Group dwarf galaxies (119 within 2 Mpc) is presented using H <jats:sc>i</jats:sc> survey data. Within the virial radius of the Milky Way (224 kpc here), 53 of 55 dwarf galaxies are devoid of gas to limits of <jats:italic>M</jats:italic> <jats:sub>H <jats:sc>i</jats:sc> </jats:sub> &lt; 10<jats:sup>4</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. Within the virial radius of M31 (266 kpc), 27 of 30 dwarf galaxies are devoid of gas (with limits typically &lt;10<jats:sup>5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>). Beyond the virial radii of the Milky Way and M31, the majority of the dwarf galaxies have detected H <jats:sc>i</jats:sc> gas and H <jats:sc>i</jats:sc> masses higher than the limits. When the relationship between gas content and distance is investigated using a Local Group virial radius, more of the nondetected dwarf galaxies are within this radius (85 ± 1 of the 93 nondetected dwarf galaxies) than within the virial radii of the Milky Way and M31. Using the Gaia proper-motion measurements available for 38 dwarf galaxies, the minimum gas density required to completely strip them of gas is calculated. Halo densities between 10<jats:sup>−5</jats:sup> and 5 × 10<jats:sup>−4</jats:sup> cm<jats:sup>−3</jats:sup> are typically required for instantaneous stripping at perigalacticon. When compared to halo density with radius expectations from simulations and observations, 80% of the dwarf galaxies with proper motions are consistent with being stripped by ram pressure at Milky Way pericenter. The results suggest that a diffuse gaseous galactic halo medium is important in quenching dwarf galaxies, and that a Local Group medium also potentially plays a role.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Due to their small mass, subsolar mass black hole binaries would have to be primordial in origin instead of the result of stellar evolution. Soon after formation in the early universe, primordial black holes can form binaries after decoupling from the cosmic expansion. Alternatively, primordial black holes as dark matter could also form binaries in the late universe due to dynamical encounters and gravitational-wave braking. A significant feature for this channel is the possibility that some sources retain nonzero eccentricity in the LIGO/Virgo band. Assuming all dark matter is primordial black holes with a delta function mass distribution, 1<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>–1<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> binaries formed in this late-universe channel can be detected by Advanced LIGO and Virgo with their design sensitivities at a rate of <jats:inline-formula> <jats:tex-math> <?CDATA ${ \mathcal O }(1)$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe939ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> yr<jats:sup>−1</jats:sup>, where 12%(3%) of events have eccentricity at a gravitational-wave frequency of 10 Hz, <jats:italic>e</jats:italic> <jats:sup>10 Hz</jats:sup> ≥ 0.01(0.1), and nondetection can constrain the binary formation rate within this model. Third generation detectors would be expected to detect subsolar mass eccentric binaries as light as 0.01<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> within this channel, if they accounted for the majority of the dark matter. Furthermore, we use simulated gravitational-wave data to study the ability to search for eccentric gravitational-wave signals using a quasi-circular waveform template bank with Advanced LIGO design sensitivity. For a match-filtering targeted search, assuming binaries with a delta function mass of 0.1(1)<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and the eccentricity distribution derived from this late-universe formation channel, 41%(6%) of the signals would be missed compared to the ideal detection rate due to the mismatch in the gravitational-wave signal from eccentricity.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Presented are the first interferometric images of cool starspots on the chromospherically active giant <jats:italic>λ</jats:italic> Andromedae. Using the Michigan Infra-Red Combiner coupled to the Center for High Angular Resolution Astronomy Array, 26 interferometric observations were made between 2008 August 17 and 2011 September 24. The photometric time series acquired at Fairborn Observatory spanning 2008 September 20 to 2011 January 20 is also presented. The angular diameter and power-law limb-darkening coefficient of this star are 2.759 ± 0.050 mas and 0.229 ± 0.111, respectively. Starspot properties are obtained from both modeled and SQUEEZE reconstructed images. The images from 2010 through 2011 show anywhere from one to four starspots. The cadence in the data for the 2010 and 2011 data sets is sufficient to measure a stellar rotation period based on apparent starspot motion. This leads to estimates of the rotation period (<jats:italic>P</jats:italic> <jats:sub>2010</jats:sub> = 61 ± 4.0 days, <jats:italic>P</jats:italic> <jats:sub>2011</jats:sub> = 54.0 ± 2.4 days) that are consistent with the photometrically determined period of 54.8 days. In addition, the inclination and position angle of the rotation axis are computed for both the 2010 and 2011 data sets; values (<jats:inline-formula> <jats:tex-math> <?CDATA $\bar{{\rm{\Psi }}}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi mathvariant="normal">Ψ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb670ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> = 21.°5, <jats:inline-formula> <jats:tex-math> <?CDATA $\bar{i}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>i</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb670ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> = 78.°0) for each are nearly identical between the two years.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In 2019 July, the IceCube experiment detected a high-energy neutrino from the direction of the powerful blazar PKS 1502+106. We perform multiwavelength and multimessenger modeling of this source, using a fully selfconsistent one-zone model that includes the contribution of external radiation fields typical of flat-spectrum radio quasars. We identify three different activity states of the blazar: one quiescent state and two flaring states with hard and soft gamma-ray spectra. We find two hadronic models that can describe the multiwavelength emission during all three states: a leptohadronic model with a contribution from photohadronic processes to X-rays and gamma-rays, and a proton synchrotron model, where the emission from keV to 10 GeV comes from proton synchrotron radiation. Both models predict a substantial neutrino flux that is correlated with the gamma-ray and soft X-ray fluxes. Our results are compatible with the detection of a neutrino during the quiescent state, based on event rate statistics. We conclude that the soft X-ray spectra observed during bright flares strongly suggest a hadronic contribution, which can be interpreted as additional evidence for cosmic-ray acceleration in the source independently of neutrino observations. We find that more arguments can be made in favor of the leptohadronic model vis-a-vis the proton synchrotron scenario, such as a lower energetic demand during the quiescent state. However, the same leptohadronic model would be disfavored for flaring states of PKS 1502+106 if no IceCube events were found from the direction of the source before 2010, which would require an archival search.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The diversity of Type II supernovae (SNe II) is thought to be driven mainly by differences in their progenitor’s hydrogen-rich (H-rich) envelope mass, with SNe IIP having long plateaus (∼100 days) and the most massive H-rich envelopes. However, it is an ongoing mystery why SNe II with short plateaus (tens of days) are rarely seen. Here, we present optical/near-infrared photometric and spectroscopic observations of luminous Type II short-plateau SNe 2006Y, 2006ai, and 2016egz. Their plateaus of about 50–70 days and luminous optical peaks (≲−18.4 mag) indicate significant pre-explosion mass loss resulting in partially stripped H-rich envelopes and early circumstellar material (CSM) interaction. We compute a large grid of <jats:monospace>MESA</jats:monospace>+<jats:monospace>STELLA</jats:monospace> single-star progenitor and light-curve models with various progenitor zero-age main-sequence (ZAMS) masses, mass-loss efficiencies, explosion energies, <jats:sup>56</jats:sup>Ni masses, and CSM densities. Our model grid shows a continuous population of SNe IIP–IIL–IIb-like light-curve morphology in descending order of H-rich envelope mass. With large <jats:sup>56</jats:sup>Ni masses (≳0.05 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>), short-plateau SNe II lie in a confined parameter space as a transitional class between SNe IIL and IIb. For SNe 2006Y, 2006ai, and 2016egz, our findings suggest high-mass red supergiant (RSG) progenitors (<jats:italic>M</jats:italic> <jats:sub>ZAMS</jats:sub> ≃ 18–22 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) with small H-rich envelope masses (<jats:inline-formula> <jats:tex-math> <?CDATA ${M}_{{{\rm{H}}}_{\mathrm{env}}}\simeq 1.7\,{M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabf6d6ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) that have experienced enhanced mass loss (<jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}\simeq {10}^{-2}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabf6d6ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>) for the last few decades before the explosion. If high-mass RSGs result in rare short-plateau SNe II, then these events might ease some of the apparent underrepresentation of higher-luminosity RSGs in observed SN II progenitor samples.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The growth of galaxy clusters is energetic and may trigger and/or quench star formation and black hole activity. The ENISALA<jats:xref ref-type="fn" rid="apjabe7f8fn1"> <jats:sup>4</jats:sup> </jats:xref> <jats:fn id="apjabe7f8fn1"> <jats:label> <jats:sup>4</jats:sup> </jats:label> <jats:p>The project is named as a tribute to the storied Enisala citadel (Dobrogea, Romania). Enisala (“new settlement,” in Turkish and Romanian) sits on top of a windswept hill, at the crossroads of the Danube Delta and the Pontus Euxinus sea (“hospitable sea,” Black Sea), forever shaped by forces of nature. It stands as a metaphor for the ever-evolving galaxy cluster environment and its profound influence on galaxy and black hole evolution. ENISALA can also be understood to stand for “ENvironmental Influence on Star formation and AGN through Line Astrophysics.</jats:p> </jats:fn> project is a collection of multiwavelength observations aimed at understanding how large-scale structure drives galaxy and black hole evolution. Here, we introduce optical spectroscopy of over 800 H<jats:italic>α</jats:italic> emission-line galaxies, selected in 14 <jats:italic>z</jats:italic> ∼ 0.15–0.31 galaxy clusters, spanning a range of masses and dynamical states. We investigate the nature of the emission lines in relation to the host galaxy properties, its location within the cluster, and the properties of the parent cluster. We uncover remarkable differences between mergers and relaxed clusters. The majority of H<jats:italic>α</jats:italic> emission-line galaxies in merging cluster fields are located within 3 Mpc of their center. A large fraction of these line emitters in merging clusters are powered by star formation irrespective of cluster-centric radius, while the rest are powered by active galactic nuclei (AGNs). Star-forming galaxies are rare within 3 Mpc of relaxed clusters and AGNs are most abundant at their outskirts (∼1.5–3 Mpc). We discover a population of star-forming galaxies with large equivalent widths and blue UV–optical colors found exclusively in the merging clusters in our sample. The widespread emission-line activity in merging clusters is likely supported by triggered activity in recently accreted, gas-rich galaxies. By contrast, our observations for relaxed clusters match established models in which black hole activity is enhanced at the virial radius and star formation is quenched within the infall region. We conclude that emission-line galaxies experience distinct evolutionary paths in merging and relaxed clusters.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Active region EUV loops are believed to trace a subset of magnetic field lines through the corona. Malanushenko et al. proposed a method, using loop images and line-of-sight photospheric magnetograms, to infer the 3D shape and field strength along each loop. McCarthy et al. used this novel method to compute the total magnetic flux interconnecting a pair of active regions observed by SDO/AIA. They adopted the common assumption that each loop had a circular cross section. The accuracy of inferred shape and circularity of cross sections can both be tested using observations of the same loops from additional vantage points as provided by STEREO/EUVI. Here we use multiple viewing angles to confirm the 3D structure of loops. Of 151 viable cases, 105 (69.5%) matched some form of visible coronal structure when viewed approximately in quadrature. A loop with a circular cross section should appear of a similar width in different perspectives. In contradiction to this, we find a puzzling lack of correlation between loop diameters seen from different perspectives, even an anticorrelation in some cases. Features identified as monolithic loops in AIA may, in fact, be more complex density enhancements. The 30.5% of reconstructions from AIA that did not match any feature in EUVI might be such enhancements. Others may be genuine loop structures, but with elliptical cross sections. We observe an anticorrelation between diameter and brightness, lending support to the latter hypothesis. Of 13 loops suitable for width analysis, 4 are consistent with noncircular cross sections, where we find anticorrelation in both comparisons.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Besides the spirals induced by the Lindblad resonances, planets can generate a family of tightly wound spirals through buoyancy resonances. The excitation of buoyancy resonances depends on the thermal relaxation timescale of the gas. By computing timescales of various processes associated with thermal relaxation, namely, radiation, diffusion, and gas–dust collision, we show that the thermal relaxation in protoplanetary disks’ surface layers (<jats:italic>Z</jats:italic>/<jats:italic>R</jats:italic> ≳ 0.1) and outer disks (<jats:italic>R</jats:italic> ≳ 100 au) is limited by infrequent gas–dust collisions. The use of the isothermal equation of state or rapid cooling, common in protoplanetary disk simulations, is therefore not justified. Using three-dimensional hydrodynamic simulations, we show that the collision-limited slow thermal relaxation provides favorable conditions for buoyancy resonances to develop. Buoyancy resonances produce predominantly vertical motions, whose magnitude at the <jats:sup>12</jats:sup>CO emission surface is of the order of 100 m s<jats:sup>−1</jats:sup> for Jovian-mass planets, sufficiently large to detect using molecular line observations with ALMA. We generate synthetic observations and describe characteristic features of buoyancy resonances in Keplerian-subtracted moment maps and velocity channel maps. Based on the morphology and magnitude of the perturbation, we propose that the tightly wound spirals observed in TW Hya could be driven by a (sub-)Jovian-mass planet at 90 au. We discuss how non-Keplerian motions driven by buoyancy resonances can be distinguished from those driven by other origins. We argue that observations of multiple lines tracing different heights, with sufficiently high spatial/spectral resolution and sensitivity to separate the emission arising from the near and far sides of the disk, will help constrain the origin of non-Keplerian motions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first 850 <jats:italic>μ</jats:italic>m polarization observations in the most active star-forming site of the Rosette Molecular Cloud (<jats:italic>d</jats:italic> ∼ 1.6 kpc) in the wall of the Rosette Nebula, imaged with the SCUBA-2/POL-2 instruments of the James Clerk Maxwell telescope, as part of the B-Fields In Star-forming Region Observations 2 (BISTRO-2) survey. From the POL-2 data we find that the polarization fraction decreases with the 850 <jats:italic>μ</jats:italic>m continuum intensity with <jats:italic>α</jats:italic> = 0.49 ± 0.08 in the <jats:italic>p</jats:italic> ∝ <jats:italic>I</jats:italic> <jats:sup>−<jats:italic>α</jats:italic> </jats:sup> relation, which suggests that some fraction of the dust grains remain aligned at high densities. The north of our 850 <jats:italic>μ</jats:italic>m image reveals a “gemstone ring” morphology, which is a ∼1 pc diameter ring-like structure with extended emission in the “head” to the southwest. We hypothesize that it might have been blown by feedback in its interior, while the <jats:italic>B</jats:italic>-field is parallel to its circumference in most places. In the south of our SCUBA-2 field the clumps are apparently connected with filaments that follow infrared dark clouds. Here, the POL-2 magnetic field orientations appear bimodal with respect to the large-scale Planck field. The mass of our effective mapped area is ∼174 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, which we calculate from 850 <jats:italic>μ</jats:italic>m flux densities. We compare our results with masses from large-scale emission-subtracted Herschel 250 <jats:italic>μ</jats:italic>m data and find agreement within 30%. We estimate the plane-of-sky <jats:italic>B</jats:italic>-field strength in one typical subregion using the Davis–Chandrasekhar–Fermi technique and find 80 ± 30 <jats:italic>μ</jats:italic>G toward a clump and its outskirts. The estimated mass-to-flux ratio of <jats:italic>λ</jats:italic> = 2.3 ± 1.0 suggests that the <jats:italic>B</jats:italic>-field is not sufficiently strong to prevent gravitational collapse in this subregion.</jats:p>