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<jats:title>Abstract</jats:title> <jats:p>Type Ia supernovae (SNe Ia) span a range of luminosities and timescales, from rapidly evolving subluminous to slowly evolving overluminous subtypes. Previous theoretical work has, for the most part, been unable to match the entire breadth of observed SNe Ia with one progenitor scenario. Here, for the first time, we apply non-local thermodynamic equilibrium radiative transfer calculations to a range of accurate explosion models of sub-Chandrasekhar-mass white dwarf detonations. The resulting photometry and spectra are in excellent agreement with the range of observed nonpeculiar SNe Ia through 15 days after the time of <jats:italic>B</jats:italic>-band maximum, yielding one of the first examples of a quantitative match to the entire Phillips relation. The intermediate-mass element velocities inferred from theoretical spectra at maximum light for the more massive white dwarf explosions are higher than those of bright observed SNe Ia, but these and other discrepancies likely stem from the one-dimensional nature of our explosion models and will be improved upon by future non-local thermodynamic equilibrium radiation transport calculations of multidimensional sub-Chandrasekhar-mass white dwarf detonations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Radio pulsar observations probe the lives of Galactic double neutron star (DNS) systems while gravitational waves enable us to study extragalactic DNS in their final moments. By combining measurements from radio and gravitational-wave astronomy, we seek to gain a more complete understanding of DNS from formation to merger. We analyze the recent gravitational-wave binary neutron star mergers GW170817 and GW190425 in the context of other DNS known from radio astronomy. By employing a model for the birth and evolution of DNS, we measure the mass distribution of DNS at birth, at midlife (in the radio), and at death (in gravitational waves). We consider the hypothesis that the high-mass gravitational-wave event GW190425 is part of a subpopulation formed through unstable case BB mass transfer, which quickly merge in ∼10–100 Myr. We find only mild evidence to support this hypothesis and that GW190425 is not a clear outlier from the radio population as previously claimed. If there are fast-merging binaries, we estimate that they constitute 8%–79% of DNS at birth (90% credibility). We estimate the typical delay time between the birth and death of fast-merging binaries to be ≈5–401 Myr (90% credibility). We discuss the implications for radio and gravitational-wave astronomy.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Due to its proximity to Earth, Jupiter of the solar system serves as a unique case study for gas-giant exoplanets. In the current Letter, we perform fits of ab initio, reflective, semi-infinite, homogeneous model atmospheres to 61 phase curves from 0.40 to 1.00 <jats:italic>μ</jats:italic>m, obtained from the Cassini spacecraft, within a Bayesian framework. We reproduce the previous finding that atmospheric models using classic reflection laws (Lambertian, Rayleigh, single Henyey–Greenstein) provide poor fits to the data. Using the double Henyey–Greenstein reflection law, we extract posterior distributions of the single-scattering albedo and scattering asymmetry factors and tabulate their median values and uncertainties. We infer that the aerosols in the Jovian atmosphere are large, irregular, polydisperse particles that produce strong forward scattering together with a narrow backscattering lobe. The near-unity values of the single-scattering albedos imply that multiple scattering of radiation is an important effect. We speculate that the observed narrow backscattering lobe is caused by coherent backscattering of radiation, which is usually associated with solar system bodies with solid surfaces and regolith. Our findings demonstrate that precise, multiwavelength phase curves encode valuable information on the fundamental properties of cloud/haze particles. The method described in this Letter enables single-scattering albedos and scattering asymmetry factors to be retrieved from James Webb Space Telescope phase curves of exoplanets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>It has been recently claimed that primordial magnetic fields could relieve the cosmological Hubble tension. Fields of sufficient strength to relieve this tension would result in a magnetic field whose Alfvén velocity, <jats:italic>v</jats:italic> <jats:sub> <jats:italic>a</jats:italic> </jats:sub>, is comparable to the speed of sound, <jats:italic>c</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>, at the start of structure formation. We consider the impact of such fields on the formation of the first cosmological objects, minihalos (&lt;10<jats:sup>6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>), forming stars with zoom-in cosmological simulations tracking a single such minihalo. We seed each simulation with present-day field strengths of 2 × 10<jats:sup>−12</jats:sup>–2 × 10<jats:sup>−10</jats:sup> G corresponding to initial ratios of Alfvén velocity to the speed of sound of <jats:italic>v</jats:italic> <jats:sub> <jats:italic>a</jats:italic> </jats:sub>/<jats:italic>c</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> ≈ 0.03−3. We find that when <jats:italic>v</jats:italic> <jats:sub> <jats:italic>a</jats:italic> </jats:sub>/<jats:italic>c</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>≪1, the effects are modest. However, when <jats:italic>v</jats:italic> <jats:sub> <jats:italic>a</jats:italic> </jats:sub> ∼ <jats:italic>c</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>, the starting time of the gravitational collapse is delayed and the duration extended as much as by Δ<jats:italic>z</jats:italic> = 2.5 in redshift. When <jats:italic>v</jats:italic> <jats:sub> <jats:italic>a</jats:italic> </jats:sub> &gt; <jats:italic>c</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>, the collapse is completely suppressed and the minihalos continue to grow and are unlikely to collapse until reaching the atomic cooling limit. Employing current observational limits on primordial magnetic fields we conclude that inflationary-produced primordial magnetic fields could have a significant impact on first star formation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Data suggest that most rocky exoplanets with orbital period <jats:italic>p</jats:italic> &lt; 100 days (“hot” rocky exoplanets) formed as gas-rich sub-Neptunes that subsequently lost most of their envelopes, but whether these rocky exoplanets still have atmospheres is unknown. We identify a pathway by which 1–1.7 <jats:italic>R</jats:italic> <jats:sub>⊕</jats:sub> (1–10 <jats:italic>M</jats:italic> <jats:sub>⊕</jats:sub>) rocky exoplanets with orbital periods of 10–100 days can acquire long-lived 10–2000 bar atmospheres that are H<jats:sub>2</jats:sub>O-dominated, with mean molecular weight &gt;10. These atmospheres form during the planets’ evolution from sub-Neptunes into rocky exoplanets. H<jats:sub>2</jats:sub>O that is made by reduction of iron oxides in the silicate magma is highly soluble in the magma, forming a dissolved reservoir that is protected from loss so long as the H<jats:sub>2</jats:sub>-dominated atmosphere persists. The large size of the dissolved reservoir buffers the H<jats:sub>2</jats:sub>O atmosphere against loss after the H<jats:sub>2</jats:sub> has dispersed. Within our model, a long-lived, water-dominated atmosphere is a common outcome for efficient interaction between a nebula-derived atmosphere (peak atmosphere mass fraction 0.1–0.6 wt%) and oxidized magma (&gt;5 wt% FeO), followed by atmospheric loss. This idea predicts that most rocky planets that have orbital periods of 10–100 days and that have radii within 0.1–0.2 <jats:italic>R</jats:italic> <jats:sub>⊕</jats:sub> of the lower edge of the radius valley still retain H<jats:sub>2</jats:sub>O atmospheres. This prediction is imminently testable with James Webb Space Telescope and has implications for the interpretation of data for transiting super-Earths.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Gravitational wave (GW) detections of binary black holes (BBHs) have shown evidence for a dearth of component black holes with masses above ∼50<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. This is consistent with expectations of a mass gap due to the existence of pair-instability supernovae (PISN). We argue that ground-based GW detectors will be sensitive to BBHs with masses above this gap, ≳120 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. With no detections, 2 yr at upgraded sensitivity (A+) would constrain the local merger rate of these BBHs on the “far side” of the PISN gap to be lower than 0.01 yr<jats:sup>−1</jats:sup>Gpc<jats:sup>−3</jats:sup>. Alternatively, with a few tens of events we could constrain the location of the upper edge of the gap to the percent level. We consider the potential impact of “interloper” black holes within the PISN mass gap on this measurement. Far side BBHs would also be observed by future instruments such as Cosmic Explorer (CE), Einstein Telescope (ET) and LISA, and may dominate the fraction of multiband events. We show that by comparing observations from ground and space it is possible to constrain the merger rate history. Moreover, we find that the upper edge of the PISN mass gap leaves an imprint on the spectral shape of the stochastic background of unresolved binaries, which may be accessible with A+ sensitivity. Finally, we show that by exploiting the upper edge of the gap, these high-mass BBHs can be used as standard sirens to constrain the cosmic expansion at redshifts of ∼0.4, 0.8, and 1.5 with LISA, LIGO-Virgo, and CE/ET, respectively. These far-side binaries would be the most massive BBHs LIGO-Virgo could detect.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The gravitational-wave (GW) detection of GW190521 has provided new insights on the mass distribution of black holes and new constraints for astrophysical formation channels. With independent claims of GW190521 having significant premerger eccentricity, we investigate what this implies for GW190521-like binaries that form dynamically. The Laser Interferometer Space Antenna (LISA) will also be sensitive to GW190521-like binaries if they are circular from an isolated formation channel. However, GW190521-like binaries that form dynamically may skip the LISA band entirely. To this end, we simulate GW190521 analogs that dynamically form via post-Newtonian binary–single scattering. From these scattering experiments, we find that GW190521-like binaries may enter the LIGO-Virgo band with significant eccentricity as suggested by recent studies, though well below an eccentricity of <jats:italic>e</jats:italic> <jats:sub>10 Hz</jats:sub> ≲ 0.7. Eccentric GW190521-like binaries further motivate the astrophysical science case for a decihertz GW observatory, such as the kilometer-scale version of the Midband Atomic Gravitational-wave Interferometric Sensor. We carry out a Fisher analysis to estimate how well the eccentricity of GW190521-like binaries can be constrained with such a decihertz detector. These eccentricity constraints would also provide additional insights into the possible environments that GW190521-like binaries form in.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent observations of the periodic fast radio burst source 180916.J0158+65 (FRB 180916) find small linear polarization position angle swings during and between bursts, with a burst activity window that becomes both narrower and earlier at higher frequencies. Although the observed chromatic activity window disfavors models of periodicity in FRB 180916 driven solely by the occultation of a neutron star by the optically thick wind from a stellar companion, the connection to theories where periodicity arises from the motion of a bursting magnetar remains unclear. In this Letter, we show how altitude-dependent radio emission from a magnetar, with bursts emitted from regions that are asymmetric with respect to the magnetic dipole axis, can lead to burst activity windows and polarization consistent with the recent observations. In particular, the fact that bursts arrive systematically earlier at higher frequencies disfavors theories where the FRB periodicity arises from forced precession of a magnetar by a companion or fallback disk, but is consistent with theories where periodicity originates from a slowly rotating or freely precessing magnetar. Several observational tests are proposed to verify/differentiate between the remaining theories, and pin down which theory explains the periodicity in FRB 180916.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The origins of most stellar streams in the Milky Way are unknown. With improved proper motions provided by Gaia EDR3, we show that the orbits of 23 Galactic stellar streams are highly clustered in orbital phase space. Based on their energies and angular momenta, most streams in our sample can plausibly be associated with a specific (disrupted) dwarf galaxy host that brought them into the Milky Way. For eight streams we also identify likely globular cluster progenitors (four of these associations are reported here for the first time). Some of these stream progenitors are surprisingly far apart, displaced from their tidal debris by a few to tens of degrees. We identify stellar streams that appear spatially distinct, but whose similar orbits indicate they likely originate from the same progenitor. If confirmed as physical discontinuities, they will provide strong constraints on the mass loss from the progenitor. The nearly universal ex situ origin of existing stellar streams makes them valuable tracers of galaxy mergers and dynamical friction within the Galactic halo. Their phase-space clustering can be leveraged to construct a precise global map of dark matter in the Milky Way, while their internal structure may hold clues to the small-scale structure of dark matter in their original host galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>AT2019wey is a Galactic low-mass X-ray binary with a candidate black hole accretor first discovered as an optical transient by ATLAS in 2019 December. It was then associated with an X-ray source discovered by SRG/eROSITA and SRG/ART-XC instruments in 2020 March. After a brightening in X-rays in 2020 August, VLA observations of the source revealed an optically thin spectrum that subsequently shifted to optically thick, as the source continued to brighten in the radio. This motivated us to observe AT2019wey with the VLBA. We found a resolved source that we interpret to be a steady compact jet, a feature associated with black hole X-ray binary systems in hard X-ray spectral states. The jet power is comparable to the accretion-disk X-ray luminosity. Here, we summarize the results from these observations.</jats:p>