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<jats:title>Abstract</jats:title> <jats:p>Galactic disks are highly responsive systems that often undergo external perturbations and subsequent collisionless equilibration, predominantly via phase mixing. We use linear perturbation theory to study the response of infinite isothermal slab analogs of disks to perturbations with diverse spatiotemporal characteristics. Without self-gravity of the response, the dominant Fourier modes that get excited in a disk are the bending and breathing modes, which, due to vertical phase mixing, trigger local phase-space spirals that are one- and two-armed, respectively. We demonstrate how the lateral streaming motion of slab stars causes phase spirals to damp out over time. The ratio of the perturbation timescale (<jats:italic>τ</jats:italic> <jats:sub>P</jats:sub>) to the local, vertical oscillation time (<jats:italic>τ</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub>) ultimately decides which of the two modes is excited. Faster, more impulsive (<jats:italic>τ</jats:italic> <jats:sub>P</jats:sub> &lt; <jats:italic>τ</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub>) and slower, more adiabatic (<jats:italic>τ</jats:italic> <jats:sub>P</jats:sub> &gt; <jats:italic>τ</jats:italic> <jats:sub> <jats:italic>z</jats:italic> </jats:sub>) perturbations excite stronger breathing and bending modes, respectively, although the response to very slow perturbations is exponentially suppressed. For encounters with satellite galaxies, this translates to more distant and more perpendicular encounters triggering stronger bending modes. We compute the direct response of the Milky Way disk to several of its satellite galaxies and find that recent encounters with all of them excite bending modes in the solar neighborhood. The encounter with Sagittarius triggers a response that is at least 1–2 orders of magnitude larger than that due to any other satellite, including the Large Magellanic Cloud. We briefly discuss how ignoring the presence of a dark matter halo and the self-gravity of the response might impact our conclusions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Delayed radio flares of optical tidal disruption events (TDEs) indicate the existence of nonrelativistic outflows accompanying TDEs. The interaction of TDE outflows with the surrounding circumnuclear medium creates quasi-perpendicular shocks in the presence of toroidal magnetic fields. Because of the large shock obliquity and large outflow velocity, we find that the shock acceleration induced by TDE outflows generally leads to a steep particle energy spectrum, with the power-law index significantly larger than the “universal” index for a parallel shock. The measured synchrotron spectral indices of recently detected TDE radio flares are consistent with our theoretical expectation. It suggests that the particle acceleration at quasi-perpendicular shocks can be the general acceleration mechanism accounting for the delayed radio emission of TDEs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Resolving physical and chemical structures in the vicinity of a protostar is of fundamental importance for elucidating their evolution to a planetary system. In this context, we have conducted 1.2 mm observations toward the low-mass protostellar source B335 at a resolution of 0.″03 with the Atacama Large Millimeter/submillimeter Array. More than 20 molecular species including HCOOH, NH<jats:sub>2</jats:sub>CHO, HNCO, CH<jats:sub>3</jats:sub>OH, CH<jats:sub>2</jats:sub>DOH, CHD<jats:sub>2</jats:sub>OH, and CH<jats:sub>3</jats:sub>OD are detected within a few tens au around the continuum peak. We find a systematic chemical differentiation between oxygen-bearing and nitrogen-bearing organic molecules by using the principal component analysis for the image cube data. The distributions of the nitrogen-bearing molecules are more compact than those of the oxygen-bearing ones except for HCOOH. The temperature distribution of the disk/envelope system is revealed by a multiline analysis for each of HCOOH, NH<jats:sub>2</jats:sub>CHO, CH<jats:sub>3</jats:sub>OH, and CH<jats:sub>2</jats:sub>DOH. The rotation temperatures of CH<jats:sub>3</jats:sub>OH and CH<jats:sub>2</jats:sub>DOH at the radius of 0.″06 along the envelope direction are derived to be 150–165 K. On the other hand, those of HCOOH and NH<jats:sub>2</jats:sub>CHO, which have a smaller distribution, are 75–112 K, and are significantly lower than those for CH<jats:sub>3</jats:sub>OH and CH<jats:sub>2</jats:sub>DOH. This means that the outer envelope traced by CH<jats:sub>3</jats:sub>OH and CH<jats:sub>2</jats:sub>DOH is heated by additional mechanisms rather than protostellar heating. We here propose the accretion shock as the heating mechanism. The chemical differentiation and the temperature structure on a scale of a few au provide us with key information to further understand chemical processes in protostellar sources.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Terrestrial planets currently in the habitable zones around M dwarfs likely experienced a long-term runaway-greenhouse condition because of a slow decline in host-star luminosity in its pre-main-sequence phase. Accordingly, they might have lost significant portions of their atmospheres including water vapor at high concentration by hydrodynamic escape induced by the strong stellar X-ray and extreme ultraviolet (XUV) irradiation. However, the atmospheric escape rates remain highly uncertain due partly to a lack of understanding of the effect of radiative cooling in the escape outflows. Here we carry out 1D hydrodynamic escape simulations for an H<jats:sub>2</jats:sub>–H<jats:sub>2</jats:sub>O atmosphere on a planet with mass of 1<jats:italic>M</jats:italic> <jats:sub>⊕</jats:sub> considering radiative and chemical processes to estimate the atmospheric escape rate and follow the atmospheric evolution during the early runaway-greenhouse phase. We find that the atmospheric escape rate decreases with the basal H<jats:sub>2</jats:sub>O/H<jats:sub>2</jats:sub> ratio due to the energy loss by the radiative cooling of H<jats:sub>2</jats:sub>O and chemical products such as OH and OH<jats:sup>+</jats:sup>: the escape rate of H<jats:sub>2</jats:sub> becomes one order of magnitude smaller when the basal H<jats:sub>2</jats:sub>O/H<jats:sub>2</jats:sub> = 0.1 than that of the pure hydrogen atmosphere. The timescale for H<jats:sub>2</jats:sub> escape exceeds the duration of the early runaway-greenhouse phase, depending on the initial atmospheric amount and composition, indicating that H<jats:sub>2</jats:sub> and H<jats:sub>2</jats:sub>O could be left behind after the end of the runaway-greenhouse phase. Our results suggest that temperate and reducing environments with oceans could be formed on some terrestrial planets around M dwarfs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present near-infrared spectra of three low-luminosity protostars and one background star in the Perseus molecular cloud, acquired using the infrared camera on board the AKARI space telescope. For the comparison with different star-forming environments, we also present spectra of the massive protostar AFGL 7009S, where the protostellar envelope is heated significantly, and the low-mass protostar RNO 91, which is suspected to be undergoing an episodic burst. We detected ice absorption features of H<jats:sub>2</jats:sub>O, CO<jats:sub>2</jats:sub>, and CO in all spectra around the wavelengths of 3.05, 4.27, and 4.67 <jats:italic>μ</jats:italic>m, respectively. For at least two low-luminosity protostars, we also detected the XCN ice feature at 4.62 <jats:italic>μ</jats:italic>m. The presence of the crystalline H<jats:sub>2</jats:sub>O ice and XCN ice components indicates that the low-luminosity protostars experienced a hot phase via accretion bursts during the past mass accretion process. We compared the ice abundances of the low-luminosity protostars with those of embedded low-mass protostars and the dense molecular clouds and cores, suggesting that their ice abundances reflect the strength of prior bursts and the timescale after the last burst.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Searching for periodic non-accelerated signals in the presence of ideal white noise using the fully phase-coherent fast-folding algorithm (FFA) is theoretically established as a more sensitive search method than the fast Fourier transform (FFT) search with incoherent harmonic summing. In this paper, we present a comparison of the performance of an FFA search implementation using <jats:monospace>RIPTIDE</jats:monospace> and an FFT search implementation using <jats:monospace>PRESTO</jats:monospace>, over a range of signal parameters with white noise and with real telescope noise from the Giant Meterwave Radio Telescope (GMRT) High Resolution Southern Sky (GHRSS) survey with the upgraded GMRT (uGMRT). We find that the FFA search with appropriate de-reddening of the time series performs better than the FFT search with spectral whitening for long-period pulsars under real GHRSS noise conditions. We describe an FFA-search pipeline implemented for the GHRSS survey looking for pulsars over a period of 0.1–100 s and up to a dispersion measure of 500 pc cm<jats:sup>−3</jats:sup>. We processed GHRSS survey data covering ∼1500 deg<jats:sup>2</jats:sup> of the sky with this pipeline. We re-detected 43 known pulsars with a better signal-to-noise ratio in the FFA search than in the FFT search. We also report the discovery of two new pulsars, including a long-period pulsar with a short duty cycle, using this FFA-search pipeline. A population of long-period pulsars with periods of several seconds or higher could help constrain the pulsar death line.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We introduce a novel machine-learning framework for estimating the Bayesian posteriors of morphological parameters for arbitrarily large numbers of galaxies. The Galaxy Morphology Posterior Estimation Network (GaMPEN) estimates values and uncertainties for a galaxy’s bulge-to-total-light ratio (<jats:italic>L</jats:italic> <jats:sub> <jats:italic>B</jats:italic> </jats:sub>/<jats:italic>L</jats:italic> <jats:sub> <jats:italic>T</jats:italic> </jats:sub>), effective radius (<jats:italic>R</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>), and flux (<jats:italic>F</jats:italic>). To estimate posteriors, GaMPEN uses the Monte Carlo Dropout technique and incorporates the full covariance matrix between the output parameters in its loss function. GaMPEN also uses a spatial transformer network (STN) to automatically crop input galaxy frames to an optimal size before determining their morphology. This will allow it to be applied to new data without prior knowledge of galaxy size. Training and testing GaMPEN on galaxies simulated to match <jats:italic>z</jats:italic> &lt; 0.25 galaxies in Hyper Suprime-Cam Wide <jats:italic>g</jats:italic>-band images, we demonstrate that GaMPEN achieves typical errors of 0.1 in <jats:italic>L</jats:italic> <jats:sub> <jats:italic>B</jats:italic> </jats:sub>/<jats:italic>L</jats:italic> <jats:sub> <jats:italic>T</jats:italic> </jats:sub>, 0.″17 (∼7%) in <jats:italic>R</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>, and 6.3 × 10<jats:sup>4</jats:sup> nJy (∼1%) in <jats:italic>F</jats:italic>. GaMPEN's predicted uncertainties are well calibrated and accurate (&lt;5% deviation)—for regions of the parameter space with high residuals, GaMPEN correctly predicts correspondingly large uncertainties. We also demonstrate that we can apply categorical labels (i.e., classifications such as <jats:italic>highly bulge dominated</jats:italic>) to predictions in regions with high residuals and verify that those labels are ≳97% accurate. To the best of our knowledge, GaMPEN is the first machine-learning framework for determining joint posterior distributions of multiple morphological parameters and is also the first application of an STN to optical imaging in astronomy.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Rotation causes an increase in a neutron star’s mass and equatorial radius. The mass and radius depend sensitively on the unknown equation of state (EOS) of cold, dense matter. However, the increases in mass and radius due to rotation are almost independent of the EOS. The EOS independence leads to the idea of neutron star universality. In this paper, we compute sequences of rotating neutron stars with constant central density. We use a collection of randomly generated EOSs to construct simple correction factors to the mass and radius computed from the equations of hydrostatic equilibrium for nonrotating neutron stars. The correction factors depend only on the nonrotating star’s mass and radius and are almost independent of the EOS. This makes it computationally inexpensive to include observations of rotating neutron stars in EOS inference codes. We also construct a mapping from the measured mass and radius of a rotating neutron star to a corresponding nonrotating star. The mapping makes it possible to construct a zero-spin mass–radius curve if the masses and radii of many neutron stars with different spins are measured. We show that the changes in polar and equatorial radii are symmetric, in that the polar radius shrinks at the same rate in which the equatorial radius grows. This symmetry is related to the observation that the equatorial compactness (the ratio of mass to radius) is almost constant on one of the constant-density sequences.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We describe representative observing scenarios for early warning detection of binary neutron star mergers with the current generation of ground-based gravitational wave detectors as they approach design sensitivity. We incorporate recent estimates of the infrastructure latency and detector sensitivities to provide up-to-date predictions. We use Fisher analysis to approximate the associated localizations, and we directly compare to Bayestar to quantify biases inherited from this approach. In particular, we show that Advanced LIGO and Advanced Virgo will detect and distribute ≲1 signal with signal-to-noise ratio greater than 15 before a merger in their fourth observing run provided they maintain a 70% duty cycle. This is consistent with previous early warning detection estimates. We estimate that 60% of all observations and 8% of those detectable 20 s before a merger will be localized to ≲100 deg<jats:sup>2</jats:sup>. If KAGRA is able to achieve a 25 Mpc horizon, 70% of these binary neutron stars will be localized to ≲100 deg<jats:sup>2</jats:sup> by a merger. As the Aundha–Hanford–KAGRA–Livingston–Virgo network approaches design sensitivity over the next ∼10 yr, we expect one (six) early warning alerts to be distributed 60 (0) s before a merger. Although adding detectors to the Hanford–Livingston–Virgo network at design sensitivity impacts the detection rate at ≲50% level, it significantly improves localization prospects. Given uncertainties in sensitivities, participating detectors, and duty cycles, we consider 103 future detector configurations so electromagnetic observers can tailor preparations toward their preferred models.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Many explosive astrophysical events, like magnetars’ bursts and flares, are magnetically driven. We consider dynamics of such magnetic explosions—relativistic expansion of highly magnetized and highly magnetically overpressurized clouds. The corresponding dynamics are qualitatively different from fluid explosions due to the topological constraint of the conservation of the magnetic flux. Using analytical, relativistic MHD as well as force-free calculations, we find that the creation of a relativistically expanding, causally disconnected flow obeys a threshold condition: it requires sufficiently high initial overpressure and a sufficiently quick decrease of the pressure in the external medium (the preexplosion wind). In the subcritical case the magnetic cloud just “puffs up” and quietly expands with the preflare wind. We also find a compact analytical solution to Prendergast’s problem—expansion of force-free plasma into a vacuum.</jats:p>