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<jats:title>Abstract</jats:title> <jats:p>Continuous gravitational waves are nearly monochromatic signals emitted by asymmetries in rotating neutron stars. These signals have not yet been detected. Deep all-sky searches for continuous gravitational waves from isolated neutron stars require significant computational expense. Deep searches for neutron stars in binary systems are even more expensive, but these targets are potentially more promising emitters, especially in the hundreds of Hertz region, where ground-based gravitational-wave detectors are most sensitive. We present here an all-sky search for continuous signals with frequency between 300 and 500 Hz, from neutron stars in binary systems with orbital periods between 15 and 60 days and projected semimajor axes between 10 and 40 lt-s. This is the only binary search on Advanced LIGO data that probes this frequency range. Compared to previous results, our search is over an order of magnitude more sensitive. We do not detect any signals, but our results exclude plausible and unexplored neutron star configurations, for example, neutron stars with relative deformations greater than 3 × 10<jats:sup>−6</jats:sup> within 1 kpc from Earth and <jats:italic>r</jats:italic>-mode emission at the level of <jats:italic>α</jats:italic> ∼ a few 10<jats:sup>−4</jats:sup> within the same distance.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Periodic nuclear transients have been detected with increasing frequency, with one such system—ASASSN-14ko—exhibiting highly regular outbursts on a timescale of 114 ± 1 days. It has been postulated that the outbursts from this source are generated by the repeated partial disruption of a star, but how the star was placed onto such a tightly bound orbit about the supermassive black hole remains unclear. Here we use analytic arguments and three-body integrations to demonstrate that the Hills mechanism, where a binary system is destroyed by the tides of the black hole, can lead to the capture of a star on a ∼114 days orbit and with a pericenter distance that is comparable to the tidal radius of one of the stars within the binary. Thus, Hills capture can produce stars on tightly bound orbits that undergo repeated partial disruption, leading to a viable mechanism for generating not only the outbursts detected from ASASSN-14ko but periodic nuclear transients in general. We also show that the rate of change of the period of the captured star due to gravitational-wave emission is likely too small to produce the observed value for ASASSN-14ko, indicating that in this system there must be additional effects that contribute to the decay of the orbit. In general, however, gravitational-wave emission can be important for limiting the lifetimes of these systems and could produce observable period decay rates in future events.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present FORS2@VLT follow-up photometry of YMCA-1, a recently discovered stellar system located 13° from the Large Magellanic Cloud (LMC) center. The deep color–magnitude diagram (CMD) reveals a well-defined main sequence (MS) and a handful of stars in the post-MS evolutionary phases. We analyze the YMCA-1 CMD by means of the automated isochrone-matching package <jats:monospace>ASteCA</jats:monospace> and model its radial density profile with a Plummer function. We find that YMCA-1 is an old (<jats:inline-formula> <jats:tex-math> <?CDATA ${11.7}_{-1.3}^{+1.7}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>11.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.7</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac6421ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> Gyr), metal-intermediate ([Fe/H] <jats:inline-formula> <jats:tex-math> <?CDATA $\simeq -{1.12}_{-0.13}^{+0.21}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>≃</mml:mo> <mml:mo>−</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1.12</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.13</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.21</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac6421ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> dex), compact (<jats:italic>r</jats:italic> <jats:sub>h</jats:sub> = 3.5 ± 0.5 pc), low-mass (<jats:italic>M</jats:italic> = 10<jats:sup>2.45±0.02</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>), and low-luminosity (<jats:italic>M</jats:italic> <jats:sub> <jats:italic>V</jats:italic> </jats:sub> = −0.47 ± 0.57 mag) stellar system. The estimated distance modulus (<jats:inline-formula> <jats:tex-math> <?CDATA ${\mu }_{0}={18.72}_{-0.17}^{+0.15}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>μ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>18.72</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.17</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.15</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac6421ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> mag), corresponding to about 55 kpc, suggests that YMCA-1 is associated with the LMC, but we cannot discard the scenario in which it is a Milky Way satellite. The structural parameters of YMCA-1 are remarkably different compared with those of the 15 known old LMC globular clusters. In particular, it resides in a transition region of the <jats:italic>M</jats:italic> <jats:sub> <jats:italic>V</jats:italic> </jats:sub>–<jats:italic>r</jats:italic> <jats:sub> <jats:italic>h</jats:italic> </jats:sub> plane, in between the ultrafaint dwarf galaxies and the classical old clusters, and close to SMASH-1, another faint stellar system recently discovered in the LMC surroundings.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Most stellar evolution models predict that black holes (BHs) should not exist above approximately 50–70 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, the lower limit of the pair-instability mass gap. However, recent LIGO/Virgo detections indicate the existence of BHs with masses at and above this threshold. We suggest that massive BHs, including intermediate-mass BHs (IMBHs), can form in galactic nuclei through collisions between stellar-mass BHs and the surrounding main-sequence stars. Considering dynamical processes such as collisions, mass segregation, and relaxation, we find that this channel can be quite efficient, forming IMBHs as massive as 10<jats:sup>4</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. This upper limit assumes that (1) the BHs accrete a substantial fraction of the stellar mass captured during each collision and (2) that the rate at which new stars are introduced into the region near the SMBH is high enough to offset depletion by stellar disruptions and star–star collisions. We discuss deviations from these key assumptions in the text. Our results suggest that BHs in the pair-instability mass gap and IMBHs may be ubiquitous in galactic centers. This formation channel has implications for observations. Collisions between stars and BHs can produce electromagnetic signatures, for example, from X-ray binaries and tidal disruption events. Additionally, formed through this channel, both BHs in the mass gap and IMBHs can merge with the SMBHs at the center of a galactic nucleus through gravitational waves. These gravitational-wave events are extreme- and intermediate-mass ratio inspirals.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this study, we model a sequence of a confined and a full eruption, employing the relaxed end state of the confined eruption of a kink-unstable flux rope as the initial condition for the ejective one. The full eruption, a model of a coronal mass ejection, develops as a result of converging motions imposed at the photospheric boundary, which drive flux cancellation. In this process, parts of the positive and negative external flux converge toward the polarity inversion line, reconnect, and cancel each other. Flux of the same amount as the canceled flux transfers to a flux rope, increasing the free magnetic energy of the coronal field. With sustained flux cancellation and the associated progressive weakening of the magnetic tension of the overlying flux, we find that a flux reduction of ≈11% initiates the torus instability of the flux rope, which leads to a full eruption. These results demonstrate that a homologous full eruption, following a confined one, can be driven by flux cancellation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>I show that gravitational scattering of dark matter objects of mass ∼10<jats:sup>4</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and speeds of ∼10 km s<jats:sup>−1</jats:sup>, provides the cross section per unit mass required in self-interacting dark matter models that alleviate the small-scale structure challenges to the collisionless cold dark matter model. For primordial objects of mass 10<jats:sup>4</jats:sup> <jats:italic>M</jats:italic> <jats:sub>4</jats:sub> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, moving at the velocity dispersion characteristic of dwarf galaxies, 10<jats:italic>v</jats:italic> <jats:sub>1</jats:sub> km s<jats:sup>−1</jats:sup>, the cross section per unit mass for gravitational scattering is <jats:inline-formula> <jats:tex-math> <?CDATA $\sim 10\,({M}_{4}/{v}_{1}^{4})\,{\mathrm{cm}}^{2}\,{{\rm{g}}}^{-1}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∼</mml:mo> <mml:mn>10</mml:mn> <mml:mspace width="0.25em" /> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msubsup> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo stretchy="false">)</mml:mo> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi>cm</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac6591ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. The steep decline in interaction with increasing velocity explains why self-interaction is not evident in data on massive galaxies and clusters of galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We map the planetary wake associated with the embedded protoplanet creating the CO kink in the disk of HD 163296. We show that the wake can be traced by a series of correlated perturbations in the peak velocity map. The sign change of the perturbations across the disk’s major axis confirms that the wake induces predominantly radial motion, as predicted by models of planet–disk interaction. These results provide the first direct confirmation of planet wakes generated by Lindblad resonances. Mapping the wake provides a constraint on the disk aspect ratio, which is required to measure the mass of the planet.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The component black holes (BHs) observed in gravitational-wave (GW) binary black hole (BBH) events tend to be more massive and slower spinning than those observed in black hole X-ray binaries (BH-XRBs). Without modeling their evolutionary histories, we investigate whether these apparent tensions in the BH populations can be explained by GW observational selection effects alone. We find that this is indeed the case for the discrepancy between BH masses in BBHs and the observed high-mass X-ray binaries (HMXBs), when we account for statistical uncertainty from the small sample size of just three HMXBs. On the other hand, the BHs in observed low-mass X-ray binaries (LMXBs) are significantly lighter than the astrophysical BBH population, but this may just be due to a correlation between component masses in a binary system. Given their light stellar companions, we expect light BHs in LMXBs. The observed spins in HMXBs and LMXBs, however, are in tension with the inferred BBH spin distribution at the &gt;99.9% level. We discuss possible scenarios behind the significantly larger spins in observed BH-XRBs. One possibility is that a small subpopulation (conservatively &lt;30%) of BBHs have rapidly spinning primary components, indicating that they may have followed a similar evolutionary pathway to the observed HMXBs. In LMXBs, it has been suggested that BHs can spin up by accretion. If LMXB natal spins follow the BBH spin distribution, we find LMXBs must gain an average dimensionless spin of <jats:inline-formula> <jats:tex-math> <?CDATA ${0.47}_{-0.11}^{+0.10}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.47</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlac64a5ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, but if their natal spins follow the observed HMXB spins, the average spin-up must be &lt;0.03.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We reanalyze the exquisite eclipsing binary data from the Kepler and TESS missions, focusing on eccentricity measurements at short orbital periods to empirically constrain tidal circularization. We calculate a circularization period of ∼6 days due to nearly circular binaries with long orbital periods (“cold core”) but find many binaries with moderate eccentricities that circularize interior to only ∼3 days (“eccentricity envelope”). We show that these features are present in previous spectroscopic surveys. We also reaffirm the statistically significant difference between the eccentricity distributions of young (&lt;1 Gyr) and old (&gt;3 Gyr) binaries. Our work introduces new methods that have the potential to reconcile theoretical predictions with observations to empirically constrain tidal circularization.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Agriculture is one of the oldest forms of technology on Earth. The cultivation of plants requires a terrestrial planet with active hydrological and carbon cycles and depends on the availability of nitrogen in soil. The technological innovation of agriculture is the active management of this nitrogen cycle by applying fertilizer to soil, at first through the production of manure excesses but later by the Haber–Bosch industrial process. The use of such fertilizers has increased the atmospheric abundance of nitrogen-containing species such as NH<jats:sub>3</jats:sub> and N<jats:sub>2</jats:sub>O as agricultural productivity intensifies in many parts of the world. Both NH<jats:sub>3</jats:sub> and N<jats:sub>2</jats:sub>O are effective greenhouse gases, and the combined presence of these gases in the atmosphere of a habitable planet could serve as a remotely detectable spectral signature of technology. Here we use a synthetic spectral generator to assess the detectability of NH<jats:sub>3</jats:sub> and N<jats:sub>2</jats:sub>O that would arise from present-day and future global-scale agriculture. We show that present-day Earth abundances of NH<jats:sub>3</jats:sub> and N<jats:sub>2</jats:sub>O would be difficult to detect, but hypothetical scenarios involving a planet with 30–100 billion people could show a change in transmittance of about 50%–70% compared to preagricultural Earth. These calculations suggest the possibility of considering the simultaneous detection of NH<jats:sub>3</jats:sub> and N<jats:sub>2</jats:sub>O in an atmosphere that also contains H<jats:sub>2</jats:sub>O, O<jats:sub>2</jats:sub>, and CO<jats:sub>2</jats:sub> as a technosignature for extraterrestrial agriculture. The technology of agriculture is one that could be sustainable across geologic timescales, so the spectral signature of such an “ExoFarm” is worth considering in the search for technosignatures.</jats:p>