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We present deep high-resolution (∼50 mas, 8 au) Atacama Large Millimeter/submillimeter Array (ALMA) 0.88 and 1.3 mm continuum observations of the LkCa 15 disk. The emission morphology shows an inner cavity and three dust rings at both wavelengths, but with slightly narrower rings at the longer wavelength. Along a faint ring at 42 au, we identify two excess emission features at ∼10σ significance at both wavelengths: one as an unresolved clump and the other as an extended arc, separated by roughly 120° in azimuth. The clump is unlikely to be a circumplanetary disk (CPD) as the emission peak shifts between the two wavelengths even after accounting for orbital motion. Instead, the morphology of the 42 au ring strongly resembles the characteristic horseshoe orbit produced in planet–disk interaction models, where the clump and the arc trace dust accumulation around Lagrangian points L₄ and L₅, respectively. The shape of the 42 au ring, dust trapping in the outer adjacent ring, and the coincidence of the horseshoe ring location with a gap in near-IR scattered light, are all consistent with the scenario of planet sculpting, with the planet likely having a mass between those of Neptune and Saturn. We do not detect pointlike emission associated with a CPD around the putative planet location (027 in projected separation from the central star at a position angle of ∼60°), with upper limits of 70 and 33 μJy at 0.88 and 1.3 mm, respectively, corresponding to dust mass upper limits of 0.02–0.03 M_⊕.

We report that object 282P/(323137) 2003 BM₈₀ is undergoing a sustained activity outburst, lasting over 15 months thus far. These findings stem in part from our NASA Partner Citizen Science project Active Asteroids (http://activeasteroids.net), which we introduce here. We acquired new observations of 282P via our observing campaign (Vatican Advanced Technology Telescope (VATT), Lowell Discovery Telescope (LDT), and the Gemini South telescope), confirming 282P was active on UT 2022 June 7, some 15 months after 2021 March images showed activity in the 2021–2022 epoch. We classify 282P as a member of the quasi-Hilda objects (QHOs), a group of dynamically unstable objects found in an orbital region similar to, but distinct in their dynamical characteristics to, the Hilda asteroids (objects in 3:2 resonance with Jupiter). Our dynamical simulations show 282P has undergone at least five close encounters with Jupiter and one with Saturn over the last 180 yr. 282P was most likely a Centaur or Jupiter-family comet (JFC) 250 yr ago. In 350 yr, following some 15 strong Jovian interactions, 282P will most likely migrate to become a JFC or, less likely, an outer main-belt asteroid orbit. These migrations highlight a dynamical pathway connecting Centaurs and JFCs with quasi-Hildas and, potentially, active asteroids. Synthesizing these results with our thermodynamical modeling and new activity observations, we find volatile sublimation is the primary activity mechanism. Observations of a quiescent 282P, which we anticipate will be possible in 2023, will help confirm our hypothesis by measuring a rotation period and ascertaining the spectral type.

The faint and ultrafaint dwarf galaxies in the Local Group form the observational bedrock upon which our understanding of small-scale cosmology rests. In order to understand whether this insight generalizes, it is imperative to use resolved-star techniques to discover similarly faint satellites in nearby galaxy groups. We describe our search for ultrafaint galaxies in the M81 group using deep ground-based resolved-star data sets from Subaru's Hyper Suprime-Cam. We present one new ultrafaint dwarf galaxy in the M81 group and identify five additional extremely low surface brightness candidate ultrafaint dwarfs that reach deep into the ultrafaint regime to M_V ∼ − 6 (similar to current limits for Andromeda satellites). These candidates' luminosities and sizes are similar to known Local Group dwarf galaxies Tucana B, Canes Venatici I, Hercules, and Boötes I. Most of these candidates are likely to be real, based on tests of our techniques on blank fields. Intriguingly, all of these candidates are spatially clustered around NGC 3077, which is itself an M81 group satellite in an advanced state of tidal disruption. This is somewhat surprising, as M81 itself and its largest satellite M82 are both substantially more massive than NGC 3077 and, by virtue of their greater masses, would have been expected to host as many or more ultrafaint candidates. These results lend considerable support to the idea that satellites of satellites are an important contribution to the growth of satellite populations around Milky Way–mass galaxies.

Recent observations of protoplanetary disks (PPDs) at submillimeter wavelengths have revealed the ubiquity of annular substructures that are indicative of pebble-sized dust particles trapped in turbulent ringlike gas pressure bumps. This major paradigm shift also challenges the leading theory of planetesimal formation from such pebbles by means of the streaming instability, which operates in a pressure gradient and can be suppressed by turbulence. Here, we conduct 3D local shearing box nonideal magnetohydrodynamic simulations of dust trapping in enforced gas pressure bumps, including dust backreaction. Under a moderate level of turbulence generated by the magnetorotational instability with ambipolar diffusion, which is suitable for outer disk conditions, we achieve quasi-steady states of dust trapping balanced by turbulent diffusion. We find strong dust clumping in all simulations near the gas pressure maxima, reaching a maximum density well above the threshold for triggering gravitational collapse to form planetesimals. A strong pressure bump concentrates dust particles toward the bump's center. With a weak pressure bump, dust can also concentrate in secondary filaments off the bump's center, due to dust backreaction, but strong clumping still occurs mainly in the primary ring around the bump's center. Our results reveal dust-trapping rings to be robust locations for planetesimal formation in outer PPDs, while they may possess diverse observational properties.

A shock or a mini-magnetosphere was once thought to be formed by the solar wind interaction with strong lunar magnetic anomalies. However, the full structure of a mini-magnetosphere has never been verified and whether a mini-magnetosphere can be completely formed remains a controversy. In this work, we present a unique multipoint observation of such an interaction by the ARTEMIS spacecraft and the Chang'E-4 rover. Both solar wind deceleration and penetration are observed by the Chang'E-4 rover on the lunar surface near the magnetic anomaly. Meanwhile, a shock is observed by the ARTEMIS spacecraft downstream from the magnetic anomaly. It is suggested that the magnetic anomaly cannot stand off the solar wind, and there is no shock but just a boundary layer near the magnetic anomaly. Accordingly, a mini-magnetosphere is not completely formed and the downstream shock observed the ARTEMIS spacecraft just corresponds to a trailing shock.

Detection of the first stars has remained elusive so far but their presence may soon be unveiled by upcoming JWST observations. Previous studies have not investigated the entire possible range of halo masses and redshifts that may help in their detection. Motivated by the prospects of detecting galaxies up to z ∼ 20 in the JWST early data release, we quantify the contribution of Population III stars to high-redshift galaxies from 6 ≤ z ≤ 30 by employing the semianalytical model a-sloth, which self-consistently models the formation of Population III and Population II stars along with their feedback. Our results suggest that the contribution of Population III stars is the highest in low-mass halos of 10⁷–10⁹M_⊙. While high-mass halos ≥10¹⁰M_⊙ contain less than 1% Population III stars, they host galaxies with stellar masses of 10⁹M_⊙ as early as z ∼ 30. Interestingly, overall the apparent magnitude of Population III stars gets brighter toward higher redshift due to the higher stellar masses, but Population III–dominated galaxies are too faint to be directly detected with JWST. Our results predict JWST can detect galaxies up to z ∼ 30, which may help in constraining the initial mass function of Population III stars and will guide observers to discern the contribution of Population III stars to high-redshift galaxies.

We use Gaia DR3 data to study the Collinder 132–Gulliver 21 region via the machine-learning algorithm StarGO and find eight subgroups of stars (ASCC 32, Collinder 132 gp 1–6, Gulliver 21) located in close proximity. Three comoving populations were identified among these eight subgroups: (i) a coeval 25 Myr old moving group (Collinder 132), (ii) an intermediate-age (50–100 Myr) group, and (iii) the 275 Myr old dissolving cluster Gulliver 21. These three populations form parallel diagonal stripe-shape overdensities in the U–V distribution, which differ from open clusters and stellar groups in the solar neighborhood. We name this kinematic structure the Collinder 132–Gulliver 21 stream, as it extends over 270 pc in the 3D space. The oldest population, Gulliver 21, is spatially surrounded by the Collinder 132 moving group and the intermediate-age group. Stars in the Collinder 132–Gulliver 21 stream have an age difference up to 250 Myr. Metallicity information shows a variation of 0.3 dex between the youngest and oldest populations. The formation of the Collinder 132–Gulliver 21 stream involves both star formation and dynamical heating. The youngest population (Collinder 132 moving group) with homogeneous metallicity is probably formed through filamentary star formation. The intermediate-age and oldest populations were then scattered by the Galactic bar or spiral structure resonance to intercept Collinder 132's orbit. Without mutual interaction between each population, the three populations are flying by each other currently and will become three distinct groups again in ∼50 Myr.

The solar magnetic activity cycle provides energy input that is released in intense bursts of radiation known as solar flares. As such, the dynamics of the activity cycle is embedded in the sequence of times between the flare events. Recent analysis shows that solar flares exhibit memory on different timescales. These previous studies showed that the time ordering of flare events is not random, but rather there is dependence between successive flares. In the present work, the clustering of flares is demonstrated through a straightforward nonparametric method where the cumulative distribution function of successive flares is compared with the cumulative distribution function of surrogate sequences of flares obtained by random permutation of flares. The random permutation is performed within rate-variable Bayesian blocks during which the flare rate is assumed to be constant. Differences between the cumulative distribution functions are substantial on a timescale around 3 hr, suggesting that flare recurrence on that timescale is more likely than would be expected if the waiting time were drawn from a nonstationary Poisson process.

Evidence of a relation between the mass accretion rate and the disk mass is established for young, Class II pre-main-sequence stars. This observational result opened an avenue to test theoretical models and constrain the initial conditions of disk formation, fundamental in the understanding of the emergence of planetary systems. However, it is becoming clear that planet formation starts even before the Class II stage, in disks around Class 0 and I protostars. We show for the first time evidence for a correlation between the mass accretion rate and the disk mass for a large sample of Class I young stars located in nearby (<500 pc) star-forming regions. We fit our sample, finding that the Class I object relation has a slope flatter than Class II stars, and the former have higher mass accretion rates and disk masses. The results are put in context of disk evolution models.

We report on the interaction of the legs of the erupting filament of 2012 August 31 and associated prominent supra-arcade downflows (P-SADs) as observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. We employ a number of image processing techniques to enhance weak interacting features. As the filament erupts, both legs stretch outwards. The positive-polarity leg also untwists and splits into two parts. The first part runs into the conjugate (negative-polarity) leg, tearing it apart. The second part then converges into the remnant of the conjugate leg, after which both weaken and finally disappear. All these episodes of interaction of oppositely oriented filament legs are followed by the appearance of P-SADs, seen in the on-disk projection to be shaped as loop tops, along with many weaker SADs. All SADs are preceded by hot supra-arcade downflowing loops. This observed evolution is consistent with the three-dimensional rr–rf (leg–leg) reconnection, where the erupting flux rope reconnects with itself. In our observations, as well as in some models, the reconnection in this geometry is found to be long lasting. It plays a substantial role in the evolution of the flux rope of the erupting filament and leads to prominent SADs.

Solar active regions that violate the Hale–Nicholson rule are rare, but once formed, they tend to be flare-productive. In this letter, we investigated the evolution of an anti-Hale region newly emerging from the active region AR 12882 with a regular Hale distribution. The entire active region became very active, producing two eruptive flares within 48 hr after the emergence of the anti-Hale region. Strong photospheric shear motions appeared in this anti-Hale region, changing its tilt angle from the north–south direction to the east–west direction. The flux emergence and shearing motions continuously injected magnetic energy and negative magnetic helicity into the upper atmosphere. Meanwhile, the upper coronal structure changed from double J-shaped to reverse S-shaped, forming a magnetic flux rope lying above the anti-Hale region. This magnetic flux rope erupted successfully, then re-formed and erupted successfully again, producing a C2.7 flare and an M1.6 flare, respectively. Moreover, a large cusp structure was observed to form next to the flaring region after the M1.6 flare. Accordingly, we conclude that the evolution of the emerging anti-Hale region provides sufficient magnetic energy and helicity for the flares, and the interaction between the emerging anti-Hale region and the preexisting Hale active region eventually promotes the flares to be eruptive.

We present a toy model for the thermal optical/UV/X-ray emission from tidal disruption events (TDEs). Motivated by recent hydrodynamical simulations, we assume that the debris streams promptly and rapidly circularize (on the orbital period of the most tightly bound debris), generating a hot quasi-spherical pressure-supported envelope of radius R_v ∼ 10¹⁴ cm (photosphere radius ∼10¹⁵ cm) surrounding the supermassive black hole (SMBH). As the envelope cools radiatively, it undergoes Kelvin–Helmholtz contraction R_v ∝ t⁻¹, its temperature rising T_eff ∝ t^1/2 while its total luminosity remains roughly constant; the optical luminosity decays as $\nu {L}_{\nu }\propto \,{R}_{v}^{2}{T}_{\mathrm{eff}}\propto {t}^{-3/2}$. Despite this similarity to the mass fallback rate ${\dot{M}}_{\mathrm{fb}}\propto {t}^{-5/3}$, envelope heating from fallback accretion is subdominant compared to the envelope cooling luminosity except near optical peak (where they are comparable). Envelope contraction can be delayed by energy injection from accretion from the inner envelope onto the SMBH in a regulated manner, leading to a late-time flattening of the optical/X-ray light curves, similar to those observed in some TDEs. Eventually, as the envelope contracts to near the circularization radius, the SMBH accretion rate rises to its maximum, in tandem with the decreasing optical luminosity. This cooling-induced (rather than circularization-induced) delay of up to several hundred days may account for the delayed onset of thermal X-rays, late-time radio flares, and high-energy neutrino generation, observed in some TDEs. We compare the model predictions to recent TDE light-curve correlation studies, finding both agreement and points of tension.

The spin properties of merging black holes observed with gravitational waves can offer novel information about the origin of these systems. The magnitudes and orientations of black hole spins offer a record of binaries' evolutionary history, encoding information about massive stellar evolution and the astrophysical environments in which binary black holes are assembled. Recent analyses of the binary black hole population have yielded conflicting portraits of the black hole spin distribution. Some works suggest that black hole spins are small but nonzero and exhibit a wide range of misalignment angles relative to binaries' orbital angular momenta. Other works conclude that the majority of black holes are nonspinning while the remainder are rapidly rotating and primarily aligned with their orbits. We revisit these conflicting conclusions, employing a variety of complementary methods to measure the distribution of spin magnitudes and orientations among binary black hole mergers. We find that the existence of a subpopulation of black holes with vanishing spins is not required by current data. Should such a subpopulation exist, we conclude that it must contain ≲60% of binaries. Additionally, we find evidence for significant spin–orbit misalignment among the binary black hole population, with some systems exhibiting misalignment angles greater than 90°, and see no evidence for an approximately spin-aligned subpopulation.

The gas surface density profile of protoplanetary disks is one of the most fundamental physical properties to understanding planet formation. However, it is challenging to determine the surface density profile observationally, because the H₂ emission cannot be observed in low-temperature regions. We analyzed the Atacama Large Millimeter/submillimeter Array (ALMA) archival data of the ¹²CO J = 3 − 2 line toward the protoplanetary disk around TW Hya and discovered extremely broad line wings due to the pressure broadening. In conjunction with a previously reported optically thin CO isotopologue line, the pressure broadened line wings enabled us to directly determine the midplane gas density for the first time. The gas surface density at ∼5 au from the central star reaches ∼10³ g cm⁻², which suggests that the inner region of the disk has enough mass to form a Jupiter-mass planet. Additionally, the gas surface density drops at the inner cavity by ∼2 orders of magnitude compared to outside the cavity. We also found a low CO abundance of ∼10⁻⁶ with respect to H₂, even inside the CO snow line, which suggests conversion of CO to less volatile species. Combining our results with previous studies, the gas surface density jumps at r ∼ 20 au, suggesting that the inner region (3 < r < 20 au) might be the magnetorotational instability dead zone. This study sheds light on the direct gas surface density constraint without assuming the CO/H₂ ratio using ALMA.

Most existing criteria derived from progenitor properties of core-collapse supernovae are not very accurate in predicting explosion outcomes. We present a novel look at identifying the explosion outcome of core-collapse supernovae using a machine-learning approach. Informed by a sample of 100 2D axisymmetric supernova simulations evolved with Fornax, we train and evaluate a random forest classifier as an explosion predictor. Furthermore, we examine physics-based feature sets including the compactness parameter, the Ertl condition, and a newly developed set that characterizes the silicon/oxygen interface. With over 1500 supernovae progenitors from 9−27 M_⊙, we additionally train an autoencoder to extract physics-agnostic features directly from the progenitor density profiles. We find that the density profiles alone contain meaningful information regarding their explodability. Both the silicon/oxygen and autoencoder features predict the explosion outcome with ≈90% accuracy. In anticipation of much larger multidimensional simulation sets, we identify future directions in which machine-learning applications will be useful beyond the explosion outcome prediction.

The first experimental study of the low-temperature kinetics of the gas-phase reaction of NH₂ with formaldehyde (CH₂O) has been performed. This reaction has previously been suggested as a source of formamide (NH₂CHO) in interstellar environments. A pulsed Laval nozzle equipped with laser-flash photolysis and laser-induced fluorescence spectroscopy was used to create and monitor the temporal decay of NH₂ in the presence of CH₂O. No loss of NH₂ could be observed via reaction with CH₂O, and we place an upper limit on the rate coefficient of <6 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ at 34 K. Ab initio calculations of the potential energy surface were combined with Rice–Rampsberger–Kassel–Marcus (RRKM) calculations to predict a rate coefficient of 6.2 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ at 35 K, consistent with the experimental results. The presence of a significant barrier, 18 kJ mol⁻¹, for the formation of formamide as a product, means that only the H-abstraction channel producing NH₃ + CHO, in which the transfer of an H atom can occur by quantum mechanical tunneling through a 23 kJ mol⁻¹ barrier, is open at low temperatures. These results are in contrast with a recent theoretical study, which suggested that the reaction could proceed without a barrier and was therefore a viable route to gas-phase formamide formation. The calculated rate coefficients were used in an astrochemical model, which demonstrated that this reaction produces only negligible amounts of gas-phase formamide under interstellar and circumstellar conditions. The reaction of NH₂ with CH₂O is therefore not an important source of formamide at low temperatures in interstellar environments.

In recent years, the discovery of increasing numbers of rocky, terrestrial exoplanets orbiting nearby stars has drawn increased attention to the possibility of studying these planets' atmospheric and surface properties. This is especially true for planets orbiting M dwarfs, whose properties can best be studied with existing observatories. In particular, the minerological composition of these planets and the extent to which they can retain their atmospheres in the face of intense stellar irradiation both remain unresolved. Here, we report the detection of the secondary eclipse of the terrestrial exoplanet GJ 1252b, obtained via 10 eclipse observations using the Spitzer Space Telescope's IRAC2 4.5 μm channel. We measure an eclipse depth of ${149}_{-32}^{+25}$ ppm, corresponding to a dayside brightness temperature of ${1410}_{-125}^{+91}$ K. This measurement is consistent with the prediction for a bare rock surface. Comparing the eclipse measurement to a large suite of simulated planetary spectra indicates that GJ 1252b has a surface pressure of ≲10 bar, i.e., substantially thinner than the atmosphere of Venus. Assuming energy-limited escape, even a 100 bar atmosphere would be lost in <1 Myr, far shorter than our gyrochronological age estimate of 3.9 ± 0.4 Gyr. The expected mass loss could be overcome by mantle outgassing, but only if the mantle's carbon content were >7% by mass—over two orders of magnitude greater than that found in Earth. We therefore conclude that GJ 1252b has no significant atmosphere. Model spectra with granitoid or feldspathic surface composition, but with no atmosphere, are disfavored at >2σ. The eclipse occurs just +1.4${}_{-1.0}^{+2.8}$ minutes after orbital phase 0.5, indicating $e\cos \omega$ = +0.0025 ${}_{-0.0018}^{+0.0049}$, consistent with a circular orbit. Tidal heating is therefore likely to be negligible with regard to GJ 1252b's global energy budget. Finally, we also analyze additional, unpublished TESS transit photometry of GJ 1252b, which improves the precision of the transit ephemeris by a factor of 10, provides a more precise planetary radius of 1.180 ± 0.078 R_⊕, and rules out any transit-timing variations with amplitudes ≳1 minute.

The IllustrisTNG simulations reproduce the observed scaling relation between the stellar specific angular momentum (sAM) j_s and mass M_s of central galaxies. We show that the local j_s–M_s relation $\mathrm{log}\ {j}_{{\rm{s}}}=0.55\ \mathrm{log}\ {M}_{{\rm{s}}}+2.77$ develops at z ≲ 1 in disk-dominated galaxies. We provide a simple model that describes well such a connection between halos and galaxies. The index of 0.55 of the j_s–M_s relation comes from the product of the indices of the ${j}_{\mathrm{tot}}\propto {M}_{\mathrm{tot}}^{0.81}$, ${M}_{\mathrm{tot}}\propto {M}_{{\rm{s}}}^{0.67}$, and j_s ∝ j_tot relations, where j_tot and M_tot are the overall sAM and mass of the halo, respectively. A non-negligible deviation from tidal torque theory, which predicts ${j}_{\mathrm{tot}}\propto {M}_{\mathrm{tot}}^{2/3}$, should be included. This model further suggests that the stellar-to-halo mass ratio of disk galaxies increases monotonically following a nearly power-law function that is consistent with the latest dynamical measurements. Biased collapse, in which galaxies form from the inner and lower sAM portion of their parent halos, has a minor effect at low redshifts. The retention factor of angular momentum reaches ∼1 in disk galaxies with strong rotations, and it correlates inversely with the mass fraction of the spheroidal component, which partially explains the morphological dependence of the j_s–M_s relation.

Much of coronal hole (CH) research is focused upon determining the boundary and calculating the open flux as accurately as possible. However, the observed boundary itself is worthy of investigation, and holds important clues to the physics transpiring at the interface between the open and closed fields. This Letter reports a powerful new method, an application of the correlation integral which we call correlation dimension mapping, by which the irregularity of a CH boundary can be objectively quantified. This method highlights the most important spatial scales involved in boundary dynamics, and also allows for easy temporal analysis of the boundary. We apply this method to an equatorial CH bounded on two sides by helmet streamers and on the third by a small pseudostreamer, which we observed at maximum cadence for an hour on 2015 June 4. We argue that the relevant spatial scales are in the range of ∼5–20 Mm, and we find that the boundary complexity depends measurably upon the nature of the neighboring closed structure. The boundary along the pseudostreamer shows signs of highly localized, intermittent-complexity variability, likely associated with abrupt changes in the magnetic topology, which would be elegantly explained by interchange reconnection. By contrast, the helmet streamer boundary supports long-lived, high-complexity regions. These findings support the recent predictions of interchange reconnection occurring at very small scales in the corona.