Table of contents

Close-in gas giants present a surprising range of stellar obliquity, the angle between a planet's orbital axis and its host star's spin axis. It is unclear whether the obliquities reflect the planets' dynamical history (e.g., aligned for in situ formation or disk migration versus misaligned for high-eccentricity tidal migration) or whether other mechanisms (e.g., primordial misalignment or planet–star interactions) are more important in sculpting the obliquity distribution. Here we present the stellar obliquity measurement of TOI-1268 (TIC-142394656, V_mag ∼ 10.9), a young K-type dwarf hosting an 8.2 day period, Saturn-sized planet. TOI-1268's lithium abundance and rotation period suggest the system age between the ages of the Pleiades cluster (∼120 Myr) and the Prasepe cluster (∼670 Myr). Using the newly commissioned NEID spectrograph, we constrain the stellar obliquity of TOI-1268 via the Rossiter–McLaughlin effect from both radial velocity and Doppler tomography signals. The 3σ upper bounds of the projected stellar obliquity ∣λ∣ from both models are below 60°. The large host star separation (a/R_⋆ ∼ 17), combined with the system's young age, makes it unlikely that the planet has realigned its host star. The stellar obliquity measurement of TOI-1268 probes the architecture of a young gas giant beyond the reach of tidal realignment (a/R_⋆ ≳ 10) and reveals an aligned or slightly misaligned system.

We present spectroscopic measurements of the Rossiter–McLaughlin effect for WASP-148b, the only known hot Jupiter with a nearby warm-Jupiter companion, from the WIYN/NEID and Keck/HIRES instruments. This is one of the first scientific results reported from the newly commissioned NEID spectrograph, as well as the second obliquity constraint for a hot Jupiter system with a close-in companion, after WASP-47. WASP-148b is consistent with being in alignment with the sky-projected spin axis of the host star, with $\lambda =-8\buildrel{\circ}\over{.} {2}_{-9\buildrel{\circ}\over{.} \,7}^{+8\buildrel{\circ}\over{.} \,7}$. The low obliquity observed in the WASP-148 system is consistent with the orderly-alignment configuration of most compact multi-planet systems around cool stars with obliquity constraints, including our solar system, and may point to an early history for these well-organized systems in which migration and accretion occurred in isolation, with relatively little disturbance. By contrast, previous results have indicated that high-mass and hot stars appear to more commonly host a wide range of misaligned planets: not only single hot Jupiters, but also compact systems with multiple super-Earths. We suggest that, to account for the high rate of spin–orbit misalignments in both compact multi-planet and isolated-hot-Jupiter systems orbiting high-mass and hot stars, spin–orbit misalignments may be caused by distant giant planet perturbers, which are most common around these stellar types.

We use a numerical Galactic chemical evolution model and find that the common envelope jet supernova (CEJSN) r-process scenario can account for both the very early average ratio of europium to iron and its evolution at later times in the the Milky Way. In the CEJSN scenario a neutron star (NS) spirals in inside a red supergiant (RSG) star all the way to the core and destroys it. According to this scenario r-process isotopes are nucleosynthesized inside neutron-rich jets that the accretion disk around the NS launches inside the core. The merger of an NS with an RSG core already takes place in the very young Galaxy. We conclude that CEJSNe can be a major contributor to r-process nucleosynthesis.

We present 12 epochs of optical spectroscopy taken across the discovery outburst of the black hole (BH) candidate MAXI J1803−298 with the Gran Telescopio Canarias and Very Large Telescope. The source followed a standard outburst evolution with hard and soft states. The system displays a triangular shape in the hardness intensity diagram, consistent with that seen in high-inclination BH transients and the previously reported detection of X-ray dips. The two epochs observed during the initial hard state exhibited asymmetric emission-line profiles, including a P-Cygni profile simultaneously detected in Hα and He i 6678, which indicates the presence of an optical wind in the system. The remaining spectra, obtained during the transition to the soft state and the subsequent decay, are instead characterized by narrower, double-peaked emission lines embedded into broad absorption components. One epoch (intermediate state) also includes near-infrared (NIR) coverage, revealing complex line profiles in the Paschen and Bracket series, which suggests that the outflow is still present during the outburst decay through the soft state. The growing list of low-mass X-ray binaries with optical and NIR outflow signatures indicates that these are common features. Furthermore, the lowest luminosity spectrum exhibits an Hα FWHM of 1570 ± 100 km s⁻¹. This, together with previous constraints on the binary parameters, allows us to favor a compact object mass of ∼3–10 M_⊙, further supporting its BH nature.

Ongoing large-scale optical time-domain surveys, such as the Zwicky Transient Facility (ZTF), are producing alerts at unprecedented rates. Analysis of transient sources has so far followed two distinct paths: archival analysis of data on transient sources at a time when they are no longer observable and real-time analysis at the time when the sources are first detected. The latter is the realm of alert brokers such as the Arizona-NOIRLab Temporal Analysis and Response to Events System (ANTARES). In this paper, we synthesize the two analysis paths and present a first systematic study of archival alert-broker data, focusing on extragalactic transients with multipeaked light curves identified in the ANTARES archive of ZTF alerts. Our analysis yields a sample of 37 such sources, including core-collapse supernovae (with two analogs of iPTF14hls), thermonuclear supernovae interacting with their surrounding circumstellar medium, tidal disruption events, luminous blue variables, and as yet unclassified objects. A large fraction of the identified sources is currently active, warranting allocation of follow-up resources in the immediate future to further constrain their nature and the physical processes at work.

While stars are often found in binary systems, brown dwarf binaries are much rarer. Brown dwarf–brown dwarf pairs are typically difficult to resolve because they often have very small separations. Using brown dwarfs discovered with data from the Wide-field Infrared Survey Explorer (WISE) via the Backyard Worlds: Planet 9 citizen science project, we inspected other, higher-resolution, sky surveys for overlooked cold companions. During this process, we discovered the brown dwarf binary system CWISE J0146−0508AB, which we find has a very small chance alignment probability based on the similar proper motions of the components of the system. Using follow-up near-infrared spectroscopy with Keck/NIRES, we determined component spectral types of L4 and L8 (blue), making CWISE J0146−0508AB one of only a few benchmark systems with a blue L dwarf. At an estimated distance of ∼40 pc, CWISE J0146−0508AB has a projected separation of ∼129 au, making it the widest-separation brown dwarf pair found to date. We find that such a wide separation for a brown dwarf binary may imply formation in a low-density star-forming region.

Using the integral field unit data from the Mapping Nearby Galaxies at Apache Point Observatory survey, we select a sample of 101 galaxies with counterrotating stellar disks and regularly rotating ionized gas disks. We classify the 101 galaxies into four types based on the features of their stellar velocity fields. The relative fractions and stellar population age radial gradients of the four types are different in the blue cloud, green valley, and red sequence populations. We suggest different formation scenarios for counterrotating stellar disks; the key factors in the formation of counterrotating stellar disks include (1) the abundance of preexisting gas in the progenitor and (2) the efficiency in angular momentum consumption.

With the aim of investigating how the magnetic field in solar active regions (ARs) controls flare activity, i.e., whether a confined or eruptive flare occurs, we analyze 106 flares of Geostationary Operational Environmental Satellite class ≥M1.0 during 2010–2019. We calculate mean characteristic twist parameters α_FPIL within the "flaring polarity inversion line" region and α_HFED within the area of high photospheric magnetic free energy density, which both provide measures of the nonpotentiality of the AR core region. Magnetic twist is thought to be related to the driving force of electric current-driven instabilities, such as the helical kink instability. We also calculate total unsigned magnetic flux (Φ_AR) of ARs producing the flare, which describes the strength of the background field confinement. By considering both the constraining effect of background magnetic fields and the magnetic nonpotentiality of ARs, we propose a new parameter α/Φ_AR to measure the probability for a large flare to be associated with a coronal mass ejection (CME). We find that in about 90% of eruptive flares, α_FPIL/Φ_AR and α_HFED/Φ_AR are beyond critical values (2.2 × 10⁻²⁴ and 3.2 × 10⁻²⁴ Mm⁻¹ Mx⁻¹), whereas they are less than critical values in ∼80% of confined flares. This indicates that the new parameter α/Φ_AR is well able to distinguish eruptive flares from confined flares. Our investigation suggests that the relative measure of magnetic nonpotentiality within the AR core over the restriction of the background field largely controls the capability of ARs to produce eruptive flares.

The two sources AGC 226178 and NGVS 3543, an extremely faint, clumpy, blue stellar system and a low surface brightness dwarf spheroidal, are adjacent systems in the direction of the Virgo cluster. Both have been studied in detail previously, with it being suggested that they are unrelated normal dwarf galaxies or that NGVS 3543 recently lost its gas through ram pressure stripping and AGC 226178 formed from this stripped gas. However, with Hubble Space Telescope Advanced Camera for Surveys imaging, we demonstrate that the stellar population of NGVS 3543 is inconsistent with being at the distance of the Virgo cluster and that it is likely a foreground object at approximately 10 Mpc, whereas the stellar population of AGC 226178 is consistent with it being a very young (10–100 Myr) object in the Virgo cluster. Through a reanalysis of the original ALFALFA H i detection, we show that AGC 226178 likely formed from gas stripped from the nearby dwarf galaxy VCC 2034, a hypothesis strengthened by the high metallicity measured with MUSE VLT observations. However, it is unclear whether ram pressure or a tidal interaction is responsible for stripping the gas. Object AGC 226178 is one of at least five similar objects now known toward Virgo. These objects are all young and unlikely to remain visible for over ∼500 Myr, suggesting that they are continually produced in the cluster.

The Parker Solar Probe (PSP) entered a region of sub-Alfvénic solar wind during encounter 8, and we present the first detailed analysis of low-frequency turbulence properties in this novel region. The magnetic field and flow velocity vectors were highly aligned during this interval. By constructing spectrograms of the normalized magnetic helicity, cross-helicity, and residual energy, we find that PSP observed primarily Alfvénic fluctuations, a consequence of the highly field-aligned flow that renders quasi-2D fluctuations unobservable to PSP. We extend Taylor's hypothesis to sub- and super-Alfvénic flows. Spectra for the fluctuating forward and backward Elsässer variables (z^±, respectively) are presented, showing that z⁺ modes dominate z⁻ by an order of magnitude or more, and the z⁺ spectrum is a power law in frequency (parallel wavenumber) f^−3/2 (${k}_{\parallel }^{-3/2}$) compared to the convex z⁻ spectrum with f^−3/2 (${k}_{\parallel }^{-3/2}$) at low frequencies, flattening around a transition frequency (at which the nonlinear and Alfvén timescales are balanced) to f^−1.25 at higher frequencies. The observed spectra are well fitted using a spectral theory for nearly incompressible magnetohydrodynamics assuming a wavenumber anisotropy ${k}_{\perp }\sim {k}_{\parallel }^{3/4}$, that the z⁺ fluctuations experience primarily nonlinear interactions, and that the minority z⁻ fluctuations experience both nonlinear and Alfvénic interactions with z⁺ fluctuations. The density spectrum is a power law that resembles neither the z^± spectra nor the compressible magnetic field spectrum, suggesting that these are advected entropic rather than magnetosonic modes and not due to the parametric decay instability. Spectra in the neighboring modestly super-Alfvénic intervals are similar.

The obliquity of a star, or the angle between its spin axis and the average orbit normal of its companion planets, provides a unique constraint on that system's evolutionary history. Unlike the solar system, where the Sun's equator is nearly aligned with its companion planets, many hot-Jupiter systems have been discovered with large spin–orbit misalignments, hosting planets on polar or retrograde orbits. We demonstrate that, in contrast to stars harboring hot Jupiters on circular orbits, those with eccentric companions follow no population-wide obliquity trend with stellar temperature. This finding can be naturally explained through a combination of high-eccentricity migration and tidal damping. Furthermore, we show that the joint obliquity and eccentricity distributions observed today are consistent with the outcomes of high-eccentricity migration, with no strict requirement to invoke the other hot-Jupiter formation mechanisms of disk migration or in situ formation. At a population-wide level, high-eccentricity migration can consistently shape the dynamical evolution of hot-Jupiter systems.

Saturn has a dynamically rich satellite system, which includes at least three orbital resonances between three pairs of moons: Mimas–Tethys 4:2, Enceladus–Dione 2:1, and Titan–Hyperion 4:3 mean-motion resonances. Studies of the orbital history of Saturn's moons usually assume that their past dynamics was also dominated solely by two-body resonances. Using direct numerical integrations, we find that three-body resonances among Saturnian satellites were quite common in the past, and could result in a relatively long-term, but finite capture time (10 Myr or longer). We find that these three-body resonances are invariably of the eccentricity type and do not appear to affect the moons' inclinations. While some three-body resonances are located close to two-body resonances (but involve the orbital precession of the third body), others are isolated, with no two-body arguments being near resonance. We conclude that future studies of the system's past must take full account of three-body resonances, which have been overlooked in the past work.

We present analysis of 17,043 proton kinetic-scale current sheets (CSs) collected over 124 days of Wind spacecraft measurements in the solar wind at 11 samples s⁻¹ magnetic field resolution. The CSs have thickness, λ, from a few tens to one thousand kilometers with typical values around 100 km, or within about 0.1–10λ_p in terms of local proton inertial length, λ_p. We found that the current density is larger for smaller-scale CSs, J₀ ≈ 6 nAm⁻² · (λ/100 km)^−0.56, but does not statistically exceed a critical value, J_A, corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units: ${J}_{0}/{J}_{A}\approx 0.17\cdot {(\lambda /{\lambda }_{p})}^{-0.51}$. The CSs are statistically force-free with magnetic shear angle correlated with CS spatial scale: ${\rm{\Delta }}\theta \approx 19^\circ \cdot {(\lambda /{\lambda }_{p})}^{0.5}$. The observed correlations are consistent with local turbulence being the source of proton kinetic-scale CSs in the solar wind, while the mechanisms limiting the current density remain to be understood.

Asteroid impacts with Earth may have played an essential role in the emergence of life on Earth through their creation of favorable niches for life, changes to the atmosphere, and delivery of water. Consequently, we suggest two potential requirements for life in an exoplanetary system: first, that the system has an asteroid belt, and second, that there is a mechanism to drive asteroids to impact the terrestrial habitable planet. Since in the solar system the ν₆ secular resonance has been shown to have been important in driving these impacts, we explore how the masses and locations of two giant planets determine the location and strength of this secular resonance. Examining observed exoplanetary systems with two giant planets, we find that a secular resonance within the asteroid belt region may not be uncommon. Hence, the solar system is somewhat special, but the degree of fine-tuning that may be necessary for the emergence of life is not excessive. Finally, with n-body simulations, we show that when the two giant planets are close to the 2:1 mean motion resonance, the asteroid belt is unstable, but this does not lead to increased asteroid delivery.

We present Keck Cosmic Web Imager observations of giant Lyα halos surrounding nine galaxy groups and clusters at 2 < z < 3.3, including five new detections and one upper limit. We find observational evidence for the cold-stream to hot-accretion transition predicted by theory by measuring a decrease in the ratio between the spatially extended Lyα luminosity and the expected baryonic accretion rate (BAR), with increasing elongation above the transition mass (M_stream). This implies a modulation of the share of BAR that remains cold, diminishing quasi-linearly (logarithmic slope of 0.97 ± 0.19, 5σ significance) with the halo to M_stream mass ratio. The integrated star formation rates (SFRs) and active galactic nucleus (AGN) bolometric luminosities display a potentially consistent decrease, albeit significant only at 2.6σ and 1.3σ, respectively. The higher scatter in these tracers suggests the Lyα emission might be mostly a direct product of cold accretion in these structures rather than indirect, mediated by outflows and photoionization from SFR and AGNs; this is also supported by energetics considerations. Below M_stream (cold-stream regime), we measure L_Lyα/BAR = 10^40.51±0.16 erg s⁻¹${M}_{\odot }^{-1}$ yr, consistent with predictions, and SFR/BAR = 10^{−0.54±0.23}: on average, ${30}_{-10}^{+20}$% of the cold streams go into stars. Above M_stream (hot-accretion regime), L_Lyα is set by M_stream (within 0.2 dex scatter in our sample), independent of the halo mass but rising 10-fold from z = 2 to 3.

The Central Molecular Zone (CMZ) contains most of the mass of our Galaxy but its star formation rate is one order of magnitude lower than in the Galactic disk. This is likely related to the fact that the bulk of the gas in the CMZ is in a warm (>100 K) and turbulent phase with little material in the prestellar phase. We present in this Letter observations of deuterium fractionation (D/H ratios) of HCN, HNC, HCO⁺, and N₂H⁺ toward the CMZ molecular cloud G+0.693–0.027. These observations clearly show, for the first time, the presence of a colder, denser, and less turbulent narrow component, with a line width of ∼9 km s⁻¹, in addition to the warm, less dense, and turbulent broad component with a line width of ∼20 km s⁻¹. The very low D/H ratio ≤6 × 10⁻⁵ for HCO⁺ and N₂H⁺, close to the cosmic value (∼2.5 × 10⁻⁵), and the high D/H ratios, >4 × 10⁻⁴ for HCN and HNC, derived for the broad component confirm the presence of high-temperature deuteration routes for nitriles. For the narrow component we have derived D/H ratios >10⁻⁴ and excitation temperatures of 7 K for all molecules, suggesting kinetic temperatures ≤30 K and H₂ densities ≥5 × 10⁴ cm⁻³, at least one order of magnitude larger than that for the broad component. The method presented in this Letter allows us to identify clouds on the verge of star formation, i.e., under prestellar conditions, toward the CMZ. This method can also be used for the identification of such clouds in external galaxies.

Understanding the origin of accretion and dispersal of protoplanetary disks is fundamental for investigating planet formation. Recent numerical simulations show that launching winds are unavoidable when disks undergo magnetically driven accretion and/or are exposed to external UV radiation. Observations also hint that disk winds are common. We explore how the resulting wind mass loss rate can be used as a probe of both disk accretion and dispersal. As a proof-of-concept study, we focus on magnetocentrifugal winds, magnetorotational instability turbulence, and external photoevapotaion. By developing a simple yet physically motivated disk model and coupling it with simulation results available in the literature, we compute the wind mass loss rate as a function of external UV flux for each mechanism. We find that different mechanisms lead to different levels of mass loss rate, indicating that the origin of disk accretion and dispersal can be determined, by observing the wind mass loss rate resulting from each mechanism. This determination provides important implications for planet formation. This work thus shows that the ongoing and future observations of the wind mass loss rate for protoplanetary disks are paramount to reliably constrain how protoplanetary disks evolve with time and how planet formation takes place in the disks.

We searched through the entire Gaia EDR3 candidate white dwarf catalog for stars with proper motions and positions that are consistent with them having escaped from the Alpha Persei cluster within the past 81 Myr, the age of the cluster. In this search we found five candidate white dwarf escapees from Alpha Persei and obtained spectra for all of them. We confirm that three are massive white dwarfs sufficiently young to have originated in the cluster. All these are more massive than any white dwarf previously associated with a cluster using Gaia astrometry, and possess some of the most massive progenitors. In particular, the white dwarf Gaia EDR3 4395978097863572, which lies within 25 pc of the cluster center, has a mass of about 1.20 solar masses and evolved from an 8.5 solar-mass star, pushing the upper limit for white dwarf formation from a single massive star, while still leaving a substantial gap between the resulting white dwarf mass and the Chandrasekhar mass.

We present three new spectra of the nearby Type Ia supernova (SN Ia) 2011fe covering ≈480–850 days after maximum light and show that the ejecta undergoes a rapid ionization shift at ∼500 days after explosion. The prominent Fe iii emission lines at ≈4600 Å are replaced with Fe i+Fe ii blends at ∼4400 Å and ∼5400 Å. The ≈7300 Å feature, which is produced by [Fe ii]+[Ni ii] at ≲400 days after explosion, is replaced by broad (≈±15,000 km s⁻¹) symmetric [Ca ii] emission. Models predict this ionization transition occurring ∼100 days later than what is observed, which we attribute to clumping in the ejecta. Finally, we use the nebular-phase spectra to test several proposed progenitor scenarios for SN 2011fe. Nondetections of H and He exclude nearby nondegenerate companions, [O i] nondetections disfavor the violent merger of two white dwarfs, and the symmetric emission-line profiles favor a symmetric explosion.

We analyze the micro-kinetic stability of the electron strahl in the solar wind depending on heliocentric distance. The oblique fast-magnetosonic/whistler (FM/W) instability has emerged in the literature as a key candidate mechanism for the effective scattering of the electron strahl into the electron halo population. Using data from the Parker Solar Probe (PSP) and Helios, we compare the measured strahl properties with the analytical thresholds for the oblique FM/W instability in the low- and high-β_∥c regimes, where β_∥c is the ratio of the core parallel thermal pressure to the magnetic pressure. Our PSP and Helios data show that the electron strahl is on average stable against the oblique FM/W instability in the inner heliosphere. Our analysis suggests that the instability, if at all, can only be excited sporadically and on short timescales. We discuss the caveats of our analysis and potential alternative explanations for the observed scattering of the electron strahl in the solar wind. Furthermore, we recommend the numerical evaluation of the stability of individual distributions in the future to account for any uncertainties in the validity of the analytical expressions for the instability thresholds.

The structure of the electron diffusion region (EDR) is essential for determining how fast the magnetic energy converts to plasma energy during magnetic reconnection. Conventional knowledge of the diffusion region assumes that the EDR is a single layer embedded within the ion diffusion region (IDR). This paper reports the first observation of two EDRs that stack in parallel within an IDR by the Magnetospheric Multiscale mission. The oblique tearing modes can result in these stacked EDRs. Intense electron flow shear in the vicinity of two EDRs induced electron Kelvin–Helmholtz vortices, which subsequently generated kinetic-scale magnetic peak and holes, which may effectively trap electrons. Our analyses show that both the oblique tearing instability and electron Kelvin–Helmholtz instability are important in three-dimensional reconnection since they can control the electron dynamics and structure of the diffusion region through cross-scale coupling.

If a significant fraction of dark matter is in the form of compact objects, they will cause microlensing effects in the gravitational wave signals observable by LIGO and Virgo. From the nonobservation of microlensing signatures in the binary black hole events from the first two observing runs and the first half of the third observing run, we constrain the fraction of compact dark matter in the mass range 10²–10⁵M_⊙ to be less than ≃50%–80% (details depend on the assumed source population properties and the Bayesian priors). These modest constraints will be significantly improved in the next few years with the expected detection of thousands of binary black hole events, providing a new avenue to probe the nature of dark matter.

Thermal instability is a fundamental process of astrophysical plasmas. It is expected to occur whenever the cooling is dominated by radiation and cannot be compensated for by heating. In this work, we conduct 2.5D radiation MHD simulations with the Bifrost code of an enhanced activity network in the solar atmosphere. Coronal loops are produced self-consistently, mainly through Joule heating, which is sufficiently stratified and symmetric to produce thermal nonequilibrium. During the cooling and driven by thermal instability, coronal rain is produced along the loops. Due to flux freezing, the catastrophic cooling process leading to a rain clump produces a local enhancement of the magnetic field, thereby generating a distinct magnetic strand within the loop up to a few Gauss stronger than the surrounding coronal field. These strands, which can be considered fundamental, are a few hundred kilometers in width, span most of the loop leg, and emit strongly in the UV and extreme UV (EUV), thereby establishing a link between the commonly seen rain strands in the visible spectrum with the observed EUV coronal strands at high resolution. The compression downstream leads to an increase in temperature that generates a plume-like structure, a strongly emitting spicule-like feature, and short-lived brightening in the UV during the rain impact, providing an explanation for similar phenomena seen with IRIS. Thermal instability and nonequilibrium can therefore be associated with localized and intermittent UV brightening in the transition region and chromosphere, as well as contribute to the characteristic filamentary morphology of the solar corona in the EUV.

The empirical relationship between the nonthermal 5 GHz radio luminosity and the soft X-ray luminosity of active stellar coronae, canonically called the Güdel–Benz relationship, has been a cornerstone of stellar radio astronomy, as it explicitly ties the radio emission to the coronal heating mechanisms. The relationship extends from microflares on the Sun to the coronae of the most active stars suggesting that active coronae are heated by a flare-like process. The relationship is thought to originate from a consistent partition of the available flare energy into relativistic charges, which emit in the radio-band via the incoherent gyrosynchrotron mechanism, and heating of the bulk coronal plasma, which emits in the X-ray band via the Bremsstrahlung mechanism. Consequently, coherent emission from stellar and substellar objects is not expected to adhere to this empirical relationship, as it is observed in ultracool dwarf stars and brown dwarfs. Here we report a population of radio-detected chromospherically active stars that surprisingly follow the Güdel–Benz relationship despite their radio emission being classified as coherent emission by virtue of its high circularly polarized fraction and high brightness temperature. Our results prompt a reexamination of the physics behind the Güdel–Benz relationship, its implication for the mechanism of coronal heating and particle acceleration in active stars, and the phenomenological connection between solar and stellar flares.

The detection of a binary neutron star (BNS) merger in 2017 through both gravitational waves and electromagnetic emission opened a new era of multimessenger astronomy. The understanding of the magnetic field amplification triggered by the Kelvin–Helmholtz instability during the merger is still a numerically unresolved problem because of the relevant small scales involved. One of the uncertainties comes from the simplifications usually assumed in the initial magnetic topology of merging neutron stars. We perform high-resolution, convergent large-eddy simulations of BNS mergers, following the newly formed remnant for up to 30 ms. Here we specifically focus on the comparison between simulations with different initial magnetic configurations, going beyond the widespread-used aligned dipole confined within each star. The results obtained show that the initial topology is quickly forgotten, in a timescale of a few milliseconds after the merger. Moreover, at the end of the simulations, the average intensity (B ∼ 10¹⁶G) and the spectral distribution of magnetic energy over spatial scales barely depend on the initial configuration. This is expected due to the small-scale efficient dynamo involved, and thus it holds as long as (i) the initial large-scale magnetic field is not unrealistically high (as often imposed in mergers studies), and (ii) the turbulent instability is numerically (at least partially) resolved, so that the amplified magnetic energy is distributed across a wide range of scales and becomes orders of magnitude larger than the initial one.

A population of more than 50 binary black hole mergers has now been observed by the LIGO and Virgo gravitational-wave observatories. While neutron stars are known to have large velocities associated with impulsive kicks imparted to them at birth in supernovae, whether black holes receive similar kicks, and of what magnitude, remains an open question. Recently, Callister et al. analyzed the binary black hole population under the hypothesis that they were all formed through isolated binary evolution and claimed that large black hole kicks (greater than 260 km s⁻¹ at 99% confidence) were required for the spin distribution of merging binary black holes to match observations. Here we highlight that a key assumption made by Callister et al.—that all secondary black holes can be tidally spun up—is not motivated by physical models and may lead to a bias in their estimate of the magnitudes of black hole kicks. We make only minor changes to the Callister et al. model, accounting for a population of wider merging binaries where tidal synchronization is ineffective. We show that this naturally produces a bimodal spin distribution for secondary black holes and that the spin–orbit misalignments observed in the binary black hole population can be explained by more typical black hole kicks of order 100 km s⁻¹, consistent with kicks inferred from Galactic X-ray binaries containing black holes. We conclude that the majority of the binary black hole population is consistent with forming through isolated binary evolution.

We present the discovery of neutral gas detected in both damped Lyα absorption (DLA) and H i 21 cm emission outside of the stellar body of a galaxy, the first such detection in the literature. A joint analysis between the Cosmic Ultraviolet Baryon Survey and the MeerKAT Absorption Line Survey reveals an H i bridge connecting two interacting dwarf galaxies (log (M_star/M_⊙) = 8.5 ± 0.2) that host a z = 0.026 DLA with log[N(H i)/cm⁻²] = 20.60 ± 0.05 toward the QSO J2339−5523 (z_QSO = 1.35). At impact parameters of d = 6 and 33 kpc, the dwarf galaxies have no companions more luminous than ≈0.05L_* within at least Δv = ±300 km s⁻¹ and d ≈ 350 kpc. The H i 21 cm emission is spatially coincident with the DLA at the 2σ–3σ level per spectral channel over several adjacent beams. However, H i 21 cm absorption is not detected against the radio-bright QSO; if the background UV and radio sources are spatially aligned, the gas is either warm or clumpy (with a spin temperature to covering factor ratio T_s/f_c > 1880 K). Observations with VLT-MUSE demonstrate that the α-element abundance of the ionized interstellar medium (ISM) is consistent with the DLA (≈10% solar), suggesting that the neutral gas envelope is perturbed ISM gas. This study showcases the impact of dwarf–dwarf interactions on the physical and chemical state of neutral gas outside of star-forming regions. In the SKA era, joint UV and H i 21 cm analyses will be critical for connecting the cosmic neutral gas content to galaxy environments.

Magnetic reconnection plays an important role in converting energy while modifying field topology. This process takes place under varied plasma conditions during which the transport of magnetic flux is intrinsic. Identifying active magnetic reconnection sites with in situ observations is challenging. A new technique, Magnetic Flux Transport (MFT) analysis, has been developed recently and proven in numerical simulation for identifying active reconnection efficiently and accurately. In this study, we examine the MFT process in 37 previously reported electron diffusion region (EDR)/reconnection-line crossing events at the day-side magnetopause and in the magnetotail and turbulent magnetosheath using Magnetospheric Multiscale measurements. The coexisting inward and outward MFT flows at an X-point provides a signature that magnetic field lines become disconnected and reconnected. The application of MFT analysis to in-situ observations demonstrates that MFT can successfully identify active reconnection sites under complex varied conditions, including asymmetric and turbulent upstream conditions. It also provides a higher rate of identification than plasma outflow jets alone. MFT can be applied to in situ measurements from both single- and multi-spacecraft missions and laboratory experiments.

Most large galaxies host supermassive black holes in their nuclei and are subject to mergers, which can produce a supermassive black hole binary (SMBHB), and hence periodic signatures due to orbital motion. We report unique periodic radio flux density variations in the blazar PKS 2131−021, which strongly suggest an SMBHB with an orbital separation of ∼0.001–0.01 pc. Our 45.1 yr radio light curve shows two epochs of strong sinusoidal variation with the same period and phase to within ≲2% and ∼10%, respectively, straddling a 20 yr period when this variation was absent. Our simulated light curves accurately reproduce the "red noise" of this object, and Lomb–Scargle, weighted wavelet Z-transform and least-squares sine-wave analyses demonstrate conclusively, at the 4.6σ significance level, that the periodicity in this object is not due to random fluctuations in flux density. The observed period translates to 2.082 ± 0.003 yr in the rest frame at the z = 1.285 redshift of PKS 2131−021. The periodic variation in PKS 2131−021 is remarkably sinusoidal. We present a model in which orbital motion, combined with the strong Doppler boosting of the approaching relativistic jet, produces a sine-wave modulation in the flux density that easily fits the observations. Given the rapidly developing field of gravitational-wave experiments with pulsar timing arrays, closer counterparts to PKS 2131−021 and searches using the techniques we have developed are strongly motivated. These results constitute a compelling demonstration that the phenomenology, not the theory, must provide the lead in this field.

The astrophysical origins of r-process elements remain elusive. Neutron star mergers (NSMs) and special classes of core-collapse supernovae (rCCSNe) are leading candidates. Due to these channels' distinct characteristic timescales (rCCSNe: prompt, NSMs: delayed), measuring r-process enrichment in galaxies of similar mass but differing star formation durations might prove informative. Two recently discovered disrupted dwarfs in the Milky Way's stellar halo, Kraken and Gaia-Sausage Enceladus (GSE), afford precisely this opportunity: Both have M_⋆ ≈ 10⁸ M_⊙ but differing star formation durations of ≈2 Gyr and ≈3.6 Gyr. Here we present R ≈ 50,000 Magellan/MIKE spectroscopy for 31 stars from these systems, detecting the r-process element Eu in all stars. Stars from both systems have similar [Mg/H] ≈ −1, but Kraken has a median [Eu/Mg] ≈ −0.1 while GSE has an elevated [Eu/Mg] ≈ 0.2. With simple models, we argue NSM enrichment must be delayed by 500–1000 Myr to produce this difference. rCCSNe must also contribute, especially at early epochs, otherwise stars formed during the delay period would be Eu free. In this picture, rCCSNe account for ≈50% of the Eu in Kraken, ≈25% in GSE, and ≈15% in dwarfs with extended star formation durations like Sagittarius. The inferred delay time for NSM enrichment is 10×–100× longer than merger delay times from stellar population synthesis—this is not necessarily surprising because the enrichment delay includes time taken for NSM ejecta to be incorporated into subsequent generations of stars. For example, this may be due to natal kicks that result in r-enriched material deposited far from star-forming gas, which then takes ≈10⁸–10⁹ yr to cool in these galaxies.

Jupiter's atmosphere is enriched with heavy elements by a factor of about 3 compared to a protosolar composition. The origin of this enrichment and whether it represents the bulk composition of the planetary envelope remain unknown. Internal structure models of Jupiter suggest that its envelope is separated from the deep interior and that the planet is not fully mixed. This implies that Jupiter's atmosphere was enriched with heavy elements just before the end of its formation. Such enrichment can be a result of late planetesimal accretion. However, in situ Jupiter formation models suggest a decreasing accretion rate with increasing planetary mass, which cannot explain Jupiter's atmospheric enrichment. In this study, we model Jupiter's formation and show that the migration of proto-Jupiter from ∼20 au to its current location can lead to late planetesimal accretion and atmospheric enrichment. Late planetesimal accretion does not occur if proto-Jupiter migrates only a few astronomical units. We suggest that if Jupiter's outermost layer is fully mixed and is relatively thin (up to ∼20% of its mass), such late accretion can explain its measured atmospheric composition. It is therefore possible that Jupiter underwent significant orbital migration followed by late planetesimal accretion.

The velocity of alpha particles relative to protons can vary depending on the solar wind type and distance from the Sun. Measurements from the previous spacecraft provided the alpha–proton differential velocities down to 0.3 au. The Parker Solar Probe (PSP) now enables insights into differential flows of the newly accelerated solar wind closer to the Sun for the first time. Here we study the difference between proton and alpha bulk velocities near PSP perihelia of encounters 3–7 when the core solar wind is in the field of view of the Solar Probe Analyzer for Ions instrument. As previously reported at larger heliospheric distances, the alpha–proton differential speed observed by PSP is greater for fast wind than the slow solar wind. We compare PSP observations with various spacecraft measurements and present the radial and temporal evolution of the alpha–proton differential speed. The differential flow decreases as the solar wind propagates from the Sun, consistent with previous observations. While Helios showed a small radial dependence of differential flow for the slow solar wind, PSP clearly showed this dependency for the young slow solar wind down to 0.09 au. Our analysis shows that the alpha–proton differential speed's magnitude is mainly below the local Alfvén speed. Moreover, alpha particles usually move faster than protons close to the Sun. The PSP crossed the Alfvén surface during its eighth encounter and may cross it in future encounters, enabling us to investigate the differential flow very close to the solar wind acceleration source region for the first time.

Using imaging and radio multi-wavelength observations, we studied the origin of two homologous accelerated electron beams and a quasiperiodic fast-propagating (QFP) wave train associated with a solar jet on 2012 July 14. The jet occurred in a small-scale fan-spine magnetic system embedded in a large-scale pseudostreamer associated with a GOES C1.4 flare, a jet-like coronal mass ejection (CME), a type II radio burst, and a type III radio burst. During the initial stage, a QFP wave train and a fast-moving on-disk radio source were detected in succession ahead of the jet along the outer spine of the fan-spine system. When the jet reached a height of about 1.3 solar radii, it underwent a bifurcation into two branches. Based on our analysis results, all the observed phenomena in association with the jet can be explained by using a fan-spine magnetic system. We propose that both the type III radio burst and the on-disk fast-moving radio source were caused by the same physical process, i.e., energetic electrons accelerated by magnetic reconnection at the null point, and these energetic electrons were propagating along the open field lines of the pseudostreamer and the closed outer spine of the fan-spine structure, respectively. Due to the bifurcation of the jet body, the lower branch along the closed outer spine of the fan-spine structure fell back to the solar surface, while the upper branch along the open field lines of the pseudostreamer caused the jet-like CME in the outer corona.