Hongping Deng

Massive moons around rocky exoplanets are expected outcomes of giant impacts, yet no exomoon has been securely confirmed. We address this problem using meshless finite-mass (MFM) simulations of giant impacts between differentiated rocky planets that include stellar tidal and Coriolis forces in a local corotating frame for orbital periods of 1–300 days, and compare these simulations with otherwise identical collisions in isolation. We find that stellar perturbations impose a severe formation-stage bottleneck on moon-forming disks. Ultra-short-period impacts leave essentially no surviving circumplanetary disk (CPD), whereas 10 day systems retain only strongly depleted, radially truncated, and dynamically compact disks. High impact velocities, expected for close-in planets, are devastating for disk generation, while retrograde impacts can slightly compensate for disk mass depletion. These results show that stellar perturbations can suppress exomoon formation around close-in rocky planets, reshaping expectations for the demographics of exomoons.

Classical pebble and core accretion theories face a severe challenge: they struggle to produce giant planets around low-mass M dwarfs, whose protoplanetary disks contain insufficient material. Yet, discoveries of such systems continue to mount. We propose flyby-induced second accretion (FISA), a new formation channel in which an M dwarf already hosting a terrestrial planet flies through a younger star’s protoplanetary disk, capturing gas and dust to build a second-generation disk. The preexisting planet then undergoes renewed pebble and gas accretion within this replenished reservoir and migrates inward to become a close-in giant. We performed hydrodynamic and core-accretion simulations to show that, under favorable encounter geometries, protoplanets around M dwarfs can grow into gas giants via this two-stage process. We apply the FISA framework to TOI-6894 b, a 0.168 Jupiter-mass planet orbiting a 0.207 solar-mass star at just 0.026 au, showing that it is a plausible outcome of such a flyby. An order-of-magnitude estimation shows that the probability for a close-in gas giant to form in an M dwarf system via the proposed disk-flyby mechanism is roughly 3 × 10 ⁻⁵ .

Calcium-aluminum-rich inclusions (CAIs) in carbonaceous chondritic meteorites are the oldest relics in the solar system. Notably, their radiogenic ages indicate a brief (100 kyr) condensation episode. In contrast, the reservoirs of the short-lived isotopes in CAIs, presumably supernovae or asymptotic giant stars, pollutes star-forming regions in giant molecular cloud complexes (GMC) over much longer (Myr) timescales. Through a series of numerical simulations, we show here the possibility that, within an extended region (~2–3 au), nearly all “presolar” CAI-loaded grains in the infalling clouds were sublimated and subsequently recondensed during the early (≲10 ⁵ yr) infall and formation phases of Class 0 disks. We adopt a set of initial conditions from a previous hydrodynamic simulation of GMC collapse and young stellar cluster formation. We analyze the evolution of the disk’s thermal distribution and dynamical structure resulting from the interaction between circumstellar disks and infalling gas. Our follow-up simulations, performed at much higher resolution, show significant and rapid changes in disk orientation and morphology due to the dynamic infall of external streamers. Warps and global spiral density waves commonly emerge, leading to intense dissipation that heats the gas to sufficiently high temperatures to sublimate prior-generation CAIs. This solid-to-gas phase transition is followed by subsequent cooling and recondensation. The CAIs contained in meteorites today may therefore be relics of the last major episode of infall onto Class 0 disks.

X-ray quasiperiodic eruptions (QPEs) represent a novel population of extreme, repeating nuclear transients whose physical origins remain debated. A defining characteristic of QPEs has been their exclusive detection in the X-ray band, with a notable absence of correlated multiwavelength counterparts. Here we report the first detection of a recurrent ultraviolet (UV) response temporally coupled to the X-ray QPE signal in the source Ansky/ZTF19acnskyy. The UV emission displays coherent periodic modulations over five consecutive cycles, systematically lagging the X-ray eruptions by 0.9 6 − 0.39 + 0.38 days, with a cross-correlation coefficient of r max ∼ 0.6 . We suggest that the detectability of this corresponding signal may be enabled by Ansky’s unusually long recurrence timescale, which could reduce the temporal smearing of the UV response seen in more rapid QPEs. The observed delay may correspond to a diffusion timescale associated with heated blobs. However, we cannot exclude the possibility that the lag corresponds to the light-crossing time associated with X-ray irradiation that originates near the central black hole and propagates to the outer UV-emitting region. While numerous QPE models have been proposed, any viable model for Ansky must be able to simultaneously explain the presence of a UV counterpart, its measured time lag, and the previously observed steadily increasing recurrence period.

We present a detailed analysis of the nature of migration of protoplanetary clumps formed via disc instability in self-consistent 3D hydrodynamical (HD) and magnetohydrodynamical (MHD) simulations of self-gravitating discs. Motivated by the complex structure of protoplanetary clumps we do not introduce sink particles. We find that the orbital evolution of the clumps has a stochastic character but also exhibits recurrent properties over many orbits. Clump migration is governed by two sources of gravitational torques: a torque originating from a region about twice the Hill sphere around each clump’s orbit, and the torque resulting from clump–clump interactions. Compared to non-magnetized companion runs, the latter are more frequent in MHD simulations, which give rise to more numerous clumps starting off at smaller masses, often below a Neptune mass. Clump–clump interactions can lead to temporary strong accelerations of migration in both directions, but integrated over time provide a lesser impact than disc-driven torques. They can also lead to clump mergers but do not cause ejections; a difference to previous works which adopted sink particles. The local “Hill torque” is responsible for the fast migration, inward or outward. Estimating the characteristic time-scales of conventional migration in our regime, we find that the disc-driven migration time-scales are in agreement with Type III migration. However, the dominant local torque is rapidly fluctuating, which reflects the turbulent nature of the flow. The resulting stochastic migration pattern is markedly different from Type III runaway migration and appears to be a distinctive feature of orbital dynamics in a fragmenting disc.

A black hole (BH) can tear apart a star that ventures within its tidal radius, producing a luminous flare as the stellar debris falls back, known as a tidal disruption event (TDE). While TDEs in quiescent galaxies are relatively well understood, identifying TDEs in active galactic nuclei (AGNs) still remains a significant challenge. We present the discovery of AT2021aeuk, a transient exhibiting dual flares within around 3 yr in a narrow-line Seyfert 1 galaxy. Multiwavelength observations triggered during the second flare in 2023 revealed an extraordinary X-ray V-shaped light curve, strongly anticorrelated with the optical light curve and accompanied by a lag of ∼40 days. This behavior is inconsistent with both supernova and pure AGN origins. In addition, a new broad component emerges in the Balmer lines during the second flare, showing a clear reverberation signal to the continuum variation. We propose that the dual flare may be linked to a repeating partial TDE (rpTDE), where the second flare results from a collision between the TDE stream and the inner accretion disk, triggering an optical flare while simultaneously partially destroying the X-ray corona. However, other mechanisms, such as a stellar-mass BH merger within an accretion disk, could produce similar phenomena, which we cannot entirely rule out. The Vera C. Rubin Observatory will be a powerful tool for further investigating the nature of such events in the future.

The origin of planetary mass objects (PMOs) wandering in young star clusters remains enigmatic, especially when they come in pairs. They could represent the lowest-mass object formed via molecular cloud collapse or high-mass planets ejected from their host stars. However, neither theory fully accounts for their abundance and multiplicity. Here, we show via hydrodynamic simulations that free-floating PMOs have a unique formation channel via the fragmentation of tidal bridges between encountering circumstellar disks. This process can be highly productive in dense clusters like Trapezium forming metal-poor PMOs with disks. Free-floating multiple PMOs also naturally emerge when neighboring PMOs are caught by their mutual gravity. PMOs may thus form a distinct population that is fundamentally different from stars and planets.