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1. Gravitational Instability and Fragmentation in Collapsar Disks Supports the Formation of Subsolar Neutron Stars

   https://doi.org/10.3847/2041-8213/ae045d  

   Chen, Yi-Xian; Metzger, Brian D (September 2025, The Astrophysical Journal Letters)

   Abstract We perform three-dimensional shearing box hydrodynamical simulations to explore the outcome of gravitational instability in the outer regions of neutrino-cooled disks such as those formed from the collapse of rotating massive stars (“collapsars”). We employ a physical equation of state and optically thin neutrino cooling and assume an electron fraction set by the balance ofe^±pair-capture reactions. Disks in a marginally stable initial state (Toomre parameterQ ≈ 1) undergo runaway cooling and fragmentation when the dimensionless cooling timescale obeysτ_cool ≡ t_coolΩ ≲ 10, where Ω is the orbital frequency; these conditions correspond to accretion rates ≳M_⊙s⁻¹on the upper end of those achieved by collapsar progenitor stars. Fragmentation leads to the formation of neutron-rich clumps (electron fractionY_e ≲ 0.1) spanning a range of masses ∼0.01–1M_⊙around the local Jeans value. Most clumps exceed the local Chandrasekhar mass $M_{\mathrm{Ch}}\propto Y_e^2$and hence will continue to collapse to nuclear densities, forming neutron stars (NSs) with subsolar masses otherwise challenging to create through ordinary stellar core collapse. Even cool disks dominated byαparticles (Y_e ≃ 0.5) can fragment and collapse into neutron-rich clumps capable of forming subsolar NSs. Although our simulations cannot follow this process directly, if the disk-formed NSs subsequently pair into binaries, the GW chirps from their rapid mergers are potentially detectable by ground-based observatories. The temporal coincidence of such a hierarchical NS merger chain with the collapsar gamma-ray burst and supernova would offer a uniquely spectacular multimessenger “symphony.” 

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2. A Solution for the Density Dichotomy Problem of Kuiper Belt Objects with Multispecies Streaming Instability and Pebble Accretion

   https://doi.org/10.3847/PSJ/ad1d5b  

   Cañas, Manuel H; Lyra, Wladimir; Carrera, Daniel; Krapp, Leonardo; Sengupta, Debanjan; Simon, Jacob B; Umurhan, Orkan M; Yang_楊, Chao-Chin 朝欽; Youdin, Andrew N (February 2024, The Planetary Science Journal)

   Abstract Kuiper Belt objects (KBOs) show an unexpected trend, whereby large bodies have increasingly higher densities, up to five times greater than their smaller counterparts. Current explanations for this trend assume formation at constant composition, with the increasing density resulting from gravitational compaction. However, this scenario poses a timing problem to avoid early melting by decay of²⁶Al. We aim to explain the density trend in the context of streaming instability and pebble accretion. Small pebbles experience lofting into the atmosphere of the disk, being exposed to UV and partially losing their ice via desorption. Conversely, larger pebbles are shielded and remain icier. We use a shearing box model including gas and solids, the latter split into ices and silicate pebbles. Self-gravity is included, allowing dense clumps to collapse into planetesimals. We find that the streaming instability leads to the formation of mostly icy planetesimals, albeit with an unexpected trend that the lighter ones are more silicate-rich than the heavier ones. We feed the resulting planetesimals into a pebble accretion integrator with a continuous size distribution, finding that they undergo drastic changes in composition as they preferentially accrete silicate pebbles. The density and masses of large KBOs are best reproduced if they form between 15 and 22 au. Our solution avoids the timing problem because the first planetesimals are primarily icy and²⁶Al is mostly incorporated in the slow phase of silicate pebble accretion. Our results lend further credibility to the streaming instability and pebble accretion as formation and growth mechanisms. 

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3. The Interstellar Interlopers

   https://doi.org/10.1146/annurev-astro-071221-054221  

   Jewitt, David; Seligman, Darryl Z. (August 2023, Annual Review of Astronomy and Astrophysics)

   Interstellar interlopers are bodies formed outside of the Solar System but observed passing through it. The first two identified interlopers, 1I/‘Oumuamua and 2I/Borisov, exhibited unexpectedly different physical properties. 1I/‘Oumuamua appeared unresolved and asteroid-like, whereas 2I/Borisov was a more comet-like source of both gas and dust. Both objects moved under the action of nongravitational acceleration. These interlopers and their divergent properties provide our only window so far onto an enormous and previously unknown galactic population. The number density of such objects is ∼0.1 AU⁻³which, if uniform across the galactic disk, would imply 10²⁵to 10²⁶similar objects in the Milky Way. The interlopers likely formed in, and were ejected from, the protoplanetary disks of young stars. However, we currently possess too little data to firmly reject other explanations. ▪ 1I/‘Oumuamua and 2I/Borisov are both gravitationally unbound, subkilometer bodies showing nongravitational acceleration. ▪ The acceleration of 1I/‘Oumuamua in the absence of measurable mass loss requires either a strained explanation in terms of recoil from sublimating supervolatiles or the action of radiation pressure on a nucleus with an ultralow mass column density, ∼1 kg m⁻². ▪ 2I/Borisov is a strong source of CO and H₂O, which together account for its activity and nongravitational acceleration. ▪ The interlopers are most likely planetesimals from the protoplanetary disks of other stars, ejected by gravitational scattering from planets. 1I/‘Oumuamua and 2I/Borisov have dynamical ages ∼10⁸and ∼10⁹years, respectively. ▪ Forthcoming observatories should detect interstellar interlopers every year, which will provide a rapid boost to our knowledge of the population. 

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4. Reactions of Aluminum Oxide Cluster Cations with Ethane: A Mass‐Spectrometric and Vibrational Spectroscopy Study

   https://doi.org/10.1002/cphc.202400427  

   Phasuk, Apakorn; Metz, Ricardo B (December 2024, ChemPhysChem)

   Abstract The pathways for the reactions of aluminum oxide cluster ions with ethane have been measured. For selected ions (Al₂O⁺, Al₃O₂⁺, Al₃O₄⁺, Al₄O₇⁺) the structure of the collisionally‐stabilized reaction intermediates were explored by measuring the photodissociation vibrational spectra from 2600 cm⁻¹–3100 cm⁻¹. Density functional theory was used to calculate features of the potential energy surfaces for the reactions and the vibrational spectra of intermediates. Generally, more than one isomer contributes to the observed spectrum. The oxygen‐deficient clusters Al₂O⁺and Al₃O₂⁺have large C−H activation barriers, so only the entrance channel complexes in which intact C₂H₆binds to aluminum are observed. This interaction leads to a substantial (~200 cm⁻¹) red shift of the C−H symmetric stretch in ethane, indicating significant weakening of the proximal C−H bonds. In Al₃O₄⁺, the complex formed by interactions with three C₂H₆is investigated and, in addition to entrance channel complexes, the C−H activation intermediate Al₃O₄H⁺(C₂H₅)(C₂H₆)₂is observed. For oxygen‐rich Al₄O₇⁺, the C₂H₆is favored to bind at an aluminum site far from the reactive superoxide group, reducing the reactivity. As expected, oxygen‐rich species and open‐shell cluster ions have smaller barriers for C−H bond activation, except for Al₃O₄⁺which is predicted and observed to be reactive. 

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5. Effects of initial density profiles on massive star cluster formation in giant molecular clouds

   https://doi.org/10.1093/mnras/stab491  

   Chen, Yingtian; Li, Hui; Vogelsberger, Mark (March 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We perform a suite of hydrodynamic simulations to investigate how initial density profiles of giant molecular clouds (GMCs) affect their subsequent evolution. We find that the star formation duration and integrated star formation efficiency of the whole clouds are not sensitive to the choice of different profiles but are mainly controlled by the interplay between gravitational collapse and stellar feedback. Despite this similarity, GMCs with different profiles show dramatically different modes of star formation. For shallower profiles, GMCs first fragment into many self-gravitation cores and form subclusters that distributed throughout the entire clouds. These subclusters are later assembled ‘hierarchically’ to central clusters. In contrast, for steeper profiles, a massive cluster is quickly formed at the centre of the cloud and then gradually grows its mass via gas accretion. Consequently, central clusters that emerged from clouds with shallower profiles are less massive and show less rotation than those with the steeper profiles. This is because (1) a significant fraction of mass and angular momentum in shallower profiles is stored in the orbital motion of the subclusters that are not able to merge into the central clusters, and (2) frequent hierarchical mergers in the shallower profiles lead to further losses of mass and angular momentum via violent relaxation and tidal disruption. Encouragingly, the degree of cluster rotations in steeper profiles is consistent with recent observations of young and intermediate-age clusters. We speculate that rotating globular clusters are likely formed via an ‘accretion’ mode from centrally concentrated clouds in the early Universe. 

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