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1. Long-term Evolution of Tightly Packed Stellar Black Holes in AGN Disks: Formation of Merging Black Hole Binaries via Close Encounters

   https://doi.org/10.3847/1538-4357/ac7c0d  

   Li, Jiaru ; Lai, Dong ; Rodet, Laetitia ( August 2022 , The Astrophysical Journal)

   We study the long-term evolution of two or more stellar black holes (BHs) on initially separated but unstable circular orbits around a supermassive BH (SMBH). Such a close-packed orbital configuration can naturally arise from BH migrations in the AGN disk. Dynamical instability of the orbits leads to recurring close encounters between two BHs, during which the BH separationr_pbecomes less than the Hill radiusR_H. In rare very close encounters, a tight merging BH binary can form with the help of gravitational wave emission. We useN-body simulations to study the time evolution of close encounters of various degrees ofcloseness. For a typical “SMBH+2BH” system, the averaged cumulative number of close encounters (withr_p≲R_H) scales approximately as ∝t^0.5. The minimum encounter separationr_pfollows a cumulative distributionP(<r_p) ∝r_pforr_p≪R_H. We obtain a semi-analytical expression for the averaged rate of binary captures that lead to BH mergers. Our results suggest that close-packed BHs in AGN disks may take a long time (≳10⁷orbits around the SMBH) to experience a sufficiently close encounter and form a bound binary. This time can be shorter if the initial BH orbits are highly aligned. The BH binary mergers produced in this scenario have high eccentricities when entering the LIGO band and broad distribution of orbital inclinations relative to the original AGN disk. We explore the effects of the gas disk and find that simple gas drags on the BHs do not necessarily lead to an enhanced BH binary capture rate.

    

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2. “Tidal Peeling Events”: Low-eccentricity Tidal Disruption of a Star by a Stellar-mass Black Hole

   https://doi.org/10.3847/1538-4357/ad11d3  

   Xin, Chengcheng ; Haiman, Zoltán ; Perna, Rosalba ; Wang, Yihan ; Ryu, Taeho ( January 2024 , The Astrophysical Journal)

   Close encounters between stellar-mass black holes (BHs) and stars occur frequently in dense star clusters and in the disks of active galactic nuclei. Recent studies have shown that in highly eccentric close encounters, the star can be tidally disrupted by the BH in a microtidal disruption event (microTDE), resulting in rapid mass accretion and possibly bright electromagnetic signatures. Here we consider a scenario in which the star might approach the stellar-mass BH in a gradual, nearly circular inspiral, under the influence of dynamical friction in a circum-binary gas disk or three-body interactions in a star cluster. We perform hydrodynamics simulations of this scenario using the smoothed particle hydrodynamics codePHANTOM. We find that under certain circumstances (for initial eccentricitye₀≳ 0.4 and penetration factorβ= 1, ore₀< 0.4 andβ≲ 0.67), the mass of the star is slowly stripped away by the BH. We call this gradual tidal disruption a “tidal-peeling event.” Additionally, we discover that some low-eccentricity microTDEs (e₀< 0.4 andβ= 1) are a new form of fast luminous transients similar to parabolic microTDEs. Depending on the initial distance and eccentricity of the encounter, these low-eccentricity microTDEs might exhibit significant accretion rates and orbital evolution distinct from those of a typical (eccentric) microTDE.

    

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3. Dynamical Friction Models for Black Hole Binary Formation in Active Galactic Nucleus Disks

   https://doi.org/10.3847/1538-4357/ad1b53  

   Qian, Kecheng ; Li, Jiaru ; Lai, Dong ( February 2024 , The Astrophysical Journal)

   Stellar-mass black holes (sBHs) embedded in gaseous disks of active galactic nuclei (AGN) can be important sources of detectable gravitational radiation for LIGO/Virgo when they form binaries and coalesce due to orbital decay. In this paper, we study the effect of dynamical friction (DF) on the formation of BH binaries in AGN disks usingN-body simulations. We employ two simplified models of DF, with the force on the BH depending on Δv, the velocity of the sBH relative to the background Keplerian gas. We integrate the motion of two sBHs initially on circular orbits around the central supermassive black hole (SMBH) and evaluate the probability of binary formation under various conditions. We find that both models of DF (with different dependence of the frictional coefficient on ∣Δv∣) can foster the formation of binaries when the effective friction timescaleτsatisfies Ω_Kτ≲ 20–30 (where Ω_Kis the Keplerian frequency around the SMBH): prograde binaries are formed when the DF is stronger (smallerτ), while retrograde binaries dominate when the DF is weaker (largerτ). We determine the distribution of both prograde and retrograde binaries as a function of initial orbital separation and the DF strength. Using our models of DF, we show that for a given sBH number density in the AGN disk, the formation rate of sBH binaries increases with decreasingτand can reach a moderate value with a sufficiently strong DF.

    

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4. Sites of Planet Formation in Binary Systems. I. Evidence for Disk−Orbit Alignment in the Close Binary FO Tau

   https://doi.org/10.3847/1538-3881/ad354d  

   Tofflemire, Benjamin M. ; Prato, Lisa ; Kraus, Adam L. ; Segura-Cox, Dominique ; Schaefer, G. H. ; Akeson, Rachel ; Andrews, Sean ; Jensen, Eric L. N. ; Johns-Krull, Christopher M. ; Zanazzi, J. J. ; et al ( April 2024 , The Astronomical Journal)

   Close binary systems present challenges to planet formation. As binary separations decrease, so do the occurrence rates of protoplanetary disks in young systems and planets in mature systems. For systems that do retain disks, their disk masses and sizes are altered by the presence of the binary companion. Through the study of protoplanetary disks in binary systems with known orbital parameters, we seek to determine the properties that promote disk retention and therefore planet formation. In this work, we characterize the young binary−disk system FO Tau. We determine the first full orbital solution for the system, finding masses of${0.35}_{-0.05}^{+0.06}{M}_{\odot }$and 0.34 ± 0.05M_⊙for the stellar components, a semimajor axis of$22{(}_{-1}^{+2})$au, and an eccentricity of$0.21{(}_{-0.03}^{+0.04})$. With long-baseline Atacama Large Millimeter/submillimeter Array interferometry, we detect 1.3 mm continuum and¹²CO (J= 2–1) line emission toward each of the binary components; no circumbinary emission is detected. The protoplanetary disks are compact, consistent with being truncated by the binary orbit. The dust disks are unresolved in the image plane, and the more extended gas disks are only marginally resolved. Fitting the continuum and CO visibilities, we determine the inclination of each disk, finding evidence for alignment of the disk and binary orbital planes. This study is the first of its kind linking the properties of circumstellar protoplanetary disks to a precisely known binary orbit. In the case of FO Tau, we find a dynamically placid environment (coplanar, low eccentricity), which may foster its potential for planet formation.

    

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5. Jets with a Twist: The Emergence of FR0 Jets in a 3D GRMHD Simulation of Zero-angular-momentum Black Hole Accretion

   https://doi.org/10.3847/1538-4357/ad0974  

   Lalakos, Aretaios ; Tchekhovskoy, Alexander ; Bromberg, Omer ; Gottlieb, Ore ; Jacquemin-Ide, Jonatan ; Liska, Matthew ; Zhang, Haocheng ( March 2024 , The Astrophysical Journal)

   Spinning supermassive black holes (BHs) in active galactic nuclei magnetically launch relativistic collimated outflows, or jets. Without angular momentum supply, such jets are thought to perish within 3 orders of magnitude in distance from the BH, well before reaching kiloparsec scales. We study the survival of such jets at the largest scale separation to date, via 3D general relativistic magnetohydrodynamic simulations of rapidly spinning BHs immersed into uniform zero-angular-momentum gas threaded by a weak vertical magnetic field. We place the gas outside the BH sphere of influence, or the Bondi radius, chosen to be much larger than the BH gravitational radius,R_B= 10³R_g. The BH develops dynamically important large-scale magnetic fields, forms a magnetically arrested disk (MAD), and launches relativistic jets that propagate well outsideR_Band suppress BH accretion to 1.5% of the Bondi rate,${\dot{M}}_{\mathrm{B}}$. Thus, low-angular-momentum accretion in the MAD state can form large-scale jets in Fanaroff–Riley (FR) type I and II galaxies. Subsequently, the disk shrinks and exits the MAD state: barely a disk (BAD), it rapidly precesses, whips the jets around, globally destroys them, and lets 5%–10% of${\dot{M}}_{\mathrm{B}}$reach the BH. Thereafter, the disk starts rocking back and forth by angles 90°–180°: the rocking accretion disk (RAD) launches weak intermittent jets that spread their energy over a large area and suppress BH accretion to ≲2%${\dot{M}}_{\mathrm{B}}$. Because the BAD and RAD states tangle up the jets and destroy them well insideR_B, they are promising candidates for the more abundant, but less luminous, class of FR0 galaxies.

    

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