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The active galactic nucleus (AGN) disc has been proposed as a potential channel for the merger of binary black holes. The population of massive stars and black holes in AGN discs captured from the nuclei cluster plays a crucial role in determining the efficiency of binary formation and final merger rate within the AGN discs. In this paper, we investigate the capture process using analytical and numerical approaches. We discover a new constant integral of motion for one object’s capture process. Applying this result to the whole population of the nuclei cluster captured by the AGN disc, we find that the population of captured objects depends on the angular density and eccentricity distribution of the nuclei clusters and is effectively independent of the radial density profile of the nuclei cluster and disc models. An isotropic nuclei cluster with thermal eccentricity distribution predicts a captured profile dN/dr ∝ r−1/4. The captured objects are found to be dynamically crowded within the disc. Direct binary formation right after the capture would be promising, especially for stars. The conventional migration traps that help pile up single objects in AGN discs for black hole mergers might not be required.

 

Recent observations of changing-look active galactic nuclei (AGNs) hint at a frequency of accretion activity not fully explained by tidal disruption events (TDEs) stemming from relaxation processes in nuclear star clusters (NSCs), traditionally estimated to occur at rates of 10⁻⁴–10⁻⁵yr⁻¹per galaxy. In this Letter, we propose an enhanced TDE rate through the AGN disk capture process, presenting a viable explanation for the frequent transitions observed in changing-look AGNs. Specifically, we investigate the interaction between the accretion disk and retrograde stars within NSCs, resulting in the rapid occurrence of TDEs within a condensed time frame. Through detailed calculations, we derive the time-dependent TDE rates for both relaxation-induced TDE and disk-captured TDE. Our analysis reveals that TDEs triggered by the disk capture process can notably amplify the TDE rate by several orders of magnitude during the AGN phase. This mechanism offers a potential explanation for the enhanced high-energy variability characteristic of changing-look AGNs.

 

We separately assess elemental abundances in active galactic nuclei's (AGNs) broad and narrow emission line regions (BLR and NLR), based on a critical assessment of published results together with new photoionization models. We find (1) He/H enhancements in some AGN, exceeding what can be explained by normal chemical evolution and confirm, (2) super-solar α abundance, though to a lesser degree than previously reported. We also reaffirm, (3) an N/O ratio consistent with secondary production, (4) solar or slightly sub-solar Fe abundance, and (5) red-shift independent metallicity, in contrast with galactic chemical evolution. We interpret (6) the larger metallicity in the BLR than NRL in terms of an in situ stellar evolution and pollution in AGN discs (SEPAD) model. We attribute (a) the redshift independence to the heavy element pollutants being disposed into the disc and accreted onto the central supermassive black hole (SMBH), (b) the limited He excess to the accretion–wind metabolism of a top-heavy population of evolving massive main sequence stars, (c) the super-solar CNO enrichment to the nuclear synthesis during their post-main-sequence evolution, (d) the large N/O to the byproduct of multiple stellar generations, and (e) the Mg, Si, and Fe to the ejecta of type II supernovae in the disc. These results provide supporting evidence for (f) ongoing self-regulated star formation, (g) adequate stellar luminosity to maintain marginal gravitational stability, (h) prolific production of seeds, and (i) dense coexistence of subsequently grown residual black hole populations in AGN discs.

 

Hydrodynamical interaction in circumbinary discs (CBDs) plays a crucial role in various astrophysical systems, ranging from young stellar binaries to supermassive black hole binaries in galactic centres. Most previous simulations of binary-disc systems have adopted locally isothermal equation of state. In this study, we use the grid-based code Athena++ to conduct a suite of two-dimensional viscous hydrodynamical simulations of circumbinary accretion on a Cartesian grid, resolving the central cavity of the binary. The gas thermodynamics is treated by thermal relaxation towards an equilibrium temperature (based on the constant − β cooling ansatz, where β is the cooling time in units of the local Keplerian time). Focusing on equal mass, circular binaries in CBDs with (equilibrium) disc aspect ratio H/R = 0.1, we find that the cooling of the disc gas significantly influences the binary orbital evolution, accretion variability, and CBD morphology, and the effect depends sensitively on the disc viscosity prescriptions. When adopting a constant kinematic viscosity, a finite cooling time (β ≳ 0.1) leads to a binary inspiral as opposed to an outspiral and the CBD cavity becomes more symmetric. When adopting a dynamically varying α-viscosity, binary inspiral only occurs within a narrow range of cooling time (corresponding to β around 0.5).

 

Abstract Mechanisms have been proposed to enhance the merger rate of stellar-mass black hole binaries, such as the Von Zeipel–Lidov–Kozai mechanism (vZLK). However, high inclinations are required in order to greatly excite the eccentricity and to reduce the merger time through vZLK. Here, we propose a novel pathway through which compact binaries could merge due to eccentricity increase in general, including in a near coplanar configuration. Specifically, a compact binary migrating in an active galactic nucleus disk could be captured in an evection resonance, when the precession rate of the binary equals the orbital period around the supermassive black hole. In our study we include precession due to first-order post-Newtonian precession as well as that due to disk around one or both components of the binary. Eccentricity is excited when the binary sweeps through the resonance, which happens only when it migrates on a timescale 10–100 times the libration timescale of the resonance. Libration timescale decreases as the mass of the disk increases. The eccentricity excitation of the binary can reduce the merger timescale by up to a factor of ∼10 3−5 . 

Mentorship has been established in the literature as being salient to degree completion for doctoral students. Mentoring primarily focuses on the extended academic development of a less experienced student by a more experienced faculty scholar. Federal governance policies have enabled greater participation in STEM by underrepresented populations, and as a result, enrollments in doctoral STEM programs by groups underrepresented in STEM have increased, but their success frequently hinges on support resources such as quality mentorship (Millett & Nettles, 2006). A substantial commitment to high-quality mentoring is needed to best prepare doctoral students for high skilled careers requiring innovation. This paper explores the perceptions of STEM doctoral faculty from three institutions in the southeastern part of the United States to understand their knowledge of STEM doctoral mentoring. This work seeks to improve STEM doctoral education by focusing on the mentorship relationship, an experience that is vital to matriculation, degree completion, and career planning Millett & Nettles, 2006). Using a qualitative multiple embedded case study design, the researchers interviewed and surveyed STEM doctoral faculty about their perceptions of STEM doctoral mentoring. This article focuses on five key findings from the qualitative interviews. STEM doctoral faculty: (a) have difficulty differentiating mentoring responsibilities from and in addition to advising; (b) have limited mentoring training opportunities; (c) see mentoring more exclusively as the development of scientific knowledge; (d) lack meaningful understanding of the role of culture in mentoring; and (e) lack deep understanding of the importance of relational connections with mentees.