分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We present a new mechanism of generating large planetary eccentricities. This mechanism applies to planets within the inner cavities of their companion protoplanetary disks. A massive disk with an inner truncation may become eccentric due to non-adiabatic effects associated with gas cooling, and can retain its eccentricity in long-lived coherently-precessing eccentric modes; as the disk disperses, the inner planet will encounter a secular resonance with the eccentric disk when the planet and the disk have the same apsidal precession rates; the eccentricity of the planet is then excited to a large value as the system goes through the resonance. In this work, we solve the eccentric modes of a model disk for a wide range of masses. We then adopt an approximate secular dynamics model to calculate the long-term evolution of the "planet + dispersing disk" system. The planet attains a large eccentricity (between 0.1 and 0.6) in our calculations, even though the disk eccentricity is quite small ($\lesssim0.05$). This eccentricity excitation can be understood in terms of the mode conversion (``avoided crossing'') phenomenon associated with the evolution of the "planet + disk" eccentricity eigenstates.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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 separation
$r_{\rm p}$ becomes less than the Hill radius $R_{\rm H}$. In the rare very
close encounters (with $r_{\rm p}$ several orders of magnitude less than
$R_{\rm H}$), a tight merging BH binary can form with the help of gravitational
wave emission. We use $N$-body simulations to study the time evolution of close
encounters of various degrees of "closeness" and the property of the resulting
binary BH mergers. For a typical "SMBH + 2 BHs" system, the averaged cumulative
number of close encounters (with $r_{\rm p} \lesssim R_{\rm H}$) scales
approximately as $\propto t^{0.5}$. The minimum encounter separation $r_{\rm
p}$ follows a linear cumulative distribution $P(
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We study the close encounters between two single black holes (BHs) embedded in an AGN disk using a series of global 2D hydrodynamics simulations. We find that when the disk density is sufficiently high, bound BH binaries can be formed by the collision of their circum-single disks. Our analysis demonstrates that, after a BH pair passes the pericenter of their relative trajectory, a gas post-collision drag may slow down the BHs, possibly forcing the two BHs to stay tightly bound. A binary formed by a close encounter can have a compact semi-major axis, large eccentricity, and retrograde orbital angular momentum. We provide a fitting formula that can accurately predict whether a close encounter can form a binary based on the gas mass and the incoming energy of the encounter. This fitting formula can be easily implemented in other long-term simulations that study the dynamical evolution of BHs in AGN disks.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Binary black hole (BBH) mergers, particularly those with component masses in the pair-instability gap, may be produced by hierarchical mergers in the disks surrounding Active Galactic Nuclei (AGN). While the interaction of an embedded BBH with an AGN disk is typically assumed to facilitate a merger, recent high-resolution hydrodynamical simulations challenge this assumption. However, these simulations often have simplified treatments for the gas thermodynamics. In this work, we model the possible consequence of various feedback from an embedded BBH with a simple model that maintains an enhanced temperature profile around each binary component. We show that when the minidisks around each BH become hotter than the background by a factor of three, the BBH orbital evolution switches from expansion to contraction. By analyzing the gravitational torque profile, we find that this change in direction is driven by a weakening of the minidisk spirals and their positive torque on the binary. Our results highlight the important role of thermodynamics around BBHs and its effect on their orbital evolution, suggesting that AGN disks could be efficient factories for BBH mergers.
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