Evolution of Binary Systems Embedded in Circumbinary Accretion Disks

Accretion;High-energy astrophysics;binaries;black holes;accretion disks;binary evolution;Physics

Bankert, JustinKrolik, Julian H. ;

Johns Hopkins University

关键词: Accretion;    High-energy astrophysics;    binaries;    black holes;    accretion disks;    binary evolution;    Physics;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/58629/BANKERT-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

In this work, I combine two studies on circumbinary accretion disks. The first study focuses on the interactions between a central equal mass binary and a surrounding retrograde circumbinary accretion disk. The second study examines the evolution of the orbital parameters (semi-major axis a and eccentricity e) of a binary on an eccentric orbit embedded in a prograde circumbinary accretion disk. In both studies, we utilize three-dimentional MHD simulations.In the first study, we find that the binary-on-disk torque is much smaller for a retrograde circumbinary disk than for a disk orbiting in the prograde direction. Thus, fluid in a retrograde circumbinary disk can accrete directly from the disk to the binary;;s domain. There is no ;;gap;; carved out of the retrograde disk. Also, retrograde disks do not develop the large-amplitude lumps generically seen near the inner edges of prograde disks. Because the disks angular momentum is directed oppositely to that of the binary, accretion onto the binary causes a rapid increase in eccentricity. Lastly we discuss the comparative rates of orbital evolution associated with accretion in prograde and retrograde circumbinary disks, finding that the principal contrast is that e-dot should be rather greater in the retrograde case.In the second study, we find that for prograde circumbinary accretion disks around binaries on eccentric orbits, as was found for the case of circular binaries, accretion occurs through transient accretion streams, which traverse the gap region between the binary and the bulk disk. However, the m = 1 ;;lump;; feature found in the circular case did not develop in any of the eccentric binary cases. Instead, we found prominent m =2 spiral density waves in the bulk disk. We observed a monotonic increase in the time averaged mass accretion rate with increasing binary orbital eccentricity. The rate of mass accretion peaks during the binary descent phase, although this trend is weaker for larger eccentricities. Even so, accretion into the binary region occurs primarily near apoapsis for all binary eccentricities. As the binary nears apoapsis, negative binary-on-disk torques extract angular momentum from accretion streams allowing material to cross the gap and pass into the binary region. Positive binary-on-disk torques propel accretion streams back into the gap region a short time (less than a binary orbit) later. This effect is more pronounced for small eccentricity binaries; smaller eccentricity binaries thus have larger populations of disk material with a high ratio of |vr/vphi|. We find that, on average, prograde circumbinary disks increase (or at least do not decrease) the net angular momentum of the central binary. Utilizing simple ballistic calculations, we find that the majority of material passing into the binary region does not impact a binary mini-disk within a binary obital time for e > 0.3. We conclude that future work must contain the binary region on the computational grid in order to accurately model the binary evolution.