White Dwarf Binaries in Globular Clusters Fred Rasio (Northwestern)
Outline WD - WD binaries only (CVs and WD - NS binaries in other talks) Theoretical predictions from: N-body simulations with binaries and stellar evolution (Hurley & Shara) N-body simulations with binaries and stellar evolution (Hurley & Shara) Binary Population Synthesis with dynamics (Ivanova, Belczynski, Fregeau & FR) Binary Population Synthesis with dynamics (Ivanova, Belczynski, Fregeau & FR) Merger rates, properties of binaries in LISA band
Why WD - WD binaries? Most common type of compact binary in any environment Close (merging) WD - WD binaries should be especially abundant in dense star clusters (Chen & Leonard 1993 ApJ 411,L75; Shara & Hurley 2002, ApJ 571, 830) Most promising LISA sources in Galactic globular clusters (Benacquista, SPZ & FR 2001, CQG, 18, 4024) Interesting merger products: Type Ia SN, NS or MSPs by “merger-induced collapse” (?)
Key Dynamical Processes Destruction of wide binaries (not just soft binaries!) Hardening of close binaries Exchange interactions (tend to replace low-mass companion by more massive companion) Most important: Interplay with stellar evolution (eg exchange interaction + CE)
Results from N-body Simulations Hurley & Shara: NBODY4 + Hurley et al. Recipes for stellar evolution on GRAPE-6 NBODY4 + Hurley et al. Recipes for stellar evolution on GRAPE-6 N = 20,000 with 10% binaries, open cluster conditions, t f = 4 Gyr N = 20,000 with 10% binaries, open cluster conditions, t f = 4 Gyr Even for these small, low-density clusters, dynamics plays a crucial role! (mainly through hardening and exchanges) Merger rate for systems with M tot > M Ch is ~15 times higher than in a system with no dynamical interactions. Implications for Type Ia SN rate, NS formation, Number of LISA sources in clusters, LISA Galactic background Cluster Field LISA merging ApJ 589, 179
Never explore parameter space !
Population Synthesis with Dynamics Combine: StarTrack (Belczynski, Kalogera et al.) for binary evolution StarTrack (Belczynski, Kalogera et al.) for binary evolution FewBody (Fregeau) for dynamical 3-body and 4-body interactions FewBody (Fregeau) for dynamical 3-body and 4-body interactions Monte Carlo generation of interactions in core Monte Carlo generation of interactions in core Simplified model of cluster dynamics: Simplified model of cluster dynamics: Simplest: constant density “box” modelSimplest: constant density “box” model Current: two-zone model with core and haloCurrent: two-zone model with core and halo Ultimate: full dynamical Monte CarloUltimate: full dynamical Monte Carlo Typical parameters: N ~ 10 5 with 50% - 100% binaries, globular cluster conditions, t f = 13 Gyr
Initial Primordial Binary Fractions… Matching observed core binary fractions today of a few percent requires close to 100% binaries initially! (Ivanova, Belczynski, Fregeau, & FR 2003)
WD - WD Binary Mergers Overall ~ 5 times more mergers with dynamics For M tot > M Ch, ~ 100 times more mergers!
Binaries in the LISA band Results from 10 independent cluster snapshots May need ~ to obtain meaningful distribution n c = 10 5 n c = 10 7