Dynamical Formation of Close Binaries during the Pre-main-sequence Phase
Abstract
Solar-type binaries with short orbital periods ({P}{close}\equiv 1{--}10 days a ≲ 0.1 au) cannot form directly via fragmentation of molecular clouds or protostellar disks, yet their component masses are highly correlated, suggesting interaction during the pre-main-sequence (pre-MS) phase. Moreover, the close binary fraction of pre-MS stars is consistent with that of their MS counterparts in the field ({F}{close}=2.1 % ). Thus, we can infer that some migration mechanism operates during the early pre-MS phase (τ ≲ 5 Myr) that reshapes the primordial separation distribution. We test the feasibility of this hypothesis by carrying out a population synthesis calculation which accounts for two formation channels: Kozai-Lidov (KL) oscillations and dynamical instability in triple systems. Our models incorporate (1) more realistic initial conditions compared to previous studies, (2) octupole-level effects in the secular evolution, (3) tidal energy dissipation via weak-friction equilibrium tides at small eccentricities and via non-radial dynamical oscillations at large eccentricities, and (4) the larger tidal radius of a pre-MS primary. Given a 15% triple-star fraction, we simulate a close binary fraction from KL oscillations alone of {F}{close}≈ 0.4 % after τ = 5 Myr, which increases to {F}{close}≈ 0.8 % by τ = 5 Gyr. Dynamical ejections and disruptions of unstable coplanar triples in the disk produce solitary binaries with slightly longer periods P ≈ 10-100 days. The remaining ≈60% of close binaries with outer tertiaries, particularly those in compact coplanar configurations with log {P}{out} (days) ≈ 2-5 ({a}{out}< 50 au), can be explained only with substantial extra energy dissipation due to interactions with primordial gas.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2018
- DOI:
- 10.3847/1538-4357/aaa6d2
- arXiv:
- arXiv:1706.09894
- Bibcode:
- 2018ApJ...854...44M
- Keywords:
-
- binaries: close;
- stars: formation;
- stars: kinematics and dynamics;
- stars: pre-main sequence;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- Accepted by ApJ