{
  "id": "1706.09894",
  "version": "v1",
  "published": "2017-06-29T18:00:00.000Z",
  "updated": "2017-06-29T18:00:00.000Z",
  "title": "Dynamical Formation of Close Binaries During the Pre-main-sequence Phase",
  "authors": [
    "Maxwell Moe",
    "Kaitlin M. Kratter"
  ],
  "comment": "Submitted to ApJ, 21 pages, 8 figures",
  "categories": [
    "astro-ph.SR",
    "astro-ph.EP"
  ],
  "abstract": "Solar-type binaries with short orbital periods ($P_{\\rm close}$ $\\equiv$ 1 - 10 days; $a$ $\\lesssim$ 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 ($F_{\\rm close}$ $=$ 2.1%). Thus we can infer that some migration mechanism operates during the early pre-MS phase ($\\tau$ $\\lesssim$ 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 eddy viscosity 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 $F_{\\rm close}$ $\\approx$ 0.4% after $\\tau$ = 5 Myr, which increases to $F_{\\rm close}$ $\\approx$ 0.8% by $\\tau$ = 5 Gyr. Dynamical ejections and disruptions of unstable coplanar triples in the disk produce solitary binaries with slightly longer periods $P$ $\\approx$ 10 - 100 days. The remaining $\\approx$60% of close binaries with outer tertiaries, particularly those in compact coplanar configurations with log $P_{\\rm out}$ (days) $\\approx$ 2 - 5 ($a_{\\rm out}$ $<$ 50 AU), can be explained only with substantial extra energy dissipation due to interactions with primordial gas.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-06-29T18:00:00.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "pre-main-sequence phase",
      "dynamical formation",
      "close binary fraction",
      "disk produce solitary binaries",
      "substantial extra energy dissipation"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 21,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}