Signatures of formation mechanisms in the multiple-star statistics A. Tokovinin CTIO/NOAO, Chile Recent results & updates on binary and multiple-star statistics Facts to be explained by the theory M.Bate et al. Toronto, May 2007

Orbital parameters as a fossil record of binary formation Relevant orbital parameters [and evolution effects] Period P (semi-major axis a) [tides, magnetic braking, mass transfer, encounters] Eccentricity e [tides, mass transfer] Mass ratio q =M2/M1 (M2) [mass transfer] Primary mass M1 [mass transfer] Multiple (N>2) systems: period and mass ratios, relative inclinations, etc. Toronto, May 2007

Binary & multiple statistics Observations, selection effects joint distribution f (P,e,q,M1) f (P,e,q,M1) is multi-dimensional f (P ) is its projection f (P )*f (e )*f (q )*f (M1) is wrong! Statistics depends on the environment, not universal Physical processes and models N-body dynamics Accretion & migration Toronto, May 2007

Periods and companion fraction Öpik’s law is still unexplained! Toronto, May 2007

Period-eccentricity relation Abt (2005), B0-B9.5 Udry et al. (1998), G V Toronto, May 2007

Eccentricity distribution f (e)=2e ?? Abt (2006), FGK SBs with massive secondaries have higher eccentricities 0.5 e=1 Ribas & Miralda-Escude (2007) Toronto, May 2007

Mass ratio Random pairing is excluded at ALL periods There is a population of twins (q ~1) among SB and VB [accretion or… mass transfer?] Brown dwarf desert (lack of small q) at short and long periods 2M0 5M0 Soderhjelm (2007) Random pairing Toronto, May 2007

Mass ratio distribution f (q,P ) f (q) ~ q0 not dynamics! Abt & Levy 1976 f (q) ~ q -1/3 Kouwenhoven et al. Toronto, May 2007

Multiples (N>2) Solar-type dwarfs type % Single 56 (57) Binary 36 (38) Triple 6 (4) N>3 2.5 (1) Tokovinin (2005) vs. DM91 (15yr later) Multiples are still being discovered within 25 parsec! Highest: N=6 (N=7 possible) Up to 4 levels of hierarchy! GJ 225.2 Toronto, May 2007

Are all close binaries triple? No! P<3d: all triple P>10d: 35% triple Fraction of triples SB period [days] Tokovinin et al. 2006: FGK-dwarfs, P=1…30 day f (q) – same! Toronto, May 2007

Period ratios in FGK multiples PL/PS=10 000 PL/PS=5 (stability) Plong [years] Migration? Gap at Pshort ~100d? Pshort [days] PL/PS~100 Toronto, May 2007

Mass ratios in multiples q3 Plong [years] Most massive component is preferentially in the close system (18% q3>1) Mass ratio of the tertiary decreases with its period? Toronto, May 2007

Relative orientation of orbital spins Spin correlation Less hierarchical triples have more correlated spins Period ratio Sterzik & Tokovinin 2002 Spin correlation is determined from the counts of co-rotating and counter-rotating systems Toronto, May 2007

Fact summary (I) Periods obey Öpik’s distribution f (logP) ~ const CSF and f (P) depend on the environment and M1 Average e~0.5, f (e) same for planets and SB SBs with larger q have larger e f (q) depends on period, no random pairing Twins with q~1 exist at short and long periods Brown dwarf desert at short and long periods Toronto, May 2007

Fact summary (II) Multiples with N>3 are common (~2.5%) All P<3d binaries are multiple Half of P~10d binaries are NOT multiple PL/PS is 5…104 for wide multiples, but >100 for close (PS<30d) multiples Most massive component prefers close sub-system Tertiary mass decreases with its period Orbital spins in triples are correlated Toronto, May 2007

Challenges Can we model f (e) from the binary-disk interaction during migration? How many proto-binaries merged? Are twins formed by a special mechanism? Who will explain the Öpik’s law? Toronto, May 2007