Multiple Star Formation at the Bottom of the IMF

1. Multiple Star Formationat the Bottom of the IMF Adam L. Kraus February 1, 2007

2. Background: Field Binaries • Solar-type binaries: • High binary frequency • Large separations • Low companion masses • Brown dwarf binaries: • Low binary frequency • Small separations • Near-equal masses Figure from Bouy et al. (2006)

3. Complications for Field Surveys • Field binary systems may have been subject to dynamical evolution • Substellar companions are intrinsically faint • Substellar masses/ages are degenerate • The field represents a composite population, drawn from all formation environments

4. Solution: Uniform Stellar Populations M44 (Praesepe)Open cluster Known quantities: • Age • Distance • FormationEnvironment Others: Taurus, Chamaeleon Upper ScorpiusOB Association

5. The Role of Mass and Environment Examined 2MASS data for 787 known members of Upper Sco, Taurus, Chamaeleon. This is a seeing-limited survey, which can detect wide systems with separations of 1.5-30.0”. Taurus+ChamI: T Associations(low density, low mass) Upper Sco: OB Association(higher density, higher mass) The wide binary frequency appears to decline with mass, as seen in the field. The wide binary frequency is also significantly lower in Upper Sco than in Taurus or Cham I.

6. Multiplicity with Keck/Palomar LGSAO • High-resolution imaging survey with LGSAO • Observing low-mass (<0.25 Msun) stars and brown dwarfs in 3 nearby clusters/associations • Goals: • Study the role of mass andenvironment in establishinglow-mass binary properties • Test whether field binaryproperties are establishedat formation or duringsubsequent dynamical evolution

7. Survey Sample: Mass Range 0.25 Msun: Minimum mass for wide binaries (>300 AU) 0.06 Msun: Minimum mass for even very close binaries(5-20 AU) Fundamental shift (100X in separation and frequency) over a narrow mass range.

8. Survey Sample: Regions Taurus-AurigaAge=1 MyrDistance=145 pcT Association PraesepeAge=600 MyrDistance=175 pcOpen Cluster Upper ScorpiusAge=5 MyrDistance=145 pcOB Association These regions sample the full range of star formation environments: a low-mass stellar association, a high-mass stellar association, and a bound open cluster.

9. Preliminary Results • We observed 65 targets and identified 10new binary systems • Many (but not all) system properties areconsistent with the field: small separationsand similar masses • Most (but not all) newly-discovered binarysystems fall above the substellar boundary;there are few BD+BD binaries 0.85”150 AU 0.05”7 AU IZ-072 V410 X-ray3

10. V410-Xray3: A Binary BD in Taurus Separation 42 mas (6 AU), mass ratio q~0.5. Mass = 0.08+0.04 Msun.

11. Separation vs Mass Circles: Newly-discovered systems.Crosses: Other known binaries in Taurus and Upper Sco.Line: Empirical Mtot-amax limit (Burgasser et al. 2003)

12. A UScoJ1606-1935 10.6” We discovered the AB system in a search for wide binary systems in 2MASS; subsequent LGSAO observations showed that A is itself a tight binary. 0.06” B Ab Mtot ~0.34 Msun (M5+M6+M5) aAB ~1500 AU aAaAb ~9 AU 2MASS Aa This system is too wide by a factor of ~10. Keck+LGSAO

13. The Orbit of MHO-Tau-8 MHO-Tau-8 was one of the first known young substellar binaries, so the time baseline is longer than average. The system orbital motion (40 degrees in 5 years) has allowed for a rough mass determination, but accurate determination of an orbit will require a few more years. Kraus et al. (2006), White et al. (in prep)

14. Summary • Are substellar binary properties a primordial feature?Probably. • Do binary frequencies continue to decline in the substellar regime?Probably. • Does binary formation occur in a single universal process, or does it depend on the environment?Environment (wide binaries) • Where is the transition between stellar and substellar binary properties?0.25-0.07 Msun • What is the functional form of the transition?