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

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

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

4. Solution: Uniform Stellar Populations Known quantities: Age Age Distance Distance Formation Environment Formation Environment M44 (Praesepe) Open cluster Upper Scorpius OB Association Others: Taurus, Chamaeleon

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 High-resolution imaging survey with LGSAO Observing low-mass (<0.25 M sun ) stars and brown dwarfs in 3 nearby clusters/associations Observing low-mass (<0.25 M sun ) stars and brown dwarfs in 3 nearby clusters/associations Goals: Goals: Study the role of mass and environment in establishing low-mass binary properties Study the role of mass and environment in establishing low-mass binary properties Test whether field binary properties are established at formation or during subsequent dynamical evolution Test whether field binary properties are established at formation or during subsequent dynamical evolution

7. Survey Sample: Mass Range 0.25 M sun : Minimum mass for wide binaries (>300 AU) 0.06 M sun : 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-Auriga Age=1 Myr Distance=145 pc T Association Praesepe Age=600 Myr Distance=175 pc Open Cluster Upper Scorpius Age=5 Myr Distance=145 pc OB 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 V410 X-ray3 IZ-072 We observed 65 targets and identified 10 new binary systems We observed 65 targets and identified 10 new binary systems Many (but not all) system properties are consistent with the field: small separations and similar masses Many (but not all) system properties are consistent with the field: small separations and similar masses Most (but not all) newly-discovered binary systems fall above the substellar boundary; there are few BD+BD binaries Most (but not all) newly-discovered binary systems fall above the substellar boundary; there are few BD+BD binaries 0.05” 7 AU 0.85” 150 AU

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

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

12. 10.6” A B Aa Ab 0.06” Keck+LGSAO UScoJ1606-1935 2MASS 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. M tot ~0.34 M sun (M5+M6+M5) a AB ~1500 AU a AaAb ~9 AU This system is too wide by a factor of ~10.

13. The Orbit of MHO-Tau-8 Kraus et al. (2006), White et al. (in prep) 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.

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 M sun What is the functional form of the transition?