1. Multiplicity Among Young Stars and Brown Dwarfs
Regional variations?
VLM regime
Binarity-disk connection?
Ray Jayawardhana
University of Toronto
Mirza Ahmic, Duy Nguyen, Ivana Damjanov
Alexis Brandeker, Marten van Kerkwijk

2. Multiplicity in the  Cha Cluster(~8 Myr)
17 stars imaged with VLT/NACO
No binaries wider than 20 AU, even though we could detect any companions > 0.07 MSun
Deficit of wide binaries, for example, in comparison to TW Hydrae association: likelihood of being same < 2 x 10-4
Stellar encounters in the present configuration cannot account for this deficit
Brandeker, Jayawardhana, Khavari, et al. (2006)

3. Multiplicity in Chamaeleon I (~2 Myr)
* Adaptive optics imaging survey in H and K with NACO on the VLT
* Able to detect nearly equal brightness pairs at ~ 6-10 AU, q=0.2 at ~30 AU, and companions down to ~15 MJupiter at wide separations
* 135 targets: all known Cha I members bright enough for optical or NIR WFS of NACO
* Found a total of 28 binaries and 7 triples
6 out of 11 previously known binaries turned out to be triples
22 new binaries identified
no quadruples or higher-order multiples found: contrary to some simulations that produce small groups that later break up
Ahmic, Jayawardhana, Brandeker, et al., in prep.

4. A few examples of Cha I multiple systems
Ahmic, Jayawardhana, Brandeker, et al., in prep.

5. Multiplicity Frequency in Cha I
Multiplicity Frequency = (B+T+Q) / (S+B+T+Q) = (28+7)/135 = 26%
Companion Star Frequency = (B+2T+3Q) / (S+B+T+Q)
= ( x 7) / 135 = 31%
Dependence of MF on SpT
Ahmic, Jayawardhana, Brandeker, et al., in prep.

6. Multiplicity in the VLM Domain in Cha I
* 26 targets in the MSun range
* Only two binaries resolved: q~1, a~20 and 26 AU, no new wide pairs
* Multiplicity frequency = 8 (+17-7 )%
- comparable to Kraus et al. (2005, 2006) results in Taurus & Upper Sco
for a combined sample of 60 young VLMOs, mf = 7 (+10-5 )%
comparable to MF for VLM objects in the field (e.g., Close et al. 2003)
no significant evolution of multiplicity between a few Myr to several Gyr?
Jayawardhana, Ahmic, Brandeker, et al. (2007)

7. Stability of VLM Binaries
binding energy (ergs)
Jayawardhana, Ahmic, Brandeker, et al. (2007)
separation (AU)

8. Puzzle: about half-dozen wide young VLM binaries are now known
(e.g., 2MASS J with a~240 AU
DENIS-P J with a~140 AU
USCO with a~120 AU
Oph with a~240 AU)
Problem: usually these have been found by chance or in small sample studies rather than in well-defined large multiplicity surveys
Solution: either wide VLM binaries are rare or there might be an environmental dependence
(Konopacky et al. recently found 5/13 binaries, two of them with a~80-90 AU range, among VLM stars in Taurus)

9. Disks and Multiplicity:  Cha (~8 Myr)
- 7/9 single stars show IR excess
only 1/6 binaries shows IR excess
- tdisk dissipation ~ 9 Myr for singles
tdisk dissipation ~ 5 Myr for binaries
Bouwman et al. (2006)
[see Brandeker et al. (2006) for AO imaging]
mid-infrared excess

10. Disks and Multiplicity: Cha I (~2 Myr)
63 stars and BDs with Spitzer mid-IR fluxes and VLT AO imaging
Of those, 15 binaries and 2 triples
Disk frequency among binaries: 35 (+15-13) %
Disk frequency among singles: 46 (+/-9) %
Of <20AU binaries, 4/5 lack IR excess
Damjanov, Jayawardhana, Scholz, et al. (2007)

11. Disks and Multiplicity: Upper Sco VLMOs (~5 Myr)
29 VLM stars and brown dwarfs with Spitzer mid-IR photometry and HST or VLT AO imaging
Of 6 with companions,
2 don’t show excess: these are the two tightest binaries at 5 & 10 AU
4 show excess: these have separations in the AU range
5 AU disk model fits the SED of the tightest pair with excess
Scholz, Jayawardhana, Wood, et al. (2007)

12. Could there be companions in even tighter orbits (unresolved with AO and HST)?

13. Radial Velocity Survey of Young Stars
* ~4 epochs with MIKE on Magellan
* ~75 targets in Cha I and ~130 targets in Taurus
Companion Detectability
Nguyen et al., in prep.

14. Examples of Varying RV
Nguyen et al., in prep.

15. Near Future Prospects
Comprehensive AO+RV surveys of multiplicity in nearby young SFRs, including the VLM regime, to explore variations (e.g., deficit of wide binaries in  Cha: Brandeker et al. 2006)
Investigations of a possible connection between disk dissipation and close companions