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

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)

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.

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

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) = (28 + 2 x 7) / 135 = 31% Dependence of MF on SpT Ahmic, Jayawardhana, Brandeker, et al., in prep.

Multiplicity in the VLM Domain in Cha I * 26 targets in the 0.02 - 0.2 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)

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

Puzzle: about half-dozen wide young VLM binaries are now known (e.g., 2MASS J1101-7732 with a~240 AU DENIS-P J161833-251750 with a~140 AU USCO-160028.4 with a~120 AU Oph 162225-2420515 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)

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

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)

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 12 - 130 AU range 5 AU disk model fits the SED of the tightest pair with excess Scholz, Jayawardhana, Wood, et al. (2007)

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

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.

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

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