Ray JayawardhanaUniversity of Toronto

Mirza Ahmic, Duy Nguyen, Ivana Damjanov

Alexis Brandeker, Marten van Kerkwijk

Multiplicity Among Young Stars and Brown Dwarfs

I. Regional variations?II.VLM regime

III. Binarity-disk connection?

Brandeker, Jayawardhana, Khavari, et al. (2006)

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

Ahmic, Jayawardhana, Brandeker, et al., in

prep.

* Adaptive optics imaging survey in H and K with NACO on the VLT* Adaptive optics imaging survey in H and K with NACO on the VLT

* Able to detect nearly equal brightness pairs at ~ 6-10 AU, * Able to detect nearly equal brightness pairs at ~ 6-10 AU, qq=0.2 at =0.2 at ~30 AU, and companions down to ~15 M~30 AU, and companions down to ~15 MJupiterJupiter at wide separations at wide separations

* 135 targets: all known Cha I members bright enough for optical or * 135 targets: all known Cha I members bright enough for optical or NIR WFS of NACONIR WFS of NACO

* Found a total of 28 binaries and 7 triples* Found a total of 28 binaries and 7 triples

6 out of 11 previously known binaries turned out to be triples6 out of 11 previously known binaries turned out to be triples

22 new binaries identified22 new binaries identified

no quadruples or higher-order multiples found: contrary to no quadruples or higher-order multiples found: contrary to some simulations that produce small groups that later break up some simulations that produce small groups that later break up

Multiplicity in Chamaeleon I (~2 Myr)

A few examples of Cha I multiple systems

Ahmic, Jayawardhana, Brandeker, et al., in prep.

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%

Multiplicity Frequency in Cha I

Ahmic, Jayawardhana, Brandeker, et al., in prep.

Dependence of MF on SpT

* 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?

Multiplicity in the VLM Domain in Cha I

Jayawardhana, Ahmic, Brandeker, et al. (2007)

separation (AU)

bin

din

g e

ne

rgy

(erg

s)

Stability of VLM Binaries

Jayawardhana, Ahmic, Brandeker, et al.

(2007)

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)

Bouwman et al. (2006)

[see Brandeker et al. (2006) for AO imaging]

mid-infrared excess

- 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

Disks and Multiplicity: Cha I (~2 Myr)

Damjanov, Jayawardhana, Scholz, et al.

(2007)

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

Disks and Multiplicity: Upper Sco VLMOs (~5 Myr)

Scholz, Jayawardhana, Wood, et al. (2007)

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

Could there be companions in even tighter orbits (unresolved with

AO and HST)?

Nguyen et al., in prep.

Companion Detectability

Radial Velocity Survey of Young Stars

* ~4 epochs with MIKE on Magellan

* ~75 targets in Cha I and ~130 targets in Taurus

Nguyen et al., in prep.

Examples of Varying RV

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