Adam L. KrausAdam L. Kraus

February 1, 2007February 1, 2007

Multiple Star FormationMultiple Star Formationat the Bottom of the IMFat the Bottom of the IMF

Background: Field BinariesBackground: Field BinariesSolar-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

Complications for Field SurveysComplications for Field Surveys

Field binary systems may have been Field binary systems may have been subject to dynamical evolutionsubject to dynamical evolution

Substellar companions are intrinsically faintSubstellar companions are intrinsically faintSubstellar masses/ages are degenerateSubstellar masses/ages are degenerateThe field represents a composite The field represents a composite

population, drawn from all formation population, drawn from all formation environmentsenvironments

Solution: Uniform Stellar PopulationsSolution: Uniform Stellar PopulationsKnown quantities:Known quantities:AgeAgeDistanceDistanceFormationFormation

EnvironmentEnvironment

M44 (Praesepe)Open cluster

Upper ScorpiusOB Association

Others: Taurus, Chamaeleon

The Role of Mass The Role of Mass and Environmentand 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.

Multiplicity with Keck/Palomar LGSAOMultiplicity with Keck/Palomar LGSAOHigh-resolution imaging survey with LGSAOHigh-resolution imaging survey with LGSAOObserving low-mass (<0.25 MObserving low-mass (<0.25 Msunsun) stars and ) stars and

brown dwarfs in 3 nearby clusters/associationsbrown dwarfs in 3 nearby clusters/associationsGoals:Goals:

Study the role of mass andStudy the role of mass andenvironment in establishingenvironment in establishinglow-mass binary propertieslow-mass binary properties

Test whether field binaryTest whether field binaryproperties are establishedproperties are establishedat formation or duringat formation or duringsubsequent dynamical subsequent dynamical evolutionevolution

Survey Sample: Mass RangeSurvey 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.

Survey Sample: RegionsSurvey Sample: RegionsTaurus-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.

Preliminary ResultsPreliminary Results

V410 X-ray3IZ-072

We observed 65 targets and identified 10We observed 65 targets and identified 10new binary systemsnew binary systems

Many (but not all) system properties areMany (but not all) system properties areconsistent with the field: small separationsconsistent with the field: small separationsand similar massesand similar masses

Most (but not all) newly-discovered binaryMost (but not all) newly-discovered binarysystems fall above the substellar boundary;systems fall above the substellar boundary;there are few BD+BD binariesthere are few BD+BD binaries

0.05”7 AU

0.85”150 AU

V410-Xray3: A Binary BD in TaurusV410-Xray3: A Binary BD in Taurus

Separation 42 mas (6 AU), mass ratio q~0.5. Mass = 0.08+0.04 Msun.

Separation vs MassSeparation vs Mass

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

10.6”A

B

Aa

Ab0.06”

Keck+LGSAO

UScoJ1606-1935UScoJ1606-1935

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.

Mtot ~0.34 Msun

(M5+M6+M5)

aAB ~1500 AU

aAaAb ~9 AUThis system is too wide by a factor of ~10.

The Orbit of MHO-Tau-8The 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.

SummarySummaryAre 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?