Transcript lgs_meeting_kraus.ppt

Multiple Star Formation
at the Bottom of the IMF
Adam L. Kraus
February 1, 2007
Background: Field Binaries
Solar-type binaries:
• High binary frequency
• Large separations
• Low companion masses
Brown dwarf binaries:
• Low binary frequency
• Small separations
• Near-equal masses
Figure from Bouy et al. (2006)
Complications for Field Surveys
Field binary systems may have been
subject to dynamical evolution
 Substellar companions are intrinsically faint
 Substellar masses/ages are degenerate
 The field represents a composite
population, drawn from all formation
environments

Solution: Uniform Stellar Populations
M44 (Praesepe)
Open cluster
Known quantities:
 Age
 Distance
 Formation
Environment
Others: Taurus,
Chamaeleon
Upper Scorpius
OB Association
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.
Multiplicity with Keck/Palomar LGSAO
High-resolution imaging survey with LGSAO
 Observing low-mass (<0.25 Msun) stars and
brown dwarfs in 3 nearby clusters/associations
 Goals:

 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
Survey Sample: Mass Range
0.06 Msun:
Minimum
mass for
even very
close binaries
(5-20 AU)
0.25 Msun:
Minimum
mass for
wide binaries
(>300 AU)
Fundamental shift
(100X in separation
and frequency) over a
narrow mass range.
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.
Preliminary Results



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
Most (but not all) newly-discovered binary
systems fall above the substellar boundary;
there are few BD+BD binaries
0.85”
150 AU
0.05”
7 AU
V410 X-ray3
IZ-072
V410-Xray3: A Binary BD in Taurus
Separation 42 mas (6 AU), mass ratio q~0.5. Mass = 0.08+0.04 Msun.
Separation vs Mass
Circles: Newly-discovered systems.
Crosses: Other known binaries in Taurus and Upper Sco.
Line: Empirical Mtot-amax limit (Burgasser et al. 2003)
A
UScoJ1606-1935
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.
Ab
Mtot
0.06”
10.6”
B
~0.34 Msun
2MASS
(M5+M6+M5)
aAB
~1500 AU
aAaAb
~9 AU
Keck+LGSAO
Aa
This system is too wide by
a factor of ~10.
The Orbit of MHO-Tau-8
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.
Kraus et al. (2006), White et al. (in prep)
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 Msun
 What is the functional form of the transition?
