Transcript Proto-Brown Dwarf Disks as Products of Protostellar Disk Encounters.

Proto-Brown Dwarf Disks as
Products of Protostellar Disk
Encounters
Sijing Shen, James Wadsley
(McMaster)
The Western Disk Workshop
May 19, 2006
Brown Dwarfs – Observational Facts
• Mass range: 0.013 ~ 0.075
Msun, below hydrogen burning
limit.
• Abundant in our galaxy (Chabrier
2002)
• Within stellar clusters (e.g.,
Luhman et al. 2000; Bejar et al. 2001)
or Field stars (e.g., Kirkpatrick et al.
2000 )
• Free-floating objects or
companion to stars
• “BD desert” -- No close BD
companion to stars with R < 3
AU (Marcy & Butler 2000)
• Binary or Multiple BDs: small
separation between
components (Simon et al.
2006)
The Pleiades cluster, where
many BDs are found
Disks around Young Brown Dwarfs
• Disks are commonly observed around young BDs (Muench et al. 2001)
• T Tauri Phase: Broad, asymmetric Hα profile (Jayawardhana et al. 2003 )
• Accretion with a lower rate vs. T-Tauri disks
(Natta et al. 2004)
• Similar percentage of BD accretors as protostellar ones (Liu et al. 2003)
(Jayawardhana et al. 2003 )
(Pascucci et al., 2003)
Brown Dwarf Formation Puzzle
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Difficulty: M << MJeans in a molecular cloud core (Bate et al. 2002)
Scenarios:
Turbulent Fragmentation (Padoan
& Nordlund 2004)
• Supersonic turbulent flow
increase the local density to form
BD mass cores by gravitational
instability
• After fragmentation, both BDs
and normal stars do not
significantly accrete from outside
(Krumholz & McKee 2005)
Competitive accretion: (Reipurth
& Clarke 2001; Bate et al. 2003)
•Both star and BDs start as
multiple “embyros”
•Accretion of outside gas
determines final mass
•BDs: ejected from the gas
reservoir and “failed” to become
star.
•Proto BD Disk will be truncated
and short-lived
BD Formation – Protostellar Disk Interaction
• Protostars and protostellar disks form first
from collapse molecular cloud cores
• Disks may co-exist in early, embedded
clusters (Whitworth et al. 1995)
• Dynamical interaction trigger instability
and fragmentation (Lin et al. 1998, Watkins et al.
1998a, b)
• Can proto-BD disks form in this way?
Collision Probability
• Early-stage protostellar disk: flared, large
R ~ 1000 AU (Yorke & Bodenheimer, 1999)
• Lifetime -- about
years
• Encounter time scale:
• Encounters are likely in clusters and depend on
star density.
• Disks in young clusters are more likely to
collide
Previous Work
Lin et al., 1998
Tidal tails + condensation
Watkins et al., (1998)
Multiple system formation
+substellar companions
Substellar objects formation
Simulating the Collisions
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Spatial resolution: about 0.2 AU ( IC: 2 AU )
Mass resolution:
Msun
Jeans Mass and Disk scale height resolved
TreeSPH code Gasoline (Wadsley et al. 2004)
“Sink particle” stars but not fragments: model
fragment gaseous evolution
• EOS:
The disk is passively heated
by the radiation from the star
• Assume efficient cooling (optical depth low in IC)
Encounter Configurations
• Coplanar, both disks
prograde
• Coplanar, both disks
retrograde
• Non-coplanar, both
disks prograde
• Non-coplanar, both
disks retrograde
A Coplanar Retrograde Disk Collision
Gas Dispersion
Shock
Formation
Inner Disk Accretion
Fragmentation From retrograde Disk Encounter
Shock layer
fragmentation
Spiral
Instability
A Non-coplanar Prograde Disk Collision
Gas dispersion
t=0
13,000 years
Gravitational
resonance
10,000 years
12,000 years
14,000 years
16, 000 years
Tidal tail
clump
Tidal tail
structure
Disk
Fragmentation
Interaction Velocities
•Encounter velocity
ven = 2.0 km/s
• Rotational velocity at
500 AU
vrot = 2.0 km/s
• For ven > 2.0 km/s
Interaction timescale less
than dynamical timescale
– No clumps
•Disk truncation is severe
Fragments – rapid rotating brown
dwarf disks!
• Mass: 0.002-0.073 Msun – substellar (Brown Dwarfs +
Planets)
• Disk-like objects
• Size: 0.3 – 18 AU; less than typical protostellar disks
• Rotating fast rrBD then vrot> 10 vbreak
• Outflows? Planets or companion BDs?
BD Outflows & Planets Observed
Outflow signature (a P Cygni-like dip) in
the Hα profile ρ Oph 102 (Whelan et al.
2005 )
Planet around young BD 2M1207
(Chauvin et al. 2005)
Accretion of Proto-BD Disks
• Low specific angular momentum gas condenses first
and forms proto-BD core within the disk-shape clump
• Viscous accretion of the high specific angular
momentum materials
• Accretion timescale:
Observation: Small central object  lower accretion rate (Natta et
al. 2004).
Assume Md = 1 MJ and take
Viscous time scale: 10 Myr
In the α disk model, α < 0.001
•
Lifetime is comparable to the Disks around protostars–
observable.
(c.f. Bate et al 2003: Short lived accretion stage?)
Orbits of Clumps
•Large range of
Eccentricity &
separation
•Unbound – Free
floating BDs
•Large bound
orbits: wide
separation BD
(Gizis et al. 2001)
•Smaller bound
orbits: Chaotic
orbital evolution:
Ejection? Multiple
BD companions?
Multiple BD Companions to Stars
• Observations: hierarchical triple GL 569B (Simon et al.
2006)
• Multiples from Simulations:
– One encounter usually produces 3 or more clumps
– Clump orbits will evolve further: captures?
– Additional fragmentation expected in proto-BD disks
Fragment Mass Distribution
(Preliminary)
• 32 clumps
• Substellar mass
1 MJ < M < 75 MJ
• Abundant in low mass
end with M < 30 MJ
• Lack of population in
mass range 35 MJ ~ 60 MJ?
• Only a few clumps have
planetary mass—
resolution limitation?
Bin size = 5 MJ
BD Abundance in Different Clusters
• Observed trend of decreasing number of BDs
with decreasing cluster density
e.g., BD population in Taurus is 2 times lower than in
the Trapezium cluster (Briceño 2002)
(cf. Turbulent fragmentation explanation:
decrease in the turbulent velocity dispersion)
• Encounter-induced BD formation: number of
BDs related to
probability of encounter configuration
number of BDs produced by encounter
Dense clusters  more encounters  more BDs
Conclusions
• Proto-BD disks are natural product of protostellar disk
encounters
• Collisions complementary to other mechanisms (e.g.
turbulent fragmentation)
• Disks are smaller but still have lifetime comparable to TTauri disks
• Excess spin initial angular momentum  Outflow or
planet
• BD Multiples can form in this way
• Statistics: abundant in lower mass end, consistent with
substellar IMF
• Future work: More cases, better statistics