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: ~ M sun, 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 ) 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 ) 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) (Pascucci et al., 2003) (Jayawardhana et al )

Brown Dwarf Formation Puzzle Difficulty: M << M Jeans 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?

Protostellar Disks HST/NICMOS images of Taurus Young Stellar Objects (Padgett et al.,1999)

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 Substellar objects formation Watkins et al., (1998) Multiple system formation +substellar companions

Resolve Jeans Mass and Disk Scale Height Real disk: scale height increase with radius Real collapse: M > M Jeans Resolve the scale height & Jeans Mass is important (Bate et al. 1997; Kim et al., 2001)

Simulating the Collisions Spatial resolution: about 0.2 AU ( IC: 2 AU ) Mass resolution: M sun 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 t = 0 10,000 years12,000 years 13,000 years 14,000 years 16, 000 years Gas dispersion Gravitational resonance Tidal tail structure Tidal tail clump Disk Fragmentation

Interaction Velocities Encounter velocity v en = 2.0 km/s Rotational velocity at 500 AU v rot = 2.0 km/s For v en > 2.0 km/s Interaction timescale less than dynamical timescale – No clumps Disk truncation is severe

Fragments – rapid rotating brown dwarf disks! Mass: M sun – substellar (Brown Dwarfs + Planets) Disk-like objects Size: 0.3 – 18 AU; less than typical protostellar disks Rotating fast r  r BD then v rot > 10 v break 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 ) 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 M d = 1 M J and take Viscous time scale: 10 Myr In the α disk model, α < 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) Fragment Mass Distribution (Preliminary) 32 clumps Substellar mass 1 M J < M < 75 M J Abundant in low mass end with M < 30 M J Lack of population in mass range 35 M J ~ 60 M J ? Only a few clumps have planetary mass— resolution limitation? Bin size = 5 M J

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 T- Tauri 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