Formation Mechanisms of Brown Dwarfs: Observations & Theories. Dan Li April 2009

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1 Formation Mechanisms of Brown Dwarfs: Observations & Theories Dan Li April 2009

2 What is brown dwarf (BD)? BD Mass : upper-limit ~ M lower-limit ~ M (?) Differences between BD and giant planet: 1) Formation mechanism 2) Elemental composition

3 Continuity between BDs and lowmass stars: IMF (Initial mass function) Clustering properties Binary statistics (multiplicity) Discs, accretion and outflows (Whitworth & Goodwin 2006)

4 IMF IMF of young cluster NGC 6611 (Oliveira et al. 2008)

5 IMF Can be better characterized in starforming regions and young clusters, in which BDs are brighter. Two constraints from observed IMF: Star to BD ratio normal stars outnumber BDs by a factor of 5~8 (Luhman et al. 2007) Minimum mass of BDs 10~20 M Jup (Luhman et al. 2007)

6 Clustering properties Spatial distributions of BDs (red crosses) and H-burning stars (blue circles) in Taurus starforming region The velocity dispersions are not distinguishable between BDs and H- burning stars, either (Luhman 2006)

7 Multiplicity properties Frequency, separation, mass radio distribution, etc. Sun-like star BD binary systems lack of close (separation < 3 AU) system - BD Desert more common at larger separations (semimajor > 100 A.U.) BD BD binary systems

8 Disks, accretion and outflows Disks (Luhman et al. 2005e)

9 Disks, accretion and outflows Accretion (Muzerolle et al. 1998, 2005)

10 Disks, accretion and outflows Jets and outflows Indicated by photometric and line profile variability, H 2 emission in UV, UV continuum excesses, and blue-shifted absorption and forbidden emission (see Luhman et al for references).

11 Formation mechanisms of BDs: (copyright by Don Dixon / cosmographica.com)

12 Formation by turbulent fragmentation Explored by Padona & Nordlund 2002: Strong, magnetic shocks produce high density post-shock gas which will have low Jeans masses and therefore form brown dwarfs directly from the turbulence Revised by Boyd & Whitworth 2005: 2-D one-shot fragmentation Star formation occurs in molecular clouds where two or more turbulent flows of sufficient density collide with sufficient ram pressure to produce a shock-compressed layer

13 Formation by turbulent fragmentation 2-D one-shot fragmentation Minimum stellar mass: solar mass (for shocked gas with temperature T ~ 10K) or even solar mass (for T ~ 6K) Cons: two or more coincident shocks are required to produce the high-density layer; not efficient enough to produce all BDs

14 Formation by disk fragmentation Bate, Bonnell & Bromm 2002; Whitworth & Stamatellos 2006; Stamatellos et al. 2007, etc. Disk fragmentation occurs when i) Disk is massive enough to be gravitational unstable, and ii) Compressional energy from condensation can radiate away efficiently

15 Formation by disk fragmentation Is convection efficient enough for cooling (for inner regions of disks around Sun-like stars, R ~ 3 to 30 A.U.)? Reason for BD Desert? Not a problem for outer regions (R > 100 A.U.) (Whitworth & Stamatellos 2006)

16 Formation by disk fragmentation Numerical simulations (Stamatellos & Whitworth 2007)

17 Formation by disk fragmentation Numerical simulations predictions: Low-mass H-burning stars forms close to the central star, while BDs form farther out in the disk a possible explanation of BD desert Most BDs forms with disk, however, most ejected BDs cannot retain their disks BDs that are companions to Sun-like stars are more likely to have discs than BDs in the field Binary properties. BD HS (H-burning star) and BD-BD binaries can exist even after ejection Existence of free-floating planetary-mass objects

18 Formation by ejection More than one protostars are formed in the collapse of a prestellar core grow by competitive accretion and interact dynamically protostars which get ejected from the core before they have time to grow to solar mass end up as BDs (Reipurth & Clarke, 2001) Even quite modest levels of turbulence are sufficient to ensure the fragmentation of the prestellar core; N-body dynamics will then almost inevitably eject one of the protostars, usually the least massive one.

19 Formation by ejection Numerical simulations (Bate, Bonnell & Bromm, 2002a, b, 2003; Delgado Donate, Clarke & Bate, 2003, 2004; Godwin, Whitworth & Ward-Thompson, 2004a,b) Low-mass disk (M DISK < 0.01 solar-mass, R DISK < 40AU) can be retained after ejection (but difficult to explain massive disks); Radial velocity distribution of ejected BDs is similar to that of the H-burning stars; Difficult to produce close BD-BD binaries

20 Formation by photo-erosion A pre-existing core of standard mass is overrun by an HII region photoerosion (Hester et al. 1996; Whitworth & Zinnecker 2004) Very inefficient one massive pre-existing prestellar core forms only one low-mass star or BD finally Can only work in the immediate vicinity of an OB star

21 Formation by binary disruption Goodwin & Whitworth 2008 BDs are initially binary companions, formed around low-mass H-burning stars. Most of these binaries are then gently disrupted by passing stars to create a largely single population of BDs and low-mass hydrogen-burning stars BDs will have velocity dispersions and spatial distributions very similar to higher-mass stars, and they will be able to retain discs, and thereby to sustain accretion and outflows. Formation by gravitational fragmentation of infalling gas into stellar clusters Bonnell et al. 2008

22 References Bate. M. R., Bonnell, I. A., Bromm, V., 2002a, MNRAS, 332, 65 Bate. M. R., Bonnell, I. A., Bromm, V., 2002b, MNRAS, 336, 705 Bate. M. R., Bonnell, I. A., Bromm, V., 2003, MNRAS, 339, 577 Bonnell, I. A., Clark, P., Bate, M. R.,2008, MNRAS, 389, 1556 Boyd, D. F. A., Whitworth, A. P., 2005, A&A, 430, 1059 Delgado Donate E. J., Clarke, C. J., Bate, M. R., 2003, MNRAS, 342, 926 Delgado Donate E. J., Clarke, C. J., Bate, M. R., 2004, MNRAS, 347, 759 Goodwin, S. P., Whitworth, A. P., 2007, A&A, 466, 943 Goodwin, S. P., Whitworth, A. P., Ward-Thompson, D., 2004a, A&A, 414, 633 Goodwin, S. P., Whitworth, A. P., Ward-Thompson, D., 2004b, A&A, 423, 169 Hester, J. J., et al. 1996, AJ, 111, 2349 Luhman, K. L., 2006, ApJ, 645, 676 Luhman, K. L., et al., 2005, ApJ, 635, 93 Luhman, K. L. et al., Protostars and Planets V (University of Arizona Press, Tucson; Eds. B. Reipurth, D. Jewitt, K. Keil), 443 Muzerolle, J., Hartmann, L., & Calvet, N., 1998, AJ, 116, 455 Muzerolle, J., et al. 2005, ApJ, 625, 906 Oliveira, J. M., Jeffries, R. D., & van Loon, J. T. 2009, MNRAS, 392, 1034 Padoan, P., Nordlund, A., 2002, ApJ, 576, 870 Reipurth, B., Clark, C. J., 2001, AJ, 122, 432 Stamatellos, D., Hubber, D. A., Whitworth, A. P., 2007, MNRAS, 328, 30 Whitworth, A. P., Goodwin, S. P., 2005, AN, 326, 899 Whitworth, A. P., Stamatellos, D., 2006, A&A, 458, 817 Whitworth, A. P., Zinnecker, H., 2004, A&A, 427, 299