Early Stages of (Low-Mass) Star Formation: The ALMA Promise

Transcription

1 Early Stages of (Low-Mass) Star Formation: The ALMA Promise Philippe André, CEA/SAp Saclay Outline Introduction: Prestellar cores and the origin of the IMF Identifying proto-brown dwarfs Bate et al Probing the binary fragmentation process Census and dynamical structure of protoclusters Conclusions

2 Classical scenario for low-mass star formation (modified from Shu et al ARA&A - cf. Protostars & Planets IV+V) A) Prestellar phase (t ~ 10 6 yr) Fragmentation and collapse B) Protostellar phase (t ~ 10 5 yr) Accretion and ejection Class 0 Bipolar flow (M * < M env ) Envelope Disk 10 4 AU Prestellar cores C) Pre-main sequence (t ~ yr) Gravitational contraction André, Motte, Bacmann 1999 T Tauri star Class I (M * > M env ) Class II ---> Class III Protoplanetary disk 0.1 pc Cold SEDs ALMA Mostly Externally heated Protostar Young main-sequence star with planets Debris disk Spitzer L bol ~ 0.1 Lo

3 Star Formation Rate on Galactic Scales: Directly Proportional to the Mass of Dense Gas Gao & Solomon 2004; Wu, Evans et al. 2005; but see Gracia-Carpio et al Prestellar dense cores are the basic units of the star formation process

4 The prestellar core mass function (CMF) resembles the IMF NGC2068 at 850 µm Salpeter s IMF (dn/dm ~ M -2.4 ) pc 0.3 M o CO clumps (Blitz 1993) Motte et al Motte et al The IMF is at least partly determined by pre-collapse cloud fragmentation (cf. turbulent fragmentation scenario - Padoan & Nordlund 2002) ALMA needed to see if this result holds in the brown dwarf regime Motte, André, Neri 1998 See also: Testi & Sargent 1998; Johnstone et al. 2000, 2001; Onishi et al. 2002; Stanke et al. 2006; Enoch et al. 2006; Alves et al And for massive cores: Beuther & Schilke 2004; Reid & Wilson 2005, 2006

5 Two paradigms for the formation/evolution of prestellar cores The classical ambipolar diffusion picture (e.g. Shu et al. 1987, 2004; Mouschovias & Ciolek 1999) The dynamic, turbulent picture (e.g. Klessen et al. 2000; Vazquez-Semadeni et al. 00; Padoan & Nordlund 02) ~ pc ~ pc t = 0.0 M * = 0% t = 1.5 M * = 10% Initially: M/Φ ~ N/B < 0.12/ G 1/2 B > B crit ~ 10 µg x (N H2 /10 21 cm -2 ) t = 2.0 M * = 30% t = 2.8 M * = 60% Slow formation and decoupling of supercritical cores within magnetically-supported clouds Strong, organized B field in cores Rapid formation of (interacting) protostellar cores from turbulencegenerated density fluctuations Weak, chaotic B field in cores

6 Turbulent fragmentation: A possible scenario for the formation of brown dwarfs (M BD < M o ) Strong shock compression due to supersonic turbulence can generate ultra-low-mass prestellar cores with M ~ M BD (e.g. Padoan & Nordlund 2004) or «(pre)proto-brown dwarfs» Proto-brown dwarfs are expected to be compact (R < AU), very dense (n > cm -3 ), and associated with large velocity jumps Easy targets for ALMA in nearby star-forming regions Alternative scenario: Dynamical ejection of stellar embryos Velocity (km/s) Density (cm -3 ) Greyscale: l.o.s. velocity dispersion Contours: column density Density and velocity profiles Whitworth et al. 2006, PPV c s Klessen et al Radius (AU)

7 Fragmentation to individual protostars: A two-step process 1. From molecular cloud to prestellar cores Turbulent, weakly-condensed molecular cloud Gravo-turbulent cloud fragmentation mediated by magnetic fields Cores (e.g. Klessen & Burkert 2000, Nakamura & Li 2005) 2. From prestellar core to protostar(s) Collapsing, weakly-turbulent prestellar core Dynamical, rotationally-driven core fragmentation modified by magnetic fields (e.g. Goodwin et al. 2006, Fromang et al. 2006) Protostellar system Ejection?

8 Implications for the IMF/CMF IMF of individual stars = convolution of the core mass function (CMF) with the distribution of object masses produced by binary fragmentation In typical cases, the IMF follows the CMF at the high-mass end but differs at the low-mass end (cf. Delgado-Donate et al. 2003) Observationally, the agreement between the system IMF and the CMF is not perfect at the lowmass end Need to understand the binary fragmentation process with ALMA to fully explain the lowmass end of the IMF Salpeter s IMF 0.3 M o system IMF CO clouds & clumps Motte, André, Neri 98

9 Formation of multiple protostellar systems Collapse simulations (Goodwin et al. 2004) 800 AU NGC1333 IRAS4 - BIMA mm Most MS & PMS stars are multiple (e.g. Duquennoy & Mayor 91, Duchêne et al. 04) Need to study the collapse phase (Class 0 objects) to constrain the fragmentation mechanism(s) and assess the importance of dynamical ejections 600 AU 5 A2 3 Today: only the widest/most massive protostellar systems are accessible With ALMA: complete samples of hundreds of protostars; orbital/proper motions (1km/s <=> pc) A Frequency and properties of systems between ~ 3 AU and ~10000 AU Looney, Mundy, Welch (2000)

10 Strong influence of the magnetic field Fromang, Hennebelle, Teyssier 2006 A&A (MHD version of «RAMSES» AMR code) With B xy Without B xy xz 400 AU

11 Critical tests of fragmentation models with ALMA B = 0 B = B crit /2 600 AU t ~ 10 4 yr (i.e. Class 0 protostellar stage) v = 2 km/s RAMSES v = 1 km/s Fromang, Hennebelle, Teyssier Simulated images with ALMA ( d = 140 pc) ALMA simulator in GILDAS software (cf. ALMA memo 386 by Pety, Gueth, Guilloteau)

12 Census and dynamical structure of protoclusters Most stars form in clusters (e.g. Lada & Lada 03) 5 ~ 0.7 pc Trapezium in JHK (SOFI - NTT) 30 Precursors: Submm protoclusters ρ Oph 1 pc Motte et al NGC2068 Muench et al pc SCUBA 850 µm Johnstone et al With ALMA at 1.3mm (27 fov) : OTF mosaicing of 30 x30 with (λ/4d) x (λ/2d) sampling --> ~ pointings ~ 1 sec/point to detect proto-brown dwarfs of ~ 3 M J (T= 10K) at 10σ in the continuum Short spacings required (SD + ACA) Total time ~ 10 hr x 4 ~ 40 hr

13 Dynamical studies using, e.g., N 2 H +, N 2 D + as tracers NGC µm continuum (JCMT) NGC2068 in N 2 H + (1-0) (IRAM 30m) pc Motte et al Klessen & Burkert Hydrodynamic simulations of protocluster evolution predict strong dynamical interactions at the intersection of filaments. ALMA can probe the corecore velocity dispersion down to ~ 100 AU scales + proper motions in the nearest protoclusters. Narrow N 2 H + ( ) lines (ΔV < 0.5 km/s) Good determination of the l.o.s. velocities Belloche et al. 2001; André et al. 2006

14 Evidence of central dynamical interactions in protoclusters: IRAM (30m+PdBI) observations of NGC2264-C (d ~ 800 pc) and comparison with SPH simulations Peretto, André, Belloche 2006 Peretto, Hennebelle, André 2006 Potential formation of massive (M > 50 M o ) core by merging of > 3 Class 0 objects With ALMA: Similar studies possible in more distant, more massive protoclusters Observations Simulations 1.2mm continuum map (IRAM 30m) Position (arcsec) Synthetic column density map Position (arcsec) N 2 H+( ) PV diagram Velocity (km/s) Velocity (km/s) PdBI+30m 5 Position (arcsec) Synthetic PV diagram PdBI+30m arcsec

15 Conclusions ALMA will revolutionize our understanding of the star formation process by making possible quantitative studies of the structure and dynamics of both individual cores and protoclusters throughout the Galaxy Key open issues that can be addressed Origin of the low-mass end of the IMF Do brown dwarfs form from ultra-low-mass cores or from binary fragmentation followed by dynamical ejection? How does the collapse of a typical prestellar core lead to a binary/multiple star system? Role of dynamical interactions in embedded star clusters and the formation of massive stars?