Early Phases of Star Formation

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1 Early Phases of Star Formation Philippe André, CEA/SAp Saclay Outline Introduction: The earliest stages of the star formation process Probing the formation and evolution of prestellar cores with Herschel Detailed studies of the collapse phase and protostellar systems with ALMA Conclusions: The Herschel and ALMA promise Taurus extinction map 10 deg ~ 25 pc dense cores 0.01 pc

2 Embedded Phases Paradigm for the formation of isolated, single low-mass stars (modified from Shu et al. 1987) Cloud fragmentation Core Collapse Accretion & ejection Courtesy T. Greene

3 Key questions on the early phases of star formation: Herschel What determines the distribution of stellar masses = the IMF? What generates prestellar cores and what governs their evolution to protostars and proto-brown dwarfs? Timescale of core/star formation? Slow, quasi-static process or fast, dynamic process? ALMA Why and how does protostellar collapse typically produce binary/multiple systems of young stars? Formation of embedded star clusters and of massive stars?

4 Prestellar Cores (t < 0) The progenitors of protostars Class 0 protostars (t > 0) Protostars in the build-up phase 0.1 pc Sizes: ~ 0.01 pc to ~ 0.1 pc (resolved by Herschel up to ~ 0.5 kpc) 0.1 pc No complete census so far Gravitationally bound (M ~ M VIR, M * = 0) Massive envelopes (M env > M * ) Greybody (T = 9K, b = 2) L1544 (Starless) Submm-only objects whose SEDs l ~ mm Greybody (T = 13K, b = 1.5) IRAM (Class 0) Herschel bands essential for luminosity and temperature determinations

5 Importance of complete surveys of these early stages The mass distribution of prestellar condensations resembles the IMF Salpeter s IMF CO clouds & clumps (Motte, André, Neri 1998; Testi & Sargent 1998; Johnstone et al. 2000; Motte et al. 2001; Onishi et al. 2002; Beuther & Schilke 2004) 0.4 M o Motte, André, Neri 1998 IMF at least partly determined by cloud fragmentation at prestellar stage Herschel & ALMA needed to see 1) if this result still holds in the brown dwarf regime and 2) whether the break at ~ 0.5 M o varies with Jeans mass

6 Importance of direct determinations of the dust properties Current mass estimates assume constant dust properties (T dust, k dust ) But both T dust and k dust are uncertain and may vary from object to object Expectations: T dust and k dust as N H2 (or A V ) Mass spectrum of r Oph prestellar condensations assuming a distribution of dust temperatures Predicted Temperature Profile (G 0 ~ 100) Salpeter s IMF Bouwman et al (also Evans et al. 01; Zucconi et al. 01; Stamatellos et al. 04) Direct temperature measurements with Herschel are crucially needed

7 Next challenge: What generates prestellar condensations in the ISM? Physics of core formation determines the IMF ==> It is crucial to get at a global view of core formation within molecular clouds Taurus 10 deg ~ 25 pc 5 Near-IR extinction map (3 resol.) 0.1 pc Padoan, Cambrésy, Langer 2002 Onishi,André et al. Probe a wide range of scales from < 0.01 pc (i.e. < 140 pc) to > 10 pc (several degrees) and a wide range of column densities from the diffuse ISM (A v < 1) to protostellar condensations (A v > ). Calls for wide-field FIR/submm dust imaging with Herschel (SPIRE+PACS), followed up by complementary (sub)mm line mapping with, e.g., ALMA

8 Herschel Photometric Survey of the Gould Belt An Herschel GT Key Project: Gould Belt SPIRE mm survey of ~ 140 deg 2 in both active and quiescent nearby (d < 0.5kpc) molecular clouds, including densest (A v > 3) part of Gould belt PACS mm imaging of ~ 15 deg 2 : nearby protoclusters, isolated dense cores, and representative, selected areas Sensitivity (5s): A v ~ 1 ~ cirrus confusion Expected immediate outcome of such a survey: A few hundred Class 0 protostars and a few thousand prestellar condensations with well-characterized temperatures, luminosities, masses (+ profiles in many cases) Good sampling of the prestellar core mass function from the substellar to the intermediate-mass regime; lifetimes as a function of mass, density, environment Unique database for follow-up kinematical/multiplicity studies with ALMA

9 Detailed Kinematical Studies of Individual Cores IRAM04191 Time Belloche et al Derived density and infall velocity profiles inconsistent with inside-out collapse (Shu77) Today: Only a few isolated cores with resolved profiles. With ALMA: Large samples

10 Formation of multiple protostellar systems Simulations of fragmentation during rotating collapse 200 AU Hennebelle, Whitworth et al. (2004) Simulated image with ALMA 0.35 Most MS & PMS stars are multiple (e.g. Duquennoy & Mayor 91, Ghez et al. 97) Need to study the collapse phase (Class 0 objects) to constrain the fragmentation mechanism(s) and assess the importance of dynamical ejections Today : only the widest/most massive protostellar systems are accessible With ALMA : complete samples of hundreds of protostars; orbital/proper motions (1km/s <=> pc) Frequency and properties of systems between ~ 3 AU and ~10000 AU

11 Fragmentation is a two-step process: 1) Turbulent cloud fragmentation generates prestellar condensations 2) The collapse of prestellar condensations produces protostellar systems The IMF results from the convolution of the condensation mass function (CMF) with the mass fraction probability distribution (MFPD) produced by binary fragmentation Example of MFPD for M cond = 1 M o Some simulations suggest that protostellar collapse typically produces small-n, unstable systems (N ~ 5) (e.g. Delgado- Donate et al. 2003). Low-mass members are ejected as single stars, leaving behind massive binaries. In this case, the IMF resembles the CMF. dn/dlogm Singles Binaries Mass (Solar) Delgado-Donate, Clarke, Bate (2003)

12 Conclusions: The Herschel and ALMA promise Wide-field unbiased surveys of nearby clouds at l ~ mm with Herschel will probe the prestellar mass distribution down to substellar (~ 0.01 M o ) masses and provide complete samples of prestellar condensations and young protostars ALMA will make possible quantitative dynamical studies of both individual cores/protostars and protoclusters throughout the Galaxy Fragmentation in multiple systems, evolution of angular momentum and accretion rate, initial conditions of protoplanetary disk growth