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9/21/12 ASTR 541 Seminar in Theoretical Astrophysics Goals of Star Formation Theory • Predict the rate of star formation – in clouds – in galaxies • Predict distribution of stellar masses • Predict properties of individual stellar systems • Quantify effects of star formation on surroundings • Explain detailed phenomenology of starforming systems at all scales 1 9/21/12 A Universe of stars… A Universe of galaxies… 2 9/21/12 Kepler Mission/Jason Rowe A Universe of exoplanets… The arena Rose?e molecular cloud/Herschel PACS/SPIRE Orion op6cal & CO 1-‐0/Tom Dame 3 9/21/12 Tools • Analytic/semi-analytic theory to identify key processes & scalings, develop idealized models • Time-dependent numerical simulations – HD, MHD, RMHD for detailed tests, models • Synthetic observables from idealized models & simulations • Observations, especially surveys, in many wavelengths: – mm lines for molecular transitions, 21 cm for HI – IR lines, IR continuum for atomic gas, dust – Optical, UV for heated, shocked, ionized regions – X-ray for magnetically-active TTSs Narrative of Star Formation 4 9/21/12 Star formation in the large Star formation begins when massive bound structures condense gravitationally out of the diffuse ISM to create giant molecular clouds (GMCs). Mul6wavelength M51/ courtesy S. Vogel 5 9/21/12 Shetty & 9/21/12 Ostriker (2006) 11 Molecular gas in the MW Panoramic view inner galaxy (BU/FCRAO) • Mtot~109M • Giant Molecular Clouds (GMCs) (~105 -‐ 10 7 M ) All stars form from molecular component 9/21/12 Sun is here Face-‐on reconstruc6on of inner galaxy from cloud veloci6es 12 6 9/21/12 Trapezium/Orion GMC • GMCs inherit turbulence from the diffuse ISM • Damping of turbulence is rapid • Maintenance of turbulence by external, internal sources under study 7 9/21/12 Ikeda et al (2002) Astrophysical Journal • GMC turbulence is highly supersonic, Alfvenic • Imposes log-normal density distribution • Combines with gravity to cause fragmentation into hierarchy of clumps and filaments Roman-‐Duval et al 2011 • GMCs and spatially defined substructures have v ~ R ½ scaling,, reflecting PS for supersonic turbulence • Intermediate-scale structures are transient 8 9/21/12 Internal dynamics of GMCs 9/21/12 17 • Densest regions become self-gravitating cores, with subsonic internal velocities • Cores form within filaments, and are clustered • Core mass spectrum appears similar to the IMF Perseus molecular cloud (courtesy A. Goodman) 9 9/21/12 • Dense cores that become magnetically supercritical undergo self-gravitating collapse • Cores first become internally stratified (from outside to inside) • Envelope infall is from inside to outside • Accretion rates decline over time • Observed core lifetimes consistent with moderate B fields • Bondi-Hoyle accretion from ambient medium can increase stellar mass after envelope infall Prestellar collapse and infall 10 9/21/12 Core formation with lowvelocity turbulence Core formation with high velocity turbulence 11 9/21/12 • Disks form interior to accretion shock • Size/mass is affected by magnetic braking • Disk accretion mechanisms: Li et al (2011) – Gravitational torques (subject to mass) – MRI (subject to ionization) – Accretion is nonsteady: FU Ori outbursts • Winds are magnetocentrifugally driven from disk surface layers • Inner portion is collimated magnetically • Impact of wind on surrounding gas sweeps up molecular outflow 12 9/21/12 • Disk is cleared away by accretion + photoevaporation • Gap opening starts in the center • Half-life is 2 Myr Heznandez et al 2007 • Growth of solids from dust to planets occurs in several stages • Solid bodies are subject to migration from aerodynamic and gravitational drag • Gas/grain instabilities likely needed to collect sufficient mass to cross “metersized” barrier 13 9/21/12 Star cluster formation Most star formation occurs in clusters, including both high and low mass stars 9/21/12 27 • Radiation pressure affects dynamics of massive star formation, for individual stars and clusters • Massive stars ionize their surroundings, creating expanding HII regions • Feedback from massive stars is crucial in destroying GMCs, limiting SF efficiency to ~ 1-5 % over cloud lifetime 14 9/21/12 Kim, Kim, & Ostriker (2011) • Supernovae are important in energizing the diffuse ISM, driving turbulence • Large-scale rate of star formation may be selfregulated to replenish turbulence, offset cooling, and maintain ISM equilibrium 15 9/21/12 16
