Chapter 6: Planetological foundations for origins of life A. Major questions in star formation: 1. What determines the stellar mass spectrum (the “initial mass spectrum” IMF)? 2. How do individual stars/disks/jets form? 3. Do all stars form in the same way? (both low and high mass stars)? 4. How does star formation affect planet formation? [what accounts for weird extrasolar planetary systems?] Star formation in the Milky Way: Stars form in massive clouds of dusty, cold, molecular gas - To detect gas - map millimetre wave emission from carbon monoxide molecule. - To detect dust - map sub-millimetre emission from dust grains (eg. Use James Clerk Maxwell Telescope – on top of Mauna Kea volcano - Hawaii) Radio Star formation in the Galaxy The Galactic Center in visible Light Optical Optical images and infrared images of the Orion Nebula IRAS satellite: sensitive at wavelengths 10 – 100 microns Orion GMC - and the Orion Nebula Cluster Most stars form as members of star clusters and not in isolation: Major clue to origin of IMF….. Johnstone et al (2000) Super-massive star clusters Star cluster in the Large Magellanic Cloud, (HST image) The Origin of Stellar Masses: Formation of Molecular Cloud Cores? • Numerous small dense gas “cores” within a clump. Individual stars form in cores – typically 0.04 pc in size (Motte et al 2001) Origin of stellar masses – have same distribution in mass as small gas cores How do nearby stars form in molecular clouds? Clouds are turbulent Turbulence produces density fluctuations that resemble rotating cores. Simulations and theory show that “turbulent fragmentation” can produce core mass spectrum. Turbulence is universal – may imply universality of the IMF Largest star formation simulation ever done: 100,000 cpu hours! - Begin with: cloud is 1.2 light-years across, contains 50 solar masses of gas. - Initial turbulence in the cloud fragments it – then gravity pulls regions together to form “cores” Turbulence and star cluster simulation - shows highly filamented structure - shows many small overdense regions which can be identified with “cores”. - cores formed through turbulent fragmentation Tilley & Pudritz (2004) Massive star formation – filamentary accretion FLASH – Adaptive Mesh Refinement (AMR) simulation: (Banerjee, Pudritz, & Anderson 2006: start with TP04 ) Collapse along filament into a forming disk... Tilley & Pudritz ‘04 – hydro simulations of turbulent fragmentation M peak 103 M J 103 M tot / nJ Simulating star formation in magnetized clouds (Tilley & Pudritz 2005) Turbulence breaks up clouds into dense cores in which stars form Gravitational collapse of core: formation of a star/disk/jet Infrared image Barnard 68 (Alves et al 2001): excellent fit with Bonner-Ebert model (pressure truncated isothermal sphere) Disks around young – and old stars Orion Proplyd – star in formation Submm image of Epsilon Eridanni Greaves et al (1998) Disks in the Orion nebula… Disk structure: reprocessing stellar radiation Submm Infrared Optical Radiative resprocessing: hydrostatic equilibrium disk models 1: Disk Surface Tds 2: Disk Interior Ti Chiang and Goldreich (1997) Chemisty in protoplanetary disks: mm wavelengths (Aikawa et al 2002): distribution of T (top row) and n (bottom row), in D’Allessio et al model. Columns correspond to 3 different accretion rates. Top: dark is low CO/H2, grey is higher: Molecular Probes of Inner Disks H2 UV, NIR, MIR H2O ro-vib OH v=1 CO v=1 CO v=2 0.1 AU 1 AU 10 AU ~1000 K ~200 K ~50 K Are the organic precursor molecules for life common in planet-forming disks? • The MIR is rich in transitions of organic molecules Hydrocarbons in massive protostar NGC 7538 Collapse & disk formation: Density (Banerjee & Pudritz, 2004) Jets are strongly correlated with disk properties… what produces jets? Magnetic fields are crucial… Collapse of a magnetized core: produce outflows by “magnetic centrifuge”. (Banerjee & Pudritz 2006) Jets as disk winds: (Banerjee & Pudritz 2006) - launch inside 0.07 AU (separated by 5 month interval) - jets rotate and carry off angular momentum of disk - spin of protostellar core at this early time? Universality – do all stars form in the same way? Brown dwarfs: observed to have disks and jets! (eg. Whelan et al 2006 for 60 MJupiter BD). Massive YSO jets: massive accretion > 0.001 solar masses/yr prevent radiation pressure from blowing away the infall: -> massive star formation by accretion picture too? (McKee & Tan 2003) - some massive stars observed to have both disks and jets.
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