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Queen Mary University of London

Queen Mary University of London. Star Formation When, Where, How Jim Emerson . Where Do Stars Form?. Spiral Galaxies contain younger Stars than Elliptical Galaxies Spiral arms – contain the brightest (hottest & youngest) stars. Our star in the Galaxy.

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Queen Mary University of London

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  1. Queen Mary University of London Star Formation When, Where, How Jim Emerson Goldsmiths Course

  2. Where Do Stars Form? • Spiral Galaxies contain younger Stars than Elliptical Galaxies • Spiral arms – contain the brightest (hottest & youngest) stars Goldsmiths Course

  3. Our star in the Galaxy Goldsmiths Course

  4. Rotation Curve (Tangential Velocity (km/sec) vs r (distance from centre - kpc) of a spiral Goldsmiths Course

  5. Differential Rotation Θ=rωθ≈const - observed ω≈const/r If spiral pattern stayed fixed expect ω=constant But it isn’t so any initial pattern should just ‘wind up’ with time Goldsmiths Course

  6. Wind up Goldsmiths Course

  7. When Do Stars Form? • arms would wind up over time • Age of galaxy ~10,000 million years • Age of solar system ~5,000 million years • Spiral arms of galaxies are delineated by O&B stars whose main sequence lifetimes are 5 million years • Therefore if they weren’t continually forming there should be none now • presence of arms NOW implies stars form continually • Observed Star Formation Rate in the Galaxy is ~ 5 solar masses/year Goldsmiths Course

  8. Where do stars Form? • As we already saw In Spiral arms • In particular in Giant Molecular Clouds • What do they from from? - matter (gas and dust) • Surveys of our galaxy for details Goldsmiths Course

  9. Planeof our Galaxy Mosaics of images along the galactic planein Visible light (above) in Infrared light (below) • Visible images affected by obscuration (absorption & scattering – collectively extinction) by dust in the interstellar medium • Infrared’s longer wavelengths relatively unaffected by dust extinction – better picture of true location of material close to a plane • Infrared also senses heat emitted from any warm dust Goldsmiths Course

  10. Scattering by dust:- Reflection nebulae Dark Cloud in Scorpius Cometary Globule Goldsmiths Course

  11. Absorption by dust: Barnard 68 • Newly formed stars are obscured by dust • Dark cloud Barnard68 is 500 light years away • Starlight is dimmed by factor of 1014 on passing through it • If between us & the Sun the Sun would be 15 times fainter than faintest star naked eye can see • Will probably collapse to form stars in 100,00 years • Cloud has~ 3Mo material Goldsmiths Course

  12. Infrared: Man striking a match • Infrared wavelengths see through dust • Infrared Wavelengths radiate heat from absorbed starlight Goldsmiths Course

  13. Emission from dust: Orion – stellar nursery:Far Infrared emission from dust (left)and visible emission from stars (right) Goldsmiths Course

  14. M16: Pillars of Star Formation • Photo-Evaporating Gaseous Globules • or EGGs • Elephant trunk globules • Ionizing radiation from stars out of top of picture erode material leaving these dense clouds. • Distance 7,000 ly • Contains about 50 stars about to form • Tallest 1 ly long Goldsmiths Course

  15. Hubble Space Telescope0.6 m SCUBA on the JCMT850 m Infrared Space Observatory 7 m Eagle Nebula Goldsmiths Course

  16. Proplyds Colours of emitting gases (disk) Continuum (see star) 17 x diameter of solar system Goldsmiths Course

  17. Dark disks seen silhouetted against glowing gas of Orion Nebula • Stars 0.3 to 1.5 solar masses • 1,000 my old • Sun is 4,500 my • Each image 30 times diameter of solar system • Disks 2-8 diameter solar system Goldsmiths Course

  18. Multiwavelength Goldsmiths Course

  19. Sky Mosaic in Visible light – Milky Way Goldsmiths Course

  20. Sky Mosaic from COBE in near IR light – Milky Way Goldsmiths Course

  21. What has to happen to form a star? • The density of the interstellar medium ranges from a few to a few 1,000 particles in volume of St Paul’s Cathedral dome • To go from even the densest such regions to a star requires a compression in density of a factor of 1034 • very hard to make any detailed computation over such a vast range of a parameter • Total mass in Molecular clouds in the Galaxy is ~109 Mo.Free fall time (tff= [4GM/r3] -1/2) for a cloud of 100 H atoms/cm3 is 4.3 106 years. => expected star formation rate of ~233 Mo/yr. • observed 5 rate is Mo/yr =>the clouds must be stabilised against collapse for ~50 (233/5) times longer than the free fall time or 2 108 years • How? Goldsmiths Course

  22. "Cores" and Outflows Giant Molecular Clouds Jets and Disks Solar System Formation The overall picture

  23. Pre-main sequence evolution Goldsmiths Course

  24. How might stars form (in theory)? • Tendency to Collapse due to • Gravity • Counteracted by material being held up by • Gas Pressure (depends on density and temperature) • Turbulent pressure (star formation starts when turbulence goes) • Magnetic field pressure (material can slip through magnetic field) • Rotation/Centrifugal force (causes • (perpendicular to rotation axis) • Inside-out collapse • support falls away and material rains down on protostar • Leading to Contracting Envelope • Material falling down onto Accretion Disk • Material accreting through disk (via ?) onto the star • Nuclear burning eventually begins in centre of star Goldsmiths Course

  25. Stages of Evolution Goldsmiths Course

  26. Details • When turbulence dissipates (much of it is supersonic so is dissipated in shocks) • Balance between gravitational PE and internal KE shows a density condensation of density n, temperature T structure bigger than a “Jeans length” RJ should collapse under gravity • RJ= 8.5 T 0.5 n-0.5 where RJ is in parsecs, T in K, n in H atoms/cm3 • or in terms of Jeans mass • MJ= 11.7 T 1.5 n-0.5 where MJ is in solar masses, T in K, n in H atoms/cm3 • and this will have a characteristic free fall time • tff= [4GM/r3] -1/2 • tff= 4.3 107 n-0.5 where tff is in years, n in H atoms/cm3 • and free fall velocity vff • vff= 0.045 n0.5 R where vff is in km/sec, n in H atoms/cm3 and R in parsecs Goldsmiths Course

  27. More details • For a monatomic gas of hydrogen sound speed a is related to Temperature T by • a = 0.06 T 0.5 where a is in km/sec and T in K • Condition for collapse can also be usefully thought of as • time for sound to cross cloud >> free fall time • as cloud then has time to collapse before a pressure gradient (travelling at the sound speed) can set up to oppose it • Once contraction starts it accelerates if cloud is isothermal • Consider cloud mass M, radius R, density n, temperature T • Gravitational force/cm3 ~ n/R2 ~ M/R5 • Pressure gradient ~ P/R ~MkT/R3 . 1/R ~ MT/R4 • so as R gets smaller gravity wins [as long as clouds stays isothermal] • But as contraction continues cloud becomes opaque and energy liberated as collapses can no longer be radiated away Goldsmiths Course

  28. Accretion rate and Luminosity • The collapse will be halted until the T rises enough that the opacity changes (dust melts, molecules dissociate, atoms ionise) and an isothermal state is reacquired. Thus on a T, L diagrams a star will move about as it heads towards the stable Hydrogen burning state where it will spend most of its life on the ‘main sequence’ (a T,L relationship for stars burning H) • Accretion rate = Jeans mass / free fall time • dM/dt = 5 10-8 T 3/2 Mo/year • Luminosity L ~ (GM*dM/dt)/R* • Star will not grow to arbitrary mass as radiation pressure on infalling material limits masses to ~100Mo • Any winds of outflows from the star may also stop its growth Goldsmiths Course

  29. Collimated winds from Young Stellar Objects • Almost all young stellar objects -- which are gaining material (equatorially) by accretion are also losing it (in the polar directions) the form of a wind or jet. • This helps solve the “angular momentum” problem. • This helps the stars become visible (drives away dust out of their disk) – except along disk equator • The jets interact with the surrounding gas often producing shocks Goldsmiths Course

  30. Conceptual idea of jet from a YSO Goldsmiths Course

  31. Episodic ejections? Goldsmiths Course

  32. Motion of a jet • Ejection speed 0.5 million mph • Episodic ejections • Disk in shadow • Flared shape • NB relative size • of Solar System Goldsmiths Course

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