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Sternentstehung - Star Formation

Sternentstehung - Star Formation. Sommersemester 2006 Henrik Beuther & Thomas Henning. 24.4 Today: Introduction & Overview 1.5 Public Holiday: Tag der Arbeit 8.5 --- 15.5 Physical processes, heating & cooling, cloud thermal structure

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Sternentstehung - Star Formation

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  1. Sternentstehung - Star Formation Sommersemester 2006 Henrik Beuther & Thomas Henning 24.4 Today: Introduction & Overview 1.5 Public Holiday: Tag der Arbeit 8.5 --- 15.5 Physical processes, heating & cooling, cloud thermal structure 22.5 Physical processes, heating & cooling, cloud thermal structure (H. Linz) 29.5 Basic gravitational collapse models I 5.6 Public Holiday: Pfingstmontag 12.6 Basic gravitational collapse models II 19.6 Accretion disks 26.6 Molecular outflow and jets 3.7 Protostellar evolution, the stellar birthline 10.7 Cluster formation and the Initial Mass Function 17.7 Massive star formation, summary and outlook 24.7 Extragalactic star formation More Information and the current lecture files: http://www.mpia.de/homes/beuther/lecture_ss06.html beuther@mpia.de

  2. Summary protostellar evolution • The “first core” contracts until temperatures are able to dissociate H2 to H. • H-region spreads outward, T and P not high enough to maintain equilibrium, • further collapse until H gets collisionally ionized. The dynamically stable • protostar has formed. • - Accretion luminosity. Definition of low-mass protostar can be “mass-gaining • object where the luminosity is dominated by accretion”. • - Structure of the protostellar envelope and effects of rotation. • - Stellar structure equations: follow numerically the protostellar and then • later the pre-main sequence evolution. • Convection and deuterium burning. • End of protostellar/beginning or pre-main sequence evolution --> birthline. • Pre-main sequence evolution in the Hertzsprung-Russel (HR) diagram. • Connection of HR diagram with protostellar and pre-main sequence • class scheme.

  3. Msun 32 10 1 0.1 0.01 Muench et al. 2002 Shu et al. 2004 Clusters and the Initial Mass Function (IMF)

  4. Shu et al. 2004 General properties of the IMF • Almost all stars form in clusters, isolated star formation is exception. • Seminal paper 1955 by E. Salpeter: linear: dN/dM ~ M-2.35 • log: d(logN)/d(logM) ~ (logM)-1.35 • derived for stars approximately larger than 1Msun. • - More detailed current description of the total IMF (e.g., Kroupa 2001): • dN/dM ~ M-a with • a = 0.3 --> 0.01 ≤ M/Msun ≤ 0.08 (brown dwarf regime) • a = 1.3 --> 0.08 ≤ M/Msun ≤ 0.5 • a = 2.3 --> 0.5 ≤ M/Msun • - Characteristic mass plateau • around 0.5 Msun • - Upper mass limit of ~ 150Msun • - Largely universally valid, in • clusters and the field in our • Galaxy as well as in the • Magellanic Clouds .

  5. Star cluster NGC3603 6 x 6 pc 1 pc diameter 10 000 stars between 0.5 & 120 Msun Stolte et al. 2006

  6. NGC3603 Mass segregation Stolte et al. 2006

  7. Westerlund 1 Brandner et al. 2006

  8. Hartmann 2002 Goodwin et al. 2002 Deviations from the IMF I: Taurus - Taurus: filamentary, more distributed mode of star formation. - The core-mass function already resembles a similar structure. Grey: 12CO

  9. Deviations from the IMF II: The Arches cluster Stolte et al. 2005

  10. Orion B South 13CO(2-1) Cloud mass distributions Kramer et al. 1996

  11. Pre-stellar core mass functions I Motte et al. 1998

  12. M-1.5 M-0.5 T from Bonnor-Ebert fits Constant T Pre-stellar core mass functions II Orion B South Johnstone et al. 2006

  13. Gravitational fragmentation Initial Gaussian density fluctuations • With the Jeans mass: • MJ = 1.0Msun (T/(10K))3/2 (nH2/(104cm-3)-1/2 • one can in principal obtain all masses. • - Unlikely to be the sole driver for IMF: • - Initial conditions have to be very spatial and • nearly always the same. • - With the usual temperatures and densities, • the most massive fragments are hard to • produce. Klessen et al. 1998

  14. (Gravo)-turbulent fragmentation I Klessen 2001 Freely decaying tubulence field Low k: large- scale driving High K: small- scale driving Driven turbulence with wave- number k (perturbations l= L/k) • - Turbulence produces complex network • of filaments and interacting shocks. • - Converging shock fronts generate • clumps of high density. • Collapse when the local Jeans-length • [lJ = (pat2/Gr0)]gets smaller than the • size of fluctuation. • - Have to collapse on short time-scale • before next shock hits the region. • --> Efficiency of star formation depends • strongly on the wave-number and • strength of the turbulence driving. • --> Large-scale less strongly driven • turbulence results in clustered • mode of star formation. • --> Small-scale srong driving results • in more low-mass protostars • and more isolated star formation.

  15. (Gravo)-turbulent fragmentation II Histogram: Gas clumps Grey: Jeans un- stable clumps Dark: Collapsed core • - 2 steps: 1.) Turbulent fragmentation --> 2.) Collapse of individual core • Large-scale driving reproduces shape of IMF. • However, under discussion whether largest fragments really remain stable • or whether they fragment further …

  16. Bonnell et al. 2004 • - Gas clump first fragments into a large number • of clumps with approximately a Jeans mass. • Hence fragmentation on smaller scales. • Then each clump subsequently accretes gas • from the surrounding gas potential. Even gas • that was originally far away may finally fall • onto the protostar. Distance of gas that is ultimately accreted. Competetive Accretion

  17. Simulation example SPH simulation. Initial conditions: Uniform density 1000Msun 1pc diameter Temperature 10K

  18. Pre-stellar core mass functions III Motte et al. 2001

  19. or fragmentation or fragmentation General properties (maybe) governing the IMF • Rather general agreement that • characteristic mass plateau must be due • to the original fragmentation processes. • At the low-mass end, fragmentation • may not be efficient enough and • dynamical ejection could help. - Are large, massive fragments stable enough to be responsible for the Salpeter tail of the IMF, or do large clumps further fragments that later competitive accretion sets the masses of the high-mass end? -- Initially, one would expect fragmentation down to the original Jeans-mass at the beginning of collapse. -- However, early accretion luminosity may heat the surrounding gas relatively far out --> This would increase the Jeans-mass and inhibit further fragmentation. --> Not finally solved yet!

  20. Tempereature variation with increasing density Larson 1985 Characteristic mass defined by thermal physics • - Jeans mass depends on T: • MJ = m1at3/(r01/2G3/2) • = 1.0Msun (T/(10K))3/2 (nH2/(104cm-3)-1/2 • The number of bound fragments is • generally similar to the number of • Jeans-masses within the cloud. • At low densities, temperature decreases with increasing density, regions • can cool efficiently via atomic and molecular line emission. • --> decreasing MJ suggests that fragmentation may be favored there. • With further increasing density gas thermally couples to dust and clouds • become partially optically thick. Cannot cool well enough anymore • --> temperature increases again. • --> MJ decreases slower, potentially inhibiting much further fragmentation. • - Regime with lowest T should then correspond to the preferred scale for • fragmentation. The Bonnor-Ebert mass at this point is about 0.5 Msun.

  21. The IMF in extreme environments Stolte et al. 2005 • - Low-mass deficiency in Arches cluster near Galactic center. • The average densities and temperatures in such an extreme • environment close to the Galactic Center are much higher • --> Gas and dust couples at higher temperatures. • --> Clouds become earlier opaque for own cooling. • --> Larger characteristic mass for the fragmentation process!

  22. Clustering around intermediate-mass stars - The clustering increases with the mass of the dominating sources.

  23. Williams et al. 2004 Shirley et al. 2003 Reid et al. 2005 Beltran et al. 2006 Log[M/Msun] Going to high-mass star formation Cumulative mass functions from single-dish surveys of massive star-forming regions resemble Salpeter-IMF. But regions sample evolving clusters?!

  24. Fragmentation of a massive protocluster • 12 clumps within • each core • Integrated masses • 98Msun (south) • 42Msun (north) • --> 80 to 90% of • the gas in halo • Clump masses • 1.7Msun to 25Msun • Column densities • 1024cm-2 -->Av~1000 Spatial filtering affects only large scale halo on scales >20’’ Assumptions: - All emission peaks of protostellar nature - Same temperature for all clumps (46K, IRAS) Caveats: - Temperature structure - Peaks due to different emission processes, e.g., outflows? Beuther & Schilke 2004

  25. Order of star formation Kumar et al. 2006 - Detection of a large fraction of embedded clusters around young High- Mass Protostellar Objects. Since the detected sources are largely class I and II, and the massive HMPO are still forming, it indicates that low- mass sources may form first and high-mass sources later.

  26. Summary • The most widespread mode of star formation is clustered. • The IMF is almost universally valid, Salpeter-like for >1Msun, characteristic • mass plateau around 0.5Msun. • - Mass segregation in clusters. • - Hertzsprung-Russel diagrams allow to estimate cluster ages. • - Anomalous IMFs (e.g., Taurus or Arches). • - Cloud mass distributions versus pre-stellar core mass distributions. • - Gravitational fragmentation. • - Gravo-turbulent fragmentation. • - Competitive accretion. • - General features of the IMF. The plateau at 0.5Msun maybe explicable by • thermal physics. • - High-Mass star formation differences? • - Order of star formation.

  27. Sternentstehung - Star Formation Sommersemester 2006 Henrik Beuther & Thomas Henning 24.4 Today: Introduction & Overview 1.5 Public Holiday: Tag der Arbeit 8.5 --- 15.5 Physical processes, heating & cooling, cloud thermal structure 22.5 Physical processes, heating & cooling, cloud thermal structure (H. Linz) 29.5 Basic gravitational collapse models I 5.6 Public Holiday: Pfingstmontag 12.6 Basic gravitational collapse models II 19.6 Accretion disks 26.6 Molecular outflow and jets 3.7 Protostellar evolution, the stellar birthline 10.7 Cluster formation and the Initial Mass Function 17.7 Massive star formation, summary and outlook 24.7 Extragalactic star formation More Information and the current lecture files: http://www.mpia.de/homes/beuther/lecture_ss06.html beuther@mpia.de

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