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Star Formation IV. Star Formation 1. Basic Aspects of Star Formation • Star Formation: The fundamental cosmic (baryonic) process Determines cosmic fate of normal matter Galaxy formation, Star Conditions for life evolution, IMF Formation Elements Planet formation (He => U) Clusters Light, K.E. of ISM black holes WS 2018/19 (AGN,CDE stellar) IV 2 1 Star formation visible on local scales 3 Stellar associations: Does the energy feedback self-regulate the SF? 2. Star Formation in Spiral Galaxies IC5333 in Hα star formation follows the gaseous spiral arms. 4 2 Star formation can also happen in diffuse, less dense HI gas at the rim of gas disks. NGC 4625 UV disk is 4x larger than optical disk. Gil de Paz et al. (2005) 6 3 2.1. “Kennicutt/Schmidt law”: 2 global relat.s n SF threshold at g 6 M SF g (1989) ApJ, ˙ n 1.4 WS 2018/19 CDE IV 8 4 1.40.15 (2.5 0.7) 104 g M yr 1kpc 2 SF 2 s 1Ms pc g SF 0.017 g g dyn WS 2018/19 CDE IV 9 Kennicutt (1998) ApJ, 498 Can we understand the SF – gas surface density relation? if large - scale SF is produced by small - scale dynamics over a large area : g g 1.5 (for const. H) SF 1 g SF 2 (G g ) but : SF could be very short WS 2018/19 CDE IV 10 5 Starbursting galaxies drop off the KS correlation; the same for mergers. Hensler (2012) ApSS Proceed. + WS 2018/19 CDE IV Kuehtreiber (2010) BSc. thesis11 2.2. Toomre‘s criterion for large-scale SF c Q 1.4 Gravitational instability of 3.36 G integrated vertical disk leads to SF 0.7 c Above a critical density the disk is crit 3.36 G gravitationally unstable. WS 2018/19 CDE IV 12 6 2.3. SF dependence on dense cloud cores Kennicutt‘s law is valid for the total amount of gas: 1.4 SF HI But SFR correl.s strongly with the dense molecular gas mass (from HCN) Gao & Solomon (2004) ApJ, 606 WS 2018/19 CDE IV 13 Genzel et al. (2010) WS 2018/19 CDE IV 14 7 WS 2018/19 CDE IV 15 WS 2018/19 CDE IV 16 8 3. Giant Molecular Clouds • Typical characteristics of GMCs: 4 6 – Mass = 10 ...10 M – Distance to nearest GMC = 140 pc (Taurus) – Typical size = 5..100 pc – Size on the sky of near GMCs = 5..20 x full moon – Average temperature (in cold parts)= 20... 30 K – Typical density = 103... 106 mol./cm3 – Typical (estimated) life time = ~107 year – Star formation efficiency = ~1%..10% • Composition of material: – 99% gas, 1% solid sub-micron particles (‘dust’) (by mass) -4 -5 – Gas: 0.9 H2/H, 0.1 He, 10 CO, 10 other molecules (by number) – Dust: Mostly silicates + carbonaceous (< m in size) • Properties of the gas: – Gas mostly in molecular form: hydrogen in H2, carbon in CO, oxygen in O (O2?), nitrogen in N2(?). – At the edges of molecular clouds: transition to atomic species. “Photo-Dissociation Regions” (PDRs). – H2 cannot be easily observed. Therefore CO often used as tracer. 9 Nearby well-studied GMCs: 4 • Taurus (dist ≈ 140 pc, size ≈ 30 pc, mass ≈10 M): Only low mass stars (~105), quiet slow star formation, mostly isolated star formation. 4 • Ophiuchus (dist ≈ 140 pc, size ≈ 6 pc, mass ≈ 10 M): Low mass stars (~78), strongly clustered in western core (stellar density 50 stars/pc), high star formation efficiency 6 • Orion (dist ≈ 400 pc, size ≈ 60 pc, mass ≈ 10 M): Cluster of O-stars at center, strongly ionized GMC, O-stars strongly affect the low-mass star formation • Chamaeleon... • Serpens... 3.1. GMC mass distribution Fukui & Kawamura (2010) Ann.Rev.A&A, 48, 547 10 3.2. typical GMCs in nearby galaxies Fukui & Kawamura (2010) Ann.Rev.A&A, 48, 547 SMC GMCs LMC GMCs , with HI M33 GMCs , with HI IC10 GMCs with HI Fukui & Kawamura (2010) Ann.Rev.A&A, 48, 547 11 3.3. Typical parameters of Galactic GMCs parameter average range in 5 4 6 mass 5·10 5·10 … 5·10 M⊙ radius 20 10 … 50 pc 3 3 density 300 100 … 10 H2 /cm temperature 10 5 … 30 K sound speed 0.2 0.15 … 0.35 km/s turb. vel. dispersion 6 2 … 10 km/s rotation 0.3 ? km/s/pc magn. field strength 50 20 … 100 Gauss lifetime 107 ? yr free fall time 2·106 106 … 3·106 yr star formation efficiency 0.01 ? 9 PS. total amount of gas in GMCs in the Galaxy: M(H2) = 2·10 M⊙ 3.4. Hierarchical Structure • Clump picture: hierarchical structure – Clouds (≥ 10 pc) – Clumps (~1 pc) • Precursors of stellar clusters – Cores (~0.1 pc) • High density regions which form individual stars or binaries • Fractal picture: clouds are scale-free V AD / 2 D 1.4 fractal dimension 12 WS 2018/19 CDE IV Ward-Thompson 25 3.5. Molecular Cloud Structure • Formation of IS clouds, • Filamentary structure, • Embedded dense clumps, • Turbulent dynamics, • Star Formation in densest clumps McKee & Ostriker (2007) ARAA, 45 13 3.6. Molecular Cores Most clumps don’t form stars. But if they do, they form many. Core mass spectrum is more interesting for predicting the stellar masses of the newborn stars. Deep 1.3 mm continuum map of Ophiuchi (140 pc) at 0.01 pc (=2000 AU) resolution. (Motte et al. 1998) Clumps 14 3.7. Clump mass spectrum Orion B: First GMC systematically surveyed for dense gas and embedded YSOs by E. Lada 1990 Survey of gas clumps Clumps in range M = 8..500 M dN M 1.6 dM dN MdN M 0.6 M 0.4 Most of mass in dln M dln M massive clumps Core mass spectrum Result of survey: dN dN M 0.6 M (1.11.5) dln M dln M for M < 0.5 M for M > 0.5 M Motte et al. 1998 15 For a video see: http://www.astro.umd.edu/~ostriker/research/clouds/project.html Larson 1981 3.8. Mol.Cloud Parameters The larger and more massive the (sub- )clouds the larger the velocity dispersion (~T). 16 Fukui & Kawamura (2010) Ann.Rev.A&A, 48, 547 Larson 1981 17 2 Isolated gas and dense particle systems 3 n G M c E pot achieve Virial equilibrium: 2T + V = 0 5 2n Rc T: kinetic (+ thermal) energy M E c 2 V: potential energy th 2 assumptions: spherical cloud, 2G M 5 2n cloud mass M c c 2 L 3 n ρ(r) r-n 5/3, n 0 1.0, n 2 Larson 1981 Fukui & Kawamura (2010) Ann.Rev.A&A, 48, 547 18 The molecular gas fraction depends on the ISM pressure: 0.92 H2/HI ∝ P after: Blitz & Rosolowsky, 2006, ApJ, 650 19 4. star formation in molecular clouds why not in atomic clouds? molecular clouds can cool to low temperatures (T = 10-30 K) and condense to high densities (n = 103-105 cm-3) atomic clouds cannot (T = 100-5000 K, n = 1-10 cm-3) low gas temperatures and high gas densities promote the possibility of star formation (Jeans criterium) some important molecules: - diffuse clouds: CH, CN (1937, 1940) 12 13 18 - mol. clouds: H2, CO ( CO, CO, C O) + + - dense cores: H2CO, NH3, CS, HCN, HCO , H2D - maser sources: OH, H2O, SiO, CH3OH (methanol) - photodiss. regions: PAHs (10% of all carbon) dual role of dust: shielding molecular clouds from UV radiation + efficient cooling agent!!! star formation in molecular clouds (ctd) 12 -4 - formation of H2: on dust surface CO/H2 ≈ 10 - destruction of H2: dissoc. energy 11.2 eV (optically thick) (binding energy 4.5 eV) 13 -6 - formation of CO: OH + C, CH + O, etc. CO/H2 ≈ 10 - destruction of CO: dissoc. energy 11.1 eV (optically thin) formation of giant molecular clouds (AV > 1): large-scale grav. instabilities (tidal limit) agglomeration of smaller clouds (spiral arms) turbulent compression of supersonic interstellar medium sources of turbulence: winds, supernovae, rotational shear destruction of molecular clouds: photodiss. regions (PDR): dissoc. of H2 and CO HII regions: ionisation of HI 20 SF clouds Stars are formed in den dense cores of molecular clouds. Necessarily, they must be shielded against the IS radiation field, in particular, against UV. critical cloud radius 13 -3 Rclτ 1 Zo T 4 10 dyn cm 5 pc Z 80 K Pext cloud cores cool (molecular lines) thermal instability Jeans unstable WS 2018/19 CDE IV 41 protostellar collapse 4. Star-formation Process Hierarchical (fractal) structure of giant molecular clouds (GMCs) protostellar fragments small scale condensations (cores) large scale condensations (clumps) diffuse molecular envelope warm atomic „skin“ H2 HI cf. Surdin & Lamzin 1998, p. 99 21 Star Formation Shrink size by 107; increase density by x 1021 ! Where planets also form • Giant Molecular Cloud Core Raw material for star birth • Gravitational Collapse & Fragmentation Proto-stars, proto-binaries, proto-clusters • Rotation & Magnetic Fields Accretion disks, jets, & outflows • Planets WS 2018/19Most may form in clusters!CDE IV C. Lada 43 4.1. Star formation by gravitational instability simple approach „collapse“ WS 2018/19 CDE IV 44 22 4.2. Evolutionary sequence of the mol. clouds. The left panels are examples of Large Magellanic Cloud (LMC) giant molecular cloud (GMC) Type I (GMC 225, LMC N J0547-7014 in Fukui et al. 2008), Type II (GMC 135, LMC N J0525-6609), Type III (the northern part of GMC 197, LMC N J0540-7008) from the top panel. Each panel presents Hα images from Kim et al.
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