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From Atoms to Stars

From Atoms to Stars. D. Li Harvard—Smithsonian CfA. Darkness Among Stars. Barnard: “ Not Holes in the Heavens ”. B68: A Prototypical Dark Clouds. Discovery of Interstellar Hydrogen. Improved Vision and Expanding Horizon. 1970: Discovery of Interstellar CO 1980: Discovery of Outflow

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From Atoms to Stars

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  1. From Atoms to Stars D. Li Harvard—Smithsonian CfA SHAO, 2003

  2. Darkness Among Stars SHAO, 2003

  3. Barnard: “Not Holes in the Heavens” SHAO, 2003

  4. B68: A Prototypical Dark Clouds SHAO, 2003

  5. Discovery of Interstellar Hydrogen SHAO, 2003

  6. Improved Vision and Expanding Horizon • 1970: Discovery of Interstellar CO • 1980: Discovery of Outflow • 1983: Launch of IRAS • 90s: HST and CHANDRA • Young stars formed in dense molecular clouds. • Stellar energetics affect the physics and chemistry of ISM SHAO, 2003

  7. "Cores" and Outflows Molecular or Dark Clouds Jets and Disks Solar System Formation Star and Planet Formation SHAO, 2003

  8. H2 Formation On Dust Grains Production rate (s-1cm-3 ) Conversion time scale (yr) H2 Dissociation By Cosmic Rays Destruction Rate(cm-3 s-1) (Williams et al. 1992) How Molecular Are Molecular Clouds? • (Hollenbach & Salpeter 1970; • Buch & Zhang 1991) SHAO, 2003

  9. HI Density In Dark Clouds • Dark Clouds Conditions: • Steady State Solutions of Atomic Hydrogen Density • n1 is independent of H2 density! • Abundance depends on density. • The amount of cold HI depends on the size of the shielded region. SHAO, 2003

  10. Galactic HI Absorption SHAO, 2003

  11. Canadian Galactic Plane Survey • CGPS HI image of ‘HI Self-Absorption’ (HISA) • CO-HISA correlation? inconclusive. • CO-H2 correlation? Inconclusive (Gibson et al. 2000) SHAO, 2003

  12. Detecting Cold HI - HINarrow Self Absorption • Called ‘self-absorption’ in order to differentiate from absorption against continuum sources • HI Self-Absorption is different from the usual meaning of ‘self’, as in the case of thick tracer like CO. • In case of HI absorption caused by dark clouds, HINSA could be a less confusing alternative. SHAO, 2003

  13. Colder Gas in CNM Phases in neutral medium (Wolfire et al. 1995) Cold Neutral Medium: T~80 K Warm Neutral Medium: T~8000 K • Existence for colder CNM (T<40 K) • Continuum absorption: Heiles 2001, Kalbella et al. 1985 • HI self-absorption (HISA): Knee & Brunt 2001 SHAO, 2003

  14. HI Self Absorption • Cold feature revealed in deficiency of emission GSH139-03-69 (Knee and Brunt, 2001 Nature) SHAO, 2003

  15. Past Observations-Technical Limitations • Haystack 120ft: 25’; 0.5km/s (Myers et al. 1978) • Green Bank 140ft: 21’; 0.4 km/s(Knapp 1974) • 76m Lovell Telescope:12’; 0.5 km/s (McCutcheon et al.1978; Montgomery et al. 1995 ) • Effelsberg 100m: 9’, 0.5 km/s (Wilson & Minn 1977; Batrla et al. 1983; Poppel et al. 1983) • Arecibo 300m: 4’, 1 km/s (Baker & Burton 1979, Bania & Lockman 1984) • VLA & DRAO: 1’ - 1.5’, 1.3 km/s (Van der Werf et al. 1988, 1989; Gibson et al. 2000) SHAO, 2003

  16. Arecibo: Upgraded Point-focused with secondary and tertiary Wider bandwidth LO and better backends Central panel patched Surface readjustment underway SHAO, 2003

  17. Arecibo HI Survey of Dark Clouds • Cold Atomic Hydrogen in Dark Clouds? • Self Absorption v.s. Narrow Line Absorption • CO Observations at FCRAO • CI Data from SWAS • The Correlation of Tracers • HI, OH, CO, 13CO, C18O, CI • Column Density of Cold HI and Its Abundance • H2Formation Models SHAO, 2003

  18. HI and OH Survey of Dark Clouds at Arecibo • Observe HI/OH simultaneously Four independent correlator boards centered on 1667MHz, 1665MHz, 1420MHz (narrow), 1420MHz (wide) • High sensitivity L wide receiver: Tsys ~ 30k Gain ~ 10k/Jy • High frequency and adequate spatial resolution • 780Hz Channel width, 0.14 km/s velocity resolution for OH, 0.16 km/s for HI • ~3.4’ FWHM beam at 21cm corresponding to 0.14pc at the distance of Taurus • Stable baseline: total powerONLY ‘ON’mode, avoid uncertainties caused by HI emission variations SHAO, 2003

  19. Survey Statistics: HI NLA Detection Rate • 23 sources with clear HI NLA (cont.) • Detection rate: 77% • Similar Turbulent content HINSA vs. OH Average non-thermal line width (km/s) OH 0.83; HI 0.84 The raw line width of HI is on average ~1km/s, less than two channels in DRAO survey. The average depth is less than 2 RMS noise in DRAO survey. SHAO, 2003

  20. Three-Component Radiative Transfer SHAO, 2003

  21. Two Useful Limiting Cases • Without foreground: absorber is equivalent to an emitting cloud at the temperature differential • With optically thin foreground and Tb=Tf cf. Van der Werf et al. 1988 SHAO, 2003

  22. Column Density of HI • Total HI column density derived from optical depth of the 21cm line: • Average HI column density: • If using C18O, the abundance [HI/H2] is 0.15% • Under the standard model, this corresponds to a cloud size ~ 1pc, about 24’ at Taurus distance • 7.2x1018 cm-2 SHAO, 2003

  23. Evidence of Cold HI: Temperature Upper Limit Estimates • The narrow line width of HI NLA indicates an upper limit of kinetic temperature • Optically thick limit in radiative transfer provides an upper limit of excitation temperature SHAO, 2003

  24. Correlation of Centroid Velocity HINLA is definitely coincide with molecules SHAO, 2003

  25. HINLA Correlations with CO and CI SHAO, 2003

  26. Extremely Good Correlation of HI Narrow Self-Absorption and Molecular Emission Lines SHAO, 2003

  27. HINSA: Conclusions • HINSA with v < 2 km s-1 is a widespread phenomenon associated with dark clouds . • The narrow line atomic hydrogen has significant column density N(HINSA)~7x1018 cm-2, making it the third most abundant species of molecular clouds after H2 and He. • HINSA is produced in regions of moderate extinctions with Av > a few. • HINSA has very low temperature ~15K and is most likely mixed with well shielded molecular gas SHAO, 2003

  28. How Fast Does Star Form? • Conventional View • Core: Ambipolar diffusion • Collapse: Inside-out • Jets: Deuterium Burning, Stellar energetics starts to take over • Accretion Disk: the termination of infall will determine the final mass of the new star. • Fast Star Formation? • Clouds form by HI stream interaction (Hartmann et al. 2001) • Stars are formed on a dynamical time scale (~Myr). • Clouds are dispersed by stellar energy output (Elmegreen 2000) SHAO, 2003

  29. Evidence of “FAST” SF SHAO, 2003

  30.  Ophiuchus SHAO, 2003

  31. Clump and Inter-Clump Structure SHAO, 2003

  32. Pursing the Elusive B Field • The ionization ratio in dark clouds < 10-6 • Zeeman effect: Polarization experiment • Less than 15 credible detection in the past 30 years • Require strong lines, and large Lande g factor • Successful tracer(s) • OH for molecular clouds • HI for CNM • CN, CCS experiments still going on => HINSA SHAO, 2003

  33. HI Fractional Abundances are ~ 10 x Greater than Steady State Values SHAO, 2003

  34. Gas-Grain Reaction Network of H2 Formation • The variable set • The parameters set SHAO, 2003

  35. HI Fractional Abundance Measures TIME since start of Atomic to Molecular Conversion SHAO, 2003

  36. Central Portion of the Taurus Molecular Cloud SHAO, 2003

  37. HMSF vs. LMSF • Spatial Distinction • LMSF region: Taurus • GMCs: Orion • “Intermediate”: Ophiuchus • “Thermal” vs.“Turbulent” Cores • Initial Conditions? • Super vs. Sub Critical • Evolutionary Paths? • No pre-main-sequence for HMSF? SHAO, 2003

  38. HMSF: Time Scale • Different Time Scales/Paths • Infall time scale: • Kelvin-Helmholtz: 1 M: tKH~107 yr, M>5MtKH <tinf no pre-main-sequence! • Different Initial Conditions • Massive cores could be supercriticalfragmentation?, cluster formation? Binary? SHAO, 2003

  39. Surreptitious Discovery SHAO, 2003

  40. From Atoms to Stars: Known and Unknown • H2 formed on dust grains in shielded regions Are molecular complexes formed quickly along with H2 gas or through congregation of long existing H2 clouds (Allen and Pringle 1997)? • Star formation happens in dense cores What are the triggering factor, density or column density? • High mass star formation happens only in a subsample of star forming clouds. Are all star formation fast? • Stellar energetics affect the physics and chemistry of their host clouds. Will the cloud be dispersed or the turbulence be resuscitated? Time scale / Mass spectrum / Dominant process SHAO, 2003

  41. How to Proceed From Here? • We have defined a large-scale survey of the Taurus region for HINSA to be carried out using ALFA – the L-band 7 element focal plane array • Map several hundred square degrees with good sensitivity • Compare with 12CO and 13CO maps from FCRAO (300 hours of telescope time already awarded) • ~ 100 hours of AO observing time required to get high sensitivity maps of COLD and WARM HI SHAO, 2003

  42. TheALFA – TAUProject • When we compare the kinematics of cold and warm atomic gas -- are they the same? • What is the relationship of the atomic and the molecular gas? • What is the characteristic timescale for dark cloud cores as traced by residual HI? • Can we develop a scenario for the evolution from atomic to molecular material and then from molecular clouds to cores to young stars? SHAO, 2003

  43. 2 degrees ~ 10 pc SIRTF Legacy Survey SIRTFNASA’s Space InfraRed Telescope Facility SHAO, 2003

  44. The COordinated Molecular Probe Line Extinction Thermal Emission Survey } COMPLETE SHAO, 2003

  45. 5 degrees (~tens of pc) SIRTF Legacy Coverage of Perseus COMPLETE Observations: 2003--Mid- and Far-IR SIRTF Legacy Observations: dust temperature and column density maps ~5 degrees mapped with ~15" resolution (at 70 m) 2002-- NICER/2MASS Extinction Mapping: dust column density maps ~5 degrees mapped with ~5' resolution 2003-- SCUBA Observations: dust column density maps, finds all "cold" source ~20" resolution on all AV>2” 2002-- FCRAO/SEQUOIA 13CO and 13CO Observations: gas temperature, density and velocity information ~40" resolution on all AV>1 Science: • Combined Thermal Emission data: dust spectral-energy distributions, giving emissivity, Tdust and Ndust • Extinction/Thermal Emission inter-comparison: unprecedented constraints on dust properties and cloud distances, in addition to high-dynamic range Ndust map • Spectral-line/Ndust Comparisons Systematic censes of inflow, outflow & turbulent motions enabled • CO maps in conjunction with SIRTF point sources will comprise YSOoutflow census >10-degree scale Near-IR Extinction, Molecular Line and Dust Emission Surveys of Perseus, Ophiuchus & Serpens SHAO, 2003

  46. Faster and Larger • Focal plane receiver arrays • FCRAO: 32 pixels • SHARC II: 384 pixels • ALFA: 7 • APEX: 300 • Large single dish and Interferometer • LMT: 50m • GBT: 100m • SMA: ~1’ • ALMA: 0.01 arcsec SHAO, 2003

  47. From Atoms to Stars H=>H2 => Cloud Complex => Core=>Protostars ALFA-Tau COMPLETE SHAO, 2003

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