Adam Ginsburg's blog

Articles by Adam Ginsburg

• PPVI Talk 2: Philippe Andre

  FROM FILAMENTARY NETWORKS TO DENSE CORES IN MOLECULAR CLOUDS: TOWARD A NEW PARADIGM FOR STAR FORMATION

  Authors:

  P. Andre (CEA Saclay, Laboratoire AIM - Service d'Astrophysique, Gif-sur-Yvette, France), J. Di Francesco (National Research Council Canada, Herzberg Institute of Astrophysics, Canada), S.-I. Inutsuka (Nagoya University, Japan), R. Pudritz (McMaster University, Origins Institute, Canada), D. Ward-Thompson (University of Central Lancashire, Jeremiah Horrocks Institute, UK), J. Pineda (University of Manchester, Jodrell Bank Centre for Astrophysics, United Kingdom)

  Abstract

  We review recent progress in our understanding of the physics controlling the earliest evolutionary phases of star formation. Since PPV seven years ago, one area that has seen the most dramatic advances has been the characterization of the link between star formation and the structure of the cold interstellar medium (ISM). In particular, extensive studies of the nearest star-forming clouds of our Galaxy with the Herschel Space Observatory have provided us with unprecedented images of the initial and boundary conditions of the star formation process. The Herschel images reveal an intricate network of filamentary structures in every interstellar cloud. The observed filaments share common properties such as their central widths - but only the densest ones contain prestellar cores, the seeds of future stars. Overall, the Herschel submillimeter data, as well as other observations from, e.g., near- IR extinction studies, favor a scenario in which interstellar filaments and prestellar cores represent two key steps in the star formation process: first supersonic turbulence stirs up the gas, giving rise to a universal web-like structure in the ISM, then gravity takes over and controls the further fragmentation of filaments into prestellar cores and ultimately protostars. The new observational results connect remarkably well with nearly a decades worth of numerical simulations and theory which have consistently shown that the ISM should be highly filamentary on all scales and that star formation is intimately connected with self-gravitating filaments. We thus attempt to synthesize a comprehensive physical picture that arises from the confrontation of recent observations and simulations. We also emphasize how the apparent complexity of cloud structure and star formation may be governed by relatively simple universal processes - from filamentary clumps to galactic scales.

  Content

  Goal: convince that filamentary structures are essential

  (Talk was relatively disorganized; this is reflected in the disorganization below)

  Filaments well-known from both observations and theory.

  • IRAS image of Taurus
  • Filamentary IRDCs
  • Turbulence, collapse, etc. all show filamentary structures in simulations

  Herschel is great!

  • G11.11-0.12 one of the densest, darkest clouds
  • Filamentary networks in Vela C
  • Sousbie's DisPerSE shows "networks of filaments"

  Universals in filaments:

  • Striations perpendicular to large-scale filaments (e.g., Taurus)

  • Serpens S shows H-band polarization perpendicular to filaments

  • similar column density profiles: fixed "scales"

    • QUESTION: If these are genuinely common size scales, can they be used as a distance tracer? [implied answer: No, they are not universal, they are observationally selected]

  • Plummer-like density profile: r^-2, not the expected cylindrical Ostriker solution r^-4

    • colder inside (non-isothermal)
    • size of central inner plateau "roughly the same"
    • same over wide range of column densities

  Possible explanation

  • 0.1 pc is the scale of turbulence

  • in Larson's law, 0.1pc is where velocity dispersion meets sound speed

  • Sharp transition (Pineda 2011; Goodman 1998?)

  • Turbulent fragmentation may cause this?

    • width of compressed filaments is the sonic scale

  • Hennebelle: Compression + shear -> filaments...

  Equilibrium with ISM?

  • unbound filaments below N ~ 5e21 (Inutsuka in prep)
  • Gravitating filaments above that sacel
  • widths centered at 0.1 +/- 0.05 pc?

  More observations (Helen Kirk et al)

  • Radial infall motions in HNC along minor axis of filament
  • gradient along filament "suggesting infall" (no, that doesn't suggest infall, at least no more than it suggests rotation)
  • Overall picture is collapse in all dimensions
  • Accretion of "background material" required to maintain "fixed widths"

  Arzoumanian 2013:

  • velocity dispersion increases with Column density
  • velocity dispersion gives mass per unit length...

  Heitsch 2013:

  • Accretion onto dense filaments
  • Most likely location in parameter space is around 0.1 pc....

  Hacar, Tafalla et al 2013 (1S036)

  • CO -> velocity-coherent "fibers" branching off of filament

  Aquila:

  • Wavelet/Curvelet decomposition

    • wavelets extract cores well
    • curvelets highlight filaments

  Konyves cores

  • core extraction comparison underway
  • Menschikov getsources, Kirk csar, Molinari cutex (csar: see this thesis? http://orca.cf.ac.uk/14483/ just used,didn't develop, csar)

  Mass vs Size diagram for starless vs starry cores

  • Simpson: model evoluationary tracks
  • Prestellar CMF resembles IMF...
  • one-to-one mapping....
  • supports fragmentation models...
  • 75% of cores form "on" filaments
  • preferentially form above A_V ~ 8
  • Instability proportional to mass per length for filaments...
  • 16 msun/pc corresponds to 160 msun/pc^2 or 1600 msun/pc^3 (2e4 cm^3)
  • threshold explained by filaments...
  • Bonnor-Ebert mass corresponding to 16 msun/pc is ~0.5 pc, corresponding to CMF peak

  Conclusions

  First you form filaments Then filaments fragment Perhaps massive stars come from filament convergence into hubs "This scenario may possibly account for the global rate of star formation on Galactic scales"

  • Q: Chris: A_V = 8. Arises naturally?
  • Q: Is there any relationship between the critical column density for SF and the column density of the parent cloud
  • A: Not really. Average column density in clouds is very low. No correlation between average column density and presence/absence of overdensities
  • Q: 75% of dense cores in filaments. Must be a large number of isolated ones. Any difference between on-core and off-core?
  • A: Yes, see Polychroni's poster. More massive on filaments.
  • Q: Tan - Universal widths. Seem to be a factor of 2 from resolution for all widths. How sure?

  • A: The physical width is constant to within a factor of 2. Angular width varies.

    Only the average width is constant

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• PPVI Talk 1: Mark Krumholz

  FORMATION OF MOLECULAR CLOUDS AND GLOBAL CONDITIONS FOR STAR FORMATION

  Authors:

  C. Dobbs (Exeter University, United Kingdom), M. Krumholz (University of California, Santa Cruz, United States), J. Ballesteros-Paredes (CRyA-UNAM, Mexico), A. Bolatto (University of Maryland, United States), Y. Fukui (Nagoya University, Japan), M. Heyer (University of Massachusetts, United States), M. Mac Low (American Museum of Natural History, United States), E. Ostriker (Princeton University, United States), E. Vazquez-Semadeni (UNAM, Morelia, Mexico)

  Abstract

  Giant molecular clouds (GMCs) are the primary reservoirs of cold, star-forming molecular gas in the Milky Way and similar galaxies, and thus any understanding of star formation must encompass a model for GMC formation, evolution, and destruction. These models are necessarily constrained by measurements of interstellar molecular and atomic gas, and the emergent, newborn stars. Both observations and theory have undergone great advances in recent years, the latter driven largely by improved numerical simulations, and the former by the advent of large-scale surveys with new telescopes and instruments. This chapter reviews the current state of the field.

  Content

  Galactic Scale

  • HI column density "hits a wall"
  • molecular gas correlated with SF
  • "depletion time" ~ 2 Gyr
  • power law index of molecular clouds: -1.5 to -2 -> most mass in big clouds
    • only in gas-rich regions
    • steeper in outer regions of galaxies: -2 to -2.5
  • typical mean surface density 100 Msun/pc^2
  • PDF of surface densities "lognormal" with powerlaw tail

  Theory

  Virial Parameter: \(\sigma_v = (\alpha_G \pi G \Sigma R / 5)^{1/2}\)

  \(\alpha_G = -2 T/ W\)

  • \(\alpha_G = 2\) -> free-fall collapse
  • 3 options: pressure confinement, virial balance, or free-fall collapse
  • Most velocity dispersion on largest scale
  • median \(\alpha_G \sim 0.1\)

  \(M/M_{crit} = \frac{M}{\phi/(4\pi G)^{1/2}} \approx 2\)

  • B-fields not strong enough to support against gravity in mol. clds (Crutcher)

  Lifetime of Molecular Clouds

  Spatial distribution + galactic dynamics.

  Method 1: compare dynamics to distribution

  • Formed in the arms and survive passage between arms? (implies lifetime ~100 Myr)

  Method 2: compare star cluster ages to spatial cloud distribution

  • lifetimes closer to 30 Myr

  Method 3: look at T Tau vs post T Tau (Hartmann 2009)

  • 3 Myr for Taurus

  Why difference?

  • size matters
  • should compare lifetimes to cloud dynamical times

  \(t_{ff} = \left(\frac{3\pi}{32 G \rho}\right)^{3/2}\)

  Efficiency

  The rate at which gas converts into stars.

  \(\epsilon_{ff} = SFR / (M_{gas}/t_{ff}) \sim 0.01\)

  • if pure free-fall, \(\epsilon_{ff} = 1\)
  • Low star formation rate
  • Lifetime << 100 \(t_{ff}\)
  • Lifetime * efficiency < 1, so low overall star formation efficiency

  Metallicity

  Break between HI and CO:

  • 10 Msun in Galaxy, local spirals
  • 50 Msun in SMC

  How do Molecular Clouds form?

  "Stories", not conclusions

  Converging Flows

  • Collision -> cooling
  • vazquez-semadeni et al
  • "local turbulence" triggers collision of warm HI
  • Can't make big clouds: limited to mean surface density * scale height? [needs more]

  Cloud Collision in Spiral Arms

  • Spiral arms are convergence points
  • Collisions unlikely in most of galaxy
  • orbits crowd in arms: collisions more likely
  • mechanism probably can't work in flocculent galaxies
  • explains counter-orbiting GMCs [are these really a common feature?]

  Gravitational Magneto-Jeans (Kim & Ostriker 2006)

  • Toomre Q instability at Q < 1.5
  • makes 10^7-10^8 Msun clouds; smaller ones must fragment
  • produces low spin clouds. Mol. Clds apparently have much less angular momentum than expected
  • easily explains "beads on a string" morphology [why?]

  Parker + Thermal instability (Mouschovias et al 2005)

  • Buoyancy draws B-fields out of plane
  • may not work with turbulence

  Chemistry: Forming H2 and CO

  • H2 dominance begins at ~10 Msun/pc^2
  • Glover and Clark: Is the molecular transition a necessary prerequisite to star formation?
  • Extincting FUV allows cooling, H2 formation

  Evolution of Molecular Clouds

  Morphology?

  • Highest intensity offset from highest column (DR21; Schneider 2001)
  • MHD collapse (Hennebelle) vs. pure turbulence (Federrath)

  Non-thermal motions in GMCs:

  • Molecular clouds are invariably turbulent
  • Reynolds number ~10^9
  • Turbulence decays: Why sigma so large?
    • Global grav collapse?
    • Internal energy injection?
    • External driving?

  External driving:

  • Accretion energy
  • Large-scale galactic shear
  • Requires feedback to get star formation right

  Internal driving:

  • Gritschneder: ejected clumps stirring up
  • Someone else: B-fields distorted by ionization, store energy

  Mol Cld Disruption

  • Dont' survive, must be disrupted....

  Why SF rate low?

  • feedback
  • or turbulence

  Disruption Scenarios

  • ionization can disrupt up to 10^5, may not work above that mass
  • turbulence slows down SF
  • turbulence must be maintained

  Scales

  • "Normal" from 10-100 msun/pc^2 (all clouds are the same; cloud counting)
  • extreme above 100 msun/pc^2 (densities in mol clds very high; mean surface density of galaxy matches individual clouds)
  • "all hell break loose" < 10 msun/pc^2

  Future

  • Obs: big surveys, better data
  • Thry: Need to combine large and small scales

  Citations

  • Leroy 2008
  • Roman-Duval 2010
  • Gratier 2012
  • Narayanan 2008: Taurus cloud CO 3-color by velocity
  • Crutcher 201
  • Koda 2009 M51
  • Kawamura 2009 LMC
  • Rosolowsky & Blitz 2005

  Questions

  • Q: Correlation with CO, H2... have there been correlations with others?
  • A: Yes, very well correlated with HCN, but no maps
  • Q Tan: Tasker showed that GMC collisions can happen on short timescales...????

  • Q Tan: B-fields relatively weak by "factor of 2, but don't believe to factor of

    2". Could B-fields still be important?

  • A: I know about your work with Tasker. Requires high surface density. Can't

    work in, e.g., M33.

  • A: B-fields. If only a few measurements, fine. But with so much data...

    (systematic??)

  • Comment: Reconciling ages in LMC vs local. Function of measurement method: LMC, use OB stars outside. Inside clouds, locally, don't use OB. Ages may be biased...
  • A: What do you call a cloud? Clump?

  • Q: Should we stop talking about virial equilibrium, start talking about

    equipartition?

  • A: They may be in equilibrium. No "true" equilibrium, but a "statistical

    equilibrium". Equipartition is uninteresting since the energies are automatically equal.

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