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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