Scales and thresholds in molecular cloud turbulence

• Applicants: Professor Ralf Klessen, Ph.D.; Privatdozent Dr. Volker Ossenkopf-Okada; Professorin Dr. Stefanie Walch-Gassner
• Subject Area: Astrophysics and Astronomy
• Term: from 2011 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 203320423

• Final Report Year: 2015

Final Report Abstract

The project studied ways for statistically comparing observations and simulations of turbulent interstellar clouds. Large data sets from Herschel continuum observations and molecular line maps had to be compared with the outcome of new high-resolution (magneto-)hydrodynamical turbulence computations that allowed to simulate equivalent maps. The statistical tools for the comparison had to be characterised in terms of their sensitivity to observational artifacts, radiative transfer effects, and the eventually relevant physical processes governing the turbulent cloud structure. New tools were developed to quantify the scale-dependent correlation between different tracers in the same region (wavelet-based weighted cross-correlation function (WWCC)) and to measure the local anisotropy of filamentary structures seen in two-dimensional maps (anisotropic wavelet analysis). The impact of observational biases and radiative transfer effects on existing statistical measures was investigated (column density PDFs, ∆-variance, structure function, bispectrum), and several measures were systematically applied to many molecular cloud observations to find commonalities and, through the comparison with turbulence simulations, physical reasons for characteristic properties seen in the measures (PDFs, clump decomposition algorithms, ∆-variance). Due to a lower funding than requested and a delay in the start of the project, not all tools that were originally envisaged have been studied. We found that the effects of unavoidable line-of-sight contamination in observational data can be corrected by carefully exploiting auxiliary information. Radiative transfer effects prevent a reliable determination of the true cloud structure structure from single line observations, but a combination of observations of lines with different critical density and optical thickness can resolve both the velocity and the density structure of turbulent interstellar clouds. The interpretation of column density PDFs allows to deduce several parameters of the turbulence in observed clouds. The PDFs typically consist of a lognormal distribution at low column densities and a power-law tail at higher densities. We showed that the transition of the PDF from lognormal shape into power-law tail occurs at the same column density for low-mass and high-mass star-forming clouds, most likely dominated by supersonic turbulence. Our measurements of the slopes of the power-law tails show that they are consistent with a structure dominated by gravitational collapse. From the clump decomposition of molecular line and continuum maps of Rosette we find that most large clumps are gravitationally instable while all small, low mass clumps are not gravitationally bound but must be transient. The criterion for the transition to gravitationally bound clumps is just the accumulated mass. High mass must unavoidably lead to star-formation. We find a critical mass limit of 10-20 solar masses. Radiative or thermal pressure or small-scale shocks on are not a main driver for clump and core formation. With the WWCC we can constrain commonalities and differences in the distribution of different species as a function of the scale, revealing the chemical transition scales for the formation and excitation of different molecular species. The derived displacement vector between HCN and CO isotopes in G333 indicates a global anisotropy. The systematic differences could indicate either a chemical transition or a strong density gradient. By comparing the masses of associated clumps seen in CO isotopes and dust we can constrain the fundamental X factor describing the ratio between column density and line intensity.

Publications

• Large scale IRAM 30 m CO-observations in the giant molecular cloud complex W43, 2013, A&A 560, 24
  Carlhoff, P.; Nguyen Luong, Q.; Schilke, P.; . . . ; Schneider, N.; Ossenkopf, V.; Simon, R.; et al.
  
• The Effects of Radiative Transfer on the Probability Distribution Functions of Molecular Magnetohydrodynamic Turbulence, 2013, ApJ 771, 122
  Burkhart, B.; Ossenkopf, V.; Lazarian, A.; Stutzki, J.
  
• The Turbulence Power Spectrum in Optically Thick Interstellar Clouds, 2013, ApJ 771, 123
  Burkhart, B.; Lazarian, A.; Ossenkopf, V.; Stutzki, J.
  
• Carbon in different phases ([CII], [CI], and CO) in infrared dark clouds: Cloud formation signatures and carbon gas fractions, 2014, A&A 571, 53
  Beuther, H.; Ragan, S. E.; Ossenkopf, V.; Glover, S.; Henning, Th.; et al.
  
• Opacity Broadening of 13 CO Linewidths and its Effect on the Variance-Sonic Mach Number Relation, 2014, ApJ 785, 1
  Correia, C.; Burkhart, B.; Lazarian, A.; Ossenkopf, V.; Stutzki, J.; et al.
  
• Detection of two powerlaw tails in the probability distribution functions of massive GMCs, 2015, MNRAS
  Schneider N., Bontemps, S., Girichidis, P., . . . ; Ossenkopf, V. et al.
  
• Understanding star formation in molecular clouds. II. Signatures of gravitational collapse of IRDCs, 2015, A&A 578, 29
  Schneider, N.; Csengeri, T.; Klessen, R. S.; Tremblin, P.; Ossenkopf, V.; et al.
  
• Understanding star formation in molecular clouds: I. Effects of contamination on the column density structure, 2015, A&A 575, 79
  Schneider N., Ossenkopf V., Csengeri T., Klessen, R. S.; Federrath, C; et al.
  
• Wavelet-based cross-correlation analysis of structure scaling in turbulent clouds, 2015
  Arshakian, T.; Ossenkopf, V.