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Properties of interstellar filaments observed with Herschel Modeling the 3D magnetic field structure inspired by Planck Doris Arzoumanian (IAS, Orsay) F. Boulanger, M. Alves, J. Soler, V. Guillet (IAS, Orsay) E. Falgarone (ENS, Paris) Physical processes in the ISM Monday 21 October 2013 Omnipresence of filamentary structures seen by Herschel both in star forming regions and in quiescent clouds Polaris André+ 2010 Men’shchikov+2010 N. Schneider+ 2013 IC5146 Musca Arzoumanian+ 2011 N. Cox+ Aquila Taurus Pipe Palmeirim+ 2013 D. Arzoumanian Orion B Peretto+ 2012 Physical processes in the ISM, MPE, Garching Könyves+ 2010 October 21, 2013 All interstellar filaments share the same inner width of ~ 0.1pc Number of filaments per bin Filament width (FWHM) [pc] Arzoumanian et al 2011 Sample of 278 filaments 0.1 Central column density NH2 Distribution of filament width [pc] [cm-2] filament radial column density profile Musca filament SPIRE 250 µm Resolution 18’’ ~ 0.02 pc at 200 pc Inner radius for r >> Rflat ρ(r) ~ r -2 1 pc beam Observed by the Herschel Gould Belt survey D. Arzoumanian Gaussian fit background Palmeirim et al. 2013 Physical processes in the ISM, MPE, Garching October 21, 2013 Increase of the total velocity dispersion with central column density of self-gravitating filaments induced by the gravitational accretion of surrounding low density material Velocity dispersion of interstellar filaments Organized magnetic field of the cloud surrounding the filament Self-gravitating star forming filaments Musca Arzoumanian et al 2013 The densest filaments grow in mass per unit length by accreting surrounding material while contracting (cf. Heitsch 2013, Hennebelle & André 2013) D. Arzoumanian Blue segments: optical polarization vectors tracing the magnetic field orientation (Pereyra & Magalhaes 2004) Physical processes in the ISM, MPE, Garching October 21, 2013 Increase of the total velocity dispersion with central column density of self-gravitating filaments induced by the gravitational accretion of surrounding low density material What is the magnetic field geometry in the filaments? How the magnetic field of the filament connects with the background field? Self-gravitating star forming filaments Musca Arzoumanian et al 2013 The densest filaments grow in mass per unit length by accreting surrounding material while contracting (cf. Heitsch 2013, Hennebelle & André 2013) D. Arzoumanian Blue segments: optical polarization vectors tracing the magnetic field orientation (Pereyra & Magalhaes 2004) Physical processes in the ISM, MPE, Garching October 21, 2013 Whole sky thermal dust emission observed by Planck • Thermal dust emission is linearly polarized due to asymmetric dust grains aligned with the magnetic field Planck collaboration D. Arzoumanian Physical processes in the ISM, MPE, Garching October 21, 2013 Planck probes the polarized thermal emission from the cold dust in the ISM Polarization fractions and polarization angles are derived from the observed Stokes parameters I, Q and U Observed Stokes parameters Derived quantities from I,Q and U Depends on the structure of the magnetic field and the properties of the dust grains Dust polarization angle +90o orientation of B field (component projected on the plane of the sky) D. Arzoumanian Physical processes in the ISM, MPE, Garching October 21, 2013 Polarization fraction vs. Column density of the interstellar medium Is it possible to describe the observed depolarization in dense regions (e.g., filaments) as an effect due to the geometry of the interstellar magnetic field? Studies previous to Planck (Whittet et al 2008) (Decrease due to loss of dust alignment in dense regions) Planck Results • The decrease of the polarization fraction with increasing column density is associated with local dispersion of polarization angles See the results from the Planck collaboration presented at ESLAB in April 2013 (Cf. F. Boulanger’s talk on Thursday) D. Arzoumanian Physical processes in the ISM, MPE, Garching October 21, 2013 Modeling the polarized thermal dust emission of interstellar filaments Methodology: Constructing synthetic I,U,Q maps of filaments in a magnetized cloud • Density profile for the filament derived from the observations, uniform density for the background • 3D Magnetic field structure: - background (uniform) - filament (uniform, helical, poloidal) Free parameters: - pairs of angles (filament + background) defining the 3D structure of the field Fixed parameters (derived from Planck observations): - maximum polarization degree (p0) - position angle of the filament in the plane of the sky Fitting the observations - Spanning all the pairs of angles describing the B field - Minimizing Χ2 z λ (optically thin emission) y ϕ x D. Arzoumanian observer Physical processes in the ISM, MPE, Garching October 21, 2013 From the 3D magnetic field of the modeled filament to the integrated Stokes parameters (I,U,Q) to the polarization fraction Depolarization in filaments due to the geometry of the field uniform field Uniform field for the background cloud + Polarization fraction helical field Characteristic profiles of polarization fraction corresponding to the 3D field geometry of the filament and the background cloud z + Polarization fraction Courtesy of Marco Padovani for producing (e.g., models described by the 3D plots of field lines Fiege & Pudritz 2000) D. Arzoumanian Physical processes in the ISM, MPE, Garching October 21, 2013 Conclusions • The observed depolarization in filaments can be described by an effect of the magnetic field geometry • Constraining the configurations of magnetic field in filaments fitting the Q and U maps observed by Planck - Uniform field? Helical field? What else? Perspectives • Understanding the physical implications of the different field geometries - Role of the magnetic field in the formation and evolution of filaments? • Statistical study of magnetic field geometries on a large sample of filaments Synthetic I map and B field vectors helical uniform Polarization fraction [%] 14 12 10 8 6 -6 D. Arzoumanian -4 -2 0 2 Radius [pc] Physical processes in the ISM, MPE, Garching 4 6 October 21, 2013