Download Properties of interstellar filaments observed with Herschel and 3D

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