Understanding the observed properties ofinterstellar filaments
Insights on the initial conditions of star formation
Doris Arzoumanian (Nagoya University)

Galactic dust emission at 353 GHz,
with plane-of-the-sky magnetic field(BPOS) orientation observed by Planck

MHD workshop, December 13, 2017, Kagoshima

Understanding the observed properties ofinterstellar filaments
Insights on the initial conditions of star formation
Galactic dust emission at 353 GHz,
with plane-of-the-sky magnetic field(BPOS) orientation observed by Planck

Herschel observations of submm dust emission
IC5146 molecular cloudStar forming
cores

Outline• Filament properties derived fromdust continuum observations
Arzoumanian et al. 2011

Special thanks to
Shu-ichiro Inutsuka
Tsuyoshi Inoue
Yoshito Shimajiri
Philippe André• Kinematics of clouds and filamentsfrom molecular line observations• Insight on the magnetic fieldstructure from dust polarization
observations

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Interstellar filaments share a common inner width of 0.1pc while theyspan a wide range in column density

Arzoumanian et al. 2011,+ in prep.

Herschel observations ofIC5146 molecular cloud

Inner radius

2pc at 400pc
Log-Log scaleBlue 70µm
Green 160µm
Red 350µm

beam

Background3 pc at 460 pc
UnboundSelf-gravitating
quiescent filaments star-forming filaments
Filament width (FWHM) [pc]Number of filaments per bin

Filament characteristic
Sample of 278 filaments
width of 0.1 pc~ 0.1 pc
Thermal Jeans length[λJ ~ cs2/(GΣ)]

0.1
Distribution of filament width [pc]Central column density NH2 [cm-2]

Doris Arzoumanian, MHD workshop, December 13, 2017, KagoshimaCombining Herschel data withhigh resolution observations of
ArTéMiS/APEX
Observations (June 2016) of Serpens Southin the Aquila rift

×3.4 higher resolution than Hersche SPIREOpen to ESO/OSO community since 2014
ArTéMiS 350 µm
APEX 12m
Focal plane
2304 pixels

André, Revéret,Könyves,
et al.
Herschel Spire
350 µmShimajiri(24.8’’)

Herschel Spire + ArTéMiS 350 µm (10.5’’)

Herschel SPIRE 350 µm (24.8’’)

ArTéMiS + SPIRE 350/450µm (10.5’’)

MJy/srResolution: 0.031pc at 260pc

Resolution: 0.013pc at 260pc

Doris Arzoumanian, MHD workshop, December 13, 2017, KagoshimaCombining Herschel data with high resolution observations of
ArTéMiS/APEX+ ArTéMiS
350 µm (10.5’’)
Herschel observations
Spire 350 µm (24.8’’)
Recent(June 2016) of HerschelSerpensSpireSouthin the Aquila
rift
ArTéMiS + SPIRE 350/450µm (10.5’’)

Herschel SPIRE 350 µm (24.8’’)

Resolution:
0.013pc

Resolution:
0.031pc at 260pc

MJy/srPreliminary analysis
Mean radial intensity profileF

Filament crest

ArTéMiS
beam SPIRE beam

Gaussian fits:
FWHM = 0.083+- 0.012pc
FWHM = 0.087+- 0.013pc
FWHMdec= 0.083+- 0.013pc
FWHMdec= 0.081+- 0.013pc

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Resolving the inner structure of filaments at distances
further than the Gould Belt clouds
Combining Herschel data with high resolution observations with ArTéMiS/APEX 12m
André, Revéret, Könyves, Arzoumanian et al. 2016

×3.4 higher resolution than Hersche SPIRE

distance~ 1.7kpc

APEX 12m

main filament:
Mline~1500 M⊙/pc

Resolution:
8’’ or 0.066 pc

Intensity [MJy/sr]

NGC6334

ArTéMiS 350 µm
Focal plane
2304 pixels

W~0.15pc

Mean radialintensity
profileArTéMiS
Gaussianbeam
fitRadius [pc]Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Filaments are also observed to be main structures of theinterstellar medium of other galaxies
ALMA observations of pc-scale filaments in the LMC
Fukui et al. 2015, Tokuda et al., in prep.
13CO(2-1)

5 pc

Res. ~1”~0.24pc
at 50kpc

Filaments may help understanding the initial conditions ofstar formation in galaxies (cf. Lada et al. 2012)
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Understanding the observed properties:
Model of non isothermal filaments
in pressure equilibrium with the ambient medium

Filament width (FWHM) [pc]Unboundquiescent filaments

Self-gravitatingstar-forming filaments

Model

Model of polytropic filaments
P ~ ργwith
γ = 0.8 andPex / kB ~ 5x104 K cm-3
Reasonable agreementwiththe observations
for subcritical filaments(Inutsuka et al. in prep.)

Central column density NH2 [cm-2]

Cf. Fischera & Martin 2012 for a model of isothermal filaments
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Tension between filament models and observations• Thermally supercritical filament with Mline > Mline,crit are unstable for radialcollapse and gravitational fragmentation (Inutsuka & Miyama 1992, 1997)• Runaway collapse of thermally supercritical filaments with Mline > 2Mline,critStar forming supercritical filaments with Mline >> 2Mline,crit are observed(Inutsuka & Miyama 1997)
Width (W)L
Filament length
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Mline=M/L

Understanding the observed properties:
Model of accreting dense/self-gravitating filaments

Model

Supercriticalfilaments

Filament width (FWHM) [pc]Subcriticalfilaments

Balance between accretiondriven turbulence(Klessen & Hennebelle 2010)

and dissipation of turbulence
due to ion-neutral friction(Hennebelle & André 2013)

Central column density NH2 [cm-2]

See also numerical simulations of gravitational
infall onto molecular filaments (Heitsch 2013)
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Hint of the evolution of supercritical filaments from Accretion ofsurrounding material channeled by the magnetic field?
Low column density filaments/striationsaligned with the magnenetic field seem tobe feeding the dense filamentRed-shifted and blue-shifted velocity gradients on
both sides of the filament trace flows of surrounding
matter being accreted onto the filament?
CO integrated intensity

Magnetic field lines

Faint filaments/striations

Palmeirim et al. 2013

Herschel column density map (Palmeirim et al. 2013) CO observations from Goldsmith et al. 2008
Magnetic field orientation from Optical/IR polarization(Heyer et al. 2008, Heils 2000, Chapman et al. 2011)

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Internal velocity structure of the B211/3 filament“Velocity coherent filaments/fibers” observed along the B211/13 filament
Palmeirim et al. 2013

TaurusHerschel NH2“Velocity coherent
filaments/fibers”
identified in
C18O(1-0)

B211

B213

Hacar et al. 2013
Fiber 12 traces the crest of B211
Fiber 20 traces the crest of B213
Possible scenario: Matter accretion in a turbulent flow can produce fiber-like structures and delay the
global radial collapse of the supercritical filament (Clarke, Whitworth et al. 2017)
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

“Velocity coherent filaments/fibers” observed along the B211/13 filament?
Palmeirim et al. 2013
TaurusHerschel NH2“Velocity coherent
filaments/fibers”
identified in
C18O(1-0)
Hacar et al. 2013

B211

B213

B211
B211

B213
Northern profile
Sourthern profile

Northern profile
Sourthern profile

Herschel column density
profile of CO fiber 20

Herschel column density
profile of CO fiber 12

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Nobeyama 45m observations of a filament on the verge of gravitational
fragmentation in the Taurus molecular cloud
Investigating the interaction between the filament and its surroundings
Herschel column density map

Arzoumanian et al., in prep.

Map: Velocity [5-6km/s]
integrated intensity of
C18O(1-0)

K km/sC18O

Contours:
Herschel NH2(5, 7, 9, 11, 13 x 1021 cm-2)

1pc

13CO

Unboundfragments

Prestellar
cores

Nobeyama 45m[4-5km/s]
13CO(1-0)[6-7km/s]
13CO(1-0)
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Multiple velocity components are observed towards thefilament
Arzoumanian et al., in prep.
PV cuts perpendicular to the filament and averaged along the crest

Herschel column
density map

Velocity

Tmb (K)

Black contours:
C18O emission
for reference~ 0.3pc

Spatial resolution 28”, Spectral res 0.07km/s
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Velocity channel maps

Multiple velocity components are observed
towards the filament, but they may correspondto extended structures surrounding the filament
Tmb (K)

13CO[7.4-9]km/s

13CO[6.4-7.4]km/s

Herschelfilament~ 0.3pc

13CO[4-5]km/s

Doris Arzoumanian, MHD workshop, December 13, 2017, KagoshimaWhy thermal dust grain emission is linearly polarized andtraces the ambiant B field structure?
• Linear polarization results from non-spherical dust grains• Dust grains have paramagnetic property and are fast spinning, so they precess
around the ambient magnetic field! These two properties make the dust polarized emission trace B field orientation(e.g., Davis & Greenstein 1951, Vaillancourt 2007, Hoang, Lazarian, Andersson et al. 2014)

Maximalaxis of J
inertiaB

E• Aspherical dust
grains align theiraxis of maximal
inertia with thelocal B-fieldorientation

observer

B
BPOS(Bperp)

BLOS
(B//)

From Andrea Bracco

B orientation = pol. direction + 90°

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

The observed polarized emission depends on the structure of themagnetic field and the polarization efficiency of dust grains
Observed Stokes parameters
The imprint of the mean field geometry ispresent in Q and U, which are integrated
quantities along the line of sight (LOS)LOS
Minimalpolarization

LOS "Maximalpolarization

BJ(I: total intensity~ column density)
Polarization
propertiesof dust grains

Polarization angle,+ 90o orientation of BPOS

Polarization properties derived
from the Stokes parameters
Polarized intensity

E
Linearly polarized thermal
dust emission

B

Polarization fraction

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

The observed polarized emission depends on the structure of themagnetic field and the polarization efficiency of dust grains
4
dust
1

2

Empirical parametrization of theobserved polarization fraction31) Dust properties: size, shape, composition (Voshchinnikov & Hirashita 2014)
2) Grain alignment efficiency with the local magnetic field (Goodman 1992, Whittet et al
2008, Hoang & Lazarian 2014)

3) Depolarization from
2 (or more) layers along the LOS

P2P

F=P/(P1+P2)(Jones et al. 1992, 2015, Myers & Goodman
1991, Planck 2015 XX, 2016 XXXIII)

LOS

POS

4) Field geometry
cos2 γ

LOS "

P1

B
p>p

(Planck 2015 XX, 2016 XXXIII)
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

What are the limitations in using thermal dust polarized
emission to trace the mangetic field structure in the ISM?
4

dust
1
Dust grainproperties

2

Empirical parametrization of theobserved polarization fraction

3
Magnetic fieldstructure

Degeneracy between all these factors: Details modelling of grainproperties and understanding of the B field structure is important

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Whole sky Planck dust polarization results on thedispersion of polarization fraction
Polarization fraction vs. gas column density

Planck 2015 XIX
Total intensity + BPOS orientation

Polarization fraction-Large scatter in p for each NH bin-Drop of p at large NH values
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Dust polarization observations with JCMT/SCUBA2-POL2
Variations of the polarization angle and the polarization fraction
for high column density regions (> 10Av)
BISTRO observations towards Oph-A
0.1pc
Kwon et al., submitted
Polarization fraction
vs.
Intensity

Image: SCUBA2 at 850µm (14.1”)
Segments: POS B-field from POL2 at 850µm (14.1”)
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Understanding the observed polarization properties:
Hint from numerical simulations of MHD turbulence
Philosophy:-Producing I, Q, U maps from MHD simulation-Computing the observable quantities, e.g., p-Assuming uniform grain alignment and dust grainproperties-Varying the line of sight integration to mimicvariation of orientation of the magnetic field withrespect to the line of sight

dust
Uniform
Dust properties

Magnetic fieldstructure

Colliding flow turbulent MHD simulation ofmolecular cloud formation

Reprojecting Bx,By,Bz of the simulation
into psi and gamma
Computing I,Q, and U maps from thesimulation cube

Doris Arzoumanian, MHD workshop, December 13, 2017, KagoshimaHennebelle et al. 2008

Understanding the observed polarization properties:
Hint from numerical simulations of MHD turbulence
Results:-Observations are well produced by numericalsimulations up to a ~10Av, with non variation of dust
grain properties-Depolarization along the LOS and the orientation of
the mean magnetic field with respect to the LOS playa role in reproducing the observations

dust
Uniform
Dust properties

Magnetic fieldstructure

Colliding flow turbulent MHD
simulation of molecular cloudformation

Hennebelle et al. 2008
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Understanding the observed polarization properties:
modelling the distribution of p and psi of the Galactic pole
Phenomenological model:

Planck XLIV 2016, Andrea Bracco- Modeling the dust polarization observations towards the Galactic pole
- Characterizing the ordered (Lon0 and Lat0) and turbulent (random) components of the
Galactic magnetic field in the solar neighbourhood- The line of sight depolarization is modeled by summing the emission over a numberof layers with independent realizations of turbulence (Bturb)- Assuming uniform grain alignment and dust grain properties
Layers with independent
turbulence along the line of sight

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Understanding the observed polarization properties:
modelling the distribution of p and psi of the Galactic pole
Results: The Galactic mean field-The observations can be modeled with (Lon0 ,Lat0)=(80o,0o), a uniform directionof the Galactic magnetic field.
same values are also infered from starlight polarization (Heiles 1996)
Stokes parameters centered on the south Galactic pole

Observations

Observed I

Q

U
Model

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Understanding the observed polarization properties:
modelling the distribution of p and psi of the Galactic pole
Results: Distributions of polarization angle and fraction with one layer model- One layer with an ordered field B0 and a turbulent component Bturb- Uniforme dust grain intrinsic polarization

1 layer model! The wide distribution of the polarization fraction cannot be
reproduced with a single layer model
Polarization fraction

Polarization angle

data
model
model

data

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Understanding the observed polarization properties:
modelling the distribution of p and psi of the Galactic pole
Results: Distributions of polarization angle and fraction with multiple layer model!The observations are well reproduced with
Model with multiple- Ratio of Bturb/ B0 ~0.8
- Uniforme dust grain intrinsic polarization pmax=26%- When integrating over 7 layers along the line of sightlayers along the lineof sight(The number of layers along the line of sight can also be understood in termsof the exponent of the power spectrum of Bturb along the line of sight)

Polarization fraction

data

pmax=26%

Polarization angle

data
model

model

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Tow layers along the line of sight: the filament and its background
Mean radial profiles of the Stokes parameters and fit of the background

U

Q

I

Two layer model to estimate
the contribution of the filament
to the total observed emission

Musca

Taurus molecular cloud

B211

tot

L1506

tot
tot
I at 353GHz+90o = BPOS angle

I at 353GHz

Doris Arzoumanian, MHD workshop, December 13, 2017, KagoshimaVariation of the polarization fraction (p)from the parent cloud to the filament- The observed polarization fraction (p) towards the filaments show a decrease withrespect to p in the background.
- For Musca and B211 the intrinsic p in the filaments is smaller than what isobserved without component separation, for L1506 p in the filament is larger thanthe observed value (filament + background)
Polarization fraction (%) across the filaments (at 5’)
Musca

B211

background

L1506

Filament+ background
Filament
Polarization fraction
I: total intensity

Planck XXXIII 2016

Polarized intensity

Doris Arzoumanian, MHD workshop, December 13, 2017, KagoshimaDepolarization from LOS integration
Superposition, along the LOS, of 2 layers withdifferent BPOS angles (filament + background)

The L1506 filament
In Taurus

Planck dust intensity map at 353GHzof the L1506 filamentin Taurus Molecular cloud

P2
P
P1

LOS "

Planck XXXIII 2016

BPOS angle [deg]Plancktotal
intensity
atmagnetic
353GHz(850µm) in MJy/sr
The segments
show theplane of the sky
field orientation.
The
lengthis
proportionaltothepolarization
fraction.
Segments: BPOS length ~ polarization fraction(10’ resolution)

Polarization fraction (%)

Filament

background
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

MJy/srVariations of the polarization angle and the polarization fraction:
Insight on the 3D magnetic field structure in the filaments?
Role of the geometry of the magnetic field in the observed 0.1 pc
width of filaments?
Increase of the magnetic pressure due to thecompression of the magnetic field lines: may
prevent the further collapse of the filament?
Magnetic field lines
in the cloud surrounding
the filament
Accretion ofsurrounding
material onto
the filament

Change of theorientation of themagnetic field lines
in the filament due
to gravitation
contraction

Planck XXXIII 2016 (arXiv:1411.2271)
The L1506 filament in the Taurus molecular cloud
Planck dust intensity map at 353GHzof the L1506 filamentin Taurus Molecular cloud

MJy/srRadial collapse of
a supercriticalfilament

The segments show the plane of the sky magnetic field orientation.
The length is proportional to the polarization fraction.

Planck total intensity at 353GHz (850µm) in MJy/sr
Segments: BPOS length ~ polarization fraction(10’ resolution)
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Modelling the magnetic field structure of a filament
Philosophy of the modelling:

Arzoumanian et al., in prep.
Present day (observed) radial density profile- Initial gas density and magnetic field uniform- Frozen-in conditions: magnetic field lines are
dragged with the collapsing gas andconcentrate towards the central part of
the filament- We consider a radially symetric configuration- The “displacement law” satisfy theconservation of mass

Initial condition

Present state

Initial uniform density

Initial magnetic field
Present magnetic field
Vector potential

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Modelling the magnetic field structure of a filament
Density map and BPOS orientation

Face on view maps of themodel filament + cloud

B field intensity and BPOS orientation

I, Q, U from the modeled cube

Polarization properties

See also Tomisaka 2015,
Auddy et al. 2017
Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

Modelling the magnetic field structure of a filament
Polarization angle and fraction across the filament

Arzoumanian et al., in prep.

-We see both variation of the polarization angle
and the decrease of the polarization fraction fromthe background to the filament as in observations! The “hour glass” magnetic field configuration may be
responsible, at least partly, for the observed decrease of
the polarization fraction in “dense” filaments

BPOS orientation across the filament= 45o
Polarization fraction across the filament[o]
For pmax=20%
Radial profile across the filament [pc]Radial profile across the filament [pc]Doris Arzoumanian, MHD workshop, December 13, 2017, KagoshimaSummary and conclusions• The Herschel results support a “filamentary paradigm” for starformation, where 0.1pc-wide supercritical filaments are themain sites of star formation, and they may govern the starformation process.

Taurus (star forming cloud)Large scale Bpos
structure from Planckpolarization
observations• Multiple velocity components observed towards filaments may
be linked to large scale structures/sheets interacting with thefilament.
• The analyses of Planck dust polarization data (in the regimeof optically thin dust emission) indicate that the observed
scatter in polarization fraction and angle can be understood asa result of depolarization due to LOS integration and orientationof the mean B field with respect to the LOS.
• It is important to analyse simultaneously polarization angles
and fractions: Both give us a hint on the magnetic field structure
along the LOS.
• Modeling dust polarization towards supercritical filaments may
give insights on their B-field, which is important to constrain
their formation, evolution, and fragmentation into star forming
cores.

Taurus B211/3
Star forming
filament
cores

Palmeirim+ 2013

Doris Arzoumanian, MHD workshop, December 13, 2017, Kagoshima

0.1 pcat 140pc