magnetic field structure in molecular clouds by polarization measurements

Magnetic Field Structure in Molecular Clouds by Polarization MeasurementsWen-Ping Chen

National Central UniversityCollaborators: C. Eswaraiah (ARIES), S. P. Lai (NTHU), C. D. Lee (NCU), C. C. Lin (NCU), A. K. Pandey (ARIES), Shuji Sato (Nagoya U), Y. H. Shi (NCU), Bohe Su (NCU), M. Tamura (NAOJ), J. W. Wang (NTHU)

Magnetic Field and Star Formation• What is the field strength and structure on

different length scales (hence densities) from the molecular cloud, core, to protostar?

• B suppresses cloud fragmentation/collapse; ambipolar diffusion reduces the strength, hence the influence of, the field.

• B collates filaments? Guides core collapse and mass outflows?

Observations in OIR

Observations in FIR to mm

Scattering by dust Dichroic extinction by aligned dust

Polarized thermal emission by dust aligned by B

Courtesy: Tamura

Stahler & Pallo 2004

The Rho Oph cloud (Vrba et al. 1976)

The Rho Oph cloud (Stahler & Palla 2004; data from Loren 1989 and Goodman et al. 1990)

Background stars should be otherwise unpolarized.

IR less extinction than the optical, so probes deeper into the cloud (more background stars) but less effective

To derive the B information, need to sort out which mechanism is at work.

To infer if cloud geometry influenced by B, need to isolate other effects, e.g., by shocks.

Polarization of Background Stars --- Dichroic extinction by thermalized, magnetically aligned dust

Our programTo probe the B structure on protostellar scales and on the inner part of

a cloud core by SMA, and soon by ALMA; disk/outflow configuration

on the outer part of a cloud core by NIR polarization (e.g., SIRPOL)

on the periphery of a cloud core by optical polarization (e.g., TRIPOL)

SIRPOL --- SIRIUS (Simultaneous IR imager for Unbiased Survey) with polarimeter

• IRSF 1.4 m telescope at SAAO• Simultaneous JHKs imaging polarimeter• FOV 7.7’, (1 K x 1 K x 3 bands, 0.45”/pix)• Imaging sensitivity, J=19.2, H=18.6,

Ks=17.3 mag (S/N=5; 60 min)• Pol sensitivity, J < 16.5, H < 15.7,

Ks < 14.5 (dP < 0.3-1%)

Tamura et al. 2006 on the Orion Nebula

The Carina nebula (NGC 3372) RA = 10:45:08.5, Dec = ‒59:52:04

is a large bright nebula powered by UV radiation from 65 O-type stars and 3 WNH stars (Smith et al. 2008), including the most massive and luminous star in the Milky Way, Eta Carinae. At a distance of 2.3 kpc, the Carina nebula is a good laboratory to study massive star formation.

RCW 57A (NGC 3576) RA = 11:11:54.8Dec = ‒61:18:26

is among the brightest Galactic HII regions, hosting many IR excess stars and some high-mass Class 0/I objects (Barbosa et al. 2003). At a distance of 2.4 kpc (Persi et al. 1994), RCW57 is also a good target to study star formation in a turbulent environment.

DSS 5 deg

Carina Nebula

RCW 57A

Only reliable measurements (ΔP < 0.5% or P/ΔP > 3 and ΔH < 0.05 mag) are included.

Carina nebula RCW 57A12CO(J = 1−0) emission (Yonekura et al. 2004)

H band

Carina nebula

[Background = foreground = Galactic] in polarization because the SIRPOL field is devoid of dense cloud. No B information

The whole region was mosaicked in the spring of 2012.

[S II]

Rcw 57A

Hour-glass shaped B threading the elongated cloud

H-band stellar polarization overlaid on the WISE 4.6 micron image

Grey lines: H-band polCentral curve: 13CODashed lines: HII region (3.4 cm) Pluses: H2O maser sourcesFilled squares: IRAS sourcesTriangles: Class IOpen squares: Class II cavities, bubbleEswaraiah+2012 prep

Foreground stars toward RCW57A and Carina Nebula with V-band polarization (Heiles 2000) and Hipparcos parallaxes (van Leeuwen 2007).

By assuming Pmax= 1.0%, max =0.55 micron, K=1.15 and by using the Serkowski's relation

P/Pmax = exp[ -K * ln2 (max/ ) ]

So the foreground polarization in NIR is negligible

PJ < 0.65 % (J=1.25 micron)PH < 0.46 % (H=1.63 micron) PKs < 0.30 % (K=2.14 micron)

P internal in RCW 57A no external perturber to shape the cloud

TRIPOL --- Tri-color Imaging Polarimeter

• Designed and fabricated by Prof. Shuji Sato of Nagoya U.

• Prototype completed in 2011, tested on Lulin one-meter telescope (LOT) , now on the 75 cm telescope at SAAO.

• Second unit completed in 2012, now as facility instrument at Lulin.

• Meant to be simple, robust, versatile, and economic, particularly suitable for small telescopes.

• Simultaneous imaging at gri bands

F10

3 Color Imaging & a Polarizer

3 channel

polarization½-plate

wire-gridbirefringenceor

g

r

i

plain imager CCD

3-CCD

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~3000 US$

SBIG

TRIPOL images of M1 (top) polarized intensity and (bottom) total intensity in g’, r’, and i’ (left to right)

TRIPOL first light images: M16 in g’ (left), r’, and i’.

HL Tau

XZ Tau

g’ r’ i’

HL Tau (B=16.02, K=7.41)

Polarization Pol Angle

g’ 15.01 +/- 0.62 84 +/- 01

r’ 14.17 +/- 0.23 87 +/- 01

i‘ 14.19 +/- 0.25 -88 +/- 0.0

XZ Tau (B=10.4, K=7.29)

g‘ 1.48 +/- 0.18 -77 +/- 03

r‘ 1.29 +/- 0.11 -65 +/- 02

i‘ 1.51 +/- 0.10 -75 +/- 01

TRIPOL images taken with the LOT in August 2011

T Tauri stars

CB3 --- a dark globule

CB 3 g band

Ward-Thompson et al. (2009)

Conclusions• Polarization is a powerful tool to infer the

magnetic field configuration in molecular clouds. • A combination of extinction (OIR) and thermal

emission (FIR to mm) measurements will yield the field structure from small- to large-length scales.

• In RCW 57A, there is possible evidence of B controlled cloud contraction.

• Full operation of TRIPOL is scheduled in December 2012 to study cores with YSOs, starless cores, etc.