POL-2 Polarimetry for the JCMT in the EAO era

Antonio Chrysostomou, Pierre Bastien and the POL-2 Team

POL-2 Commissioning Team

P. Bastien, A. Chrysostomou, D. Berry, D. Johnstone, P. Friberg, G. Savini, S. Coudé, M. Houde, J. Greaves

Overview

o Quick update from commissioning

o A science case for a POL-2 surveyo Magnetic fields in molecular cloudso Dust grain physicso More for discussion at this meeting

o Conclusions

POL-2 installation at JCMT

July 2010

‘Blade’ housing

containing the spinning waveplate,

analyser and calibrating wire-grid polarisers

Commissioning Updateo On-sky commissioning: March-September 2013o Demonstrated that the instrument works and functions

as a polarimeter, although some issues revealed o cross-talk between harmonics (under control, we think we

understand this)o instrumental polarisation (IP) needs to be better understoodo a simple model (based on the membrane) yields good fit to

planetary data but there is strong scatter presento telescope IP component is expected to vary across the focal

planeo stability of SCUBA-2 detectors…

o Basic observing mode (stare & spin) works, others not tested

o Data reduction works – can make maps and remove IP (once we understand it!)

Mars IP measurement

Systematic errors introduce a sensitivity

floor

OMC-1 Commissioning Data

SCUBA pol.Matthews et al. (2009)

Survey options for POL-2o What is the science case for a POL-2

survey?o here I choose the two obvious cases

o Mapping of magnetic fieldso characterisation of magnetised

turbulence in molecular clouds and star forming regions

o the role of filamentary structureo Dust grain physics

o testing grain alignment mechanisms

Mapping magnetic fields

o Polarisation → geometry of the field in plane of the sky (POS)

o Can obtain 3D maps of the magnetic field by including spectral information (Houde et al. 2002):o Dust polarimetry → B in POSo Zeeman splitting of spectral lines →

strength of B along l-o-so Ion-to-neutral molecular line-width ratio

angle between B and l-o-s (Houde et al. 2000)

The orientation ofthe projection of the magnetic field in the POS is shown by the vectors. The viewing angle is given by the length of the vectors (using the scale shown in the bottom right corner).Contours & gray scale:total continuum flux.

Houde et al. 2002CSO & Hertz, 350 µm

Mapping magnetic fields

o Characterisation of magnetised turbulence in molecular clouds and star forming regions

Context: need to hold up clouds & prevent rapid collapse (otherwise SF efficiency is too high)

o Both turbulence and magnetic fields are invokedo Can differentiate contribution from each by

calculating the angular dispersion to evaluate contributions of turbulent and ordered field components (Hildebrand et al. 2009, Houde et al. 2009)

Mapping magnetic fields

o The angular dispersion method allows determination of (Houde et al. 2009):o turbulent correlation lengtho turbulent/ordered field energy ratioo plane of the sky ordered field strengtho turbulent power spectrum

SCUBAPOL polarisation map @ 850 µmOMC-2 & OMC-3

Angular dispersion function:Angular dispersions vs. distance, displacement between pairs of magnetic field vectors → dispersion about large-scale fields

Poidevin et al. (2010)

Higher turbulent component

Mapping magnetic fields

o Herschel has shown us that filaments appear to be ubiquitous in SF regions

Pipe Nebula,

(Peretto et al. 2012,

A&A, 541, A63)

Andre et al 2013 (arXiv1312.6232)PPVI in press

Lupus I with BLASTpolMatthews et al 2014, ApJ, 784,

116

Palmeirim et al. 2013 A&A 550, A38

Orion A North,Salji et al. 2014

DR21/Cygnus,Natario et al. in prep.

Dust grain physics

o Grain alignment theories:o Mechanical (Gold 1952) ‒ NO, too weako Paramagnetic relaxation (Davis & Greenstein

1951) & enhancements (e.g., Purcell 1979; ferromagnetic, or suprathermal rotation) ‒ Difficult to reconcile with observations

o Radiative torques (RT) (Dolginov & Mytrophanov 1976, …, Hoang & Lazarian 2008, 2009) ‒ Observational support: Whittet+2008; Andersson & Potter 2010 → in the NIR

Predictions of RT alignmentMolecular cloud environment Alignment?

1 Cores with embedded YSOs Yes, very efficient

2 Cores without a protostar (or starless cores)

Decreases with τ (or AV) above a threshold

o Grain alignment is sensitive to ratio (λ/ a )o Reddening of external radiation field due to dust extinction

removes short wavelength photons necessary to align smaller grains in grain size distribution.

o With progressively fewer aligned grains, expect value of P/τ to drop

o Whittet et al. (2008) found this adequate to explain their observations up to AV ~ 10 mag.

2-comp. model withuniform + turbulent

mag. field

Whittet et al. (+)NIR data reaches to AV ~ 10 Jones et al. (2014, subm.)

Starless cores

(SCUBA-pol)

Matthews et al. (2009)

K-band polarimetry

Deep K-band polarimetry extends data to AV ~50, and

seems to fit to Whittet model.

Submm data extends reach to AV = 100+

Model assumes degree ofalignment decreases asa function of optical depth.

Change in slope indicative of grain alignment weakening deeper into the molecular cloud when no protostar is present

Jones et al. (2014, subm.)

Options for a polarimetry survey

o Include best regions from existing JLS/Herschel surveys

o Meet 2 major goalso mapping B-fields in SF regionso testing grain alignment mechanisms

o Variety of SF regions to allow statisticso include regions with YSOs and starless

cores, enough to do statistics

Would require ~ 200 hours …?

Conclusions

o Polarimetry with POL-2 offers opportunity too improve our understanding of magnetic

fields and turbulence in star-forming regions

o tests grain alignment mechanisms

o Best done with dedicated polarisation survey of modest scope, crafted to meet science goals