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!)
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
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