1. The low frequency Galactic polarisation foreground Xiaohui Sun & Wolfang Reich MPIfR 23.05.2011

2. foreground polarisation causes trouble Jelic et al. 2010 Galactic foreground 0.15% Leakage of PI to I (dotted) + reionisation (solid) 1 rad m -2  229 deg at 150 MHz

3. WSRT observations at 150MHz Bernardi et al. 2009 Fan region 8 bands: 139.3 141.5 143.7 145.9 148.1 150.3 152.5 154.7 bandwidth: 2.5 MHz Frequency resolution: 9.8 kHz (tapering) Resolution (PI): 4.2 arcmin 1 mJy/beam = 1 K rms fluctuation: ~ 7 K

4. learn from observations: RM synthesis Brentjens & de Bruyn 2005 from Heald

5. data: Bernardi et al. (2009) RM synthesis software: Brentjens clean software: Heald φ: −9 – 2 rad m -2 resolution: 3.44 rad m -2 max φ extent: 0.85 rad m -2 −9 −5 −4 −3−2−1 012 −8−7 −6

6. Missing large-scale components 2 – 4 K at 408 MHz (Brouw & Spoelstra 1976) spectral index of –2.5  24 – 49 K at 150 MHz Faraday depth very small (Spoelstra 1984) φ<0.8 rad m -2 otherwise depolarisation  assuming φ=0 Structures not influenced if |φ|>2 rad m -2

7. −9 −5−4 −3−2 −8−7 −6 A ring structure!

8. An emission feature? NO! -- the intrinsic φ~0 -- only appear at certain φ Properties of the ring structure -- centre-filled to ring-like -- intensity twice as the surrounding -- inner radius about 1 degree -- outer radius of about 2 degree

9. HAMMURABI code (Waelkens et al. 2009) pixel Understanding observations by simulations 3D-emssion models Sun et al. 2008; Sun & Reich 2009; Sun & Reich 2010 Code modifications Cartesian coordinates Output RM cubes directly Add faraday screens Zoom-in of a patch

10. Observations imply a Faraday screen FD from Simulations negative monotonously decreasing versus distance Constrains on FS positive smaller at centre FD too large too small distance too far too near

11. FS facts D: 200-250 pc R: 6.4 pc Shell: 2 pc Φ: 1.5--3.7 rad m -2 -- RM synthesis ~ 1/λ 2 min maximal FD extent 0.85 rad m -2 needs frequency > 400 MHz (Effelsberg 300-800 MHz GMIMS survey) -- Geometry of the FS multi-FSs? (Bernardi et al. 2009)

12. Previous observations Haverkorn et al. 2003 Schnitzeler et al. 2007 Difficulties observations: influence of large-scale emission simulations: add local emission

13. Summary Nature of the FS related with the star HD 20336 -- B2V -- D: 246±37 pc (parallax) -- wind-blow bubble? although disfavored by Haverkorn et al. (2003) not clear -- a magnetic structure (Haverkorn et al. 2003) need 300-800MHz observations (GMIMS) RM synthesis + simulations can constrain distance and other parameters! nature of FSs remains unclear

14. Thank you! 谢谢！