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Proceedings Paper

A mid-infrared polarization capability for the ELT
Author(s): Christopher M. Wright; Ralf Siebenmorgen; Bringfried Stecklum; Michael Sterzik; Hans-Ulrich Käufl
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Paper Abstract

Imaging- and spectropolarimetry in the thermal infrared (~ 5-30 μm) can inform us about two important open questions in modern astrophysics - namely the role of magnetism in the formation of stars, and the life-cycle of cosmic dust. These are key questions outlined in the document "A Science Vision for European Astronomy" by de Zeeuw & Molster (2007). Thermal IR polarimetry is the only technique that can peer into the heart of star forming cores, where an infant star heats its immediate surroundings to temperatures of several hundred Kelvin. The polarization itself is induced by a preferential alignment of the spin axis of cosmic dust grains, a process ultimately controlled by the ambient magnetic field. The spectrum is sensitively dependent on the grain optical properties, structure and shape, thus providing information not otherwise obtainable by conventional spectroscopy. The MIRI instrument on the JWST will not have a polarimetry mode, thus leaving open the possibility of an ELT mid-IR instrument being able to make substantial progress on these fundamental issues. Before describing the advantages of a mid-IR spectropolarimeter on an ELT, we first present some preliminary results from our polarization observations with the TIMMI2 mid-IR instrument between 2004 and 2006. The experience gained with TIMMI2 - in terms of technical issues and observing strategy - will inform the design of any future instrument. Following this we will describe the science that could be done with an ELT instrument, and some of the basic design parameters. For instance, with a resolution of ~ 70 milli-arcseconds (FWHM at 10 μm) it will become possible to resolve the magnetic field configuration in the circumstellar disks and bipolar outflows of young stars at a spatial scale of less than 10 AU in the nearest star formation regions. This will strongly constrain hydromagnetic models - the favoured means of extracting angular momentum and allowing accretion to proceed - for bipolar jets emanating from a range of compact astrophysical objects. Further, with a resolving power of order 200, and sensitivity of 100σ in 1 hour integration on a 0.5 mJy point source, the evolution of cosmic dust - and the governing physical and chemical processes - from its formation in old stellar outflows to its deposition in planet-forming disks, will become amenable to detailed polarization studies.

Paper Details

Date Published: 9 July 2008
PDF: 13 pages
Proc. SPIE 7014, Ground-based and Airborne Instrumentation for Astronomy II, 701429 (9 July 2008); doi: 10.1117/12.787899
Show Author Affiliations
Christopher M. Wright, The Univ. of New South Wales@Australian Defence Force Academy (Australia)
Ralf Siebenmorgen, European Southern Observatory (Germany)
Bringfried Stecklum, Thüringer Landessternwarte Tautenburg (Germany)
Michael Sterzik, European Southern Observatory (Germany)
Hans-Ulrich Käufl, European Southern Observatory (Germany)

Published in SPIE Proceedings Vol. 7014:
Ground-based and Airborne Instrumentation for Astronomy II
Ian S. McLean; Mark M. Casali, Editor(s)

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