Studies in mid-infrared spectropolarimetry - II. An atlas of spectra
C. H. Smith, C. M. Wright, D. K. Aitken, P. F. Roche, J. H. Hough;
MNRAS, 2000, 312, 327
ABSTRACT:We present 8-13μm spectropolarimetry of 55 sources and 16-22μm spectropolarimetry of six of these.
This represents a substantial fraction of star
formation regions that can be observed in this way with current technology on 4-m
class telescopes (i.e.
brighter than about 20Jy at 10μm in a 4-arcsec
beam).
Most of the sources are embedded young stellar objects (YSOs), Hii regions
containing sites of star formation or bipolar protoplanetary nebulae (PPN),
although a few other sources (e.g.
NGC 1068, MWC 349) are also included.
The
majority have oxygen-rich chemistry but there are three carbon-rich
sources.
Many of the oxygen-rich sources show deep silicate absorption overlying
featureless or optically thin silicate emission.
Absorptive polarization with
polarization per optical depth (paτ)~=1-3 per cent is common, and
many also show evidence for an emissive polarization component as well,
although pure polarization in emission is rare.
The observed ranges of
paτ and pe are very similar, rather surprising in view of their
origin from very different environments.
Typically the absorptive
polarization profiles are similar to the archetypal silicate polarization found in
OMC1 BN, but an exception is AFGL 2591, which displays an additional narrow
polarization feature at 11.2μm, which has been attributed to annealed
silicates.
Many of the intensity absorption spectra also show an inflection near
11.2μm, which might also be attributable to annealed silicates.
The
carbon-rich sources have nearly featureless polarizations in the 0.5-1per cent
range, which we ascribe to dichroism in carbon-based grains; this is the first
evidence that such grains can be aligned.
In two of these sources the
polarization appears to be caused by absorption by SiC.
A few of the polarization
spectra have no straightforward interpretation.
KEYWORDS: magnetic fields, techniques: polarimetric, stars: formation, dust, extinction, infrared: ism: continuum<x>.</x>
PERSOKEY:polarisation, ,
CODE: smith2000