Studies in mid-infrared spectropolarimetry - II. An atlas of spectra

We present 8-13 mu m spectropolarimetry of 55 sources and 16-22 mu m spectr opolarimetry of six of these. This represents a substantial fraction of sta r formation regions that can be observed in this way with current technolog y on 4-m class telescopes (i.e. brighter than about 20 Jy at 10 mu m in a 4 -arcsec beam). Most of the sources are embedded young stellar objects (YSOs ), H II regions containing sites of star formation or bipolar protoplanetar y 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 abs orption overlying featureless or optically thin silicate emission. Absorpti ve polarization with polarization per optical depth (p(a)/tau) similar or e qual to 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 p(a)/tau and p(e) are very similar, rather sur prising in view of their origin from very different environments. Typically the absorptive polarization profiles are similar to the archetypal silicat e polarization found in OMC1 BN, but an exception is AFGL 2591, which displ ays an additional narrow polarization feature at 11.2 mu m, which has been attributed to annealed silicates. Many of the intensity absorption spectra also show an inflection near 11.2 mu m, which might also be attributable to annealed silicates. The carbon-rich sources have nearly featureless polari zations in the 0.5-1 per cent range, which we ascribe to dichroism in carbo n-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 absorptio n by SiC. A few of the polarization spectra have no straightforward interpr etation.