7.3. Polarization Percentages and Physical Parameters
Magnetic fields play a significant role in the collapse of protostellar cloudlets. But they are difficult to observe, and it appears that submillimeter polarimetry is perhaps the best way to observe them reliably. This new technique is in its infancy. A significant improvement is needed to put on solid ground the observational constraints inferred from theoretical models. For example, the observational errors need to be improved from typically 0.3% down to 0.1% which is now becoming possible. In the following, I describe briefly the tentative physical correlations which have been obtained so far, concerning the submillimeter/Extreme IR polarization percentage. These correlations have important implications for testing the many theories of star formation with magnetic fields.
7.3.1. Percentage Correlation 1 (Pol. percent vs grain chemistry)
The ratio of the polarization amplitude of dust emission at two different wavelengths varies with grain composition, i.e., silicate grains give a constant ratio of polarization percentages while graphite grains give a polarization percentage increasing at longer wavelengths (e.g., Hildebrand 1988). Thus graphite/metallic grains seem to predominate near DR21, since at 800 µm one sees only 2.5% polarization while at 1100 µm one sees 4% ±1% polarization (quite a change, e.g., Minchin and Murray 1994). New observations of DR21 at 1300 microns indicate 1.7% ±0.2%, supporting silicate graphite grains (Glenn et al. 1997a). Silicate grains seem to predominate near OMC-1, since at 100 µm one also sees 3.7% polarization while at 1300 µm one sees 4.6% polarization (about the same, e.g., Leach et al. 1991). More observational work remains to be done to firm up this correlation.
7.3.2. Percentage Correlation 2 (Pol. percent vs angular distance)
A simple trend in polarization percentage as a function of angular distance from a source center has been claimed. A small decrease in polarization percentage, relative to its immediate surroundings, termed a "polarization hole", has often been observed; it has been variously attributed physically to "a change in the magnetic field alignment" within the beam (e.g., Minchin and Murray 1994; Aitken et al. 1997; Rao et al. 1998) or to "gas-grain collisions" near the dust density maximum (e.g., Minchin et al. 1996). However, a calibrated plot of percentage versus telescope beam size for DR21 (see Fig. 3 in Glenn et al. 1997a) shows no evidence of a decrease or increase, from HPBW = 13" out to 42". Yet Greaves et al. (1997) find that the polarization percentage increases from 1% for nearby objects (at ~ 100 pc) up to 4% for more distant ones (at ~ 400 pc). More observational work remains to be done to firm this up.
7.3.3. Percentage Correlation 3 (Pol. percent versus time evolution)
Two opposite variations of polarization percentage with source age or time have been claimed. "The magnetic field becomes more ordered as a young stellar object moves towards the main sequence" (Holland et al. 1996), and "the magnetic field lines must first be more ordered around younger protostars" (Greaves et al. 1997). In this last reference, the polarization percentage decreases from 4% for young objects (outflow opening angle ~ 10° ; small ratio of bolometric luminosity / 1.3mm luminosity) down to 1% for older objects (outflow opening angle ~ 60°; large ratio of bolometric luminosity / 1.3mm luminosity).