- Background: Using near-IR polarimetry, Goodman et al. (1995) found that polarization does not rise with extinction inside dark clouds, and therefore is an ineffective probe of magnetic fields inside those clouds. The near-IR study could not probe very low extinctions, where the polarization should rise with extinction, and it also focused only on the dark clouds themselves. Our aim in the present study was to determine how polarization depends on extinction, over a large range of extinction, in a "controlled" environment, where the distance to the extinguishing dust and the field inclination are relatively constant. We expected to, and did (see below), find a break in the p-AV plots, where polarization ceases to rise with extinction. This break is an indication of the answer to the "Question" below.
- The Question: How high in extinction, near dark clouds, can one safely go before low polarization efficiency causes background starlight polarimetry to fail as a magnetic field probe?
- The Answer: About 1.3 magnitudes of visual extinction, in the immediate vicinity of the (Taurus) dark clouds (see below).
- The Evidence: Polarization-Extinction relations based on photometry, spectroscopy, and polarimetry for stars along a constant R.A. cut through the Taurus dark cloud complex.
The figure on the left shows an extinction (AV)
map of the region in the Taurus dark cloud
complex we studied. This map was obtained
using co-added images of the flux density
from the Infrared Astronomical Satellite (IRAS)
Sky Survey Atlas. Here we show the position of
the 36 stars in our sample for which we have
obtained the necessary photometric, spctroscopic,
and polarimetric data to enter them on the
polarization vs. extinction plot shown below
(see figure below for legend). Notice that there
are stars in high extinction regions associated
with obvious dark cloud peaks (i.e., most of the green dots),
and stars in high and low extinction regions far from the
clouds (i.e., most of the blue and purple dots).
Please click on the plot to see a larger
version.
This plot is the ``Evidence'' on which we base the
``Answer'' above. Here we plot the observed
relation between polarization (p) and extinction,
where AV=3.1EB-V. The lines are least square
linear fits, weighted by the uncertainty in the
polarization. The dashed line is the fit to points
representing stars background to the low density
ISM (purple triangles and blue circles). The solid
line is the fit to the points representing the stars
background to dark clouds (green rectangles).
The intersection of these two lines at about 1.3
mag shows where the linear relation between
p and AV that exists in the low density ISM
breaks down for stars background to dark clouds.
CONCLUSION: In regions like Taurus,
it is safe
to interpret the polarization of background starlight
as a representation of the plane-of-the-sky
projected magnetic field only up to the 1.3±0.2 mag
"edge" of the dark cloud.