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In theory, the application of a reddening correction should be
straightforward. For example, one method is to derive the spectral
energy distribution (SED) for a target star and compare it to the SED of a
dereddened standard star. One can multiply the standard SED by
various extinction coefficients (A
), so the residual of the
target with respect to
standard is minimized. However, since I only have five filter bands to work
with and the U band data are very noisy, I lack sufficient
leverage to use this method. Nonetheless, B through I data can still be
useful. Temperature can be determine via the B-V color
(Gilliland 1985):
Likewise, the R-I color can be converted to temperature (Laird 1985):
Here 
is defined in the usual way
(
).
The relations are both valid for dwarfs between 4000K and 7000K.
PMS stars are sub giants, a luminosity class which is not well
understood. The spectral type assigned to a star of a given
temperature can differ by 3 subtypes, depending on luminosity class,
with the largest differences being seen for G stars.
However, the difference in color index between luminosity classes
III and V for a
given temperature is less than 0.03 for B-V and R-I.
The spectral subtype I derived here is based on the observed color index.
The difference in the
color temperatures thus obtained is a function of the reddening.
By adjusting the reddening to minimize the difference between the
B-V and R-I color temperatures, an estimate to extinction can be
made. The extinction is determined to be the A
needed to make
to B-V and R-I temperatures the same.
This methodology was tested against the background stars
in the Orion OB1a fields. The results produced small extinctions
to the background stars of between A
and A
.
However, when the same techniques were applied to the
X--ray data, very unusual results were obtained. Extinctions of
A
or < 0 were not uncommon. These results are considered
spurious and were likely the effect
of photospheric contamination by the active chromosphere.
Such unusual extinctions could be the result
of chromospheric activity which was primarily detected in the B band
and therefore made the star appear warmer in the blue than in the red.
The high extinctions could be the result of strong chromospheric H
line
emission which is observed in the R--band and can make the R-I
color temperature artificially high. Given coverage into the infrared,
temperature and be independently derived from the V-K color so long
as the stars lack any near-IR excess (Carney 1983).
Alternatively, spectral typing gives an independent assessment of
temperature. The most accurate extinction calculation
requires both IR and spectral data.