V. Andretta et Ms. Giampapa, A METHOD FOR ESTIMATING THE FRACTIONAL AREA COVERAGE OF ACTIVE REGIONS ON DWARF F-STARS AND G-STARS, The Astrophysical journal, 439(1), 1995, pp. 405-416
The D-3 (lambda 5876) and lambda 10830 lines arising from triplet leve
ls in neutral helium appear in absorption in active (plage) regions on
the Sun and, by implication, in the active regions on Sun-like (F-ear
ly K) stars. These features either do not occur, or appear only very w
eakly, in the quite solar (or stellar) photosphere. Hence, these diagn
ostics are ideal tracers of magnetic regions outside of cool spots. Th
e appearance of D-3 and lambda 10830 in absorption immediately suggest
s that these lines can be utilized to infer the fractional area covera
ge, or filling factor, of active regions on stellar surfaces if their
intrinsic absorption strengths in these regions are known. In particul
ar, a meaningful lower limit to the active region filling factor can b
e deduced if the maximum absorption equivalent width (W-max) in D-3 or
lambda 10830 as either appears in stellar analogs of solar plages can
be estimated. We develop this approach by constructing a grid of mode
l chromospheres based on the VAL C model of the quiet solar chromosphe
re. This thermal structure is superposed on published models for F and
G dwarf photospheres. We solve for the non-LTE ionization of hydrogen
to infer chromospheric electron densities. We then perform a multilev
el, non-LTE computation of the helium triplet lines in the sequence of
model chromospheres, taking into account the potential effects of cor
onal XUV back radiation on the line formation. We conservatively estim
ate that W-max approximate to 100-150 m Angstrom for D-3 in both F and
G dwarfs. The implied lower limits to the filling factor of plagelike
regions can be similar to 20% among active solar-type stars. We exten
d this approach by investigating a method by which the actual filling
factor can be deduced through a study of the joint response of D-3 and
lambda 10830 to chromospheric nonradiative heating. We emphasize that
our filling factor estimates indicate the area coverage at the height
of formation of the helium triplet lines in the active chromosphere.
Because of field line spreading with height, filling factors based on
chromospheric lines are expected to exceed estimates based on purely p
hotospheric lines. Finally, we discuss the relative importance of coll
isional and photoionization processes in the formation of these import
ant diagnostics.