A METHOD FOR ESTIMATING THE FRACTIONAL AREA COVERAGE OF ACTIVE REGIONS ON DWARF F-STARS AND G-STARS

The D-3 (lambda 5876) and lambda 10830 lines arising from triplet levels in neutral helium appear in absorption in active (plage) regions on the Sun and, by implication, in the active regions on Sun-like (F-early K) stars. These features either do not occur, or appear only very weakly, in the quite solar (or stellar) photosphere. Hence, these diagnostics are ideal tracers of magnetic regions outside of cool spots. The appearance of D-3 and lambda 10830 in absorption immediately suggests that these lines can be utilized to infer the fractional area coverage, or filling factor, of active regions on stellar surfaces if their intrinsic absorption strengths in these regions are known. In particular, a meaningful lower limit to the active region filling factor can be 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 model chromospheres based on the VAL C model of the quiet solar chromosphere. 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 multilevel, non-LTE computation of the helium triplet lines in the sequence of model chromospheres, taking into account the potential effects of coronal XUV back radiation on the line formation. We conservatively estimate 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 extend 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 photospheric lines. Finally, we discuss the relative importance of collisional and photoionization processes in the formation of these important diagnostics.