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A method for estimating the fractional area coverage of active regions on dwarf F and G stars
The D3 (lambda 5876) and lambda 10830 lines arising fromtriplet levels in neutral helium appear in absorption in active (plage)regions on the Sun and, by implication, in the active regions onSun-like (F-early K) stars. These features either do not occur, orappear only very weakly, in the quite solar (or stellar) photosphere.Hence, these diagnostics are ideal tracers of magnetic regions outsideof cool spots. The appearance of D3 and lambda 10830 inabsorption immediately suggests that these lines can be utilized toinfer the fractional area coverage, or filling factor, of active regionon stellar surfaces if their intrinsic absorption strengths in theseregions are known. In particular, a meaningful lower limit to the activefilling factor can be deduced if the maximum absorption equivalent width(Wmax) in D3 or lambda 10830 as either appears instellar analogs of solar plages can be estimated. We develop thisapproach by constructing a grid of model chromospheres based on the VALC model of the quiet solar chromosphere. This thermal structure issuperposed on published models for F and G dwarf photospheres. We solvefor the non-LTE ionization of hydrogen to infer chromospheric electrondensities. We then perform a multilevel, non-LTE computation of thehelium triplet lines in the sequence of model chromospheres, taking intoaccount the potential effects of coronal XUV back radiation on the lineformation. We conservatively estimate that Wmax approximately= 100-150 mA for D3 in both F and G dwarfs. The implied lowerlimits to the filling factor of plagelike regions can be approximately20% among active solar-type stars. We extend this approach byinvestigating a method by which the actual filling factor can be deducedthrough a study of the joint response of D3 and lambda 10830to chromospheric nonradiative heating. We emphasize that our fillingfactor estimates indicate the area coverage at the height of formationof the helium triplet lines in the active chromosphere. Because of fieldline spreading with height, filling factors based on chromospheric linesare expected to exceed estimates based on purely photospheric lines.Finally, we discuss the relative importance of collisional andphotoionization processes in the formation of these importantdiagnostics.
| Fifth fundamental catalogue. Part 2: The FK5 extension - new fundamental stars
The mean positions and proper motions for 3117 new fundamental starsessentially in the magnitude range about 4.5 to 9.5 are given in thisFK5 extension. Mean apparent visual magnitude is 7.2 and is on average2.5 magnitudes fainter then the basic FK5 which has a mean magnitude of4.7. (The basic FK5 gives the mean positions and proper motions for theclassical 1535 fundamental stars). The following are discussed: theobservational material, reduction of observations, star selection, andthe system for the FK5 extension. An explanation and description of thecatalog are given. The catalog of 3117 fundamental stars for the equinoxand epoch J2000.0 and B1950.0 is presented. The parallaxes and radialvelocities for 22 extension stars with large forecasting effects aregiven. Catalogs used in the compilation of the FK5 fundamental catalogare listed.
| Activity in early F-type stars in the Hyades
Observations of the equivalent width of D3 He I A 5876 in 14main-sequence stars in the Hyades show that stellar activity in thesestars is closely similar to that seen in early F-type field stars. Theonset of activity occurs at about (B - V) = 0.30 m, and the activitylevel in stars bluer than (B - V) = 0.45 m does not correlate withrotation. There is no evidence of a decline of activity with increasingage during the rather narrow time interval spanned by the main-sequencelifetimes of early F-type stars. The sharp minimum in lithium abundancethat is seen in Hyades stars of middle F spectral type occurs at aboutthe color that marks the boundary between temperature regimes whereactivity does and does not correlate with rotation.
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