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Surface Activity on Sun-like Stars (1Jun94) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 38, 1 June 1994) Observations show that late-type stars which are otherwise characterized by essentially identical photospheric properties can exhibit a significant range in their levels of chromospheric and coronal emission. The detailed physical processes responsible for outer atmospheric heating in the late-type stars are poorly understood. However, studies of the Sun and solar-type stars indicate that `activity' is spatially associated with sites of emergent magnetic flux. The magnetic fields themselves are widely regarded as the surface manifestations of an interior dynamo mechanism which, in a general way, involves the interaction between rotation, convection, and an extant magnetic field. It is believed that the solar cycle is a result of such a process. The further development of stellar dynamo theory - the theory of the origin and evolution of magnetic fields in the Sun and Sun-like stars - requires a knowledge of magnetic field properties. In particular, the difference between an `active' star and a `quiet' star or, equivalently, a solar active region and the quiet Sun, may be due to the differences in the characteristics of the magnetic field that, in turn, influence the structure of the chromosphere and corona. Recent observational investigations suggest that the primary difference between active and quiet stars is in the fractional area coverage, or `filling factor,' of magnetic regions analogous to solar plages on the stellar surface. In order to verify and extend this tentative result, we must measure the filling factor of magnetic complexes on stars. Since we cannot spatially resolve the surfaces of nearby, main-sequence stars, we must rely on the analysis of spectroscopic diagnostics to infer active region filling factors. Vincenzo Andretta (U. of Naples, Italy, and a former NSO summer student and visiting thesis student) and [Figure not included] Correlations computed by Andretta and Giampapa between the strengths of lambda 10830 vs. lambda 5876 (D3) for several filling factors for G and F dwarfs. The solid points are from sources in the literature. Arrows indicate upper limits. In the case of solar-type (G) stars, fractional area coverages of up to 80% are indicated for some particularly active objects. Mark Giampapa have been working on a novel approach for estimating active region filling factors on solar-type stars. Their method involves observations of the He I triplet lines at 5876 A (D3) and 10830 A, respectively, combined with model computations of the intrinsic strengths of these features in stellar active regions. These lines appear in absorption in plages on the Sun and, by implication, in the active regions on Sun-like (F - early K) stars. The D3 line and lambda 10830 either do not appear, or occur only very weakly, in the quiet solar (or stellar) photosphere. Hence, these spectral features are ideal tracers of magnetic regions. In order for their method to work, the maximum strength that these uniquely useful lines can attain in the active regions associated with the magnetic fields on the surfaces of Sun-like stars must be estimated. Andretta and Giampapa use a combination of observations and theory to calibrate the maximum strength of the helium lines in magnetic regions. They have constructed a grid of stellar model chromospheres based on models of the solar chromosphere. The chromospheric thermal structures are then superposed on models of F and G star photospheres, and the strengths of the helium lines computed using a radiative transfer code. Using data from the McMath-Pierce FTS on Kitt Peak and the Jack Evans Facility at Sacramento Peak, Andretta and Giampapa find that their computations are in good agreement with what is seen in solar active regions. From their results as applied to stars, they find that the coverage of magnetic, plage-like regions on the surfaces of stars that are more active than the Sun can exceed 20%. Andretta and Giampapa are now refining this method in an effort to deduce the actual filling factor, rather than just lower limits, of stellar magnetic regions using observations of the helium triplet lines. Some preliminary results of this effort are shown in the figure. In this illustration the theoretical correlation between the strengths of D3 and lambda 10830 for several filling factors are shown. Also plotted are the observed equivalent widths of these lines for a number of G and F dwarfs. As can be seen, fractional area coverages of up to 80% are indicated in some active, solar-type stars.
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