The Sun in Time: Rotation-Activity Relations for Solar Analogues of Different Ages

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Session 45 -- Stellar Activity I
Display presentation, Tuesday, 10, 1995, 9:20am - 6:30pm

[45.02] The Sun in Time: Rotation-Activity Relations for Solar Analogues of Different Ages

J.D. Dorren (U.Penn.), E.F. Guinan \& L.E. DeWarf (Villanova U.)

The extensive body of ultraviolet (IUE ), and X-ray (Einstein and ROSAT ) observations of late-type stars is employed to investigate the coronal, transition-region (TR), and chromospheric emission of single solar-type stars. By considering only stars in a restricted range of spectral types (F5 V - G8 V) with measured rotation periods, we focus on the role of rotation in determining activity levels, since the exclusion of K stars significantly limits the range of variation of the other properties (mass, radius, temperature, and particularly convection zone depth ). There is, however, a wide spread of rotation rates (P$_{rot} = 1.5-45$ days) and ages (70 Myr to 9.5 Gyr), and consequently a wide range of magnetic activity. These stars thus constitute a test of the effect on the stellar dynamo of varying rotation rates, keeping all other parameters approximately constant. We have derived accurate relations between X-ray, C IV, and Mg II emission and rotation period for stars in this range of spectral types. Moreover, there appears to be little difference in the relations obeyed by the entire group and those of the subset of sunlike stars (G0 V - G5 V). Since this smaller group may be considered to be solar proxies, we have, in effect, also determined empirical relationships between the historical solar magnetic activity (coronal, TR, and chromospheric) and the Sun's rotation period, which is related to solar age. We have combined our results with existing rotation-age relations to obtain rotation-activity-age relations. In addition, photoelectric photometry shows a correlation between the mean light curve amplitude and rotation period, which indicates how the photospheric (starspot) activity depends on rotation. The use of rotation period rather than projected rotational velocity significantly reduces the uncertainties in these relations.

This research is supported by NASA NAG5-2160, NAG5-2494, and NSF AST 86-16362.

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