Guinan and DeWarf, with M. Güdel (PSI & ETHZ, Switzerland) and
J.D. Dorren (Edinburgh, Scot.), continue with the Sun in Time project,
a coordinated multiwavelength study of several nearby, single,
solar-type stars selected as proxies for the Sun at several stages in
its evolution from ZAMS (Zero Age Main-Sequence) to TAMS (Terminal Age
Main-Sequence). An 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 with spectral types close to solar.
By considering only main-sequence stars in a restricted range of
spectral types (F5 V - G8 V) with measured rotation periods, they can
focus on the role of rotation in determining activity levels, since the
exclusion of K stars significantly limits the variation range of other
properties such as mass, radius, temperature and, particularly,
convection zone depth. There is still a wide spread of rotation rates
(P
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 the
rotation rate, keeping all other parameters approximately constant.
They have derived precise relations between X-ray (corona), C IV
(transition region), and Mg II (chromosphere) 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 G0 V - G5 V stars that are very close
to solar. Since this smaller group may be considered to be proxies for
the Sun at different ages, they 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. These results have been combined with existing
rotation-age relations to obtain rotation-activity-age relations. In
addition, photoelectric photometry of several of these stars shows a
correlation between the mean light curve amplitude and the rotation
period, which indicates how the photospheric (starspot) activity (areal
extent) depends on rotation. The use of rotation period rather than
projected rotational velocity significantly reduces the uncertainties in
these relations.