The Sun in Time
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X-Ray, FUV, and UV Observations of α Centauri B: Determination of Long-term Magnetic Activity Cycle and Rotation Period
10 October 2010, ApJ, 722, 343
L.E. DeWarf, K.M. Datin & E.F. Guinan
Abstract:
Over the last couple of decades we have been carrying out a study of stellar
magnetic activity, dynamos, atmospheric physics, and spectral irradiances
from a sample of solar-type G0-5 V stars with different ages. One of the
major goals of this program is to study the evolution of the Sun's X-ray
through NUV spectral irradiances with age. Of particular interest is the
determination of the young Sun's elevated levels of high-energy fluxes
because of the critical roles that X-ray (coronal) through FUV (transition
region (TR), chromospheric) emissions play on the photochemical and
photoionization evolution (and possible erosion) of early, young planetary
atmospheres and ionospheres. Motivated by the current exoplanetary search
missions (such as Kepler and CoRoT, along with the planned
Space Interferometry mission and Darwin/Terrestrial Planet Finder
mission) that are hunting for earth-size planets in the habitable zones
(liquid water) of nearby main-sequence G-M stars, we are expanding our
program to cooler, less luminous, but very importantly, much more numerous
main-sequence K-type stars, such as α Centauri B. The long life
(2-3× longer than our Sun) and slow evolution of K stars provide nearly
constant energy sources for possible hosted planets. This program
parallels our "Sun in Time" program, but extends the study to stars with
deeper convective zone depths. Presented here are X-ray (coronal;
ROSAT, Chandra, XMM), UV (TR; IUE), NUV
(chromospheric; IUE), and recently acquired FUV (TR/chromospheric;
FUSE Cycles 7/8) observations of the K1 V star α Cen B
(HD 128621; V = 1.33; (B-V) = +0.88; τ = 5.6 ± 0.6 Gyr).
These combined high-energy measures provide a more complete look into the
nature of α Cen B's magnetic activity and X-UV radiances. We find that
α Cen B has exhibited significant long-term variability in X-ray
through NUV emission fluxes, indicating a solar-like long-term activity cycle
of Pcycle = 8.84±0.4 years. In addition, analysis of the
short-term rotational modulation of mean light due to the effects of
magnetically active regions has yielded a well-determined rotation period of
Protation = 36.2±1.4 days. α Cen B is the only old
main sequence K star with a reliably determined age and rotation period, and
for early K-stars, as in the case of the Sun for G2 V stars, is an important
calibrator for stellar age/rotation/activity relations.
The Sun in Time
