PDF (for b&w printout) - Living Reviews in Solar Physics

The Sun in Time: Activity and Environment 37
For the most active stars, LX is somewhat suppressed, for reasons that are not well understood.
The stars are in a “super-saturated” regime (Randich et al. 1996, see discussion in Güdel 2004).
The overall rotation and activity history of a solar analog star from ZAMS to the terminal
stages of its MS life thus proceeds roughly as follows:
• A star enters on the MS as a fast rotator although the rotation period is not unique, depending
on the previous star-disk interaction history. The star thus typically begins its MS life in the
saturation regime where LX ≈ (2 – 4)×10 33 erg s −1 . Although it spins down as a consequence
of its magnetic-wind mass loss, there is little evolution of the X-ray (and UV, FUV, EUV)
output.
• At an age of around 20 – 200 Myr, the rotation period has decreased sufficiently so that the
star’s corona drops out of the saturation regime (e.g., Patten and Simon 1996; James and
Jeffries 1997, and Section 17.5.2 in Güdel 2004). For a G star, saturation holds as long as
P < 1.5 – 2 d.
• Convergence of rotation periods has been achieved at ages of a few 100 Myr, and by that
time a solar analog shows P ≈ 4 – 5 d and LX ≈ 10 29 erg s −1 , i.e., less than 10% of the
saturation value.
• From then on, the luminosity decay law holds, because the rotation period is a function of
age and increases monotonically. This decay law has been observationally tested until the
terminal stages of the MS life of a solar analog.
5.5.2 The coronal temperature in time
The non-flaring corona of the contemporary Sun shows temperatures of a few MK. Active regions
tend to be hotter than the quiet corona, and consequently, the average coronal temperature during
the Sun’s activity maximum is higher than at minimum. Peres et al. (2000) have studied the fulldisk
solar coronal emission measure distribution to find peak temperatures of ≈ 1 MK and ≈ 2 MK
during minimum and maximum, respectively. The corresponding X-ray luminosities amount to
LX ≈ 3 × 10 26 erg s −1 and LX ≈ 5 × 10 27 erg s −1 , respectively. These solar observations suggest
that regions of higher magnetic activity, but also episodes of higher overall magnetic activity, not
only produce higher-luminosity plasma but also higher temperatures. Does this reflect in coronae
of stars with widely varying activity levels?
The spectral evolution of the X-ray and EUV (coronal) Sun in time is illustrated in Figures 15,
16, and 17. These figures of course support the trend of decreasing flux with increasing age as
described in Section 5.5.1, but they reveal other important features:
These spectral features clearly suggest higher temperatures in more active, younger solar
analogs.
• In Figure 15, the EUV spectrum shows prominent lines of highly ionized Fe (Fe xxii, xxiii)
in the young EK Dra but these lines rapidly disappear in older stars, where lines of Fe xv
and Fe xvi predominate. This is a signature of hotter overall temperatures in the younger,
more active solar analogs.
• In Figure 16, the X-ray spectra of the youngest, most active solar analogs (47 Cas, EK Dra),
again reveal strong lines from highly ionized Fe and also a prominent Ne x line (at 12.1 ˚A);
these lines are formed by plasma with temperatures of order 10 MK. In contrast, in the
least active star shown (β Com), lines of Fe xvii, Ne ix, and O vii dominate. These lines are
formed at temperatures of 2 – 5 MK.
Living Reviews in Solar Physics
http://www.livingreviews.org/lrsp-2007-3