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The Sun in Time: Activity and Environment 13
Table 2: The “Sun in Time” project: Relevant observations.
Instrument Wavelength range Energy range References a
( ˚A) (keV)
XMM-Newton (CCDs) 0.83 – 83 0.15 – 15 1
XMM-Newton (gratings) 6.0 – 38.0 0.33 – 2.1 1
ASCA (CCDs) 1.2 – 31 0.4 – 10 2
ROSAT (PSPC) 5.2 – 124 0.1 – 2.4 2, 3, 4, 5
EUVE (gratings) 80 – 760 b 0.016 – 0.15 5, 6, 7, 8
FUSE (gratings) 920 – 1180 9
HST (gratings) 1150 – 1730 8
IUE (gratings) 1150 – 1950 10
UBVRI photometry 3500 – 8300 11, 12
VLA (continuum) 3.6 cm/8.4 GHz 2, 3, 13, 14, 15, 16
a References: 1 Telleschi et al. (2005); 2 Güdel et al. (1997b); 3 Güdel et al. (1995c); 4 Dorren et al. (1995); 5
Ayres (1997); 6 Güdel et al. (1997a); 7 Audard et al. (1999); 8 Ribas et al. (2005); 9 Guinan et al. (2003); 10
Dorren and Guinan (1994a); 11 Messina and Guinan (2002); 12 Messina and Guinan (2003); 13 Güdel et al.
(1994); 14 Güdel et al. (1995b); 15 Gaidos et al. (2000); 16 Güdel and Gaidos (2001).
b Wavelengths longer than ≈ 360 ˚A are subject to strong interstellar absorption.
3.2 Overview of stellar sample
The program uses a homogeneous sample of single nearby G0-5 V MS stars (and one G2 IV
subgiant) that all have known rotation periods and well-determined physical properties, including
temperatures, luminosities, metal abundances and ages (Table 3). Observations cover spectral
ranges from radio to X-rays (excluding the 360 – 920 ˚A range, which is a region of very strong
absorption by the interstellar medium). The principal stellar sample is given in Table 3; the
samples used for particular studies may vary somewhat, depending on the availability of appropriate
observational data. The parameters collected in this table are from Dorren and Guinan (1994a),
Güdel et al. (1997b), Güdel et al. (1998b), Güdel and Gaidos (2001), Guinan and Ribas (2002),
Ribas et al. (2005), and Telleschi et al. (2005). Bolometric luminosities have been derived from
absolute visual magnitudes, MV, by applying standard bolometric corrections. I briefly summarize
the magnetic-activity properties of a few important “Young Suns” that will be discussed extensively
in this paper (further notes can be found in Ribas et al. 2005).
EK Draconis = HD 129333 is an exemplary near-ZAMS solar analog, representing the very
active young Sun at the time when planetary atmospheres first formed. EK Dra is a member of
the Pleiades Moving Group at a distance of 34 pc. König et al. (2005) estimated an age of only
35 Myr, and a mass of (0.9 – 1.0)±0.1 M⊙. EK Dra’s optical and UV properties have been studied in
detail by Dorren and Guinan (1994a). With a rotation period of 2.7 d, it reveals extreme magnetic
activity, showing a photospheric spot coverage of 6% (Dorren and Guinan 1994a, larger fractions
were given by König et al. 2005), transition-region line fluxes 20 – 100 times larger than the Sun’s
(Dorren and Guinan, 1994a), a very hot X-ray emitting corona with a luminosity ≈ 1000 times
the present-day solar X-ray output (Güdel et al., 1997b; Telleschi et al., 2005), and an extremely
luminous non-thermal radio corona exceeding radio luminosities of strong solar flares by orders
of magnitude (Güdel et al., 1994, 1995c). EK Dra shows rotational modulation from spots in
the optical, and due to inhomogeneous coronal active regions also in the radio and X-ray regimes
(Güdel et al., 1995c). It further shows strong evidence for a ≈ 10 yr activity cycle (Dorren and
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