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 . 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 Guinan, 1994a; Dorren et al., 1995; Güdel, 2004; Järvinen et al., 2005). Duquennoy and Mayor (1991) reported a low-mass (M-type) companion star, whose mass was found to be (0.5 ± 0.1) , and Metchev and Hillenbrand (2004) discussed evidence for another 0.2 companion although the latter has been questioned by König et al. (2005). Note that UV, EUV, X-ray, and radio emissions are likely to be dominated by the G star, given the low mass(es) of the companion(s).
47 Cas B = HR 581 = HD 12230 is a somewhat mysterious solar analog in a binary system at a distance of 33.5 pc. It has not been revealed in the optical regime given the primary F0 V star’s overwhelming light. But astrometric, X-ray and radio properties constrain it to be an early-to-mid G-type star, and its likely membership in the Pleiades Moving Group suggests a near-ZAMS age (Güdel et al., 1995a, 1998a). It shows X-rays and radio emission at an even more extreme level than EK Dra, perhaps induced by the even faster rotation; the rotation period has been inferred from periodic X-ray modulation to be about 1 d (Güdel et al., 1995a, 1998a; Telleschi et al., 2005).
UMa = HD 72905 and Ori = HD 39587: These are two members of the Ursa Major Stream (King et al., 2003), with an estimated age of 300 Myr (Soderblom and Mayor, 1993). While UMa is considered to be single, Ori is orbited by an M-type companion with an orbital period of about 14 yr (Han and Gatewood, 2002) and a mass of 0.15 (König et al., 2002), suggesting that most observational signatures of magnetic activity are dominated by the primary.
Cet = HD 20630 is a Hyades-age solar analog. Its age is not well known but is indirectly inferred from its rotation period and its X-ray luminosity (Ribas et al., 2005).
|Star||HD||Dist.||Spectr.||Teff||Mass||Radius||MV||Lbol||log LX||log (LX||log LRd||P||Age||Age indicator,|
|47 Cas B||12230||33.5||G V||...||...||...||...||...||30.31||...||14.91||1.0?||0.1||Pleiades Moving Group|
|EK Dra||129333||33.9||G0 V||5870||1.06||0.91||4.96||0.90||29.93||–3.61||14.18||2.75||0.1||Pleiades Moving Group|
|UMa||72905||14.3||G1 V||5850||1.03||0.96||4.87||0.97||29.10||–4.47||12.67||4.68||0.3||Ursa Major Stream|
|HN Peg||206860||18.4||G0 V||5970||1.06||0.99||4.68||1.14||29.12||–4.52||...||4.86||0.3||Rotation-Age Relationshipe|
|Ori||39587||8.7||G1 V||5890||1.01||1.02||4.71||1.13||28.99||–4.65||...||5.08||0.3||Ursa Major Stream|
|BE Cet||1835||20.4||G2 V||5740||0.99||1.02||4.83||1.02||29.13||–4.46||...||7.65||0.6||Hyades Moving Group|
|Cet||20630||9.2||G5 V||5750||1.02||0.93||5.02||0.86||28.79||–4.73||12.42||9.2||0.75||Rotation-Age Relationshipf|
|Com||114710||9.2||G0 V||6000||1.10||1.10||4.45||1.41||28.21||–5.52||12.53||12.4||1.6||Rotation-Age Relationship|
|15 Sge||190406||17.7||G5 V||5850||1.01||1.10||4.56||1.29||28.06||–5.64||...||13.5||1.9||Rotation-Age Relationship|
|Sun||–||1 AU||G2 V||5777||1.00||1.00||4.83||1.00||27.30||–6.29||...||25.4||4.6||Isotopic Dating on Earth|
|18 Sco||146233||14.0||G2 V||5785||1.01||1.03||4.77||1.08||...||...||...||23||4.9||Isochrones|
|Cen A||128620||1.4||G2 V||5800 g||1.10h||1.22h||4.34||1.60||27.12||–6.67||...||30||5 – 6||Isochrones, Rotation|
|16 Cyg A||186408||21.6||G1.5 V||5790||1.00||1.16||4.29||1.38||...||...||...||35||8.5||Isochrones|
a Parameters mostly collected 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).
b Stellar distances are from the Hipparcos Catalogue (Perryman et al., 1997)
c LX refers to the 0.1 – 2.4 keV band as measured by ROSAT.
e Same rotation period as Ursa Major stream G0V members.
f Possible member of the Hyades Moving Group.
g From Chmielewski et al. (1992).
h From Kervella et al. (2003) based on interferometric observations.
i Isochrone age from Dravins et al. (1998); LX normalized to 1 .
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