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 Å 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 (1994aJump To The Next Citation Point), Güdel et al. (1997bJump To The Next Citation Point), Güdel et al. (1998bJump To The Next Citation Point), Güdel and Gaidos (2001Jump To The Next Citation Point), Guinan and Ribas (2002Jump To The Next Citation Point), Ribas et al. (2005Jump To The Next Citation Point), and Telleschi et al. (2005Jump To The Next Citation Point). 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. 2005Jump To The Next Citation Point).

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. (2005Jump To The Next Citation Point) 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 (1994aJump To The Next Citation Point). With a rotation period of 2.7 d, it reveals extreme magnetic activity, showing a photospheric spot coverage of 6% (Dorren and Guinan 1994aJump To The Next Citation Point, larger fractions were given by König et al. 2005Jump To The Next Citation Point), transition-region line fluxes 20 – 100 times larger than the Sun’s (Dorren and Guinan, 1994aJump To The Next Citation Point), a very hot X-ray emitting corona with a luminosity ≈ 1000 times the present-day solar X-ray output (Güdel et al., 1997bJump To The Next Citation PointTelleschi et al., 2005Jump To The Next Citation Point), and an extremely luminous non-thermal radio corona exceeding radio luminosities of strong solar flares by orders of magnitude (Güdel et al., 1994Jump To The Next Citation Point1995cJump To The Next Citation Point). 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., 1995cJump To The Next Citation Point). It further shows strong evidence for a ≈ 10 yr activity cycle (Dorren and Guinan, 1994aJump To The Next Citation PointDorren et al., 1995Jump To The Next Citation PointGüdel, 2004Jump To The Next Citation PointJärvinen et al., 2005Jump To The Next Citation Point). Duquennoy and Mayor (1991) reported a low-mass (M-type) companion star, whose mass was found to be (0.5 ± 0.1) M ⊙, and Metchev and Hillenbrand (2004) discussed evidence for another 0.2 M ⊙ companion although the latter has been questioned by König et al. (2005Jump To The Next Citation Point). 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., 1995aJump To The Next Citation Point1998aJump To The Next Citation Point). 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., 1995aJump To The Next Citation Point1998aTelleschi et al., 2005Jump To The Next Citation Point).

π1 UMa = HD 72905 and χ1 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 π1 UMa is considered to be single, χ1 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 M ⊙ (König et al., 2002), suggesting that most observational signatures of magnetic activity are dominated by the primary.

κ1 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., 2005Jump To The Next Citation Point).


Table 3: The “Sun in Time” samplea.
Star HD Dist. Spectr. Teff Mass Radius MV Lbol log LX log (LX log LRd P Age Age indicator,
  no. (pc)b Type (K) (M ⊙) (R⊙ ) (m) (L ⊙) (erg/s)c /Lbol) (erg/Hz/s) (d) (Gyr) Membership
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
π1 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
χ1 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
κ1 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
β 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
α 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
β Hyi 2151 7.5 G2 IV 5774 1.10 1.92 3.43 3.70 27.18 –6.41 ... ∼28 6.7 Isochronesi
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 (1994aJump To The Next Citation Point), Güdel et al. (1997bJump To The Next Citation Point), Güdel et al. (1998b), Güdel and Gaidos (2001Jump To The Next Citation Point), Guinan and Ribas (2002Jump To The Next Citation Point), Ribas et al. (2005Jump To The Next Citation Point), and Telleschi et al. (2005Jump To The Next Citation Point).

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.

d For radio observations of further solar analogs, see Güdel et al. (1994Jump To The Next Citation Point) and Güdel and Gaidos (2001Jump To The Next Citation Point).

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 R⊙.



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