Overview of stellar sample

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 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) $M ⊙$ , and Metchev and Hillenbrand (2004 ) discussed evidence for another 0.2 $M ⊙$ 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

$π1$ 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., 2005

Table 3:

The “Sun in Time” sample^a.






Star	HD	Dist.	Spectr.	T_eff	Mass	Radius	M_V	L_bol	log L_X	log (L_X	log L_R^d	P	Age	Age indicator,
	no.	(pc)^b	Type	(K)	$(M ⊙ )$	$(R ⊙)$	(m)	( $L⊙$ )	(erg/s)^c	/L_bol)	(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
$1 π$ 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 Relationship^e
$χ1$ 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
$1 κ$ 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 Relationship^f
$β$ 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	1.10^h	1.22^h	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