3.1 Goals of the “Sun in Time” project

Solar analogs with different ages and therefore activity levels play an important role in our understanding of the past magnetic evolution of the Sun. For stars with masses < 1.5 M ⊙ ∼ and ages of at least a few 100 Myr, angular momentum loss by a stellar wind brakes rotation in such a way that it is nearly uniquely determined by the stellar age (Soderblom et al., 1993Jump To The Next Citation Point). The only independent variable, the stellar age, then determines the rotation period and, through the internal magnetic dynamo, magnetic activity at all levels of the stellar atmosphere, probably including characteristics of the stellar wind (Wood et al., 2002Jump To The Next Citation Point2005Jump To The Next Citation PointKulikov et al., 2006Jump To The Next Citation Point). The study of a series of near-solar-mass stars back to ages close to the ZAMS age will therefore be sufficient to reconstruct the history of our Sun and the interaction of its magnetic activity with its environment. This is the goal of the observational “Sun in Time” program (see, e.g., Dorren and Guinan 1994bJump To The Next Citation Point).

Specifically, the “Sun in Time” program was established to study the long-term magnetic evolution of the Sun during the entire MS lifetime. The primary aims of the program are, (1) to test dynamo models of the Sun in which rotation is the only significant variable parameter, and (2) to determine the spectral irradiance of the Sun over its MS lifetime. Key studies within the “Sun in Time” program comprise the following:

A summary of the instruments used for the observations and their wavelength (or energy) ranges covered is given in Table 2. Various project summaries can be found in Dorren and Guinan (1994bJump To The Next Citation Point), Güdel et al. (1998bJump To The Next Citation Point), Guinan and Ribas (2002Jump To The Next Citation Point), Guinan et al. (2002), and Güdel (2003).


Table 2: The “Sun in Time” project: Relevant observations.
Instrument Wavelength range Energy range Referencesa
  (Å) (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 – 760b 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. (2005Jump To The Next Citation Point); 2 Güdel et al. (1997bJump To The Next Citation Point); 3 Güdel et al. (1995cJump To The Next Citation Point); 4 Dorren et al. (1995Jump To The Next Citation Point); 5 Ayres (1997Jump To The Next Citation Point); 6 Güdel et al. (1997a); 7 Audard et al. (1999Jump To The Next Citation Point); 8 Ribas et al. (2005Jump To The Next Citation Point); 9 Guinan et al. (2003Jump To The Next Citation Point); 10 Dorren and Guinan (1994aJump To The Next Citation Point); 11 Messina and Guinan (2002Jump To The Next Citation Point); 12 Messina and Guinan (2003Jump To The Next Citation Point); 13 Güdel et al. (1994Jump To The Next Citation Point); 14 Güdel et al. (1995bJump To The Next Citation Point); 15 Gaidos et al. (2000Jump To The Next Citation Point); 16 Güdel and Gaidos (2001Jump To The Next Citation Point).

b Wavelengths longer than ≈ 360 Å are subject to strong interstellar absorption.



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