Goals of the “Sun in Time” project

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., 1993

). 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., 2002

, 2005

; Kulikov et al., 2006

). 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 1994b

).

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:

the optical (photospheric) and UV “Sun in Time”, the latter studied with IUE (Messina and Guinan, 2002 , 2003 ; Dorren and Guinan, 1994a ), to characterize level and extent of surface magnetic activity;
the FUV “Sun in Time” as studied with FUSE (Guinan et al., 2003 ), to infer transition-region magnetic activity;
the EUV “Sun in Time” as studied with EUVE (Ayres, 1997 ; Güdel et al., 1997a ; Audard et al., 1999 ; Ribas et al., 2005 ), to obtain information on coronal activity;
the X-ray “Sun in Time” as studied by ROSAT and ASCA (Güdel et al., 1997b , 1995c ; Dorren et al., 1995 ), to obtain full coverage in coronal temperatures and characterize flares;
the X-ray “Sun in Time” as studied spectroscopically with XMM-Newton (Telleschi et al., 2005 ), to study the thermal stratification and composition of coronae of solar analogs;
spectral evolution of the XUV “Sun in Time” (Ribas et al., 2005 ), to find systematics across the UV-to-X-ray range;
the radio “Sun in Time” as studied with the VLA (Güdel et al., 1994 , 1995b ,c , 1997b ; Gaidos et al., 2000 ; Güdel and Gaidos, 2001 ), to measure the production of non-thermal particles in magnetic coronae;
magnetic cycles of the “Sun in Time” (Messina and Guinan, 2002 ), measured in the optical or in X-rays;
differential rotation of the “Sun in Time” (Messina and Guinan, 2003 ) as seen in starspot measurements.

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 (1994b ), Güdel et al. (1998b ), Guinan and Ribas (2002 ), Guinan et al. (2002 ), and Güdel (2003 ).

Table 2:

The “Sun in Time” project: Relevant observations.






Instrument	Wavelength range	Energy range	References^a
	( 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)	80–760^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 Å are subject to strong interstellar absorption.