AAS 205th Meeting, 9-13 January 2005
Session 11 Brown Dwarfs and Exoplanets
Poster, Monday, January 10, 2005, 9:20am-6:30pm, Exhibit Hall

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[11.04] Probing the Sun's Hidden Magnetic Past: The Two Billion Year-Old Solar Proxy 15 Sge and its Brown Dwarf Companion

M.J. Dulude, E.F. Guinan, L.E. DeWarf, G.P. McCook (Villanova University)

Recently, 15 Sge was discovered to host a dim, cool L6 ± 1.5 brown dwarf companion with a mass of ~58-71 Mj and a distance of ~14-21 AU from the host star (see Liu et al 2002; Bocccaletti et al. 2003). 15 Sge (HR 7276; G1V; V = +5.80 mag; \pi (Hipp) = 56.6 mas) has been on the Villanova Sun-in-Time program for over 15 years. As part of the Sun-in-Time program, this young solar proxy has been observed at X-ray, EUV, FUV-NUV, and Optical wavelengths to study its coronal, chromospheric, and starspot activity. Its rotation period of 13.9 ± 0.4 days is well determined from photometry (light variations of starspots) and from Mt. Wilson Ca II H+K studies. From our Age-Rotation-Activity relations, its age is 1.9 ± 0.3 Gyrs. This value agrees well with an age of 2.0 ± 0.5 Gyr estimated from recent stellar evolution codes. 15 Sge is extremely critical because of its age. It serves as an important proxy for our Sun at a time in the early solar system when primitive life had just established a foothold on Earth and when Mars may have been warm, wet, and suitable for life. From the available data, we computed the XUV spectral irradiances. These can be used to model the radiation effects of the younger Sun's stronger X-ray (~10x present) and FUV (2-3x present) radiation on paleoplanetary atmospheres. The large XUV solar fluxes may have had major effects on the younger planets' ionospheres and resulting mass loss. The stronger FUV fluxes may have been influential in photochemical reactions that could affect life. We will also discuss the effects of star's strong XUV emission on its brown dwarf companion, and the importance of the refined age determination of 15 Sge. Using Age-Rotation-Activity relations pins down the age of its brown dwarf companion, and allows comparisons to current brown dwarf models. This research is supported in part by grants from NASA and NSF for which we are very grateful.

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