The Night Time Sun: X-Ray Observations of the Solar Twin 18 Scorpii

Since the study by Porto de Mello & da Silva(1997,ApJ,482) the nearby 5.5-mag G2V star 18 Scorpii has been considered one of the best solar twins, being a near-perfect match to our Sun in all physical characteristics(T_eff, R, log(g), M_V, metallicity, luminosity, and chromospheric CaII H&K emissions). Also, 18 Sco has a rotation period of P_Rot=22.7±0.5 days(Petit et al. 2008), which is very close to that of the Sun. In addition, ongoing CaII H&K observations carried out at Lowell Observatory indicate a possible 7-11yr. activity cycle. However, until our X-ray observations with XMM-Newton in Aug 2005, 18 Sco had never been observed in this spectral region. The analysis of these measurements yield an X-ray luminosity and coronal plasma temperature of L_X=8±1.5ergs/s and T_Corona 1.5-2 MK. These data were taken near mid-cycle and match very closely with those of the Sun(L_X 6-30ergs/s T_Corona 2 MK). In addition to these measures, interferometric angular diameter measures with the Palomar Testbed Interferometer yield a stellar diameter of 0.975±0.162 R/R_Sun. Fitting the star's observed properties to current evolution models indicates a mass of M=0.98±0.05 M_Sun and an age of 4-5 Gyr. This age estimate (4.4±0.4 Gyr) is in excellent agreement with an age inferred from age-rotation-activity relations from the Sun in Time project. These observations demonstrate that 18 Sco is a bonafide solar twin out through the X-ray portion of the spectrum. Such a distinction makes is a very important star to use as a proxy "Night Time” Sun for standardization purposes, an excellent candidate for asteroseismic studies, and as a target for searches for life-bearing terrestrial planets.

This research is supported by grants from NASA/FUSE, NSF/RUI and by the Villanova University Research for Undergraduates Award Program, which we gratefully acknowledge.