Coronal Evolution of the Sun in Time: High‐Resolution X‐Ray Spectroscopy of Solar Analogs with Different Ages

Abstract

We investigate the long-term evolution of X-ray coronae of solar analogs based on high-resolution X-ray spectroscopy and photometry with XMM-Newton. Six nearby main-sequence G stars with ages between ≈ 0.1 Gyr and ≈ 1.6 Gyr and rotation periods between ≈ 1 d and 12.4 d have been observed. We use the X-ray spectra to derive coronal element abundances of C, N, O, Ne, Mg, Si, S, and Fe and the coronal emission measure distribution (EMD). We find that the abundances change from an inverse-First Ionization Potential (FIP) distribution in stars with ages around 0.1 Gyr to a solar-type FIP distribution in stars at ages of 0.3 Gyr and beyond. This transformation is coincident with a steep decline of non-thermal radio emission. The results are in qualitative agreement with a simple model in which the stream of electrons in magnetic fields suppresses diffusion of low-FIP ions from the chromosphere into the corona. The coronal emission measure distributions show shapes characterized by power-laws on each side of the EMD peak. The latter shifts from temperatures of about 10 MK in the most rapidly rotating, young stars to temperatures around 4 MK in the oldest target considered here. The power-law index on the cooler side of the EMD exceeds expected slopes for static loops, with typical values being 1.5-3. We interpret this slope with a model in which the coronal emission is due to a superposition of stochastically occurring flares, with an occurrence rate that is distributed in radiated energy E as a power-law, dN/dE ∝ E −α , as previously found for solar and stellar flares. We obtain the relevant power-law index α from the slope of the high-temperature tail of the EMD. Our EMDs indicate α ≈ 2.2 − 2.8, in excellent agreement with values previously derived from light curves of magnetically active stars. Modulation with time scales reminiscent of flares is found in the light curves of all our targets. Several strong flares are also observed. We use our α values to simulate light curves and compare them with the observed light curves. We thus derive the range of flare energies required to explain the light-curve modulation. More active stars require a larger range of flare energies than less active stars within the framework of this simplistic model. In an overall scenario, we propose that flaring activity plays a larger role in more active stars. In this model, the higher flare rate is responsible both for the higher average coronal temperature and the high coronal X-ray luminosity, two parameters that are indeed found to be correlated.

Photospheric and coronal abundances in solar-type stars: the peculiar case ofτ Bootis

“…The coronal abundances of the same two stars were derived by Sanz-Forcada et al (2004) and by Telleschi et al (2005) 4 respectively, with an analysis of XMM-Newton data similar to ours. We then computed the coronal/photospheric abundance ratios of all the solar-type dwarfs in our sample, excluding HD 283572 because of its peculiar evolutionary phase and extreme activity level.…”

“…7 shows a comparison of its EMD distribution vs. temperature (Method 1) with those of other four G-K stars: the Sun (G2V, Peres et al 2000), κ 1 Ceti (G5V, Telleschi et al 2005), Eri (K2V, Sanz-Forcada et al 2004), andHD 283572 (G2IV, Scelsi et al 2005).…”

Magnetic activity, high-energy radiation and variability: from young solar analogs to low-mass objects

“…Studies have used X-rays and extreme ultraviolet radiation to find α mostly in the range of 2-2.5 for G-M dwarfs (Audard et al 2000;Kashyap et al 2002;Güdel et al 2003) and for T Tauri stars (Stelzer et al 2007), supporting the view that moderate flares are the dominant heating source for these active coronae. This observation may indeed be the basis for the correlation between radio and soft X-ray emission in magnetically active stars and flares, the high level of apparently constant emission being the result of a large number of comparatively small flares not individually detected in the light curves (Telleschi et al 2005).…”

“…The relations are well represented by power laws, as illustrated in Figure 4. From band-integrated luminosities and individual lines (see Telleschi et al 2005 for X-ray lines) the following decay laws are found for chromospheric, transition region UV, transition region FUV, EUV (including the softest X-rays at 20-100Å), and soft X-rays (1-20Å), where we also use the rotation law in Eq. (2.2): …”

Long-term stellar variability

“…Finally, the active regions and flares in the upper chromosphere, transition regions and coronae as well as solar-like winds have been studied thanks to the UV, and EUV data from, e.g., IUE, HST, GALEX, EUVE and FUSE (e.g., Jordan et al 1987;Ayres 1995, Pagano et al 2004, Ribas et al 2005, Wood et al 2005, X-ray data from, e.g., Einstein, Rosat, ASCA, Beppo-SAX, Chandra, and XMM (e.g., Favata & Micela 2003 and reference therein, Telleschi et al 2005, Stelzer 2007), and radio observations from VLA, VLBI, VLBA, and single-dishes (e.g., Güdel & Benz 1993, Benz & Güdel 1994, Umana et al 1995, Trigilio et al 1998.…”