EVOLUTION OF THE SOLAR ACTIVITY OVER TIME AND EFFECTS ON PLANETARY ATMOSPHERES. II. κ¹Ceti, AN ANALOG OF THE SUN WHEN LIFE AROSE ON EARTH

Abstract

The early evolution of Earth's atmosphere and the origin of life took place at a time when physical conditions at the Earth were radically different from its present state. The radiative input from the Sun was much enhanced in the high-energy spectral domain, and in order to model early planetary atmospheres in detail, a knowledge of the solar radiative input is needed. We present an investigation of the atmospheric parameters, state of evolution, and high-energy fluxes of the nearby star κ 1 Cet, previously thought to have properties resembling those of the early Sun. Atmospheric parameters were derived from the excitation/ionization equilibrium of Fe i and Fe ii, profile fitting of Hα, and the spectral energy distribution. The UV irradiance was derived from Far-Ultraviolet Spectroscopic Explorer and Hubble Space Telescope data, and the absolute chromospheric flux from the Hα line core. From careful spectral analysis and the comparison of different methods, we propose for κ 1 Cet the following atmospheric parameters: T eff = 5665 ± 30 K (Hα profile and energy distribution), log g = 4.49 ± 0.05 dex (evolutionary and spectroscopic), and [Fe/H] = +0.10 ± 0.05 (Fe ii lines). The UV radiative properties of κ 1 Cet indicate that its flux is some 35% lower than the current Sun's between 210 and 300 nm, it matches the Sun's at 170 nm, and increases to at least 2-7 times higher than the Sun's between 110 and 140 nm. The use of several indicators ascribes an age to κ 1 Cet in the interval ∼0.4-0.8 Gyr and the analysis of the theoretical Hertzsprung-Russell diagram (H-R) suggests a mass ∼1.04 M . This star is thus a very close analog of the Sun when life arose on Earth and Mars is thought to have lost its surface bodies of liquid water. Photochemical models indicate that the enhanced UV emission leads to a significant increase in photodissociation rates compared with those commonly assumed of the early Earth. Our results show that reliable calculations of the chemical composition of early planetary atmospheres need to account for the stronger solar photodissociating UV irradiation.

Shallow extra mixing in solar twins inferred from Be abundances

“…For the two youngest stars, we also used other age indicators, as described in Monroe et al (2015). In short, we adopted a rotational age (0.5 ± 0.2 Gyr) for HD 20630, which excellently agrees with Ribas et al (2010), who obtained 0.6 ± 0.2 Gyr using different indicators. For HD 202826 a rotation period is not available to estimate its age, hence we used its chromospheric activity, X-ray luminosity, and isochrones.…”

High-resolution spectropolarimetry of κ Cet: A proxy for the young Sun

“…From a careful spectral analysis by Ribas et al (2010) and the comparison of different methods these authors give for κ 1 Cet the following atmospheric parameters: Teff = 5665 ± 30 K (Hα profile and energy distribution), log g = 4.49 ± 0.05 dex (evolutionary model and spectroscopy) and [Fe/H] = +0.10 0.05 (Fe II lines). In the left panel of Fig.…”

Accurate and homogeneous abundance patterns in solar-type stars of the solar neighbourhood: a chemo-chronological analysis

“…The UV radiation excess from the chromosphere of an active star can scape to the photosphere and cause departures from LTE due to an ionisation imbalance. The induced overionisation is commonly manifested by differences either in excitation and photometric effective temperatures, or in ionisation and evolutionary surface gravities (see Porto de Mello et al 2008;Ribas et al 2010 agreement with each other. Therefore, only the difference in surface gravity and the large value of microturbulence velocity are possible signs that an overionisation is taking place in the photosphere of this active star.…”