The extreme ultraviolet and X-ray Sun in Time: High-energy evolutionary tracks of a solar-like star

Abstract

Aims. We aim to describe the pre-main-sequence and main-sequence evolution of X-ray and extreme-ultaviolet radiation of a solarmass star based on its rotational evolution starting with a realistic range of initial rotation rates. Methods. We derive evolutionary tracks of X-ray radiation based on a rotational evolution model for solar-mass stars and the rotationactivity relation. We compare these tracks to X-ray luminosity distributions of stars in clusters with different ages. Results. We find agreement between the evolutionary tracks derived from rotation and the X-ray luminosity distributions from observations. Depending on the initial rotation rate, a star might remain at the X-ray saturation level for very different time periods, from ≈10 Myr to ≈300 Myr for slow and fast rotators, respectively. Conclusions. Rotational evolution with a spread of initial conditions leads to a particularly wide distribution of possible X-ray luminosities in the age range of 20-500 Myr, before rotational convergence and therefore X-ray luminosity convergence sets in. This age range is crucial for the evolution of young planetary atmospheres and may thus lead to very different planetary evolution histories.

Suppressed Far-UV Stellar Activity and Low Planetary Mass Loss in the WASP-18 System*

“…For all stars other than the Sun, direct measurements of the EUV flux are not possible because of absorption by neutral hydrogen in the ISM. To circumvent this problem, various indirect methods and scaling relations have been developed to estimate the XUV flux on the basis of accessible observables, such as X-ray fluxes (Sanz-Forcada et al 2011;Chadney et al 2015), Lyα fluxes (Linsky et al 2014), and stellar rotational velocities (Wood et al 1994;Johnstone et al 2015;Tu et al 2015).…”

The Evolution of Stellar Rotation and the Hydrogen Atmospheres of Habitable-Zone Terrestrial Planets

“…Tu et al (2015) combined their rotation tracks with an empirical relation between rotation and L X (5-100 Å) derived by Wright et al (2011), and the conversion between L X and L EUV (100-920 Å) derived by Sanz-Forcada et al (2011), to predict evolutionary tracks for L X and L EUV . For the pre-main-sequence phase, they used a time-dependent saturation threshold calculated using the stellar evolution models of Spada et al (2013).…”

“…As a star ages, its activity declines due to spin-down (Güdel et al 1997;Vidotto et al 2014). Due to the fact that a starʼs rotation evolves differently depending on its initial rotation rate, a starʼs activity level is not uniquely determined by its mass and age (Johnstone et al 2015a;Tu et al 2015). Solar mass stars at ages of 1 Myr have a large distribution of rotation rates, ranging from a few to a few tens of times faster than the current solar rotation rate (Herbst et al 2002;Bouvier et al 2014, p. 433;Matt et al 2015).…”

“…At this age, due to their internal structures, all stars lie above the saturation threshold in rotation. Tu et al (2015) found that the age at which a star falls out of saturation varies between ∼10 Myr and several hundred Myr depending on its initial rotation rate.…”

DYNAMICAL ACCRETION OF PRIMORDIAL ATMOSPHERES AROUND PLANETS WITH MASSES BETWEEN 0.1 AND 5 M⊕ IN THE HABITABLE ZONE

“…The values were calculated by Johnstone et al (2015), who combined a static lower atmosphere model with a hydrodynamic upper atmosphere model to derive scaling laws for the mass loss rate as a function of planetary mass, atmospheric mass, and input stellar XUV flux. Using the stellar XUV evolutionary tracks derived by Tu et al (2015), they calculated the amount of atmosphere lost by hydrogen atmospheres with a range of planetary masses and initial atmospheric masses. The numbers given in Table 2 correspond to the assumption that the star started out its life as an average rotator.…”