X-ray activity of the young solar-like star Kepler-63 and the structure of its corona

Abstract

The X-ray satellite XMM-Newton has so far revealed coronal cycles in seven solar-like stars. Of these, the youngest stars ϵ Eridani (~400 Myr) and ι Horologii (~600 Myr) display the shortest X-ray cycles and the smallest amplitudes, defined as the variation of the X-ray luminosity between the maximum and minimum of the cycle. The X-ray cycle of ϵ Eridani was characterised by applying a novel technique that allowed us to model the corona of a solar-like star in terms of magnetic structures, such as those observed on the Sun (active regions, cores of active regions, and flares), at various filling factors. The high surface coverage of the magnetic structures on ϵ Eridani (65%–95%) that emerged from that study was suggested to be responsible for the low cycle amplitude in the X-ray band. It was also hypothesised that the basal surface coverage with magnetic structures may be higher on the corona of solar-like stars while they are young. To investigate this hypothesis, we started the X-ray monitoring campaign of Kepler-63 in 2019. The star had been observed once before, in 2014, with XMM-Newton. With an age of 210 ± 45 Myr and a photospheric cycle of 1.27 yr, Kepler-63 is the youngest star so far to be observed in X-rays in order to reveal its coronal cycle. Our campaign comprised four X-ray observations of Kepler-63 spanning 10 months (i.e. three-fifths of its photospheric cycle). The long-term X-ray light curve did not reveal a periodic variation of the X-ray luminosity, but a factor two change would be compatible with the considerable uncertainties in the low signal-to-noise data for this relatively distant star. In the case of ϵ Eridani, we describe the coronal emission measure distribution (EMD) of Kepler-63 with magnetic structures such as those observed on the Sun. The best match with the observations is found for an EMD composed of cores and flares of GOES Class C and M following the canonical flare frequency distribution. More energetic flares are occasionally present but they do not contribute significantly to the quasi-stationary high-energy component of the emission measure probed with our modelling. This model yields a coronal filling factor of 100%. This complete coverage of the corona with X-ray-emitting magnetic structures is consistent with the absence of an X-ray cycle, confirming the analogous results derived earlier for ϵ Eridani. Finally, combining our results with the literature on stellar X-ray cycles, we establish an empirical relation between the cycle amplitude LX,max/LX,min and the X-ray surface flux, FX,surf. From the absence of a coronal cycle in Kepler-63, we infer that stars with higher X-ray flux than Kepler-63 must host an EMD that comprises a significant fraction of higher energy flares than those necessary to model the corona of Kepler-63, that is, flares of Class X or higher. Our study opens a new path for studies of the solar-stellar analogy and the joint exploration of resolved and unresolved variability in stellar X-ray light curves.

Evolution of X-Ray Activity in <25 Myr Old Pre-main Sequence Stars

“…Our 7-25 Myr old l-PMS stars (averaged across all masses) have Chandra-based median hot temperature component of ∼40 MK (Table 5) and fit nicely into the ASCAbased hot temperature versus age relation of Güdel et al (1997). And so do the 440 and 210 Myr old solar-mass stars ò Eri and Kepler-63, whose XMM-Newton-based hot plasma temperatures range between 9 and 12 MK (Coffaro et al 2020(Coffaro et al , 2022. Since the heights of X-ray flaring coronal structures on young stars, older active stars, and the Sun positively correlate with plasma temperature (Figure 7 in , it is reasonable to propose that the X-ray coronas of our solar-mass l-PMS stars are more extended than those of older (t > 70 Myr) solar-mass stars, but are not as large as the coronas of solar-mass e-PMS stars.…”

“…Our future work on X-ray super-flares from l-PMS stars will provide more quantitative treatment of this issue. Coffaro et al (2020Coffaro et al ( , 2022) also find that the X-ray cycle amplitudes of ò Eri and Kepler-63 are the smallest (compared to several older solar-mass stars with known X-ray cycles) and their surfaces may be profusely (60%-100%) covered by solartype X-ray emitting magnetic structures (e.g., active region cores and flares). Careful analyses of mass-stratified X-ray spectral stacking, mass-stratified super-flare energies and frequencies, and rotation rates for l-PMS stars need to be performed to examine whether the solar-mass l-PMS stars exhibit similar X-ray surface saturation.…”

X-Ray, Near-ultraviolet, and Optical Flares Produced by Colliding Magnetospheres in the Young High-eccentricity Binary DQ Tau

“…The absence of evidence for long-term variability in the X-ray characteristic emission of the 1-million-year-old DQ Tau aligns with the notion that younger stars possess larger active regions and more extended X-ray coronal structures (Getman et al 2022b;Coffaro et al 2022;Getman et al 2023), which may mitigate the appearance of magnetic dynamo cycling.…”

“…For example, Wargelin et al (2017) conducted X-ray analyses on several mature stars and observed a decrease in the amplitude of quiescent variability as X-ray activity increased. Coffaro et al (2020Coffaro et al ( , 2022 discovered that ò Eri, a star approximately 440 million years old, and Kepler-63, a star approximately 210 million years old, exhibited the shortest X-ray cycles and smallest X-ray amplitudes when compared to several older solar-mass stars known to have X-ray cycles. Additionally, their findings suggested that the surfaces of these stars may be extensively (around 60%-100%) covered by solar-type X-ray-emitting magnetic structures, such as active region cores and flares.…”