Search for: All records

Chromospheric Ca II activity cycles are frequently found in late-type stars, but no systematic programs have been created to search for their coronal X-ray counterparts. The typical time scale of Ca II activity cycles ranges from years to decades. Therefore, long-lasting missions are needed to detect the coronal counterparts. The XMM-Newton satellite has so far detected X-ray cycles in five stars. A particularly intriguing question is at what age (and at what activity level) X-ray cycles set in. To this end, in 2015 we started the X-ray monitoring of the young solar-like star ɛ Eridani, previously observed on two occasions: in 2003 and in early 2015, both by XMM-Newton . With an age of 440 Myr, it is one of the youngest solar-like stars with a known chromospheric Ca II cycle. We collected the most recent Mount Wilson S-index data available for ɛ Eridani, starting from 2002, including previously unpublished data. We found that the Ca II cycle lasts 2.92 ± 0.02 yr, in agreement with past results. From the long-term XMM-Newton lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric Ca II cycle. The average X-ray luminosity is 2 × 10 28 erg s −1 , with an amplitude that is only a factor of 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution of ɛ Eridani in terms of solar magnetic structures: active regions, cores of active regions, and flares covering the stellar surface at varying filling fractions. Combinations of these three types of magnetic structures can only describe the observed X-ray emission measure of ɛ Eridani if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona of ɛ Eridani last longer than their solar counterparts. We ascribe this to the lower metallicity of ɛ Eridani. Our analysis also revealed that the X-ray cycle of ɛ Eridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle of ɛ Eridani is thus explained by the high percentage of magnetic structures on its surface. 

Context . The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3 R ⊕ and 3.0 M ⊕ . It is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets. Aims . To prepare for future studies, we aim to thoroughly characterise the planetary system with new accurate and precise data collected with state-of-the-art photometers from space and spectrometers and interferometers from the ground. Methods . We collected light curves of seven new transits observed with the CHEOPS space mission and new radial velocities obtained with MAROON-X at the 8.1 m Gemini North telescope and CARMENES at the 3.5 m Calar Alto telescope, together with previously published spectroscopic and photometric data from the two spectrographs and TESS. We also performed near-infrared interferometric observations with the CHARA Array and new photometric monitoring with a suite of smaller telescopes (AstroLAB, LCOGT, OSN, TJO). This extraordinary and rich data set was the input for our comprehensive analysis. Results . From interferometry, we measure a limb-darkened disc angular size of the star Gl 486 at θ LDD = 0.390 ± 0.018 mas. Together with a corrected Gaia EDR3 parallax, we obtain a stellar radius R * = 0.339 ± 0.015 R ⊕ . We also measure a stellar rotation period at P rot = 49.9 ± 5.5 days, an upper limit to its XUV (5-920 A) flux informed by new Hubble /STIS data, and, for the first time, a variety of element abundances (Fe, Mg, Si, V, Sr, Zr, Rb) and C/O ratio. Moreover, we imposed restrictive constraints on the presence of additional components, either stellar or sub-stellar, in the system. With the input stellar parameters and the radial-velocity and transit data, we determine the radius and mass of the planet Gl 486 b at R p = 1.343 −0.062 +0.063 R ⊕ and M p = 3.00 −0.12 +0.13 M ⊕ , with relative uncertainties of the planet radius and mass of 4.7% and 4.2%, respectively. From the planet parameters and the stellar element abundances, we infer the most probable models of planet internal structure and composition, which are consistent with a relatively small metallic core with respect to the Earth, a deep silicate mantle, and a thin volatile upper layer. With all these ingredients, we outline prospects for Gl 486 b atmospheric studies, especially with forthcoming James Webb Space Telescope ( Webb ) observations.