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COCOA-PUFS is an energy-diverse, time-domain study of the ultra-fast spinning, heavily spotted, yellow giant FK Com (HD117555; G4 III). This single star is thought to be a recent binary merger, and is exceptionally active by measure of its intense ultraviolet and X-ray emissions, and proclivity to flare. COCOA-PUFS was carried out with Hubble Space Telescope in the UV (120-300 nm), using mainly its high-performance Cosmic Origins Spectrograph, but also high-precision Space Telescope Imaging Spectrograph; Chandra X-ray Observatory in the soft X-rays (0.5-10 keV), utilizing its High-Energy Transmission Grating Spectrometer; together with supporting photometry and spectropolarimetry in the visible from the ground. This is an introductory report on the project. FK Com displayed variability on a wide range of time scales, over all wavelengths, during the week-long main campaign, including a large X-ray flare; super-rotational broadening of the far-ultraviolet hot-lines (e.g., Si IV 139 nm (T~80,000 K) together with chromospheric Mg II 280 nm and C II 133 nm (10,000-30,000 K); large Doppler swings suggestive of bright regions alternately on advancing and retreating limbs of the star; and substantial redshifts of the epoch-average emission profiles. These behaviors paint a picture of a highly extended, dynamic, hot (10 MK) coronal magnetosphere around the star, threaded by cooler structures perhaps analogous to solar prominences, and replenished continually by surface activity and flares. Suppression of angular momentum loss by the confining magnetosphere could temporarily postpone the inevitable stellar spindown, thereby lengthening this highly volatile stage of coronal evolution.
FK Comae is a rapidly rotating magnetically active star, the light curve of which is modulated by cool spots on its surface. It was the first star where the flip-flop phenomenon was discovered. Since then, flip-flops in the spot activity have been re
Recently, we presented a general model for the light curves of chromospherically active stars, where the observed light curve is interference of two real constant period light curves of long-lived starspots. In this first paper, we make six specific
For seven decades, the widely held view has been that the formation, the migration and the decay of short-lived starspots explain the constantly changing light curves of chromospherically active stars. Our hypothesis is that these deceptive observed
We study the connection between the chromospheric and photospheric behaviour of the active late-type star FK Comae. We use spot temperature modelling, light curve inversion based on narrow- and wide-band photometric measurements, Halpha observations
We present a three-dimensional simulation of the corona of an FK Com-type rapidly rotating G giant using a magnetohydrodynamic model that was originally developed for the solar corona in order to capture the more realistic, non-potential coronal stru