Magnetic activity in HD 111456, a young F5–6 main-sequence star

We present spectroscopic and photometric observations of the young F5–6 V field star HD 111456 which exhibits emission cores in the $\ion{Ca}{ii}$ H and K lines and a filling in the H α line. Absorption lines of $\ion{He}{i}$ D 3 and $\ion{Li}{i}$ are clearly detected and we give a new value of $v\sin i=41.5$ km s -1 . Although the limit of magnetic activity detectability at optical wavelengths occurs around F5–6 spectral types, HD 111456 shows unusual chromospheric activity. High UV fluxes in the $\ion{C}{ii}$, $\ion{C}{iv}$, $\ion{Si}{ii}$ and $\ion{Si}{iv}$ lines and a strong X-ray emission confirm that HD 111456 is one of the most active F5–6 V star. The $\ion{Ca}{ii}$ and H α chromospheric emission fluxes do not show any detectable variation on time scales of few days, as well as uvby photometry. A possible explanation in terms of a very high level of magnetic activity and a homogeneous distribution of active regions, both at chromospheric and photospheric level, or a very low inclination of the rotation axis with respect to the line of sight is proposed. From 1998 to 2001 the radial velocity varies indicating a possible binary system, but the spectral distribution of HD 111456 is better reproduced by a single normal F5 V star, and composite spectra simulations ($\ion{Ca}{ii}$, H α ) with a normal inactive F star and a very active low-mass K-M star are not able to reproduce the observed emissions.