The young active star SAO 51891 (V383 Lacertae)^*

¹ INAF – Catania Astrophysical Observatory, via S. Sofia 78, 95123 Catania, Italy
² INAF – Arcetri Astrophysical Observatory, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: kbiazzo@arcetri.inaf.it
³ ESO – European Southern Observatory, Karl-Schwarzschild-Str. 3, 85748 Garching bei München, Germany
⁴ INAF – Capodimonte Astronomical Observatory, via Moiariello 16, 80131 Napoli, Italy
⁵ Ege University, Science Faculty, Astronomy and Space Sciences Department, 35100 Bornova, Izmir, Turkey
⁶ Strasbourg Astronomical Observatory, Université de Strasbourg & CNRS (UMR 7550), 11 rue de l'Université, 67000 Strasbourg, France
⁷ Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA

Received: 7 December 2008
Accepted: 18 March 2009

Abstract

Aims. The aim of this work is to investigate the surface inhomogeneities of a young, late-type star, SAO 51891, at different atmospheric levels, from the photosphere to the upper chromosphere, analyzing contemporaneous optical high-resolution spectra and broad-band photometry.

Methods. The full spectral range of FOCES@CAHA (R 40 000) is used to perform the spectral classification and to determine the rotational and radial velocities. The lithium abundance is measured to obtain an age estimate. The  photometric bands are used to construct the spectral energy distribution (SED). The variations in the observed BV fluxes and effective temperature are used to infer the presence of photospheric spots and observe their behavior over time. The chromospheric activity is studied applying the spectral subtraction technique to Hα, Ca ii H & K, Hϵ, and Ca ii IRT lines.

Results. We find SAO 51891 to be a young K0-1V star with a lithium abundance close to the Pleiades upper envelope, confirming its youth (~100 Myr), which is also inferred from its kinematical membership of the Local Association. No infrared excess is detected from analysis of its SED, limiting the amount of remaining circumstellar dust. We detect a rotational modulation of the luminosity, effective temperature, Ca ii H & K, Hϵ, and Ca ii IRT total fluxes. A simple spot model with two main active regions, about 240 K cooler than the surrounding photosphere, fits the observed light and temperature curves very well. The small-amplitude radial velocity variations are also well reproduced by our spot model. The anti-correlation of light curves and chromospheric diagnostics indicates chromospheric plages spatially associated with the spots. The largest modulation amplitude is observed for the Hϵ flux suggesting that this line is very sensitive to the presence of chromospheric plages.

Conclusions. SAO 51891 is a young active star, lacking significant amounts of circumstellar dust or any evidence of low mass companions, and displays the typical phenomena produced by magnetic activity. The spots are larger and warmer than those in less active main-sequence stars. If some debris is still present around the star, it will only be detectable by future far-infrared and sub-mm observations (e.g., Herschel or ALMA). The RV variation produced by the starspots has an amplitude comparable with those induced by Jupiter-mass planets orbiting close to the host star. SAO 51891 is another good example of an active star in which the detection of planets may be hampered by the high activity level.