Evidence of a massive planet candidate orbiting the young active K5V star BD+20 1790*,**

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Context. BD+20 1790 is a young active, metal-rich, late-type K5Ve star. We have undertaken a study of stellar activity and kinematics for this star over the past few years. Previous results show a high level of stellar activity, with the presence of prominence-like structures, spots on the surface, and strong flare events, despite the moderate rotational velocity of the star. In addition, radial velocity variations with a semi-amplitude of up to 1 km s(-1) were detected. Aims. We investigate the nature of these radial velocity variations, in order to determine whether they are due to stellar activity or the reflex motion of the star induced by a companion. Methods. We have analysed high-resolution echelle spectra by measuring stellar activity indicators and computing radial velocity (RV) and bisector velocity spans. Two-band photometry was also obtained to produce the light curve and determine the photometric period. Results. Based upon the analysis of the bisector velocity span, as well as spectroscopic indices of chromospheric indicators, Ca II H & K, H alpha, and taking the photometric analysis into account, we report that the best explanation for the RV variation is the presence of a substellar companion. The Keplerian fit of the RV data yields a solution for a close-in massive planet with an orbital period of 7.78 days. The presence of the close-in massive planet could also be an interpretation for the high level of stellar activity detected. Since the RV data are not part of a planet search programme, we can consider our results as a serendipitous evidence of a planetary companion. To date, this is the youngest main sequence star for which a planetary candidate has been reported.
© ESO 2010. We thank Calar Alto Observatory for allocation of director’s discretionary time to this programme. This work was supported by the Spanish Ministerio de Educación y Ciencia (MEC) under grant AYA2005-02750, Ministerio de Ciencia e Innovación (MICINN) under grant AYA2008-06423-C03-03, and the Comunidad de Madrid under PRICIT project S-0505/ESP-0237 (ASTROCAM). M.C.G.O. acknowledges financial support from the European Commission in the form of a Marie Curie Intra European Fellowship (PIEF-GA- 2008-220679). M.H.O. and G.A.E. thank Dr. Chriss Moss, support astronomer at the LT for his help and patience. Also M.H.O. thanks Dr. Santos Pedraz, support astronomer at the Calar Alto Observatory for his help with DDT run. M.H.O. is grateful to Dr. José Antonio Caballero for valuable discussions, and also Dr. Laurence R. Doyle for his suggestions that was the initial inspiration for this work. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. The authors gratefully acknowledge the valuable comments and suggestions of an anonymous referee that helped to improve the paper.
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