Magnetic field, differential rotation and activity of the hot-Jupiter-hosting star HD 179949

HD 179949 is an F8V star, orbited by a giant planet at similar to 8 R? every 3.092 514 d. The system was reported to undergo episodes of stellar activity enhancement modulated by the orbital period, interpreted as caused by starplanet interactions (SPIs). One possible cause of SPIs is the large-scale magnetic field of the host star in which the close-in giant planet orbits. In this paper we present spectropolarimetric observations of HD 179949 during two observing campaigns (2009 September and 2007 June). We detect a weak large-scale magnetic field of a few gauss at the surface of the star. The field configuration is mainly poloidal at both observing epochs. The star is found to rotate differentially, with a surface rotation shear of dO= 0.216 +/- 0.061 rad d-1, corresponding to equatorial and polar rotation periods of 7.62 +/- 0.07 and 10.3 +/- 0.8 d, respectively. The coronal field estimated by extrapolating the surface maps resembles a dipole tilted at similar to 70 degrees. We also find that the chromospheric activity of HD 179949 is mainly modulated by the rotation of the star, with two clear maxima per rotation period as expected from a highly tilted magnetosphere. In 2009 September, we find that the activity of HD 179949 shows hints of low-amplitude fluctuations with a period close to the beat period of the system.