Starspots on the fastest rotators in the ѕ’ Pic moving group

D. García-Alvarez, A.F. Lanza, S. Messina, J.J. Drake, F. van Wyk, R.R. Shobbrook, C.J. Butler, D. Kilkenny, J.G. Doyle, and V.L. Kashyap

Fig. 5. Upper panel: The first light curve of HD 199143 (filled dots) with the corresponding synthetic light curve from the ME best fit spot distribution (solid line). The flux is given in relative units, assuming as the reference value (1.0) the brightest observation of the star (see Sect. 2). Middle panel: the residuals between the observations and the model in relative units. Lower panel: The ME spot map of the star in orthographic projection, showing the disc of the star as it would be seen by an observer on the Earth at the labelled rotation phases (see the text).

Aims. We carried out high-resolution spectroscopy and BV(I)C photometric monitoring of the two fastest late-type rotators in the nearby β Pictoris moving group, HD 199143 (F7V) and CDтИТ64°1208 (K7V). The motivation for this work is to investigate the rotation periods and photospheric spot patterns of these very young stars, with a longer term view to probing the evolution of rotation and magnetic activity during the early phases of main-sequence evolution. We also aim to derive information on key physical parameters, such as rotational velocity and rotation period.

Methods. We applied maximum entropy (ME) and Tikhonov regularizing (TR) criteria to derive the surface spot map distributions of the optical modulation observed in HD 199143 (F7 V) and CDтИТ64°1208 (K7 V). We also used cross-correlation techniques to determine stellar parameters such as radial velocities and rotational velocities. Lomb-Scargle periodograms were used to obtain the rotational periods from differential magnitude time series.

Results. We find periods and inclinations of 0.356 days and 21.5 deg for HD 199143, and 0.355 days and 50.1 deg for CDтИТ64°1208. The spot maps of HD 199143 obtained from the ME and TR methods are very similar, although the latter gives a smoother distribution of the filling factor. Maps obtained at two different epochs three weeks apart show a remarkable increase in spot coverage amounting to тИ® 7% of the surface of the photosphere over a time period of only тИ® 20 days. The spot maps of CDтИТ64°1208 from the two methods show good longitudinal agreement, whereas the latitude range of the spots is extended to cover the whole visible hemisphere in the TR map. The distributions obtained from the first light curve of HD 199143 show the presence of an extended and asymmetric active longitude with the maximum filling factor at longitude тИ® 325°. A secondary active longitude is present at тИ® 100°. The spotted area distributions on CDтИТ64°1208 show two active longitudes separated by about 180°, which is not unusual on such very active stars.

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Last Revised: 2011 August 5th