## Surface differential rotation and prominences of the Lupus post T Tauri star RX J1508.6-4423

### J.-F. Donati, M. Mengel, B.D. Carter, A.C. Cameron, R. Wichmann

#### MNRAS 316, 699 (2000) 943 kbyte .ps.gz file or 790 kbyte PDF file

We present in this paper a spectroscopic monitoring of the Lupus post T Tauri star RX J1508.6-4423 carried out at two closely separated epochs (1998 May 06 & 10) with the UCL Echelle Spectrograph on the 3.9 m Anglo-Australian Telescope. Applying Least-Squares Deconvolution and maximum entropy image reconstruction techniques to our sets of spectra, we demonstrate that this star features on its surface a large cool polar cap with several appendages extending to lower latitudes, as well as one spot close to the equator. The images reconstructed at both epochs are in good overall agreement, except for a photospheric shear that we interpret in terms of latitudinal differential rotation. Given the spot distribution at the epoch of our observations, differential rotation could only be investigated between latitude 15 and 60 degrees. We find in particular that the observed differential rotation is compatible with a solar-like law (i.e. with rotation rate decreasing towards high latitudes proportionally to sin^2l where l denotes the latitude) in this particular latitude range. Assuming that such a law can be extrapolated to all latitudes, we obtain that the equator of RX J1508.6--4423 does one more rotational cycle than the pole every 50+-10 d, implying a photospheric shear 2 to 3 times stronger than that of the Sun.

We also discover that the H$\alpha$ emission profile of RX J1508.6-4423 is most of the time double peaked and strongly modulated with the rotation period of the star. We interpret this rotationally modulated emission to be due to a dense and complex prominence system whose circumstellar distribution is obtained through maximum entropy Doppler tomography. These maps show in particular that prominences form a complete and inhomogeneous ring around the star, just outside the corotation radius. We use the total Halpha and Hbeta emission flux to estimate that the mass of the whole prominence system is about 10^20 g. From our observation that the whole cloud system surrounding the star is regenerated in less than 4 d, we conclude that the braking timescale of RX J1508.6-4423 is shorter than 1 Gyr and that prominence expulsion is thus likely to contribute significantly to the rotational spindown of young low-mass stars.