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
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.