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

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 and 10) with the UCL Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Applying least-squares convolution 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 latitudes 15° and 60°. 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 sin2 l, where l denotes the latitude) in this particular latitude range. Assuming that such a law can be extrapolated to all latitudes, we find 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.\ud \ud We also discover that the Hα 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 as being caused by a dense and complex prominence system, the circumstellar distribution of which is obtained through maximum entropy Doppler tomography. These maps show in particular that prominences form a complete and inhomogeneous ring around the star, precisely at the corotation radius. We use the total Hα and Hβ 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 time-scale 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

Inverse Compton backscattering source driven by the multi-10 TW laser installed at Daresbury

Inverse Compton scattering is a promising method to implement a high brightness, ultra-short, energy tunable X-ray source at accelerator facilities. We have developed an inverse Compton backscattering X-ray source driven by the multi-10 TW laser installed at Daresbury. Hard X-rays, with spectral peaks ranging from 15 to 30 keV, depending on the scattering geometry, will be generated through the interaction of laser pulses with electron bunches delivered by the energy recovery linac machine, initially known as energy recovery linac prototype and subsequently renamed accelerators and lasers in combined experiments. X-ray pulses containing 9 107 photons per pulse will be created from head-on collisions, with a pulse duration comparable to the incoming electron bunch length. For transverse collisions 8 106 photons per pulse will be generated, where the laser pulse transit time defines the X-ray pulse duration. The peak spectral brightness is predicted to be 1021 photons/(s mm2 mrad2 0.1% Dl/l)