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

Abstract

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. 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 1020 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.

ACTIVITY ON THE M STAR OF QS Vir

“…Changes of the position of the corresponding feature will occur accordingly with the regeneration/evolution of the gas cloud and the geometry of the magnetic field of the M star at that location. Donati et al (2000) measured a flux ratio of Hα/Hβ ∼ 10 for the gas cloud of the post T Tauri star RX J1508.6−4423; our respective value is Hα/Hβ ∼ 14 for the equivalent structure, in agreement with this interpretation.…”

The differential rotation of G dwarfs

“…Marsden et al (2005) report a surface differential rotation of 0.025 ± 0.015 rad/day for R58, while Jeffers & Donati (2009b) find the much larger value of 0.138 ± 0.011 rad/day. Donati et al (2000) find a surface differential rotation δΩ = 0.12 ± 0.02 rad/day for LQ Lup. Jeffers & Donati (2009a) determined the surface rotation of HD 171488 using the Zeeman-Doppler imaging technique and found a large surface differential rotation of 0.5 rad/day.…”

“…Stellar differential rotation can be inferred from the light curves of rotating spotted stars (see, e.g. Henry et al 1995;Messina & Guinan 2003), by monitoring magnetic activity (Donahue et al 1996), spectroscopically (Reiners & Schmitt 2003), or by Doppler imaging (Barnes et al 2000;Donati et al 2000).…”

Abundance anomalies in pre-main-sequence stars

“…1 may also be compared to our models. The star HD 104237 has a mass of about 2.3 M (T eff = 8000 K), a luminosity of about 1.42 L (Böhm et al 2004) and an approximate age of 2 Myr (van den Ancker et al 1998), as well as a magnetic field of about 50 G (Donati et al 1997(Donati et al , 2000. It is an HAeBe star for which Acke & Waelkens (2004) found that Si, Cr and Fe abundances were solar, which agrees with our results since even the heavier 2.50W5E-14 model is roughly normal until 3 Myr (see Fig.…”