Simulated imaging of the upper atmosphere of active stars

Abstract

We have applied the principles of indirect stellar surface imaging (Doppler imaging) for simulated mapping of active regions in the upper atmosphere of cool active stars. Our tests illustrate how synthetic distributions at the chromosphere or transition region level can be recovered from time series of high resolution profiles of UV lines, as satellites like IUE, HST or PRISMA could collect them. Due to the large width of intrinsic profiles in such regions of the atmosphere, we find that only stars with a projected equatorial velocity υ sin i larger than about 20 km s^-1 can be mapped with this method. For all other stars, only large scale equatorial features can be recovered. Increasing spectrum quality improves the reconstruction significantly, especially in the case of high latitude features. We also find that data should span several successive rotational cycles in order to minimise the impact at the reconstruction of all intrinsic random variations of UV lines formed in the chromosphere or transition region. Finally, we show how mapping can be affected by the existence of flows in the upper atmosphere.