SIMBAD references

Maximum entropy image reconstruction (Doppler imaging) is applied to HII 686 (K4 V, vsini=64 km.s^–1) and HII 3163 (K0 V, vsini=70 km.s^–1), two zero-age main-sequence stars of the Pleiades cluster. Their surface temperature distributions are determined in order to study the nature of magnetic activity in ultrafast rotators of known evolutionary status. Specific intensity line profiles used in the Doppler imaging (DI) algorithm are synthesized. The sensitivity of the reconstructions to the choice of input stellar and atomic data is investigated. The quality of the fit to the average disk-integrated line profile is found to be more important to DI analysis than the exact parameters used to generate the fit when the shape of the disk-integrated line profile is dominated by rotational broadening. Fifteen absorption lines are used to generate 15 individual reconstructions of each Pleiades target. Averaging reduces noise in the reconstructions. However, simulations reveal that spurious features tend to cluster systematically at subobserver longitudes and can therefore persist in the images even after averaging several reconstruction together. Such ``phase-ghosting'' becomes more severe as the signal-to-noise ratio decreases and does not appear to depend on the spot morphology. HII 686 shows one predominant feature at 77° latitude, which is 860 K cooler than the surrounding photosphere. HII 3163 shows a predominant feature centered at approximately 70° latitude, which has two temperature minima (780 K cooler than the photosphere) separated by approximately 130° in longitude. However, photometric observations suggest that this double-lobed morphology is a consequence of poor phase sampling. One temperature minimum located midway in longitude better reproduces the observed light curve. We are unable to make any reliable detections of low-latitude features on the surface of either star. The results are discussed in light of recent theoretical observations and existing image reconstructions of similar objects. No correlations are found among reconstructions that could be attributed to line characteristics: equivalent width, excitation potential, or depth of formation. This is also true for the Li I and K I resonance lines. Despite their temperature sensitivity, they yield results that are consistent with those determined from lines that are not temperature sensitive. For each object, the chromospheric residual emission (Hα, Ca II 8498 Å) is measured as a function of rotational phase. Maximum emission does not consistently occur at the phase of spot transit.