Abstract
Many of the Mira stars observed with adequate spatial resolution show detectable asymmetry. This asymmetry can be caused by an asymmetric stellar photosphere and/or asymmetric envelope around the star and can be the origin of asymmetries in the subsequent planetary nebula. In this paper, we present the results of long baseline interferometric observations of the Mira-type star U Ori at 1.51 (H2O band), 1.64 (pseudocontinuum), and 1.78 (H2O band) μm in 2005. We performed model-independent image reconstruction of the envelope around the star using measured visibilities and closure phases. The images show asymmetric structure of the U Ori envelope that is similar to the structure of 22 GHz H2O masers obtained by Vlemmings et al. in 2003. Further comparison of near infrared images with available radio maps gives some evidence for differential rotation of the envelope with rotational velocities varying between 3 and 5 km s–1. Finally, we discuss the geometric and kinematic structure of the U Ori envelope based on a model of an almost face-on expanding and rotating disk around the star.
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