MONITORING Hα EMISSION AND CONTINUUM OF UXORs: RR Tauri

The Maria Mitchell Observatory, in collaboration with the Astrokolkhoz Observatory, started a program of photometric monitoring of UX Ori-type stars (UXORs) with narrowband interference filters (IFs; augmented with the traditional broadband filters) aimed at separating the Hα emission variations from those of the continuum. We present the method of separation and the first results for RR Tau obtained in two seasons, each roughly 100 days long (2010 Winter-Spring and 2010 Fall-2011 Spring). We confirm the conclusion from previous studies that the Hα emission in this star is less variable than the continuum. Although some correlation between the two is not excluded, the amplitude of Hα variations is much smaller (factors of 3-5) than that of the continuum. These results are compatible with Grinin's model of UXORs, which postulates the presence of small obscuring circumstellar clouds as the cause of the continuum fading, as well as the presence of a circumstellar reflection/emission nebula, larger than the star and the obscuring clouds, which is responsible for Hα emission and the effect of the 'color reversal' in deep minima. However, the results of both our broadband and narrowband photometry indicate that the obscuration model may be insufficient to explain all of the observations. Disk accretion, the presence of stellar or (proto) planetary companion(s), as well as the intrinsic variations of the star, may contribute to the observed light variations. We argue, in particular, that the Hα emission may be more closely correlated with the intrinsic variations of the star than with the much stronger observed variations caused by the cloud obscuration. If this hypothesis is correct, the close monitoring of Hα emission with IFs, accessible to small-size telescopes, may become an important tool in studying the physical nature of the UXORs' central stars.