The Evolutionary Status of UX Orionis–Type Stars

Abstract

We present measurements of 1.3 mm continuum emission from a sample of UX Ori stars (UXOrs). The UXOrs are pre-main-sequence stars, typically of intermediate mass, and they are distinguished from other pre-main-sequence stars by their large photometric and polarimetric variations that are thought to be due to variable extinction by circumstellar dust. Transient optical and UV redshifted absorption lines are a second defining characteristic, and have been interpreted in terms of the disruption of infalling cometary bodies. Our new millimeter fluxes are used to derive masses of circumstellar dust, MCSD. We combine these measurements with data from the literature to examine a sample of 30 pre-main-sequence systems with spectral types F0-B9, i.e., Herbig Ae stars, about half of which display UXOr characteristics. We find no systematic differences in MCSD between the two groups of stars. Moreover, no statistically significant correlation between MCSD and stellar age is found, and the amplitude of photometric variability appears to be independent of age. We propose that UXOr phenomena do not characterize a more evolved environment. They are probably common to the majority of stars in our sample, but are observed only when the line of sight is close to the equatorial plane of an aspheric circumstellar nebula.