Observations of R Coronae Borealis stars in decline - Empirical arguments for dust formation near the stellar surface

We present new IUE observations which monitor the spectroscopic evolution of RY Sgr through a deep decline. These data, along with other IUE, visible, and infrared observations of declines in RY Sgr, R CrB, and V854 Cen, are considered in an attempt to understand the dust formation process and the source of the narrow- and broad-line emission seen during declines. Spectroscopic, photometric, and polarimetric data covering 10 declines of these stars have been analyzed. The evolution of the UV emission spectrum is similar to that found in the visible. Previous models of dust formation assume that dust forms around 20 stellar radii, where gas ejected from the star is cool enough to condense. However, the spectral evolution, and the dispersal times of dust implied by the light curve are consistent with dust forming within a few stellar radii and moving outward due to radiation pressure. The expanding dust cloud eclipses the photosphere and parts of the emission line region, causing the observed spectral evolution.