Light curves for 10 R Coronae Borealis stars for longer than a century: secular evolution, dip statistics, and a general model for the shape of isolated light-curve dips

Abstract

ABSTRACT R Coronae Borealis stars (RCBs) are cool supergiants that display non-periodic deep dips in brightness. Recently, a group of ‘Hot RCB stars’ has been discovered to be fast evolving across the HR (Hertzsprung-Russell) diagram, as these stars leave the RCB region, with brightness changes at the rate of ∼1 mag per century. Perhaps cool RCB stars can also be seen evolving, either increasing in temperature as they evolve to become Hot RCB stars, or perhaps increasing in luminosity as the stars arrive at the RCB region. To seek these changes, the only possible method is to extract archival data going back more than a century, looking for the brightness changes associated with the evolution. I have measured and extracted 323 464 mag (mostly from the Harvard plates and from the American Association of Variable Star Observers) for 10 cool RCB stars, all with over a century for the light curves, all consistently calibrated to a modern magnitude system. For times away from any dips, these light curves are flat to within the typical uncertainty of ±0.10 mag per century. That is, I see no significant evolution. I also have collected a large data base of light-curve dips and their properties. From this, the light curves for all the well-observed isolated dips have the same shape, featuring a flat slope for the few days immediately after the minima. Further, I derive a general model for the shape of the light curve for all isolated RCB dips, with a simple equation accurately describing the observed recovery to maximum light.