2024JAVSO..52....3R

In recent years low resolution spectroscopy has become available for use in crowd-sourced campaigns. One type of target that could benefit from a crowd-sourced photometry and spectroscopy campaign is Mira-type variables undergoing significant changes in their light curves in any combination of amplitude, average luminosity, period, and shape. These types of Miras have been previously studied and are hypothesized to be undergoing a helium shell flash. Previously published spectroscopic studies showed that R Cen, a dual-maximum Mira undergoing significant changes to its light curve, cycled between a spectral type of about M4.5 at maximum light to M9+ at the deeper of the two minima. Our results, from 63 spectra taken over the course of a full pulsation cycle, show that the spectral type at maximum light is still about M4.5, while at minimum light the spectrum now only goes as late as between M6 and M7. This lends support to the hypothesis that R Cen is undergoing a shell flash and is in the increasing energy output phase of the flash cycle. Some helium shell flashes result in a dredge-up of thermonuclear-processed materials from the deep interior into the stellar atmosphere. Our results show that major changes in spectral type from M to S to C can be detected with very modest observing equipment. The ability to detect both types of changes shows that low resolution spectroscopy, which can be undertaken by amateurs, clearly has scientific value.