A Partnership in Observational and Computational Astronomy

POCA Research

SC State astronomers Jennifer Cash and Donald Walter are conducting a long term study of RV Tauri and Semiregular variables in collaboration with NASA astronomer Steve Howell and NOAO astronomer Ken Hinkle. This study uses ground-based photometric and spectroscopic data as well as space based observations with the NASA Kepler Observatory.

RV Tauri stars are pulsating yellow supergiants whose light curves are characterized by alternating deep and shallow minima. The time from one deep minimum to the next deep minimum, known as the formal period, is typically in the range of 30 to 150 days. During their pulsation cycle, they change several spectral subtypes between minimum and maximum light. RV Tauri stars are old, low mass supergiants that lie between the Cepheid and Mira-type variables on the Hertzsprung-Russell (HR) diagram. These stars may be executing blue loops around the Asymptotic Giant Branch (AGB) or in transition from the AGB to white dwarfs. Since this transition is not well understood, RV Tauri stars stand as a potential bridge across this evolutionary gap.

Semiregular variables are giant or supergiant stars of intermediate or late spectral types. While they display some periodicity in their light curves, at other times their light output is irregular or nonperiodic. Their periods typically range from 20 to more than 2000 days. On the HR diagram, they are located between the RV Tauri types and the Mira variables.

A light curve from the AAVSO database for the RV Tauri type R Sct.
Note the characteristic alternating deep and shallow minima.


The Data Set

Our ground-based photometry comes from the database of the American Association of Variable Star Observers (AAVSO). Our ground-based spectra were acquired at the Coude Feed Telescope at Kitt Peak National Observatory. We have high signal-to-noise spectra of most of our stars from 2002 to the present. Our space-based photometry comes from the Kepler Observatory. We acquired new observations in Cycles 2 and 4 and are using the Kepler archives to fill in the gaps from other quarters.

One of our spectra taken by the Coude Feed Telescope of Z UMa. Note the variety of features, including metallic and Balmer emission lines, molecular emission bands and absorption lines such as Ca II H and K lines.