RV Tauri star
The alternate deep and shallow minima of RV Tauri stars may sometimes be replaced by spells of total irregularity. AC Herculis, whose light curve is shown here, is a case in point; it has a magnitude from 7 to 8.5.
Location of AC Herculis in the sky.
Approximate position of RV Tauri stars on the Hertzsprung-Russell diagram. Credit: AAVSO.
RV Tauri is a luminous yellow supergiant pulsating variable, the light curve of which shows alternating deep and shallow minima with a period (measured between one deep minimum and the next) of 30 to 150 days and a brightness range of up to four magnitudes. The spectral type is typically F to G at minimum and G to K at maximum. RV Tau stars seems to be intermediate between the Cepheid variables and Mira variables. They probably represent the low mass, and at least in some cases the low metallicity, portion of stars that are in transition from the asymptotic giant branch (AGB) to white dwarfs. Because of their previously high mass-loss rates, many will probably become planetary nebulae. Others, however, may evolve so slowly that the envelopes may dissipate before becoming photoionized. Since the transition from the AGB to the white dwarf stage of stellar evolution isn't well understood, RV Tau stars stand as a potential bridge across this evolutionary gap. This post-AGB phase of stellar evolution is short, astronomically speaking, lasting only a few thousand years.
There are two main varieties of RV Tau stars: the RVa types, of which R Scuti is an example, maintain a roughly constant mean brightness; RVb types, which include RV Tau itself, have long-term (600- to 1,500-day) periodicity. Infrared studies suggest that RV Tau stars have dusty circumstellar shells, which may be initiated by pulsation via a shock wave. Based on the seemingly smooth transition between the RVa and RVb stars, the two groups may not be physically distinct. The RVb stars may be in an active phase in which the dust shell is replenished by dust formation close to the star. The dust may be swept out with this gaseous outflow, and in the absence of fresh dust production the star will become an RVa, with a much less dense shell. The RVa-types may have thinner dust shells or have concentration of dense dust located at large radii.