RV RV Tauri stars

- These are radially pulsating supergiants having spectral types F-G at maximum light and K-M at minimum. The light curves are characterized by the presence of double waves with alternating primary and secondary minima that can vary in depth so that primary minima may become secondary and vice versa. The complete light amplitude may reach 3"'-4m in V. Periods between two adjacent primary minima (usually called formal periods) lie in the range 30''-150'' (these are the periods appearing in the Catalog). Two subtypes, RVa and RVb, are recognized: RVa (subtype) - RV Tauri variables that do not vary in mean magnitude. RVb (subtype) - RV Tauri variables that periodically vary in mean magnitude with periods from 600 to 1500 days and amplitudes up to 2'" in V. GCVS

RV Tauri12 stars appear to be post-asymptotic giant branch13 (AGB) stars, evolving from the red giant to the white dwarf phase on a time-scale of thousands of years. They are spectral type F or G when brightest and G or early K when faintest. The visual light curves display double waves with alternating primary and secondary dips to minimum light that can vary in depth. The primary minimum brightness may become secondary minimum brightness.

The double-wave light curves probably result from a resonance between the fundamental mode of pulsation and the first overtone mode. The amplitude in visual light is usually between one and two magnitudes although it may exceed three magnitudes. The periods between two adjacent primary minimum declines, usually called the formal period, is approximately 30-

Observation

"A' Bright stars fift Mixed amplitude

Mixed periods <3> CCD or PEP

"Within the GCVS the subtypes RVa and RVb are listed as RVA and

RVB. This should be avoided because of the possible confusion with the spectroscopic subtypes (i.e. A, B, and C).

"Taurus, in Greek mythology, the snow-white bull which, carried

Europa off, only to be revealed as Zeus in disguise.

"The second time that a star climbs the giant branch, it has a graphical track on the MR diagram that is crudely asymptotic to the first track and so is called the asymptotic giant branch.

Figure 4.8. Light curve 0(iheRV-lype variable swr, SZMon. Cycle phase is indicated along ¡he horizontal axis. Doto ^dedbyrt« KIPPARCOS mission Used wilt, permission

Figure 4.8. Light curve 0(iheRV-lype variable swr, SZMon. Cycle phase is indicated along ¡he horizontal axis. Doto ^dedbyrt« KIPPARCOS mission Used wilt, permission

150 days. The cause of the slow variation in the mean amplitude, known as the RVb phenomenon, is not understood.

Light curves for RV Tauri stars are semiregular, and sizable variations are seen from one cycle to the next (Figure 4.8). The longer-period stars have a tendency to be less regular than the shorter-period stars. The phase of the U-B and B-V color curves precedes that of the visual light curve by up to a quarter of a period. Interchanges of deep and shallow minimum brightness occasionally occur and may be abrupt or gradual. Stretches of irregular or chaotic behavior have also been recorded.

The RV Tau stars are subdivided as a result of their long-term behavior. Those stars with clear long-term variability are classed as RVb and those without such variations as RVa. The RVb type are periodic with periods in the range of hundreds to thousands of days. The RV Tau stars can be oxygen- or carbon-rich and have also been subdivided into groups A, B, and C on the basis of their spectra.

The aging process of RV Tauri stars is equivocal. They have extended atmospheres, are undergoing mass loss, and TiO absorption bands are occasionally observed in the optical spectra near minimum light which is also the coolest phase. The TiO absorption bands indicate a later spectral type, perhaps beginning at M2. Dust-shells, as evidenced by their strong infrared emissions, surround some RV Tau stars. They may be AGB stars executing blue loops within the HR diagram following a helium-shell flash, or they could be post-AGB stars in the process of losing the last remnant of their atmospheres as they turn into white dwarfs.

RV Tauri stars are closely related to type II Cepheids, also found in metal-deficient globular clusters, and they occupy the same instability strip in the HR diagram as lower luminosity and shorter-period stars. They also

90 Observing Variable St^,

have similarities to the semiregular variables, in particular SRd and the UU Her groups.

Additional examples of RV Tauri variable stars are DF Cyg, AC Her, U Mon, R Set and RV Tau. R Scuti is classed as an RVa because of its period of 146.5 days.

The observed vs calculated (O-C diagrams) light curves of RV Tauri stars are known to be dominated by the effects of random cycle-to-cycle fluctuations in the period, as is the case in Mira stars.

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