Astron. Astrophys. 336, L17-L20 (1998)

5. Discussion

In this letter the binary nature of AC Her is deduced from our CORAVEL radial velocity set. The close correlation between the chemical deficiencies of the photosphere and the condensation temperature shows that the chemical patterns observed in AC Her are due to depletion rather than internal nucleosynthesis and dredge-ups. These depletion patterns are also observed in the extremely iron deficient binary post-AGB stars (Van Winckel et al., 1995 and references therein) and strengthens further the fact that binarity is a necessary condition for the depletion process to occur (Waters et al. 1992).

The properties of the circumstellar dust shell of AC Her closely resemble those of the Red Rectangle (RR): both stars have (1) prominent oxygen-rich crystalline silicates (Waters et al. 1998; Molster et al. (in preparation), (2) remarkably weak CO rotational line emission with a small velocity width (Jura et al. 1995; Bujarrabal et al. 1988), suggesting that the gas is depleted, (3) very strong millimeter continuum flux from large dust grains (van der Veen et al. 1994; Shenton et al. 1995). These properties suggest that the dust is highly processed. In analogy with the RR, the dust in AC Her is likely also stored in a long-lived disc. This implies that no conclusion can be derived about the time that has elapsed since AC Her has left the AGB from the inner radius of the dust shell.

Since the depletion patterns are observed in all field RV Tauri stars of type A and B studied till now (Gonzalez et al. 1997b) the binary nature of all these RV Tauri stars should be addressed together with the absence of the depletion patterns in globular cluster RV Tauri stars and field RV Tauri objects of class C (Gonzalez & Lambert 1997; Giridhar et al. 1998). Since direct detection of binary motions is made difficult by the high amplitude pulsations, we propose indirect methods which include physical, geometrical and chemical studies of the circumstellar material to probe for the presence of dust discs. The very short post-AGB lifetime inferred by Jura (1986) for the RV Tauri objects detected by IRAS together with the low gas-to-dust ratio observed by Bujarrabal et al. (1988) for 4 objects give already an indication that indeed the binary fraction among RV Tauri stars might be very high but a more detailed investigation is clearly needed.

We can conclude that our radial velocity monitoring campaign confirms the binary nature of AC Her and that the photospheric chemical depletion patterns, presence of a population of large grains, low gas-to-dust ratio and the presence of crystalline silicates indicates that the circumstellar dust around AC Her is highly processed and trapped in a long-lived circumbinary disc similar to the oxygen rich dust-disc in the Red Rectangle.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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