Fluorine in R Coronae Borealis Stars

R Coronae Borealis (RCB) stars comprise a sequence of hydrogen deficient supergiants with effective temperatures from about Teff = 3500 K, as represented by Z UMi and DY Per, to about 19,500 K, as represented by DY Cen. The characteristic of H-deficiency is shared by the H-deficient cool carbon (HdC) stars at low temperatures and by the extreme helium (EHe) stars at high temperatures. The sequence HdC - RCB - EHe in the (Teff , log g) plane reflects a close evolutionary connection.

If HdC, RCB, and EHe stars share a common heritage, the expectation is that their atmospheric compositions should show some common features (Pandey et al. 2004; Rao 2005). Just five HdC, about 40 RCB (Zaniewski et al. 2005), and 21 EHe stars are known in the Galaxy. It is through a study of their compositions that one hopes to test theoretical ideas about the origins of these extremely rare stars.

Currently, two scenarios remain in contention to account for these H-poor high luminosity stars. According to the first, a final He-shell flash in a post-AGB star on the white dwarf cooling track creates a H-poor luminous star. This is dubbed the ‘final flash’ (FF) scenario. In the second scenario, an H-poor star is formed from a merger of a He white dwarf with a C-O white dwarf. In a close binary system, accretion of the He white dwarf by the C-O white dwarf may lead to a H-poor supergiant with the C-O white dwarf as its core. This is called the ‘double degenerate’ (DD) scenario.

A determination of which scenario provided which star rests in large part on the observed chemical composition of a star’s atmosphere and theoretical predictions about the FF and DD products. Evidence from elemental abundances, especially the H, C, N, and O abundances, suggests that the RCB and EHe stars evolved via the DD rather than the FF route (Pandey et al. 2001; Saio & Jeffery 2002; Pandey et al. 2006). The convincing and essentially incontrovertible evidence, that the DD scenario led to the HdCs and some cool RCBs was presented by Clayton et al. (2007) with their discovery that the 18 O was very abundant in their atmospheres. Presence of this isotope of oxygen was attributed to nucleosynthesis occurring during and following accretion of the He-rich material onto the C-O white dwarf.

Determination of the oxygen isotopic ratios demands a cool star with the CO vibration rotation bands in its spectrum. The majority of RCBs and all of the EHes are too hot for CO to contribute to their spectra (Tenenbaum et al. 2005). An alternative tracer of nucleosynthesis during a merger may be provided by the fluorine abundance. Considerable enrichment of EHe stars with F was discovered by Pandey (2006) from detection and analysis of about a dozen F I lines in optical spectra of cool EHe stars. Calculation by Clayton et al. (2007) suggest that F synthesis is possible in the DD scenario. Here, we report on a search for F I lines in spectra of RCBs.

High-resolution optical spectra of RCBs at maximum light obtained at the Vainu Bappu Observatory and at the W.J. McDonald Observatory in Texas, USA were examined for suitability for this project. F I lines are identified and analysed for the first time in the spectra of RCB stars. Sakurai’s object, a final He-shell flash product, shows no detectable F I lines.

The abundance of fluorine in RCB and EHe stars is about 1000 times the solar abundance. The present challenge is to show that the DD scenario includes the possibility of robustly increasing the F abundance to the observed levels of 1000 times the solar abun- dance. This work has been done by Gajendra Pandey in collaboration with David Lambert and N. Kameswara Rao and has been published in The Astrophysical Journal of February 20, 2008.

Spectra of RCBs showing F I lines

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