Isotopic ratios in the red giant component of the recurrent nova T Coronae Borealis

Abstract

We report the determination of abundances and isotopic ratios for C, O and Si in the photosphere of the red giant component of the recurrent nova T Coronae Borealis from new 2.284--2.402 $\mu$m and 3.985--4.155 $\mu$m spectroscopy. Abundances and isotopic ratios in the photosphere may be affected by (i) processes in the red giant interior which are brought to the surface during dredge-up, (ii) contamination of the red giant, either during the common envelope phase of the binary evolution or by material synthesised in recurrent nova eruptions, or a combination of the two. We find that the abundances of C, O and Si are reasonably consistent with the expected composition of a red giant after first dredge-up, as is the ${}^{16}$O/${}^{17}$O ratio. The ${}^{28}$Si/${}^{29}$Si ratio is found to be $8.6\pm 3.0$, and that for ${}^{28}$Si/${}^{30}$Si is $21.5\pm 3.0$. The ${}^{12}$C/${}^{13}$C ratio ($10\pm 2$) is somewhat lower than expected for first dredge-up. The ${}^{16}$O/${}^{18}$O ratio ($41\pm 3$) is highly inconsistent with that expected either from red giant evolution ($\sim 550$) or from contamination of the red giant by the products of a nova thermonuclear runaway. In particular the C and O isotopic ratios taken in combination are a puzzle. We urge confirmation of our results using spectroscopy at high resolution. We also encourage a thorough theoretical study of the effects on the secondary star in a recurrent nova system of contamination by ejecta having anomalous abundances and isotopic ratios.

Accepted for publication in Monthly Notices of the Royal Astronomical Society on August 26th. 11 pages, 10 figures

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