Sodium enhancement in evolved cataclysmic variables

We present follow-up spectroscopy of 21 cataclysmic variables (CVs) with evolved secondaries and ongoing or recently terminated mass transfer. Evolutionary models predict that the secondaries should have anomalous surface abundances owing to nuclear burning in their cores during their main-sequence evolution and subsequent envelope stripping by their companion white dwarfs. To test these models, we measure sodium (Na) abundances of the donors from the Fraunhofer 'D' doublet. Accounting for interstellar absorption, we find that all objects in our sample have enhanced Na abundances. We measure 0.3 dex less than or similar to [Na/H] less than or similar to 1.5 dex across the sample, with a median [Na/H] = 0.956 dex, i.e. about an order of magnitude enhancement over solar values. To interpret these values, we run Modules for Experiments in Stellar Astrophysics binary evolution models of CVs in which mass transfer begins just as the donor leaves the main sequence. These generically predict Na enhancement in donors with initial donor masses greater than or similar to 1 M-circle dot, consistent with our observations. In the models, Na enrichment occurs in the donors' cores via the NeNa cycle near the end of their main-sequence evolution. Na-enhanced material is exposed when the binaries reach orbital periods of a few hours. Donors with higher initial masses are predicted to have higher Na abundances at fixed orbital period owing to their higher core temperatures during main-sequence evolution. The observed [Na/H] values are on average approximate to 0.3 dex higher than predicted by the models. Surface abundances of evolved CV donors provide a unique opportunity to study nuclear burning products in the cores of intermediate-mass stars.

Automated summary

In this study, follow-up spectroscopy of 21 cataclysmic variables (CVs) with evolved secondaries and ongoing or recently terminated mass transfer was conducted. The sodium (Na) abundances of the donors were measured to test evolutionary models, and it was found that all objects in the sample had enhanced Na abundances. Binary evolution models of CVs were used to interpret the results, which predicted Na enhancement in the donors' cores near the end of their main-sequence evolution. The observed [Na/H] values were on average approximately 0.3 dex higher than predicted by the models.