KP Cyg: an Unusual Metal-rich RR Lyr Type Star of Long Period

TL;DR

This study reveals that KP Cyg, a long-period star previously classified as RR Lyrae, is unusually metal-rich and may be a short-period CWB star with surface pollution from helium burning products.

Contribution

It provides the first detailed chemical abundance analysis of KP Cyg, challenging its classification and suggesting it is a CWB star rather than an RR Lyrae.

Findings

KP Cyg is very metal-rich ([Fe/H] = +0.18) unlike typical RR Lyrae stars.

KP Cyg shows strong carbon and nitrogen excesses, indicating surface pollution from helium burning.

Similar C and N enhancements are observed in other short-period variable stars.

Abstract

We present the results of a detailed spectroscopic study of the long period ($P=0.856$ days) RR Lyrae star, KP Cyg. We derived abundances of many chemical elements including the light species, iron-group elements and elements of the s-processes. Most RR Lyrae stars with periods longer than 0.7 days are metal-deficient objects. Surprisingly, our results show that KP Cyg is very metal rich ([Fe/H] $= +0.18\pm 0.23$). By comparison with a number of short period ($P=1\sim 6$ days), metal-rich CWB stars, we suggest that KP Cyg may be a very short period CWB star (BL Her star) rather than an RR Lyrae star. As seen in some CWB stars, KP Cyg shows strong excesses of carbon and nitrogen in its atmosphere. This indicates that the surface of KP Cyg has been polluted by material that has undergone helium burning (to enhance carbon) and proton capture (to transform carbon into nitrogen). We also note that UY CrB, whose period is 0.929 days, also shows an enhancement of C and N, and that two carbon cepheids of short period, V553 Cen and RT TrA, show similar excesses of carbon and nitrogen.