Exploring the History of Stellar Mergers with Chemistry: Examining the Origins of Massive <i>α</i> -enriched Stars Using Carbon Isotope Ratios

Abstract Recently discovered massive α -enriched (MAE) stars have surface chemistry consistent with members of the older, thick-disk Milky Way population but high masses (∼1.5–3 M ⊙ ) that suggest these stars are young. The origin of MAE stars is not fully understood, although binary interactions are likely an important formation pathway. To better constrain the history of MAE stars, we have measured metallicities, carbon isotope ratios, and CNO abundances in 49 red clump (RC) stars and four red giants. Our sample included thin-disk, thick-disk, and MAE stars to best constrain different formation scenarios. We observed our sample stars using the Tull Spectrograph on the McDonald 2.7 m telescope and derived abundances using synthetic spectra created by the Turbospectrum radiative transfer code. Overall, we find that 10 of our RC MAE stars are consistent with the average thick-disk carbon isotope ratio of 12 C/ 13 C = 8.2 ± 3.4. We find 5 MAE stars that have significantly higher carbon isotope ratios ( 12 C/ 13 C &gt; 15) than stars at similar metallicities. A total of 2 of the anomalous stars show abundance patterns consistent with asymptotic giant branch mass transfer, while 3 MAE stars have 12 C/ 13 C ratios similar to massive, single RC stars and show no clear signs of binarity from radial velocity monitoring or from the Gaia RUWE measurement. Overall, we find that carbon isotope ratio measurements provide a unique constraint when discerning the possible origins of MAE stars.