The Carbon-Dependent Binary Frequency of CEMP-no Stars
Studies of the oldest and most metal-poor stars in the Milky Way confirm a common chemical signature of high carbon abundances and subsolar neutron-capture enrichment. The so-called CEMP-no stars are speculated to be bona fide population II stars, potentially tracing the nucleosynthesis of the first stars in the universe. However, constraining the binary nature of CEMP-no stars is crucial for understanding their origins. The binary fraction of CEMP-no stars has tentatively been found to vary with carbon enhancement, independent of metallicity. Here we present the results of radial-velocity monitoring of 30 CEMP-no stars over five years, increasing the total number of CEMP-no stars with a constrained binary status by ${\sim}30\%$ in order to better investigate the CEMP-no binary fraction as a function of carbon. Combining our results with literature data, we find an overall binary frequency of $50^{+13}_{-13}\%$ among high-carbon ($A(\rm{C})\ge7.3$) CEMP-no stars, compared to $18^{+5}_{-4}\%$ for low-carbon ($A(\rm{C}) < 7.3$) stars, establishing a statistically significant increase in the CEMP-no binary frequency as a function of carbon for the first time. Our results indicate no correlation between metallicity and binary frequency. Additionally, we derive orbital parameters for three new CEMP-no stars in binary systems and estimate secondary masses for these stars as well as seven binaries from the literature. We discuss these mass estimates in the context of investigating the nature of the unseen companions and the plausibility of a mass-transfer formation channel for high-carbon CEMP-no stars.