Carbon-enhanced metal-poor ( CEMP ) stars span a wide range of stellar populations , from bona fide second-generation stars to later-forming stars that provide excellent probes of binary mass transfer and stellar evolution .
Here we analyse 11 metal-poor stars ( 8 of which are new to the literature ) , and demonstrate that 10 are CEMP stars .
Based on high signal-to-noise ( SNR ) X-Shooter spectra , we derive abundances of 20 elements ( C , N , O , Na , Mg , Ca , Sc , Ti , Cr , Mn , Fe , Ni , Sr , Y , Ba , La , Ce , Pr , Nd , and Eu ) . From the high SNR spectra , we could trace the chemical contribution of the rare earth elements ( REE ) from various possible production sites , finding a preference for metal-poor low-mass asymptotic giant branch ( AGB ) stars of 1.5 M _ { \odot } in CEMP- s stars , while CEMP- r / s stars may indicate a more massive AGB contribution ( 2–5 M _ { \odot } ) .
A contribution from the r -process – possibly from neutron star – neutron star mergers ( NSM ) , is also detectable in the REE stellar abundances , especially in the CEMP- r / s sub-group rich in both s low and r apid neutron-capture elements .
Combining spectroscopic data with Gaia DR2 astrometric data provides a powerful chemodynamical tool for placing CEMP stars in the various Galactic components , and classifying CEMP stars into the four major elemental-abundance sub-groups , dictated by their neutron-capture element content .
The derived orbital parameters indicate that all but one star in our sample ( and the majority of the selected literature stars ) belong to the Galactic halo .
They exhibit a median orbital eccentricity of 0.7 , and are found on both prograde and retrograde orbits .
We find that the orbital parameters of CEMP-no and CEMP- s stars are remarkably similar in the 98 stars we study . A special case is the CEMP-no star ( HE~0020-1741 ) , with very low Sr and Ba content , which possesses the most eccentric orbit among the stars in our sample , passing close to the Galactic centre .
Finally , we propose an improved scheme to sub-classify the CEMP stars , making use of the Sr/Ba ratio , which can also be used to separate very metal-poor stars from CEMP stars .
We explore the use of [ Sr/Ba ] vs. [ Ba/Fe ] in 93 stars in the metallicity range -4.2 \lesssim [ Fe/H ] < -2 . We show that the Sr/Ba ratio can also be successfully used for distinguishing CEMP- s , CEMP- r / s and CEMP-no stars .
The Sr/Ba ratio is also a powerful astro-nuclear indicator , since the metal-poor AGB stars exhibit very different Sr/Ba ratios , compared to fast rotating massive stars and NSM , and it is reasonably unbiased by NLTE and 3D corrections .