On the Origin of Neutron-capture Elements in r -I and r -II Stars: A Differential-abundance Analysis

We present a strictly line-by-line differential analysis of a moderately r -process-enhanced star ( r -I: HD 107752) with respect to a strongly r -process-enhanced star ( r -II: CS 31082-0001) to investigate the possible common origin of their heavy-element nucleosynthesis with high-precision abundances. This study employs European Southern Observatory data archive high-resolution and high-signal-to-noise spectra taken with the Ultraviolet and Visual Echelle Spectrograph Very Large Telescope spectrograph. Considering only the lines in common in both spectra, we estimate differential abundances of 16 light/Fe-peak elements and 15 neutron-capture elements. Abundances of O, Al, Pr, Gd, Dy, Ho, Er, and a detection of Tm in HD 107752, are presented for the first time. We found three distinct features in the differential-abundance pattern. Nearly equal abundances of light elements up to Zn are present in both stars, indicating a common origin for these elements; in addition to no noticable odd–even differential pattern. Regarding the neutron-capture elements, the r -I star exhibits mildly depleted light r -process elements and more depleted heavier r -process elements relative to r -II star. We also show that, among the r -I and r -II stars, the ratio of lighter-to-heavier r -process elements (e.g., [(Sr, Y, Zr)/Eu]) exhibits a decreasing trend with respect to the overall r -process enhancement, forming a continuous sequence from the r -I to the r -II stars. Finally, we discuss the necessity of multiple sites for the formation of r -I stars.