OBSERVATIONAL CONSTRAINTS ON FIRST-STAR NUCLEOSYNTHESIS. II. SPECTROSCOPY OF AN ULTRA METAL-POOR CEMP-no STAR

We report on the first high-resolution spectroscopic analysis of HE. 0020-1741, a bright (V - 12.9), ultra metal-poor ([Fe/H]= -4.1), carbon- enhanced ([C/Fe]= +1.7) star selected from the Hamburg/ESO Survey. This star exhibits low abundances of neutron-capture elements ([Ba/Fe]= -1.1) and an absolute carbon abundance A (C) = 6.1; based on either criterion, HE 0020-1741 is subclassified as a carbon-enhanced metal-poor star without enhancements in neutron-capture elements (CEMP-no). We show that the light-element abundance pattern of HE. 0020-1741 is consistent with predicted yields from a massive (M = 21.5 M circle dot), primordial-composition, supernova (SN) progenitor. We also compare the abundance patterns of other ultra metal-poor stars from the literature with available measures of C, N, Na, Mg, and Fe abundances with an extensive grid of SN models (covering the mass range 10-100 M circle dot), in order to probe the nature of their likely stellar progenitors. Our results suggest that at least two classes of progenitors are required at[Fe/H]< -4.0, as the abundance patterns for more than half of the sample studied in this work (7 out of 12 stars) cannot be easily reproduced by the predicted yields.