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 HE0020-1741,
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, HE0020-1741 is sub-classified
as a CEMP-no star. We show that the light-element abundance pattern of
HE0020-1741 is consistent with predicted yields from a massive (M = 21.5 Mo),
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 Mo - 100 Mo), 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.
