We discuss the detailed composition of 28 extremely metal-poor ( EMP ) dwarfs , 22 of which are from the Hamburg/ESO Survey , based on Keck Echèlle spectra .
Our sample has a median [ Fe/H ] of -2.7 dex , extends to -3.5 dex , and is somewhat less metal-poor than was expected from [ Fe/H ] ( HK , HES ) determined from low resolution spectra .
Our analysis supports the existence of a sharp decline in the distribution of halo stars with metallicity below [ Fe/H ] = -3.0 dex .
So far no additional turnoff stars with \mbox { [ Fe / H ] } < -3.5 have been identified in our follow up efforts .

For the best observed elements between Mg and Ni , we find that the abundance ratios appear to have reached a plateau , i.e .
[ X/Fe ] is approximately constant as a function of [ Fe/H ] , except for Cr , Mn and Co , which show trends of abundance ratios varying with [ Fe/H ] .
These abundance ratios at low metallicity correspond approximately to the yield expected from Type II SN with a narrow range in mass and explosion parameters ; high mass Type II SN progenitors are required .
The dispersion of [ X/Fe ] about this plateau level is surprisingly small , and is still dominated by measurement errors rather than intrinsic scatter .
These results place strong constraints on the characteristics of the contributing SN .

The dispersion in neutron-capture elements , and the abundance trends for Cr , Mn and Co are consistent with previous studies of evolved EMP stars .

We find halo-like enhancements for the \alpha -elements Mg , Ca and Ti , but solar Si/Fe ratios for these dwarfs .
This contrasts with studies of EMP giant stars , which show Si enhancements similar to other \alpha -elements .
Sc/Fe is another case where the results from EMP dwarfs and from EMP giants disagree ; our Sc/Fe ratios are enhanced compared to the solar value by \sim 0.2 dex .
Although this conflicts with the solar Sc/Fe values seen in EMP giants , we note that \alpha -like Sc/Fe ratios have been claimed for dwarfs at higher metallicity .

Two dwarfs in the sample are carbon stars , while two others have significant C enhancements , all with ^ { 12 } C/ ^ { 13 } C \sim 7 and with C/N between 10 and 150 .
Three of these C-rich stars have large enhancements of the heavy neutron capture elements , including lead , which implies a strong s -process contribution , presumably from binary mass transfer ; the fourth shows no excess of Sr or Ba .