Nine 20 M _ { \odot } models were computed with metallicities ranging from solar , through Z = 10 ^ { -5 } ( [ Fe/H ] \sim -3.1 ) down to Z = 10 ^ { -8 } ( [ Fe/H ] \sim -6.1 ) and with initial rotational velocities between 0 and 600 km s ^ { -1 } to study the impact of initial metallicity and rotational velocity [ Hirschi ( 2006 ) ] .
The very large amounts of ^ { 14 } N observed ( \sim 0.03 M _ { \odot } ) are only produced at Z = 10 ^ { -8 } ( PopII 1/2 ) .
The strong dependence of the ^ { 14 } N yields on rotation and other parameters like the initial mass and metallicity may explain the large scatter in the observations of ^ { 14 } N abundance .
The metallicity trends are best reproduced by the models with \upsilon _ { ini } / \upsilon _ { c } \sim 0.75 , which is slightly above the mean observed value for OB solar metallicity stars .
Indeed , in the model with \upsilon _ { ini } =600 km s ^ { -1 } at Z = 10 ^ { -8 } , the ^ { 16 } O yield is reduced due to strong mixing .
This allows in particular to reproduce the upturn for C/O and a slightly decreasing [ C/Fe ] , which are observed below [ Fe/H ] \sim -3 .