We calculate a grid of star models with and without the effects of axial rotation for stars in the mass range between 2 and 60 M _ { \odot } for the metallicity Z = 10 ^ { -5 } .
Star models with initial masses superior or equal to 9 M _ { \odot } were computed up to the end of the carbon–burning phase .
Star models with masses between 2 and 7 M _ { \odot } were evolved beyond the end of the He–burning phase through a few thermal pulses during the AGB phase .
Compared to models at Z = 0.02 , the low Z models show faster rotating cores and stronger internal \Omega –gradients , which favour an important mixing of the chemical elements .
The enhancement of N/C at the surface may reach 2 to 3 orders of magnitude for fast rotating stars .
Surface enrichments may make the evolved stars less metal poor than they were initially .
In very low Z models , primary nitrogen is produced during the He–burning phase by rotational diffusion of ^ { 12 } C into the H–burning shell .
A large fraction of the primary ^ { 14 } N escapes further destruction and enters the envelope of AGB stars , being ejected during the TP–AGB phase and the formation of a planetary nebula .
The intermediate mass stars of very low Z are the main producers of primary ^ { 14 } N , but massive stars also contribute to this production ; no significant primary nitrogen is made in models at metallicity Z =0.004 or above .
We calculate the chemical yields in He , C , N , O and heavy elements and discuss the chemical evolution of the CNO elements at very low Z .
Remarkably , the C/O vs O/H diagram is mainly sensitive to the interval of stellar masses , while the N/O vs O/H diagram is mainly sensitive to the average rotation of the stars contributing to the element synthesis .
The presently available observations in these diagrams seem to favour contributions either from stars down to about 2 M _ { \odot } with normal rotation velocities or from stars above 8 M _ { \odot } but with very fast rotation .
hysical data and processes : nucleosynthesis – Stars : interiors , evolution , rotation – Stars : early–types , AGB