Evolution and CNO yields of Z=10^-5 stars and possible effects on CEMP production

Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate their C, N, and O yields. Using the Monash University Stellar Evolution code we computed and analysed the evolution of stars of metallicity Z = 10^-5 and masses between 4 and 9 M_sun, from their main sequence until the late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Our model stars experience a strong C, N, and O envelope enrichment either due to the second dredge-up, the dredge-out phenomenon, or the third dredge-up early during the TP-(S)AGB phase. Their late evolution is therefore similar to that of higher metallicity objects. When using a standard prescription for the mass loss rates during the TP-(S)AGB phase, the computed stars lose most of their envelopes before their cores reach the Chandrasekhar mass, so our standard models do not predict the occurrence of SNI1/2 for Z = 10^-5 stars. However, we find that the reduction of only one order of magnitude in the mass-loss rates, which are particularly uncertain at this metallicity, would prevent the complete ejection of the envelope, allowing the stars to either explode as an SNI1/2 or become an electron-capture SN. Our calculations stop due to an instability near the base of the convective envelope that hampers further convergence and leaves remnant envelope masses between 0.25 M_sun for our 4 M_sun model and 1.5 M_sun for our 9 M_sun model. We present two sets of C, N, and O yields derived from our full calculations and computed under two different assumptions, namely, that the instability causes a practically instant loss of the remnant envelope or that the stars recover and proceed with further thermal pulses. Our results have implications for the early chemical evolution of the Universe.Comment: 12 pages, 13 figures, accepted for publication in A&

Interconnection

SIGLEAvailable from British Library Document Supply Centre-DSC:Vf98/0340 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

The evolution and C, N and O yields of intermediate-mass Z = 10-5 stars in isolation and in close binary systems

We have computed the evolution of Z = 10−5 stars of masses between 4 and 9 M , from their main sequence till the late TP-(S)AGB phase.We use a recent version of the Mount Stromlo Stellar Evolution code, in which molecular opacities include the effects of variable C/O abundances ratio, [1]. By computing hundreds (or thousands) of thermal pulses, we have been able either to remove the bulk of the stellar envelopes or to obtain stellar cores very close to MCh. Using [2] prescription for the mass loss rates the computed stars lose their envelopes before their cores reach MCh. This would forbid the occurrence of SN 1.5 for Z = 10−5 stars. Nevertheless the results by [3] suggest that the former prescription might overestimate the mass-loss rates. Therefore we have decreased the rates by [2]. For all the cases we present, even a decrease of one order of magnitude let the stellar cores reach MCh before the envelope is lost. Therefore the occurrence of SN1.5 at Z = 10 −5 and their potential contribution to the chemical evolution of the Universe should not be discarded. We consider the combined effects of the deep/corrosive 2 nd dredge-up and Roche Lobe Overflow (RLOF) during the E-AGB to help to constrain the contribution of massive Z = 10 −5 AGB stars to the CEMPs problem. Our results have implications for the chemical evolution of the Universe and might provide another piece for the puzzle of the CEMPs problem.Peer Reviewe

Evolution and CNO yields of Z = 10-5 stars and possible effects on carbon-enhanced metal-poor production

Aims. Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate the C, N, and O yields of these stars. Methods. Using the Monash University Stellar Evolution code MONSTAR we computed and analysed the evolution of stars of metallicity Z = 10-5 and masses between 4 and 9 M¿, from their main sequence until the late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Results. Our model stars experience a strong C, N, and O envelope enrichment either due to the second dredge-up process, the dredge-out phenomenon, or the third dredge-up early during the TP-(S)AGB phase. Their late evolution is therefore similar to that of higher metallicity objects. When using a standard prescription for the mass loss rates during the TP-(S)AGB phase, the computed stars are able to lose most of their envelopes before their cores reach the Chandrasekhar mass (mCh), so our standard models do not predict the occurrence of SNI1/2 for Z = 10-5 stars. However, we find that the reduction of only one order of magnitude in the mass-loss rates, which are particularly uncertain at this metallicity, would prevent the complete ejection of the envelope, allowing the stars to either explode as an SNI1/2 or become an electron-capture SN. Our calculations stop due to an instability near the base of the convective envelope that hampers further convergence and leaves remnant envelope masses between 0.25 M¿ for our 4 M¿ model and 1.5 M¿ for our 9 M¿ model. We present two sets of C, N, and O yields derived from our full calculations and computed under two different assumptions, namely, that the instability causes a practically instant loss of the remnant envelope or that the stars recover and proceed with further thermal pulses. Conclusions. Our results have implications for the early chemical evolution of the Universe and might provide another piece for the puzzle of the carbon-enhanced EMP star problem