9 research outputs found
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