994 research outputs found
Light element abundances in carbon-enhanced metal-poor stars
We model the evolution of the abundances of light elements in carbon-enhanced
metal-poor (CEMP) stars, under the assumption that such stars are formed by
mass transfer in a binary system. We have modelled the accretion of material
ejected by an asymptotic giant branch star on to the surface of a companion
star. We then examine three different scenarios: one in which the material is
mixed only by convective processes, one in which thermohaline mixing is present
and a third in which both thermohaline mixing and gravitational settling are
taken in to account. The results of these runs are compared to light element
abundance measurements in CEMP stars (primarily CEMP-s stars, which are rich in
-processes elements and likely to have formed by mass transfer from an AGB
star), focusing on the elements Li, F, Na and Mg. None of the elements is able
to provide a conclusive picture of the extent of mixing of accreted material.
We confirm that lithium can only be preserved if little mixing takes place. The
bulk of the sodium observations suggest that accreted material is effectively
mixed but there are also several highly Na and Mg-rich objects that can only be
explained if the accreted material is unmixed. We suggest that the available
sodium data may hint that extra mixing is taking place on the giant branch,
though we caution that the data is sparse.Comment: Accepted for publication in MNRAS. 9 figures, 1 tabl
The effects of thermohaline mixing on low-metallicity asymptotic giant branch stars
We examine the effects of thermohaline mixing on the composition of the
envelopes of low-metallicity asymptotic giant branch (AGB) stars. We have
evolved models of 1, 1.5 and 2 solar masses from the pre-main sequence to the
end of the thermally pulsing asymptotic giant branch with thermohaline mixing
applied throughout the simulations. In agreement with other authors, we find
that thermohaline mixing substantially reduces the abundance of helium-3 on the
upper part of the red giant branch in our lowest mass model. However, the small
amount of helium-3 that remains is enough to drive thermohaline mixing on the
AGB. We find that thermohaline mixing is most efficient in the early thermal
pulses and its efficiency drops from pulse to pulse. Nitrogen is not
substantially affected by the process, but we do see substantial changes in
carbon-13. The carbon-12 to carbon-13 ratio is substantially lowered during the
early thermal pulses but the efficacy of the process is seen to diminish
rapidly. As the process stops after a few pulses, the carbon-12 to carbon-13
ratio is still able to reach values of 10^3-10^4, which is inconsistent with
the values measured in carbon-enhanced metal-poor stars. We also note a
surprising increase in the lithium-7 abundance, with log epsilon(Li-7) reaching
values of over 2.5 in the 1.5 solar mass model. It is thus possible to get
stars which are both C- and Li-rich at the same time. We compare our models to
measurements of carbon and lithium in carbon-enhanced metal-poor stars which
have not yet reached the giant branch. These models can simultaneously
reproduced the observed C and Li abundances of carbon-enhanced metal-poor
turn-off stars that are Li-rich, but the observed nitrogen abundances still
cannot be matched.Comment: Accepted for publication in MNRAS. 12 pages, 7 figure
The evolution of low-metallicity asymptotic giant branch stars and the formation of carbon-enhanced metal-poor stars
We investigate the behaviour of asymptotic giant branch (AGB) stars between
metallicities Z = 10-4 and Z = 10-8 . We determine which stars undergo an
episode of flash-driven mixing, where protons are ingested into the intershell
convection zone, as they enter the thermally pulsing AGB phase and which
undergo third dredge-up. We find that flash-driven mixing does not occur above
a metallicity of Z = 10-5 for any mass of star and that stars above 2 M do not
experience this phenomenon at any metallicity. We find carbon ingestion (CI),
the mixing of carbon into the tail of hydrogen burning region, occurs in the
mass range 2 M to around 4 M . We suggest that CI may be a weak version of the
flash-driven mechanism. We also investigate the effects of convective
overshooting on the behaviour of these objects. Our models struggle to explain
the frequency of CEMP stars that have both significant carbon and nitrogen
enhancement. Carbon can be enhanced through flash-driven mixing, CI or just
third dredge up. Nitrogen can be enhanced through hot bottom burning and the
occurrence of hot dredge-up also converts carbon into nitrogen. The C/N ratio
may be a good indicator of the mass of the primary AGB stars.Comment: 15 pages, 13 figures, 1 table, accepted by MNRA
Population Synthesis of Binary Carbon-enhanced Metal-poor Stars
The carbon-enhanced metal-poor (CEMP) stars constitute approximately one
fifth of the metal-poor ([Fe/H] ~< -2) population but their origin is not well
understood. The most widely accepted formation scenario, invokes mass-transfer
of carbon-rich material from a thermally-pulsing asymptotic giant branch
(TPAGB) primary star to a less massive main-sequence companion which is seen
today. Recent studies explore the possibility that an initial mass function
biased toward intermediate-mass stars is required to reproduce the observed
CEMP fraction in stars with metallicity [Fe/H] < -2.5. These models also
implicitly predict a large number of nitrogen-enhanced metal-poor (NEMP) stars
which is not seen. We investigate whether the observed CEMP and NEMP to
extremely metal-poor (EMP) ratios can be explained without invoking a change in
the initial mass function.
We confirm earlier findings that with current detailed TPAGB models the large
observed CEMP fraction cannot be accounted for. We find that efficient third
dredge up in low-mass (less than 1.25Msun), low-metallicity stars may offer at
least a partial explanation to the large observed CEMP fraction while remaining
consistent with the small observed NEMP fraction.Comment: 20 pages, 23 figures, accepted for publication in A&
Carbon Rich Extremely Metal Poor Stars: Signatures of Population-III AGB stars in Binary Systems
We use the Cambridge stellar evolution code STARS to model the evolution and
nucleosynthesis of zero-metallicity intermediate-mass stars. We investigate the
effect of duplicity on the nucleosynthesis output of these systems and the
potential abundances of the secondaries. The surfaces of zero-metallicity stars
are enriched in CNO elements after second dredge up. During binary interaction,
such as Roche lobe overflow or wind accretion, metals can be released from
these stars and the secondaries enriched in CNO isotopes. We investigate the
formation of the two most metal poor stars known, HE 0107-5240 and HE
1327-2326. The observed carbon and nitrogen abundances of HE 0107-5240 can be
reproduced by accretion of material from the companion-enhanced wind of a seven
solar star after second dredge-up, though oxygen and sodium are underproduced.
We speculate that HE 1327-2326, which is richer in nitrogen and strontium, may
similarly be formed by wind accretion in a later AGB phase after third
dredge-up.Comment: 16 pages, 1 figure, 7 tables, accepted by MNRA
Mass Action Modeling of Catalysis Through Reaction Flux Estimation
The complexity of biochemical networks necessitates the use of computational and mathematical frameworks to accurately characterize and study these systems. However, modern frameworks developed for this task have inadequacies that limit their accuracy or scalability. In this report, a mathematical model of the canonical enzyme substrate binding network is developed, and, using estimated true and maximal reaction rates, a methodology utilizing principles of flux balance analysis is developed to deduce the individual reaction rate constants in the network. It is then shown that these two reaction rates are not sufficient to unambiguously define a mass action kinetic model of this network. Nevertheless, the methodology developed greatly reduces the degrees of freedom of the system, and, as a result, the solution space of the network can be examined computationally and analytically revealing several non-intuitive sensitivities
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