arXiv subjects

Richard J. Stancliffe

Publications and source records attributed to Richard J. Stancliffe.

47 records · Page 3Linked to original sources

arXiv2009

The depletion of carbon by extra mixing in metal-poor giants

There is an apparent dichotomy between the metal-poor ([Fe/H] <= -2) yet carbon-normal giants and their carbon-rich counterparts. The former undergo significant depletion of carbon on the red giant branch after they have undergone first dredge-up, whereas the latter do not appear to experience significant depletion. We investigate this in the context that the extra mixing occurs via the thermohaline instability that arises due to the burning of helium-3. We present the evolution of [C/Fe], [N/Fe] and carbon-12/carbon-13 for three models: a carbon-normal metal-poor star, and two stars that have accreted material from a 1.5 solar mass AGB companion, one having received 0.01 solar masses of material and the other having received 0.1 solar masses. We find the behaviour of the carbon-normal metal-poor stars is well reproduced by this mechanism. In addition, our models also show that the efficiency of carbon-depletion is significantly reduced in carbon-rich stars. This extra-mixing mechanism is able to reproduce the observed properties of both carbon-normal and carbon-rich stars.

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arXiv2009

Modelling the binary progenitor of supernova 1993J

We have developed a detailed stellar evolution code capable of following the simultaneous evolution of both stars in a binary system, together with their orbital properties. To demonstrate the capabilities of the code we investigate potential progenitors for the Type IIb supernova 1993J, which is believed to have been an interacting binary system prior to its primary exploding. We use our detailed binary stellar evolution code to model this system to determine the possible range of primary and secondary masses that could have produced the observed characteristics of this system, with particular reference to the secondary. Using the luminosities and temperatures for both stars (as determined by Maund et al. 2004) and the remaining mass of the hydrogen envelope of the primary at the time of explosion, we find that if mass transfer is 100 per cent efficient the observations can be reproduced by a system consisting of a 15 solar mass primary and a 14 solar mass secondary in an orbit with an initial period of 2100 days. With a mass transfer efficiency of 50 per cent, a more massive system consisting of a 17 solar mass primary and a 16 solar mass secondary in an initial orbit of 2360 days is needed. We also investigate some of the uncertainties in the evolution, including the effects of tidal interaction, convective overshooting and thermohaline mixing.

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arXiv2009

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.

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arXiv2009

Why do low-mass stars become red giants?

We revisit the problem of why stars become red giants. We modify the physics of a standard stellar evolution code in order to determine what does and what does not contribute to a star becoming a red giant. In particular, we have run tests to try to separate the effects of changes in the mean molecular weight and in the energy generation. The implications for why stars become red giants are discussed. We find that while a change in the mean molecular weight is necessary (but not sufficient) for a 1 solar mass star to become a red giant, this is not the case in a star of 5 solar masses. It therefore seems that there may be more than one way to make a giant.

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arXiv2008

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 $s$-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.

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arXiv2008

Thermohaline mixing and gravitational settling in carbon-enhanced metal-poor stars

We investigate the formation of carbon-enhanced metal-poor (CEMP) stars via the scenario of mass transfer from a carbon-rich asymptotic giant branch (AGB) primary to a low-mass companion in a binary system. We explore the extent to which material accreted from a companion star becomes mixed with that of the recipient, focusing on the effects of thermohaline mixing and gravitational settling. We have created a new set of asymptotic giant branch models in order to determine what the composition of material being accreted in these systems will be. We then model a range of CEMP systems by evolving a grid of models of low-mass stars, varying the amount of material accreted by the star (to mimic systems with different separations) and also the composition of the accreted material (to mimic accretion from primaries of different mass). We find that with thermohaline mixing alone, the accreted material can become mixed with between 16 and 88 per cent of the pristine stellar material of the accretor, depending on the mass accreted and the composition of the material. If we include the effects of gravitational settling, we find that thermohaline mixing can be inhibited and, in the case that only a small quantity of material is accreted, can be suppressed almost completely.

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arXiv2007

Carbon-enhanced metal-poor stars and thermohaline mixing

One possible scenario for the formation of carbon-enhanced metal-poor stars is the accretion of carbon-rich material from a binary companion which may no longer visible. It is generally assumed that the accreted material remains on the surface of the star and does not mix with the interior until first dredge-up. However, thermohaline mixing should mix the accreted material with the original stellar material as it has a higher mean molecular weight. We investigate the effect that this has on the surface abundances by modelling a binary system of metallicity Z=0.0001 with a 2 solar mass primary star and a 0.74 solar mass secondary star in an initial orbit of 4000 days. The accretion of material from the wind of the primary leads to the formation of a carbon-rich secondary. We find that the accreted material mixes fairly rapidly throughout 90% of the star, with important consequences for the surface composition. Models with thermohaline mixing predict very different surface abundances after first dredge-up compared to canonical models of stellar evolution.

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arXiv2006

Galactic Sodium from AGB Stars

Galactic chemical evolution models which include sodium from type II supernovae alone underestimate the abundance of sodium in the interstellar medium by a factor of two to three over about 3 dex in metallicity and predict a flat behaviour in the evolution of [Na/Fe] at super-solar metallicities. Conversely, recent observations of stars with [Fe/H]~+0.4 suggest that [Na/Fe] increases at high metallicity. We have combined stellar evolution models of asymptotic giant branch and Wolf-Rayet stars with the latest supernova yields in an attempt to resolve these problems ... and have created many more.

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arXiv2006

Mass loss and yield uncertainty in low-mass asymptotic giant branch stars

We investigate the uncertainty in surface abundances and yields of asymptotic giant branch (AGB) stars. We apply three different mass loss laws to a 1.5 solar mass star of metallicity Z=0.008 at the beginning of the thermally pulsing asymptotic giant branch (TP-AGB) phase. Efficient third dredge-up is found even at very low envelope mass, contrary to previous simulations with other evolution codes. We find that the yield of carbon is uncertain by about 15% and for most other light elements the yield is uncertain at the level of 20-80%. For iron group elements the uncertainty varies from around 30% for the more abundant species to over a factor of two for the less abundant radioactive species, like iron-60. The post-AGB surface abundances for this mass and metallicity are much more uncertain due to the dilution of dredged-up material in differing envelope masses in the later stages of the models. Our results are compared to known planetary nebula (PN) and post-AGB abundances. We find that the models are mostly consistent with observations but we are unable to reproduce observations of some of the isotopes.

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arXiv2006

Does simultaneous solution matter for stellar evolution codes?

A version of the STARS stellar evolution code has been developed that uses a non-simultaneous solution of the equations of stellar structure and evolution. In all other respects it is identical to the normal, fully simultaneous version. It is therefore possible to test the dependence of the solution on how the equations are solved. Two cases are investigated: a 5Msol and a 3Msol star, both of metallicity Z=0.02. Prior to the asymptotic giant branch, the models are almost identical. However once thermal pulses start, the two methods of solution yield diverging results with the non-simultaneous technique predicting longer interpulse periods. This is traced to difficulties associated with hydrogen burning caused by the use of a moving mesh. It is shown that, with careful control of the temporal resolution, the results of the simultaneous technique can be recovered.

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arXiv2004

Third Dredge-up in Low Mass Stars: Solving the LMC Carbon Star Mystery

A long standing problem with asymptotic giant branch (AGB) star models has been their inability to produce the low-luminosity carbon stars in the Large and Small Magellanic Clouds. Dredge-up must begin earlier and extend deeper. We find this for the first time in our models of LMC metallicity. Such features are not found in our models of SMC metallicity. The fully implicit and simultaneous stellar evolution code STARS has been used to calculate the evolution of AGB stars with metallicities of Z=0.008 and Z=0.004, corresponding to the observed metallicities of the Large and Small Magellanic Clouds, respecitively. Third dredge-up occurs in stars of 1Msol and above and carbon stars were found for models between 1Msol and 3Msol. We use the detailed models as input physics for a population synthesis code and generate carbon star luminosity functions. We now find that we are able to reproduce the carbon star luminosity function of the LMC without any manipulation of our models. The SMC carbon star luminosity function still cannot be produced from our detailed models unless the minimum core mass for third dredge-up is reduced by 0.06Msol.

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