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C. A. Tout

Publications and source records attributed to C. A. Tout.

At least 37 records · Page 2Linked to original sources
arXiv2004

The convective Urca process

One possible fate of an accreting white dwarf is explosion in a type Ia supernova. However, the route to the thermonuclear runaway has always been uncertain owing to the lack of a convective model consistent with the Urca process. We derive a formalism for convective motions involving two radial flows. This formalism provides a framework for convective models that guarantees self-consistency for chemistry and energy budget, allows time-dependence and describes the interaction of convective motions with the global contraction or expansion of the star. In the one-stream limit, we reproduce several already existing convective models and allow them to treat chemistry. We also suggest as a model easy to implement in a stellar evolution code. We apply this formalism to convective Urca cores in Chandrasekhar mass white dwarfs. We stress that in degenerate matter, nuclear reactions that change the number of electrons strongly influence the convective velocities. We point out the sensitivity of the energy budget on the mixing. We illustrate our model by computing {\it stationary} convective cores with Urca nuclei. We show that even a very small mass fraction of Urca nuclei ($10^{-8}$) strongly influences the convective velocities. Finally, we present preliminary computations of the late evolution of a close to Chandrasekhar mass C+O white dwarf including the convective Urca process.

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arXiv2004

A two-stream formalism for the convective Urca process

We derive a new formalism for convective motions involving two radial flows. This formalism provides a framework for convective models that guarantees consistency for the chemistry and the energy budget in the flows, allows time-dependence and accounts for the interaction of the convective motions with the global contraction or expansion of the star. In the one-stream limit the formalism reproduces several existing convective models and allows them to treat the chemistry in the flows. We suggest a version of the formalism that can be implemented easily in a stellar evolution code. We then apply the formalism to convective Urca cores in Chandrasekhar mass white dwarfs and compare it to previous studies. We demonstrate that, in degenerate matter, nuclear reactions that change the number of electrons strongly influence the convective velocities and we show that the net energy budget is sensitive to the mixing. We illustrate our model by computing stationary convective cores with Urca nuclei. Even a very small mass fraction of Urca nuclei (as little as $10^{-8}$) strongly influences the convective velocities. We conclude that the proper modelling of the Urca process is essential for determining the ignition conditions for the thermonuclear runaway in Chandrasekhar-mass white dwarfs.

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arXiv2004

Reconstructing the evolution of white dwarf binaries: further evidence for an alternative algorithm for the outcome of the common-envelope phase in close binaries

We determine the possible masses and radii of the progenitors of white dwarfs in binaries from fits to detailed stellar evolution models and use these to reconstruct the mass-transfer phase in which the white dwarf was formed. We confirm the earlier finding that in the first phase of mass transfer in the binary evolution leading to a close pair of white dwarfs, the standard common-envelope formalism equating the energy balance in the system, does not work. An algorithm equating the angular momentum balance can explain the observations. This conclusion is now based on ten observed systems rather than three. With the latter algorithm the separation does not change much for approximately equal mass binaries. Assuming constant efficiency in the standard α-formalism and a constant value of γ, we investigate the effect of both methods on the change in separation in general and conclude that when there is observational evidence for strong shrinkage of the orbit, the γ-algorithm also leads to this. We then extend our analysis to all close binaries with at least one white dwarf component and reconstruct the mass transfer phases that lead to these binaries. We find that all observations can be explained with a single value of γ, making the γ-algorithm a useful tool to predict the outcome of common-envelope evolution. We discuss the consequences of our findings for different binary populations in the Galaxy, including massive binaries, for which the reconstruction method cannot be used (abbriged).

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arXiv2004

Stellar Chemical Signatures And Hierarchical Galaxy Formation

ABRIDGED: To compare the chemistries of stars in the Milky Way dSph galaxies with stars in the Galaxy, we have compiled a large sample of Galactic stellar abundances from the literature. As found in previous studies, the [alpha/Fe] ratios of most stars in the dSph galaxies are generally lower than similar metallicity Galactic stars in this extended sample. Our kinematically selected stars confirm that this is true for all major stellar components of the Galaxy, including the halo, thin disk, and thick disk. There is marginal overlap in the low [alpha/Fe] ratios between dSph stars and Galactic halo stars on extreme retrograde orbits, but this is not supported by other element ratios, like [Ba/Y]. Thus, the full chemical signature of most of the dSph stars are distinct from the main components of the Galaxy. This result rules out continuous merging of low mass galaxies similar to these dSph satellites during the formation of the Galaxy, which is discuss further. The new environments offered by the dSph galaxies also allow us to examine fundamental assumptions related to nucleosynthesis. The metal-poor stars ([Fe/H] < -1.8) in the dSph galaxies have [alpha/Fe] ratio that are consistent with predictions from the alpha-process (alpha-rich freeze out). This may also explain the very low Y abundances, and could suggest a separate r-process site for this light (first peak r-process) element. In stars with higher metallicities ([Fe/H] > -1.8), contributions from the s-process are expected; [Ba/Y] is still much higher in the dSph stars than similar metallicity Galactic stars though, consistent with s-process contributions from only the low metallicity AGB stars in dSph galaxies. Finally, the Na-Ni trend in Galactic halo stars is confirmed, but discuss this in terms of nucleosynthesis, and not the accretion of dSphs.

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arXiv2004

The Progenitors of Core-Collapse Supernovae

We present maps of the nature of single star progenitors of supernovae and their remnants in mass and metallicity space. We find our results are similar to others but we have gone further in varying the amount of mixing and using various mass-loss schemes to see how the maps change. We find that extra-mixing, in the form of convective overshooting, moves boundaries such as the minimum mass for a supernova or WR star to lower masses. We also find that the pre-WR mass-loss determines the shape of our maps. We find that different mass-loss rates lead to quite different results. We find that the rise in luminosity at 2nd dredge-up places quite tight constraints on the masses of some progenitors and in particular the progenitor of supernova 2003gd.

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arXiv2004

Deep Dredge-up in Intermediate-Mass TP-AGB Stars

We present results of the evolution of AGB stars of 3M_sol and 5M_sol with solar metallicity calculated with the Eggleton stellar evolution code (STARS), which has a fully implicit and simultaneous method for solving for the stellar structure, convective mixing and nuclear burning. We introduce the concept of a viscous mesh in order to improve the numerical stability of the calculations. For the 5M_sol star, we evolve through 25 thermal pulses and their associated third dredge-up events. We obtain a maximum helium luminosity of 1.7x10^9 L_sol and significantly deep dredge-up after the second pulse. Strong hot-bottom burning is observed after the 5th pulse. The 3M_sol model is evolved through 20 thermal pulse events and we find third dredge-up after the 7th pulse. During the 14th pulse sufficient carbon has been brought to the surface to produce a carbon star. We find that dredge-up and the transformation into a carbon star occur at significantly smaller core masses (0.584M_sol and 0.608M_sol, respectively) than in previous calculations for 3M_sol.

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arXiv2004

Detection of a red supergiant progenitor star of a type II-plateau supernova

We present the discovery of a red supergiant star that exploded as supernova 2003gd in the nearby spiral galaxy M74. The Hubble Space Telescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9 months before the supernova explosion and subsequent HST images confirm the positional coincidence of the supernova with a single,resolved star which is an 8 +4/-2 solar mass red supergiant. This confirms both stellar evolution models and supernova theories which predict that type II-Plateau supernovae have cool red supergiants as their immediate progenitor stars.

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arXiv2003

A more detailed look at the Opacities for Enriched Carbon and Oxygen Mixtures

We have included opacity tables in our stellar evolution code that enable us to accurately model the structure of stars composed of mixtures with carbon and oxygen independently enhanced relative to solar. We present tests to demonstrate the effects of the new tables. Two of these are practical examples, the effect on the evolution of a thermally pulsing asymptotic giant branch star and a Wolf-Rayet Star. The changes are small but perceptible.

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arXiv2003

Mass limits for the progenitor star of supernova 2001du and other type II-P supernovae

The supernova SN2001du in the galaxy NGC1365 (19+/-2Mpc), is a core-collapse event of type II-P. Images of this galaxy, have been taken with HST approximately 6.6 years before discovery and include the supernova position on the WFPC2 field of view. We have observed the supernova with the WFPC2 to allow accurate differential astrometry of SN2001du on the pre-explosion frames. There is a marginal detection (3-sigma) of a source close to the supernova position on the prediscovery V-band frame, however it is not precisely coincident and we do not believe it to be a robust detection of a point source. We conclude that there is no stellar progenitor at the supernova position and derive sensitivity limits of the prediscovery images which provide an upper mass limit for the progenitor star. We estimate that the progenitor had a mass of less than 15M_sol. We revisit two other nearby SNe II-P which have high quality pre-explosion images, and refine the upper mass limits for the progenitor stars. Finally we compile all the direct information available for the progenitors of eight nearby core-collapse supernovae and compare their mass estimates. These are compared with the latest stellar evolutionary models of pre-supernova evolution which have attempted to relate metallicity and mass to the supernovae type. Reasonable agreement is found for the lower mass events (generally the II-P), but some discrepancies appear at higher masses. (abridged).

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arXiv2001

The nature of the progenitor of the Type II-P supernova 1999em

We present high quality ground-based VRI images of the site of the Type II-P SN1999em (in NGC1637) taken before explosion, which were extracted from the CFHT archive. We determine a precise position of the SN on these images to an accuracy of 0.17''. The host galaxy is close enough (7.5 +/- 0.5 Mpc) that the bright supergiants are resolved as individual objects, however we show that there is no detection of an object at the SN position before explosion that could be interpreted as the progenitor star. By determining the sensitivity limits of the VRI data, we derive bolometric luminosity limits for the progenitor. Comparing these to standard stellar evolutionary tracks which trace evolution up to the point of core carbon ignition, we initially derive an upper mass limit of approximately 12M_sol. However we present evolutionary calculations that follow 7-12M_sol stars throughout their C-burning lifetime and show that we can restrict the mass of the progenitor even further. Our calculations indicate that progenitors initially of 8-10M_sol, undergoing expected mass loss, can also be excluded because a second dredge up sends them to somewhat higher luminosities than a star of initially 12M_sol. These results limit the progenitor's initial main-sequence mass to a very narrow range of 12 +/- 1 M_sol. We discuss the similarities between the Type II-P SNe 1999em and 1999gi and their progenitor mass limits, and suggest that SN Type II-P originate only in intermediate mass stars of 8-12M_sol, which are in the red supergiant region and that higher mass stars produce the other Type II sub-types. (Abridged).

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arXiv2001

An upper mass limit for the progenitor of the TypeII-P supernova SN1999gi

Masses and progenitor evolutionary states of TypeII supernovae remain almost unconstrained by direct observations. Only one robust observation of a progenitor (SN1987A) and one plausible observation (SN1993J) are available. Neither matched theoretical predictions and in this Letter we report limits on a third progenitor (SN1999gi). The Hubble Space Telescope has imaged the site of the TypeII-P supernova SN1999gi with the WFPC2 in two filters (F606W and F300W) prior to explosion. The distance to the host galaxy (NGC3184) of 7.9Mpc means that the most luminous, massive stars are resolved as single objects in the archive images. The supernova occurred in a resolved, young OB association 2.3kpc from the centre of NGC3184 with an association age of about 4Myrs. Follow-up images of SN1999gi with WFPC2 taken 14 months after discovery determine the precise position of the SN on the pre-explosion frames. An upper limit of the absolute magnitude of the progenitor is estimated (M_v >= -5.1). By comparison with stellar evolutionary tracks this can be interpreted as a stellar mass, and we determine an upper mass limit of 9(+3/-2)M_solar. We discuss the possibility of determining the masses or mass limits for numerous nearby core-collapse supernovae using the HST archive enhanced by our current SNAP programme.

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arXiv2001

Russell Lecture: Dark Star Formation and Cooling Instability

Optically thin cooling gas at most temperatures above 30K will make condensations by pressure pushing material into cool dense regions. This works without gravity. Cooling condensations will flatten and become planar/similarity solutions. Most star formation may start from cooling condensations - with gravity only important in the later stages. The idea that some of the dark matter could be pristine white dwarfs that condensed slowly on to planetary sized seeds without firing nuclear reactions is found lacking. However, recent observations indicate fifty times more halo white dwarfs than have been previously acknowledged; enough to make the halo fraction observed as MACHOS. A cosmological census shows that only 1% of the mass of the Universe is of known constitution.

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arXiv2000

Observational Implications of Precessing Protostellar Discs and Jets

We consider the dynamics of a protostellar disc in a binary system where the disc is misaligned with the orbital plane of the binary, with the aim of determining the observational consequences for such systems. The disc wobbles with a period approximately equal to half the binary's orbital period and precesses on a longer timescale. We determine the characteristic timescale for realignment of the disc with the orbital plane due to dissipation. If the dissipation is determined by a simple isotropic viscosity then we find, in line with previous studies, that the alignment timescale is of order the viscous evolution timescale. However, for typical protostellar disc parameters, if the disc tilt exceeds the opening angle of the disc, then tidally induced shearing within the disc is transonic. In general, hydrodynamic instabilities associated with the internally driven shear result in extra dissipation which is expected to drastically reduce the alignment timescale. For large disc tilts the alignment timescale is then comparable to the precession timescale, while for smaller tilt angles $δ$, the alignment timescale varies as $(\sin δ)^{-1}$. We discuss the consequences of the wobbling, precession and rapid realignment for observations of protostellar jets and the implications for binary star formation mechanisms.

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arXiv2000

Comprehensive analytic formulae for stellar evolution as a function of mass and metallicity

We present analytic formulae that approximate the evolution of stars for a wide range of mass and metallicity. Stellar luminosity, radius and core mass are given as a function of age, M and Z, for all phases from the zero-age main-sequence up to, and including, the remnant stages. For the most part we find continuous formulae accurate to within 5% of detailed models. These formulae are useful for purposes such as population synthesis that require very rapid but accurate evaluation of stellar properties, and in particular for use in combination with N-body codes. We describe a mass loss prescription that can be used with these formulae and investigate the resulting stellar remnant distribution.

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arXiv1998

Accretion Disc Evolution in Single and Binary T Tauri Stars

We present theoretical models for the evolution of T Tauri stars surrounded by circumstellar discs. The models include the effects of pre-main-sequence stellar and time dependent disc evolution, and incorporate the effects of stellar magnetic fields acting on the inner disc. For single stars, consistency with observations in Taurus-Auriga demands that disc dispersal occurs rapidly, on much less than the viscous timescale of the disc, at roughly the epoch when heating by stellar radiation first dominates over internal viscous dissipation. Applying the models to close binaries, we find that because the initial conditions for discs in binaries are uncertain, studies of extreme mass ratio systems are required to provide a stringent test of theoretical disc evolution models. We also note that no correlation of the infra-red colours of T Tauri stars with their rotation rate is observed, in apparent contradiction to the predictions of simple magnetospheric accretion models.

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arXiv1996

Tidal Decay of Close Planetary Orbits

The 4.2-day orbit of the newly discovered planet around 51~Pegasi is formally unstable to tidal dissipation. However, the orbital decay time in this system is longer than the main-sequence lifetime of the central star. Given our best current understanding of tidal interactions, a planet of Jupiter's mass around a solar-like star could have dynamically survived in an orbit with a period as short as $\sim10\,$hr. Since radial velocities increase with decreasing period, we would expect to find those planets close to the tidal limit first and, unless this is a very unusual system, we would expect to find many more. We also consider the tidal stability of planets around more evolved stars and we re-examine in particular the question of whether the Earth can dynamically survive the red-giant phase in the evolution of the Sun.

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