Towards Multiple-Star Population Synthesis

The multiplicities of stars, and some other properties, were collected recently by Eggleton & Tokovinin, for the set of 4559 stars with Hipparcos magnitude brighter than 6.0 (4558 excluding the Sun). In this paper I give a numerical recipe for constructing, by a Monte Carlo technique, a theoretical ensemble of multiple stars that resembles the observed sample. Only multiplicities up to 8 are allowed; the observed set contains only multiplicities up to 7. In addition, recipes are suggested for dealing with the selection effects and observational uncertainties that attend the determination of multiplicity. These recipes imply, for example, that to achieve the observed average multiplicity of 1.53, it would be necessary to suppose that the real population has an average multiplicity slightly over 2.0. This numerical model may be useful for (a) comparison with the results of star and star cluster formation theory, (b) population synthesis that does not ignore multiplicity above 2, and (c) initial conditions for dynamical cluster simulations

It's EZ to Evolve ZAMS Stars: A Program Derived from Eggleton's Stellar Evolution Code

"Evolve ZAMS", "EZ" for short, is derived from Peter Eggleton's stellar evolution program. The core of EZ is a stripped down, rewritten version of a subset of Eggleton's code, specialized to handle single star evolution from the zero-age main sequence until forced to stop by an event such as a helium flash or a crystallizing core. The procedure and data interfaces to the program are designed to be easy to use while still providing a wide range of function. EZ is written in Fortran 95 following current programming practices and can be downloaded from http://theory.kitp.ucsb.edu/~paxton/.Comment: 2 pages. To appear in PASP. Download tar file with source code, data, and instructions for building EZ from http://theory.kitp.ucsb.edu/~paxton/ -- website has more information and pdf's for many plots of stellar evolutio

Evolution of Intermediate-Mass Black Hole X-Ray Binaries

The majority of the ultraluminous X-ray sources (ULXs) in external galaxies are believed to be accreting black holes in binary systems; some of the black holes could be as massive as \sim 100-1000 \ms. We have performed evolution calculations for intermediate-mass black hole X-ray binaries, assuming they are formed in dense star clusters via tidal capture. The results are compared with those for stellar-mass black holes X-ray binaries. We find that these two types of black holes may have similar companion stars and binary orbits if observed as ULXs. However, intermediate-mass black holes seem to be favored in explaining the most luminous ULXs. We also discuss the possibilities of transient behavior and beamed emission in the evolution of these binary systems.Comment: 11 pages, 3 figures. Accepted for publication in ApJ

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).Comment: Replaced with accepted version to appear in ApJ, 30 pages, inc. 6 figure

The Zero Age Main Sequence of WIMP burners

We modify a stellar structure code to estimate the effect upon the main sequence of the accretion of weakly interacting dark matter onto stars and its subsequent annihilation. The effect upon the stars depends upon whether the energy generation rate from dark matter annihilation is large enough to shut off the nuclear burning in the star. Main sequence WIMP burners look much like protostars moving on the Hayashi track, although they are in principle completely stable. We make some brief comments about where such stars could be found, how they might be observed and more detailed simulations which are currently in progress. Finally we comment on whether or not it is possible to link the paradoxically young OB stars found at the galactic centre with WIMP burners.Comment: 4 pages, 3 figs. Matches published versio

Deep Mixing of He-3: Reconciling Big Bang and Stellar Nucleosynthesis

Low-mass stars, ~1-2 solar masses, near the Main Sequence are efficient at producing He-3, which they mix into the convective envelope on the giant branch and should distribute into the Galaxy by way of envelope loss. This process is so efficient that it is difficult to reconcile the low observed cosmic abundance of He-3 with the predictions of both stellar and Big Bang nucleosynthesis. In this paper we find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell, where a nuclear reaction lowers the mean molecular weight slightly. Thus we are able to remove the threat that He-3 production in low-mass stars poses to the Big Bang nucleosynthesis of He-3.Comment: Accepted by Science, and available from Science Express onlin

Compulsory Deep Mixing of 3He and CNO Isotopes in the Envelopes of low-mass Red Giants

Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the 3He(3He,2p)4He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of 3He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades' standing. This mixing mechanism, which we call `$\delta\mu$-mixing', operates rapidly (relative to the nuclear timescale of overall evolution, ~ 10^8 yrs) once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Pop I stars between 0.8 and 2.0\Msun develop 12C/13C ratios of 14.5 +/- 1.5, while Pop II stars process the carbon to ratios of 4.0 +/- 0.5. In stars less than 1.25\Msun, this mechanism also destroys 90% to 95% of the 3He produced on the main sequence.Comment: Final accepted version (submitted to Astrophys J in Jan 2007...

Approximate input physics for stellar modelling

We present a simple and efficient, yet reasonably accurate, equation of state, which at the moderately low temperatures and high densities found in the interiors of stars less massive than the Sun is substantially more accurate than its predecessor by Eggleton, Faulkner & Flannery. Along with the most recently available values in tabular form of opacities, neutrino loss rates, and nuclear reaction rates for a selection of the most important reactions, this provides a convenient package of input physics for stellar modelling. We briefly discuss a few results obtained with the updated stellar evolution code.Comment: uuencoded compressed postscript. The preprint are also available at http://www.ast.cam.ac.uk/preprint/PrePrint.htm