Evolution of binary stars and the effect of tides on binary populations

We present a rapid binary evolution algorithm that enables modelling of even the most complex binary systems. In addition to all aspects of single star evolution, features such as mass transfer, mass accretion, common-envelope evolution, collisions, supernova kicks and angular momentum loss mechanisms are included. In particular, circularization and synchronization of the orbit by tidal interactions are calculated for convective, radiative and degenerate damping mechanisms. We use this algorithm to study the formation and evolution of various binary systems. We also investigate the effect that tidal friction has on the outcome of binary evolution. Using the rapid binary code, we generate a series of large binary populations and evaluate the formation rate of interesting individual species and events. By comparing the results for populations with and without tidal friction we quantify the hitherto ignored systematic effect of tides and show that modelling of tidal evolution in binary systems is necessary in order to draw accurate conclusions from population synthesis work. Tidal synchronism is important but because orbits generally circularize before Roche-lobe overflow the outcome of the interactions of systems with the same semi-latus rectum is almost independent of eccentricity. It is not necessary to include a distribution of eccentricities in population synthesis of interacting binaries, however, the initial separations should be distributed according to the observed distribution of semi-latera recta rather than periods or semi-major axes.Comment: 36 pages, 12 figures, to be published in the Monthly Notices of the Royal Astronomical Societ

Direct N-body Modelling of Stellar Populations: Blue Stragglers in M67

We present a state-of-the-art N-body code which includes a detailed treatment of stellar and binary evolution as well as the cluster dynamics. This code is ideal for investigating all aspects relating to the evolution of star clusters and their stellar populations. It is applicable to open and globular clusters of any age. We use the N-body code to model the blue straggler population of the old open cluster M67. Preliminary calculations with our binary population synthesis code show that binary evolution alone cannot explain the observed numbers or properties of the blue stragglers. On the other hand, our N-body model of M67 generates the required number of blue stragglers and provides formation paths for all the various types found in M67. This demonstrates the effectiveness of the cluster environment in modifying the nature of the stars it contains and highlights the importance of combining dynamics with stellar evolution. We also perform a series of N = 10000 simulations in order to quantify the rate of escape of stars from a cluster subject to the Galactic tidal field.Comment: 26 pages, 18 figures, accepted for publication in MNRA

A Complete N-body Model of the Old Open Cluster M67

The old open cluster M67 is an ideal testbed for current cluster evolution models because of its dynamically evolved structure and rich stellar populations that show clear signs of interaction between stellar, binary and cluster evolution. Here we present the first truly direct N-body model for M67, evolved from zero age to 4 Gyr taking full account of cluster dynamics as well as stellar and binary evolution. Our preferred model starts with 12000 single stars and 12000 binaries placed in a Galactic tidal field at 8.0 kpc from the Galactic Centre. Our choices for the initial conditions and for the primordial binary population are explained in detail. At 4 Gyr, the age of M67, the total mass has reduced by 90% as a result of mass loss and stellar escapes. The mass and half-mass radius of luminous stars in the cluster are a good match to observations although the model is more centrally concentrated than observations indicate. The stellar mass and luminosity functions are significantly flattened by preferential escape of low-mass stars. We find that M67 is dynamically old enough that information about the initial mass function is lost, both from the current luminosity function and from the current mass fraction in white dwarfs. The model contains 20 blue stragglers at 4 Gyr which is slightly less than the 28 observed in M67. Nine are in binaries. The blue stragglers were formed by a variety of means and we find formation paths for the whole variety observed in M67. Both the primordial binary population and the dynamical cluster environment play an essential role in shaping the population. A substantial population of short-period primordial binaries (with periods less than a few days) is needed to explain the observed number of blue stragglers in M67.Comment: 32 pages, 17 figures, submitted to MNRA

Could Edge-Lit Type Ia Supernovae be Standard Candles

The progenitors of Type Ia supernovae (SNe Ia) have not been identified. Though they are no longer fashionable we investigate the consequences if a significant number of SNe Ia were edge-lit detonations (ELDs) of carbon/oxygen white dwarfs that have accreted a critical mass of helium. Our best understanding of the Phillips relation between light curve speed and peak luminosity assigns both these phenomena to the amount of $^{56}$Ni produced. In ELDs there are two sites of $^{56}$Ni synthesis. If the peak luminosity is determined primarily by the C/O ratio in the core it is primarily a function of its progenitor's initial mass. If the light curve decay speed is determined by the total mass of iron group elements ejected this is a function of the total mass of the ELD at the time of explosion. In general these two masses are correlated and an empirical relation between peak luminosity and light curve shape can be expected. However when we perform population synthesis for progenitors of different metallicities we find a systematic shift in this relation that would make distant SNe Ia fainter than those nearby. The abundances of alpha-rich isotopes, such as $^{44}$Ca, in the solar system indicate that only about 40 per cent of SNe Ia are edge-lit so any systematic effect that could be present would be correspondingly diluted. If we examine only the small subset of ELDs that accrete from a naked helium star, rather than a He white dwarf, the systematic effect disappears.Comment: 14 pages, 3figure

A critical test of stellar evolution and convective core overshooting by means of zeta-Aur systems

No description supplie

A new synthetic model for asymptotic giant branch stars

Publisher's version/PDFWe present a synthetic model for thermally pulsing asymptotic giant branch (TPAGB) evolution constructed by fitting expressions to full evolutionary models in the metallicity range 0.0001 [less than or equal to] Z [less than or equal to] 0.02. Our model includes parametrizations of third dredge-up and hot-bottom burning with mass and metallicity. The Large Magellanic Cloud and Small Magellanic Cloud carbon star luminosity functions are used to calibrate third dredge-up. We calculate yields appropriate for galactic chemical evolution models for [superscript 1]H, [superscript 4]He, [superscript 12]C, [superscript 13]C, [superscript 14]N, [superscript 15]N, [superscript 16]O and [superscript 17]O. The initial–final mass relation is examined for our stars and found to fit to within 0.1 M[subscript circled dot] of the observations. We also reproduce well the white dwarf mass function for masses above about 0.58 M[subscript circled dot]. The new model is to be implemented in a rapid binary star evolution code

The Spatial Distribution of Coalescing Neutron Star Binaries: Implications for Gamma-Ray Bursts

We find the distribution of coalescence times, birthrates, spatial velocities, and subsequent radial offsets of coalescing neutron stars (NSs) in various galactic potentials accounting for large asymmetric kicks introduced during a supernovae. The birthrates of bound NS-NS binaries are quite sensitive to the magnitude of the kick velocities but are, nevertheless, similar (~10 per Galaxy per Myr) to previous population synthesis studies. The distribution of merger times since zero-age main sequence is, however, relatively insensitive to the choice of kick velocities. With a median merger time of ~100 Myr, we find that compact binaries should closely trace the star formation rate in the Universe. In a range of plausible galactic potentials (M_galaxy > 3 x 10^10 M_solar) the median radial offset of a NS-NS mergers is less than 10 kpc. At a redshift of z=1 (with H_0 = 65 km/s/Mpc and Omega = 0.2), this means that half the coalescences should occur within ~1.3 arcsec from the host galaxy. In all but the most shallow potentials, ninety percent of NS-NS binaries merge within 30 kpc of the host. We find that although the spatial distribution of coalescing neutron star binaries is consistent with the close spatial association of known optical afterglows of gamma-ray bursts (GRBs) with faint galaxies, a non-negligible fraction (~15 percent) of GRBs should occur well outside (>30 kpc) dwarf galaxy hosts. Extinction due to dust in the host, projection of offsets, and a range in interstellar medium densities confound the true distribution of NS-NS mergers around galaxies with an observable set of optical transients/galaxy offsets.Comment: Accepted to MNRAS (12 Jan 1999