Martin E. Beer

The light curve of the companion to PSR B1957+20

We present a new analysis of the light curve for the secondary star in the eclipsing binary millisecond pulsar system PSR B1957+20. Combining previous data and new data points at minimum from the Hubble Space Telescope, we have 100 per cent coverage in the R-band. We also have a number of new K s -band data points, which we use to constrain the infrared magnitude of the system. We model this with the eclipsing light-curve (ELC) code. From the modelling with the ELC code we obtain colour information about the secondary at minimum light in BVRI and K. For our best-fitting model we are able to constrain the system inclination to 65° ± 2° for pulsar masses ranging from 1.3 to 1.9 M⊙. The pulsar mass is unconstrained. We also find that the secondary star is not filling its Roche lobe. The temperature of the unirradiated side of the companion is in agreement with previous estimates and we find that the observed temperature gradient across the secondary star is physically sustainable.

How special is the Solar system

Most mechanisms proposed for the formation of planets are modified versions of the mechanism proposed for the solar system. Here we argue that, in terms of those planetary systems which have been observed, the case for the solar system being a typical planetary system has yet to be established. We consider the possibility that most observed planetary systems have been formed in some quite different way. If so, it may be that none of the observed planetary systems is likely to harbour an earth-like planet.

Wolf-Rayet and O star runaway populations from supernovae

We present numerical simulations of the runaway fractions expected amongst O and WolfRayet star populations resulting from stars ejected from binaries by the supernova of the companion. Observationally the runaway fraction for both types of star is similar, prompting the explanation that close dynamical interactions are the main cause of these high-velocity stars. We show that, provided that the initial binary fracti on is high, a scenario in which twothirds of massive runaways are from supernovae is consistent with these observations. Our models also predict a low frequency of runaways with neutron star companions and a very low fraction of observable Wolf-Rayet‐compact companion systems.

Mass transfer in eccentric binaries: the new oil-on-water smoothed particle hydrodynamics technique

To measure the onset of mass transfer in eccentric binaries, we have developed a two-phase smoothed particle hydrodynamics (SPH) technique. Mass transfer is important in the evolution of close binaries, and a key issue is to determine the separation at which mass transfer begins. The circular case is well understood and can be treated through the use of the Roche formalism. To treat the eccentric case, we use a newly developed two-phase system. The body of the donor star is made up from high-mass water particles, whilst the atmosphere is modelled with low-mass oil particles. Both sets of particles take part fully in SPH interactions. To test the technique, we model circular mass-transfer binaries containing a 0.6 M-circle dot donor star and a 1 M-circle dot white dwarf; such binaries are thought to form cataclysmic variable ( CV) systems. We find that we can reproduce a reasonable CV mass-transfer rate, and that our extended atmosphere gives a separation that is too large by approximately 16 per cent, although its pressure scale height is considerably exaggerated. We use the technique to measure the semimajor axis required for the onset of mass transfer in binaries with a mass ratio of q = 0.6 and a range of eccentricities. Comparing to the value obtained by considering the instantaneous Roche lobe at pericentre, we find that the radius of the star required for mass transfer to begin decreases systematically with increasing eccentricity.

The planet in M4: implications for planet formation in globular clusters

We consider the formation and evolution of the planetary system PSR B1620‐26 in the globular cluster M4. We propose that as M4 is a very low-metallicity environment the standard model of planet formation around main-sequence stars through the accretion of gas on to metallic rocky cores should not be applied. Consequently the previously suggested methods for formation are unlikely. We propose that the planet formed through the interaction of a passing star with a circumbinary disc during the common-envelope phase of the evolution of the inner binary. This formation route is favoured by dense stellar systems such as globular clusters. Ke yw ords: planets and satellites: formation ‐ planetary systems: protoplanetary discs ‐ pulsars: individual: PSR B1620‐26.

An alternative to common envelope evolution

We investigate the evolution of interacting binaries where the donor star is a low-mass giant more massive than its companion. It is usual to assume that such systems undergo common envelope (CE) evolution, where the orbital energy is used to eject the donor envelope, thus producing a closer binary or a merger. We suggest instead that because mass transfer is super-Eddington even for non-compact companions, a wide range of systems avoid this type of CE phase. The accretion energy released in the rapid mass-transfer phase unbinds a significant fraction of the giants envelope, reducing the tendency to dynamical instability and merging. We show that our physical picture accounts for the success of empirical parametrizations of the outcomes of assumed CE phases.

The population of black widow pulsars

We consider the population of black widow pulsars (BWPs). The large majority of these are members of globular clusters. For minimum companion masses 0.1 M⊙ are systems relaxing to equilibrium after a relatively rece nt capture event. We point out that all binary millisecond pulsars (MSPs) with orbital periods P < 10 hr are BWPs (our line of sight allows us to see the eclipses in 10 out of 16 cases). This implies that recycled MSPs emit either in a wide fan beam or a pencil beam close to the spin plane. Simple evolutionary ideas favour a fan beam.

Red giant depletion in globular cluster cores

We investigate the observed depletion of red giants in the cores of post-core-collapse globular clusters. In particular, the evolutionary scenario we consider is a binary consisting of two low-mass stars which undergoes two common-envelope phases. The first common-envelope phase occurs when the primary is a red giant resulting in a helium white dwarf and main-sequence star in a detached binary. The second common-envelope phase occurs shortly after the secondary becomes a red giant. During the second common-envelope phase, the degenerate helium cores merge resulting in a core mass greater than the helium burning limit and the formation of a horizontal branch star. We show that stellar encounters enhance this evolutionary route in post-core-collapse clusters. These encounters increase the population of binary secondaries which would have evolved on to the red giant branch in the recent past.

A general three‐dimensional fluid dynamics code for stars in binary systems

We describe the theory and implementation of a three-dimensional fluid dynamics code that we have developed for calculating the surface geometry and circulation currents in the secondaries of interacting binary systems. The main method is based on an Eulerian-Lagrangian scheme to solve the advective and force terms in Eulers equation. Surface normalized spherical polar coordinates are used to allow the accurate modelling of the surface of the star, as is necessary when free surfaces and irradiation effects are to be considered. The potential and its gradient are expressed as sums of Legendre polynomials, which allows a very efficient solution of Poissons equation. The basic solution scheme, based on operator splitting, is outlined, and standard numerical tests are presented.

The optical a IR lightcurve of PSR B1957+20

We present a new analysis of the light curve of the secondary star in the PSR B1957+20 system. Combining previous data and new data points at minimum from the Hubble Space Telescope, we have 100% coverage in the R-band. We also have a number of new Ks band data points, which we use to constrain the IR magnitude of the system. We model this with the Eclipsing Light Curve code. From the modeling we obtain colour information about the secondary at minimum in BVRI & K. For our best fit model we are able to constrain the system inclination to 66.6 ± 2.1 for pulsar masses ranging from 1.35 – 1.9 M . The pulsar mass is unconstrained. We also find that the secondary is not filling its Roche lobe, which has important consequences for evolutionary models of this system. The temperature of the un-irradiated side of the companion is in agreement with previous estimates.