Lynnette M. Dray

Winds from massive stars: implications for the afterglows of γ‐ray bursts

Recent observations suggest that long-duration γ-ray bursts (GRBs) and their afterglows are produced by highly relativistic jets emitted in core-collapse explosions. The pre-explosive ambient medium provides a natural test for the most likely progenitors of GRBs. Those stars that shed their envelopes most readily have short jet crossing times and are more likely to produce a GRB. We construct a simple computational scheme to explore the expected contribution of the presupernova ejecta of single Wolf–Rayet (WR) stars to the circumstellar environment. Using detailed stellar tracks for the evolution of massive stars, we discuss the effects that the initial main-sequence mass, metallicity, rotation and membership in a binary system have on the ambient medium. We extend the theory of GRB afterglows in winds to consider the effect of the relativistic fireball propagating through the WR ejecta. Specific predictions are made for the interaction of the relativistic blast wave with the density bumps that arise when the progenitor star rapidly loses a large fraction of its initial mass or when the ejected wind interacts with the external medium and decelerates. A re-brightening of the afterglow with a spectrum redder than the typical synchrotron spectrum (as seen in GRB 970508, GRB 980326 and GRB 000911) is predicted. We also calculate the luminosity of the reflected echo that arises when circumstellar material Compton-scatters the prompt radiation, and examine the spectral signatures expected from the interaction of the GRB afterglow with the ejected medium.

On the metallicity dependence of high-mass X-ray binaries

It is commonly assumed that high mass X-ray binary (HMXB) populations are little-affected by metallicity. However, the massive stars making up their progenitor systems depend on metallicity in a number of ways, not least through their winds. We present simulations, well-matched to the observed sample of Galactic HMXBs, which demonstrate that both the number and the mean period of HMXB progenitors can vary with metallicity, with the number increasing by about a factor of three between solar and SMC metallicity. However, the SMC population itself cannot be explained simply by metallicity effects; it requires both that the HMXBs observed therein primarily sample the older end of the HMXB population, and that the star formation rate at the time of their formation was very large.

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.

On rejuvenation in massive binary systems

We introduce a set of stellar models for massive stars whose evolution has been affected by mass transfer in a binary system, at a range of metallicities. As noted by other authors, the effect of such mass transfer is frequently more than just rejuvenation. We find that, whilst stars with convective cores which have accreted only H-rich matter rejuvenate as expected, those stars which have accreted He-rich matter (e.g. at the end stages of conservative mass transfer) evolve in a way that is qualitatively similar to rejuvenated stars of much higher metallicity. Thus, the effects of non-conservative evolution depend strongly on whether He-rich matter is amongst the portion accreted or ejected. This may lead to a significant divergence in binary evolution paths with only a small difference in initial assumptions. We compare our models to observed systems and find approximate formulae for the effect of mass accretion on the effective age and metallicity of the resulting star.

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.

On the metallicity dependence of HMXBs

It is commonly assumed that high mass X-ray binary (HMXB) populations are little-affected by metallicity. However, the massive stars making up their progenitor systems depend on metallicity in a number of ways, not least through their winds. We present simulations, well-matched to the observed sample of Galactic HMXBs, which demonstrate that both the number and the mean period of HMXB progenitors can vary with metallicity, with the number increasing by about a factor of three between solar and SMC metallicity. However, the SMC population itself cannot be explained simply by metallicity effects; it requires both that the HMXBs observed therein primarily sample the older end of the HMXB population, and that the star formation rate at the time of their formation was very large.

Single Stars and Supernovae from Wolf‐Rayet Secondaries

We investigate the population of single Wolf‐Rayet (WR) stars in the Milky Way and Magellanic Clouds which may have resulted from massive binary evolution. Observationally, the binary fraction amongst Wolf‐Rayet stars is much smaller than that of O stars, their direct progenitors, suggesting that many single WR stars might once have been part of a binary system. We present a grid of stellar models aimed at studying the evolution of the secondary in the case that the supernova explosion of the primary unbinds the binary to see if a suitable population may be produced in this manner. Whilst transferring a significant amount of mass between the stars in such a system is difficult to achieve — a contact binary often resulting instead — the accretion of He‐enriched matter onto those secondaries which avoid contact changes their subsequent evolution in a more complex manner than simple rejuvenation and increases the liklihood that they will subsequently undergo a WR phase. If the initial binary fraction is high...