A picture of a star

Ross Church - Research

A complete list of my publications can be found at ADS.

Selected papers

Mass transfer in white dwarf-neutron star binaries

We perform hydrodynamic simulations of mass transfer in binaries that contain a white dwarf and a neutron star (WD-NS binaries), and measure the specific angular momentum of material lost from the binary in disc winds. By incorporating our results within a long-term evolution model, we measure the long-term stability of mass transfer in these binaries. We find that only binaries containing helium white dwarfs (WDs) with masses less than a critical mass of 0.2 M undergo stable mass transfer and evolve into ultracompact X-ray binaries. Systems with higher mass WDs experience unstable mass transfer, which leads to tidal disruption of the WD. Our low critical mass compared to the standard jet-only model of mass-loss arises from the efficient removal of angular momentum in the mechanical disc winds, which develop at highly super-Eddington mass-transfer rates. We find that the eccentricities expected for WD-NS binaries when they come into contact do not affect the loss of angular momentum, and can only affect the long-term evolution if they change on shorter time-scales than the mass-transfer rate. Our results are broadly consistent with the observed numbers of both ultracompact X-ray binaries and radio pulsars with WD companions. The observed calcium-rich gap transients are consistent with the merger rate of unstable systems with higher mass WDs.

Published as Mass transfer in white dwarf-neutron star binaries, Bobrick, A., Davies M. B., Church R. P. (2017) MNRAS 467 3556 (ADS, arXiv)

Galactic dynamics and the orbit of M67

Context: Could the velocity spread, increasing with time, in the Galactic disk be explained as a result of gravitational interactions of stars with giant molecular clouds (GMCs) and spiral arms? Do the old open clusters high above the Galactic plane provide clues to this question?
Aims: We explore the effects on stellar orbits of scattering by inhomogeneities in the Galactic potential due to GMCs, spiral arms and the Galactic bar, and whether high-altitude clusters could have formed in orbits closer to the Galactic plane and later been scattered.
Methods: Simulations of test-particle motions are performed in a realistic Galactic potential. The effects of the internal structure of GMCs are explored. The destruction of clusters in GMC collisions is treated in detail with N-body simulations of the clusters.
Results: The observed velocity dispersions of stars as a function of time are well reproduced. The GMC structure is found to be significant, but adequate models produce considerable scattering effects. The fraction of simulated massive old open clusters, scattered into orbits with |z| > 400 pc, is typically 0.5%, in agreement with the observed number of high-altitude clusters and consistent with the present formation rate of massive open clusters.
Conclusions: The heating of the thin Galactic disk is well explained by gravitational scattering by GMCs and spiral arms, if the local correlation between the GMC mass and the corresponding voids in the gas is not very strong. Our results suggest that the high-altitude metal-rich clusters were formed in orbits close to the Galactic plane and later scattered to higher orbits. It is possible, though not very probable, that the Sun formed in such a cluster before scattering occurred.

Published as Gravitational scattering of stars and clusters and the heating of the Galactic disk, Gustafsson, B., Church R. P., Davies M. B., Rickman, H. (2016) A&A 593 85 (ADS, arXiv)

Numerics of thermohaline mixing

In recent years much interest has been shown in the process of thermohaline mixing in red giants. In low- and intermediate-mass stars this mechanism first activates at the position of the bump in the luminosity function, and has been identified as a likely candidate for driving the slow mixing inferred to occur in these stars. One particularly important consequence of this process, which is driven by a molecular weight inversion, is the destruction of lithium. We show that the degree of lithium destruction, or in some cases production, is extremely sensitive to the numerical details of the stellar models. Within the standard 1D diffusion approximation to thermohaline mixing, we find that different evolution codes, with their default numerical schemes, can produce lithium abundances that differ from one another by many orders of magnitude. This disagreement is worse for faster mixing. We perform experiments with four independent stellar evolution codes, and derive conditions for the spatial and temporal resolution required for a converged numerical solution. The results are extremely sensitive to the time-steps used. We find that predicted lithium abundances published in the literature until now should be treated with caution.

Published as On the numerical treatment and dependence of thermohaline mixing in red giants, Lattanzio, J. C., Siess, L., Church R. P., Angelou, G., Stancliffe, R. J., Doherty, C. L., Stephen, T., Campbell, S. W. (2015) MNRAS 446 2673 (ADS, arXiv)

Thorne-Żtkow objects

The very bright red star HV2112 in the Small Magellanic Cloud could be a massive Thorne-Żytkow object (TŻO), a supergiant-like star with a degenerate neutron core. With a luminosity of over 105 L, it could also be a super asymptotic giant branch (SAGB) star, a star with an oxygen/neon core supported by electron degeneracy and undergoing thermal pulses with third dredge up. Both TŻOs and SAGB stars are expected to be rare. Abundances of heavy elements in HV2112's atmosphere, as observed to date, do not allow us to distinguish between the two possibilities based on the latest models. Molybdenum and rubidium can be enhanced by both the irp-process in a TŻO or by the s-process in SAGB stars. Lithium can be generated by hot bottom burning at the base of the convective envelope in either. HV2112's enhanced calcium could thus be the key determinant. Neither SAGB stars nor TŻOs are known to be able to synthesize their own calcium but it may be possible to produce it in the final stages of the process that forms a TŻO, when the degenerate electron core of a giant star is tidally disrupted by a neutron star. Hence, it is more likely, on a fine balance, that HV2112 is indeed a genuine TŻO.

Published as HV2112, a Thorne–Żytkow object or a super asymptotic giant branch star, Tout C. A., Żtkow A. N., Church R. P., Lau H. B., Doherty C. L., Izzard R. I. MNRAS 445 L36 (ADS, arXiv)

Physics of thermohaline mixing

Stars ascending the red giant branch develop an inversion in mean molecular weight (μ) owing to the burning of 3He in the region immediately above their hydrogen-burning shells. This inversion may drive thermohaline mixing and thereby be responsible for the extra mixing which is observationally indicated on the red giant branch. In this paper, we investigate the physical influences that determine the mass and temperature at which the inversion in μ develops. We find that it depends most strongly on the thermal structure of the envelope - the profiles of density and temperature in the region of the star immediately above the shell - and is otherwise relatively insensitive to abundances and nuclear reaction rates. The changes in the effects of thermohaline mixing as stars proceed up the giant branch can mostly be understood in terms of their changing thermal structure, driven by their increasing core mass.

Published as , Church R. P., Lattanzio J., Angelou G., Tout C. A., Stancliffe R. J. (2014) MNRAS 443 977 (ADS)

Constraints on the birth cluster of the Sun

We use N-body simulations of star cluster evolution to explore the hypothesis that short-lived radioactive isotopes found in meteorites, such as Al-26, were delivered to the Sun's protoplanetary disc from a supernova. We cover a range of cluster formation parameter space including clusters with primordial substructure and those with smooth profiles, and a range of initial virial ratios from collapsing clusters to expanding associations. Each model cluster has 2100 stars, and contains one massive 25 M star which explodes as a supernova at about 6.6 Myr. We determine the number of Solar-type stars that are within 0.1 - 0.3 pc of the supernova, which is the distance required to enrich the protoplanetary disc with the 26-Al abundances found in meteorites. Typically only about 25 per cent of clusters contain enriched, unperturbed 'singletons' -- stars that have had no strong dynamical encounters that might have disrupted the solar system -- and usually only 1 - 2 in each cluster. Summing together simulations with identical initial conditions, we find that about 1 per cent of all G-dwarfs are enriched, unperturbed singletons.

Published as Supernova enrichment and dynamical histories of solar-type stars in clusters, Parker R. J., Church R. P., Davies M. B., Meyer M. R. (2013) MNRAS 437 946 (ADS, arXiv)

Gamma-ray bursts in compact binaries

We consider a popular model for long-duration gamma-ray bursts, in which the progenitor star, a stripped helium core, is spun up by tidal interactions with a black- hole companion in a compact binary. We perform population synthesis calculations to produce a representative sample of such binaries, and model the effect that the companion has on material that falls back on to the newly-formed black hole. Taking the results of hydrodynamic models of black-hole formation by fallback as our starting point, we show that the companion has two main effects on the fallback process. First, a break forms in the accretion curve at around 10 000 s. Secondly, subsequent to the break, we expect to see a flare of total energy around 0.1 foe. We predict that the break time is set largely by the semi-major axis of the binary at the time of explosion, and that this correlates negatively with the flare energy. Although comparison with observations is non-trivial, we show that our predicted break times are comparable to those found in the X-ray light curves of canonical long-duration gamma-ray bursts. Similarly, the flare properties that we predict are consistent with the late-time flares observed in a sub-sample of bursts.

Published as The properties of gamma-ray bursts in massive compact binaries, Church R. P., Kim C., Levan A. J., Davies M. B. (2012) MNRAS 425 470 (ADS, arXiv)

Galactic offsets of short gamma-ray bursts

We present the observed offsets of short-duration gamma-ray bursts (SGRBs) from their putative host galaxies and compare them with the expected distributions of merging compact object binaries, given the observed properties of the hosts. We find that for all but one burst in our sample the offsets are consistent with this model. For the case of bursts with massive elliptical host galaxies, the circular velocities of the hosts' haloes exceed the natal velocities of almost all our compact object binaries. Hence, the extents of the predicted offset distributions for elliptical galaxies are determined largely by their spatial extents. In contrast, for spiral hosts, the galactic rotation velocities are smaller than typical binary natal velocities and the predicted burst offset distributions are more extended than the galaxies.

One SGRB, 060502B, apparently has a large radial offset that is inconsistent with an origin in a merging galactic compact binary. Although it is plausible that the host of GRB 060502B is misidentified, our results show that the large offset is compatible with a scenario where at least a few per cent of SGRBs are created by the merger of compact binaries that form dynamically in globular clusters.

Published as Implications for the origin of short gamma-ray bursts from their observed positions around their host galaxies Church R. P., Levan A. J., Davies M. B., Tanvir N. (2011), MNRAS, 413 2004 (arXiv, ADS)

Black hole spin in Cygnus X-1

To date, there have been several detections of high-mass black hole binaries in both the Milky Way and other galaxies. For some of these, the spin parameter of the black hole has been estimated. As many of these systems are quite tight, a suggested origin of the spin is angular momentum imparted by the synchronous rotation of the black hole progenitor with its binary companion. Using Cygnus X-1, the best studied high-mass black hole binary, we investigate this possibility. We find that such an origin of the spin is not likely, and our results point rather to the spin being the result of processes during the collapse.

Published as On the origin of black hole spin in high-mass black hole binaries: Cygnus X-1 Axelsson M., Church R. P., Davies M. B., Levan A. J., Ryde F. (2011) MNRAS 412 2260 (arXiv, ADS)

Mass transfer in eccentric binaries

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 solar mass donor star and a 1 solar mass 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.

Published as Mass transfer in eccentric binaries; the new Oil-on-Water SPH technique, Church R. P., Dischler J., Davies M. B., Tout C. A. Tout, Adams T., Beer M. E. (2009), MNRAS 395 1127 (arXiv, ADS)

Nucleosynthesis in intermediate-mass stars

We present MONTAGE, a post-processing nucleosynthesis code that combines a traditional network for isotopes lighter than calcium with a rapid algorithm for calculating the s-process nucleosynthesis of the heavier isotopes. The separation of those parts of the network where only neutron-capture and beta-decay reactions are significant provides a substantial advantage in computational efficiency. We present the yields for a complete set of s-process isotopes for a 3 solar mass, Z = 0.02 stellar model, as a demonstration of the utility of the approach. Future work will include a large grid of models suitable for use in calculations of Galactic chemical evolution.

Published as MONTAGE: AGB nucleosynthesis with full s-process calculations, Church R. P., Cristallo S., Lattanzio J. C., Stancliffe R. J., Straniero O., Cannon R. C. (2009) PASA 26 217 (arXiv, ADS)

Stellar evolution in N-body simulations

An N-body code containing live stellar evolution through combination of the software packages NBODY6 and STARS is presented. Operational details of the two codes are outlined and the changes that have been made to combine them discussed. We have computed the evolution of clusters of 104 stars using the combined code and we compare the results with those obtained using NBODY6 and the synthetic stellar evolution code SSE. We find that, providing the physics package within stars is set up correctly to match the parameters of the models used to construct SSE, the results are very similar. This provides a good indication that the new code is working well. We also demonstrate how this physics can be changed simply in the new code with convective overshooting as an example. Similar changes in SSE would require considerable reworking of the model fits. We conclude by outlining proposed future development of the code to include more complete models of single stars and binary star systems.

Published as N-body simulations with live stellar evolution , Church R. P., Tout C. A., Hurley J. R. (2009) PASA 26 92 (arXiv, ADS)

The workings of a stellar evolution code

Models of stellar clusters link the theoretical gravitational N-body problem to the study of real astrophysical systems. Such models require a description of the stars contained within the cluster. Stars are interesting objects in their own right, and the study of stellar evolution is important across astronomy, from the formation of exotic objects such as X-ray binaries and gamma-ray bursts to measuring the ages of galaxies.

The physical processes important for and qualitative results of stellar evolution theory are discussed elsewhere in this book. Here the technical problem of computing the structure and evolution of the stars is considered. How can we solve the set of differential equations that describe the interior of a star to obtain a model of its physical properties? A brief mention will be made of some of the uncertainties in stellar physics and how they affect the results obtained.

Published as The Workings of a Stellar Evolution Code , in The Cambridge N-Body Lectures, Lecture Notes in Physics 760 (Springer Berlin Heidelberg 2008) p. 333 (ADS)

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