arXiv subjects

J. E. Pringle

Publications and source records attributed to J. E. Pringle.

At least 55 records · Page 3Linked to original sources

arXiv2011

Disc instability in RS Ophiuchi: a path to Type Ia supernovae?

We study the stability of disc accretion in the recurrent nova RS Ophiuchi. We construct a one-dimensional time-dependent model of the binary-disc system, which includes viscous heating and radiative cooling and a self-consistent treatment of the binary potential. We find that the extended accretion disc in this system is always unstable to the thermal-viscous instability, and undergoes repeated disc outbursts on ~10-20yr time-scales. This is similar to the recurrence time-scale of observed outbursts in the RS Oph system, but we show that the disc's accretion luminosity during outburst is insufficient to explain the observed outbursts. We explore a range of models, and find that in most cases the accretion rate during outbursts reaches or exceeds the critical accretion rate for stable nuclear burning on the white dwarf surface. Consequently we suggest that a surface nuclear burning triggered by disc instability may be responsible for the observed outbursts. This allows the white dwarf mass to grow over time, and we suggest that disc instability in RS Oph and similar systems may represent a path to Type Ia supernovae.

astro-ph.HE↗

arXiv2011

The properties of the ISM in disc galaxies with stellar feedback

We perform calculations of isolated disc galaxies to investigate how the properties of the ISM, the nature of molecular clouds, and the global star formation rate depend on the level of stellar feedback. We adopt a simple physical model, which includes a galactic potential, a standard cooling and heating prescription of the ISM, and self gravity of the gas. Stellar feedback is implemented by injecting energy into dense, gravitationally collapsing gas, but is independent of the Schmidt-Kennicutt relation. We obtain fractions of gas, and filling factors for different phases of the ISM in reasonable ageement with observations. Supernovae are found to be vital to reproduce the scale heights of the different components of the ISM, and velocity dispersions. The GMCs formed in the simulations display mass spectra similar to the observations, their normalisation dependent on the level of feedback. We find ~40 per cent of the clouds exhibit retrograde rotation, induced by cloud-cloud collisions. The star formation rates we obtain are in good agreement with the observed Schmidt-Kennicutt relation, and are not strongly dependent on the star formation efficiency we assume, being largely self regulated by the feedback. We also investigate the effect of spiral structure by comparing calculations with and without the spiral component of the potential. The main difference with a spiral potential is that more massive GMCs are able to accumulate in the spiral arms. Thus we are able to reproduce massive GMCs, and the spurs seen in many grand design galaxies, even with stellar feedback. The presence of the spiral potential does not have an explicit effect on the star formation rate, but can increase the star formation rate indirectly by enabling the formation of long-lived, strongly bound clouds.

astro-ph.GA↗

arXiv2011

Tidal Warping of Be Star Decretion Discs

Rapidly rotating Be stars are observed as shell stars when the decretion disc is viewed edge on. Transitions between the two implies that the discs may be warped and precessing. Type II X-ray outbursts are thought to occur when the warped disc interacts with the fast stellar wind. We suggest that tides from a misaligned companion neutron star can cause the observed effects. We make numerical models of a Be star decretion disc in which the spin of the Be star is misaligned with the orbital axis of a neutron star companion. Tidal torques from the neutron star truncate the disc at a radius small enough that the neutron star orbit does not intersect the disc unless the eccentricity or misalignment is very large. A magnetic torque from the Be star that is largest at the equator, where the rotation is fastest, is approximated by an inner boundary condition. There are large oscillations in the mass and inclination of the disc as it moves towards a steady state. These large variations may explain the observed changes from Be star to Be shell star and vice-versa and also the Type II X-ray outbursts. We find the tidal timescale on which the disc warps, precesses and reaches a steady state to be around a year up to a few hundred years. If present, the oscillations in mass and disc inclination occur on a fraction of this timescale depending on the orbital parameters of the binary. The timescales associated with the tidal torque for observed Be star binaries suggest that these effects are important in all but the longest period binaries.

astro-ph.SR↗

arXiv2011

Why are most molecular clouds not gravitationally bound?

The most recent observational evidence seems to indicate that giant molecular clouds are predominantly gravitationally unbound objects. In this paper we show that this is a natural consequence of a scenario in which cloud-cloud collisions and stellar feedback regulate the internal velocity dispersion of the gas, and so prevent global gravitational forces from becoming dominant. Thus, while the molecular gas is for the most part gravitationally unbound, local regions within the denser parts of the gas (within the clouds) do become bound and are able to form stars. We find that the observations, in terms of distributions of virial parameters and cloud structures, can be well modelled provided that the star formation efficiency in these bound regions is of order 5 - 10 percent. We also find that in this picture the constituent gas of individual molecular clouds changes over relatively short time scales, typically a few Myr.

astro-ph.GA↗

arXiv2010

Retrograde Accretion and Merging Supermassive Black Holes

We investigate whether a circumbinary gas disc can coalesce a supermassive black hole binary system in the centre of a galaxy. This is known to be problematic for a prograde disc. We show that in contrast, interaction with a retrograde circumbinary disc is considerably more effective in shrinking the binary because there are no orbital resonances. The binary directly absorbs negative angular momentum from the circumbinary disc by capturing gas into a disc around the secondary black hole, or discs around both holes if the binary mass ratio is close to unity. In many cases the binary orbit becomes eccentric, shortening the pericentre distance as the eccentricity grows. In all cases the binary coalesces once it has absorbed the angular momentum of a gas mass comparable to that of the secondary black hole. Importantly, this conclusion is unaffected even if the gas inflow rate through the disc is formally super--Eddington for either hole. The coalescence timescale is therefore always $\sim M_2/\dot M$, where $M_2$ is the secondary black hole mass and $\dot M$ the inflow rate through the circumbinary disc.

astro-ph.HE↗

arXiv2010

A Common Envelope Binary Star Origin of Long Gamma-ray Bursts

The stellar origin of gamma-ray bursts can be explained by the rapid release of energy in a highly collimated, extremely relativistic jet. This in turn appears to require a rapidly spinning highly magnetised stellar core that collapses into a magnetic neutron star or a black hole within a relatively massive envelope. They appear to be associated with type Ib/c supernovae but, with a birthrate of around 10^{-6}-10^{-5} per year per galaxy, they are considerably rarer than such supernovae in general. To satisfy all these requirements we hypothesize a binary star model that ends with the merging of an oxygen neon white dwarf with the carbon-oxygen core of a naked helium star during a common envelope phase of evolution. The rapid spin and high magnetic field are natural consequences of such a merging. The evolution that leads to these progenitors is convoluted and so naturally occurs only very rarely. To test the hypothesis we evolve a population of progenitors and find that the rate is as required. At low metallicity we calculate that a similar fraction of stars evolve to this point and so would expect the gamma-ray burst rate to correlate with the star formation rate in any galaxy. This too is consistent with observations. These progenitors, being of intermediate mass, differ radically from the usually postulated high-mass stars. Thus we can reconcile observations that the bursts occur close to but not within massive star associations.

astro-ph.SR↗

arXiv2010

Age distributions of star clusters in spiral and barred galaxies as a test for theories of spiral structure

We consider models of gas flow in spiral galaxies in which the spiral structure has been excited by various possible mechanisms: a global steady density wave, self-gravity of the stellar disc and an external tidal interaction, as well as the case of a galaxy with a central rotating bar. In each model we estimate in a simple manner the likely current positions of star clusters of a variety of ages, ranging from ~ 2 Myr to around 130 Myr, depending on the model. We find that the spatial distribution of cluster of different ages varies markedly depending on the model, and propose that observations of the locations of age-dated stellar clusters is a possible discriminant between excitation mechanisms for spiral structure in an individual galaxy.

astro-ph.GA↗

arXiv2010

The Accretion Disc Dynamo in the Solar Nebula

The nearest accretion disc to us in space if not time was the protosolar nebula. Remnants of this nebula thus potentially offer unique insight into how discs work. In particular the existence of chondrules, which must have formed in the disc as small molten droplets, requires strong and intermittent heating of disc material. We argue that this places important constraints on the way gravitational energy is released in accretion discs, which are not met by current shearing--box simulations of MRI--driven dynamos. A deeper understanding of accretion energy release in discs may require a better model for these dynamos.

astro-ph.SR↗

arXiv2010

The observable effects of tidally induced warps in protostellar discs

We consider the response of a protostellar disc to a tidally induced warp and the resultant changes in the spectral energy distribution (SED). We argue that for typical protostellar disc parameters the warp is communicated through the disc in a wave-like fashion. We find that the main effects of the warp tend to be at large radii (greater than 30 AU) and, for sufficiently small viscosity, can be quite long-lived. This can result in non-uniform illumination of the disc at these radii and can induce significant changes to the SED at wavelengths greater than 100 microns.

astro-ph.SR↗

arXiv2009

Black hole mergers: the first light

The coalescence of supermassive black hole binaries occurs via the emission of gravitational waves, that can impart a substantial recoil to the merged black hole. We consider the energy dissipation, that results if the recoiling black hole is surrounded by a thin circumbinary disc. Our results differ significantly from those of previous investigations. We show analytically that the dominant source of energy is often potential energy, released as gas in the outer disc attempts to circularize at smaller radii. Thus, dimensional estimates, that include only the kinetic energy gained by the disc gas, underestimate the real energy loss. This underestimate can exceed an order of magnitude, if the recoil is directed close to the disc plane. We use three dimensional Smooth Particle Hydrodynamics (SPH) simulations and two dimensional finite difference simulations to verify our analytic estimates. We also compute the bolometric light curve, which is found to vary strongly depending upon the kick angle. A prompt emission signature due to this mechanism may be observable for low mass (10^6 Solar mass) black holes whose recoil velocities exceed about 1000 km/s. Emission at earlier times can mainly result from the response of the disc to the loss of mass, as the black holes merge. We derive analytically the condition for this to happen.

astro-ph.HE↗

arXiv2009

Supernova Kicks and Misaligned Microquasars

The low-mass X-ray binary microquasar GRO J1655-40 is observed to have a misalignment between the jets and the binary orbital plane. Since the current black hole spin axis is likely to be parallel to the jets, this implies a misalignment between the spin axis of the black hole and the binary orbital plane. It is likely the black holes formed with an asymmetric supernova which caused the orbital axis to misalign with the spin of the stars. We ask whether the null hypothesis that the supernova explosion did not affect the spin axis of the black hole can be ruled out by what can be deduced about the properties of the explosion from the known system parameters. We find that this null hypothesis cannot be disproved but we find that the most likely requirements to form the system include a small natal black hole kick (of a few tens of km/s) and a relatively wide pre-supernova binary. In such cases the observed close binary system could have formed by tidal circularisation without a common envelope phase.

astro-ph.HE↗

arXiv2009

Chaotic star formation and the alignment of stellar rotation with disc and planetary orbital axes

We investigate the evolution of the relative angle between the stellar rotation axis and the circumstellar disc axis of a star that forms in a stellar cluster from the collapse of a turbulent molecular cloud. This is an inherently chaotic environment with variable accretion, both in terms of rate and the angular momentum of the material, and dynamical interactions between stars. We find that the final stellar rotation axis and disc spin axis can be strongly misaligned, but this occurs primarily when the disc is truncated by a dynamical encounter so that the final disc rotation axis depends simply on what fell in last. This may lead to planetary systems with orbits that are misaligned with the stellar rotation axis, but only if the final disc contains enough mass to form planets. We also investigate the time variability of the inner disc spin axis, which is likely to determine the direction of a protostellar jet. We find that the jet direction varies more strongly for lighter discs, such as those that have been truncated by dynamical interactions or have suffered a period of rapid accretion. Finally, we note that variability of the angular momentum of the material accreting by a star implies that the internal velocity field of such stars may be more complicated than that of aligned differential rotation.

astro-ph.SR↗

arXiv2009

Discovery of CIV Emission Filaments in M87

Gas at intermediate temperature between the hot X-ray emitting coronal gas in galaxies at the centers of galaxy clusters, and the much cooler optical line emitting filaments, yields information on transport processes and plausible scenarios for the relationship between X-ray cool cores and other galactic phenomena such as mergers or the onset of an active galactic nucleus. Hitherto, detection of intermediate temperature gas has proven elusive. Here, we present FUV imaging of the "low excitation" emission filaments of M87 and show strong evidence for the presence of CIV 1549 A emission which arises in gas at temperature ~10^5K co-located with Halpha+[NII] emission from cooler ~10^4K gas. We infer that the hot and cool phases are in thermal communication, and show that quantitatively the emission strength is consistent with thermal conduction, which in turn may account for many of the observed characteristics of cool core galaxy clusters.

astro-ph.CO↗

arXiv2009

Hydrodynamic Simulations of M51 and the interaction with NGC 5195

The M51 galaxy is one of best examples of grand design structure in galaxies. The most likely origin of the spiral arms in M51 is the ongoing interaction with NGC 5195. Here we report recent calculations which model the orbit of M51 and NGC 5195. These calculations, which for the first time focus on the gas dynamics, are able to reproduce even detailed features in M51, as well as demonstrate the nature of the spiral structure of M51.

astro-ph.GA↗

arXiv2009

The Shape of an Accretion Disc in a Misaligned Black Hole Binary

We model the overall shape of an accretion disc in a semi-detached binary system in which mass is transfered on to a spinning black hole the spin axis of which is misaligned with the orbital rotation axis. We assume the disc is in a steady state. Its outer regions are subject to differential precession caused by tidal torques of the companion star. These tend to align the outer parts of the disc with the orbital plane. Its inner regions are subject to differential precession caused by the Lense-Thirring effect. These tend to align the inner parts of the disc with the spin of the black hole. We give full numerical solutions for the shape of the disc for some particular disc parameters. We then show how an analytic approximation to these solutions can be obtained for the case when the disc surface density varies as a power law with radius. These analytic solutions for the shape of the disc are reasonably accurate even for large misalignments and can be simply applied for general disc parameters. They are particularly useful when the numerical solutions would be slow.

astro-ph.HE↗

arXiv2009

Black hole mergers: can gas discs solve the `final parsec' problem?

We compute the effect of an orbiting gas disc in promoting the coalescence of a central supermassive black hole binary. Unlike earlier studies, we consider a finite mass of gas with explicit time dependence: we do not assume that the gas necessarily adopts a steady state or a spatially constant accretion rate, i.e. that the merging black hole was somehow inserted into a pre--existing accretion disc. We consider the tidal torque of the binary on the disc, and the binary's gravitational radiation. We study the effects of star formation in the gas disc in a simple energy feedback framework. The disc spectrum differs in detail from that found before. In particular, tidal torques from the secondary black hole heat the edges of the gap, creating bright rims around the secondary. These rims do not in practice have uniform brightness either in azimuth or time, but can on average account for as much as 50 per cent of the integrated light from the disc. This may lead to detectable high--photon--energy variability on the relatively long orbital timescale of the secondary black hole, and thus offer a prospective signature of a coalescing black hole binary. We also find that the disc can drive the binary to merger on a reasonable timescale only if its mass is at least comparable with that of the secondary black hole, and if the initial binary separation is relatively small, i.e. $a_0 \lesssim 0.05$ pc. Star formation complicates the merger further by removing mass from the disc. In the feedback model we consider, this sets an effective limit to the disc mass. As a result, binary merging is unlikely unless the black hole mass ratio is $\la 0.001$. Gas discs thus appear not to be an effective solution to the `last parsec' problem for a significant class of mergers.

astro-ph.CO↗

arXiv2009

Supernova Kicks and Misaligned Be Star Binaries

Be stars are rapidly spinning B stars surrounded by an outflowing disc of gas in Keplerian rotation. Be star/X-ray binary systems contain a Be star and a neutron star. They are found to have non-zero eccentricities and there is evidence that some systems have a misalignment between the spin axis of the star and the spin axis of the binary orbit. The eccentricities in these systems are thought to be caused by a kick to the neutron star during the supernova that formed it. Such kicks would also give rise to misalignments. In this paper we investigate the extent to which the same kick distribution can give rise to both the observed eccentricity distribution and the observed misalignments. We find that a Maxwellian distribution of velocity kicks with a low velocity dispersion, $σ_k \approx 15\rm km s^{-1}$, is consistent with the observed eccentricity distribution but is hard to reconcile with the observed misalignments, typically $i \ge 25^\circ$. Alternatively a higher velocity kick distribution, $σ_k = 265 \rm km s^{-1}$, is consistent with the observed misalignments but not with the observed eccentricities, unless post-supernova circularisation of the binary orbits has taken place. We discuss briefly how this might be achieved.

astro-ph.SR↗

arXiv2009

RS Ophiuchi: Thermonuclear Explosion or Disc Instability?

Sokoloski et al (2008) have recently reported evidence that the recurrent nova RS Ophiuchi produced a pair of highly collimated radio jets within days of its 2006 outburst. This suggests that an accretion disc must be present during the outburst. However in the standard picture of recurrent novae as thermonuclear events, any such disc must be expelled from the white dwarf vicinity, as the nuclear energy yield greatly exceeds its binding energy. We suggest instead that the outbursts of RS Oph are thermal--viscous instabilities in a disc irradiated by the central accreting white dwarf. The distinctive feature of RS Oph is the very large size of its accretion disc. Given this, it fits naturally into a consistent picture of systems with unstable accretion discs. This picture explains the presence and speed of the jets, the brightness and duration of the outburst, and its rise time and linear decay, as well as the faintness of the quiescence. By contrast, the hitherto standard picture of recurrent thermonuclear explosions has a number of severe difficulties. These include the presence of jets, the faintness of quiescence, and the fact the the accretion disc must be unstable unless it is far smaller than any reasonable estimate.

astro-ph.EP↗