On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160)  Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

/pdf/the-observation-of-gravitational-waves-from-a-binary-black-58srpupgg9.pdf

The Observation of Gravitational Waves from a Binary Black Hole Merger

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 semilatus 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 semilatera recta rather than periods or semimajor axes.

https://researchbank.swinburne.edu.au/file/ab4c7d3f-f3a0-4cb3-bb4f-9790d3f33b19/1/PDF (Accepted manuscript).pdf

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

Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory and the standard theory of magnetostatic support, from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that, although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic-scale star formation. It suggests that individual star-forming cores are likely not quasistatic objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. Global star formation in galaxies appears to be controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

Control of star formation by supersonic turbulence

Although fundamental for astrophysics, the processes that produce massive stars are not well understood. Large distances, high extinction, and short timescales of critical evolutionary phases make observations of these processes challenging. Lacking good observational guidance, theoretical models have remained controversial. This review offers a basic description of the collapse of a massive molecular core and a critical discussion of the three competing concepts of massive star formation: ▪  monolithic collapse in isolated cores ▪  competitive accretion in a protocluster environment ▪  stellar collisions and mergers in very dense systems We also review the observed outflows, multiplicity, and clustering properties of massive stars, the upper initial mass function and the upper mass limit. We conclude that high-mass star formation is not merely a scaled-up version of low-mass star formation with higher accretion rates, but partly a mechanism of its own, primarily owing to the role of stellar mass ...

Toward Understanding Massive Star Formation

Although fundamental for astrophysics, the processes that produce massive stars are not well understood. Large distances, high extinction, and short timescales of critical evolutionary phases make observations of these processes challenging. Lacking good observational guidance, theoretical models have remained controversial. This review offers a basic description of the collapse of a massive molecular core and a critical discussion of the three competing concepts of massive star formation: 
- monolithic collapse in isolated cores 
- competitive accretion in a protocluster environment 
- stellar collisions and mergers in very dense systems 
We also review the observed outflows, multiplicity, and clustering properties of massive stars, the upper initial mass function and the upper mass limit. We conclude that high-mass star formation is not merely a scaled-up version of low-mass star formation with higher accretion rates, but partly a mechanism of its own, primarily owing to the role of stellar mass and radiation pressure in controlling the dynamics.

LoTr 1 is a planetary nebula thought to contain an intermediate-period binary central star system (that is, a system with an orbital period, P, between 100 and, say, 1500 d). The system shows the signature of a K type, rapidly rotating giant and most likely constitutes an accretion-induced post-mass-transfer system similar to other PNe, such as LoTr 5, WeBo 1 and A70. Such systems represent rare opportunities to further the investigation into the formation of barium stars and intermediate-period post-asymptotic giant branch systems – a formation process still far from being understood. Here, we present the first detailed analyses of both the central star system and the surrounding nebula of LoTr 1 using a combination of spectra obtained with Very Large Telescope-Focal Reducer and low dispersion Spectrograph, Anglo-Australian Telescope-UCL Coude Echelle Spectrograph and New Technology Telescope-European Southern Observatory Multi-Mode Instrument, as well as SuperWASP (Wide Angle Search for Planets) photometry. We confirm the binary nature of the central star of LoTr 1 that consists of a K1 III giant and a hot white dwarf. The cool giant does not present any sign of s-process enhancement but is shown to have a rotation period of 6.4 d, which is a possible sign of mass accretion. LoTr 1 also presents broad double-peaked Hα emission lines, whose origin is still unclear. The nebula of LoTr 1 consists in two slightly elongated shells, with ages of 17 000 and 35 000 yr, respectively, and with different orientations. As such, LoTr 1 present a very different nebular morphology than A70 and WeBo 1, which may be an indication of difference in the mass-transfer episodes.

Two rings but no fellowship: LoTr 1 and its relation to planetary nebulae possessing barium central stars

The chemically peculiar B star $\phi$ Phe was, until very recently, considered a triple system, even though the data were not conclusive and the orbits rather uncertain. Very recent results by Korhonen et al. (2013) provided a revised orbit, different from the then available astrometric Hipparcos orbit. 
Additional spectroscopic data, obtained with the BESO spectrograph at Cerro Armazones, confirm the newly found orbit, even though the resulting radial velocities do not allow to improve on the recent orbit. We combine the latter with the Hipparcos measurements to secure the astrometric orbit, and derive the inclination of the system. Using evolutionary tracks, we can finally constrain all the parameters of the two components in this system. 
We confirm the mass of the primary, 3 M$_\odot$, and find that the companion has a mass of 0.9 M$_\odot$. The inclination of the system is $i=93^{\circ} \pm 4.7^{\circ}$, and is potentially eclipsing; we predict the time of the next conjunction. Given that the eccentricity of the orbit and the exact value of the semi-amplitude of the radial velocity relies on just one set of points, we also urge observers to measure radial velocities at the next periastron passage in April 2015.

The multiplicity of \phi\ Phe revisited

Dense circumstellar nebulae in wide binary central stars

Binarity and mass transfer appear to play a key role in the shaping and, possibly, formation of planetary nebulae (PNe), thereby explaining the large fraction of axisymmetric morphologies. I present the binary hypothesis for PNe and its current status. Recent discoveries have led to a quadrupling of the number of post-common envelope binary central stars of PNe compared to the end of the last century, thereby allowing us to envisage statistical studies. Moreover, these binary systems let us study in detail the mass transfer episodes before and after the common envelope, and I present the evidences for mass transfer - and accretion - prior to the common envelope phase.

When a couple's squabbling leads to cosmic bubbles

The Direct Simulation Monte Carlo (DSMC) method, developed originally to calculate rarefied gas dynamical problems, is applied to continuous flow including shocks assuming that the Knudsen number is sufficiently small. In particular, we study the formation of spiral shocks in the accretion disc of a close binary system. The method involves viscosity and thermal conduction automatically, and can thus simulate turbulent viscosity.

Application of the DSMC method to astrophysical flows

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