Why Halley did not discover proper motion and why Cassini did

In 1717 Halley compared contemporaneous measurements of the latitudes of four stars with earlier measurements by ancient Greek astronomers and by Brahe, and from the differences concluded that these four stars showed proper motion. An analysis with modern methods shows that the data used by Halley do not contain significant evidence for proper motion. What Halley found are the measurement errors of Ptolemaios and Brahe. Halley further argued that the occultation of Aldebaran by the Moon on 11 March 509 in Athens confirmed the change in latitude of Aldebaran. In fact, however, the relevant observation was almost certainly made in Alexandria where Aldebaran was not occulted. By carefully considering measurement errors Jacques Cassini showed that Halleys results from comparison with earlier astronomers were spurious, a conclusion partially confirmed by various later authors. Cassinis careful study of the measurements of the latitude of Arcturus provides the first significant evidence for proper motion.

Using electromagnetic observations to aid gravitational-wave parameter estimation of compact binaries observed with LISA II: The effect of knowing the sky position

In this follow-up paper, we continue our study of the effect of using knowledge from electromagnetic observations in the gravitational wave (GW) data analysis of Galactic binaries that are predicted to be observed by the new textit{Laser Interferometer Space Antenna} in the low-frequency range, $10^{-4} :mathrm{Hz}<f<1 :mathrm{Hz}$. In the first paper, we have shown that the strong correlation between amplitude and inclination can be used for mildly inclined binaries to improve the uncertainty in amplitude, and that this correlation depends on the inclination of the system. In this paper we investigate the overall effect of the other orientation parameters, namely the sky position and the polarisation angle. We find that after the inclination, the ecliptic latitude of the source has the strongest effect in determining the GW parameter uncertainties. We ascertain that the strong correlation we found previously, only depends on the inclination of the source and not on the other orientation parameters. We find that knowing the sky position of the source from electromagnetic data can reduce the GW parameter uncertainty up to a factor of $sim 2$, depending on the inclination and the ecliptic latitude of the system. Knowing the sky position and inclination can reduce the uncertainty in amplitude by a factor larger than 40. We also find that unphysical errors in the inclinations, which we found when using the Fisher matrix, can affect the corresponding uncertainties in the amplitudes, which need to be corrected.

Gravitational waves from compact binaries

In this review, I give a summary of the history of our understanding of gravitational waves and how compact binaries were used to transform their status from mathematical artefact to physical reality. I also describe the types of compact (stellar) binaries that LISA will observe as soon as it is switched on. Finally, the status and near future of LIGO, Virgo and GEO are discussed, as well as the expected detection rates for the Advanced detectors, and the accuracies with which binary parameters can be determined when BH/NS inspirals are detected.

Masses and envelope binding energies of primary stars at the onset of a common envelope

We present basic properties of primary stars that initiate a common envelope (CE) in a binary, while on the giant branch. We use the population-synthesis code described in Politano et al. (2010) and follow the evolution of a population of binary stars up to the point where the primary fills its Roche lobe and initiates a CE. We then collect the properties of each system, in particular the donor mass and the binding energy of the donors envelope, which are important for the treatment of a CE. We find that for most CEs, the donor mass is sufficiently low to define the core-envelope boundary reasonably well. We compute the envelope-structure parameter {lambda_mathrm{env}} from the binding energy and compare its distribution to typical assumptions that are made in population-synthesis codes. We conclude that {lambda_mathrm{env}} varies appreciably and that the assumption of a constant value for this parameter results in typical errors of 20--50%. In addition, such an assumption may well result in the implicit assumption of unintended and/or unphysical values for the CE parameter {alpha_mathrm{CE}}. Finally, we discuss accurate existing analytic fits for the envelope binding energy, which make these oversimplified assumptions for {lambda_mathrm{env}}, and the use of {lambda_mathrm{env}} in general, unnecessary.

Parameter estimation for signals from compact binary inspirals injected into LIGO data

During the fifth science run of the Laser Interferometer Gravitational-wave Observatory (LIGO), signals modelling the gravitational waves emitted by coalescing non-spinning compact-object binaries were injected into the LIGO data stream. We analysed the data segments into which such injections were made using a Bayesian approach, implemented as a Markov-chain Monte-Carlo technique in our code SPINspiral. This technique enables us to determine the physical parameters of such a binary inspiral, including masses and spin, following a possible detection trigger. For the first time, we publish the results of a realistic parameter-estimation analysis of waveforms embedded in real detector noise. We used both spinning and non-spinning waveform templates for the data analysis and demonstrate that the intrinsic source parameters can be estimated with an accuracy of better than 1-3% in the chirp mass and 0.02-0.05 (8-20%) in the symmetric mass ratio if non-spinning waveforms are used. We also find a bias between the injected and recovered parameters, and attribute it to the difference in the post-Newtonian orders of the waveforms used for injection and analysis.

Common Envelope Mergers: A Possible Channel for Forming Single sdB Stars

We quantify an evolutionary channel for single sdB stars based on mergers of binaries containing a red giant star and a lower mass main sequence or brown dwarf companion in our Galaxy. Population synthesis calculations that follow mergers during the common envelope phase of evolution of such systems reveal a population of rapidly rotating horizontal branch stars with a distribution of core masses between 0.32 Mo - 0.7 Mo that is strongly peaked between 0.47 Mo - 0.54 Mo. The high rotation rates in these stars are a natural consequence of the orbital angular momentum deposition during the merger and the subsequent stellar contraction of the merged object from the tip of the red giant branch. We suggest that centrifugally enhanced mass loss facilitated by the rapid rotation of these stars may lead to the formation of single sdB stars for some of these objects.

Parameter estimation of spinning binary inspirals using Markov-chain Monte Carlo

We present a Markov-chain Monte-Carlo (MCMC) technique to study the source parameters of gravitational-wave signals from the inspirals of stellar-mass compact binaries detected with ground-based gravitational-wave detectors such as LIGO and Virgo, for the case where spin is present in the more massive compact object in the binary. We discuss aspects of the MCMC algorithm that allow us to sample the parameter space in an efficient way. We show sample runs that illustrate the possibilities of our MCMC code and the difficulties that we encounter.