Methods of searching for planets around pulsars

We analyse different methods of searching for planets around neutron stars by timing observations of pulsars. To this end, we study a few interesting models describing TOA residual variations that are observed, or could be observed, and which can mimic planets. We carry out a detailed theoretical analysis of the behaviour of these methods in the situations mentioned. We show that it is very helpful to look at these phenomena as some kind of quasi-periodic variations of residuals of time of arrival of pulsar pulses. We demonstrate that such a model-independent approach leads to promising conclusions that can be useful when analysing timing observations of pulsars to find planets or to prove that observed phenomena are of planetary origin.     

Debris discs around nearby solar analogues

An unbiased search for debris discs around nearby Sun-like stars is reported. 13 G-dwarfs at 12–15 parsec distance were searched at 850 μm wavelength, and a disc is confirmed around HD 30495. The estimated dust mass is  0.008 M_⊕  with a net limit  ≲0.0025 M_⊕  for the average disc of the other stars. The results suggest there is not a large missed population of substantial cold discs around Sun-like stars – HD 30495 is a bright rather than unusually cool disc, and may belong to a few hundred Myr old population of greater dust luminosity. The far-infrared and millimetre survey data for Sun-like stars are well fitted by either steady state or stirred models, provided that typical comet belts are comparable in size to that in the Solar system.     

A search for radio pulsars around low-mass white dwarfs

Low-mass white dwarfs can be produced either in low-mass X-ray binaries by stable mass transfer to a neutron star, or in a common envelope phase with a heavier white dwarf companion. We have searched eight low-mass white dwarf candidates recently identified in the Sloan Digital Sky Survey for radio pulsations from pulsar companions, using the Green Bank Telescope at 340 MHz. We have found no pulsations down to flux densities of 0.6–0.8 mJy kpc⁻² and conclude that a given low-mass helium-core white dwarf has a probability of  <0.18 ± 0.05  of being in a binary with a radio pulsar.     

Time-dependent models of the structure and stability of self-gravitating protoplanetary discs

Angular momentum transport within young massive protoplanetary discs may be dominated by self-gravity at radii where the disc is too weakly ionized to allow the development of the magneto-rotational instability. We use time-dependent one-dimensional disc models, based on a local cooling time calculation of the efficiency of transport, to study the radial structure and stability (against fragmentation) of protoplanetary discs in which self-gravity is the sole transport mechanism. We find that self-gravitating discs rapidly attain a quasi-steady state in which the surface density in the inner disc is high and the strength of turbulence very low (  α∼ 10⁻³  or less inside 5 au). Temperatures high enough to form crystalline silicates may extend out to several astronomical units at early times within these discs. None of our discs spontaneously develop regions that would be unambiguously unstable to fragmentation into substellar objects, though the outer regions (beyond 20 au) of the most massive discs are close enough to the threshold that fragmentation cannot be ruled out. We discuss how the mass accretion rates through such discs may vary with disc mass and with mass of the central star, and note that a determination of the     relation for very young systems may allow a test of the model.     

The use of genetic algorithms to model protoplanetary discs

The protoplanetary discs of T Tauri and Herbig Ae/Be stars have previously been studied using geometric disc models to fit their spectral energy distribution (SED). The simulations provide a means to reproduce the signatures of various circumstellar structures, which are related to different levels of infrared excess. With the aim of improving our previous model, which assumed a simple flat-disc configuration, we adopt here a reprocessing flared-disc model that assumes hydrostatic, radiative equilibrium. We have developed a method to optimize the parameter estimation based on genetic algorithms (GAs). This paper describes the implementation of the new code, which has been applied to Herbig stars from the Pico dos Dias Survey catalogue, in order to illustrate the quality of the fitting for a variety of SED shapes. The star AB Aur was used as a test of the GA parameter estimation, and demonstrates that the new code reproduces successfully a canonical example of the flared-disc model. The GA method gives a good quality of fit, but the range of input parameters must be chosen with caution, as unrealistic disc parameters can be derived. It is confirmed that the flared-disc model fits the flattened SEDs typical of Herbig stars; however, embedded objects (increasing SED slope) and debris discs (steeply decreasing SED slope) are not well fitted with this configuration. Even considering the limitation of the derived parameters, the automatic process of SED fitting provides an interesting tool for the statistical analysis of the circumstellar luminosity of large samples of young stars.     

Metallicity, debris discs and planets

We investigate the populations of main-sequence stars within 25 pc that have debris discs and/or giant planets detected by Doppler shift. The metallicity distribution of the debris sample is a very close match to that of stars in general, but differs with >99 per cent confidence from the giant planet sample, which favours stars of above average metallicity. This result is not due to differences in age of the two samples. The formation of debris-generating planetesimals at tens of au thus appears independent of the metal fraction of the primordial disc, in contrast to the growth and migration history of giant planets within a few au. The data generally fit a core accumulation model, with outer planetesimals forming eventually even from a disc low in solids, while inner planets require fast core growth for gas to still be present to make an atmosphere.     

Gravitationally unstable protoplanetary discs

The possibility that protoplanetary gaseous discs are dynamically unstable to axisymmetric and non-axisymmetric gravity perturbations (e.g. those produced by spontaneous disturbances) with characteristic scales larger than the vertical scale height is discussed analytically, using a local Wentzel–Kramers–Brillouin (WKB) approach. It is shown that such discs might be clumpy, and these gravitationally bound clumps may later collapse to become giant planets ('hot Jupiters'). The chief aim in this paper is to underscore a fact of vital importance for application in the planetary formation process: gravitationally unstable non-axisymmetric (spiral) perturbations can effectively transport both the angular momentum and the mass in a spatially inhomogeneous disc.     

Spectral features in gamma-rays expected from millisecond pulsars

In the advent of next generation gamma-ray missions, we present general properties of spectral features of high-energy emission above 1 MeV expected for a class of millisecond, low magnetic field (∼10⁹ G) pulsars. We extend polar-cap model calculations of Rudak & Dyks by including inverse Compton scattering events in an ambient field of thermal X-ray photons and by allowing for two models of particle acceleration. In the range between 1 MeV and a few hundred GeV, the main spectral component is the result of curvature radiation of primary particles. The synchrotron component arising from secondary pairs becomes dominant only below 1 MeV. The slope of the curvature radiation spectrum in the energy range from 100 MeV to 10 GeV strongly depends on the model of longitudinal acceleration, whereas below ∼100 MeV all slopes converge to a unique value of 4/3 (in a ν ℱ _ν convention). The thermal soft X-ray photons, which come either from the polar cap or from the surface, are Compton upscattered to a very high energy domain and form a separate spectral component peaking at ∼1 TeV. We discuss the observability of millisecond pulsars by future high‐energy instruments and present two rankings relevant for GLAST and MAGIC. We point to the pulsar J0437−4715 as a promising candidate for observations.