arXiv subjects

Dave Syer

Publications and source records attributed to Dave Syer.

4 recordsLinked to original sources

arXiv1996

Non-axisymmetric, scale-free, razor-thin discs

Galaxies exhibit a variety of non-axisymmetric structure (bars, spiral structure, lopsided structure, etc.). These suggest the following general problem: what are the possible stationary configurations of a two-dimensional self-gravitating fluid other than an axisymmetric razor-thin disc? We address a modest component of this problem: we seek non-axisymmetric razor-thin discs of two-dimensional barotropic fluid that are stationary in an inertial frame. We distinguish between `razor-thin' and `two-dimensional,' applying the latter term to the equation of state. Furthermore we assume that our systems are scale-free, which reduces the partial differential equations describing the system to ordinary differential equations. We also allow for the presence of an axisymmetric background potential. This simple and highly idealized problem already exhibits a rich variety of solutions, the richest being for m=1 symmetry.

astro-ph↗

arXiv1995

Low Mass Companions to T~Tauri Stars: a Mechanism for Rapid-rise FU~Orionis Outbursts

We show that outside-in disc instabilities, which can produce rapid-\null rise FU Orionis outbursts in T~Tauri systems, are a natural consequence of the existence of protoplanetary protostellar companions. The \pcomp{} would be formed through gravitational instability on a dynamical timescale ($\sim 10^5\y$), and provided it was formed at less than a few \AU{}, it would be swept in to $\sim15\Rsun$ in less than $10^5\y$. A companion of mass $10^{-2}$\Msun at this radius would act as a flood gate which stores material upstream and periodically releases it following the ignition of the thermal ionisation instability.

astro-ph↗

arXiv1995

Harmonic Function Expansion of Nearly Oblate Systems

We show how to develop an expansion of nearly oblate systems in terms of a set of potential-density pairs. A harmonic (multipole) structure is imposed on the potential set at infinity, and the density can be made everywhere regular. We concentrate on a set whose zeroth order functions describe the perfect oblate spheroid of de Zeeuw (1985). This set is not bi-orthogonal, but it can be shown to be complete in a weak sense. Poisson's equation can be solved approximately by truncating the expansion of the potential in such a set. A simple example of a potential which is not one of the basis functions is expanded using the symmetric members of the basis set up to fourth order. The basis functions up to first order are reconstructed approximately using 10,000 particles to show that this set could be used as part of an $N$-body code.

astro-ph↗

arXiv1995

Satellites in Discs: Regulating the Accretion Luminosity

We demonstrate, using a simple analytic model, that the presence of a massive satellite can globally modify the structure and emission properties of an accretion disc to which it is tidally coupled. We show, using two levels of numerical approximation, that the analytic model gives reasonable results. The results are applicable to two astrophysical situations. In the case of an active galactic nucleus, we consider the case of a $\sim 10^3\Msun$ compact companion to the central black-hole and show that it could modulate the emitted spectrum on a timescale of $\sim10^5$ years. In the case of a T Tauri accretion disc, a satellite such as a sub-dwarf or giant planet could modify the disc spectral energy distribution over a substantial fraction of the T Tauri star lifetime.

astro-ph↗