Research Interests and Experience

General Research Interests

My research interests are focussed on the theory and modelling of accretion processes as well as star and planet formation mechanisms. I am particularly interested in the stability properties of accretion discs, the generation of MHD turbulence and associated angular momentum transport, disk - planet interactions and the origin and characteristics of jets and winds from accreting systems.

Research Projects Conducted/ in progress

For my PhD project I conducted a linear analysis of the vertical structure and growth of the magnetorotational instability (MRI). This is a fundamental topic, because this instability is the leading mechanism to generate MHD turbulence within the disk and, as such, it measures the ability of the magnetic field to remove angular momentum from the disk material so accretion can proceed. Knowledge of the configuration and properties of the turbulent zones is important also because of their relevance for planet formation/migration theories and models. The method incorporated vertical stratification and non-ideal MHD regimes appropriate for weakly ionised accretion discs, such as protostellar and quiescent dwarf novae systems.

The conductivity was treated as a tensor and, as a first approximation, was assumed to be constant with height. I obtained solutions for the structure and growth rate of global unstable modes for different conductivity regimes, strengths of the magnetic coupling and the initial magnetic field, which was assumed to be vertical. This study revealed that, for a weak magnetic coupling, the Hall effect causes perturbations to grow faster and act over a more extended cross-section of the disc than those obtained using the ambipolar diffusion approximation. It also showed that ambipolar diffusion perturbations peak higher above the midplane than those including Hall conductivity, a consequence of the action of competing growth rates at different heights in this regime. Finally, we showed that Hall conductivity determines the growth of the MRI in fluid conditions satisfied over a wide range of radii in weakly ionised discs, reducing the extent of the magnetic `dead zone'. As a result, considerable accretion can take place in regions closer to the midplane, despite the weak magnetic coupling, rather than in the surface regions, which have a much stronger coupling, but significantly less fluid density. The results of this study are published in Monthly Notices of the Royal Astronomical Society (Salmeron & Wardle 2003, M.N.R.A.S., 345:992-1008).

In a second part of this work, a realistic height-dependent conductivity tensor was evaluated under the assumption that dust grains have settled towards the midplane of the disc, an assumption valid for late evolutionary stages of accretion. Ionisation sources included cosmic rays, x-rays from the central protostar and the effect of radioactive elements within the disc. The vertical structure and growth rate of unstable modes were obtained for a range of radial positions within the disc and different strengths of the initial magnetic field. Results of this study indicate that the MRI can grow at a significant fraction of its ideal rate for a wide range of magnetic field strengths and radial locations. At 1 AU, for example, unstable modes are found for 1 mG < B < 10 G. Some of these modes grow in regions close to the disc midplane. These results have been published in Salmeron & Wardle, 2005, M.N.R.A.S., 361: 45-69. In a final part of this project I computed these solutions for a disc where dust grains are well mixed with the gas phase. When a population of 0.1 micron grains is assumed to be present, perturbations grow at 10 AU for B < 10 mG. We estimate that the figure for R = 1 AU would be of order 400 mG. These results are yet unpublished.

I am now working on obtaining local disc-wind solutions which incorporate the vertical structure of the disc, in particular a conductivity tensor varying with height. I expect that this study will help to clarify the critical effect of the disc structure on the properties of winds launched from protostellar accretion discs. Through this analysis it will also be possible to explore the viability of such winds at distances of the order of a few AU from the central object as well as obtain a realistic estimate of the wind mass loss rate. I also expect to model the fractions of angular momentum transported via the MRI and outflows, and investigate the implications to planet formation and migration.

Finally, as an aeronautical engineer, I have also completed a number of research projects in the aviation industry. For my honours thesis, I developed a method and the computer codes to design a remotely-piloted vehicle to be used by the Venezuelan Navy to test the automatic shooting systems of Mariscal Sucre - type frigates. Also, as an Aviation Analyst, I developed a new method to quantify the probability of collisions of large aircraft operating in parallel taxiways. The method was, essentially, an ``extreme value'' model, as it is expected that the extreme deviations that would contribute to the probability of collisions will be beyond the range of observed deviations during normal operations.

Future Research

I am interested in conducting research and collaborating with colleagues in the fields of disk/accretion dynamics, star and planet formation and related phenomena. I find the analysis of MHD turbulence and angular momentum transport in accretion discs, as well as the dynamics of jets and winds from protostellar and compact objects, both important and interesting research topics.

The development of detailed disc models, with a realistic treatment of the relevant microphysics, including radiation dynamics and non-ideal processes, where applicable, is crucial for our understanding of accretion mechanisms. The analysis of sufficiently detailed accretion disc models will enable us to generate useful predictions, which could potentially be tested against observations.