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Research Interests

Orion's belt, comprising three massive stars: (left-to-right) [[simbad:Alnitak]], [[simbad:Alnilam]] and [[simbad:Mintaka]]. This is a reduced-size version of a stunning [ APOD] image created by Davide De Martin from the Digitized Sky Survey.

Orion's belt, comprising three massive stars: (left-to-right) Alnitak, Alnilam and Mintaka. This is a reduced-size version of a stunning APOD image created by Davide De Martin from the Digitized Sky Survey.

I have quite a variety of research interests, but most of them fall under the aegis of the astrophysics of massive stars. Although quite rare, massive stars are a dominant source of light in galaxies. Their high surface temperatures mean that much of this light is emitted as ultraviolet photons, which ionize the gas in adjacent star-forming regions to produce beautiful nebulae such as the ones seen in the image to the right.

These UV photons also drive fast and dense wind outflows from massive stars, whose energy and momentum sculpt huge bubbles in the interstellar medium. This feedback continues throughout the stars' brief lifetimes, until they finally explode as supernovae, enriching the interstellar medium with nucleosynthetically-processed material and triggering new waves of star formation. This underscores the pivotal role played by massive stars in governing the evolution of the gas, dust, and stellar populations composing galaxies.

Massive stars fascinate me because of the many opportunities they offer for studying dynamical phenomena — winds, oscillations, rotation, magnetic fields — in unprecedented detail. In the following sections, I outline some of the research projects involving these phenomena, that I've been undertaking with my collaborators.

  1. Non-Radial Oscillations
  2. Circumstellar Magnetospheres
  3. Radiative Transfer
  4. Spectral Compositing

Non-Radial Oscillations

As massive stars evolve across the main sequence, they inevitably must pass through one or more instability strips. These are regions of the Hertzsprung-Russell diagram where global oscillations are excited in stars' interiors, by a naturally occurring heat engine that converts thermal energy into mechanical energy. The oscillations — which are akin to terrestrial earthquakes — perturb the surface geometry and brightness distribution of a star, leading to distinctive periodic variations in the star's observable properties (brightness, line profiles).

By comparing the measured frequencies of these variations against theoretical predictions, it becomes possible to leverage the oscillations to probe the interior of a star, much as an terrestrial geologist studies earthquakes to determine the structure of the Earth deep beneath our feet. This technique of asteroseismology has already proven remarkably successful in revealing the internal structure of the Sun. With the advent of space-based seismic facilities such as MOST and CoRoT, and most recently with the launch of Kepler, we are arguably now enjoying the golden age of asteroseismology.

Circumstellar Magnetospheres

A simulation of the circumstellar matter distribution of σ Ori E, as predicted by the Rigidly Rotating Magnetosphere model.

A simulation of the circumstellar matter distribution of σ Ori E, as predicted by the Rigidly Rotating Magnetosphere model.

Massive stars aren't expected to harbor magnetic fields, owing to the absence of the sub-surface convection zone to act as a field-generating dynamo. In spite of this, strong (~kG) fields are observed in a number of objects, including the helium-strong chemically peculiar stars (e.g., σ Ori E), the helium-weak stars (e.g., 36 Lyn) and a few more-massive O-type stars (e.g., θ1 Ori C). With new generations of high-resolution spectropolarimeters such as ESPaDOnS, the list of magnetic massive stars continues to grow; recent additions include V* NU Ori and V* LP Ori (see Petit et al. 2008).

My interests in magnetic massive stars focus around the observation and theory of magnetospheres — circumstellar environments where the dynamics of wind outflows are significantly affected by the presence of the magnetic field. In the standard magnetically confined wind shock (MCWS) paradigm developed by Babel & Montmerle (1997), a sufficiently strong field can channel the supersonic wind into a head-on collision with itself. The resulting shocks heat the wind material to high temperatures (~107 K), explaining why observations of many magnetic massive stars (e.g., Gagné et al. 2005) reveal them to be moderately hard (≥ 1 keV) X-ray sources.

I've been involved in developing a variety of models that are proving useful in interpreting observations of massive-star magnetospheres:

Radiative Transfer

To come

Spectral Compositing

To come

Updated 2009-10-13 12:28:48