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 above.

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.

The Massive Stars Group at UW-Madison undertakes research into dynamical phenomena of massive stars — winds, oscillations, rotation, and magnetic fields. Primarily, our focus is on the theory side of these phenomena, but we place a strong emphasis on matching theoretical predictions (typically, in the form of computer models) against observational data. Hence, we maintain strong collaborative ties with the wider massive-star community, and in particular are deeply involved in the MiMeS (Magnetism iMassive Stars) Project.

The following topics are researched extensively in this project run by Richard Townsend.

Magnetospheres & Winds

Massive stars aren’t expected to harbor magnetic fields, due to their lack of a 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; τ Sco, shown above). 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).

Within the Massive Stars Group, our primary focus is on the observation and theory of magnetospheres — the 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.

Pulsation & Rotation

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.

GPU Computing