My research specialty is the structure and evolution of massive stars. Many massive stars are rotating much faster than the Sun, which adds to the challenge of studying them. At very high rotation speeds the stars become oblate spheroids and must be modelled in two dimensions.  Using 2D stellar structure and pulsation models, my research attempts to understand the interior of these rapidly rotating massive stars. My goal is to be able to self-consistently model the evolution of a massive star from the main sequence to the end of it's life.

Asterosiesmology

Asteroseismology is the main tool I use to probe the structure of massive stars.  Because rotating stars are not spherically symmetric, the pulsation frequencies change, and this must be modelled in two dimensions.  With sufficiently detailed models, the pulsation frequencies can be used to simultaneously constrain the age, mass, rotation rate, and convective core overshoot parameter of a particular star. 

Binary Stars

Many (perhaps even most) stars appear to have a companion.  Observations of a binary system can be used to constrain stellar parameters much more accurately than can be done for single stars. However, in any given binary system, the angle of inclination with respect to the plane of the sky is not known, so many of the parameters that can be determined are actually lower limits.  If we are lucky enough to view a system that undergoes eclipses, we can constrain the stellar parameters much more accurately, as we know the inclination angle must be close to 90^o.  In the best possible case, we can combine photometric and spectroscopic data for an eclipsing binary to completely determine the properties of the stars.  Using data of variable stars in close eclipsing binary systems observed by Kepler, I am trying to determine the parameters of the individual stars as accurately as possible. Many of the eclipsing binary systems observed by Kepler are extremely faint, so acquiring ground based spectroscopy is challenging.  My goal is to be able to use the parameters determined from fits to the photometric data combined with the pulsation frequencies to accurately constrain the stellar parameters without requiring spectroscopic observations.

Luminous Blue Variables

Luminous Blue Variables (LBVs) are a phase of stellar evolution characterized by large outbursts ejecting large quantities of mass.  This phase of evolution is poorly understood, and the mechanism driving the outbursts is not known.  I am investigating radial pulsation as one possible driving mechanism.  Models including time-dependent convection show that a more realistic treatment of convection can drive pulsations and "mini-outbursts" in massive stars, possibly driving increased mass loss.

Mass Loss

Massive stars lose significant amounts of mass through radiatively driven winds throughout their lifetimes.  Observations and simulations of winds show that the winds are often clumpy and asymmetric, but few studies have been done on how the mass loss affects the star itself.  Using 2D stellar evolution models that include mass loss as a function of position on the surface, I can study how the star evolves in terms of temperature, luminosity, and angular momentum.  Preliminary studies have shown that compared to the more traditional spherically symmetric mass loss, the differences can be significant.