Exploring the Universe with White Dwarf Stars:
the First Year of the Freshman Research Initiative Astronomy Stream
D. E. Winget, M. H. Montgomery, A. Allen, R. Falcon, J. Gomez, J. Ellis,
V. Havanur, K. Luecke, K. Melin, G. Miller, C. Rendon.
Department of Astronomy, University of Texas, Austin TX 78712, USA
Overview
This is the first year that students form the Freshman Research Initiative
(FRI) have been incorporated into the white dwarf research group at UTAustin. The focus of our group is on using white dwarf stars to study
fundamental questions in physics and astronomy. White dwarfs are stars at
the end of their lives. They have exhausted their energy reserves and have
shrunk to about the size of the Earth. They are supported by electron
degeneracy pressure and slowly cool by radiating their energy into space.
While they are faint, their simple structure makes them ideal astrophysical
laboratories for studying many questions, such as
What is the age of the universe?
● What is the mass of the (hypothetical) axion particle?
● How and when does crystallization occur in a dense stellar plasma?
● How does convection operate in a high-gravity environment?
● Are neutrinos emitted at the expected rate in white dwarf interiors?
● Do white dwarf systems contain orbiting planets?
●
Addressing these questions involves the application of specific
observational techniques, data reduction methods, and theoretical
modelling. The FRI students participate in all of these areas.
The Projects
Each student in the program has worked on one or more different research
projects. These projects have covered a wide variety of areas. For instance,
one project involved revamping our online web pages so that they once
again are a real-time view of the observations as we make them at the
telescope, as shown below (http://rocky.as.utexas.edu/argosonline/page2.html):
Light curve (top panel) and its fourier transform (bottom panel) of the
pulsating white dwarf GD154
Several of the projects have involved reducing archival data. In the figure
above, a judicious choice of aperture sizes has resulted in data with a
significantly higher signal-to-noise ratio; this allows us to find additional
pulsation modes, aiding in the seismology of this star.
Many white dwarf pulsators exhibit extreme stability in their pulsations,
allowing us to predict exactly when a pulse should arrive. An orbiting
planet induces a reflex motion of the white dwarf, and due to light-traveltime effects can produce periodic variations in the arrival times of the
pulses. In addition to the known candidate GD66, we have identified a new
candidate, PG1354+0108, shown below. If planetary in origin, this data
would indicate an orbital period of ~ 12 years.
Another project has focused on new ways of visualizing our data and the
pulsations, an example of which is shown to the far left. Precise light
curves also allow us to model convection in these objects. Using the
observed nonlinear light curves, we model the temperature perturbations
on the stellar surface and their effect on the convection zone depth (left);
this provides constraints on the physics of convection in these objects as
well as on the surface distribution of the temperature variations. Finally,
we have begun to make 2D simulations of convection in white dwarfs
(above), with the ultimate goal or running 3D simulations. This will
allow us to make a direct comparison between our convective light curve
fits and numerical hydrodynamic simulations of convection.