For stars made of ordinary gas, stellar seismology offers the opportunity to use resonant mechanical properties to derive astronomically interesting information about stellar interior structure and composition. For white dwarfs seismology offers similar opportunities as well as the chance to learn fundamental physics. For example, it is suspected that the electron degenerate cores of white dwarfs crystallize during cooling, but the calculations that suggest crystallization require observational confirmation.
In this project, Dr. Clemens will identify non-radial pulsation modes in ZZ Ceti stars (pulsating white dwarfs). This first step in their seismological analysis will be accomplished through two new methods. For the brighter targets (V< 15.5), time-resolved spectroscopy will be obtained with the Goodman spectrograph on the Southern Observatory for Astrophysical Research (SOAR) Telescope. Line profile variations will be used to measure their spherical degree, even at lower signal-to-noise ratios. For the fainter stars, time series photometry on the Goodman Spectrograph or the SOAR Optical Imager will be used to measure the amplitudes of combination modes, which can be used to identify their spherical degree. It is expected that the modes of 10-20 of the most interesting new ZZ Ceti stars discovered by the Sloan Digital Sky Survey will be identified during the three years of this effort.
This project will help support a diverse group of graduate and undergraduate students engaged in state-of-the-art astronomical research in University of North Carolina at Chapel Hill's Goodman Laboratory. The Goodman Laboratory specializes in giving students experience in astronomical instrumentation and the training of the next generation of instrument builders is a priority of the community.
" from August 15, 2007 through December, 2011. Three graduate students received training under this grant, in the major subject areas listed below. Non-radial Oscillations in Radio Pulsars Pulsars are spinning stars made of neutrons, and about as dense as the nucleus of an atom. They emit radio waves in a beam, like a lighthouse, and the pulses have interesting structure called "subpulses". We developed a new model for subpulse variations in classical pulsars that treats them as pulsations (Clemens & Rosen 2004, 2008) and we applied the model to the pulsar PSR B0943+10 (Rosen & Clemens 2008). If correct, this model may lead to way to measure the physical state of neutron star material, which cannot be created in a laboratory and is only accessible by astronomical observations. Studies of Pulsating DQ White Dwarfs White dwarfs are the final state of stars like the sun after they run out of fuel. Many of them show oscillations that allow us to probe their internal structure. Montgomery et al. (2008) discovered a new class of carbon atmosphere whate dwarfs that apparently show oscillatory behavior. This represents the first new class of white dwarf pulsators discovered in 25 years. We used funding from AST-0707381 to discover the second, third, fourth, and fifth members of the class (Barlow et al. 2008, Dunlap et al. 2010a,b). These stars are likely related to white dwarfs that explode as supernovae, and they hold the promise of extending the physics explored by WD seismology, while illuminating an interesting and important evolutionary route for making white dwarf stars. Fundamental Parameters of Hot Subdwarfs Hot subdwarf stars are a fairly rare kind of remnant formed when stars with companions become red giants and lose much of their mass in an interaction with the second star. We have discovered a number of new and interesting variable hot subdwarfs and conducted follow-up measurements to measure masses and radii via three different methods. Taken together, our work represents a nearly 15% increase in direct mass and radius measurements for hot subdwarfs. These measurements are important because they tie down the observed endpoint of formation models for these stars, which lie at the extreme blue end of the horizontal branch in the H-R diagram. In 2010, we discovered a periodic change in the pulsations of the hot subdwarf CS1246. These changes are consistent with changes in the light travel time from the star to the earth as the star orbits an unseen companion. The best-fit orbit using a subdwarf mass derived from pulsations (Barlow et al. 2010) showed the companion to have a mass near 0.12 Msun. We calculated that this orbit should generate reflex motion in CS 1246 with a maximum velocity of around 16.6 km s-1 To confirm this discovery, in late 2010 we constructed a new 2100 l/mm volume phase holographic (VPH) grating, and delivered it to the Goodman Spectrograph at the SOAR telescope in Chile. . Between November 2010 and May 2011, we acquired spectra at 19 epochs using the new grating and found radial velocities consistent with the predictions of the O-C pulse timings (Barlow et al. 2011b, see figure 1 below). The 2100 l/mm grating has also been used effectively by other projects at SOAR, especially the RESOLVE project for determining galaxy redshifts and kinematics (http://resolve.astro.unc.edu, Kannappan & Wei 2008). The higher resolving power and efficiency of the grating has made SOAR competitive with Keck and the VLT for obtaining very small velocity dispersions in ultra-compact dwarfs (see Norris & Kannappan 2011). With this project we demonstrated both our ability to manufacture custom gratings to enable specific science, and the broader impact of this approach.
