As the study of pulsating subdwarf B (sdB) stars has developed, one overriding question has emerged: do they pulsate in high-degree modes where the number of nodal lines is greater than three? Models need to include such modes to account for the observed number of pulsations, but they are less likely to be observed because of geometric cancelation across the stellar surface. This has constrained the usefulness of the models for inferring the interior conditions of sdB stars. There is currently no answer to this controversial question about high-degree modes, but techniques such as multicolor photometry and time-series spectroscopy can be applied to provide that answer. To address these issues, this project will 1) obtain high-speed multicolor observations of at least six pulsating sdB stars; 2) obtain time-series spectroscopic data of at least two stars; 3) generate perturbed atmospheric models to interpret the observations; and 4) generate evolutionary models to match constrained modes and test the physics within sdB stars. Multicolor photometry is especially sensitive to high-degree modes, yet can be obtained from modest-sized telescopes. This work will critically test the current generation of models by determining if the predicted high-degree modes exist, and will produce models incorporating the latest theories to match the observations.
Increasing the accuracy of sdB models will increase understanding of the cores inside most horizontal branch stars, and mass constraints will test evolution scenarios. The research involves undergraduate students, who will obtain, reduce, and analyze data and disseminate the results at conferences and in papers.
This grant was to perform asteroseismological studies of a special type of late-evolution star. These are helium fusing stars which are similar to what the core of our Sun will be like in approximately another five billion years. About ten percent of these stars pulsate; that is they vibrate periodically in a manner which can be used to discern their internal structure (using seismology). Prior to the onset of our work, models had been produced which predicted these stars had specific pulsations known as high-degree modes, which should be difficult to observe. Our project sought to detect those pulsations, should they exist, as well as to generally constrain and test the models, which tell us the internal conditions of these stars. Intellectual merit of the proposed activity: Our main goal was to test structural models of these late-evolution stars. We used two different and complimentary techniques; we obtained three-color and radial velocity data which can be used to identify pulsation modes (especially useful for high-degree modes), and examined Kepler spacecraft photometric data. During our project, we proposed for and were awarded seven different week-long observing runs at Kitt Peak National Observatory to obtain multicolor and/or radial velocity data. Though some of the data are still being analyzed, we did not detect any high-degree modes, and successfully constrained several pulsation modes. Observationally, the project was a great success, though some of the data still require interpretation. NASA's Kepler spacecraft also provided new and exciting results which have been used to constrain stellar models. Kepler was a unique instrument which observed, nearly without break, over 150,000 stars, about 20 of which were pulsating evolved stars of interest to us. Though there is no color information, the parts-per-million precision and continuous observations spanning several years was sufficient to detect some unexpected results. Important results of our examination of Kepler data include finding period spacings among the pulsations. Prior to Kepler, models had predicted highly stratified stars of carbon/oxygen centers surrounded by helium cores, surrounded by a thin hydrogen atmosphere, all significantly layered. However, evenly spaced pulsation periods require a non-stratified star, as layers trap some of the pulsations. Additionally, the period spacings, along with frequency multiplets, allowed us to identify many of the pulsations as low-degree modes. In addition, we were able to determine the spin period of several of these stars, many of which are in binaries with a companion star. Gravity from the companion star should tidally lock the stars such that one side always faces the other. However, we measured spin periods much longer than the orbital periods of the two stars; a surprising discovery. We also used Kepler data to detect a feature of special relativity called Doppler beaming. During the portion of its orbit around its companion star when it is coming toward us, special relativity predicts, and we measured, that a star will be brighter than when it is moving away from us. Prior to the onset of this project, structural models of these stars were unconstrained by the observations. Now, that situation is completely reversed and new physics can be incorporated into the models from which we can learn more about physics within stars. Broader impacts resulting from this project: The stars of this study (called subdwarf B stars) were believed to be the cores of most helium fusing (called horizontal branch) stars. Our hot stars were the atmosphere-stripped centers, but they should exist within nearly every helium fusing star. What was speculation is now fact as the same period spacings have been found at both ends of the horizontal branch. So what we learn about subdwarf B stars applies to nearly all stars (surely there will be exceptions) along the horizontal branch. The layers we are seeing in our subdwarf B stars are typically hidden by thick and complex atmospheres and so these pulsators allow us a direct investigation into stellar cores that are still fusing helium. Additionally, because of their high temperatures and gravities, they are excellent laboratories for studying radiative levitation and gravitational settling. This is particularly important as iron and/or nickel levitation is the driving mechanism, and so constraints on pulsation produce constraints on levitation. Already our results indicate that diffusion must be more effective than previously thought, or else these stars would become highly stratified; a prediction which is at odds with our observational results. In addition to the broad range of scientific impacts, this was an RUI grant and so included undergraduate students in all phases of the research. During our grant, a total of seven undergraduate and three high school students participated in the work. These students appear as co-authors on our papers, they presented results at local, national, and even international meetings; interacting with other astronomers and applying what they learn in their courses to our data. Additionally, we work within an international collaboration of astronomers and the students involved in this project also worked with those astronomers. Four students traveled to Kitt Peak to help obtain data and one even reported at a Kepler conference when the data were still proprietary.
