OU Astrophysicists Discover a Trio of Pulsating, Mixed-Atmosphere, Low-Mass White Dwarfs
Light curves (left) and Fourier amplitude spectra (right) for the three new pulsating low-mass white dwarfs. The red tick marks denote the significant frequencies which lie above the detection threshold of four times the average noise level.
A team of OU astrophysicists, lead by postdoctoral fellow Dr. Alexandros Gianninas, undergraduate student Brandon Curd, and Prof. Mukremin Kilic, recently discovered radial and non-radial pulsations in a trio of low-mass white dwarf stars. The initial analysis of these low-mass white dwarfs showed that their atmospheres, the topmost layer of the star, contained large amounts of helium mixed with hydrogen. Following these findings, detailed pulsation models were computed and they revealed that the large quantities of helium could create the physical conditions necessary for pulsations to occur.
Based on these results, the group obtained several hours of optical timeseries photometry for the three candidates at the 8m Gemini-North telescope in Hawaii and the FLWO 1.5m telescope in Arizona. The resultant light curves and Fourier transforms are pictured above and clearly show the detection of pulsations with periods in the range 320-590 seconds. Similar pulsating low-mass white dwarfs have been discovered in recent years but this discovery presents the first compelling evidence that helium is indeed the crucial ingredient that makes the observed pulsations possible in the first place. Extensive follow-up time series observations of these low-mass white dwarfs, possibly with the Hubble Space Telescope, will permit a detailed asteroseismological analysis which will place strong constraints on the thickness of the hydrogen layer at the surface of the star. This quantity is especially important since the amount of hydrogen at the surface regulates the cooling of the white dwarf and is a key parameter in the evolutionary models for low-mass white dwarfs.
These results are published in Gianninas et al. 2016, Astrophysical Journal Letters, 822, L27.