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Helium’s Role in the Pulsation of Early White Dwarfs

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Before low-medium mass stars become white dwarfs they pulsate wildly and eventually spew their outer layers into space – often forming beautiful planetary nebulae. The same stars are predicted to continue pulsating during their transformation to a white dwarf, if they have helium in their atmospheres. A team from the University of Oklahoma used Gemini North, in conjunction with the 1.2-meter FLWO telescope in Arizona, to observe a much-sought-after link between these pulsations and helium in the star’s atmospheres.

The researchers studied a trio of low mass white dwarf precursors, each with a mass less than one-third the mass of our Sun, and with pulsations ranging from approximately 5-10 minutes. According to team leader Dr. Alexandros Gianninas these GMOS-N observations appear to confirm the predictions of models based on non-adiabatic pulsation theory that predict the helium connection. “The nature of the observed pulsations matches almost perfectly with the predictions of our models,” said Gianninas. “Helium is the crucial ingredient that allows these stars to pulsate; models that don't include it don't predict pulsations. Our discovery represents the first concrete proof that these soon-to-be white dwarfs must still have helium at or near the surface.” The team plans to continue with additional observations to pinpoint the thickness of the hydrogen layer, and how it interacts with the helium, to better understand the dynamics of the oscillations.

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.

Dr. Alexandros Gianninas is a postdoctoral fellow at the University of Oklahoma and was assisted in this work by undergraduate student Brandon Curd, Professor Mukremin Kilic, Professor Gilles Fontaine at Université de Montréal and Dr. Warren Brown at the Smithsonian Astrophysical Observatory.

The team’s results are published in The Astrophysical Journal Letters, 822, L27.

Paper Abstract:

We report the discovery of pulsations in three mixed-atmosphere, extremely low-mass white dwarf (ELM WD, M ≤ 0.3 M⊙) precursors. Following the recent discoveries of pulsations in both ELM and pre-ELM WDs, we targeted pre-ELM WDs with mixed H/He atmospheres with high-speed photometry. We find significant optical variability in all three observed targets with periods in the range 320–590 s, consistent in timescale with theoretical predictions of p-mode pulsations in mixed-atmosphere ≈0.18 Me He-core pre-ELM WDs. This represents the first empirical evidence that pulsations in pre-ELM WDs can only occur if a significant amount of He is present in the atmosphere. Future, more extensive, timeseries photometry of the brightest of the three new pulsators offers an excellent opportunity to constrain the thickness of the surface H layer, which regulates the cooling timescales for ELM WDs.

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