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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.Hi-res image
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 1.5-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 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.
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
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.