Diagnostics of the pulsations before (red), during (green) and after (blue) the seventh outburst observed on PG 1149+057. The top panel shows the K2 light curve, while the bottom panels show the FT of each subset. Signals exceeding the significance thresholds (see text) are marked with appropriate-colored dots and show differences in the excited pulsation spectra. Amplitudes are essentially all lower after outburst, but subsequently grow (see Figure 5). Significant pulsations are also visible during the brightening event (green squares), and appear at shorter periods (higher frequencies) and higher amplitudes. The top right panel shows the window function of each subset. Credit: http://arxiv.org/pdf/1507.06319.pdf
Research confirms rapid brightening events in otherwise normal pulsating white dwarfs (burnt-out core of an evolved star, an extremely dense star almost entirely made up of carbon and oxygen). In addition to the regular rhythm from pulsations they expected on the white dwarf PG1149+057, which cause the star to get a few percent brighter and fainter every few minutes, the researchers also observed something completely unexpected every few days: arrhythmic, massive outbursts, which broke the star’s regular pulse and significantly heated up its surface for many hours.
Led by Dr JJ Hermes, the astronomers targeted Kepler spacecraft on a specific star in constellation Virgo, PG1149+057 is roughly 120 light years from Earth. White dwarfs have been known to pulsate for decades, and some are exceptional clocks, with pulsations that have kept nearly perfect time for more than 40 years. Pulsations are believed to be a naturally occurring stage when a white dwarf reaches the right temperature to generate a mix of partially ionized H atoms at its surface.
That mix of excited atoms can store up and then release energy, causing the star to resonate with pulsations characteristically every few minutes. Astronomers can use the regular periods of these pulsations just like seismologists use earthquakes on Earth, to see below the surface of the star into its exotic interior. Astronomers targeted PG1149+057, hoping to learn more about its dense core. In the process, they caught a new glimpse at these unexpected outbursts.
“These are highly energetic events, which can raise the star’s overall brightness by more than 15% and its overall temperature by more than 750 degrees in a matter of an hour,” said Dr Hermes. “For context, the Sun will only increase in overall brightness by about 1% over the next 100 million years.”
Recently, the Kepler also witnessed the first example of these strange outbursts while studying another white dwarf, KIC 4552982. There is a narrow range of surface temperatures where pulsations can be excited in white dwarfs, and so far irregularities have only been seen in the coolest of those that pulsate.
“That both stars exhibiting this new outburst phenomenon are right at the temperature where pulsations shut down suggests that the outbursts could be the key to revealing the missing physics in our pulsation theory.” Given the similarity between the first two stars to show this behaviour, they suspect it might have to do with how the pulsation waves interact with themselves, perhaps via a resonance.
“Ultimately, this may be a new type of nonlinear behaviour that is triggered when the amplitude of a pulsation passes a certain threshold, perhaps similar to rogue waves on the open seas here on Earth, which are massive, spontaneous waves that can be many times larger than average surface waves,” said Dr Hermes. http://www2.warwick.ac.uk/newsandevents/pressreleases/dying_star_suffers
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