Scientists are constantly finding new ways to look at things, and that’s especially true for objects that represent an outlier of their specific type. Adjectives like “biggest”, “brightest”, or “fastest spinning” all seem to attract scientific studies - perhaps because they’re an easier sell to funding agencies. No matter the reason, that means we typically get a lot of good science on specific objects that represent their particular class of objects well, and a new paper from Ozcan Caliskan from Istanbul University in Turkey hits that nail on the head when it comes to the most massive known white dwarf star.
A white dwarf is what gets left behind after a star has burned through its fusion fuel. Essentially they are the final form of the majority of stars in our universe, meaning they are already abundant, but will grow even more so as the universe ages. There are plenty of interesting things to study about white dwarfs, but one particular characteristic was of interest for this paper - some of them pulsate.
In particular, there is a sub-group of white dwarf stars that pulsate, but also have hydrogen-rich atmospheres, that are known as ZZ Ceti variables. The most massive of these stars is WD J004917.14-252556.81, or WD J0049-2525 for (slightly) short(er). Located about 315 light years away in the southern constellation Sculptor, WD J0049-2525 has already been the subject of several observational campaigns that have attempted to calculate the major characteristics of the star.
Fraser discusses the white dwarf cooling process - and how weird it is.Scientists already knew its mass of about 1.29 solar masses, making it the most massive ZZ Ceti variable known. They also calculated that the core of the star was around 99% crystallized, meaning the core has cooled to a point where the repulsion of the positively charged nuclei pushes against the gravitational field of the star’s mass, creating a “solid” inner core. That same cooling process puts the star into the ZZ Ceti “instability strip”, which is the temperature at which they start pulsating.
Dr. Caliskan and his team used three different telescopes - the New Technology Telescope, based at the La Silla Observatory in Chile, the Apache Point Observatory in New Mexico, and the Gemini South Telescope also in Chile - to collect data on the brightness of WD J0049-2525 over the course of the last two years. When all the data was collected, they ended up with a treasure-trove of observations that they went to work with statistical analysis on.
The outcome of that work was a discovery of 13 different “pulsation modes” - which represent oscillations in the star’s core that are reflected by changes in its brightness over long periods. Previously, scientists had known about two pulsation modes, which represented “dipole” and “quadrupole” spacings of oscillation frequencies. However, with only those two pulsation modes, it was hard for scientists to tease out details about the star's interior. With 13 of them, there is a much more statistically rich signal for them to do so.
Fraser discusses how white dwarfs crystallize.And that is exactly what they did - calculations of WD J0049-2525’s mass, it’s crystallization, and even the composition of its core (which is primarily made up of oxygen and neon, surrounded by a thin film of hydrogen), were all calculated using the detailed provided by its oscillation patterns. These findings will contribute to scientist’s wider understanding of stellar evolution, and in particular that of stars that end up as super-massive white dwarfs.
Further observations would again be helpful, as there is one characteristic of WD J0049-2525 that remains unresolved - its orbital period. The authors came up with values of either 7.28 hours (which appears more likely) or 16.03 hours, however there would need to be follow-up observations to confirm either of these values. It is unclear when those follow-up observations would be forthcoming, but, at least for now, science understands even more about this particular type of extreme star, and what needs to be done to learn more about it.
Learn More:
Phys.org - Asteroseismology study uncovers new pulsation modes in ultra-massive white dwarf
O Caliskan et al - Asteroseismology of WD J004917.14-252556.81, the Most Massive Pulsating White Dwarf
UT - White Dwarfs Pause Their Cooling, Giving Planets a Second Chance for Habitability