Rare stellar wedding

February 14, 2022
Double white dwarf merger
Artist's illustration of a double white dwarf merger. Image Credit: N. Reindl.

While hunting for hot stars with the Large Binocular Telescope in Arizona, we have discovered a completely new type of star. While normal stellar surfaces are composed of hydrogen and helium, the analysis - led by Klaus Werner from the University of Tübingen and involving several members of our team - revealed that surfaces of two stars are covered in carbon and oxygen - the ash of a helium nuclear fusion. This is a very exotic composition for a star and the situation becomes more puzzling because the new stars have temperatures and radii that indicate they are still burning helium in their cores. These results have now been published as a letter in the Monthly Notices of the Royal Astronomical Society. Published back to back with this article, a second paper from a group of astronomers from the University of La Plata and the Max Planck Institute for Astrophysics in Garching offers a possible explanation for their formation. Their work shows that these newly discovered stars might have formed in a very rare kind of stellar merger event between two white dwarf stars. For binary systems formed with very specific masses, a carbon- and oxygen-rich white dwarf might be disrupted and accreted on top of a helium-rich white dwarf one, leading to the formation of these stars.

Press release: Royal Astronomical Society, University of Potsdam, University of Tübingen

Strong magnetic field discovered in a hot subdwarf star

February 10, 2022
Zeeman splitting
Zeeman-split He II 4686 Å line in the X-shooter spectrum of newly discovered and strongly magnetic hot subdwarf J0809-2627. Image Credit: A&A.

As the stars evolve, so do their magnetic fields. While impressive, the magnetic fields of stars such as the Sun are weak compared to the extremely strong magnetic fields observed for about 20% of white dwarf stars, which can reach strengths of hundreds of megagauss (MG). The origin of these strong fields yet remains unclear. Matti Dorsch, a PhD student in our group, and several members of our team have recently discovered a strong magnetic field in a hot subdwarf star (about 340 kG). For the first time, they modeled the spectral lines of this prototypical magnetic subdwarf, which are split by the linear Zeeman effect (see the figure above). This analysis, published as a letter in A&A, shows that this star will likely evolve to become an extremely magnetic white dwarf.

Looking into the cradle of the grave

January 5, 2022
The observed Hβ line (red) can be understood as a single broad absorption from the stellar photosphere filled in by a double peak emission from the circumstellar material. Image Credit: A&A.

Joris Vos (former Humboldt fellow in our group) and several members of our team, have recently discovered a new type of hot subdwarf, named J22564-5910. It is rapidly spinning and gas is still present in the systems. All observational evidence points towards J22564-5910 being observed very shortly after the merger phase that formed it. It is the first post-merger star observed this early after the merger event, and as such it is very valuable system to test merger theories. The study got now published in A&A.

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