Annual Meeting of the German Astronomical Society

September 17, 2022
euroWD conference group picture
Hot stars in the Gaia era splinter at the Annual Meeting of the German Astronomical Society.

From September 12 to 16 we attended the Annual Meeting of the German Astronomical Society in Bremen. Two members of our group, Nicole Reindl and Veronika Schaffenroth, organised - together with two other German colleagues - a splinter meeting dedicated to "Hot stars in the Gaia era". During this splinter many of our students had the opportunity to present the results of their work and we could learn about the challenges involved in analyzing other types of hot stars, such as Wolf-Rayet stars, or hot supergiants and main-sequence stars. Finally, Veronika Schaffenroth gave an excellent plenary talk on the impact of TESS on our understanding of compact binary stars.

European Workshop on White Dwarfs in Tübingen

August 31, 2022
euroWD conference group picture
Conference group picture.

From August 14 to 19 we attended the European Workshop on White Dwarfs in Tübingen to hear and discuss what will happen to hot subdwarfs in a later stage of their lives.

Welcome to Matti Dorsch

July 7, 2022
Welcome Matti

We are very happy to welcome Matti Dorsch as a new research fellow in our group. He will be analysing photometric and spectroscopic data of hot subdwarf stars.

Hot subdwarf meeting in Liege

June 24, 2022
sdOB10.0 conference group picture
Conference group picture.

From June 13 to 17 we attended the 10th meeting on hot subdwarfs and related objects in Liege, Belgium. It was a great pleasure to meet many old and new friends, to discuss about exciting, new scientific results, and to spent some cheerful evenings together.

New hot subwarf catalogs released

June 14, 2022
sdOBs Gaia EDR3
The known hot sub luminous stars shown on the Gaia colour-magnitude diagram. Image credit: A&A.

We have used the great improvement in the quality of the European Space Agency's Gaia space observatory's astrometric and photometric measurements in the latest data release to increase to 61,585 the number of hot subdwarf candidates. This increase of 55% compared to our previous catalogue from 2019 has been made possible by improving the detection and selection methods and criteria. We have also used the latest data from the LAMOST spectroscopic survey from the National Astronomical Observatories, China to update and expand the catalogue of known hot subdwarf stars. This latest version contains 6,616 hot subluminous stars representing an increase of 742 stars compared to our previous release of the catalogue in 2020.
Hot subdwarf stars are very old objects that are thought to require stellar interactions to evolve to their current state. One evolutionary path requires that the outer hydrogen layers of a star are stripped through interaction with a stellar partner to expose its small, hot, and dense core. Studying hot subdwarfs gives us an insight into the binary interactions required to create such objects as well as allowing us to directly study the cores of stars without the outer hydrogen layers hiding their secrets. These catalogues have been presented in a paper by Rick Culpan, Dr. Ingrid Pelisoli, Dr. Nicole Reindl, and Prof. Stephan Geier together with colleagues from the European Southern Observatory which has now been published in A&A.

Any yet they move (or not)

May 12, 2022
Welcome Matti
Image credit: A&A.

In a paper published in A&A we performed a radial velocity (RV) variability study of a sample of 646 hot subdwarfs with multi-epoch radial velocities based on spectra from large area surveys, which turned out to be quite inhomogeneous across the studied parameter ranges. A striking feature is the completely dissimilar behaviour of He-poor and He-rich hot subdwarfs. While the former have a high fraction of close binaries, almost no significant RV variations could be detected for the latter. This has led us to the conclusion that there is likely no evolutionary connection between these subtypes. On the other hand, intermediate He-rich- and extreme He-rich sdOB/Os are more likely to be related. Furthermore, we conclude that the vast majority of this population is formed via one or several binary merger channels. The coolest hot subdwarfs tend to show fewer and smaller RV-variations. These objects might constitute a new subpopulation of binaries with longer periods and late-type or compact companions. The RV-variability properties of the extreme horizontal branch (EHB) and corresponding post-EHB populations of the He-poor hot subdwarfs match and confirm the predicted evolutionary connection between them. Stars found below the canonical EHB at somewhat higher surface gravities show large RV variations and a high RV variability fraction. These properties are consistent with most of them being low-mass EHB stars or progenitors of low-mass helium white dwarfs in close binaries.

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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