News (2020)

Observational evidence that binary interaction is always required to form hot subdwarf stars

October 26, 2020
Period Histogram
Rotational period distribution of main sequence stars in wide binary sytems with a hot subdwarf (black) compared to the rotational period distribution of field main sequence stars (blue). Image Credit: A&A.

Hot subdwarfs require an episode of enhanced mass-loss to be formed. Binary evolution is most commonly evoked to explain such an episode, and in fact most hot subdwarfs are found in binaries. However, the existence of single hot subdwarfs has prompted suggestions that they could form without interaction with a companion. In a recent paper with contributions from many members of our group, we provide observational evidence that interaction is in fact always required to form hot subdwarfs. Analysing data from the TESS mission, we found that hot subdwarf companions in wide composite binaries show signs of having accreted mass and angular momentum in the past, leading to shorter rotation periods than field main sequence stars (see figure). Additionally, using data from the Gaia data release 2, we showed that the fraction of hot subdwarfs in even wider binaries, not predicted to interact, is much smaller than for their progenitors, which suggests that they cannot form in non-interacting binaries. The work, lead by Ingrid Pelisoli, got now published in in Astronomy&Astrophysics.

Lessons from an unlikely couple

October 23, 2020
Artist's impression of UCAC2 46706450 (Image Credit: Nicole Reindl)
Artist's impression of UCAC2 46706450 (Image Credit: Nicole Reindl).

White dwarfs with an F, G, or K-type companion are relatively rarely known systems in comparison to thousands of M stars with white dwarf companions. Several members of our group had the pleasure to investigate - together with Klaus Werner and others - one of these systems in more detail. The study, now published in Astronomy&Astrophysics, showed that the system, named UCAC2 46706450, consists of two very unlikely and special stars. The UV source turned out to be one of the hottest white dwarfs known and has an effective temperature of 105,000K. The cool star, one the other hand, was found to be chromospherically active and amongst the most rapidly rotating sub-giants known. The reason for its rapid rotation remains elusive, though, the authors speculate it could have formed during a stellar merger.

The Stars and Antiquity

October 8, 2020
The Astronomer
The Astronomer by Johannes Vermeer.

Interested in the roots of astronomy? Then the lecture series "The Stars and Antiquity" might be something for you. We organize this lecture series together with our colleague Filippo Carla-Uhink, who is professor for ancient history at the University of Potsdam. The talks will be given by experts both in history of science and astronomy. It is planned that most of the lectures will take place at the Neuen Palais, House 9, Room 1.12. Online participation is also possible. If you want to take part in the lecture series or in individual lectures, we ask you to register in advance at The program can be found here.

Stellar Interactions

September 27, 2020
Stellar Interactions
Conference setting in times of Corona.

On September 24 and 25 we hosted - together with our colleagues from the AIP and the Thüringer Landessternwarte Tautenburg - a very succesful splinter on Stellar Interactions as part of the Annual Meeting of the Astronomische Gesellschaft 2020. We had 32 talks covering the broad field of stellar interactions. In a Corona-friendly setting, the splinter partly took place at our institute.

Observed binary populations reflect the Galactic history

September 25, 2020
P-M-ratio relation
Schematic plot showing where wide hot subdwarf B binaries formed from the different Galactic sub-populations are located in the period mass ratio diagram (Image Credit: A&A).

Wide hot subdwarf B binaries with main-sequence companions are outcomes of stable mass transfer from evolved red giants. The orbits of these binaries show a strong correlation between their orbital periods and mass ratios. A new study led by Joris Vos, Humbolt research fellow in our group, now found a binary evolution model which can explain the observed correlation. The work has been published as highlight-paper in Astronomy&Astrophysics and demonstrates, for the first time, how the metallicity history of the Milky Way is imprinted in the properties of the observed post-mass transfer binaries. It shows that Galactic chemical evolution is an important factor in binary population studies of interacting systems containing at least one evolved low-mass component.
Press release: University of Potsdam

Congratulations to Tilaksingh Pawar

September 16, 2020
Corona-friendly champagne reception on the roof of our institute.

Today Tilaksingh Pawar successfully completed his master's thesis entitled "Variations in the light curves of hot subdwarfs". In October he will join the GeoPlanet Doctoral school at the Nicolaus Copernicus Astronomical Center in Torun, Poland, and work with Prof. Maciej Konacki and Dr. Krzysztof Helminiak on detached eclipsing binaries.

All-clear in space: Cuddling inside a nebula does not have to end in a catastrophe

June 24, 2020
Cuddling inside a nebula
Artist's impression of the two merging central stars inside the planetary nebula Henize 2-428 (Image Credit: Nicole Reindl).

Supernova type Ia explosions are important astronomical tools to measure cosmic distances. However, the nature of the progenitor systems that leads to this kind of explosion is still elusive. The only definite candidate for a progenitor system was the binary nucleus nestled in the heart of the planetary nebula Henize2-428. The two central stars are actually so close that they even share a common envelope and will merge within a few hundred million years. Back in 2015, an article published in Nature claimed a total mass of the two stars of about 1.8 times that of the Sun, which is high enough for triggering a supernova type Ia explosion. Contrary to this, a careful re-analysis of the system now shows that no supernova explosion is to fear. The study was led by Nicole Reindl, open-topic research fellow in our group, and the results of this work have now been published as a highlight-paper in Astronomy&Astrophysics. Together with her international team, which also includes two other members of our group, she revealed that a contamination of the observations by diffuse interstellar bands has led to a significant overestimation of the masses of the two central stars. It was found that the total mass of the system actually only slightly exceeds one solar mass, far too low to trigger a supernova Ia explosion. Thus, the race to find a definite supernova type Ia progenitor system is now re-opened.
Press release: University of Potsdam

New version of the hot subdwarf catalog

March 1st, 2020
Hot subdwarfs
Sky distribution of the objects in the catalog in equatorial coordinates (Image Credit: A&A).

Stephan Geier's updated version of the hot subdwarf catalog has now been published in Astronomy&Astrophysics. This new version of the catalog was motivated by substantial new discoveries of hot subdwarfs by ongoing spectroscopic surveys and the availability of new all-sky data from ground-based photometric surveys and the Gaia mission Data Release 2. The catalog contains 5874 unique sources including 528 previously unknown hot subdwarfs and provides multi-band photometry, astrometry from Gaia, and classifications based on spectroscopy and colours. In addition to that atmospheric parameters of 2187 stars and radial velocities of 2790 stars from the literature are provided.

When David poses as Goliath - A stripped helium star solves the massive black hole mystery

January 10th, 2020
Image Credit: CC0 Public Domain

Stellar black holes form when massive stars end their life in a dramatic collapse. Observations have shown that stellar black holes typically have masses of about ten times that of the Sun, in accordance with the theory of stellar evolution. Recently, a Chinese team of astronomers claimed to have discovered a black hole as massive as 70 solar masses, which, if confirmed, would severely challenge the current view of stellar evolution. The publication immediately triggered theoretical investigations as well as additional observations by other astrophysicists. Among those to take a closer look at the object was a team of astronomers from the Universities of Erlangen-Nürnberg as well as some members of our group. They discovered that it may not necessarily be a black hole at all, but possibly a massive neutron star or even an 'ordinary' star. The results have now been published as a highlight-paper in Astronomy&Astrophysics.
Press releases: University of Erlangen, University of Potsdam

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