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An international research team with the participation of HITS discovered a “dormant” black hole in a binary-star system outside our galaxy. These black holes are very difficult to find because they do not emit X-ray radiation. Analysing data from six years of observations with the European Southern Observatory’s (ESO) Very Large Telescope (VLT), the team studied nearly 1,000 stars until they found the dormant black hole. The results have now been published in the journal “Nature Astronomy”.

They call themselves the “Black Hole Police”, and they aim to discover and study black holes in the universe: a group of astrophysicists from seven countries. After years of detective work, they have identified a so-called “dormant” stellar-mass black hole located in our neighbouring galaxy, the Large Magellanic Cloud. The team also found that the companion star that gave rise to the black hole disappeared without evidence of a strong supernova explosion. The discovery is based on six years of observations with the European Southern Observatory’s (ESO) Very Large Telescope (VLT) and has now been published in the journal “Nature Astronomy”.

“We have found a ‘needle in a haystack’”, says first author Tomar Shenar (University of Amsterdam), “VFTS 243 is the first ‘dormant’ stellar-mass black hole discovered outside our galaxy.” It has at least nine times the mass of our Sun and orbits a hot, blue star with 25 times the mass of the Sun. To find it, the researchers studied nearly 1,000 massive stars in the Tarantula Nebula of the Large Magellanic Cloud.

Stellar-mass black holes form when massive stars collapse under their own gravity at the end of their lives. If this collapsing star is in a binary system, the black hole will then be orbiting a luminous companion star. Such black holes often accrete material from their companion and can emit powerful X-rays, but a “dormant” black hole does not. Therefore, it is very difficult to find them. Astronomers suspect that there are many dormant black holes, maybe a few per cent of massive stars have such invisible companions.

Also involved in the study was HITS group leader Fabian Schneider (Stellar Evolution Theory group), an international expert on stellar evolution and binary systems. When massive stars explode in supernovae, their cores collapse and can forge neutron stars – a form of ultracompact matter. “In most cases, these neutron stars receive a kick of several hundreds of kilometres per second and are shot away from their explosion site into interstellar space”, explains Fabian Schneider. “The black hole in VFTS 243 did not receive such a kick, which suggests that its progenitor star directly collapsed into a black hole without signs of a strong supernova explosion.” This finding will also help understand the formation histories of the many black-hole mergers that are nowadays observed thanks to gravitational-wave astronomy.

The team utilized observations covering about six years: they consist of data from the VLT FLAMES Tarantula Survey (VFTS; led by Chris Evans, United Kingdom Astronomy Technology Centre, STFC, Royal Observatory, Edinburgh; now at the European Space Agency) obtained from 2008 and 2009, and additional data from the Tarantula Massive Binary Monitoring programme (TMBM; led by Hugues Sana, KU Leuven), obtained between 2012 and 2014. Together with other researchers, they just met in June at HITS for a workshop to discuss new avenues.

For more information, pictures and videos see the ESO press release at https://www.eso.org/public/news/eso2210/ and the HITS news at https://www.h-its.org/2022/07/19/set-black-hole/.

Publication: Shenar T et al: An X-ray quiet black hole born with a negligible kick in a massive binary of the Large Magellanic Cloud: Nature Astronomy, 18 July 2022. DOI: 10.1038/s41550-022-01730-y
https://www.nature.com/articles/s41550-022-01730-y

Astrophysicist Fabian Schneider, head of the junior group “Stellar Evolution Theory” (SET) at the Heidelberg Institute for Theoretical Studies (HITS), has received the Ludwig Biermann Award of the Astronomische Gesellschaft (AG), the professional German national association of astronomy and astrophysics.

The AG promotes activities in science and research, fosters the exchange of information between its members, and supports the dissemination of science to the public and through education. The Ludwig Biermann Award is granted in recognition of outstanding young astronomers. This year’s award recognizes Fabian Schneider for his work in studying the evolution of massive stars, binary stars, and supernovae, which has led to numerous and widely cited publications. “In addition, his research achievements and pioneering projects have enabled him to compete for several prestigious grants, and he is considered an internationally recognized expert in his field,” the AG stated.

“This prize fills us with great joy as it adds to the series of awards that Fabian Schneider has already received,” says HITS Scientific Director Frauke Gräter. Fabian Schneider has spent his career investigating the turbulent lives of massive stars. The astrophysicist conducted research as a “Hintze Research Fellow” at Oxford University after earning his doctorate in Bonn, received a postdoctoral research fellowship at Christ Church College (also in Oxford), and then joined the Center for Astronomy at Heidelberg University as a Gliese Fellow. While in Heidelberg, Fabian Schneider was also a visiting scientist at HITS in the PSO group headed by Friedrich Röpke. Last year, he was awarded an ERC Starting Grant by the European Science Council ERC. With funds of about 1.5 million euros, he has been building his own junior research group at HITS since January 2021.

Binary massive stars: Cosmic powerhouses

Astrophysicists are particularly interested in massive stars. These stars are cosmic powerhouses that explode in spectacular supernovae and leave behind some of the most exotic forms of matter: neutron stars and black holes. Mergers of neutron stars and black holes are now routinely observed thanks to gravitational-wave observatories. But there are still a lot of questions that remain unanswered.

Together with his group, Fabian Schneider is investigating these questions with a particular focus on binary stars. These stars form the vast majority of all massive stars. During their lifetime, they can reach a stage during which their outer layers are transferred onto a companion star. This mass transfer profoundly changes the evolution of both stars and even leads to the merger of both binary components in about 25 percent of massive stars.

Astronomische Gesellschaft press release.

[For the original news release, see here.]

Fabian Schneider leads the new research group “Stellar Evolution Theory” (SET) at the Heidelberg Institute for Theoretical Studies (HITS). The astrophysicist explores the turbulent life of massive binary stars and their explosive deaths in supernovae. He was awarded an ERC Starting Grant of about € 1.5 million by the European Research Council (ERC). He will use the funds to establish his own junior research group at HITS.

Stars are the basic building blocks of the visible Universe. Astrophysicists are particularly interested in massive stars. They are cosmic powerhouses, exploding in spectacular supernovae and leaving behind some of the most exotic forms of matter: neutron stars and black holes. Mergers of neutron stars and black holes are now routinely observed thanks to gravitational wave observatories. But there are still a lot of questions that remain unanswered.

Astrophysicist Fabian Schneider investigates the turbulent lives of massive stars. Since 1 January 2021, he has been the leader of the new research group “Stellar Evolution Theory” at the Heidelberg Institute for Theoretical Studies (HITS). He had successfully applied for an ERC Starting Grant from the European Research Council (ERC), and he is now establishing his own junior research group. HITS now consists of 13 research groups, four of them in the field of astronomy.  

Bonn – Oxford – Heidelberg: a scientific journey

Fabian Schneider studied physics at the University of Bonn and completed his PhD in astrophysics at the Argelander-Institute for Astronomy in 2015. He then joined the Department of Physics of the University of Oxford as a “Hintze Fellow”, where he did research on massive stars, their magnetic fields and supernovae. During this time, he started to collaborate with Friedrich Röpke from HITS, a joint venture that became even more intense after he moved to Heidelberg in 2018. Since then, Schneider has been a “Gliese Fellow” at the Center for Astronomy of Heidelberg University and, at the same time, visiting scientist in the PSO group at HITS. In October 2019, he published a study in “Nature” on the origin of magnetic fields in stellar mergers, together with colleagues from Garching and Oxford. In 2020, he was awarded an ERC Starting Grant and decided to choose HITS as the host institution for his new group.

When massive stars collide

The new SET group focuses on massive binary stars, which make up the majority of massive stars. During their lifetime, they can reach a stage where their outer layers are transferred onto their companion. The mass transfer profoundly changes the evolution of both stars. For example, if a star loses its envelope in a mass-transfer phase, it may explode in a supernova and produce a neutron star rather than collapsing into a black hole at the end of its life. In about 25% of massive stars, this mass-transfer even leads to a merger of both binary components.

About the ERC Starting Grants 2020

The European Research Council (ERC) is the premier European funding organisation for excellent frontier research. Every year, it selects and funds the very best, creative researchers of any nationality and age, to run projects based in Europe. The ERC offers four core grant schemes: The ERC Starting Grant helps early-career scientists and scholars to build their own teams and conduct pioneering research across all disciplines.

From a total of 3,272 applications, only 436 (13.3%) were selected for funding in this round. The new grantees will be based in 25 countries across Europe, with Germany (88 grants) as a top location. For the Starting Grants, the EU provides funding worth in total €677 million. The grants are part of the EU’s Research and Innovation programme, Horizon 2020.

See the ERC press release for more information:
https://erc.europa.eu/news/StG-recipients-2020

All neutron stars are magnetic, but some are more magnetic than others. The latter, so-called magnetars, are the strongest magnets in the Universe. The reason for their exceptionally large magnetic field is most probably that they formed in supernovae of already highly magnetized stars. But how do these massive stars acquire their large magnetic field? A team of astrophysicists from Germany and the UK may now have solved the more than 70-year-old conundrum of the origin of strong magnetic fields in massive stars. With the help of large computer simulations, they developed a model which shows that these can be formed in stellar mergers. The results were published in the scientific journal Nature.

Our Universe is threaded by magnetic fields. “We know that the Sun has a turbulent envelope in which its magnetic field is continuously generated. But more massive stars do not have such an envelope. Still, about 10 percent have a strong, large-scale surface magnetic field whose origin has eluded us since their discovery in 1947,” says Fabian Schneider from Heidelberg University in Germany, first author of the study. It is these stars that astronomers believe to form highly magnetic neutron stars when they explode in supernovae.

Novel code shows production of magnetic fields

“Already over a decade ago, it was suggested that strong magnetic fields might be produced when two stars collide”, says Sebastian Ohlmann from the Max Planck Society in Garching, Germany. “But up until now, we had not been able to test this hypothesis, because we did not have the necessary computational tools.” This time, the team utilised the novel AREPO code and ran it on computing clusters of the Heidelberg Institute for Theoretical Studies (HITS). They showed that a strong magnetic field is indeed produced thanks to the strong shear and the large turbulence present in the merger of two stars. Stellar mergers occur frequently, and it is thought that about 10 percent of all massive stars in the Milky Way are the products of stellar mergers – a good match with the occurrence rate of magnetic stars.

Double effect of stellar mergers

When stars merge, they appear younger than they really are. This phenomenon is well known, and such stars are called blue stragglers. “In 2016, we realised that the magnetic star Tau Scorpii (τ Sco) is a blue straggler and could show that, if τ Sco was a merger product, it would explain its anomalously young age,” remembers Philipp Podsiadlowski from the University of Oxford, UK. “Back then, we suggested that this star may also have obtained its strong magnetic field in the merger process and our new simulations demonstrate exactly this.”

At the end of its life, τ Sco will explode in a supernova when its core collapses and most probably leave behind a highly magnetized neutron star. “These magnetars are thought to have the strongest magnetic fields in the Universe – up to one hundred million times stronger than the strongest magnetic field ever produced by humans,“ says Friedrich Röpke from HITS. “Our simulations show that the generated magnetic field could be sufficient to explain the exceptionally strong magnetic fields inferred to exist in magnetars,” adds Fabian Schneider. “It makes our model a promising channel to explain the origin of such extremely strong magnetic fields. It is great to see that this idea now seems to work out so beautifully.”

Further information

The original science publication in Nature (DOI 10.1038/s41586-019-1621-5), “Stellar mergers as the origin of magnetic massive stars” by Fabian Schneider, Sebastian Ohlmann, Philipp Podsiadlowski,
Friedrich Röpke, Steven Balbus, Rüdiger Pakmor, and Volker
Springel, can be found here: https://www.nature.com/articles/s41586-019-1621-5. If you want to have a look at the paper, you can read it via this link: https://rdcu.be/bTOtS

I have also written a “Behind the paper” article for the Nature Research Communities which can be found here.

The paper is accompanied by press/news releases from Heidelberg University, Oxford University, the Heidelberg Institute for Theoretical Studies and the Max Planck Society.

An excellent illustration of τ Sco’s surface magnetic field topology can be found here.

Information about the massive binary star VFTS 352 with the surfaces of the two stars already touching each other and heading for their merger may be found in this press release. Observations of two stellar mergers in the Milky Way, V838 Mon and V1309 Sco, can be found here and here.

Images of the merger simulation

Birth of a magnetic star. The two snapshots of a simulation shown here mark the birth of a magnetic star such as Tau Scorpii. The image is a cut through the orbital plane where the colouring indicates the strength of the magnetic field and the hatching represents its field lines. Image credits: Ohlmann/Schneider/Röpke

Movies of the merger simulation

The first movie illustrates the evolution of the absolute magnetic field strength in the orbital plane of the merger while the second movie shows the gas density (ligther colours are for stronger magnetic fields and denser gases).

 

Massive star merger (magnetic field) from Fabian Schneider on Vimeo.

Massive star merger (density) from Fabian Schneider on Vimeo.

An international team of Astronomers have revealed an astonishing overabundance of very massive stars in the gigantic star-forming region 30 Doradus in our neighbouring galaxy the Large Magellanic Cloud. This discovery has far-reaching consequences for our understanding of how stars transformed the pristine Universe into the one we live in today. The results were published in the journal Science.

The team used the multi-object spectroscopic capabilities of ESO’s Very Large Telescope [1] to observe nearly 1,000 massive stars [2] in the star-forming region 30 Doradus (Dor) [3] as part of the VLT-FLAMES Tarantula Survey (VFTS) [4]. “We were absolutely surprised when we realised that 30 Dor has apparently formed many more massive stars than expected”, says the lead author Fabian Schneider from the University of Oxford (UK).

The team used detailed analyses of about 250 stars with masses between 15 and 200 times the mass of our Sun to determine the birth mass distribution of stars in 30 Dor, the so-called initial mass function (IMF) [5]. “We have not only been surprised by the sheer number of massive stars but also that their IMF is densely sampled up to 200 solar masses”, says Hugues Sana from the University of Leuven (Belgium), a co-author of the study. Until recently, the existence of stars up to 200 solar masses was highly disputed and the study shows that a maximum birth mass of stars of 200-300 solar masses appears likely.

In most parts of the Universe where astronomers looked so far, stars are more rare the more massive they are and most of the stellar mass is in low mass stars. Since the pioneering work of Edwin Salpeter in 1955, the general consensus was that a universal IMF, the so-called Salpeter IMF, could describe the distribution of stellar birth masses almost everywhere. However, measuring the IMF of massive stars is extremely difficult – primarily because of the scarcity of such stars – and there are only a handful of places in the local Universe where this can be done.

The team turned to 30 Dor, the biggest local star-forming region, which is about 180,000 light years away and hosts some of the most massive stars ever found. They determined the masses of massive stars with unique observational, theoretical and statistical tools. This allowed them to derive the most accurate high-mass end of the IMF to date from such a large sample of very massive stars and to show that massive stars are much more abundant than previously thought. “In fact, our results suggest that most of the stellar mass is actually no longer in low mass stars but a significant fraction is in high mass stars”, says Chris Evans from STFC’s UK Astronomy Technology Centre, the principal investigator of the VFTS and a co-author of the study.

Stars are cosmic engines and, for example, have produced most chemical elements heavier than helium – from the oxygen we breathe every day to the iron in our blood. During their lives, massive stars produce copious amounts of ionising radiation and kinetic energy through strong stellar winds and supernova explosions. The ionising radiation of massive stars was crucial for the re-brightening of the Universe after the so-called Dark Ages [6] and their mechanical feedback drives the evolution of galaxies. “To quantitatively understand all these feedback mechanisms and hence the role of massive stars in the Universe, we need to know how many of these behemoths are born”, explains Philipp Podsiadlowski, a co-author of the study from the University of Oxford (UK).

“Our results have far reaching consequences for the understanding of our Cosmos: there might be 70% more supernovae, a tripling of the chemical yields and towards four times the ionising radiation from massive star populations. Also, the formation rate of black holes might be increased by 180%, directly translating into a corresponding increase of binary black hole mergers that have recently been detected via their gravitational wave signals”, explains Fabian Schneider the significance of their findings.

The team’s research leaves many open questions, which they intend to investigate in the future: how universal are the findings and what are the consequences of this for the evolution of our Cosmos and the occurrence of supernovae and gravitational wave events?

 

Notes

[1] Very Large Telescope of the European Southern Observatory: http://www.eso.org/public/teles-instr/paranal-observatory/vlt/

[2] A massive star is often defined as having a mass of more than 8-10 times the mass of the Sun. This means that they can explode in spectacular supernova explosions at the end of their lives, thereby forming some of the most exotic objects in the Universe – neutron stars and black holes. Massive stars produce enormous feedback from strong stellar winds, supernova explosions and ionising radiation, and are thus very important for the evolution of our Cosmos.

[3] 30 Dor aka the Tarantula nebula is a truly gigantic and remarkable stellar nursery located 180,000 light years away in our neighbouring galaxy the Large Magellanic Cloud. If it were at the same distance as the Orion nebula, it would cover a size of 60 full moons on the night sky and would even cast shadows. 30 Dor hosts several record-holding stars such as the most massive and fastest spinning stars ever observed. It is a nearby analogue of extreme starburst events in the early Universe and thus a stepping-stone for the understanding of the distant Universe.

[4] VLT-FLAMES Tarantula Survey: http://www.roe.ac.uk/~cje/tarantula/

[5] The stellar initial mass function (IMF) describes the distribution of birth masses of stars and is thus a crucial ingredient for many areas in astrophysics. It provides the relative fraction of massive stars and predicts that only less than 1% of all stars are born with masses in excess of 10 times that of the Sun. Also, it is believed that most of the stellar mass is in low mass stars but the results of the IMF in 30 Dor might suggest that this is not always true. On average, forming stars with a total of 100 solar masses results in on average one star that will explode in a supernova.

[6] The so-called Dark Ages denote the period in the evolution of our Universe where no stars or galaxies existed. This epoch ended about 150 million years after the Big Bang when the first stars and galaxies were formed. The light from these stars and galaxies illuminated (and ionised) the Universe to its current state. Because of their extreme ionising radiation, massive stars are thought to be key players in this re-brightening of the Universe after the Dark Ages.

 

More information

The results were published in the paper “An excess of massive stars in the local 30 Doradus starburst” in the journal Science on 5^th January 2018.

The team of astronomers consists of Fabian Schneider (University of Oxford, UK), Hugues Sana (KU Leuven, Belgium), Chris Evans (UK Astronomy Technology Centre, Edinburgh, UK), Joachim Bestenlehner (University of Sheffield, UK), Norberto Castro (University of Michigan, USA), Luca Fossati (Austrian Academy of Sciences, Graz, Austria), Götz Gräfener (University of Bonn, Germany), Norbert Langer (University of Bonn, Germany), Oscar Ramírez-Agudelo (UK Astronomy Centre, Edinburgh, UK), Carolina Sabín-Sanjulían (University of La Serena, Chile), Sergio Simón-Díaz (IAC, Tenerife, Spain), Frank Tramper (European Space Astronomy Centre, Madrid, Spain), Paul Crowther (University of Sheffield, UK), Alex de Koter (University of Amsterdam, The Netherlands), Selma de Mink (University of Amsterdam, The Netherlands), Paul Dufton (Queen’s University Belfast, UK), Maria Garcia (CSIC-INTA, Madrid, Spain), Mark Gieles (University of Surrey, UK), Vincent Hénault-Brunet (Herzberg Astronomy & Astrophysics, Victoria, Canada), Artemio Herrero (IAC, Tenerife, Spain), Rob Izzard (University of Cambridge, UK), Venu Kalari (Universidad de Chile, Chile), Danny Lennon (European Space Astronomy Centre, Madrid, Spain), Jesus Maíz Apellániz (CSIC-INTA, ESAC campus, Spain), Nevy Markova (Bulgarian Academy of Sciences, Smoljan, Bulgaria), Paco Najarro (CSIC-INTA, Madrid, Spain), Philipp Podsiadlowski (University of Oxford, UK), Joachim Puls (LMU München, Germany), William Taylor (UK Astronomy Centre, Edinburgh, UK), Jacco van Loon (University of Keele, UK), Jorick Vink (Armagh Observatory, UK) and Colin Norman (Johns Hopkins University, Baltimore, USA).

 

Other media resources related to 30 Doradus

• Hubble’s 22nd birthday picture of 30 Doradus: http://www.spacetelescope.org/news/heic1206/
• Zoom-in video into Tarantula nebula and fly-over: http://www.spacetelescope.org/videos/heic1206b/

 

Related topics

1. “Stars Just Got Bigger”, https://www.eso.org/public/news/eso1030/
2. “Hubble unveils monster stars”, https://www.spacetelescope.org/news/heic1605/
3. “Final Kiss of Two Stars Heading for Catastrophe”, http://www.eso.org/public/news/eso1540/
4. “Hubble catches heavyweight runaway star speeding from 30 Doradus”, https://www.spacetelescope.org/news/heic1008/
5. “VLT Finds Fastest Rotating Star”, http://www.eso.org/public/news/eso1147/

 

Image credit: NASA, ESA, ESO, D. Lennon and E. Sabbi (ESA/STScI), J. Anderson, S. E. de Mink, R. van der Marel, T. Sohn, and N. Walborn (STScI), N. Bastian (Excellence Cluster, Munich), L. Bedin (INAF, Padua), E. Bressert (ESO), P. Crowther (Sheffield), A. de Koter (Amsterdam), C. Evans (UKATC/STFC, Edinburgh), A. Herrero (IAC, Tenerife), N. Langer (AifA, Bonn), I. Platais (JHU) and H. Sana (Amsterdam)

30 Doradus, once thought to be the 30th brightest star in the constellation Doradus in the southern hemisphere, is now known to be a massive star-forming region harbouring several record-holding objects such as the fastest spinning and most massive stars known. Sometimes also called the Tarantula nebula, 30 Dor is located in our neighbouring galaxy, the Large Magellanic Cloud. At its heart lies the massive and young star cluster R136.

We have now dissected this star cluster using ultraviolet capabilities of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble space craft, finding nine stars being at least 100 times more massive than our Sun. Together, these nine stars outshine our Sun by a factor of 30 million and totally dominate the light and feedback of stars in this region. Several of them exceed the 150 solar mass limit once thought to be the maximum birth mass of stars. Some of these behemoths may have been produced by merging lower mass stars but the sheer number of these monsters makes it unlikely that all of them have such an origin. It therefore seems that nature has found ways to form such objects, challenging our theories of star formation.

To learn more about these fascinating stars, the following resources may be helpful:

• Hubble press release
• Scientific article published in Monthly Notices of the Royal Astronomical Society
• NASA Hubble Hangouts on Youtube

I am very glad to announce that I was awarded the Phd prize 2015 for the best Phd thesis in physics and astronomy for my thesis entitled “Statistical Analyses of Massive Stars and Stellar Populations” by the Foundation for Physics and Astronomy in Bonn. A press release (in German) can be found here. My warmest thanks to my supervisors Norbert Langer and Robert Izzard, the foundation, its donors and the committee, and all my colleagues and friends!

It is my great pleasure to announce the launch of the BONNSAI web-service accompanying the online publication of the corresponding paper (Schneider et al., A&A, 570, A66) describing the method and its capabilities in detail. BONNSAI is a Bayesian method that allows users to match all available obserables of stars simultaneously to stellar models while taking prior knowledge properly into account to determine fundamental stellar parameters such as mass and age. It is equipped with robust Bayesian goodness-of-fit tests and provides full posterior probability distributions for inferred stellar parameters. The web-service runs on a dedicated server and is absolutely easy to use: just select a set of stellar models, provide observables, specify prior knowledge (if available) and let BONNSAI do the rest. A BONNSAI batch mode is also available on request.

Stars do not live alone–they mostly come in pairs in which mass can be transferred from one to the other star! In the Science issue from 27th July 2012, we show that the evolution of the most massive stars in the Universe, the O-stars, is dominated by binary interactions. Observations of up to ten years with the UVES spectrograph attached to the Very Large Telescope (VLT) revealed that almost three quarters of all O-stars within six Galactic star clusters have a close companion such that 70% of all O-stars will exchange mass with their companion and only 30% of all O-stars are effectively single stars. This has severe consequences for populations of massive stars, for example in star forming galaxies, supernova types and gamma-ray bursts.
To find out more, have a look into the paper and/or the press releases.

H. Sana et al., Science, DOI: 10.1126/science.1223344

University Bonn press release
ESO press release
STScI press release