The irresistible pull – when massive stars collide

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.

Weighing massive stars in nearby galaxy reveals excess of heavyweights

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 5th 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

 

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)

Hubble unveils monster stars

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:

Award for the best Phd thesis

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!

Launch of the BONNSAI web-service

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.

Binary Interaction Dominates the Evolution of Massive Stars

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