Supernova iPTF13bvn: First evidence of a hydrogen-deficient supernova progenitor

An artist’s conception of a binary progenitor system of the supernova iPTF13bvn. 

Larger diameter but smaller mass (4 times mass of the Sun) helium star shown on the left is to explode. 

The companion star shown on the right is a hydrogen rich star and 30 times mass of the Sun. 

Credit: Kavli Institute for the Physics and Mathematics of the Universe

A group of researchers led by Melina Bersten of Kavli IPMU recently presented a model that provides the first characterization of the progenitor for a hydrogen-deficient supernova.

Their model predicts that a bright hot star, which is the binary companion to an exploding object, remains after the explosion.

To verify their theory, the group secured observation time with the Hubble Space Telescope (HST) to search for such a remaining star.

Their findings, which are reported in the October 2014 issue of The Astronomical Journal, have important implications for the evolution of massive stars.

For years astronomers have searched for the elusive progenitors of hydrogen-deficient stellar explosions without success.

However, this changed in June 2013 with the appearance of supernova iPTF13bvn and the subsequent detection of an object at the same location in archival images obtained before the explosion using the HST.

The interpretation of the observed object is controversial. The team led by Bersten presented a self-consistent picture using models of supernova brightness and progenitor evolution.

In their picture, the more massive star in a binary system explodes after transferring mass to its companion.

One of the challenges in astrophysics is identifying which star produces which supernova. This is particularly problematic for supernovae without hydrogen, which are called Types Ib or Ic, because the progenitors have yet to be detected directly.

However, the ultimate question is: "How do progenitor stars remove their hydrogen-rich envelopes during their evolution?"

Two competing mechanisms have been proposed. One hypothesizes that a strong wind produced by a very massive star blows the outer hydrogen layers, while the other suggests that a gravitationally bound binary companion star removes the outer layers.

The latter case does not require a very massive star. Because these two scenarios predict vastly different progenitor stars, direct detection of the progenitor for this type of supernova can provide definitive clues about the preferred evolutionary path.

When young Type Ib supernova iPTF13bvn was discovered in nearby spiral galaxy NGC 5806, astronomers hoped to find its progenitor.

Inspecting the available HST images did indeed reveal an object, providing optimism that the first hydrogen-free supernova progenitor would at last be identified.

Due to the object's blue hue, it was initially suggested that the object was a very hot, very massive, evolved star with a compact structure, called a "Wolf-Rayet" star.

NB: Using models of such stars, a group based in Geneva was able to reproduce the brightness and color of the pre-explosion object with a Wolf-Rayet star that was born with over 30 times the mass of the Sun and died with 11 times the solar mass.

Supernova iPTF13bvn discovered in nearby spiral galaxy NGC 5806. 

Credit: Jean Marie Llapasset

"Based on such suggestions, we decided to check if such a massive star is consistent with the supernova brightness evolution," says Melina Bersten.

However, the results are inconsistent with a Wolf-Rayet star; the exploding star must have been merely four times the mass of the Sun, which is much smaller than a Wolf-Rayet star.

"If the mass was this low and the supernova lacked hydrogen, our immediate conclusion is that the progenitor was part of a binary system," adds Bersten.

More information: The Astronomical Journal, 148:68 (6pp), 2014 October. DOI: 10.1088/0004-6256/148/4/68

Xenoturbella bocki worm Keeps Its Position as Progenitor of Humankind

Xenoturbella bocki worm. 

Credit: Hiroaki Nakano

Researchers are arguing about whether or not the Xenoturbella bocki worm is the progenitor of humankind, but new studies indicate that this is actually the case.

Swedish researchers from the University of Gothenburg and the Gothenburg Natural History Museum are involved in the international study.

The results have been published in the journal of Nature Communications.

The Xenoturbella bocki worm is a one-centimetre long worm with a simple body plan that is only found regularly by the west coast of Sweden.

The worm lacks a brain, sexual organs and other vital organs.

Zoologists have long disagreed about whether or not the Xenoturbella bocki worm holds a key position in the animal tree of life.

If it does have a key position, it is very important for the understanding of the evolutionary development of organs and cell functions, such as stem cells, for example.

The question is therefore not only important in the field of biology, but also for potential biomedical applications.

Matthias Obst

"It's absolutely fantastic that one of the key evolutionary organisms in the animal kingdom lives right on the doorstep of the University of Gothenburg's Centre for Marine Research, and this is actually the only place in the whole world where you can do research on the creature," says Matthias Obst from the Department of Biological and Environmental Sciences at the University of Gothenburg.

Genetic studies indicate that the Xenoturbella bocki worm belongs to the group of deuterostomes, the exclusive group to which human's belongs.

"So maybe we're more closely related to the Xenoturbella bocki worm, which doesn't have a brain, than we are to lobsters and flies, for example," says Matthias Obst.

Even though the worm does not particularly resemble man, development biologists have referred to the fact that the early embryonic development of the worm may display similarities with the group to which man belongs.

But the problem has been that no one has previously been able to see the development of the creature.

But now a group of researchers at the Sven Lovén Centre for Marine Sciences and the Gothenburg Natural History Museum have succeeded in doing what no one else has done before: to isolate newly born little Xenoturbella bocki worms.

"And these new-born worms revealed absolutely no remnants at all of advanced features! Instead, they exhibit similarities with quite simple, ancient animals such as corals and sponges," says Matthias Obst.

The studies also reveal the value of the University of Gothenburg's marine stations for important basic research.

"The Lovén Centre at the University of Gothenburg is the only place in the whole world where you can study this paradoxical animal (in Swedish called 'Paradox worm'). That's one reason why researchers come from all over the world to Gullmarsfjorden to solve one of the great mysteries in the evolution of animal life," says Matthias Obst.

Reference 
Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha. Nature Communications, 2013; 4: 1537 DOI: 10.1038/ncomms2556