Supernova 2011DH: Bright blue star the last piece of a supernova puzzle

The image on the left was taken in 2009, and on the right on July 8, 2011. 

Credit: Conrad Jung

A team led by Gastón Folatelli at the Kavli IPMU, the University of Tokyo, has found evidence of a hot binary companion star to a yellow supergiant star, which had become a bright supernova.

The existence of the companion star had been predicted by the same team on the basis of numerical calculations.

This finding provides the last link in a chain of observations that have so far supported the team's theoretical picture for this supernova.

The results are published in the Astrophysical Journal Letters and have wide implications for our knowledge of binary systems and supernova mechanisms.

"One of the most exciting moments in my career as an astronomer was when I displayed the newly arrived HST images and saw the object right there, where we had anticipated it to be all along" said Gastón Folatelli , who led the efforts to obtain the new Hubble Space Telescope (HST) observations.

The question of how massive stars spend their short lives until they become supernovae is of great interest for astrophysicists.

According to the standard theory, which is only applicable to isolated stars, only cool and extended (red supergiants) or hot and blue (Wolf-Rayet stars) are able to become supernovae.

However, growing evidence suggests that most massive stars are not lonely singles but they belong to close binary systems with profuse interactions.

Episodes of mass transfer between the members of binary star systems affect the way the stars evolve, meaning that there are a great many more potential scenarios for the final stages of supernova progenitors.

The nearby supernova SN 2011dh, which occurred in 2011 in the well-known whirlpool galaxy M51, which is about 24 million light-years away from the earth, presented an excellent example that could not be explained by the standard theory.

What appeared to be a yellow supergiant star was detected at the location of the supernova in images obtained before the explosion, but yellow supergiant stars in isolation were not thought capable of becoming supernovae.

Controversy arose in the astronomy community with several experts proposing that the actual progenitor must have been an unseen bright blue object, such as a Wolf-Rayet stars.

However, the team led by Melina C. Bersten at Kavli IPMU and Omar Benvenuto at the University of La Plata, Argentina, showed that the exploding star must have been extended, like a yellow supergiant, and that it must have belonged to a binary system (see web release on September 28 2012: http://www.ipmu.jp/node/1404).

"We produced detailed models that self-consistently explained every property of SN 2011dh through the explosion of a yellow supergiant star in a binary system," remarked Bersten.

In March 2013, the proposal of Benvenuto, Bersten and collaborators was given substantial support when the disappearance of the yellow supergiant was observed, indicating that it and not a bright blue star was the exploding object (see web release on Apr. 5 2013: http://www.ipmu.jp/node/1537).

"At that time there was just one piece of the puzzle missing to confirm our model: we had to find the companion star that, according to our calculations, was a hot, compact object," said Benvenuto.

Images in the top row depict an artist's conception of the supernova explosion process. 

The corresponding images below were taken with the Hubble Space Telescope. 

Left: Just before the supernova explosion. 

A yellow supergiant is shining. 

Middle: The supernova exploding (the bottom image shows the fading supernova after the explosion). 

Right: A bright blue star observed. 

Credit: Top image: Kavli IPMU, Bottom image: NASA /Kavli IPMU /Gastón Folatelli

With that goal, the group set out to obtain HST observing time, which was granted in 2013 and recently executed on August 7, 2014.

Images were obtained in the ultraviolet regime, where the companion star was expected to be most clearly visible.

A point source was clearly detected in the new images at the exact location of the supernova (see announcement in http://www.astronomerstelegram.org/?read=6375).

"To our excitement, the object had the properties predicted by the models," explained Schuyler Van Dyk, of Caltech, who was in charge of the image analysis.

Folatelli and collaborators judged it unlikely that the detection was due to some other contaminating source.

More information: Gastón Folatelli, Melina C. Bersten, Omar G. Benvenuto, Schuyler D. Van Dyk, Hanindyo Kuncarayakti, Keiichi Maeda, Takaya Nozawa, Ken'ichi Nomoto, Mario Hamuy, and Robert M. Quimby, "A Blue Point Source at the Location of Supernova 2011dh," Astrophysical Journal Letters. doi:10.1088/2041-8205/793/2/L22

Stellar behemoth self-destructs in a Type IIb supernova

A star in a distant galaxy explodes as a supernova: while observing a galaxy known as UGC 9379 (left; image from the Sloan Digital Sky Survey; SDSS) located about 360 million light years away from Earth, the team discovered a new source of bright blue light (right, marked with an arrow; image from the 60-inch robotic telescope at Palomar Observatory). 

This very hot, young supernova marked the explosive death of a massive star in that distant galaxy. 

A detailed study of the spectrum (the distribution of colors composing the light from the supernova) using a technique called "flash spectroscopy" revealed the signature of a wind blown by the aging star just prior to its terminal explosion, and allowed scientists to determine what elements were abundant on the surface of the dying star as it was about to explode as a supernova, providing important information about how massive stars evolve just prior to their death, and the origin of crucial elements such as carbon, nitrogen and oxygen. 

Credit: Avishay Gal-Yam, Weizmann Institute of Science

Our Sun may seem pretty impressive: 330,000 times as massive as Earth, it accounts for 99.86 percent of the Solar System's total mass; it generates about 400 trillion trillion watts of power per second; and it has a surface temperature of about 10,000 degrees Celsius. Yet for a star, it's a lightweight.

The real cosmic behemoths are Wolf-Rayet stars, which are more than 20 times as massive as the Sun and at least five times as hot.

Because these stars are relatively rare and often obscured, scientists don't know much about how they form, live and die.

But this is changing, thanks to an innovative sky survey called the intermediate Palomar Transient Factory (iPTF), which uses resources at the National Energy Research Scientific Computing Center (NERSC) and Energy Sciences Network (ESnet), both located at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), to expose fleeting cosmic events such as supernovae.

For the first time ever, scientists have direct confirmation that a Wolf-Rayet star, sitting 360 million light years away in the Bootes constellation, died in a violent explosion known as a Type IIb supernova.

Using the iPTF pipeline, researchers at Israel's Weizmann Institute of Science led by Avishay Gal-Yam caught supernova SN 2013cu within hours of its explosion.

They then triggered ground- and space-based telescopes to observe the event approximately 5.7 hours and 15 hours after it self-destructed.

These observations are providing valuable insights into the life and death of the progenitor Wolf-Rayet.

"Newly developed observational capabilities now enable us to study exploding stars in ways we could only dream of before."

"We are moving towards real-time studies of supernovae," says Gal-Yam, an astrophysicist in the Weizmann Institute's Department of Particle Physics and Astrophysics.

He is also the lead author of a recently published Nature paper on this finding.

"This is the smoking gun. For the first time, we can directly point to an observation and say that this type of Wolf-Rayet star leads to this kind of Type IIb supernova," says Peter Nugent, who heads Berkeley Lab's Computational Cosmology Center (C3) and leads the Berkeley contingent of the iPTF collaboration.

"When I identified the first example of a Type IIb supernova in 1987, I dreamed that someday we would have direct evidence of what kind of star exploded."

"It's refreshing that we can now say that Wolf-Rayet stars are responsible, at least in some cases," says Alex Filippenko, Professor of Astronomy at UC Berkeley. Both Filippenko and Nugent are also co-authors on the Nature paper.

More information: Paper: dx.doi.org/10.1038/nature13304