The first evidence that a yellow supergiant became a supernova

The nature and diversity of the progenitor star or progenitor system of core-collapse supernovae is an important and open question in the field of astrophysics.

By University of Tokyo, Kashiwa, Japan — Published: October 1, 2012

Figure 1: M51 Galaxy before(left) and after(right) the eruption of SN 2011dh. The image on the left was taken in 2009, and on the right July 8, 2011. // Credit: Conrad Jung

A group of researchers led by Melina Bersten from the Kavli Institute in Kashiwa, Japan, has presented evidence that the yellow supergiant (YSG) star found at the location of Supernova 2011dh in the famous nearby galaxy M51 was indeed the supernova progenitor. The team also produced a self-consistent model to explain how a star of such characteristics exploded. In the model, the exploding YSG star was a member of a close binary system. The scientists further predict the detection of the companion star as a definitive test of their hypothesis.

The nature and diversity of the progenitor star or progenitor system of core-collapse supernovae is an important and open question in the field of astrophysics. It is believed that most massive stars explode when the stars become red supergiants or, alternatively, compact blue sun — so-called Wolf-Rayet stars. Recent detections of yellow supergiant stars as possible progenitors of supernovae have posed serious questions on our understanding of the evolution of massive stars.

Figure 2: Artist's conception of the progenitor system SN 2011dh. The system consists of a blue star and a yellow supergiant. // Credit: Kavli IPMU/Aya Tsuboi

Due to its proximity, SN 2011dh in M51 was one of the brightest and best-studied supernovae of 2011 (Figure 1). It was classified as a type IIb supernova — that is it showed hydrogen lines in the spectrum at early times and then evolved into a helium-dominated spectrum. This is indicative of a progenitor star that has lost most of its hydrogen-rich envelope prior to the explosion.

By searching through archival images obtained with the Hubble Space Telescope before the supernova explosion, two groups of astronomers independently detected a source at a location closely matching that of the supernova. Photometry of this pre-supernova source is compatible with a YSG star. If it is, indeed, the progenitor of the supernova, the question arises as to how such a star would undergo an explosion. Stellar evolution models predict that stars massive enough to produce a supernova explosion by core collapse should end their lives as red supergiants for the lower mass range or as a compact blue star for the larger masses. The YSG phase is an intermediate, short-lived stage in evolutionary models of single stars, during which no supernova explosion is expected to occur. Moreover, based on early optical emission and radio observations of SN 2011dh, some scientists claimed that the actual progenitor must have been a compact object. Therefore, the detected YSG star could have been a companion of the exploded star, or even an unrelated object that matched the projected supernova location by chance.

Figure 3: Theoretical light curve for a yellow supergiant (yellow) and blue compact (blue) progenitor compared with the observations of SN 2011dh (cyan points). From the figure, it is clear that the progenitor of SN 2011dh needs to be a yellow supergiant in order to reproduce the observation.

In this work, the researchers have presented evidence that the progenitor was an extended object with radius compatible with that of a YSG star (Figure 2), contrary to the previous theoretical models. This was done by modeling the early-time optical emission using hydrodynamical calculations. As shown in Figure 3, the observed light curve is reproduced only from the YSG star hypothesis. Now, these findings led to two new questions: (1) How did the star lose most of its hydrogen envelope? and (2) How could a YSG star of those characteristics explode?

Related with question (1), there are two main mechanisms proposed for stars to get rid of their outer layers: strong winds and mass transfer in interacting binary systems. For the former mechanism, it is believed that the star should have a mass at the time of birth larger than approximately 25 times the mass of the Sun for the wind to be strong enough. Hydrodynamical models, however, set a strong constraint on the mass of the progenitor star. They found that its core could not have been more massive than eight times the mass of the Sun, which implies that the initial mass of the entire star was less than 25 times the mass of the Sun. With this possibility ruled out, they tested the alternative of a system in which the progenitor transferred mass to a close companion, thereby losing most of its outer envelope. The binary scenario has the advantage of introducing a natural mechanism for lower mass stars to expel their envelopes.

Figure 4: Artist's conception of binary systems of (left) two blue stars and (right) a blue star and a yellow supergiant star. // Credit: Kavli IPMU/Aya Tsuboi

With the aim of solving both questions above, Bersten’s team carried out stellar evolution calculations for a system of two massive stars in a close orbit in which phases of mass transfer occur. By assuming a system of stars with initial masses of 16 and 10 times the mass of the Sun, and an initial period of 125 days (Figure 4), they were able to obtain a configuration for the mass-donor star — the one that eventually explodes — that matches closely the observations of the YSG object in the pre-supernova images and has a core mass that is consistent with their hydrodynamic modeling. Moreover, the amount of hydrogen left in the envelope of the exploding star was in the correct range to classify the supernova as type IIb. This is in contrast to a single star that evolves to a red supergiant (Figure 5).

Finally, the binary models predict that the accreting companion object should be a massive blue star at the moment of the supernova explosion. Because of its high surface temperature, the companion star should emit mostly in the ultraviolet range, with negligible contribution to the total flux of the system in the optical range. The companion was faint enough so that it was not detected in the pre-supernova images of the space telescope. Their calculations thus predict that in the near future, once the supernova has faded enough, the companion could be discovered with deep observations in the blue range of the spectrum. This would constitute a definitive test for the validity of the model.

Figure 5: Artist's conception of (left) a single blue star and (right) a red supergiant star. A single blue star is considered to become a red supergiant before a supernova explosion. // Credit: Kavli IPMU/Aya Tsuboi

“The present results reveal the necessity and importance of further studying the evolution and explosion of binary stars,” Berstenr said. “I look forward to the observation that will confirm our prediction.”

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SHIRLEY VUILLE from FLORIDA said:

These researchers claim that mass transfer from this supergiant to a smaller companion can trigger an explosion of the larger star (supported by computer simulations). However, it is not at all clear to me how mass loss from a supergiant can cause an instability that would lead to a core collapse supernova. In fact, in the conventional (red supergiant) type II supernova scenario, the star’s greatly distended envelope has no influence whatsoever on what’s happening in the core. What triggers the explosion?

Submitted: 1/24/2013 4:45:50 PM (CST)

STEPHEN ARMSTRONG from CALIFORNIA said:

David, they are not sure how a YSG star can even explode, much less WHY this one did.

Richard, what a great idea to mine the data archives of GALEX and the like for pre-boom glows!

Submitted: 10/9/2012 1:33:11 AM (CST)

RICHARD L COLE from MICHIGAN said:

This seems to fall under the rubric of, “Just when you (scientists) think you understand a phenomenon an exception comes along that doesn’t fit your theories.” How many such have there been just this year in these newsletters and the magazine?

Fig. 1 suffers from the image on the right being nowhere near as sharp as the image on the left. Surely some Hubble (HST) time was commandeered to image M51 (and M101) after last summer’s supernovae. There are also ultraviolet space telescopes; old data from these could be mined to look for a blue companion before the explosion, and new UV observations (in addition to the blue ones mentioned in the text) made to see if such a star, if it existed, is still there.

Submitted: 10/5/2012 1:32:28 PM (CST)

JAMES LEHMANN from NEW YORK said:

As we imaged this supernova last year it's apparent magnitude seemed as bright as those of Milky Way stars in the foreground. That led us to wonder indeed at the size and true magnitude of this event millions of light years away so long ago!

Submitted: 10/5/2012 12:57:32 PM (CST)

DAVID LECLERE from INDIANA said:

I missed the explanation for why the YSG star exploded. What is the mechanism? Did I not read the article correctly?

Fascinating article. Binary systems sound really interesting. So many things can happen!

Submitted: 10/3/2012 10:39:23 AM (CST)

STEPHEN ARMSTRONG from CALIFORNIA said:

Cool! An invisible companion!

Submitted: 10/3/2012 3:16:02 AM (CST)

BILL SIMPSON from LOUISIANA said:

Blow up Figure 1. Look at the right image. Go to the bright star just to the left of the top of the vertical line designating the supernova star discussed in this article. It looks like it has a faint circle around it. It is more evident of the left image of the galaxy in Figure 1. I wonder if it is a real ring of gas, or shock wave or what? It sure looks round, but if it was real, it would be enormous, so it probably isn't. You can really see it on the left side image of the galaxy.
Someone should have mentioned the diameter of that supernova fireball. You wouldn't need to do any Christmas shopping anywhere near that sucker.
I think I read that many stars have companions, so these mass transfer supernovas are quite common.
I was reading an article on the great unsolved mysteries of physics. One said that it can't be disproved that there isn't an infinite number of everything, including you and me. Does that mean an infinite number of me just had to take a half of an antacid pill? Seems a bit strange, doesn't it? But infinite is a hard concept to grasp. Those math and physics geniuses cause all kind of trouble.

Submitted: 10/2/2012 11:22:38 AM (CST)

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The first evidence that a yellow supergiant became a supernova

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