Disappearing Stars Confirm Supernova Origins

Artist’s rendering of SN 1993J, where a red supergiant supernova progenitor star (left) is exploding after having transferred about seven solar masses of hydrogen gas to the blue companion star (right). Credit: ESA

Astronomers have caught two stars in the disappearing acts that link them to type II supernova events.

Type II supernovae are widely believed to result from the internal collapse and explosion of massive stars, about nine times the size of our sun. But precious few observations have actually confirmed the relationship.

Now, researchers have spotted two parent stars that showed up in supernovae “before” images — but not in images taken after the blasts. 

“The disappearance of the progenitors confirms that these two supernovae were produced by Red Supergiants,” write co-authors Justyn Maund and Stephen Smartt. Their new paper is out in this week’s issue of Science.

SN 2003gd. Credit: Gemini Observatory

So far only one star has been shown to have disappeared after it exploded — the star that exploded as SN 1987A in the Local Group of galaxies. Seven other stars have been spotted in the neighborhoods of type II supernovae before they went off, but none of them has been shown to have disappeared, Maund and Smartt write.

Maund is affiliated with both the University of Copenhagen in Denmark and the University of California at Santa Cruz, and Smartt is from Queen’s University Belfast in the UK. They used the Hubble Space Telescope and the Gemini Telescope to observe the two supernovae.

The progenitor of SN 2003gd, an M-supergiant star in the galaxy M74, “is no longer observed at the SN location,” they found. They estimated 2003gd is seven times the mass of the sun, which they acknowledge “is at the lower end of the mass range considered theoretically possible to produce core-collapse events.” They said there’s enough uncertainty in the object’s mass that it could be greater than seven solar masses — but even if it’s not, several other stars in the low end of the range are suspected of exploding as supernovae.

The co-authors are also careful to point out that dust from the supernova is still visible, and, “One could argue that the star identified as the progenitor was a neighboring star that is now obscured by dust formation.” But their work indicates that the explosion wasn’t dusty enough to obscure a star as luminous as SN 2003gd’s parent. They believe the progenitor star has truly disappeared — although further confirmation will come as the dust continues to clear.

SN 1993J is a truly exceptional case. The K-supergiant star that exploded in that supernova is also no longer present, the authors report — but its B-supergiant binary companion is still observed. 

The model for the binary system was of a progenitor star 15 times the mass of the sun, with a slightly less massive binary companion. The progenitor star evolved faster, and transferred some of its mass onto the binary companion, including a substantial amount of its hydrogen envelope. The binary companion grew to 22 times the mass of the sun. The interaction happened over about 250 years and affected the supernova explosion to such an extent that SN 1993J became known as one of the most peculiar supernovae ever seen.

The site of SN 1993J was imaged several times over the 2 to 13 years after the explosion with Hubble and a handful of other telescopes. By the 2004 observation, the red portion of the SN spectral energy distribution had faded below the level of the red spectral energy of the binary progenitor system, “ruling out the continued presence of the K-supergiant star and, hence, confirming it as the progenitor of SN 1993J,” the authors wrote.

They said soon the blue part of the supernova’s spectrum will fade, opening up a window for observations of the remaining companion star.

The authors conclude that their “simple, but time-consuming” method “leaves no doubt that the two stars were the progenitors of the supernovas, SN 2003gd and SN 1993J, and confirms that type II supernovas are birthed from Red Supergiants, as predicted.”