NASA's Hubble Space Telescope (HST) has obtained the best images yet of a mysterious mirror- imaged pair of rings of glowing gas that are encircling the site of the stellar explosion supernova 1987A.
One possibility is that the two rings might be "painted" by a high-energy beam of radiation or particles, like a spinning light- show laser beam tracing circles on a screen.
The source of the radiation might be a previously unknown stellar remnant that is a binary companion to the star that exploded in 1987. Images taken by Hubble show a dim object in the position of the suspected source of the celestial light show.
"The Hubble images of the rings are quite spectacular and unexpected," says Dr. Chris Burrows of the European Space Agency and the Space Telescope Science Institute in Baltimore, Maryland. Burrows used Hubble's Wide Field Planetary Camera 2 to image the rings in February 1994.
The striking Hubble picture actually shows three rings. The smaller "center" ring of the trio was seen previously. The larger pair of outer rings were also seen in ground-based images, but the interpretation was not possible until the higher resolution Hubble observations.
Though all of the rings probably are inclined to our view (so that they appear to intersect), they probably are in three different planes. The small bright ring lies in a plane containing the supernova; the two rings lie in front and behind it.
To create the beams illuminating the outer rings, the remnant would need to be a compact object such as a black hole or neutron star with a nearby companion. Material falling from the companion onto the compact object would be heated and blasted back into space along two narrow jets, along with a beam of radiation. As the compact object spins it might wobble or precess about its axis, like a child's top winding down. The twin beam would then trace out great circles like jets of water from a spinning lawn sprinkler.
If the rings are caused by a jet, however, the beams are extremely narrow (collimated to within one degree). This leads Burrows to conclude: "This is an unprecedented and bizarre object. We have never seen anything behave like this before."
The jet model explains why the rings appear to be mirror imaged, and why they appear to be symmetrical about a point offset from the center of the explosion.
Burrows got the idea for the beam explanation when he noticed that where a ring appears brighter, an equally bright region appears on opposite ring. By connecting lines between the similar clumps on opposite rings Burrows found a common center. However, it is offset from the heart of the supernova ejecta. When Burrows did a detailed inspection of the HST image, he found a dim object which may be the source of the beam at the predicted location. The object is about 1/3 light-year from the center of the supernova explosion.
From previous HST observations and images at lower resolution taken at ground-based observatories, astronomers had expected to see an hourglass-shaped bubble being blown into space by the supernova's progenitor star. "The rings are probably on the surface of the hourglass shape," says Burrows. The hourglass was formed by a wind of slow-moving gas that was ejected by the star when it was a red supergiant, and a much faster wind of gas that followed during the subsequent blue supergiant stage. The hourglass was produced by the fact that the stellar wind from the red giant was denser in the equatorial plane of the star. When the star reached the blue supergiant stage, the faster winds tended to break out at the poles of the star.
Energetic radiation from the supernova explosion illuminated the dense gaseous material in the equatorial "waist" of the hourglass, causing it to glow - thus explaining the central bright ring. However, the two outer rings might be painted on the surface of the hourglass by a very different process, by the beams from the stellar remnant.
Further observations with HST will study any further changes that might occur. In particular, if a remnant companion really exists, it should collide with the supernova's expanding cloud of ejecta in the mid 1990s.
The observations were led by Dr. Chris Burrows in collaboration with the WFPC 2 Investigation Definition Team. The supernova is 169,000 light years away, and lies in the dwarf galaxy called the Large Magellanic Cloud, which can be seen from the southern hemisphere.
