Credit: NASA, NOAO, H. Bond and K. Exter (STScI/AURA)

SuWt 2 is a planetary nebula, consisting of a bright, nearly edge-on glowing ring of gas. Planetary nebulae are the remains of dead Sun-like stars, formed when a star sheds its outer gaseous envelope during the end throes of its lifetime. Intriguingly, this fascinating object became a cosmic crime scene. Astronomers are engaged in an interstellar CSI (crime scene investigation), searching for a missing white dwarf star within this nebula.

When a Sun-like star (a star with a mass similar to that of the Sun) dies, it sheds its outer gaseous envelope into space, forming a glowing colorful cloud, the planetary nebula. The core of the collapsing star, which has been the nuclear furnace that generated the star’s radiation through its lifetime, shrinks and turns into a white dwarf.

SuWt 2 is about 6,500 light years away in the direction of the southern constellation Centaurus. SuWt 2, the glowing remnants of a dead star, is believed to glow due to the energetic ultraviolet radiation of the core of the star, but, amazingly, the white dwarf is missing!

The mystery deepened when researchers observed the nebula in ultraviolet in the early 1990's with NASA's International Ultraviolet Explorer satellite, expecting to find signs of a faint but very hot star, but no ultraviolet radiation was detected, however.

Instead, at the center of the nebular ring are two candidate objects: two tightly spaced stars that orbit a common center of mass every five days, but neither of them is a white dwarf. These stars are hotter than our Sun, but they are still not hot enough to make the nebula glow. Only an intense flux of ultraviolet radiation, such as that emitted by white dwarfs, could power the nebula.

The study is being conducted by a team led by Katrina Exter and Howard Bond of the Space Telescope Science Institute (STScI). They extensively observed the brightness and spectrum of the binary and showed that both stars may have started to expand and evolve into red giant stars. (A red giant is a luminous huge star that is much larger in size, but cooler in temperature than the Sun. Due to its low temperature of about 3,000 C, it shines in red light.)

The two stars also appear to be rotating more slowly than expected; they would be in a locked rotation, i.e., the same sides of the stars are always turned to each other, but this is not observed.

The astronomers propose a simple interpretation for the facts at the scene: the stars at the center of SuWt 2 were born as members of a system of three stars, with the two stars circling each other tightly and a more massive star orbiting further out.

The massive star evolved to become a red giant, which only then swallowed the pair of stars. Confined inside the red giant in what astronomers term a "common envelope," the pair spiraled down toward the core, causing the envelope to spin faster.

Eventually, the outer layers of the red giant were expelled in the plane of the orbit, forming the ring-shaped nebula now observable. The unusually slow spins of the two stars may have been a consequence of being engulfed within their massive companion.

Ultraviolet radiation from the hot bare core of the red giant would have stimulated the nebula to glow. If the mass of the giant's core were high enough, the core would then shrink and cool off rapidly to a faint white dwarf, which might explain its absence.

The ground-based observations were obtained with telescopes at the Cerro Tololo Inter- American Observatory, Chile; the New Technology Telescope at the European Southern Observatory, Chile; the Anglo-Australian Telescope, Australia; and the South African Astronomical Observatory. The results were presented recently at the 212th meeting of the American Astronomical Society (AAS) in St. Louis.

Further Reading

Hubble Site
http://hubblesite.org/
A Dying Sun-like Star
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Aymen Mohamed Ibrahem
Senior Astronomy Specialist