Thursday, September 27, 2007

The Ant Nebula

The Ant Nebula (Planetary Nebula Menzel 3, or Mz3) - from Hubble

The Ant Nebula (Mz3) is located about 5 000 light-years away. The central star is as bright as 10 000 Suns and has a temperature of 35 thousand degrees Celsius. It is the last phase before this solar-like star will become a white dwarf.

From ground-based telescopes, the so-called "ant nebula" resembles the head and thorax of a garden-variety ant. This dramatic 2001 NASA/ESA Hubble Space Telescope image, reveals the "ant's" body as a pair of fiery lobes protruding from a dying, Sun-like star.

The ejection of gas from the dying star at the center of Mz 3 has intriguing symmetrical patterns unlike the chaotic patterns expected from an ordinary explosion. Scientists using Hubble would like to understand how a spherical star can produce such prominent, non-spherical symmetries in the gas that it ejects.

One possibility is that the central star of Mz 3 has a closely orbiting companion that exerts strong gravitational tidal forces, which shape the outflowing gas. For this to work, the orbiting companion star would have to be close to the dying star, about the distance of the Earth from the Sun. At that distance the orbiting companion star wouldn't be far outside the hugely bloated hulk of the dying star. It's even possible that the dying star has consumed its companion, which now orbits inside of it.

A second possibility is that, as the dying star spins, its strong magnetic fields are wound up into complex shapes. Charged winds moving at speeds up to 1000 kilometers per second from the star, much like those in our Sun's solar wind but millions of times denser, are able to follow the twisted field lines on their way out into space. These dense winds can be rendered visible by ultraviolet light from the hot central star or from highly supersonic collisions with the ambient gas that excites the material into florescence.

The frugal Cosmic Ant - from ESO

These new images revealing the Ant Nebula disc, which cannot be detected with a single 8.2-m VLT Unit Telescope, were uncovered in the interferometric mode, through the MID-infrared Interferometric instrument (MIDI). Interferometry works by combining the light of two or more telescopes, so that they act as a single, giant telescope, as large as the entire group.

With ESO's Very Large Telescope Interferometer (VLTI), when combining two of the 8.2-m Unit Telescopes, up to 25 times finer detail can be observed than with the individual telescopes.

The observations reveal a flat, nearly edge-on disc whose major axis is perpendicular to the axis of the bipolar lobes. The disc extends from about 9 times the mean distance between the Earth and the Sun (9 Astronomical Units or 9 AU) to more than 500 AU.

At the distance of the Ant Nebula, this corresponds to having detected structures that subtend an angle of only 6 milli-arcseconds. This is similar to distinguishing a two-storey building on the Moon.

The dust mass stored in the disc appears to be only one hundred thousandth the mass of the Sun and is a hundred times smaller than the mass found in the bipolar lobes.

Team leader Olivier Chesneau, from the Observatoire de la Côte d'Azur (France) suggests "We must therefore conclude that the disc is too light to have a significant impact on the outflowing material and cannot explain the shape of the Ant Nebula. Instead, it looks more like this disc is some remnant of the material expelled by the star."

The observations also provide unquestionable evidence that the disc is primarily composed of amorphous silicate. "This," says Chesneau, "most likely indicates that the disc is young, perhaps as young as the planetary nebula itself."

The astronomers favour the possibility that the large quantity of material in the lobes was propelled by several large-scale events, triggered with the help of a cool stellar companion. The solution of the mystery thus resides in the core of the system, and requires better characterisation of the hot central star and its putative companion, currently hidden from our view by the dusty disc.
VLT Interferometer detects disc around aged star from ESO
Nasa's Galex witnesses a spiral galaxy being stripped of its star

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