Astronomers Discover Disk in Heart of Ant Nebula

In an effort to discover what shaped the intriguing Ant Nebula, astronomers from the European Southern Observatory (ESO) trained the powerful Very Large Telescope Interferometer on the Ant Nebula and found a "skinny" disk.

Interferometry combines the light of two or more telescopes so that they act as a single, giant telescope with power multiplied by the number of telescopes in the combined group. ESO's Very Large Telescope Interferometer combines two 8.2-m Unit Telescopes to empower observations with up to 25 times finer detail than is possible with the individual telescopes.

Regarding the function of nebular disks in the formation of asymmetrical shapes in nebulae, particularly the Ant Nebula, team leader Olivier Chesneau, from the Observatoire de la Côte d'Azur, France explains, "The challenge is to actually detect these discs...Most astronomical instruments do not have a sharp enough view to find, let alone study them. The Very Large Telescope Interferometer however, with its exceptionally high spatial resolution, is a powerful disc-hunter."

Planetary nebulae are glowing structures of gas cast off by solar-like stars at the ends of their lives. In the Ant Nebula, astronomers have uncovered a flat, nearly edge-on disk (edge-on relates to view from earth) of silicates in the heart of the magnificent nebula. However, the disk seems too 'skinny' to explain the physics behind how the nebula got its intriguing ant-like shape: a central star core with two asymmetrical gaseous lobes (bipolar lobes) extending to the north and south of it.

The observations reveal that the disk's major axis is perpendicular to the axis of the bipolar lobes: the axis of the lobes is nearly north by south and the axis of the disk nearly west by east. The disk extends from proximity to the central core at about 9 Astronomical Units, or AU, (9 times the mean distance between the Earth and the Sun) to more than 500 AU.

Astronomers additionally found that the dust mass stored in the disk, which obstructs the view of the central core, 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: the disk mass is significantly smaller than that in the bipolar lobes.

"We must therefore conclude that the disk is too light to have a significant impact on the outflowing material and cannot explain the shape of the Ant Nebula", says Chesneau. "Instead, it looks more like this disk is some remnant of the material expelled by the star."

Observations with the Very Large Telescope also provide solid, unquestionable evidence that the nebula disk is primarily composed of amorphous silicate, as each element has it's own spectrometry signature. "This," says Chesneau, "most likely indicates that the disk is young, perhaps as young as the planetary nebula itself." Silicates, which appear in two flavors, are minerals composed of silicon and oxygen. They appear as amorphous, which is asymmetrical, and crystalline, which is symmetrical, flavors.

The question is: How can a spherical star produce such complex structures as are seen in the Ant Nebula, especially if the disk is too skinny to have produced a division of the stars' gaseous nebula? Many astronomers think the answer to the question lies in a basic understanding of the nature of the disk surrounding the central star. Indeed, this holds for all nebula; by their nature, the disks associated with nebula bear witness to the phenomena that lead to the asymmetrical structures of such planetary nebulae (disks formed of gas and dust, called circumstellar, or protoplanetary disks, are also found around young stars, as in the Orion Nebula).

Astronomers at ESO generally favor the possibility that the large quantity of material in the Ant Nebula's lobes was propelled outward by several large-scale events. These events may have been triggered by an orbiting cool stellar companion--as opposed to an orbiting planetary companion.

The solution of the mystery of how the Ant Nebula was formed thus resides in the core of the nebula system, and requires better characterization of the hot central star and its putative, or reputed, companion, which is currently hidden from view in the Ant Nebula by the newly discovered, nearly edge-on, dusty disk.

Nicknamed the "Chamber of Horrors" in the late 1950s because of its striking morphology, the Ant Nebula has a bright core, three nested pairs of bipolar lobes and an interior ring-like outflow. The Ant Nebula is located about 5 000 light-years away and the central star is as bright as 10, 000 Suns with a temperature of 35, 000 degrees Celsius. It is the last phase before this solar-like star will become a white dwarf.

Note: The results are presented in a Letter to the Editor published by the research journal Astronomy and Astrophysics ("A Silicate Disk in the Heart of the Ant" by O. Chesneau et al.). The team is composed of O. Chesneau and A. Spang (Observatoire de la Côte d'Azur, France), F. Lykou, E. Lagadec, and A.A. Zijlstra (University of Manchester, UK), B. Balick (University of Washington, Seattle, USA), M. Matsuura (NAOJ, Tokyo, Japan), N. Smith (University of California, USA), and S. Wolf (Max-Planck-Institute for Astronomy, Heidelberg, Germany).

"The Frugal Cosmic Ant; Skinny dusty disc found in the heart of the Ant Nebula," European Southern Observatory, European Organisation for Astronomical Research in the Southern Hemisphere