A team of researchers, led by Spain's National Research Council (CSIC) has discovered a second ring around the Butterfly Nebula (NGC 6302) situated some 3,400 light years from Earth. The data, compiled by the ALMA Observatory, in Chile's Atacama desert, indicate that this a younger ring which is expanding more quickly, and faces a different direction to the original. The work has been published in the magazine Astronomy & Astrophysics.
"Nebulous planets are formed by slowly dying stars, stars with low to intermediate mass (having up to eight solar masses). They pass through various phases in which the asteroid grows, augmenting its radius and ejecting the material of which it is composed. What eventually remains at the centre is the dense nucleus of the dead star, or white dwarf, surrounded by dust and gas," explains Miguel Santander, a CSIC scientific investigator at Madrid's Institute of Material Science.
Around the nucleus of some nebulae, there is a gas and dust ring normally associated with extreme symmetry, although it is not known whether this is due to the star's winds, the presence of a partner, or to magnetic fields. These rings tend to be very thick and dense. In the case of the Butterfly Nebula, the creation process of the main ring began around 5,000 years ago and lasted around 2,000 years. Later, between 3,600 and 4,700 years ago, the bipolar jets, known as lobes, were created. These give the nebula its characteristic hour-glass shape.
"This nebula does not possess one single axis of symmetry. Around 2,200 years ago, another jet appeared from the nucleus, this time with a separate symmetry. In other words, a third younger lobe exists with a different axis to the older main lobes. In parallel, and at a similar time, though unknown until now, another structure was formed- a second ring," adds the researcher.
An unexpected discovery
The initial aim of the Molecular Astrophysics Group research team which Santander leads was to look into whether there existed small rotating discs made up of gas and dust around some evolved stars. This nebula was one of the objects selected for the study. However, what they discovered was something completely different.
"Initially, whilst observing within the visible range of the electromagnetic spectrum, we saw an arc-shaped filament wrapped in the main lobes. Yet, the data from ALMA, which operates in the millimetre and submillimetre range of light, confirmed that this is a younger ring than the first. Not only is it expanding more rapidly, it is also facing another direction" notes the investigator.
Although not the first discovery of a nebula surrounded by various rings set at varying degrees of inclination, this is the first time that there is estimated to be quite a difference in the ages between rings. Furthermore, secondary rings on other nebulae are almost as massive as the main rings, and, in this case, the secondary ring has a mass of only 2.8 Jupiter masses (the main ring is, in proportion, much more massive).
The researchers are deliberating over a number of theories into the origins of this second ring of material. The first suggests the existence of a three-part system in which one of the stars destabilised the group as it went through its red giant phase. The other two stars could then have produced the new ring.
The second hypothesis suggests the ring may well have resulted from the destruction of a giant gas planet which could have been orbiting too closely to the star as it evolved into becoming a red giant. "Both scenarios are speculative and subsequent study will be required in order to confirm or dispel these hypotheses" concludes Miguel Santander.
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