18 October 2016 | Fleming 1


Credit: ESO/H. Boffin| Click image to enlarge


One of many planetary nebulae in the southern constellation Centaurus, Fg 1 (or Fleming 1) is a unique example that is of great interest to professional astronomers studying stellar death. This emerald green glowing orb was discovered by the Scottish astronomer Williamina Fleming in 1910. Easily observable in large telescopes especially with an OIII filter, the strikingly beautiful pair of jets are only visible in deep astrophotographs with hydrogen-alpha exposures and can also be photographed with amateur instruments.

The bipolar jet structure was first discovered in deep images taken by professional astronomers in 1993. Only recently has the true nature of the jets been elucidated and further studies have revealed they are the result of a binary central star! One of many outstanding problems awaiting to be solved in planetary nebula studies are the wide variety of bipolar and asymmetric morphologies that a majority (approximately 80%) of the currently known 3500 planetary nebulae in the Milky Way (in addition to extragalactic examples found in other galaxies) are known to exhibit. Common sense would dictate that spherical stars would produce round nebulae but however this is not the case. Another curious observation is that younger planetary nebulae seem to display a vast array of complex structures whilst a large number of known spherical planetary nebulae appear to be more evolved with an older age and a lower surface brightness. These cosmic butterflies present a wondrous conundrum, how the ejected gas is shaped has been a matter of debate for three decades with proposed shaping mechanisms including magnetic fields and stellar winds.

In recent years, the binary hypothesis has gained more favour in explaining the origins of planetary nebula morphologies and multiple systematic searches and long term observations have led to a significant increase in the number of known PN binaries. Some astronomers have taken the hypothesis even further and proposed that not only are binary central stars responsible for the shaping of the ejected gas but are also necessary to produce a planetary nebula in the first place! An observational technique to try and identify binary central stars is to propose that certain low ionization structures such as jets and knots are morphological traits of binarity. Detailed analysis of a sample of planetary nebulae exhibiting such structures have shown a link between the presence of these structures being indicative of a binary central star.

At an estimated distance of 7800 light years, the jets in Fg 1 have a total length of 9 light years. The point symmetric nature of the jets and analysis of the individual knots in the jets have shown they correspond perfectly with the position of the central star, thereby providing a link between their formation and the binary star. Spectroscopy of the central star leading to a measurement of its velocities inferred the presence of a companion with an estimated orbital period of a little over a day. Adding to its rarity is that the central star is a double-degenerate system, which is a term used to describe a pair of white dwarf stars.

The S-shaped configuration of the jet shows it is an example of a bipolar rotating episodic jet (BRET) with a precessing outflow axis linked to time dependent activity of the central star. Another odd curiosity is the age of the bipolar jet being far greater than the main central shell, which suggests their formation preceded the ejection of the gas that produced the main planetary nebula shell. The outermost parts of the jet have estimated to have been ejected 16,000 years ago in comparison to an age of 5000 years for the main nebular body. An accretion disk around the binary star has been invoked to explain the formation mechanism of the jets. Deep observations have also revealed an extremely faint OIII envelope around the jets, which is likely to arise from the ejected material generating shocks in the surrounding ambient interstellar medium.

Complex bipolar jet systems have been found in a few other planetary nebulae including the well known Necklace Nebula and the lesser known ETHOS 1, Longmore 16 and Sab 41. Despite their small sizes, they are capable of being photographed by both professional observatories and amateur astrophotographers. Another strange example of a planetary nebula BRET system is KjPn 8, which might have been formed in an intermediate luminosity optical transient event.

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