Gemini Legacy image of the complex planetary nebula Sh2-71 as imaged by the Gemini Multi-Object Spectrograph on Gemini North on Mauna Kea in Hawai‘i. The long-assumed central star is the brightest star near the center, but some astronomers wonder if the much dimmer and bluer star (just to the right and down a bit) might be the parent of this beautiful object. The image is composed of three narrow-band images, and each is assigned a color as follows: H-alpha (orange), HeII (blue) and OIII (cyan). Each image is 15 minutes in duration, the field-of-view is 5.3 x 3.6 arcminutes, and the image is rotated 110 degrees clockwise from north up, east left. Image credit: Gemini Observatory/AURA

Not long ago the Gemini Observatory aimed its 8-meter telescopes in the direction of the constellation of Aquila for a routine investigation.To everyone’s surprise, it ended up uncovering an enigma. Sh2-71 was originally discovered in 1946 by Rudolph Minkowski. Viewable in most amateur telescopes, this reasonably bright planetary nebula was cataloged by Stewart Sharpless in 1959 and now, over half a century later, has become the topic of astronomical debate. Researchers have always assumed it was created from the death of the central binary star system, but Gemini’s imaging techniques may just prove that assumption wrong. What exactly is spawning this beautiful planetary nebula?

As we know, they are formed when a star reaches the end of its life span and begins sloughing off material. In the Gemini Legacy image, there is a binary star near the center of Sh2-71 – the one assumed to be responsible for the gaseous envelope. However, Gemini’s observation has brought a new light to the forefront – a dimmer, bluer star just to the right and down a bit from what seems to be the obvious progenitor. What makes this case even more strange is the fact that the smaller star would be a better candidate as the nebula’s parent.

Why so? In the case of this Sharpless object, it would appear the bright central star isn’t radiating enough ultraviolet light to cause the surrounding gas to glow so brightly… but the blue star does. However, the binary star would be a better match for the nebula’s asymmetrical appearance. As of now, astronomers don’t know if the blue star is binary or not. To deepen the mystery, the bright star’s spectroscopic companion may also be hot enough to excite the gas in keeping with original theory.

A research team, led by Australian astronomers David Frew and Quentin Parker (Macquarie University, Sydney) are studying the dimmer, bluer star to understand its nature. “At the assumed distance to the nebula (roughly 1 kiloparsec or about 3,260 light-years), the faint star has about the right brightness to be the fading remnant of the nebula’s progenitor star,” says Frew.

Is your head swimming yet? Then there’s still more. The brighter binary star isn’t ordinary. It’s one that shows a dominant hydrogen-alpha emission as observed in some types of planetary nebula. According to Frew, this star is also unlikely to be a chance projection or alignment with the nebula, “So there could be at least three stars in this system,” he says.

Even if we don’t concentrate on which stars could be responsible for this planetary portrait, there’s morphology to consider as well… another complex issue. “The nebula presents a multi-polar structure and several pairs of bipolar lobes at different orientations,” says Luis Miranda of Spain’s Instituto de Astrofísica de Andalucía (CSIC) who has also studied this object extensively. “These lobes most certainly formed at different times and likely involved a binary progenitor – in particular with mass-transfer and multiple episodes of mass ejection along an axis where the orientation changes with time.”

Are we finished yet? Not hardly. To make the riddle even more complex, the Isaac Newton Telescope Photometric HydrogenAlpha Survey of the Northern Galactic Plane Survey added another twist. Parker and Romano Corradi (Instituto de Astrofisica de Canarias, Spain) have recently discovered faint outer wisps and lobes surrounding the planetary on deep hydrogen-alpha images.

The diaphanous material flows over several arcminutes, suggesting the mass loss history of Sh2-71 is a lot more complex that meets the eye. Miranda agrees, noting that the nebula’s structure is difficult to explain without a binary pair for parents. “The chaotic morphology of Sh2-71 implies that very complex processes have been involved in its formation,” says Miranda.

For now, it’s a grand encrypted message just waiting on the right astronomers to decode.

Original Story Source: Gemini Observatory News Release.

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