UK Astronomers Survey Galactic Graveyard

An unprecedented source of planetary nebulae, the disk-like relics of elderly, dying stars, has been discovered in the southern part of our Milky Way galaxy.

With about 1000 planetary nebulae found so far and many more still to be discovered, the number of aged stars in their death throes revealed by the new survey is rapidly overtaking the entire population discovered over the entire sky during the last 75 years.

The cosmic graveyard is revealed in deep survey images taken in ‘H-alpha’ light with the UK Schmidt Telescope at the Anglo-Australian Observatory (AAO/UKST) in New South Wales, Australia. The survey was led by Quentin Parker (Institute for Astronomy, Royal Observatory Edinburgh) and Steven Phillipps (University of Bristol)

Dr. Parker will be showing the colourful new images obtained during his survey to the UK National Astronomy Meeting in Bristol on Wednesday 10 April 2002, and explaining their significance for improving our understanding of star formation and evolution.

“So far we have identified 1000 new planetary nebulae from visual scans of 70 percent of the southern Galactic plane,” said Parker. “This number is now increasing rapidly as the plates are systematically scrutinised by the SuperCOSMOS facility at the Royal Observatory, Edinburgh, so that more compact, fainter candidates are being found.”

According to Parker, the doubling of the known population of planetary nebulae will have a significant impact on many aspects of research into stellar evolution and Galactic structure.

“Finding evolved planetary nebulae and their central stars can help us understand stellar evolution during the critical transition phase between the nebula and a white dwarf,” he said. “There is a currently a severe paucity of observational data of evolved planetary nebulae which our new catalogue should help address.”

The highlights from the new sample include identification of some rare and unusual objects:

* 8 Wolf-Rayet stars (exceptionally hot stars rich in carbon or nitrogen) have been discovered at the centres of new planetary nebulae from follow-up spectroscopy. They include what may be the first nitrogen-rich central star yet found in our Galaxy. Only 56 Wolf-Rayet stars are currently known.

* The discovery of several halo planetary nebulae travelling at more than 300 km/s towards the Galactic centre.

* The discovery of a planetary nebula in an old open star cluster. This is an extremely rare and valuable find as the known cluster (and hence planetary nebula) distance enables accurate estimates of planetary nebula parameters.

* The identification of large numbers of evolved planetary nebulae, many with angular sizes from 100arcseconds to 8arcminutes.

* The discovery of several close pairs of planetary nebulae, with separations of less than 2arcminutes.

* The discovery of additional shells, ansae (‘handle-shaped’ clouds) and lobes around many known planetary nebulae. This may help to solve the problem of missing mass in planetary nebulae, since they provide evidence of previous episodes of material being ejected into space. Their new angular dimensions should lead to re-evaluation of many distance estimates.

* The discovery of two faint equidistant lobes either side of the well known butterfly-like planetary nebula NGC2899, probably making this object one of the largest and closest planetary nebulae to the Sun.

* The discovery of several hundred planetary nebulae in the Galactic Bulge region – the densely populated region of stars close to the centre of the Galaxy.

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Dr. Quentin A Parker alphagalileo

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