New method to estimate more accurate distances between planetary nebulae and the Earth
The solution, presented by these authors, is both simple and elegant. More accurate distances between the most common type of "planetary nebulae" and the Earth can be estimated simply with three sets of data: firstly, the size of the object on the sky taken from the latest high resolution surveys; secondly, an accurate measurement of how bright the object is in the red hydrogen-alpha emission line; and thirdly, an estimate of the dimming toward the nebula caused by so called interstellar-reddening. The resulting so-called "surface brightness -- radius relation' has been robustly calibrated using more than 300 planetary nebulae whose accurate distances have been determined via independent and reliable means (e.g. trigonometric parallax measurements of their central stars).
The second author of this research Professor Parker, who is also the Head of the Physics Department in HKU, explained: 'the basic technique is not new but what marks out this work from what has gone before is the use of the most up-to-date and reliable measurements of all of those crucial properties'. This is combined with the use of the authors' own robust techniques to effectively remove "doppelgangers" and mimics that have seriously contaminated previous planetary nebulae catalogues, which added considerable scatter to previous statistical distance scales.
Incredibly, the new distance scale works over a factor of more than six powers of ten in surface brightness. The technique can provide distances accurate to 20 percent, which a major advance on previous estimators that can have errors of a factor of two or more. "In the past, the old distance scales worked fairly well for small planetary nebulae but got systematically worse for the larger nebulae. Ours is the first scale to be able to estimate distances for all planetary nebulae. As big planetary nebulae are the most common, we will use our new scale in making an unbiased census of planetary nebulae in the Milky Way, which will then help answer some important research questions." Dr Frew added.
This latest research by HKU astronomers promises a new era in our ability to study and understand this fascinating if brief period in the final stages of the lives of low- and mid-mass stars.
Source: The University of Hong Kong [April 13, 2016]