AsianScientist (Apr. 22, 2016) – Astronomers in Hong Kong have come up with a more accurate way of estimating distances to the thousands of planetary nebulae that are dispersed across our Galaxy. This technique, published in Monthly Notices of the Royal Astronomical Society, gives planetary nebulae a more meaningful physical presence and improves our ability to study and understand the final stages of the lives of low- and mid-mass stars.
The discovery is the result of ten years of research work by first author Dr. David Frew, Professor Quentin Parker and Dr. Ivan Bojicic based at the University of Hong Kong.
Ghostly and beautiful planetary nebulae have nothing to do with planets, but acquired this name because their glowing spheres of ionized gas resembled planets to early observers. They are the colorful, ejected shrouds of dying stars, which offer a brief window into the history of many stars’ lives, including that of our Sun.
Their stunning shapes make them popular with the wider public, but understanding their scientific significance has been hindered by poor distance estimates.
In the present study, the authors show that more accurate distances between the most common types of planetary nebulae and the Earth can be estimated 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—for example, trigonometric parallax measurements of their central stars.
“Measuring distances to galactic planetary nebulae has been an intractable problem for many decades, because of the extremely diverse nature of both the nebulae themselves and their central stars,” said Frew.
“However, understanding their true nature and physical characteristics depends crucially on knowing their distance. With our significantly improved distance estimates, we can finally provide more meaningful values for many key parameters of scientific interest.”
Notably, the new distance scale works over a factor of more than six powers of ten in surface brightness. Thus, the technique can provide distances accurate to 20 percent, which is a significant advantage over previous estimators.
“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,” said Frew.
“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.”
The article can be found at: Frew et al. (2016) The Hα Surface Brightness–Radius Relation: a Robust Statistical Distance Indicator for Planetary Nebulae.
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Source: University of Hong Kong.
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