A way of estimating more accurate distances to the thousands of so-called planetary nebulae dispersed across our Galaxy has been announced by a team of three astronomers based at the University of Hong Kong: Dr David Frew, Prof Quentin Parker and Dr Ivan Bojicic. The scientists publish their results in Monthly Notices of the Royal Astronomical Society.
Despite their name, planetary nebulae
have nothing to do with planets. They were described as such by early
astronomers whose telescopes showed them as glowing disc-like objects.
We now know that planetary nebulae are actually the final stage of activity of stars like our Sun. When they reach the end of their lives,
these stars eject most of their atmosphere into space, leaving behind a
hot dense core. Light from this core causes the expanding cloud of gas
to glow in different colours as it slowly grows, fading away over tens
of thousands of years.
There are thousands of planetary nebulae in our Galaxy
alone, and they provide targets for professional and amateur
astronomers alike, with the latter often taking spectacular images of
these beautiful objects. But despite intense study, scientists have
struggled to measure one of their key properties – their distance.
Dr Frew, lead author on the paper, said: "For many decades, measuring
distances to Galactic planetary nebulae has been a serious, almost
intractable problem because of the extremely diverse nature of the
nebulae themselves and their central stars. But finding those distances
is crucial if we want to understand their true nature and physical
properties."
The solution presented by the astronomers is both simple and elegant.
Their method requires only an estimate of the dimming toward the object
(caused by intervening interstellar gas and dust), the projected size
of the object on the sky (taken from the latest high resolution surveys)
and a measurement of how bright the object is (as obtained from the
best modern imaging).
The resulting so-called 'surface-brightness relation' has been
robustly calibrated using more than 300 planetary nebulae whose accurate
distances have been determined via independent and reliable means. Prof
Parker explained that, "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 three 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 and added considerable errors to other distance measurements.
The new approach works over a factor of several hundred thousand in
surface brightness, and allows astronomers to measure the distances to
planetary nebulae up to 5 times more accurately than previous methods.
"Our new scale is the first to accurately determine distances for the
very faintest planetaries" said Dr Frew. "Since the largest nebulae are
the most common, getting their distances right is a crucial step".
A
comparison of the distance scales of two highly evolved nebulae,
numbered (1) PuWe 1, (2) Abell 21. Previous distance scales were often
inaccurate for the largest, most evolved planetary nebulae, which are
the most common type in the Galaxy. The left panel shows the physical
sizes of two nearby nebulae, presented at a common scale and using the
authors' new calculations. The scale bar represents 4 light years. The
right panel shows the physical sizes calculated from a commonly used
older distance scale, which considerably underestimates the distances
and hence sizes of these objects. Credit: NOAO/AURA/NSF, Ivan Bojicic,
David Frew, Quentin Parker (HKU)
Planetary nebulae are a fascinating if brief stage in the life of a low-
to middle-weight star. Being able to better measure distances and hence
the sizes of these objects will give scientists a far better insight
into how these objects form and develop, and how stars as a whole evolve
and die.
Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307
Mob: +44 (0)7802 877 699
rm@ras.org.uk
Dr Sam Lindsay
Royal Astronomical Society
Tel: +44 (0)20 7734 3307
sl@ras.org.uk
Dr Morgan Hollis
Royal Astronomical Society
Tel: +44 (0)20 7734 3307
mh@ras.org.uk
Dr David Frew
The University of Hong Kong
Mob: +852 9415 2556
djfrew@hku.hk
Prof Quentin Parker
The University of Hong Kong
Mob: +852 5669 9166
quentinp@hku.hk
Dr Ivan Bojicic
The University of Hong Kong
Mob: +852 6682 7976
ibojicic@hku.hk
A collage showing 22 individual planetary nebulae artistically arranged in approximate order of physical size. The scale bar represents 4 light years. Each nebula's size is calculated from the authors' new distance scale, which is applicable to all nebulae across all shapes, sizes and brightnesses. The very largest planetary nebula currently known is nearly 20 light years in diameter, and would cover the entire image at this scale. Credit: ESA/Hubble & NASA, ESO, Ivan Bojicic, David Frew, Quentin Parker
A comparison of the distance scales of two highly evolved nebulae, numbered (1) PuWe 1, (2) Abell 21. Previous distance scales were often inaccurate for the largest, most evolved planetary nebulae, which are the most common type in the Galaxy. The left panel shows the physical sizes of two nearby nebulae, presented at a common scale and using the authors' new calculations. The scale bar represents 4 light years. The right panel shows the physical sizes calculated from a commonly used older distance scale, which considerably underestimates the distances and hence sizes of these objects. Credit: NOAO/AURA/NSF, Ivan Bojicic, David Frew, Quentin Parker (HKU)
The new work appears in "The Hα surface brightness - radius relation: a robust statistical distance indicator for planetary nebulae", David J. Frew, Quentin A. Parker and Ivan S. Bojicic, Monthly Notices of the Royal Astronomical Society, Oxford University Press. A copy of the paper is available from http://mnras.oxfordjournals.org/lookup/doi/10.1093/mnras/stv1516.
The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3900 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter via @royalastrosoc
Media contacts
Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307
Mob: +44 (0)7802 877 699
rm@ras.org.uk
Dr Sam Lindsay
Royal Astronomical Society
Tel: +44 (0)20 7734 3307
sl@ras.org.uk
Dr Morgan Hollis
Royal Astronomical Society
Tel: +44 (0)20 7734 3307
mh@ras.org.uk
Science contacts
Dr David Frew
The University of Hong Kong
Mob: +852 9415 2556
djfrew@hku.hk
Prof Quentin Parker
The University of Hong Kong
Mob: +852 5669 9166
quentinp@hku.hk
Dr Ivan Bojicic
The University of Hong Kong
Mob: +852 6682 7976
ibojicic@hku.hk
Images and captions
A collage showing 22 individual planetary nebulae artistically arranged in approximate order of physical size. The scale bar represents 4 light years. Each nebula's size is calculated from the authors' new distance scale, which is applicable to all nebulae across all shapes, sizes and brightnesses. The very largest planetary nebula currently known is nearly 20 light years in diameter, and would cover the entire image at this scale. Credit: ESA/Hubble & NASA, ESO, Ivan Bojicic, David Frew, Quentin Parker
A comparison of the distance scales of two highly evolved nebulae, numbered (1) PuWe 1, (2) Abell 21. Previous distance scales were often inaccurate for the largest, most evolved planetary nebulae, which are the most common type in the Galaxy. The left panel shows the physical sizes of two nearby nebulae, presented at a common scale and using the authors' new calculations. The scale bar represents 4 light years. The right panel shows the physical sizes calculated from a commonly used older distance scale, which considerably underestimates the distances and hence sizes of these objects. Credit: NOAO/AURA/NSF, Ivan Bojicic, David Frew, Quentin Parker (HKU)
Further information
The new work appears in "The Hα surface brightness - radius relation: a robust statistical distance indicator for planetary nebulae", David J. Frew, Quentin A. Parker and Ivan S. Bojicic, Monthly Notices of the Royal Astronomical Society, Oxford University Press. A copy of the paper is available from http://mnras.oxfordjournals.org/lookup/doi/10.1093/mnras/stv1516.
Notes for editors
The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3900 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter via @royalastrosoc
Source: Royal Astronomical Society (RAS)
