One of the largest spheres in our Galaxy is giving valuable clues about the chemical composition of stars by its very shape. Planetary nebula Abell 39, now six light-years across, was once a sun-like star's outer atmosphere expelled thousands of years ago. The nearly perfect spherical nature of Abell 39 allows astronomers to accurately estimate how much relative material is actually absorbing and emitting light. Observations indicate that Abell 39 contains only about half of the oxygen found in the Sun, an intriguing but not surprising confirmation of the chemical differences between stars. The reason why the central star is slightly off center by 0.1 light-years is currently unknown. Abell 39 lies about 7000 light years away, although several galaxies millions of light years away can be seen through and aroun作者: 跨越地平线 时间: 2007-2-4 21:18
This nebula has the rare property of being extremely round and limb-brightened, exactly as the perfect, simple, theoretical planetary nebula ought to. A high resolution version of this nebula was taken at the WIYN 3.5-m telescope.
Shape: spherical shell, like a bubble
Distance: approx. 7,000 light years
Location: Hercules constellation
RA: 00 28 20.00 DEC: -11 23 24.0
Size: 6 light years across
Abundances/Description: It contains about half the amount of oxygen as the Sun. The central star is slightly off center but the cause of this shift is unknown. Note that you can see small distant galaxies through the image of the nebula.作者: 跨越地平线 时间: 2007-2-4 21:23
The large planetary nebula, Abell 39, appears on the sky as a beautiful, nearly perfect spherical shell, and photometric measurements support that interpretation (see: ). Its very simple geometry offers a special opportunity to test nebular photoionization models with one of the usual uncertainties removed.
Based on imaging in 0.7" seeing at [OIII] 5007 with the WIYN 3.5-m telescope, it has a diameter of 155", as defined by the peak intensity point on the rim, and is circular to within 0.5". The shell thickness that matches the observations best, when modeled with a thin spherical shell projected onto the sky, is 10". A faint, high excitation halo exists outside the main shell, extending the diameter to 190".
Optical spectra obtained at the KPNO Mayall 4-m telescope, combined with UV spectra from the Hubble Space Telescope and STIS, were used to constrain photoionization models (CLOUDY 94.01) to derive the parameters of the nebula and its central star. The star has Teff = 150,000K and a mass ~.6 solar masses. From the photoionization models, the nebula is found to be a factor of 3-4 lower than Solar in metallicity. This result is not unusual by itself, but it does serve as a caution for abundance determinations using the ``ionization correction factor'' (ICF) method, which erroneously predicts a nearly normal abundance. The discrepancy arises from the uniformly high ionization state of helium t作者: 跨越地平线 时间: 2007-2-4 21:26
The simple spherical geometry of planetary nebula Abell 39 will help astronomers identify the source of very serious errors in measuring the chemical composition of dying stars.
"The truly spherical nature of this beautiful nebula helps us eliminate a common confusion concerning the actual three-dimensional geometry of most nebulae," says George Jacoby, director of the Wisconsin-Indiana-Yale-NOAO (WIYN) Observatory and co-author of a study with Gary Ferland of University of Kentucky and Kirk Korista of Western Michigan University.
The butterfly-like shape of many nebulae, and filaments or clumps of dense gas within them, cause starlight to penetrate the nebulae unevenly, depending on the density and thickness of the clumps, as well as the distance of the gas from the star. Only in rare cases can the nebula's geometry be deduced from indirect measurements in order to model the interactions.
As reported today in San Diego at the 197th meeting of the American Astronomical Society, researchers found that the star that produced Abell 39 had only half the amount of oxygen found in the Sun. This result is not especially unusual, because the Sun has a relatively enriched composition of heavy elements. But for some stars, the same sorts of analyses have yielded compositions that can disagree with each other by a factor of three or more.
"Such discrepancies are totally unsatisfactory for developing a detailed picture of how chemical elements were built up over time in our作者: 浩瀚 时间: 2007-2-4 21:33
看不懂作者: 巴刹拿 时间: 2007-2-5 06:12
翻译来翻译下
). Its very simple geometry offers a special opportunity to test nebular photoionization models with one of the usual uncertainties removed.