The following document lists the file abstract/PVHOOF_T2PROBLM.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
Accurate determinations of abundances have been hampered by many problems. The most important is the discrepancy between abundances derived from recombination lines and collisionally excited lines. For some nebulae huge discrepancies are found, even exceeding a factor of 10. One of the explanations that was proposed to explain this discrepancy was to assume the presence of electron temperature fluctuations in the nebula. These fluctuations are parameterized by the t^2 parameter and hence this problem is usually referred to as the t^2-problem. Electron temperature fluctuations increase the strength of collisionally excited emission lines. This effect is stronger for lines originating from higher energy levels. Hence, optical lines will increase more in strength than infrared fine-structure lines, which remain virtually unaffected. The fluctuations will also affect line diagnostic ratios and this will lead to an overestimation of the electron temperature. Both these effects combined will lead to an underestimation of the abundances based on optical lines. Since recombination lines are not affected by the t^2 effect (just like infrared fine-structure lines), the abundances derived from recombination lines should be roughly equal to the abundances derived from collisionally excited infrared lines, but higher than the abundances derived from collisionally excited optical lines. This theory can be tested by comparing the abundances derived from an optical spectrum and an infrared spectrum. This test was done for the planetary nebula NGC 6153 by X.-W. Liu. It showed that the t^2 effect might be present, but that it was not nearly strong enough to explain the difference between the abundances derived from recombination lines and collisionally excited lines in this nebula. Hence the problem of explaining this abundance discrepancy is still largely unsolved. In this proposal we wish to obtain an SWS spectrum of the nebula with the largest known abundance discrepancy: the galactic bulge nebula M2-23. With the SWS spectrum we will be able to test the t^2 hypothesis for this nebula. This test is important to confirm the results for NGC 6153 and possibly refute the t^2 hypothesis. The SWS spectrum may also be instrumental in identifying the true cause for the abundance discrepancy.