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Conclusions

We have measured the manganese abundance in 24 HgMn stars using a curve-of-growth technique, using 6 well characterised Mn I and Mn II lines. A more detailed spectrum synthesis analysis for a selection of stars confirms the accuracy of this simpler method when a few good unblended lines of moderate strength are used. The results agree well with the UV abundances of SD93 for all the isolated stars and some of the binaries. The remaining binary stars show a higher abundance in the visible region than the UV, which is to be expected, since SD93 did not account for binarity. The remarkably small scatter amongst the abundances derived for individual lines, and the excellent agreement with the UV results, show that careful selection of a few well characterised lines is a powerful tool for a quick and accurate abundance analysis. The similarity between the abundances derived for Mn I and Mn II may imply that we see very little in the way of non-LTE effects. The results support the comparison by SD93 of the predictions of diffusion theory by Alecian & Michaud [1981] with abundance trends for the upper envelope of abundances as a function of . This is another detailed prediction of the theory which has been confirmed observationally.

We have presented strong evidence that hyperfine structure is the cause of the widened line profiles and is mainly responsible for the line-strength anomalies seen in Mn II 4326 and 4206, and we have shown that hfs can be a major source of systematic error if not taken into account in abundance analyses. The other Mn lines studied must have very much narrower hfs structures, to give such consistent results. We were able to derive crude ad hoc structures, but a laboratory study of hfs in Mn II is urgently needed if more precise results are to be obtained.



next up previous
Next: Acknowledgements Up: Mn Abundances in HgMn Stars Previous: Discussion



Michael Dworetsky
Sat Oct 17 15:41:58 BST 1998