H. U. Kaufl

Publications (2)0 Total impact

• Source

  Available from: arxiv.org

  Article: Sulphur and zinc abundances in Galactic halo stars revisited

  P. E. Nissen, C. Akerman, M. Asplund, D. Fabbian, F. Kerber, H. U. Kaufl, M. Pettini
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  ABSTRACT: High resolution UVES spectra of 40 main-sequence stars with -3.3 < [Fe/H] < -1.0 are used to derive S, Fe and Zn abundances from lines in the 400 - 950 nm region. For one star we also present novel observations of the SI triplet at 1.046 micron carried out with the ESO VLT CRIRES spectrograph. Comparison of sulphur abundances from the weak and strong SI lines provides important constraints on non-LTE effects. The high sulphur abundances reported by others for some metal-poor stars are not confirmed; instead, when taking non-LTE effects into account, the Galactic halo stars distribute around a plateau at [S/Fe] = +0.2 dex with a scatter of 0.07 dex only. This indicates that sulphur in Galactic halo stars has been made by alpha-capture processes in massive SNe. The observed scatter in S/Fe is, however, much smaller than predicted from current stochastic models of the chemical evolution of the early Galaxy, suggesting that either the models or the calculated yields of massive SNe should be revised. [Zn/Fe] is close to zero for metallicities in the range -2.0 < [Fe/H] < -1.0 but increases to a level of [Zn/Fe] = +0.15 dex in the range -2.7 < [Fe/H] < -2.0. At still lower metallicities [Zn/Fe] rises steeply to a value around [Zn/Fe] = +0.5 dex at [Fe/H] = -3.2. We also examine the behaviour of S/Zn and find that departures from the solar ratio are significantly reduced at all metallicities if non-LTE corrections to the abundances of these two elements are adopted. This effect, if confirmed, would reduce the usefulness of the S/Zn ratio as a diagnostic of past star-formation activity, but would bring closer together the values measured in damped Lyman-alpha systems and in Galactic stars. Comment: 18 pages and 16 figures. Accepted for publication in A&A. Minor changes relative to version 1 plus additional text and figures in appendix B
  02/2007;
• Source

  Available from: adsabs.harvard.edu

  Article: Sakurai's object: ISO witnesses stellar evolution in ``real-time''

  F. Kerber, J. A. D. L. Blommaert, S. Kimeswenger, M. A. T. Groenewegen, H. U. Kaufl
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  ABSTRACT: We present ISOCAM observations of Sakurai's Object covering the wavelength range of 4 to 15 μm in seven filter bands. The photometry shows that over the period from February 1997 to February 1998 the flux over the whole range has increased by a factor of about ten. On basis of the ISOCAM and ground-based data we conclude that this increase is the result of recent mass loss from Sakurai's object and the formation of hot dust around it. Using a spherically symmetric dust radiative transfer model we get a very preliminary quantitative result for the mass loss rate of a few 10-7 Modot/year, a rather typical value for stars on the Asymptotic Giant Branch. This is in agreement with the notion that Sakurai's object is retracing its own evolutionary history as a consequence of a final helium flash.
  02/1999; 427:357.