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Session 15 - Planetary Nebulae and White Dwarfs.
Display session, Wednesday, January 07
We present observations in the Far-Ultraviolet of the hot DA star HZ 43 obtained during the ORFEUS-SPAS II mission using the Berkeley Spectrometer in November 1996. A fit of the Ly \alpha to Ly \epsilon line profiles with our grid of LTE synthetic spectra give an effective temperature of 50296 \pm 333 K and a log g of 7.85 \pm 0.03. This is in agreement with independent optical determinations (see Finley et al. 1997, ApJ, 488, 375). We have also measured the b-value of interstellar HI in the line of sight of HZ 43 by fitting the Ly 8 to Ly 12 lines. Fixing log N(HI) to 17.91 (Dupuis et al. 1995, ApJ, 455, 574), we find a b-value of 9 km/s. This spectrum is used to put stringent upper limits on the abundance of C, N, Si, S, P, Cl. These elements have relatively strong absorption lines in the FUV and are predicted to be abundant because of radiative acceleration in HZ 43 atmosphere. We have determined that the upper limits on the abundances are: C/H<-8.0, N/H<-8.0, Si/H<-8.5, S/H<-7.0, P/H<-8.5, and Cl/H<-8.5 These upper limits are at least one order of magnitude below the equilibrium abundances predicted by the radiative levitation calculation of Chayer et al. (1995, ApJ, 454, 429). This is comparable to the helium under-abundance observed in the EUVE spectrum (Barstow et al. 1995, MNRAS, 274L, 31). We speculate that weak winds dying out with cooling may interfere with radiative levitation by depleting the atmosphere of HZ 43 of its heavy element content. This work is supported by NASA grant NAG5-696.
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