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Astron. Astrophys. 334, 618-632 (1998)

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2. Observations

Our HST observations of PG 1159 stars started in cycle 1 with PG 1159-035 using FOS, because the GHRS was not available in that cycle due to the failure of the side 1 power supply. Three more objects (PG 1424 [FORMULA] 535, PG 1707 [FORMULA] 427, PG 1520 [FORMULA] 525) were scheduled already in cycle 3. However, none of them was observed due to the first service mission. The spectrum of PG 1159-035 was already presented by Werner & Heber (1993), the analysis will however be repeated here with our latest models for the sake of consistency. After demonstrating the high precision obtained in the determination of the effective temperature we re-applied for HST time in cycle 5, this time with an extended target list. After the first refurbishment mission we could then use the GHRS spectrograph which was better suited for our needs. All observations were obtained in the fp-split-four mode to reduce the fixed-pattern noise. Final spectra were obtained after the standard pipeline extraction. We smoothed the spectra with a 0.5 Å Gauss profile to reduce the noise which results in a spectral resolution of 0.9 Å for the GHRS and 1.1 Å for FOS data. Details of the observations are listed in Table 1.


Table 1. List of HST and IUE observations. [FORMULA] and E(B-V) are determined from the HST spectra. The flux is given in 10-13 erg/(cm-2 s-1 Å-1)

All optical observations except one have been performed during the past ten years at the German-Spanish Astronomical Center on Calar Alto, Spain, using the 3.5 m telescope equipped with the TWIN or the Boller & Chivens spectrograph. The dispersion varies from 26 Å/mm to 72 Å/mm resulting in a resolution of 1 Å to 3.5 Å. In the TWIN spectrograph the red and blue channels are separated by a dichroic splitting the light beam at 5500 Å. Observations of the PG 1159 stars found in the Hamburg-Schmidt survey are described in more detail by Heber et al. (1996) and Dreizler et al. (1994a). Spectra of PG 1424 [FORMULA] 535, PG 1707 [FORMULA] 427, PG 1520 [FORMULA] 525, and PG 1159-035 were already presented and analyzed by WHH. MCT 0130-1937 was observed by Thomas Rauch (Tübingen, kindly provided for this analysis) at ESO with the ESO Multi Mode Instrument attached to the NTT. A dichroic was used to feed two separate channels. Data reduction was performed in Kiel, Bamberg and Tübingen using standard MIDAS or IRAF routines. For consistency checks we evaluate IUE low- and high-resolution data selecting suitable spectra form the Final Archive. Image numbers and exposure times are listed in Table 1. Looking at Fig. 1 it is clear that the resolution and the S/N of the IUE spectra is too low to tackle the problems posed in the introduction. The two important indicators for the effective temperature, the O IV multiplet at 1340 Å and the C III multiplet at 1170 Å are invisible in the IUE spectrum due to the low resolution. A more detailed comparison between HST and IUE low resolution spectra reveals a slight calibration problem. The HST flux redwards of 1380 Å is up to 10% lower than the IUE flux and our model spectra flux. The same result is obtained if we compare the HST spectra with HUT spectra (Kruk & Werner 1998). For the final fits we therefore applied a correction function to our HST data calibrated with PG 0122 [FORMULA] 200.

[FIGURE] Fig. 1. Comparison of the IUE low-resolution and HST GHRS spectra of HS 0704 [FORMULA] 6153. Top: HST GHRS spectrum; middle: HST GHRS spectrum degraded to the IUE resolution; bottom: IUE low-resolution spectrum. The vertical lines indicate the positions of the important [FORMULA] indicators (C III 1177 Å; O IV 1340 Å; O V 1371 Å). The strong Ly [FORMULA] geocoronal emission line is omitted for clarity.

In Table 2 we list our stellar line identifications from the GHRS spectra. All major features can be attributed to either stellar or interstellar origin (see Verner et al. 1996 or Morton 1991 for interstellar line lists). As expected, we see several lines of C III and C IV as well as O IV, O V, and O VI. Unexpected was the detection of N IV and N V lines in the pulsating PG 1159 stars. The spectral resolution is too low for a separation of stellar and interstellar lines by velocity, all interstellar lines except possibly N V are, however, from low ionization stages (e.g. N I, O I, Si II, S II) and can therefore be clearly distinguished from the high ionization stages of the PG 1159 stars. There are three cases where a safe assignment is more difficult: The contribution of the interstellar N V resonance doublet to the stellar one (see Sect. 4.2.) and the possible identification of a Mg IV line at 1191 Å. According to calculations of Rauch (1997) Mg IV lines could be expected at effective temperatures of PG 1159 stars. However, there should be another Mg IV line at 1307 Å, with comparable line strength and originating from near-by levels, which is not detected. The 1190 Å line is possibly blended by interstellar S III and Si II which would, however, require far too high abundances for these elements if no other contribution is accounted for. A possible stellar magnesium contribution can therefore only be determined with detailed modeling. This is beyond the scope of this paper. Our strategic O V line at 1371 Å might be blended by interstellar Ni II. The separation of 1.2 Å is large enough to be resolved by the employed GHRS setup. In the final fit of MCT 0130-1937, PG 0122 [FORMULA] 200, and PG 2131 [FORMULA] 066 the weak contribution of the interstellar Ni II line can be seen in the blue wing of the O V line.


Table 2. Identification of stellar spectral lines in HST GHRS spectra; 'p' denotes lines visible only in pulsating, 'h' lines in "hot" ([FORMULA] [FORMULA] 110 000 K), and 'c' lines in "cool" ([FORMULA] [FORMULA] 110 000 K) PG 1159 stars; 'b' denotes lines possibly blended by interstellar lines originating from low inonization stages (except N V) within 1 Å.

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998