6090( 4) - 08/11/95 14:13 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6090 Version: 4 Check-in Date: ********** 1.Proposal Title: UV spectroscopy of two ultra-hot white dwarf candidates ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Hot Stars GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Klaus Werner Institut fuer Theoretische Physik und Sternwarte, Universitaet Kiel CoI: Ulrich Heber Sternwarte Bamberg, Astronomisches N Institut der Universitaet Erlangen- CoI: Stefan Dreizler Sternwarte Bamberg, Astronomisches N Institut der Universitaet Erlangen- CoI: Dieter Reimers Sternwarte Hamburg N ------------------------------------------------------------------------------------ 6. Abstract We propose GHRS UV spectroscopy of two outstanding stars which define a completely new spectroscopic class. The optical spectra are dominated by ultra-high ionization absorption ( lines of carbon and oxygen (CVI, OVII, OVIII). In comparison, the very hot (T(eff)>100 000 K) PG 1159 stars exhibit lines from CIV and OVI. First exploratory non- LTE model calculations indicate that the effective temperature of these objects must exceed half a million degrees temperatures by far exceed those of the hottest stars known. The surface gravities are high (log g>8), hence these object are probably compact post-AGB stars which are just about to enter the white dwarf cooling sequence. Comparison with evolutionary tracks suggests masses just below the Chandrasekhar limit. The proposed HST observations are needed to verify the nature of these stars by detailed spectrum synthesis in order to precisely determine the stellar parameters. ------------------------------------------------------------------------------------ 6090( 4) - 08/11/95 14:13 - [ 2] Observations Description ------------------------ We propose to observe HE0504-2408 and HS0713+3958 with the GHRS. For the brighter star (HE0504-2408) three different wavelength regions shall be covered by the spectra. In the case of the fainter star we restrict ourselves to the bluest wavelength region. Generally, a resolution of about 1 Angstrom is sufficient, because the lines are expected to be broad like the optical lines. We therefore want to use the low resolution grating G140L. For one exposure centered near 3000 Angstrom we must use the medium resolution grating G270M since G140L is not sensitive there. A S/N of 35 is aimed at. This requirement is reduced to almost 20 in the G270M case because appropriate binning can be performed. 1150-1436 Angstrom, t(exp)=28 min (HE0504-2408) and t(exp)=80 min (HS0713+3958 which has a UV flux 3 times lower than HS0713+3958) with grating G140L. According to Tab. 2 an OVIII and a CVI line will be detected. Also, the Lyman-alpha line is covered from which we can find the ISM column density towards to objects. This is required for analyses of X-ray observations which we are planning to perform. 1494-1781 Angstrom, t(exp)=48 min with grating G140L, covering numerous high ionization lines. 2956-- 3000 Angstrom, t(exp)=48 min with grating G270M. This covers the OVIII n=7-->8 line. We have recently detected this line in emission in the hot DO white dwarf KPD0005+5106 (see Fig. 3). As mentioned above, this DO also shows optical OVIII emissions. From the presence of optical OVIII absorptions in our targets we strongly expect the 2978 Angstrom line in absorption. The higher spectral resolution may be used to resolve fine structures in the line profile which might be indicative for a stellar wind. The first wavelength range can be exposed during one and two orbits for HE0504-2408 and HS0713+3958 respectively, after guide star and target aquisition. The second and third wavelength ranges are exposed for HE0504-2408 (only) during the 2nd and 3rd orbits after re-aquisition. The number of orbits for this whole proposal is then 5. Table 3 summarizes our exposure time estimates. The absolute UV fluxes of the stars are known from IUE low resolution spectra we have obtained earlier. begintableh caption Calculated exposure times to achieve S/N=35 with G140L (S/N=18 with G270M in the 3000Angstrom\ region) for HE0504-2408. centering wavelength& F_Lambda (IUE) /10^-13 & grating & sensitivity & count rate & t_ exp , Angstrom & erg/cm^2/sec/Angstrom & & 10^11 & per sec & minutes , 1150 & 3.5 & G140L &21. & 0.73 & 28 , 1700 & 1.3 & G140L &32. & 0.42 & 48. , 3000 & 0.40& G270M &27. & 0.11 & 48. , endtabular endtable Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6090( 4) - 08/11/95 14:13 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6090( 4) - 08/11/95 14:13 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HE0504-2408 STAR,WDO RA= 05H 06M 18.16S +/- 0.1S, J2000 V = 15.2 +/- 0.2 DEC=-24D 04' 1.9" +/- 0.1", B-V=-0.3 +/-0.1 PLATE-ID=049Y 2 HS0713+3958 STAR,WDO RA= 07H 13M 36.00S +/- 0.1S, B1950 V = 16.2 +/- 0.2 DEC=+39D 58' 47.0" +/- 0.1", B-V=-0.3 +/-0.1 PLATE-ID=00VU 6090( 4) - 08/11/95 14:13 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HE0504-2408 HRS ACQ 2.0 MIRROR-N1 SEARCH- 1 5.4S ONBOARD ACQ FOR 2 SIZE=3,BRIGHT=R ETURN Comments: STEP-TIME=0.6S, EXPECT 400 CNTS ------------------------------------------------------------------------------------------------------------------------------------ 2 HE0504-2408 HRS ACCUM 2.0 G140L 1150- FP- 1 1414.4S 1436 SPLIT=STD,COMB= FOUR,DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ 3 HE0504-2408 HRS ACCUM 2.0 G140L 1494- FP- 1 2611.2S 1781 SPLIT=STD,COMB= FOUR,DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ 4 HE0504-2408 HRS ACCUM 2.0 G270M 2956- FP- 1 2393.6S 3000 SPLIT=STD,COMB= FOUR,DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ 6090( 4) - 08/11/95 14:13 - [ 6] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HS0713+3958 HRS ACQ 2.0 MIRROR-N1 SEARCH- 1 11.7S ONBOARD ACQ FOR 2 SIZE=3,BRIGHT=R ETURN Comments: STEP-TIME=1.3S, EXPECT 400 CNTS ------------------------------------------------------------------------------------------------------------------------------------ 2 HS0713+3958 HRS ACCUM 2.0 G140L 1150- FP- 1 4243.2S 1436 SPLIT=STD,COMB= FOUR,DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ 6090( 4) - 08/11/95 14:13 - [ 7] Summary Form for Proposal 6090 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SEARCH-SIZE=3 BRIGHT=RETURN DOPPLER=ON COMB=FOUR FP-SPLIT=STD ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Hot Stars ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N1 G140L G270M ------------------------------------------------------------------------------------------------------------------------------------ Target Names HE0504-2408 HS0713+3958 ------------------------------------------------------------------------------------------------------------------------------------