6463( 3) - 02/15/96 09:17 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6463 Version: 3 Check-in Date: 15-Feb-1996 09:06:59 1.Proposal Title: The origin of the peculiar hybrid PG 1159 stars ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle HOT STARS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Dr. Stefan Dreizler Institut fuer Astronomie und Astrophysik der Universitaet CoI: Dr. Ulrich Heber Astronomisches Institut der Universitaet N Erlangen--Nuernberg CoI: Dr. Ralf Napiwotzki Astronomisches Institut der Universitaet N Erlangen--Nuernberg CoI: Dr. Klaus Werner Lehrstuhl Astrophysik, Universitaet N Potsdam ------------------------------------------------------------------------------------ 6. Abstract The PG 1159 stars form a new class of hydrogen deficient pre white dwarfs (PWD) representing the hottest stage of PWD evolution. Quantitative spectral analyses of most known PG 1159 stars have been carried out by the proposers from optical, UV (IUE, HST) and EUV (EUVE) observations. It has been shown that these stars have atmospheres dominated by C and He. In the hottest stars also a significant amount of O is present. The proposed targets, HS2324+3944, NGC7094, and Abell43 define a small group of peculiar PG 1159 stars. Unlike all other PG 1159 stars hydrogen is detected in their spectra. Stellar parameters derived by the investigators from optical spectra suggest that, as a consequence of a lower mass, they went through a less drastic mass-loss phase than the ordinary PG 1159 stars. In order to verify that hypothesis a more precise determination of the effective temperature is required. This is not possible from optical analyses alone. We demonstrate that a great improvement on the determination of the effective temperatures can be achieved by HST-GHRS spectra. The spectra will be analyzed with new highly sophisticated NLTE line blanketed model atmospheres, which allow to include many elements up to the iron group self- consistently. A precise placement of HS2324+3944, NGC7094, and Abell43 in the HR-diagram holds the key for understanding the evolutionary history of the PG 1159 class. ------------------------------------------------------------------------------------ 6463( 3) - 02/15/96 09:17 - [ 2] Observations Description ------------------------ We propose the following observations: Each star shall be observed with the GHRS using grating G140L in the wavelength range 1150-1437 Angstrom. NGC7094 is bright enough to expose additionally a spectrum in the wavelength range 1463- 1750 Angstrom within one orbit. The primary aim is to determine the effective temperature as precise as possible as well as to determine the oxygen abundance. A simultaneous analysis of the OV 1371 Angstrom line together with OIV lines (1340 Angstrom) for NGC7094, and Abell43 and OVI lines for HS2324+3944, NGC7094 and Abell43 is the only means to determine the O abundance and will reduce the uncertainties in T(eff) considerably. Fig.15 in WHH and Fig.2 in this proposal predict an extreme temperature sensitivity of the oxygen lines. It follows that the determination of T(eff) with a precision of 2% equivalent widths can be measured to an accuracy of about 10%. The first spectral range also covers the NV, 1240 Angstrom resonance line. Due to the high effective temperature it is the only means to determine the nitrogen abundance in HS2324+3944 (no other N lines are detectable). Due to a possible contamination with interstellar NV the nitrogen abundance in HS2324+3944 can be an upper limit only. For the three cooler stars we predict the presence of several (weaker) NIV lines serving as consistency checks for the derived nitrogen abundances. Many other lines of CIV and OVI can be identified in the first wavelength range (see WHH) and will allow to determine abundances with unprecedented precision. The second wavelength range for NGC7094 additionally covers the CIV resonance line, the HeII 1640 Angstrom line, and the NIV 1718 Angstrom line. All three lines can be used to additionally constrain the carbon, helium, and nitrogen abundance. Because of the high gravity, all spectral features are relatively broad and shallow. Hence a spectral resolution of about 1 Angstrom is sufficient, which can be reached with grating G140L and the large science aperture. A high S/N ratio is necessary, however, in order to perform detailed line profile fits (S/N~100 at 1350 Angstrom is aimed at). In the second wavelength range for NGC7094 a lower S/N is acceptable, since we expect strong carbon and helium lines. Exposure times and S/N have been calculated from available (or scaled) IUE fluxes, visibility periods, overhead times and the sensitivities given in the handbook. Two orbit (54 min/52 min visibility) are necessary for HS2324+3944, NGC7094, and Abell43 each. Subtracting the overhead time for guide star acquisition and re--acquisition as well as target acquisition (8 + 11 + 5 = 23min.) 84 minutes of exposure time remain for HS2324+3944, 79 minutes remain for Abell43. The 79 minutes for NGC7094 will be split in 33 minutes for the bluer and 46 minutes for the redder wavelength range. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6463( 3) - 02/15/96 09:17 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6463( 3) - 02/15/96 09:17 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 STAR-NGC7094 STAR,PG1159 STAR, RA=21H 36M 52.98S +/- J2000 V = 13.7+/-0.1 0.2",DEC=12D 47' 19.1" +/- E(B-V) = 0.1+/-0.03 0.2",PLATE-ID=02KW F(1350)=1.2+/-0.1e-12 F(1550)=7.9+/-0.1e-13 Comments: GSC Coordinates; Color_Excess from Burstein and Heiles 1982, AJ 87, 1165;Flux(1350A/1550A) from IUE spectrum; 2 STAR- STAR,PG1159 STAR, RA=23H 27M 15.88S +/- J2000 V = 14.8+/-0.2 2324+3944 0.2",DEC=40D 01' 24.7" +/- E(B-V) = 0.15+/-0.03 0.3",PLATE-ID=02S2 F(1350)=2.0+/-0.1e-13 Comments: GSC Coordinates; Color_Excess from Burstein and Heiles 1982, AJ 87, 1165;Flux(1350A) from IUE spectrum; 3 STAR-ABELL43 STAR,PG1159 STAR, RA=17H 53M 32.29S +/- J2000 V = 14.8+/-0.1 0.3",DEC=10D 37' 23.65" +/- E(B-V) = 0.18+/-0.03 0.3",PLATE-ID=00JA F(1350)=2.0+/-0.1e-13 Comments: DSS-phaseII Coordinates; Color_Excess from Burstein and Heiles 1982, AJ 87, 1165;Flux(1350A) from IUE spectrum; 6463( 3) - 02/15/96 09:17 - [ 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 STAR- HRS ACQ 2.0 MIRROR-N1 SEARCH- 1 5.4S ONBOARD ACQ FOR 2-3 NGC7094 SIZE=3,BRIGHT=R ETURN Comments: STEP-TIME=0.6s, EXPECT 400 CNTS ------------------------------------------------------------------------------------------------------------------------------------ 2 STAR- HRS ACCUM 2.0 G140L 1150- FP- 1 1305.6S NGC7094 1437 SPLIT=STD,COMB= FOUR,DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ 3 STAR- HRS ACCUM 2.0 G140L 1463- FP- 1 2828.8S NGC7094 1750 SPLIT=STD,COMB= FOUR,DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ 6463( 3) - 02/15/96 09:17 - [ 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 STAR- HRS ACQ 2.0 MIRROR-N1 SEARCH- 1 21.6S ONBOARD ACQ FOR 2 2324+3944 SIZE=3,BRIGHT=R ETURN Comments: STEP-TIME=2.4s, EXPECT 400 CNTS ------------------------------------------------------------------------------------------------------------------------------------ 2 STAR- HRS ACCUM 2.0 G140L 1150- FP- 1 4460.8S 2324+3944 1437 SPLIT=STD,COMB= FOUR,DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ 6463( 3) - 02/15/96 09:17 - [ 7] Visit: 03 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 STAR- HRS ACQ 2.0 MIRROR-N1 SEARCH- 1 21.6S ONBOARD ACQ FOR 2 ABELL43 SIZE=3,BRIGHT=R ETURN Comments: STEP-TIME=2.4s, EXPECT 400 CNTS ------------------------------------------------------------------------------------------------------------------------------------ 2 STAR- HRS ACCUM 2.0 G140L 1150- FP- 1 4243.2S ABELL43 1437 SPLIT=STD,COMB= FOUR,DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ 6463( 3) - 02/15/96 09:17 - [ 8] Summary Form for Proposal 6463 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 2-3 ONBOARD ACQ FOR 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N1 G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names STAR-NGC7094 STAR-2324+3944 STAR-ABELL43 ------------------------------------------------------------------------------------------------------------------------------------