6450( 4) - 02/20/96 18:09 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6450 Version: 4 Check-in Date: 20-Feb-1996 18:06:29 1.Proposal Title: Hydrogen and metal abundances in the cool helium-rich white dwarf Ross640 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle HOT STARS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Detlev Koester Institut fuer Astronomie und Astrophysik CoI: Nicole Allard Observatoire Paris-Meudon N ------------------------------------------------------------------------------------ 6. Abstract The metal traces found in a few cool helium-rich white dwarfs (spectral type DZ) cannot be primordial, but must have been accreted from the interstellar matter, since their lifetimes in the atmospheres is limited to about a million years (diffusion timescales). We want to obtain high S/N UV spectra of Ross640, which is one of the brightest DZ, and only one of two objects at low Teff showing both metals and H. Abundances will be determined with model atmosphere techniques and compared with the predictions of the two-phase accretion scenario proposed by the Montreal group. This is the most advanced explanation today; however, comparisons between observation and theory are currently severely limited by the lack of reliable and accurate abundance determinations in these faint objects. HST observations of the second object with similar characteristics (L745-46A), which have recently been analyzed by us, demonstrate the huge progress that can be made with FOS now. ------------------------------------------------------------------------------------ 6450( 4) - 02/20/96 18:09 - [ 2] Observations Description ------------------------ Because of the requirement of high S/N and good calibration we plan to use the 4.3 aperture and an elaborate (4-stage- peakup) target acquisition to an accuracy of 0."04. We estimate that this will take about 1.5 orbits. The remainder of the second orbit will be used for a low resolution (G160L) spectrum of the range 1600 - 2300 Angstrom. From the IUE spectrum and model calculations, which we have used to estimate exposure times, we know that the flux will decrease steeply towards shorter wavelengths below 2300 Angstrom. The S/N will correspondingly fall from about 100 to 10, which is good enough to define the shape of the Lyman alpha far wing and show the stronger line features. The third orbit will, after target reacquisition, be used for a high resolution ( 2 Angstrom, G270H) high S/N spectrum of the most important region from 2300 to 3000 Angstrom. The S/N for 50 min exposure will be >= 100 over the whole region, ideal for line identifications and detailed model atmosphere analysis. The total request is thus 3 orbits. begintabularlll Cycle 6 & Primary Orbits:3 & Exposures:2, endtabular Real Time Justification ----------------------- none none Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6450( 4) - 02/20/96 18:09 - [ 3] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 WD1626+368 STAR,LATE-TYPE RA=16H 28M 25.72S +/- J2000 V = 13.84+/-0.03 DEGENERATE,DZ 0.3",DEC=36D 46' 2.6" +/- B-V = 0.18+/-0.03 0.3",PLATE-ID=01RW Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) B1982.39 -0.04087 0.7476 0.061 6450( 4) - 02/20/96 18:09 - [ 4] 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 WD1626+368 FOS/RD ACQ/BIN 4.3 MIRROR 1 0.82S ONBOARD ACQ FOR 2 ARY ------------------------------------------------------------------------------------------------------------------------------------ 2 WD1626+368 FOS/RD ACQ/PEA 0.5 MIRROR SCAN-STEP- 1 1.0S ONBOARD ACQ FOR 3 K X=0.29,SCAN- STEP- Y=0.29,SEARCH- SIZE-X=3,SEARCH Comments: STAGE 1 of 3 STAGE PEAKUP -SIZE-Y=3 ------------------------------------------------------------------------------------------------------------------------------------ 3 WD1626+368 FOS/RD ACQ/PEA 0.3 MIRROR SCAN-STEP- 1 1.0S ONBOARD ACQ FOR 4 K X=0.17,SCAN- STEP- Y=0.17,SEARCH- SIZE-X=3,SEARCH Comments: STAGE 2 of 3 STAGE PEAKUP -SIZE-Y=3 ------------------------------------------------------------------------------------------------------------------------------------ 4 WD1626+368 FOS/RD ACQ/PEA 0.3 MIRROR SCAN-STEP- 1 3.0S ONBOARD ACQ FOR 5 K X=0.052,SCAN- STEP- Y=0.052,SEARCH- SIZE-X=5,SEARCH Comments: STAGE 3 of 3 STAGE PEAKUP -SIZE-Y=5 ------------------------------------------------------------------------------------------------------------------------------------ 5 WD1626+368 FOS/RD ACCUM 4.3 G270H 2800 1 39.5M MAX DUR 39.5M NO SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 6 WD1626+368 FOS/RD ACCUM 4.3 G270H 2800 1 1330S NO SPLIT ------------------------------------------------------------------------------------------------------------------------------------ 7 WD1626+368 FOS/RD ACCUM 4.3 G160L 1700- 1 15.0M NO SPLIT MAX DUR 15M 2400 ------------------------------------------------------------------------------------------------------------------------------------ 6450( 4) - 02/20/96 18:09 - [ 5] Summary Form for Proposal 6450 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/RD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/BINARY ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SCAN-STEP-X=0.29 SCAN-STEP-Y=0.29 SEARCH-SIZE-X=3 SEARCH-SIZE-Y=3 SCAN-STEP-Y=0.17 SEARCH-SIZE-Y=5 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category HOT STARS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2 ONBOARD ACQ FOR 3 ONBOARD ACQ FOR 4 ONBOARD ACQ FOR 5 MAX DUR 39.5M NO SPLIT NO SPLIT MAX DUR 15M ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G270H G160L ------------------------------------------------------------------------------------------------------------------------------------ Target Names WD1626+368 ------------------------------------------------------------------------------------------------------------------------------------