7473( 4) - 05/21/97 07:26 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7473 Version: 4 Check-in Date: 21-May-1997 07:25:45 1.Proposal Title: HST observations of the DAB white dwarf HS 0209+0832: testing explanations for the ``DB gap'' ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle HOT STARS AND STELLAR GO 7 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: STEFAN JORDAN INSTITUT FUER ASTRONOMIE UND ASTROPHYSIK CoI: Detlev Koester Institut fuer Astronomie und Astrophysik N der Universitaet Kiel ------------------------------------------------------------------------------------ 6. Abstract White dwarfs are basically divided into two spectral classes, the hydrogen-rich DAs and the helium-rich non-DAs. However, between 28 000 and 45 000 K not a single non- DA star exists, although there are many helium-rich stars above and below this ``DB gap''. Currently, the only scenario to explain this gap is the assumption that every non-DA must contain or accrete a small amount of hydrogen, which, during the evolution to lower temperatures, floats up to the surface and transforms the stars into DAs. At 28 000 K these objects become convectively unstable and the hydrogen layer can disappear by dilution, if it is very thin (~ 10^-14 Msun); in this case the star would become a helium-rich white dwarf (spectral type DB) again. HS 0209+0832 is the first white dwarf at DB gap temperatures in which helium (~ 2 resolution and high S/N UV observations with HST would enable us to accurately determine the atmospheric parameters, especially the helium and metal content) of this unique object. The results will be important tests for the ultra-thin layer hypothesis and the theories of accretion from the interstellar medium. ------------------------------------------------------------------------------------ 7473( 4) - 05/21/97 07:26 - [ 2] Observations Description ------------------------ Exposure times could be calculated very accurately using the STIS Exposure Time Calculator since the flux in the UV and optical is well known from IUE and optical observations. We plan to cover the spectral range from 1200 to 5500 Angstrom with the following exposures: E140M, 1150-1735 Angstrom, resolution binned to 0.1 Angstrom, S/N=30: 30 min; G230LB, 1685-3065 Angstrom, S/N=100: 10 min; G430L, 2900-5700 Angstrom, S/N=100: 5 min. Together with the necessary overhead these observations will fit into two orbits of about 52 min time available. Real Time Justification ----------------------- None. Calibration Justification ------------------------- Only standard calibrations are required. Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7473( 4) - 05/21/97 07:26 - [ 3] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HS0209+0832 STAR,DA RA=02H 12M 4.75S +/- J2000 V = 13.72+/-0.5 0.2",DEC=08D 46' 51.7" +/- F(1250)=1.35+/-0.2e-12 0.2",PLATE-ID=002L F(1500)=8.0+/-0.3e-13 F(2000)=3.0+/-0.3e-13 F(3000)=7.5+/-1.0e-14 F(4000)=2.8+/-1.0e-14 F(5000)=1.3+/-0.5e-14 F(6000)=6.7+/-1.0e-15 F(7000)=3.7+/-0.5e-15 7473( 4) - 05/21/97 07:26 - [ 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 HS0209+0832 STIS/C ACQ F28X50L MIRROR 1 0.5 S ONBOARD ACQ FOR 2-6 CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 HS0209+0832 STIS/C ACQ/PEA 0.2X0.2 MIRROR 1 1 S ONBOARD ACQ FOR 3-6 CD K ------------------------------------------------------------------------------------------------------------------------------------ 3 HS0209+0832 STIS/F ACCUM 0.2X0.2 E140M 1425 1 2075 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 4 HS0209+0832 STIS/F ACCUM 0.2X0.2 E140M 1425 1 1140 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 5 HS0209+0832 STIS/C ACCUM 52X0.5 G230LB 2375 CR-SPLIT=2 1 10 M CD ------------------------------------------------------------------------------------------------------------------------------------ 6 HS0209+0832 STIS/C ACCUM 52X0.5 G430L 4300 CR-SPLIT=2 1 5 M CD ------------------------------------------------------------------------------------------------------------------------------------ 7473( 4) - 05/21/97 07:26 - [ 5] Summary Form for Proposal 7473 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=2 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category HOT STARS AND STELLAR CORPSES ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2-6 ONBOARD ACQ FOR 3-6 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR E140M G230LB G430L ------------------------------------------------------------------------------------------------------------------------------------ Target Names HS0209+0832 ------------------------------------------------------------------------------------------------------------------------------------