8636( 3) - 09/08/00 17:50 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8636 Version: 3 Check-in Date: 02-Mar-2000 16:03:25 1.Proposal Title: Metal abundances in very hot DA white dwarfs -- a test of diffusion theory ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Ralf Napiwotzki Universitat Erlangen-Nurnberg CoI: Dr. Stefan Haas Goddard N CoI: Dr. Stefan Dreizler Tuebingen N CoI: Prof. Ulrich Heber Dr. Remeis-Sternwarte N ------------------------------------------------------------------------------------ 5. Abstract The photospheric abundances of white dwarfs are controlled by diffusion processes like gravitational settling and radiative levitation. UV, EUV and X-ray observations revealed a large metallicity spread from star to star, which cannot be explained by state-of-the-art stationary diffusion calculations. Additional processes, e.g. a weak stellar wind, have been invoked. A contrasting pair of extremely hot DA white dwarfs, recently discovered by the HS survey, has very similar temperature and gravity, but probably very different metal abundances. HS 0505+0112 is unique because it is the only H-rich hot white dwarf known to show optical C lines. Balmer line fitting yields further evidence that the heavy element abundances of HS0505+0112 must be high. In contrast HS 0615+6535 displays Balmer lines only and fitting their line profiles is consistent with an unusually low amount of metals. We propose to determine abundances of C, N, O, Si and Fe and Ni directly from STIS UV spectra by means of fully metal line blanketed NLTE model atmospheres. Indications of mass loss will also be searched for providing us with an important check of the diffusion theory in white dwarfs. Both stars are also ideally suited for a test, whether the C, N, O line opacity can account for the Balmer line discrepancy noted for the analysis of very hot H-rich stars. ------------------------------------------------------------------------------------ 8636( 3) - 09/08/00 17:50 - [ 2] Observations Description ------------------------ We will obtain short-slit (0.2* 0.06) medium dispersion ultraviolet spectroscopy for two hydrogen-rich hot white dwarfs in the E140M mode. This mode covers the range 1150- 1700 Angstrom. Besides prominent C, N, and O lines this range contains many Fe V, Fe VI and Fe VII as well as Ni V and Ni VI lines suitable for spectroscopic analysis. Since we want to use the full resolving power for the sharp lines of Fe and Ni, we will do our observations with the 0" .06 slit. We simulated STIS spectra with different S/N ratios (cf.\ Fig. 2). Most critical are the weak lines of Fe and Ni. However, since many lines are available a S/N of little less than 20 is sufficient for an abundance analysis. We used the STIS exposure time calculator to determine the exposure times necessary to achieve a S/N ~ 20 (per resolution element) at 1300Angstrom. We selected the spectrum of the 82000 K sdO star BD+28^degrees4211 for this purpose. The guide star aquisition takes 6 min., the point source acquisition 6 min. plus 6 min. for the aquisition peakup exposure, and the guide star reaquisition 5 min. The first MAMA exposure in each visit takes 8 min. HS 0615+6535 is in the continous viewing zone. The length of orbit for HS 0505+0112 is 52 min. Therefore, 4 orbits are neccesary for HS 0615+6535 and 5 for HS 0505+0112, yielding a S/N > 18 in the most important spectral region 1250 Angstrom < Lambda < 1400 Angstrom. begintable captionEstimated exposure times begintabularlcrrrr Target & Flux at 1250Angstrom & T(eff) & log g & exposure & central , & & & & time & wavelength, & erg/cm^2/s/Angstrom & K & & s & Angstrom , HS 0505+0112 & 2.0 \cdot 10^-13 & 80000 & 7.0 & 12800 & 1250 Angstrom , HS 0615+6535 & 1.2 \cdot 10^-13 & 93000 & 7.2 & 21500 & 1250 Angstrom , endtabular endtable Real Time Justification ----------------------- One target (HS 0615+6535) lies in the continuous viewing zone and, hence, can be observed for 96 min. several times in the year. Since it is also the faintest of our programme stars, the S/N will benefit from the longer exposure time. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8636( 3) - 09/08/00 17:50 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HS0505+0112 STAR, DA RA = 05H 08M 30.83S +/- 0.4", J2000 V = 15.2+/-0.3 DEC = +01D 16' 39.3" +/- F-CONT(1250)=2.0+/-0.4e-13 0.4"PLATE-ID=02O7 Coordinate Source: GUIDE_STAR_CATALOG 2 HS0615+6535 STAR, DA RA = 06H 20M 30.14S +/- 0.5", J2000 V = 15.5+/-0.3 DEC = +65D 34' 21.7" +/- F-CONT(1250)=1.2+/-0.25e-13 0.5"PLATE-ID=034M Coordinate Source: GSC_SURVEY_PLATE 8636( 3) - 09/08/00 17:50 - [ 4] Visit: 01 Visit Priority: 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 HS0505+0112 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 1 S CD P Comments: Expected total counts: 850ADU ------------------------------------------------------------------------------------------------------------------------------------ 2 HS0505+0112 STIS/C ACQ/PEA 0.2X0.0 MIRROR 1 1.9 S CD K 6 Comments: yields a minimum of 2500ADU total (for 50% slit throughput) ------------------------------------------------------------------------------------------------------------------------------------ 3 HS0505+0112 STIS/F ACCUM 0.2X0.0 E140M 1425 1 2002 S UV-MAM 6 A ------------------------------------------------------------------------------------------------------------------------------------ 4 HS0505+0112 STIS/F ACCUM 0.2X0.0 E140M 1425 4 2899 S UV-MAM 6 A ------------------------------------------------------------------------------------------------------------------------------------ 8636( 3) - 09/08/00 17:50 - [ 5] Visit: 02 Visit Priority: Visit Requirements: CVZ 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 HS0615+6535 STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 1.5 S CD P Comments: Expected total counts: 960ADU ------------------------------------------------------------------------------------------------------------------------------------ 2 HS0615+6535 STIS/C ACQ/PEA 0.2X0.0 MIRROR 1 2.5 S CD K 6 Comments: yields a minimum of 2500ADU total (for 50% slit throughput) ------------------------------------------------------------------------------------------------------------------------------------ 3 HS0615+6535 STIS/F ACCUM 0.2X0.0 E140M 1425 4 5175 S UV-MAM 6 A ------------------------------------------------------------------------------------------------------------------------------------ 8636( 3) - 09/08/00 17:50 - [ 6] Summary Form for Proposal 8636 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 0.2X0.06 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR E140M ------------------------------------------------------------------------------------------------------------------------------------ Target Names HS0505+0112 HS0615+6535 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 ------------------------------------------------------------------------------------------------------------------------------------