6442( 3) - 02/06/96 13:19 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6442 Version: 3 Check-in Date: 06-Feb-1996 13:14:13 1.Proposal Title: First Ultraviolet Asteroseismology of a Pulsating DB White Dwarf ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle HOT STARS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: S. O. Kepler Universidade Federal do Rio Grande do Sul CoI: Edward L. Robinson University of Texas at Austin N CoI: Detlev Koester University of Kiel N ------------------------------------------------------------------------------------ 6. Abstract The use of white dwarfs to test calculations of stellar evolution and to measure the history of star formation in the galaxy depends on accurate knowledge of white dwarf interiors. White dwarf pulsations give the most detailed picture of their interiors, but the picture is correct only if the indices of the pulsation modes have been identified correctly. Standard mode identifications are based on matching observed pulsation periods to periods determined from complex theoretical calculations. These methods are circular, prone to mis- identifications, and do not work for stars with few excited pulsations. Other methods of mode identifications must be used to avoid these problems. The wavelength dependence of pulsation amplitudes at ultraviolet wavelengths gives an unambiguous and model-independent signature of the pulsation index. We propose to measure the ultraviolet amplitudes of the pulsations in the pulsating DB white dwarf PG1351+489 with FOS. We choose this star because (1) There are no model- independent mode identifications for any DB white dwarf; (2) The observations will independently test earlier conclusions that some DB white dwarfs rotate differentially and have low- mass helium surface layers; (3) The observations will help define the temperature scale for DB white dwarfs; (4) PG1346+489 has the simplest light curve of any pulsating DB white dwarf, so we avoid the serious complications caused by beating between modes. ------------------------------------------------------------------------------------ 6442( 3) - 02/06/96 13:19 - [ 2] Observations Description ------------------------ We will use the FOS/BL and the G160L grating in the RAPID readout mode to obtain time-resolved spectrophotometry of PG1351+489 from 1150 Angstroms to 2500 Angstroms. A sampling interval of 10 seconds will resolve all features in the light curve while maintaining a good duty cycle for the FOS. We will use the upper 1 arcsec square aperture to avoid a scratch on the photocathode. We will use a 3-stage peak-up instead of a 4-stage peak-up even though a 3-stage peak-up in the 1 arcsec aperture does not center a point source in the aperture well enough to avoid variable light loss due (presumably) to breathing over the HST orbit. We do this because the flux variations introduced by breathing will compromise the data less than the poor signal to noise ratio caused by spending too much time on target acquisition. We estimate the required on-target integration time thusly: At 4200 Angstroms the fractional semi-amplitude of the 489.5 s pulsation is ~7.0% The pulsation is probably an l = 1 or l = 2 mode. From the calculations shown in Figure 1, we estimate that the amplitude will be 14.7% at 1400 Angstroms if it is an l = 1 mode and 17.5% if it is an l = 2 mode. To distinguish the two modes with certainty (~ 5 Sigma), the semi- amplitude in the ultraviolet must be measured with an accuracy no poorer than 0.5%, which requires an 4 * 10^4 photons per resolution element. From the SYNPHOT program we estimate that FOS/BL and the G160L grating will detect ~0.13 photons/s/Angstrom at 1400 Angstroms from PG1351+489. Because the ultraviolet spectrum of DB white dwarfs are almost featureless, we can reduce the resolution of the data to 48 Angstroms (by coadding diodes in blocks of 7) without significant loss of information. We need 6300 seconds of on-target integration time to accumulate 4 * 10^4 photons per 48 Angstroms resolution element. At a declination of 49 degrees, PG1351+489 is visible 56 minutes per orbit, of which 50 minutes are available after target re-acquisition. From discussions with the experts at STScI, we understand that the 2-stage peakup will require 43 min and will take ~1.0 orbits; and the on-target integration time requires at least 2.1 orbits. Thus we require 4 orbits for the project. Real Time Justification ----------------------- No special requirements. We will observe PG1351+489 with CCD and photoelectric photometers at McDonald Observatory to obtain a contemporaneous measurement of its optical light curve. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6442( 3) - 02/06/96 13:19 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: Edward L. Robinson*Department of Astronomy*University of Texas*Austin, TX 78712-1083 Ship Via: Email: elr@aries.as.utexas.edu , ------------------------------------------------------------------------------------ 6442( 3) - 02/06/96 13:19 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 EM-UMA STAR,DB,PULSATING RA=13H 53M 9.85S +/- J2000 B = 16.38+/-0.5 VARIABLE, 0.020S,DEC=48D 40' 21.2" +/- TYPE=DB2 0.30",PLATE-ID=008S Comments: Flux(1500)=1.5 +/- 0.1 E-14 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) B1983.1 0.0 0.0 0.0 6442( 3) - 02/06/96 13:19 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 EM-UMA FOS/BL ACQ/BIN 4.3 MIRROR 1 26.82S ONBOARD ACQ FOR 20 ARY ------------------------------------------------------------------------------------------------------------------------------------ 20 EM-UMA FOS/BL ACQ/PEA 0.25- G270H SCAN-STEP- 1 35.5S ONBOARD ACQ FOR 25-50 K PAIR-A Y=0.11,SCAN- STEP- X=0.11,SEARCH- SIZE-Y=4,SEARCH Comments: -SIZE-X=4 ------------------------------------------------------------------------------------------------------------------------------------ 25 EM-UMA FOS/BL RAPID 1.0- G160L READ- 1 340S PAIR-A TIME=10,SUB- STEP=4,COMB=YES ,STEP- PATT=SINGLE ------------------------------------------------------------------------------------------------------------------------------------ 30 EM-UMA FOS/BL RAPID 1.0- G160L READ- 1 2840S PAIR-A TIME=10,COMB=YE S,STEP- PATT=SINGLE,SUB -STEP=4 ------------------------------------------------------------------------------------------------------------------------------------ 40 EM-UMA FOS/BL RAPID 1.0- G160L READ- 1 2840S PAIR-A TIME=10,COMB=YE S,STEP- PATT=SINGLE,SUB -STEP=4 ------------------------------------------------------------------------------------------------------------------------------------ 50 EM-UMA FOS/BL RAPID 1.0- G160L READ- 1 2620S PAIR-A TIME=10,COMB=YE S,STEP- PATT=SINGLE,SUB -STEP=4 ------------------------------------------------------------------------------------------------------------------------------------ 6442( 3) - 02/06/96 13:19 - [ 6] Summary Form for Proposal 6442 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/BL ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/BINARY ACQ/PEAK RAPID ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SCAN-STEP-Y=0.11 SCAN-STEP-X=0.11 SEARCH-SIZE-Y=4 SEARCH-SIZE-X=4 STEP-PATT=SINGLE SUB-STEP=4 COMB=YES READ-TIME=10 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category HOT STARS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 20 ONBOARD ACQ FOR 25-50 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G270H G160L ------------------------------------------------------------------------------------------------------------------------------------ Target Names EM-UMA ------------------------------------------------------------------------------------------------------------------------------------