! $Id: 6654,v 9.1 1996/07/09 14:19:54 pepsa Exp $ Proposal_Information ! Section 4 Title: Ultraviolet Spectroscopy of Hot DB White Dwarfs Proposal_Category: GO Scientific_Category: HOT STARS Cycle: 6 Investigators PI_name: Francois Wesemael PI_Institution: Universite de Montreal CoI_Name: Pierre Bergeron CoI_Institution: Bell Northern Research Contact: ! Y or N (designate at most one contact) CoI_Name: Alain Beauchamp CoI_Institution: C.A.E. Electronics, Ltd. Contact: ! Y or N (designate at most one contact) Abstract: ! Free format text (please update) Any tentative explanation of the origin of the various families of white dwarf stars requires first a complete understanding of their basic physical properties (effective temperature, surface gravity, surface composition). In order to refine the calibration of the effective temperature scale of the hot, helium-line (DB) white dwarfs, we propose to secure FOS/GHRS energy distributions in the 1200--3250 Angstrom\ range for two of these stars. This includes the hottest known DB star, WD 0112+104, as well as one hot, pulsating (V777 Her) star. The specific goal of these observations is twofold: i) To derive an accurate ultraviolet temperature, T^UV_ eff, which can be compared to that derived from our detailed analysis of the optical spectrum. Because we believe that existing discrepancies between current estimates of these quantities are related to the presence of small traces of hydrogen in the photosphere, which remain invisible in the optical spectrum, we also intend ii) to use the observed Ly-alpha\ region to estimate, or constrain, the hydrogen abundance in these objects. Limits are not available from the ground for DB stars above (T_ eff >~ 20,000 K), as the Balmer lines fade rapidly for increasingly lower hydrogen abundances. By contrast, the wings of Ly-alpha\ remain useful for detecting hydrogen even at low abundance values. Questions ! Free format text (please update) Observing_Description: Medium-resolution spectrophotometric observations at S/N (square)uig 20 of the energy distribution of hot DB stars between 1600 and 3250 Angstrom\ will be obtained with a combination of FOS/RD, G190H and G270H dispersers, and high throughput ( 4.3) aperture. This combination will allow, furthermore, coverage of the He II Lambda1640 transition at (square)uig 1.5 Angstrom\ resolution. For the blue data, which include observations of Ly-alpha, we have explored two possibilities: FOS/BL+G130H through aperture 4.3; and GHRS Side 1+G140L+LSA in the 1100--1385 and 1385--1670 Angstrom\ ranges. While less efficient, the latter combination was considered more appropriate, in that the 1.74 arcsec square LSA is large enough for accurate spectrophotometry (95\% throughput), but small enough to reduce geocoronal Ly-alpha\ contamination. Because the geocoronal component is expected to be superposed onto interstellar absorption, our analysis will be restricted to the wings of the photospheric Ly-alpha\ profile. As an example, we give below the details of our exposure-time estimates for the brightest of our objects, WD 0112+104 (V=15.36), assuming S/N=20 for all spectra: smallskip # \quad&& # \quad noalign noalign Configuration & f_Lambda & S_lambda^-1 & N_Lambda & t_exp & ergs cm^-2, s^-1, Angstrom^-1 && cts, s^-1, diode^-1 & noalign noalign noalign HRS/G140L LSA Lambda_o=1285Angstrom\ & 1.1* 10^-13 & 6.3* 10^-14 & 1.76 & 228 s HRS/G140L LSA Lambda_o=1530Angstrom\ & 7.6* 10^-14 & 1.3* 10^-13 & 0.58 & 688 s FOS/RD G190H & 4.7* 10^-14 & 1.1* 10^-14 & 4.15 & 97 s FOS/RD G270H & 1.7* 10^-14 & 1.9* 10^-15 & 9.30 & 43 s noalign noalign Real_Time_Justification: Optical spectra for our two targets have already been secured at the Steward Observatory 2.3m telescope, and have been analyzed. Calibration_Justification: ! Move appropriate text from Real_Time_Justification Additional_Comments: Fixed_Targets ! Section 5.1 Target_Number: 1 Target_Name: WD0112+104 Alternate_Names: Description: STAR,DB Position: RA=1H14M37.68S+/-0.01S,DEC=10D41'05.45"+/-0.1",PLATE-ID=00GR Equinox: J2000 RV_or_Z: RA_PM: ! Units are seconds of time per year Dec_PM: ! Units are seconds of arc per year Epoch: Annual_Parallax: Flux: V=15.36,B-V=-0.14,F(1700)=6.8E-14 Comments: Target_Number: 2 Target_Name: WD1654+160 Alternate_Names: Description: STAR,DB Position: RA=16H56M57.52S+/-0.01S,DEC=15D56'26.41"+/-0.1",PLATE-ID=02D2 Equinox: J2000 RV_or_Z: RA_PM: ! Units are seconds of time per year Dec_PM: ! Units are seconds of arc per year Epoch: Annual_Parallax: Flux: V=16.5+/-0.3,B-V=-0.14,F(1700)=1.9E-14 Comments: Visits ! Section 6 Visit_Number: 1 Visit_Requirements: ! Section 7.1 ! PCS MODE [Fine | Gyro] ! GUIDing TOLerance ! DROP TO GYRO IF NECESSARY [NO REACQuisition] ! ORIENTation TO ! ORIENTation TO FROM ! ORIENTation TO FROM NOMINAL ! SAME ORIENTation AS ! CVZ ! PARallel ! SCHEDulability ! AFTER [BY [TO ]] ! AFTER ! BEFORE ! BETWEEN AND ! GROUP WITHIN