9732( 5) - 06/05/03 08:03 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9732 Version: 5 Check-in Date: 05-Jun-2003 08:02:19 1.Proposal Title: Detecting the hottest white dwarf in a dwarf nova: V446 Her as a laboratory for irradiated accretion discs ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 12 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Dr. Matthias R. Schreiber Observatoire de Strasbourg CoI: Dr. Boris T. Gaensicke University of Southampton N ------------------------------------------------------------------------------------ 5. Abstract V446 Her (Nova Her 1960) is an object with very unique combination of properties: it experienced 40 years ago a nova eruption and started regular dwarf nova outburst activity 10-20 years ago. We suggest that the white dwarf, heated to several 100.000K during the thermonuclear surface burning event, suppressed through irradiation of the accretion disc the thermal instability underlying dwarf nova outbursts for the first 20-30 years following the nova eruption. The gradual cooling of the white dwarf results in a continous decrease of the irradiating flux and, as a result, at present the outer parts of the accretion disc are thermally unstable and produce the observed dwarf nova outbursts. V446 Her provides, hence, the exceptional opportunity to study the structure of a strongly irradiated and thermally unstable accretion disc. To make rigorous use of V446 Her as a laboratory for irradiated accretion discs, it is mandatory to know the temperature of the still extremely hot white dwarf illuminating the disc. We propose to obtain STIS UV spectroscopy of V446 Her in order to detect the photospheric emission of the white dwarf, and to derive accurately its temperature. With this parameter at hand, we will be able to determine the dynamical importance of disc irradiation. Using our HST/STIS data of V446 Her, we will derive an observational calibration of the essential parameter in the framework of irradiated accretion discs i.e. the reprocessing efficiency beta, with considerable importance not only for our understanding of CV's but also of Super Soft Sources, X-Ray binaries, and AGN. ------------------------------------------------------------------------------------ 9732( 5) - 06/05/03 08:03 - [ 2] Observations Description ------------------------ Our scientific goal, detecting the hot white dwarf in V446 Her and to derive its temperature, requests two observations: (1) a high-quality FUV spectrum that allows, if necessary, to disentangle accretion disc and white dwarf contribution, and (2) a NUV spectrum that provides a good value for the interstellar reddening from the observed 2200A bump. The gratings to be used are G140L (1150-1730A) and G230L (1570-3180A). We opt for the 52"x0.2" aperture to optimize both spectral resolution and throughput. We have estimated the necessary exposure times with the following assumptions: a conservative estimate of Twd=75000K, a white dwarf radius Rwd=8e8cm, a distance of d=1kpc, and a reddening of E(B-V)=0.33 (Thorstensen + Taylor 2000). We then scaled a solar-abundance white dwarf model spectrum for Twd, Rwd, and d, and used it as input for the STIS exposure time calculator, applying the estimate for the reddening. In order to achieve S/N >=10, we need 2 HST orbits each for the FUV and NUV range. The observations will be carried out as follows: Orbit 1+2: G140L grating, 52"x0.2" aperture. Overheads are guide star acq. (6min), target acq. (6min), STIS instrument setup/readout (8min), and guide star re-acq. (5min). This leaves ~80min exposure time, providing a S/N >10 for 1200A< lambda <1500A and S/N 5 in the remaining wavelength range. Orbit 3+4: G230L grating, 52"x0.2" aperture. Overheads are 2xguide star re-acq. (10min), and STIS instrument setup/readout (8min). This leaves ~86min exposure time, providing a S/N~8-12 over most of the covered wavelength range. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9732( 5) - 06/05/03 08:03 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 V446-HER STAR, DWARF NOVA RA=18H57M21.61S +/- 0.3", J2000 V = 16.5+/-1.0 DEC=+13D14'29.7" +/- 0.3", PLATE-ID=02UZ Coordinate Source: GSC_SURVEY_PLATE 9732( 5) - 06/05/03 08:03 - [ 4] Visit: 01 Visit Priority: Visit Requirements: PCS MODE FINE SCHED 30% 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 V446-HER STIS/C ACQ F28X50L MIRROR 1 8 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 V446-HER STIS/F TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=400 1 2260 S UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 3 V446-HER STIS/F TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=400 1 2878 S UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 4 V446-HER STIS/N TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=400 1 2878 S UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 5 V446-HER STIS/N TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=400 1 2878 S UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 9732( 5) - 06/05/03 08:03 - [ 5] Summary Form for Proposal 9732 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=400 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE FINE SCHED 30% ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140L G230L ------------------------------------------------------------------------------------------------------------------------------------ Target Names V446-HER ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 2376 ------------------------------------------------------------------------------------------------------------------------------------