7362( 5) - 08/11/98 09:06 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7362 Version: 5 Check-in Date: 11-Aug-1998 09:05:07 1.Proposal Title: Demystifying the SW Sex stars ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Christian Knigge Space Telescope Science Institute CoI: D. W. Hoard University of Washington N CoI: Knox Long Space Telescope Science Institute N CoI: Vik Dhillon Royal Greenwich Observatory N CoI: Paula Szkody University of Washington N ------------------------------------------------------------------------------------ 5. Abstract We propose to use the Space Telescope Imaging Spectrograph to observe the SW Sex star DW UMa with medium spectral and high time-resolution in the far-ultraviolet for three consecutive full binary cycles. Only 7 HST orbits are required to achieve this goal because our target is located in the continuous viewing zone. The resulting data set will allow us to test the three models that have been proposed to account for the puzzling orbital phase-dependent behaviour of this high- inclination cataclysmic variable in optical spectra and thus lay to rest the controversy surrounding the SW Sex phenomenon. The complete orbital phase-coverage we will achieve is necessary to confidently rule out unsuccessful models. It will also make DW UMa an important addition to the very small number of cataclysmic variables for which phase-resolved ultraviolet spectroscopy of a full binary orbit has been performed. No bona fide SW Sex star has ever been observed with HST. Our proposed observations will rectify this important omission and achieve our scientific goals in an extremely efficient manner. ------------------------------------------------------------------------------------ 7362( 5) - 08/11/98 09:06 - [ 2] Observations Description ------------------------ We propose to use the Space Telescope Imaging Spectrograph (STIS) to obtain time-resolved FUV spectroscopy of the SW Sex star DW UMa for three full binary orbits. This can be achieved in only 7 HST orbits because DW UMa lies in the heart of the CVZ and has a relatively short binary period of 3.3 h. The medium resolution G140L grating in conjunction with the 52" * 0.2" slit will provide the optimal combination of spectral coverage (1150 Angstrom -- 1735 Angstrom), spectral resolution (1.2 Angstrom ~eq 250 km s^-1) and sensitivity for our purpose. In particular, this set-up will allow us to observe FUV continuum shape, as well as the shapes and phase-dependences of all three major wind- formed UV resonance lines (N v 1240 Angstrom, Si iv 1397 Angstrom, C iv 1549 Angstrom). The resolution of the G140L grating will be sufficient to resolve features as narrow as the interesting narrow (FWHM ~eq 300 km s^-1) absorption reversals that have recently been detected in the C iv wind line of another nova-like variable, UX UMa, with HST (Mason et al. 1995). Our spectral coverage will further include the He ii 1640 Angstrom line (which may also be mainly wind- formed and will provide a revealing comparison with its optical counterpart, He ii 4686 Angstrom), and a complex of Fe ii features around 1600- 1700 Angstrom which, if seen in absorption, will be an important diagnostic of conditions in the veiling material. Phase resolution around the entire binary orbit is required because phase dependence is a crucial aspect of the SW Sex phenomenon in the optical and is expected to remain so in the UV. The models outlined above all predict particular variations of emission and absorption features around the orbital cycle, and to test them rigorously full phase coverage will be essential. Based on DW UMa's UV flux in IUE spectra, we estimate that we will achieve a S/N of about 10 per resolution element (~eq1.2 Angstrom) at a continuum wavelength of 1450 Angstrom in about 130 s. This corresponds to an excellent phase resolution of about 0.01 even before averaging over several cycles, sufficient to observe even abrupt changes in the spectrum reliably. The necessity of observing the system for more than one binary orbit arises primarily because the SW Sex stars exhibit considerable variability in their optical light curves, sometimes even between successive orbital cycles (Shafter et al. 1988; Thorstensen et al. 1991; Hoard & Szkody 1996). By observing several binary orbits, we will determine if such behaviour is also present in the UV and, if so, what components of the accreting system are responsible. If inter-cycle variations are only moderate and/or can be accounted for in the analysis, our coverage of several orbits will allow us to average over cycles in looking for changes in our spectra as a function of orbital phase. Averaging over cycles will also reduce the effects of flickering -- quasi- random oscillations with significant amplitudes seen in the light curves of many CVs -- allowing us to construct much more reliable eclipse maps of DW UMa's accretion disk. HST has never been used to observe any bona fide SW Sex star, nor to follow an eclipsing high-state non-magnetic CV (dwarf- novae in outburst or nova-like variables) around a full binary cycle in the UV. Our proposed observations will rectify both of these important omissions at once and do so in an extremely efficient manner. Real Time Justification ----------------------- A crucial element of our proposal is to observe an SW Sex star for several full binary orbits. The scheduling difficulties related to this requirement are vastly reduced by selecting a target located in the CVZ. To illustrate this, we note that a minimum of 2 time-critical visits of 3 orbits each would be required to cover a single full binary cycle of a binary system with period identical to DW UMa, but not located not the CVZ (accounting for acquisitions and overheads and assuming a 52 min visibility period). By contrast, we will observe DW UMa for three full binary cycles in only a single 7-orbit visit. There is a total of 10 CVZ scheduling opportunities at DW UMa's position, with a total duration of 150 orbits. We have already obtained medium resolution (2Angstrom), ground-based, phase-resolved optical spectra of six SW Sex stars, including DW UMa, using the Double Imaging Spectrograph (DIS) on the Apache Point Observatory (APO) 3.5m telescope. The DW UMa data is from 1995 March 7 & 8 UT, and spans the wavelength range 4200--5800Angstrom. This includes essentially all of the important lines on which the classification of SW Sex stars rests (HAlpha, HBeta, HGamma, HDelta, He i 4921Angstrom, He i 5876Angstrom, He ii 4686Angstrom). In addition, we will apply for simultaneous ground-based phase-resolved spectroscopy of DW UMa at both APO and the 4.2m William-Herschel-Telescope (WHT). For the latter we plan to use the ISIS double -beam spectrograph to obtain red and blue spectra covering HAlpha, HBeta, HGamma, He i and He ii at a resolution of approximately 0.7 Angstrom. Thus, a large set of optical data will be available, to both complement and compare with the UV data we will obtain with HST. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7362( 5) - 08/11/98 09:06 - [ 3] Data Distribution Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7362( 5) - 08/11/98 09:06 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 DW-UMA STAR, NOVA-LIKE RA=10H 33M 52.930S +/- 3", J2000 V = 14.4+/-1.4 DEC=58D 46' 55.000" +/- 3", E(B-V) = 0.0+/-0.1 PLATE-ID=01S0 B-V = 0.06 U-B = -0.88 F(1450)=2e-14 7362( 5) - 08/11/98 09:06 - [ 5] Visit: 01 Visit Priority: Visit Requirements: CVZ On Hold Comments: Additional Comments: DW UMa's ephemeris is T(mid-eclipse)=HJD 2 446 229.00696 + 0.13660649 E where E is the cycle count. Since DW UMa is an eclipsing system, target acquisition near mid-eclipse phases should be avoided. Also, good phase coverage of eclipses and binary orbital phases near 0.5 is essential to the success of the program. Ideal starting phases (defining this as the binary orbital phase at the beginning of GS acquisition) would be between 0.5-0.6 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DW-UMA STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 1.0 S ONBOARD ACQ FOR 3 CD P Comments: DW UMa's ephemeris is T(mid-eclipse)=HJD 2 446 229.00696 + 0.13660649 E where E is the cycle count. Since DW UMa is an eclipsing system, target acquisition near mid-eclipse phases should be avoided. Also, good phase coverage of eclipses and binary orbital phases near 0.5 is essential to the success of the program. Ideal starting phases (defining this as the binary orbital phase at the beginning of GS acquisition) would be between 0.5-0.6 ------------------------------------------------------------------------------------------------------------------------------------ 2 DW-UMA STIS/C ACQ/PEA 52X0.2 MIRROR 1 1.0 S ONBOARD ACQ FOR 3 CD K Comments: DW UMa's ephemeris is T(mid-eclipse)=HJD 2 446 229.00696 + 0.13660649 E where E is the cycle count. Since DW UMa is an eclipsing system, target acquisition near mid-eclipse phases should be avoided. Also, good phase coverage of eclipses and binary orbital phases near 0.5 is essential to the success of the program. Ideal starting phases (defining this as the binary orbital phase at the beginning of GS acquisition) would be between 0.5-0.6 ------------------------------------------------------------------------------------------------------------------------------------ 3 DW-UMA STIS/F TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1000 1 6536 S UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 4 DW-UMA STIS/F TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1000 1 6536 S UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 5 DW-UMA STIS/F TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1000 1 6536 S UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 7362( 5) - 08/11/98 09:06 - [ 6] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 6 DW-UMA STIS/F TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1000 1 6536 S UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 7362( 5) - 08/11/98 09:06 - [ 7] Summary Form for Proposal 7362 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT BUFFER-TIME=1000 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ONBOARD ACQ FOR 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names DW-UMA ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 ------------------------------------------------------------------------------------------------------------------------------------