9791( 4) - 05/16/03 07:03 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9791 Version: 4 Check-in Date: 16-May-2003 07:01:42 1.Proposal Title: Curing the SW Sex Syndrome ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 12 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Dr. Christian Knigge University of Southampton CoI: Ms. Sofia Araujo-Betancor University of Southampton N CoI: Dr. Knox S. Long Space Telescope Science Institute N CoI: Dr. D. W. Hoard SIRTF Science Center, Caltech N CoI: Prof. Paula Szkody University of Washington N CoI: Dr. Vik Dhillon University of Sheffield N CoI: Dr. Thomas R. Marsh Southampton University N CoI: Dr. Boris T. Gaensicke University of Southampton N CoI: Prof. Philip A. Charles University of Southampton N CoI: Dr. Robert I. Hynes University of Texas at Austin N ------------------------------------------------------------------------------------ 5. Abstract Cataclysmic variables (CVs) are outstanding accretion laboratories. As a result, insights gleaned from CVs have critically shaped our conceptual picture of accretion processes in astrophysical systems on all scales. In particular, the accretion disks found in nova-like cataclysmic variables (NLCVs) have long been thought of as *the* classic examples of standard, steady-state disks.However, there is a group of NLCVs -- the SW Sex stars -- whose remarkable properties clearly do NOT fit the standard disk model. Many other NLCVs also show some symptoms of the SW~Sex syndrome, as does at least one neutron star low-mass X-ray binary system. The existence of the SW Sex stars throws serious doubt on the validity of the standard picture in NLCVs and other disk-accreting systems. It is therefore vital to uncover the cause of this phenomenon. The SW Sex star DW UMa is ideally suited for an HST study that will achieve this goal. Because DW UMa is located in the CVZ, we can use STIS to obtain time-resolved, far-ultraviolet spectroscopy of the system for two complete, consecutive binary orbits. The resulting data set will allow us to resolve the SW Sex controversy by distinguishing clearly between all competing models. ------------------------------------------------------------------------------------ 9791( 4) - 05/16/03 07:03 - [ 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 two full binary orbits. This can be achieved in only 5 HST orbits because DW UMa lies in the heart of the CVZ and has a relatively short binary period of 3.3 h. There are 2 CVZ scheduling opportunities at DW UMa's position, which amount to a total of 23 orbits. Both CVZ opportunities are long enough for our purposes. 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 ~ 250 km/s) 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 ~300 km/s) 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 high state spectra, we estimate that we will achieve a S/N of about 10 per resolution element (~1.2 Angstrom) at a continuum wavelength of 1450 Angstrom in less than 100s. This corresponds to a phase resolution of better than 0.01 even before averaging over several cycles, sufficient to observe even abrupt changes in the spectrum reliably. We will also be able to search for variability on even shorter time-scales by rebinning the data onto a coarser wavelength grid. The necessity of observing the system for at least two binary orbits 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 for two binary orbits, we will be able to (i) determine if such behaviour is also present in the UV; (ii) identify the responsible system components; (iii) isolate the purely phase-dependent aspects of the variability in our spectra. The last point is particularly important to eclipse mapping: much more reliable eclipse maps will result if the effects of flickering -- the non-periodic, short time-scale variability common to all CVs -- can be removed from the data by averaging. HST has never been used to obtain time-resolved UV observations of any bona fide SW Sex star in its high state. Our proposed observations will rectify this important emission in an extremely efficient manner. Real Time Justification ----------------------- An essential ingredient of our proposal is that the observations will cover two 9791( 4) - 05/16/03 07:03 - [ 3] 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 if DW UMa were not in the CVZ (accounting for acquisitions and overheads and assuming a 52 min visibility period). By contrast, we will observe DW UMa for *two* full binary cycles in only a single 5 orbit visit. There are 2 CVZ scheduling opportunities at DW UMa's position, which amount to a total of 23 orbits. Both CVZ opportunities are long enough for our purposes. We will also obtain simultaneous, phase- resolved, optical spectroscopy of DW UMa, using one or both of the ARC 3.5m telescope at the Apache Point Observatory (APO) and the 2.4m Hiltner telescope at the MDM observatory. The ground-based spectroscopy will cover all of the spectral lines on which the classification of SW Sex stars rests (Halph, Hbeta, Hgamma, Hdelta, He I 4921 Angstrom, He I 5876 Angstrom, He II 4686 Angstrom). We will also use the McGraw-Hill 1.3m telescope at the MDM observatory to obtain simultaneous optical photometry of DW UMa with high time resolution. It is extremely unlikely that we will catch DW UMa in a low state again, since SW Sex stars spend almost all of their time in the high state. We will nevertheless carry out an intensive photometric monitoring campaign on the system in the weeks and months leading up to the scheduled HST observations. For this purpose, we will use the 0.76m telescope at the University of Washington's Manastash Ridge Observatory and the McGraw-Hill 1.3m telescope at the MDM observatory. However, we stress again that there is every reason to expect that our Cycle 12 observations will find the system in the normal high state. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9791( 4) - 05/16/03 07:03 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 DW-UMA STAR, NOVA-LIKE, RA=10H33M52.93S +/- 0.2", J2000 V = 14.5+/-1.0 ACCRETION DISK, DEC=+58D46'55.0" +/- 0.2", E(B-V) = 0.0+/-0.1 INTERACTING BINARY PLATE-ID=01S0 B-V = 0.06 U-B = -0.88 F-CONT(1450)=2e-14 F-LINE(1550)=1.2e-13 Coordinate Source: GUIDE_STAR_CATALOG Comments: DW UMa is an eclipsing variable. V=14.5 is typical out of eclipse. The eclipses are about 1.5 mag deep. APT does not allow magnitude uncertainties larger than 1 mag, however, so this could not readily be specified. 9791( 4) - 05/16/03 07:03 - [ 5] Visit: 01 Visit Priority: Visit Requirements: PCS MODE FINE CVZ On Hold Comments: Additional Comments: DW UMa's ephemeris is T(mid-eclipse)=HJD 2 446 229.00691 + 0.136606526 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 1 1 S CD P Comments: DW UMa's ephemeris is T(mid-eclipse)=HJD 2 446 229.00691 + 0.136606526 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/F TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1000 1 6536 S UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 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 ------------------------------------------------------------------------------------------------------------------------------------ 9791( 4) - 05/16/03 07:03 - [ 6] Summary Form for Proposal 9791 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1000 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE FINE CVZ ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names DW-UMA ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 ------------------------------------------------------------------------------------------------------------------------------------