Proposal_Information Title: TIME RESOLVED UV SPECTROPHOTOMETRY OF THE DWARF NOVA HT CAS Proposal_Category: GO Scientific_Category: Hot Stars Cycle: 5 Investigators PI_name: Janet Wood PI_Institution: Keele University CoI_Name: William Welsh CoI_Institution: Keele University Contact: N CoI_Name: Edward Robinson CoI_Institution: University of Texas Contact: N CoI_Name: Keith Horne CoI_Institution: University of Utrecht Contact: N Abstract: ! Free format text (please update) The dwarf nova HT Cas, which has an orbital period of 106 minutes, is one of the few dwarf novae in which the white dwarf and the accretion disk are eclipsed. As such, it is one of the few systems for which detailed information can be obtained about the structure of the disk and the structure of the boundary layer between the disk and the white dwarf. Indeed, because the white dwarf plus boundary layer eclipse is so prominent a feature in this system, HT Cas provides an outstanding opportunity to make progress in understanding boundary layers. We request time on HST to obtain UV spectrophotometry of HT Cas with FOS at high time resolution (3.2 and 6.4s). We will observe complete orbital cycles of HT Cas in the continuous viewing zone. The observations through eclipse will be used to eclipse map the disk in the continuum and lines and to study the white dwarf and boundary layer. The data outside eclipse will be used to Doppler map the lines and investigate the ``Fe-curtain'', if one is present. We will also study the quasi-periodic oscillations and the flickering. Outside eclipse we will determine the spectrum of the flickering and QPOs, and in eclipse we will calculate a flicker map of the disk and determine where the flickering originates. Questions ! Free format text (please update) Observing_Description: Using the FOS in RAPID mode (3.2 and 6.4 sec READ--TIME), we wish to observe a minimum of 8 orbits of HT Cas. The rapid readout is required to resolve the fastest variations, and is only indirectly constrained by the S/N per spectrum. Eight HT Cas orbits are required so that statistical fluctuations do not dominate in our analysis. Note that because of the random nature of flickering, a single very high S/N eclipse is far inferior to several lower S/N eclipses. By co--adding the data we will 1) average out the flickering and allow us to map the steady--state disk (e.g. see Horne et al. 1991) and 2) to compute the rms variations at each phase of the eclipse to actually compute an eclipse mapping of the flickering itself. We stress that the total exposure time is dictated by our need for orbital phase coverage and the need to average out the effects of flickering, and not by the S/N. To cover eight orbits of HT Cas we need 848 minutes of viewing. Given a 96 minute HST orbit, we compute a need for 10 spacecraft orbits (including one orbit for target and guide star acq.). Because we are in a CVZ it does not matter what orbital phase we begin observing at, therefore time critical observations are not necessary to catch the eclipses. We will use the BLUE digicon and G160L grating because of its long wavelength (1140--2500Angstrom) and simultaneous order zero U--band coverage (Eracleous, et al 1994). This configuration will be used for 7.5 of the 10 spacecraft orbits, the remaining 2.5 will use the G400H grating (3240--4822Angstrom). This grating will allow simultaneous coverage of the Balmer jump, higher order Balmer lines, HeII 4686 and the Ca H+K lines, all of which are important diagnostics of the disk conditions, and it will allow intercalibration with contemporaneous ground--based observations. HT Cas has a V magnitude of 16.4 outside of eclipse. Using an IUE spectrum of HT Cas (F_Lambda ~ 4E-15 erg/s/cm^2/Angstrom) we used the HST simulator XCAL (aka synphot) to compute predicted count rates. With the G160L grating we expect 1 c/s/diode at 2000Angstrom, which equals ~49 c/3.25s/100Angstrom. For shorter wavelengths, the count rate drops by about a factor of 2. Since we will be co-- adding eight orbits together, the S/N grows to between ~13 and 20. This is adequate for our needs, but if necessary, we have the freedom to boost the S/N by binning in wavelength or time. Real_Time_Justification: HT Cas lies in an excellent CVZ position (6 possible visits per year, up to 80 hours of continuous visibility per visit). We request CVZ observations for three reasons: 1) we require full orbit coverage, and only through CVZ observations is it possible to get continuous coverage at all orbital phases (the orbital period of HT Cas is 106 minutes). Because CVs can vary on timescales less than an hour, it is best to have all the observations taken as close as possible. A CVZ observation does this naturally. 2) CVZ observations will optimize the efficiency of the observing run. Given that eclipse coverage is highest priority, to meet our allocated time we would have to sacrifice data (and science) at other orbital phases if we had an inefficient program. With only one guide star and target acquisition (instead of 6), we save a huge amount of overhead, as well as gain nearly 40 minutes of observing time per HST orbit. 3) CVZ observations increase spacecraft scheduling efficiency by an enormous amount, since otherwise we would require at least 6 time critical visits to observe 6 eclipses. We will attempt to obtain contemporaneous ground--based observations of HT Cas from McDonald Observatory. The success of the project does not in any way depend on ground--based observations. Calibration_Justification: Additional_Comments: The TAC believed that using 5s (or shorter) integrations rather than 6.18s integration times would reduce the efficiency of our observations by 20%. This is not true, as the efficiency is changed by only 6% (from 77 to 71%). Also, for READ-TIMEs less than 6.18s we will be using SUB-STEP=2, which dramatically increases efficiency. Hence to obtain the time resolution needed to resolve the fastest variations (boundary layer ingress/egress and the flickering) we will use READ-TIMEs of 6.4 and 3.2s, both giving effiencies BETTER than what could be obtained with 6.18s READ-TIME. Note that spectra with 3.2s READ-TIME and 77.5% duty cycle have already been taken with the FOS (GO-5498). See the Instrument Science Report Cal/FOS-124 written by Welsh, et al. (Welsh is a co-I on this proposal). Exposure times for target acquisition are based on expected mid-eclipse brightness, and are non-critical. These are a factor of ~7.6 longer than required for non-critical peakup outside eclipse, and therefore can be used during any phase. Exposure times may be adjusted, but READ-TIMEs must not be changed. Fixed_Targets Target_Number: 1 Target_Name: HT-CAS Description: star, dwarf nova Position: RA=1H 10M 13.11S +/- 0.4", DEC=60D 4' 36.0" +/- 0.4", PLATE-ID=01MU Equinox: 2000.0 Flux: V = 16.4 +/- 0.5 B-V = -0.08 +/- 0.16 E(B-V) = 0.03 +/- 0.05 F(2000) = 4 +/- 2 E-15 U-B = -1.13 +/- 0.03 Comments: These values are for quiescence out of eclipse. Mid-eclipse has V=18.6, B-V=0.3, and U-B=-1.3. HT Cas also has low states where it is fainter by 1 mag in V and (very rare) outbursts where it is several mags brighter. Visits Visit_Number: 1 Visit_Requirements: CVZ PERIOD .0736472039D AND ZERO-PHASE JD2443727.93775 Visit_Comments: A gap in the continuous observations due to a passage through the SAA is acceptable. Please try to schedule so that SAA passage occurs during the G160L observations (exposure number 4). Though not critical, it is best to avoid target acquisition during eclipse. With a ~59min G.S. acquisition and FOS peakup, the eclipse can be avoided if the visit starts within a 36min window centered on phase 0.225 (i.e. we recommend that G.S. target acquisition start at orbital phase 0.225 +/- 0.175). For HT Cas, the ephemeris for eclipse center is (heliocentric) HJD = 2443727.93775 + 0.0736472039E. Though not a constraint, the CVZ window that occurs in late September 1995 is best for simultaneous ground-based coverage. Exposure_Number: 1 Target_Name: HT-CAS Config: FOS/BL Opmode: ACQ/PEAK Aperture: 4.3 Sp_Element: MIRROR Optional_Parameters: SEARCH-SIZE-X=1 SEARCH-SIZE-Y=3 SCAN-STEP-Y=1.23 Number_of_Iterations: 1 Time_Per_Exposure: 11S Special_Requirements: ONBOARD ACQ FOR 2; PHASE .05 TO .3 Exposure_Number: 2 Target_Name: HT-CAS Config: FOS/BL Opmode: ACQ/PEAK Aperture: 1.0-PAIR-A Sp_Element: MIRROR Optional_Parameters: SEARCH-SIZE-X=6 SEARCH-SIZE-Y=2 SCAN-STEP-X=0.61 SCAN-STEP-Y=0.61 STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 12.1S Special_Requirements: ONBOARD ACQ FOR 3 Exposure_Number: 3 Target_Name: HT-CAS Config: FOS/BL Opmode: ACQ/PEAK Aperture: 0.25-PAIR-A Sp_Element: MIRROR Optional_Parameters: SEARCH-SIZE-X=5 SEARCH-SIZE-Y=5 SCAN-STEP-X=0.17 SCAN-STEP-Y=0.17 STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 13.2S Special_Requirements: ONBOARD ACQ FOR 4-8 Exposure_Number: 4 Target_Name: HT-CAS Config: FOS/BL Opmode: RAPID Aperture: 1.0 Sp_Element: G400H Optional_Parameters: READ-TIME=6.4 SUB-STEP=4 Number_of_Iterations: 1 Time_Per_Exposure: 13216.0S ! 2065 spectra = 2^11+17 = 2.08 cycles Special_Requirements: NO SPLIT Comments: must not be split by SAA Exposure_Number: 5 Target_Name: HT-CAS Config: FOS/BL Opmode: RAPID Aperture: 1.0-PAIR-A Sp_Element: G160L Optional_Parameters: READ-TIME=3.2 SUB-STEP=2 STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 28499.2S Comments: ok to change exp. time but not READ-TIME Exposure_Number: 6 Target_Name: HT-CAS Config: FOS/BL Opmode: RAPID Aperture: 1.0-PAIR-A Sp_Element: G160L Optional_Parameters: READ-TIME=3.2 SUB-STEP=2 STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 3680S Comments: ok to change exp. time but not READ-TIME Exposure_Number: 7 Target_Name: HT-CAS Config: FOS/BL Opmode: RAPID Aperture: 1.0-PAIR-A Sp_Element: G160L Optional_Parameters: READ-TIME=3.2 SUB-STEP=2 STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 3680S Comments: ok to change exp. time but not READ-TIME Exposure_Number: 8 Target_Name: HT-CAS Config: FOS/BL Opmode: RAPID Aperture: 1.0-PAIR-A Sp_Element: G160L Optional_Parameters: READ-TIME=3.2 SUB-STEP=2 STEP-PATT=SINGLE Number_of_Iterations: 1 Time_Per_Exposure: 3680S Comments: ok to change exp. time but not READ-TIME Data_Distribution Medium: 8MM ! 8MM or 6250BPI or 1600BPI Blocking_Factor: 10 ! 10 or 1 Ship_To: PI_Address ! STSCI or PI_Address or Ship_Via: UPS ! UPS (2-day) or OVERNIGHT