Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 3820c Report Date: 09-May-96:18:52 Version: ********** Check-in Date: ********** 1.Proposal Title: INSTABILITIES IN ACCRETION DISCS AND THE OUTBURSTS OF DWARF NOVAE RETAKE OF SAFING ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID STELLAR ASTROPHYSICS GO 2380 Sub Category ERUPTIVE BINARIES ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Dr. Keith Horne STSCI USA 3013384964 ------------------------------------------------------------------------------------ 7. Abstract We will use the HST with the FOS to observe eclipses of a dwarf nova at 5 epochs in the quiescent period between outbursts. From the eclipse data we will determine the secular evolution of the white dwarf, the accretion disc, and the bright spot. This evidence will be a clean test of the two competing theories for the instability which triggers dwarf nova outbursts. In the disc instability model the transition of the disc from a cool to hot state triggers the outburst, whereas in the red star instability model the cool binary companion transfers a short burst of material into the disc which then becomes brighter. During quiescence the disc instability model predicts an increasing accretion rate and hence an increasing ultraviolet flux, whereas the red star model predicts a decreasing accretion rate and ultraviolet flux. Therefore the variation of the ultraviolet flux with time will distinguish which of the two current models is correct. Only the HST is able to resolve the rapid variations seen in an eclipsing dwarf nova, and therefore determine the ultraviolet flux from the accretion disc. The observations that we propose will also probe the nature of the boundary layer between the disc and the white dwarf, a region too small and hot to be well constrained by any previous observations. In particular, we will measure the extent of heating of the white dwarf by the boundary layer, and the cooling ------------------------------------------------------------------------------------ 8. Scientific Key Words: WHITE DWARF DWARF NOVA ACCRETION BOUNDARY LAYER INTER- ACTING BINARY ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 6.80 par: 0 10. Num targs pri: 1 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: FOS ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 3820c PI: Dr. Keith Horne Proposal Title: INSTABILITIES IN ACCRETION DISCS AND THE OUTBURSTS OF DWARF NOVAE RETAKE OF SAFING ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ P.I. Keith Horne STSCI USA Tom Marsh STSCI USA ------------------------------------------------------------------------------------ 3. Description of proposed observations. We will take time-resolved FOS spectra of OY Car, covering a series of eclipses during the quiescent phase between two outbursts. We request Target of Opportunity status so that the observations can start several days after an outburst is reported by variable star observers of the RASNZ. Our priorities are to measure the relative contributions of the white dwarf, disc and bright spot to the ultraviolet flux and to see how they change during the quiescent period. To do this we must cover the eclipse of each component and so our exposure times are driven by orbital phase coverage NOT signal to noise ratio. To ensure that a long timebase is used we have put the exposures into five groups with defined periods in between. Other arrangements are possible, for example uniform coverage but it is not possible to specify in a simple manner. An example of an unacceptable arrangement is for all the eclipses to be observed in less than three groups. Our exact specification is designed to cover two timescales of around 8 days and 40 days. ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. The HST is the only telescope in existence that can observe at space ultraviolet wavelengths with sufficient sensitivity to time-resolve the sharp eclipse features. Analysis of optical photometry has established a coherent set of parameters for OY Car, with the orbital inclination accurate to better than a degree and the mass ratio to 3 percent, and it has led to the developement of techniques that will be needed to decompose the ultraviolet light curves. This work provides a firm base for our study but it does not answer the problems that we wish to investigate. IUE observations demonstrate a secular decrease in ultraviolet flux during quiescence, but IUE cannot distinguish between a cooling white dwarf and a decreasing accretion rate. The fraction of flux contributed by different sources can only be measured with the HST. OY Car has been detected by IUE during quiescence at a flux level of 0.5 mJy at 2400 A, anf therefore the experiment proposed will work. ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. For each eclipse, we need to cover all the contact points, and get a sufficient range outside eclipse to establish baselines for flux measurements. This requires a minimum of 24 (??) around mid-eclipse. The dates of our observations are chosen to cover two time-scales. IUE observations have shown a decrease over 8 days after an outburst. If this is white dwarf cooling, we can expect the same time- scale for OY Car. Three epochs is the minimum needed to detect and confirm any secular trend. Therefore the groups of eclipses at 3, 6 and 10 days after outburst are designed to detect this post-outburst cooling. We must also measure trends over the whole of the quiescent period, which typically lasts 40 to 70 days. The eclipse samples at 10, 25 and 40 days are sensitive to this long time- scale. The main source of noise will be the random flickering characteristic of cataclysmic variables, and in order to reduce and calibrate this, we will observe three eclipses at each epoch. Page 3 ------------------------------------------------------------------------------------ 6. Description of special calibration exposures. Time-critical, uninterrupted observations are needed for two reasons. First, each exposure must cover all the eclipse contacts of a given eclipse (therefore time-critical). Since these all occur within 15 minutes of each other, the exposure must be continuous. Second, we must cover longer 8 and 40 day timescales of variation. Many arrangements of the 14 eclipses are possible, however the limited variety of specifications available has limited our selection. In words, 8 eclipses should be spread from 3 to 10 days after outburst and the remaining 6 should cover 15 to 40 days. Finally, to ensure full coverage of a quiescent period, the observations should start within 5 days of the detection of an outburst. Note that although the outbursts are only predictable to within about 30 days, the the eclipse ephemerides are known precisely. ------------------------------------------------------------------------------------ 7. Data reduction and analysis plans. The data will initially be reduced with the standard software at STScI. The data analysis techniques that we have developed for optical eclipse dta will then be deployed. The first priority in the analysis will be to produce light curves for each eclipse for three sections of continuum covering the full range of wavelengths observed. The resulting light curves will then be decomposed to measure the fluxes from the white dwarf, disc and bright spot as in figure 2. We will also produce light curves of the Lyman-alpha absorption and any emission lines detected. These analyses will give us the white dwarf and disc fluxes at three wavelengths and also the strength of Lyman-alpha absorption as a function of time since the outburst. At this stage we will be able to determine which outburst model is supported by the data. The data will be further analysed with model atmospheres to deduce the temperature and size of the emission region on the white dwarf. Emission lines will be studied to determine their distribution in the system, in particular to see if there is any evidence for a wind from the disc during quiescence. ------------------------------------------------------------------------------------ 11. Address Information Name: KEITH HORNE Category: PI Institution: SPACE TELESCOPE SCIENCE INSTITUTE Address: 3700 SAN MARTIN DRIVE City: BALTIMORE State: MD Zip Code: 21218 Country: USA Telephone: Telex (or e-mail): ------------------------------------------------------------------------------------ TARGET LIST a) Fixed Targets ID = 3820c [ 4] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data No | Name | Description | Position |Eqnx | Vel. |Prblm|REF| | | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------ 1 OY-CAR STAR; ECLIPSING BINARY; RA = 10H 6M 22.47S +/- 2000 1 V = 14.0 +/- 2.0, E(B-V) = 0.0 DWARF NOVA; 0.5", 2 F(1500) = 6 +/- 3 E-15 VARIABLE; INTERACTING DEC = -70D 14' 4.9" +/- 3 F(2500) = 6 +/- 3 E-15 BINARY; CV; 0.5", PLATE-ID=0196 Comments: VARIABLE STAR. V MAGNITUDE GIVEN CORRESPONDS TO FULL RANGE FROM QUIESCENCE TO OUTBURST AWAY FROM THE ECLIPSE. UV FLUXES SPECIFIED FOR QUIESCENCE ONLY. ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 3820c [ 5] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 DEFINE OY-CAR FOS/BL ACQ/ 4.3 MIRROR FAINT=1800 1 3S 1 ONBOARD ACQ FOR 2 ECLPS BINARY BRIGHT=720000 CYCLE 1 / 1-16 Comments: BRIGHT AND FAINT SET VERY WIDE BECAUSE FIELD IS SIMPLE BUT OBJECT IS LIKELY TO CHANGE BRIGHTNESS DURING THE COURSE OF THE OBSERVATIONS. ------------------------------------------------------------------------------------------------------------------------------------ 2 DEFINE OY-CAR FOS/BL RAPID 1.0 G160L 1500 READ-TIME=6.18 1 39M 1 NON-INT ECLPS ZERO-PHASE JD2443993.55321 +/ - 1S PERIOD 0.0631209239D +/- 0.0000000005D Comments: THE EXPOSURE TIME IS DETERMINED PHASE 0.829 +/- BY PHASE COVERAGE, NOT SIGNAL- 0.03 TO-NOISE. AT WORST MUST COVER PHASES 0.95 TO 0.10 ------------------------------------------------------------------------------------------------------------------------------------ 3 USE AT 16-DEC-91 +/- ECLPS 30D Comments: EARLY ECLIPSE TEST;RESCHEDULE AS SOON AS POSSIBLE. FIRST TRY FAILED DUE TO FOS SAFING. ------------------------------------------------------------------------------------------------------------------------------------ 4 USE TARG OF OPP / 4-16 ECLPS GROUP 4-6 WITHIN 2D Comments: MUST GET ONTO THE TARGET WITHIN 2D +/- 2D OF DETECTION OF OUTBURST BY AMATEUR OBSERVERS ------------------------------------------------------------------------------------------------------------------------------------ 5 USE ECLPS ------------------------------------------------------------------------------------------------------------------------------------ 6 USE ECLPS ------------------------------------------------------------------------------------------------------------------------------------ 7 USE AFTER 4 BY 3D +/- ECLPS 18H GROUP 7-8 WITHIN 2D ------------------------------------------------------------------------------------------------------------------------------------ 8 USE ECLPS ------------------------------------------------------------------------------------------------------------------------------------ 9 USE AFTER 7 BY 4D +/- ECLPS 1D GROUP 9-11 WITHIN 3D ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 3820c [ 6] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 USE ECLPS ------------------------------------------------------------------------------------------------------------------------------------ 11 USE ECLPS ------------------------------------------------------------------------------------------------------------------------------------ 12 USE AFTER 10 BY 15D +/ ECLPS - 4D GROUP 12-13 WITHIN 4D ------------------------------------------------------------------------------------------------------------------------------------ 13 USE ECLPS ------------------------------------------------------------------------------------------------------------------------------------ 14 USE AFTER 12 BY 15D +/ ECLPS - 4D GROUP 14-16 WITHIN 5D ------------------------------------------------------------------------------------------------------------------------------------ 15 USE ECLPS ------------------------------------------------------------------------------------------------------------------------------------ 16 USE ECLPS ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 3820c [ 7] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/BL ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/BINARY RAPID ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FAINT=1800 BRIGHT=720000 READ-TIME=6.18 ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 6.80 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR ASTROPHYSICS ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category ERUPTIVE BINARIES ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2 ; CYCLE 1 / 1-16 NON-INT ; ZERO-PHASE JD2443993.55321 +/- 1S ; PERIOD 0.0631209239D +/- 0.0000000005D ; PHASE 0.829 +/- 0.03 AT 16-DEC-91 +/- 30D; TARG OF OPP / 4-16 ; GROUP 4-6 WITHIN 2D AFTER 4 BY 3D +/- 18H ; GROUP 7-8 WITHIN 2D AFTER 7 BY 4D +/- 1D ; GROUP 9-11 WITHIN 3D AFTER 10 BY 15D +/- 4D ; GROUP 12-13 WITHIN 4D AFTER 12 BY 15D +/- 4D ; GROUP 14-16 WITHIN 5D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G160L ------------------------------------------------------------------------------------------------------------------------------------ Target Names OY-CAR ------------------------------------------------------------------------------------------------------------------------------------