5829( 13) - 01/25/96 13:54 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5829 Version: 13 Check-in Date: 25-Jan-1996 10:59:51 1.Proposal Title: The Activity of Periodic Comet Schwassmann-Wachmann 1 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Solar System GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Paul D. Feldman The Johns Hopkins University CoI: Michael F. A'Hearn University of Maryland N CoI: Gian Paolo Tozzi Osservatorio Astrofisico di Arcetri N CoI: Harold A. Weaver Space Telescope Science Institute N ------------------------------------------------------------------------------------ 6. Abstract We propose spectroscopic observations of periodic comet Schwassmann-Wachmann 1 (SW1), a distant comet in a nearly circular orbit at ~6 AU, to observe carbon monoxide fluorescence emission in the CO Fourth Positive Band system. CO was recently successfully detected in SW1 in two independent radio searches at 1.3 mm wavelength. The production rate derived from these observations indicates that the CO ultraviolet emission should be readily detectable with the GHRS. The goal of the investigation is to determine if CO is, in fact, responsible for the activity of this comet, or whether CO2 is the dominant volatile in the nucleus and the observed CO simply a dissociation product of CO2. We will also use the FOS to measure (or set sensitive upper limits on) the coma abundance of OH and CO2+. ------------------------------------------------------------------------------------ 5829( 13) - 01/25/96 13:54 - [ 2] Observations Description ------------------------ We propose to observe comet SW1 during February 1996 when SW1 will be at opposition and hence at minimum geocentric distance, in order to maximize the flux at Earth. We note that at this time SW1 is approaching aphelion, but this does not seriously degrade the expected signal as the orbit eccentricity of SW1 is only ~4% For this initial program we have decided not to attempt a target-of -opportunity observation at the time of an outburst because of the short duration of the brightening relative to the HST response time. Moreover, our estimates of exposure time given below are based on the abundance of CO derived from the millimeter detection made when the comet was quiescent (Senay and Jewitt 1994). Although the ephemeris for SW1 is well known, we will begin the observation sequence with a PC image using the F702W filter to determine the precise position of the comet's center of brightness, which can then be uplinked to the spacecraft to provide a pointing correction to the ephemeris in the spacecraft memory. The image will also provide quantitative information, at a resolution not possible from the ground, of the distribution of the dust surrounding the nucleus. Two or three exposures, totalling ~1000 seconds, will provide adequate signal-to-noise, based on our recent experience in imaging comet Shoemaker-Levy 9 (1993e), whose visual magnitude was 1-2 magnitudes fainter than SW1, and permit cosmic ray removal. The data of Senay and Jewitt (1994) are used to estimate the expected slit-averaged brightness (in the GHRS Large Science Aperture) of the strongest CO bands in the G140L grating bandpass that we have chosen, 1295 - 1580 Angstrom . They derive a CO production rate for two possible cases, fluorescence and thermal equilibrium of the rotational levels, respectively, of 3.1 * 10^28 and 6.2 * 10^28 molecules/s, the latter assuming a temperature of 80 K. We adopt a rough mean value of 5 * 10^28 molecules s^-1, and assume a radial outflow model with an outflow velocity of 0.2 km/s, again a compromise between the various widths and the blue-shift given by Senay and Jewitt. This model assumes a uniformly vaporizing sphere and is not strictly correct in this case. However, if the observed 0.4 km/s blue- shift is indicative of hemispherical outgassing from the side towards the sun, the CO column density in the LSA will actually be considerably higher than that derived from the model. The fluorescence efficiencies and lifetimes used in the model are appropriate for solar minimum values of the solar ultraviolet flux. The results are given in Table 1. For the C I multiplet we used the vectorial model of Festou (1981), but here the parameters are more uncertain than for the CO. In general, in cometary spectra the C I emissions are much stronger than the CO bands (see Fig. reffig1), but this is not the case at ~6 AU where the photodestruction rate of CO is considerably smaller than its value at 1 AU. Table 1: GHRS Side 1 Observations _____________________________________________________________________ Feature Wavelength (A) Brightness in LSA S/N for (Rayleighs) 120 minutes _____________________________________________________________________ CO (1,0) 1509 14 5.0 CO (2,0) 1478 16 6.7 CO (3,0) 1447 9 4.5 C I 1561 10 2.5 _____________________________________________________________________ The signal-to-noise ratio for the GHRS observation (also given in Table 1) was estimated using the equation for Case 2 in section 3.4.5.1 of the GHRS Handbook 5.0. Our experience with two recent programs that utilized the GHRS Side 1 with G140L to observe Jupiter and Europa (Program IDs 5373 and 5376) shows that the S/N actually obtained on these extended sources is very closely given by this equation, and so we are confident that CO can be detected in the ultraviolet from comet SW1 with the proposed two-hour integrations. Since dark time is required for the GHRS observations, and 24 minutes of dark time is available per orbit, this will require 5 orbits. It also appears that the S/N can be further enhanced using the new option FLYLIM described in section 8.6.3 of the Handbook, but we have not pursued this possibility at the present time. For the FOS observations of OH to determine the water production rate, we have demonstrated that we can reach a 3-sigma limit of 2 * 10^27 molecules/s with a 60-minute integration for a comet at a heliocentric distance of 5.45 AU (Weaver et al. 1994b). This was based on an outflow velocity of 0.4 km/s adopted for analysis of our July 1993 observations of comet Shoemaker- Levy 9 (1993e), and so represents a fairly conservative upper limit that is a factor of 25 below the mean CO production rate given by Senay and Jewitt. Real Time Justification ----------------------- Dark time is required for the GHRS observations because our spectral range includes the terrestrial airglow emission of O I lambda1304, which is also one of the cometary emissions that we wish to observe. We also require that the observation be made when comet SW1 is close to opposition in order to maximize the ultraviolet flux from the comet. This occurs in Cycle 5 during February 1996. Note that the two requirements are, in fact, the same since dark time is optimized when the comet is at opposition. Ground-based monitoring of the activity of comet SW1 will be provided by co-I A'Hearn and his collaborators at Lowell Observatory and at Perth, Australia. We intend to seek coordinated CO millimeter observations as well. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 5829( 13) - 01/25/96 13:54 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5829( 13) - 01/25/96 13:54 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: SW1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET SCHWASSMANN-WACHMANN-1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | TYPE=COMET,Q=5.7484583,E=0.0440579,I=9.3 | 8499,O=312.82689,W=46.24130,T=09-Sep- | 1989:15:15:27.0,EQUINOX=J2000,EPOCH=27- | Apr-1996 | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | RANGE SW1 EARTH LT 5.5 | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | R = 18.0+/-0.5 | May be in outburst. Minimum range is B-V = 0.65 | 5.3 AU (opposition). Requires V-R = 0.5 | ephemeris correction. R magnitude | given is for 0.7 arc-second radius | aperture. | ------------------------------------------------------------------------------------------------------------------------------------ 5829( 13) - 01/25/96 13:54 - [ 5] Visit: 01 Visit Requirements: BETWEEN 23-JAN-96 AND 05-APR-96 SEQ 1-3 WITHIN 24H GUID TOL 0.2" On Hold Comments: Additional Comments: Visit should be completed in one orbit. Uplink offset correction to comet ephemeris before Visit 2. RANGE and BETWEEN requirements define same window. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 11 SW1 WFPC2 IMAGE PC1-FIX F702W CR-SPLIT = NO 1 120S SAVE OFFSET SW1 Comments: Determines ephemeris correction for visits 2- 3. ------------------------------------------------------------------------------------------------------------------------------------ 12 SW1 WFPC2 IMAGE PC1-FIX F702W CR-SPLIT = NO 1 480S min dur 480s ------------------------------------------------------------------------------------------------------------------------------------ 13 SW1 WFPC2 IMAGE PC1-FIX F702W cr-split=0.5,cr 1 1200S -tolerance=0 ------------------------------------------------------------------------------------------------------------------------------------ 5829( 13) - 01/25/96 13:54 - [ 6] Visit: 02 Visit Requirements: BETWEEN 23-JAN-96 AND 05-APR-96 GUID TOL 0.2" On Hold Comments: Additional Comments: Uplink required for ephemeris offset after visit 1. Use "remember offset" for all exposures this visit. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 21 SW1 FOS/RD ACQ 4.3 MIRROR 1 480S USE OFFSET SW1 ------------------------------------------------------------------------------------------------------------------------------------ 22 SW1 FOS/RD ACCUM 4.3 G270H 1 24M USE OFFSET SW1 EXPAND no split end orbit ------------------------------------------------------------------------------------------------------------------------------------ 23 SW1 FOS/RD ACCUM 4.3 G270H 1 15M USE OFFSET SW1 EXPAND no split ------------------------------------------------------------------------------------------------------------------------------------ 24 SW1 FOS/RD ACCUM 4.3 G270H 1 15M USE OFFSET SW1 EXPAND no split end orbit ------------------------------------------------------------------------------------------------------------------------------------ 5829( 13) - 01/25/96 13:54 - [ 7] Visit: 03 Visit Requirements: GUID TOL 0.2" On Hold Comments: Additional Comments: Use "remember offset" from visit 1 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 31 SW1 HRS ACCUM 2.0 G140L 1295- fp-split=std 2 14.5M USE OFFSET SW1 1575 ------------------------------------------------------------------------------------------------------------------------------------ 33 SW1 HRS ACCUM 2.0 G140L 1295- fp-split=std 6 15M USE OFFSET SW1 1575 Comments: Should take 3 orbits. ------------------------------------------------------------------------------------------------------------------------------------ 5829( 13) - 01/25/96 13:54 - [ 8] Visit: 04 Visit Requirements: after 1 by 12h to 24h GUID TOL 0.2" On Hold Comments: Additional Comments: Visit should be completed in one orbit. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 41 SW1 WFPC2 IMAGE PC1-FIX F702W CR-SPLIT = NO 1 120S ------------------------------------------------------------------------------------------------------------------------------------ 42 SW1 WFPC2 IMAGE PC1-FIX F702W CR-SPLIT = NO 1 480S ------------------------------------------------------------------------------------------------------------------------------------ 43 SW1 WFPC2 IMAGE PC1-FIX F702W cr-split=0.5,cr 1 1200S -tolerance=0.0 ------------------------------------------------------------------------------------------------------------------------------------ 5829( 13) - 01/25/96 13:54 - [ 9] Summary Form for Proposal 5829 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 FOS/RD HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT = NO cr-tolerance=0 fp-split=std cr-tolerance=0.0 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Solar System ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 23-JAN-96 AND 05-APR-96 SEQ 1-3 WITHIN 24H GUID TOL 0.2" SAVE OFFSET SW1 min dur 480s BETWEEN 23-JAN-96 AND 05-APR-96 GUID TOL 0.2" USE OFFSET SW1 USE OFFSET SW1 EXPAND no split end orbit USE OFFSET SW1 EXPAND no split after 1 by 12h to 24h GUID TOL 0.2" ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F702W MIRROR G270H G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names SW1 ------------------------------------------------------------------------------------------------------------------------------------