Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 4545c Report Date: 09-May-96:19:41 Version: ********** Check-in Date: ********** 1.Proposal Title: OUTGASSING FROM CERES -- CYCLE 3 HIGH ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID SOLAR SYSTEM GO Sub Category MINOR PLANETS ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Michael F. A'hearn UNIVERSITY OF MARYLAND USA ------------------------------------------------------------------------------------ 7. Abstract We propose to search for outgassing from Ceres by searching for emission by OH as recently reported by A'Hearn and Feldman using data data taken with IUE (1992 Icarus 98,54-60). If this emission can be confirmed with the higher sensitivity of HST, our picture of Ceres will have to be fundamentally changed. ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 4.07 par: 0 10. Num targs pri: 1 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: FOS ------------------------------------------------------------------------------------ 12. Special sched req: Time Critical obs. ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 4545c PI: Michael F. A'hearn Proposal Title: OUTGASSING FROM CERES -- CYCLE 3 HIGH ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Pi Michael F. A'Hearn UNIVERSITY OF MARYLAND USA Paul D. Feldman JOHNS HOPKINS UNIVERSITY USA Lucy A. Mcfadden UNIVERSITY OF MARYLAND USA Harold A. Weaver STSCI USA ------------------------------------------------------------------------------------ 3. Description of proposed observations. We will use FOS red side with grating G270H. We will use the 4.3 arcsec aperture to maximize signal. This will yield a resolution of 1.4 arcsec perpendicular to the dispersion before convolution with the PSF, significantly higher than available with IUE which has a PSF of at least 3 arcsec and for which we must integrate a slit length of at least 10 arcsec in order to obtain a significant detection. We propose to observe adjacent to two opposite limbs of Ceres to explore the spatial variability and at two distances both to explore the radial distribution and to adequately deal with scattered light from Ceres. We propose to use the acquisition spectra of Ceres, obtained in the 'Hartig maneuver' to provide a spectrum of Ceres itself with high S/N and identical spectral resolution. This will be scaled and fit to the scattered light in the offset exposures to provide proper subtraction of the scattered light. In addition, we will use this spectrum to search for geologically significant spectral features. At each limb, we will position Ceres just outside the 4.3- arcsec aperture (top and bottom edges) and obtain spectra through both the 'top' and 'bottom' 1.4x4.3 strips of the 4.3 arcsec aperture of FOS. The aperture closer to Ceres will be somewhat contaminated by light from the PSF of Ceres but will enable us to explore a spatial region not accessible with IUE. The aperture further from Ceres will be positioned at the distance at which we previously detected OH using IUE. The IUE observation was severely contaminated with scattered light at this larger distance. With HST, the scattered light should be very small for the more distant position. On one side of Ceres we will obtain data at 3 radial distances corresponding to the 3 strips in the 4.3 arcsec aperture in an to better define the radial profile of OH. Page 3 ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. This proposal is for a followup to a dramatic discovery made with IUE and which requires far higher sensitivity than achievable with IUE. Further IUE observations are planned but they will only have sufficient sensitivity if the OH is brighter than for our one positive detection. Ground-based observations are also planned (already scheduled with the NTT at ESO) but, due in part to the huge atmospheric attenuation at these wavelengths, the estimated limit to our sensitivity is only comparable to that with IUE. Our recent observations of comet Shoemaker-Levy have shown that we can reach surface brightnesses of about 4 Rayleighs (S/N~2.5) with exposure times of order 1/2 hour using the 4.3 arcsec aperture of FOS. This is consistent with the following calculation: The OH band, with natural width about 20-25 Angstroms, is convolved with a rectangular slit of width roughly 25 Angstroms. We assume a trapezoidal profile with base 50 Angstroms and top 10 Angstroms. A total brightness of 1 Rayleigh yields a peak count rate at the center of the band ~0.0023 counts per second per diode whereas the dark current should be 0.010 counts per sec per diode. If we ignore the scattered light from Ceres, a surface brightness of 3 Rayleighs should yield S/N~3 in one hour for the peak pixel. Integration over a range of wavelenghts will increase the S/N to 10 for the whole band although this ignores the noise induced by subtracting the baseline. We should be able to detect a brightness of 1-2 Rayleighs with a one-hour exposure and resolve the band for a brightness of 3 Rayleighs. Since Ceres itself is very bright, we propose to obtain spectra at 2(3) radial distances, the larger of which should be uncontaminated by scattered light. If we can successfully remove the scattered light, which will dominate the closer exposure, we may be able to detect the higher surface brightness that one would expect to see closer to Ceres. ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. Since the fluorescence efficiency of OH varies with heliocentric distance and since the spatial distribution of OH should drop with distance from Ceres, we need to minimize both geocentric and heliocentric distances. The geocentric distance is a minimum in late October 1993 while the heliocentric distance is decreasing slowly throughout cycle 3. We also require that the offsets be generally in the direction of the north and south poles of Ceres. Since the pole of Ceres is certainly generally north-south and, according to recent measurements by a French group, probably projected at PA=323 during fall 1993, we are limited to times when the solar direction is roughly (+/- 30 degrees) orthogonal to this direction. The combined constraints limit us to a short window in late summer before opposition and a wider window in late fall after opposition. All observations must be contiguous in order to simplify the interpretation. ------------------------------------------------------------------------------------ 7. Data reduction and analysis plans. Preliminary reductions will be carried out at STScI with final reductions carried out in parallel at JHU and at UMd. Modelling for a positive detection will be carried out at UMd elaborating on the very simple exospheric model described by A'Hearn and Feldman. ------------------------------------------------------------------------------------ 8. Additional comments or special requests. Page 4 ------------------------------------------------------------------------------------ 9. Description of previous HST work. P3064:HST Observations of Comet Levy (1990c). No relation. P2481:HST Observations of Periodic Comets. No relation. P2442:Cometary Parent Molecules- Target of Opportunity. No relation. P2483:The Volatile Composition of New Comets -Target of Opportunity. No relation P3707:HST Observations of Periodic Comets. No relation. P3064: We successfully obtained images of comet Levy prior to implementation of moving target tracking capabilities. We showed that the photometric variability was correlated with outflow of a shell of dust at a surprisingly low velocity, of order 0.2 km/s, thus implying that the particles reflecting the continuum are very large. We showed that, even at 0.1 arcsec resolution, there were no jets visible thus showing that the variability is likely to be due to a quite different source than in the case of comet Halley where the variability was caused by jets turning on and off. P2481: We obtained FOS spectra of comet P/Hartley 2. The most dramatic result was detection of the CO Cameron bands, never before seen in a comet. These bands are produced when CO2 photodissociates to form CO in an excited, metastable state. The intensity is therefore a new tracer of the CO2 abundance, the only tracer observable from Earth until ISO is launched. P2442 and 2483: The first data, spectra with HRS and FOS respectively, were obtained only in the week preceding writing this proposal. The reductions are not yet complete. P3707: No data have yet been obtained for this program. Inner Coma Imaging of Comet Levy (1990c) with the Hubble Space Telescope. Weaver, H. A., M. F. A'Hearn, P. D. Feldman, C. Arpigny, W. A. Baum, J. C. Brandt, R. M. Light, and J. A. Westphal (1992). Icarus, 97, 85- 98. (ref:P3064) The results from P2481 have been presented at meetings and published as an abstract but a complete paper has not yet been submitted for publication. An additional paper discussing models for the data obtained under P3064 is in manuscript form by Samarasinha et al. but has not yet been submitted for publication. ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). The institutions of the investigators will supply all computing facilities needed for the analysis of the data and for interpretive modelling. The institutions will also provide the salary costs for the investigators. All institutions operate networks on which reduction software, e.g., STSDAS/ IRAF/IDL, is readily available. ------------------------------------------------------------------------------------ 11. Address Information Name: Michael F. A'Hearn Category: PI Institution: University of Maryland Address: Department of Astronomy University of Maryland City: College Park State: MD Zip Code: 207422421 Country: USA Telephone: 301-405-6076 Telex (or e-mail): ------------------------------------------------------------------------------------ TARGET LIST b)Solar System Targets ID = 4545c [ 5] ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: 1-CERES ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: ASTEROID 0001 ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = 1(CERES) | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | WND1 = 28-AUG-93 TO 10-SEP-93 | WND2 = 03-NOV-93 TO 22-DEC-93 | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |V = 8.0 +/- .5 | GOAL OF SETTING WINDOW IS TO KEEP | | GOOD SPATIAL RESOLUTION, I.E. TO | | STAY NEAR OPPOSITION, BUT TO ALSO | | FORCE THE +/- Y OFFSETS WITH FOS TO | | BE WITHIN 30 DEGREES OF THE | | PROJECTED ROTATION AXIS OF CERES | | WHICH IS AT TRUE PA = 323 DURING | | THIS PERIOD. THIS ASSUMES THAT +/- | | Y IS PERPENDICULAR TO THE SUN-COMET | | LINE. | ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 | TARGET NAME: CERES-OFF-1 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: OFFSET CERES ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = 1(CERES), | TYPE = POS_ANGLE, ACQ = 0.3 | RAD = 3.7, | ANG = 90., | REF = SUN | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | WND1 = 28-AUG-93 TO 10-SEP-93 | WND2 = 03-NOV-93 TO 22-DEC-93 | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |V = 8.0 +/- .5 | FLUXNUM_1 IS CERES FOR ACQUISITION. 2 |SURF-LINE(3085) = 4.E-17 | FLUXNUM_2 IS AT OFFSET FOR SPECTRAL 3 |W-LINE(3085) = 20. | EXPOSURE. OFFSET IS IN FOS -Y | | DIRECTION | ------------------------------------------------------------------------------------------------------------------------------------ TARGET LIST b)Solar System Targets ID = 4545c [ 6] ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 | TARGET NAME: CERES-OFF-2 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: OFFSET CERES ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD = 1(CERES), | TYPE = POS_ANGLE, ACQ = 0.3 | RAD = 3.7, | ANG = 270., | REF = SUN | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | WND1 = 28-AUG-93 TO 10-SEP-93 | WND2 = 03-NOV-93 TO 22-DEC-93 | ------------------------------------------------------------------------------------------------------------------------------------ REF | DATA | COMMENTS | | 1 |V = 8.0 +/- .5 | FLUXNUM_1 IS CERES FOR ACQUISITION. 2 |SURF-LINE(3085) = 4.E-17 | FLUXNUM_2 IS AT OFFSET FOR SPECTRAL 3 |W-LINE(3085) = 20. | EXPOSURE. OFFSET IS IN FOS -Y | | DIRECTION | ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 4545c [ 7] ------------------------------------------------------------------------------------------------------------------------------------ 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 1-CERES FOS/RD IMAGE 4.3 G270H 3090 Y-SIZE=5 1 15M 30 1 1 SEQ 1-3 NOGAP Y-SPACE=128 3M RT ANALYSIS FOR 2 COMB=YES CYCLE 3 SUB-STEP=4 Comments: GET FIVE FOS SPECTRA AT DIFFERENT Y DEFLECTIONS TO FIND POSITION OF CERES. THESE SPECTRA WILL ALSO BE USED TO FIT SCATTERED LIGHT OF CERES. ------------------------------------------------------------------------------------------------------------------------------------ 2 CERES-OFF-1 FOS/RD IMAGE 4.3 G270H 3090 Y-SIZE=2 8 15M 5 2 1 CYCLE 3 Y-SPACE=512 1H COMB=YES SUB-STEP=4 Comments: TWO FOS SPECTRA WILL BE OBTAINED WITH 1.4X4.3 APER. CENTERED 2.6 AND 4.8 ARCSEC FROM CERES. ADJUST EXPOSURE TO FILL ORBIT. MAY NEED TO SWITCH TO ACCUM MODE PENDING INFO FROM FOS SCIENTISTS. ------------------------------------------------------------------------------------------------------------------------------------ 2.9 1-CERES FOS/RD IMAGE 4.3 G270H 3090 Y-SIZE=5 1 15M 30 1 1 RT ANALYSIS FOR 3 Y-SPACE=128 3M CYCLE 3 COMB=YES SUB-STEP=4 Comments: GET FIVE FOS SPECTRA AT DIFFERENT Y DEFLECTIONS TO FIND POSITION OF CERES. THESE SPECTRA WILL ALSO BE USED TO FIT SCATTERED LIGHT OF CERES. ------------------------------------------------------------------------------------------------------------------------------------ 3 CERES-OFF-2 FOS/RD IMAGE 4.3 G270H 3090 Y-SIZE=2 8 15M 5 2 1 CYCLE 3 Y-SPACE=512 1H COMB=YES SUB-STEP=4 Comments: TWO FOS SPECTRA WILL BE OBTAINED WITH 1.4X4.3 APER. CENTERED 2.6, 3.7 AND 4.8 ARCSEC FROM CERES. ADJUST EXPOSURE TO FILL ORBIT. MAY NEED TO SWITCH TO ACCUM MODE PENDING INFO FROM FOS SCIENTISTS. ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 4545c [ 8] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/RD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters Y-SIZE Y-SPACE COMB SUB-STEP ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 4.07 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category SOLAR SYSTEM ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category MINOR PLANETS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SEQ 1-3 NOGAP; RT ANALYSIS FOR 2; CYCLE 3; RT ANALYSIS FOR 3; ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G270H ------------------------------------------------------------------------------------------------------------------------------------ Target Names 1-CERES CERES-OFF-1 CERES-OFF-2 ------------------------------------------------------------------------------------------------------------------------------------