5835( 4) - 04/16/96 15:41 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5835 Version: 4 Check-in Date: 16-Apr-1996 15:36:49 1.Proposal Title: The Structure of the Inner Coma of Comet Chiron: Imaging the Exopause ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Solar System GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Karen J. Meech Institute for Astronomy CoI: Marc W. Buie Lowell Observatory N CoI: Michael J. Belton National Optical Astronomy Obs. N ------------------------------------------------------------------------------------ 6. Abstract We propose to use the Faint Object Camera to obtain a series of images of the inner coma of 2060 Chiron. The radial brightness distribution obtained from these images will be used (i) to locate and map the exopause structure detected during cycle 2 observations at R_N ~ 1,200 km, (ii) to obtain radial color variations in the extent of the coma, (iii) to look for changes as a function of time in the structure of the coma associated with rotation and shell structure. The existence of the exopause boundary indicates gravitational control by the nucleus over the structure of the inner coma. When combined with color information which will yield constraints on particle sizes - both for the inner coma from HST and for the outer coma from ground -based observations - we will be able to constrain the mass of Chiron. We plan to obtain three series of observations, near perihelion (1996, February 13/14), near its minimum geocentric distance (1996, April 1) and at the end of cycle 5 - to eliminate the possibility that the structure is caused by ejected shells of material from the nucleus. At the minimum geocentric distance, the exopause boundary should be located at a projected distance between 0.20-0.24 arcsec from the nucleus. ------------------------------------------------------------------------------------ 5835( 4) - 04/16/96 15:41 - [ 2] Observations Description ------------------------ With these observations we face the challenge of imaging a low surface- brightness feature next to a bright object. The exopause boundary we will image should be at a projected distance of 0.22 arcsec from the nucleus at minimum geocentric distance (Delta=7.458 AU) on the first of April 1996. From simulations of the Chiron nucleus plus a 2-component powerlaw coma, we find that we must detect the inner coma with a S/N of at least 10 percent in order to accurately measure the distance from the nucleus at which the change in slope occurs, and hence get a firm density limit for Chiron. From the previous PC observations, we found that the surface brightness of the exopause feature was approximately 5-6 mag fainter than Chiron's total magnitude. This requires a relatively long exposure which must be broken into many short exposures to avoid saturating the core. It is essential that we do not saturate the core of the images so that we can model the contribution of the nucleus to the total PSF. Whereas the FOC would provide the highest resolution for these observations, Chiron will greatly exceed the peak counting rate in its linear detection regime unless we use neutral density filters. Based on this, we originally selected the Planetary Camera (PC). The benefits of using the PC are that we (1) collect more photons in the alloted time, (2) could collect our own PSF data, and (3) can get both red and blue images which are important for getting information about the grain sizes in the inner coma. However, the image scale is poor, with undersampled images. Using the FOC, however will give us well-sampled PSFs and excellent image scale. At minimum geocentric distance, assuming a diameter of 180km for Chiron, the nucleus will subtend 2.4 FOC pixesl, and our simulations show that an unjitteres image may have information directly related to Chiron's size. This alone, is a fairly compelling reason to switch to the FOC. In addition, because the detector is photon counting, there is no read noise, and even though we will have to throw away photons with the ND filters, we have the potential to get the inner coma detection in single FOC images. Assuming Hv=6.2 and B-V=0.632, at opposition in 1996, Bobs=15.83. We predict 6.25 counts/sec in the peak pixel, assuming a model with a radius of 90 km, and the exopause boundary at 1,200 km with the coma contributing 20% of the light. To maintain the intensity near linear, we will required F2ND (T=0.19). Twenty pixels from the center we will be getting 1.5e-3 cnts/sec for the coma, which means the coma is readily detectable in a single long exposure. We require the longest possible wavelength coverage to model color differences between the inner coma (HST) and outer coma (from ground-based observations) in order to assess the grain sizes in the exopause (models are currently being developed). Because the efficiency and effective resolution drop significantly in the red for the FOC, we have selected the F410M filter (medium band v) for the long wavelength end, and the F275W,F278M combination for the short wavelength end (to suppress the red leak). Both filters are at the peak efficiency for the detector. Assuming a nearly solar color for Chiron at this wavelength, and the reduction in the peak transmission of a factor of 10 for the filter combination, no neutral density filters will be necessary for the F275W,F278M combination. Likewise the reduced transmission/efficiency of the F190M,F195W filter combination allows us to observe without ND filters. We have selected 3 visits of 4 orbits each and will alternate filters 1 filter of each per orbit. This will give us temporal coverage over 1 full rotational period per visit in 2 colors. The first visit is at perihelion, the second at minimum geocentric distance, and the 3rd at least 2 weeks after the second visit - out until we begin to lose significant resolution on the coma because of the increasing geocentric distance (approximately 53 days past minimum geocentric distance). For the final visit, we will take 1 F410M image per orbit and then alternate between 2 shorter filters, the F275W,F278M combination and F190M,F195W to get additional color information in the inner coma. Real Time Justification ----------------------- These observations are time-critical. Chiron's orbital period is approximately 50 years, and the early April 1996 observations will be the highest resolution we will get this apparition. In addition, we expect the comet to be most active at perihelion, around 1996 February 13, and presumably the exopause will be intrinsically the brightest and most observable. Observations are desired around these dates +/- few days. The 3rd visit only has the requirement of a long time baseline with respect to the other two visits, to rule out the possibility that the coma slope change is an expanding shell of ejected material. We plan to maintain an extensive ground-based observing program to monitor the brightness fluctuations of Chiron, and in particular to monitor the extent, morphology and color of the outer coma. In particular, we will attempt to coordinate the ground-based observations with the HST time for the coma color information. The observations will be carried out by the PI and Co-I's using the facilities on Mauna Kea, at Lowell Observatory and at the Kitt Peak National Observatory. Calibration Justification ------------------------- Additional Comments ------------------- Our simulations show that understanding and removing the PSF effects will be critical to getting an answer regarding structure in the inner coma. We are interested in removing the small angle PSF scattering component in the 0.1-0.5 arcsec range, and since there has not been a systematic study of TinyTim FOC intensity profiles in these ranges we request that a suitable PSF be provided for our filters. (Based upon communication with Robert Jedrzejewski). We also request that our observations near perihelion and minimum geocentric distance be coordinated with that of Stern (GO5843) who will be taking spectra of Chiron. We hope to use his continuum reflectivity to extend the wavelength coverage for information about the grain scattering properties. ------------------------------------------------------------------------------------ 5835( 4) - 04/16/96 15:41 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5835( 4) - 04/16/96 15:41 - [ 4] TARGET LIST b)Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: 2060-CHIRON ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: COMET CHIRON ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=2060 | | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 15.2+/-0.1 | Coordinate observations with Stern B-V = 0.63+/-0.01 | GO5843 | ------------------------------------------------------------------------------------------------------------------------------------ 5835( 4) - 04/16/96 15:41 - [ 5] Visit: 01 Visit Requirements: BETWEEN 12-FEB-96:18:00:00 AND 14-FEB-96:18:00:00 On Hold Comments: Additional Comments: Observations within 2 days of perihelion Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 15M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 2 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 15M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 3 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 17M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 4 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 17M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 5 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 18M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 6 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 18M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 7 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 18M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 8 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 18M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 5835( 4) - 04/16/96 15:41 - [ 6] Visit: 51 Visit Requirements: On Hold Comments: Additional Comments: Repeat of 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 15M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 2 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 15M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 3 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 17M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 4 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 17M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 5 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 18M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 6 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 18M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 7 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 18M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 8 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 18M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 5835( 4) - 04/16/96 15:41 - [ 7] Visit: 02 Visit Requirements: BETWEEN 30-MAR-96:07:41:00 AND 03-APR-96:07:41:00 On Hold Comments: Additional Comments: Observations within 2 days of minimum geocentric distance Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 15M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 2 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 15M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 3 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 17M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 4 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 17M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 5 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 18M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 6 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 18M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 7 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 18M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 8 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 18M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 5835( 4) - 04/16/96 15:41 - [ 8] Visit: 03 Visit Requirements: AFTER 02 BY 14D TO 53D On Hold Comments: Additional Comments: Final followup observations Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 15M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 2 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 15M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 3 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 17M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 4 2060-CHIRON FOC/96 IMAGE 512X512 F550M,F1N 1 17M NO SPLIT END ORBIT D ------------------------------------------------------------------------------------------------------------------------------------ 5 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 18M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 6 2060-CHIRON FOC/96 IMAGE 512X512 F275W,F27 1 18M NO SPLIT END ORBIT 8M ------------------------------------------------------------------------------------------------------------------------------------ 7 2060-CHIRON FOC/96 IMAGE 512X512 F410M,F2N 1 18M NO SPLIT D ------------------------------------------------------------------------------------------------------------------------------------ 8 2060-CHIRON FOC/96 IMAGE 512X512 F550M,F1N 1 18M NO SPLIT END ORBIT D ------------------------------------------------------------------------------------------------------------------------------------ 5835( 4) - 04/16/96 15:41 - [ 9] Summary Form for Proposal 5835 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOC/96 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Solar System ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 12-FEB-96 18 00 00 AND 14-FEB-96 18 00 00 NO SPLIT NO SPLIT END ORBIT BETWEEN 30-MAR-96 07 41 00 AND 03-APR-96 07 41 00 AFTER 02 BY 14D TO 53D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F410M,F2ND F275W,F278M F550M,F1ND ------------------------------------------------------------------------------------------------------------------------------------ Target Names 2060-CHIRON ------------------------------------------------------------------------------------------------------------------------------------