7381( 5) - 11/19/98 10:12 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7381 Version: 5 Check-in Date: 19-Nov-1998 10:12:13 1.Proposal Title: Thackeray's globules in IC 2944: HAlpha imaging with WFPC2 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Bo Reipurth University of Colorado CoI: Steve Heathcote National Optical Astronomy Observatories Y CoI: Guillermo Tenorio-Tagle Instituto de Astrofisica de Canarias N CoI: Patrice Corporon Laboratoire d'Astrophysique de Grenoble N ------------------------------------------------------------------------------------ 5. Abstract We propose to use WFPC2 to observe a remarkable group of small globules discovered by Thackeray in the southern HII region IC 2944. These globules are unique because they lie almost exactly on the line of sight from the ionizing OB cluster to the earth, and hence are seen starkly silhouetted against the HII region. Small globules like these are believed to be remnant molecular cores left behind when an HII region expands eating into the surrounding molecular cloud. Thus in IC 2944 we are witnessing the ultimate fate of structures like the elephant trunks in M 16 recently studied by Hester et al. (1996) Our detailed groundbased study of Thackeray's globules shows that they are fragmented on scales all the way down to the resolution limit. Three processes appear to be responsible for the ongoing destruction of these globules: evaporation, ablation and fragmentation. Of these, only evaporation is reasonably well understood. Thanks to the fortuitous geometry, imaging these globules with HST through an H-Alpha filter will enable us to obtain, for the first time, detailed observational information on the fragmentation and ablation processes. ------------------------------------------------------------------------------------ 7381( 5) - 11/19/98 10:12 - [ 2] Observations Description ------------------------ We propose to observe a section of IC 2944 chosen to include as many of Thackeray's globules as possible. We will employ the F656N filter, because the bright rims of the globules emit principally in H-Alpha, and also because the smallest globules are seen only in contrast to the bright H-Alpha emission of the background HII region. The largest globule will be centered on WF4. A roll angle of 335D then includes the most interesting of the neighbouring globules within the entire WFPC2 field of view. A +/-5D range around this value is acceptable, although we would prefer a roll angle as close as possible to the center of this range. Alternate orients are given in the visit comments which could be used in the event of scheduling problems. However, these alternates entail a compromise over which of the neighbouring globules are included. Another key issue in positioning the field of view is avoidence of bright stars just off the CCDs which would cause scattered light. HD 101205 (V=6.45) lies ~25" south of the field well outside the zone of avoidence. Some stars with V~12 unavoidably fall close to the edge of the chips. However, calculations show that these will not produce serious stray light problems given the narrow filter and short eposure times of our observations. Since the globules are seen in projection, the critical parameter for their detection and measurement is the S/N ratio achieved in the surrounding nebula background. We have estimated the mean count rate which will be obtained in the nebula background by scaling from our ground based images using the known throughput of HST+WFPC2 and the ground based instrumentation. Previous experience with WFPC2 has shown that this method gives very dependable results. From this, taking into account RON and dark current, we estimate that our total exposure time will yield a S/N ratio of ~65:1 per pixel in the nebular background and ~27:1 per pixel in the dark core of the globules. We will divide our total exposure time into nine sub -exposures to avoid serious charge overflow from the bright (V ~ 10-11) stars in the field and to permit the detection and correction of cosmic ray events. We will dither the telescope between sub-exposures in order to maximize our resolution, and eliminate defects such as hot pixels. Real Time Justification ----------------------- None. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7381( 5) - 11/19/98 10:12 - [ 3] Data Distribution Paper Products: Media: DAT Blocking Factor: 10 Ship To: S. R. Heathcote*CTIO, AURA Inc.*Mariategui 2438*Providencia- Santiago*Chile*(Tel 011-56-2-2047394) Ship Via: Email: sheathcote@noao.edu ------------------------------------------------------------------------------------ 7381( 5) - 11/19/98 10:12 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 IC2944- ISM, Dark Cloud RA = 11H 38M 15.5S +/- 0.3", DEC J2000 SURF-LINE(6563)=1.7+/-0.5e-13 GLOBULE1 = -63D 21' 4.0" +/- 0.3", PLATE- V = 10.66+/-0.01 ID=06AG B-V = 0.04+/-0.01 Comments: SURF-LINE gives the integrated H-alpha surface brightness of IC2944 against which the globule is silhouetted. Core of globule is ~5 times fainter. V and B-V are for the brightest star in the WFPC2 field CPD-62 2167. 7381( 5) - 11/19/98 10:12 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENTation 330D TO 340D On Hold Comments: Additional Comments: Alternate Orients: (1) 150 - 180 on WF2-FIX; (2) 35-50 on WF3-FIX Position 11H 38M 15.5S -63D 21' 10.0" Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 IC2944-GLOB WFPC2 IMAGE WF4-FIX F656N 6563 DITHER-TYPE=BOX,CR-SPLIT=NO 2 160 S ULE1 ------------------------------------------------------------------------------------------------------------------------------------ 2 IC2944-GLOB WFPC2 IMAGE WF4-FIX F656N 6563 CR-SPLIT=NO 1 260 S ULE1 ------------------------------------------------------------------------------------------------------------------------------------ 7381( 5) - 11/19/98 10:12 - [ 6] Summary Form for Proposal 7381 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WF4-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters DITHER-TYPE=BOX CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENTation 330D TO 340D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F656N ------------------------------------------------------------------------------------------------------------------------------------ Target Names IC2944-GLOBULE1 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 6563 ------------------------------------------------------------------------------------------------------------------------------------