9141( 3) - 10/25/01 09:43 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9141 Version: 3 Check-in Date: 25-Oct-2001 09:42:17 1.Proposal Title: Fine Scale Temperature Fluctuations in Gaseous Nebulae ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: C. O'Dell Vanderbilt University CoI: Dr. Manuel Peimbert Institute for Astronomy, UNAM N ------------------------------------------------------------------------------------ 5. Abstract In spite of superb theoretical models for photoionized Galactic and Planetary Nebulae there is a basic problem, known as the t^2 problem, which prevents the accurate determination of relative abundances of the elements. Nebulae are in practice more inhomogeneous in electron temperature than theoretically expected. The purpose of this proposal is to determine just how inhomogeneous the two closest bright exemplars of their class are and what is the source of those inhomogeneities. This will be done by imaging the Orion and Eskimo nebulae in filter sets that will allow determination of O III 5007 and 4363 Angstrom\ flux ratios, which is a well calibrated electron temperature determinant. We'll use the detailed method of WFPC2 filter calibration developed by the PI, which is necessary since most of the emission line filters do not adequately isolate the emission lines. We'll then compare the zones of high and low electron temperature with the known features in the nebulae, in order to determine if they arise from the best candidates (shock heating and/or shadow cooling), or an unanticipated process. From these results we expect to be able to estimate the uncertainties in the abundances derived for more distant objects and to suggest corrections for reducing those uncertanties. ------------------------------------------------------------------------------------ 9141( 3) - 10/25/01 09:43 - [ 2] Observations Description ------------------------ The two pointings selected should adequately address the problem. In the case of NGC 2392, WFPC2 will cover the entire object. In the case of the single field in Orion, we have selected a pointing that will include both the two regions that have been observed from the ground and will be used for calibration and also has numerous examples of the two best bets for explaining the t^2 problem, i.e. cool shadowed regions and hot shocked zones. In order to obtain the emission line ratio maps necessary for achieving the scientific goals of this program, it is necessary to possess a set of coaligned images in a set of WFPC2 filters. The key filters are F502N and F437N, which are intended to isolate the electron temperature sensitive emission lines of O III at 5007 Angstrom\ and 4363 Angstrom. However, these filters do not adequately isolate those lines and also transmit the underlying continuum. The F437N filter was included in WFPC2 for exactly this type of problem, i.e. determining the electron temperatures from O III emission line ratios, but has not actually been used for that because of the difficulty of extracting accurate spectrophotometric data. The general procedure for extracting emission line surface brightness from the WFPC2 filters has been recently described by O'Dell &\ Doi (1999, PASP, 111, 1316) where they show how to handle the problem of the underlying continuum (basically, by scaling from observations of the nearby continuum in a filter free of strong emission lines) and contamination by non- primary lines. Using the nomenclature of that paper, the equation for deriving the surface brightness in the 5007 line, using the F469N filter for the continuum reference will be beginequation S_5007=fracN_502t_502AOmega s_502T^502_5007(1- fracN_469N_502\fract_502t_469\fracs_502s_469\fracE_502E_469r_ 502) endequation where N values are the observed counts (in ADU) and t's are exposure times. The important thing to understand is that the A Omega s_502T^502_5007 term is determined by calibration against a standard region within the nebula and the subtractive (correction terms) are based on the observations and the ratio of prelaunch filter and WFPC2 characteristics, which are well defined numbers. The analogous equation for the 4363 surface brightness will be more complex, because of applying a correction for the small contamination by the HGamma 4340 line which falls in the tail of the transmission profile of the F437N filter. That correction is based on the measured intensity of the HBeta line, since WFPC2 does not have a filter that isolates HGamma. beginequation S_4363=fracN_437t_437AOmega s_437T^437_43631- fracN_487N_437\fract_437t_487\fracT^437_4340T^487_4861\fracs_ 437s_487T_H(1- \fracN_469N_487\fract_487t_469\fracs_487s_469\fracE_487E_469r _487)- \fracN_469N_437\fract_437t_469\fracE_437E_469\fracs_437s_469r _437 endequation We will rederive the absolute sensitivity coefficients for all of the lines in this program from our observations in this program. This approach is vastly better than the commonly used method (e.g. Moore et al. 2000, AJ, 119, 2991) which employs absolute calibrations determined immediately after WFPC2 installation and makes no corrections for adjacent contaminating lines and the underlying continuum. In the case of NGC 1976 the scattered light continuum is about five times stronger than the purely atomic continuum. This means that correction for the continuum is very important. Using the flux ratios of Baldwin et al. (1991, ApJ, 580) one can predict that about 80\ one can scale adequately from F547M observations (a bandpass free of strong emission lines), but in the case of NGC 1976, one needs to determine the color of the continuum by F469N observations. Normally F469N is used to isolate the He II recombination line at 4686 Angstrom, but since NGC 1976 has been determined to lack that line because of its low ionization, F469N becomes a useful continuum color determinant, which is quite important due to the strength of the continuum. We'll observe NGC 1976 in F502N and F437N to measure the key O III lines, in F547M and F469N to determine the continuum corrections to those filters, and in F487N to get the surface brightness in HBeta necessary for correcting F437N for the HGamma emission line. In Orion we have 1" resolution extinction maps determined from radio/optical fluxes (O'Dell &\ Yusef-Zadeh, 2000, AJ, 120, 382) to allow correction for interstellar extinction. The exposure times for each filter have been calculated by using the published spectrophotometry and scaling from previous WFPC2 images of NGC 1976, under the requirement of a total ADU (gain=7) count of 1000. The orbit requirement is driven by the F437N and F469N filters, each requiring three orbits. Calibration of the filters will be done by isolating sections within the WFPC2 image that have accurate published spectrophotometry. Although the filters other than F437N and F469N have been calibrated before, they can be redone from the same images. As references, we'll use the studies of Baldwin et al. and Esteban et al. (1998, MNRAS, 295, 401), as shown in Figure 2(left). This is essentially the same method adopted by O'Dell &\ Doi for the previous on-orbit filter calibration. In the case of NGC 2392 we only need to add observations in F437N since the nebula will have recently been imaged in all of the other emission line and continuum filters required through a Hubble Heritage program (G0-8499) and the PI's Cycle 9 program (GO-8726). NGC 2392 will have detectable He II 4686 Angstrom\ emission in the F469N filter, but the atomic continuum is sufficiently weak and its inherent flux distribution is theoretically well known, so that adequate scaling from the F547M observations will be possible. The extinction corrections will be made from HAlpha/HBeta ratios. The surface brightness of the nebula is sufficiently high to reach the desired total signal of 1000 in one orbit. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9141( 3) - 10/25/01 09:43 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC1976-S ISM, HII REGION, HERBIG RA = 5H 35M 15.77S +/- 0.01S, 2000 SURF-LINE(6563)=1.1+/-0.1e-10 -HARO OBJECT, EMISSION DEC = -05D 23' 48.0" +/- 0.1" LINE NEBULA Coordinate Source: OTHER_SOURCE Comments: Surface brightness given is for the entire line. 25"S, 10"W of Theta1C Ori. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 2 NGC2392 ISM, Planetary Nebula RA=07H 29M 10.77S +/- 1", 2000 SURF-LINE(6563)=6.6+/-0.5e-5 DEC=20D 54' 42.5" +/- 1" Coordinate Source: OTHER_SOURCE Comments: Above value is for NGC 7293. Exposures for other nebulae in this program are determined from earlier WFPC2 observations. 9141( 3) - 10/25/01 09:43 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 44.0D TO 46.0D On Hold Comments: Additional Comments: All observations should be made at the same ORIENT value. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 101 NGC1976-S WFPC2 IMAGE PC1-FIX F487N CR-SPLIT=NO 3 350 S ------------------------------------------------------------------------------------------------------------------------------------ 102 NGC1976-S WFPC2 IMAGE PC1-FIX F502N CR-SPLIT=NO 3 160 S ------------------------------------------------------------------------------------------------------------------------------------ 103 NGC1976-S WFPC2 IMAGE PC1-FIX F547M CR-SPLIT=NO 3 230 S ------------------------------------------------------------------------------------------------------------------------------------ 104 NGC1976-S WFPC2 IMAGE PC1-FIX F656N CR-SPLIT=NO 2 140 S ------------------------------------------------------------------------------------------------------------------------------------ 105 NGC1976-S WFPC2 IMAGE PC1-FIX F658N CR-SPLIT=NO 2 200 S ------------------------------------------------------------------------------------------------------------------------------------ 9141( 3) - 10/25/01 09:43 - [ 5] Visit: 02 Visit Priority: Visit Requirements: ORIENT 44.0D TO 46.0D On Hold Comments: Additional Comments: All observations should be made at the same ORIENT value. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 106 NGC1976-S WFPC2 IMAGE PC1-FIX F469N CR-SPLIT=NO 2 1100 S ------------------------------------------------------------------------------------------------------------------------------------ 107 NGC1976-S WFPC2 IMAGE PC1-FIX F437N CR-SPLIT=NO 4 1100 S ------------------------------------------------------------------------------------------------------------------------------------ 9141( 3) - 10/25/01 09:43 - [ 6] Visit: 03 Visit Priority: Visit Requirements: ORIENT 44.0D TO 46.0D On Hold Comments: Additional Comments: All observations should be made at the same ORIENT value. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 108 NGC1976-S WFPC2 IMAGE PC1-FIX F437N CR-SPLIT=NO 2 1100 S ------------------------------------------------------------------------------------------------------------------------------------ 109 NGC1976-S WFPC2 IMAGE PC1-FIX F469N CR-SPLIT=NO 4 1100 S ------------------------------------------------------------------------------------------------------------------------------------ 9141( 3) - 10/25/01 09:43 - [ 7] Visit: 04 Visit Priority: Visit Requirements: ORIENT 71.001D TO 71.002D On Hold Comments: Additional Comments: All observations should be made at the same ORIENT value. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 201 NGC2392 WFPC2 IMAGE WF3 F469N 4694 CR-SPLIT=NO 4 500 S Comments: This is based on the pointing from program GO 8726. ------------------------------------------------------------------------------------------------------------------------------------ 9141( 3) - 10/25/01 09:43 - [ 8] Summary Form for Proposal 9141 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX WF3 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 44.0D TO 46.0D ORIENT 71.001D TO 71.002D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F487N F502N F547M F656N F658N F469N F437N ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC1976-S NGC2392 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 4694 ------------------------------------------------------------------------------------------------------------------------------------