6434( 4) - 06/27/96 14:40 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6434 Version: 4 Check-in Date: 27-Jun-1996 14:38:47 1.Proposal Title: UV Imaging of S Andromedae (SN 1885) in M31 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle INTERSTELLAR MEDIUM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: ANDREW HAMILTON UNIVERSITY OF COLORADO CoI: Robert A. Fesen Dartmouth College N ------------------------------------------------------------------------------------ 6. Abstract We propose WFPC2 UV imaging of the remnant of S Andromedae, the 1885 supernova which exploded in the bulge of M31. Optical HST images obtained by us early in Cycle 5 show the remnant as a 0''.6 diameter (= +/- 9,000 : km : s^-1) absorption spot silhouetted against the bulge of M31. Unexpectedly, the absorption appears to be mainly caused by Ca II LambdaLambda3934,3968 (H & K) and not by Fe I 3860 as earlier suspected. We propose WFPC2 F255W imaging to detect the expected 0.5--1 Msun \ of iron ejecta in the remnant through background light absorption by the Fe II resonance line complexes at 2350 and 2600 Angstrom. A detection in the UV would: (1) confirm the presence of iron in this suspected, albeit peculiar, Type Ia supernova, (2) determine the Fe II expansion velocity, and (3) provide valuable information on the mass of Fe II in this 110 year old remnant. ------------------------------------------------------------------------------------ 6434( 4) - 06/27/96 14:40 - [ 2] Observations Description ------------------------ We will image S And using the F255W 2597 +/- 197 Angstrom \\ filter on WFC The WFC is preferable to FOC here because of its lower effective detector background for this extended (0''.6 diameter) faint source. The surface brightness of the center of M31 at 2600 Angstrom \\ observed through the 154 : arcsec^2 aperture of IUE is 6.76 * 10^-15 : erg : s^-1 \\: cm^-2 : ang^-1 (Burstein et al. 1988, ApJ, 328, 440; cf. Bertola et al. 1995, ApJ, 438, 680). We estimate that the surface brightness at the position of S And is about half this value, and we adopt 3 * 10^-15 : erg \\: s^-1 : cm^-2 : ang^-1 per 154 : arcsec^2. This translates into a surface brightness per \\ln Lambda of wavelength interval at S And's position of 6.6 * 10^-3 \\: phot : s^-1 : cm^-2 : arcsec^ -2. Dark counts and readout noise are significant for this observation. Sky brightness is negligible (about 0.01 of the signal). To reduce the dark counts, we especially request the WF2 chip, which has the lowest dark count rate (WFPC2 Handbook, Figure 4.5; the WF2 has the highest readout noise, Table 4.3, but its lower dark count still gives it the edge). To reduce the contribution of readout noise, we propose (a) to use WFPC2 in AREA mode, (WFPC2 Handbook, Section 2.8) where counts are read out with 2 * 2 on-chip binning, and (b) to take the longest possible exposures, using a full orbit for each single exposure. At declination 41^degrees, orbital visibility is 54 minutes (Phase II Proposal Instructions Table 2). Subtracting 15 minutes of overhead time (WFPC2 Handbook, Section 2.7) leaves an exposure time of 39 minutes, which quantizes to 2300 seconds (WFPC2 Handbook, Table 2.3). We request that exposures not be split into shorter units, in spite of the increased risk of cosmic ray hits. In subsequent orbits, with 8 minutes of overhead time, the exposure time is 46 minutes, which quantizes to 2700 seconds. Below we will calculate numbers for a 2500 second exposure. In a 2500 sec exposure, cosmic rays will affect approximately 4.6 percent of pixels (WFPC2 Handbook, Section 4.2.9). At 1.8 events per second, there is a 19 percent probability of the `center' of a cosmic ray event hitting somewhere inside the 0''.6 diameter spot of S And during a 2500 second exposure. Thus cosmic rays pose a significant but not insurmountable problem. To allow adequate removal of cosmic rays, we request 3 separate exposures. To convert the above surface brightness to counts through the F255W filter, we multiply by the dimensionless efficiency \\int Q T d Lambda / Lambda = 0.00078 (WFPC2 handbook Table 6.2), by the HST primary area 45238.9 : cm^2, by the binned pixel area 0''.2 * 0''.2, and by the exposure time 2500 sec. This gives 23 counts of signal per 2 * 2 binned pixel. The dark count rate with the WF2 chip is 0.0025 : cts : s^-1 : pix^-1 (WFPC2 handbook Table 4.2) which yields 25 counts per 2 * 2 binned pixel in a 2500 sec exposure. Thus the dark count rate is comparable to the signal. The noise per 2 * 2 binned pixel in each exposure comes from the signal, the dark count, and the readout noise of 5.51 electrons for the WF2 chip (again, the sky count is negligible). The noise is then ( 23 + 25 + 5.51^2 )^1/2 = 8.9 counts. The resulting S/N ratio is therefore 23/8.9 = 2.6 in each 2 * 2 binned pixel. At 0''.6 diameter, S And is extended over approximately 7 of the 2 * 2 binned pixels. Thus the overall S/N for the region of S And is 7^1/2 times the S/N for each binned pixel, yielding a S/N ratio of 6.9 from each 2500 second exposure. To remove cosmic ray hits, we request three exposures, each exposure taking one orbit. To improve flat-fielding (correction for dark counts), we request that the exposures be dithered by 10 pixels (1'') in each of the x and y directions between each exposure. Real Time Justification ----------------------- The special requests made in the previous section are: begindescription em (1) use of the low dark count WF2 chip; em (2) 2 * 2 pixel on-chip binning; em (3) exposures should fill out a full orbit, and not be split for cosmic ray removal. enddescription Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6434( 4) - 06/27/96 14:40 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6434( 4) - 06/27/96 14:40 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 S-AND EXT-MEDIUM,SNR RA = 0H 42M 43.04S +/- 0.01S, J2000 SURF(V) = 16.60+/-0.1 DEC = +41D 16' 4.49" +/- 0.1" SURF(B) = 17.63+/-0.1 SURF-CONT(2600)=2.0+/-0.6e-17 6434( 4) - 06/27/96 14:40 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 S-AND WFPC2 IMAGE WF2 F255W CR-SPLIT=NO,SUM=2x2, 1 2700 S DITHER-TYPE=LINE, DITHER-LINE-STEPS=3, DITHER-LINE-SPACING=0.564 ------------------------------------------------------------------------------------------------------------------------------------ 6434( 4) - 06/27/96 14:40 - [ 6] Summary Form for Proposal 6434 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO SUM=2x2 DITHER-TYPE=LINE DITHER-LINE-STEPS=3 DITHER-LINE-SPACING=0.564 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category INTERSTELLAR MEDIUM ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F255W ------------------------------------------------------------------------------------------------------------------------------------ Target Names S-AND ------------------------------------------------------------------------------------------------------------------------------------