! Proposal 5878, submission 5 ! PI: Edward L. Fitzpatrick ! Received Tue Aug 20 13:22:57 EDT 1996 ! From: fitz@astro.Princeton.EDU ! Proposal 5878, submission 2 ! PI: Edward L. Fitzpatrick ! Received Tue Mar 21 15:04:06 EST 1995 ! From: fitz@astro.Princeton.EDU ! Hubble Space Telescope Cycle 5 (1995) Phase II Proposal 5878 ! Edward L. Fitzpatrick, P.I. ! $Id: 5878,v 12.1 1996/08/20 18:26:39 pepsa Exp $ ! Proposal_Information Title: The Dust-Phase Abundances of Si, Fe, and Mg Towards HD93521 in the Galactic Halo Proposal_Category: GO Scientific_Category: Interstellar Medium Cycle: 5 Investigators PI_name: Edward L. Fitzpatrick PI_Institution: Princeton University Observatory Abstract: Three orbits are requested to obtain high-resolution, high S/N observations with the GHRS echelle gratings of the interstellar Mg II LambdaLambda1239.9, 1240.4; Si II Lambda1808.0; Fe II Lambda2260.8; and Mg II LambdaLambda2796.4, 2803.5 absorption lines towards the star HD93521 in the Galactic halo. The goal of the observations is to measure precise column densities of Si II, Fe II, and Mg II in the nine interstellar clouds along this line of sight. Previous observations have suggested that interstellar silicate grains have a core - mantle structure, with the core consisting of Fe_2SiO_4 and the mantle of Mg_2SiO_4. This simple picture has been challenged by the suggestion that the theoretical f-values currently adopted for the Mg II LambdaLambda1239.9, 1240.4 lines upon which most Mg II results are based --- are a factor of ~5 too small. The proposed program aims to resolve the f-value issue and provide a detailed description of the composition and structure of interstellar silicate grains from the Si, Fe, and Mg depletions and dust-phase abundances derived from the measured column densities. Questions Observing_Description: THE PROPOSED OBSERVATIONS: The proposed program requires observations at four separate wavelengths with the GHRS echelle gratings. These observations are summarized in the following table: Exposure Central Exposure Spectral Number Grating Wavelength Time Features -------- ------- ---------- --------- -------- 1 ECH-A 1240.0 A 40 min Mg II 1240 2 ECH-B 1808.0 A 40 min Si II 1808 3 ECH-B 2260.8 A 8 min Fe II 2261 4 ECH-B 2800.0 A 7 min Mg II 2800 The Small Science Aperture (SSA) will be used for all observations in order to achieve the highest spectral resolution. There is no restriction, other than sensible use of spacecraft time, on the order in which the exposures are to be obtained. A wavelength calibration exposure (WAVECAL) will be requested either before or after the ECH-A exposure and both before and after the set of ECH-B exposures. The total exposure times listed above are computed to yield 10,000 total counts per data point in the stellar continuum adjacent to the interstellar features of interest, based on the post-COSTAR GHRS sensitivity and assuming the use of a GHRS substepping pattern which yields 2 data points per resolution element (STEP-PATT=6). The observations will be performed using the FP-SPLIT routine (FP SPLIT=DSFOUR), so that each exposure will be broken into 4 individual subexposures, all obtained with slightly different rotations of GHRS grating carousel. (The 40 minute exposures of Si II Lambda1808.0 and Mg II LambdaLambda1239.9, 1240.4 will actually each be performed as 2 sets of FP- SPLITS, yielding 8 subexposures each. This will eliminate the blurring due to thermal or geomagnetic effects which may affect excessively long exposures.) The individual subexposures will be combined as described in the Appendix of Paper II to eliminate the effects of fixed pattern noise on the final, summed spectrum. The final spectra for all 4 sets of absorption lines, after the removal of the scattered light background (see Paper II Appendix), will have S/Nlevels approaching the photon noise limit of 100:1. JUSTIFICATION FOR EXPOSURE TIME REQUESTS: The requested observations of Si II Lambda1808.0 and Mg II Lambda1239.9 are repeats of lower S/N exposures made on these lines for the Cycle 0 program GTO1071, and analyzed in Paper I. These data, which each have S/N levels in the continua of ~25:1, are shown in Figure 1. The small tick marks near the top of the figure indicate the velocities of the nine principal absorbing clouds. The Mg II column densities quoted in Paper I were derived from the Mg II Lambda1239.9 line shown in Figure 1 and had typical uncertainties (ignoring, for the moment, the f-value problem) of +/-60%, with 2 components undetected. The Lambda1240.4 line, which is contained in the same GHRS exposure, is a factor of 2 weaker and was not used in Paper I. The proposed observations will allow relative cloud-to-cloud Mg II column densities to be determined using both of the weak far-UV lines and with a precision of +/-5%. The absolute values of the Mg II column densities (and any correction required to the Hibbert et al. f-values) will be determined by scaling the relative values to fit the profiles of the the strong near-UV Mg II Lambda2796.7, 2803.5. These lines, which will both fit in a single ECH-B exposure, have not been observed previously with the GHRS in its high- resolution mode. The requested S/N of 100:1 is required to define precisely the stellar continuum and the shapes of the profile wings, which are crucial for the comparison with the relative column densities. Note that a low-S/N (~17:1) Small Science Aperture observation of the region around these lines was obtained with the intermediate resolution G270M grating during the Science Verification phase (Z0K6523CT). This observation has been examined and demonstrates that the shape of the stellar continuum in the neighborhood of the Mg II lines is simple, and may be normalized easily using a low- order polynomial; however, the resolution and S/N of these data are far too low to use to investigate the Mg II f-value issue. The Si II column densities given in Paper I are based primarily on the Lambda1808.0 data in Figure 1 which have been shifted down by 0.3 units for clarity) and have uncertainties as large as +/-30% some clouds. The requested observations with S/N = 100 will decrease these uncertainties to the +/-5% level for all clouds. The new data will be combined with the existing observations for the column density determination. The Fe II column densities in Paper I were derived mainly from Fe II Lambda2374.5. The strongest Fe II component observed, which is one of the interesting high-velocity features, was saturated and has a column density uncertainty of at least 40%. Anmother component has an uncertainty of +/-25%. The requested observations pof the weaker Fe II Lambda2260.8 line (which should be approximately as strong as Si II Lambda1808.0 in Figure 1), with S/N = 100, will solve the saturation problem and, when analyzed in conjunction with Lambda2374.5, will yield Fe II column densities for all the individual clouds with precisions of +/-5%. The 40 minute observations of Si II Lambda1808.0 and Mg II LambdaLambda1239.9, 1240.4, plus their associated overhead time, will each require a full GHRS orbit. The shorter observations of Fe II Lambda2260.8 and Mg II Lambda2796.7, 2803.5 require only a fraction of an orbit, and could be scheduled efficiently to appear following the initial acquisition of HD93521. Real_Time_Justification: There are no special requirements. Calibration_Justification: There are no coordinated and/or supporting observations. Additional_Comments: There are no additional comments. ! ! ***** TARGET LIST FOLLOWS ****** ! Fixed_Targets ! Target data updated 8/3/95 - AFB Target_Number: 1 Target_Name: HD93521 Alternate_Names: GSC3009-02547 Description: STAR, MAIN SEQUENCE O Position: RA = 10H 48M 23.47S +/- 0.01S, DEC = 37D 34' 13.1" +/- 0.1" Equinox: J2000 Epoch: 2000.000 Flux: F-CONT(1300) = 3.2 +/- 0.9 E-10, V = 7.0 +/- 0.1, B-V = -0.28, TYPE = O9V Comments: Coordinates are from Hipparcos INCA ! ! ***** VISIT/EXPOSURE LOGSHEET FOLLOWS ***** ! ! This is a template for a single visit containing a single exposure ! Repeat exposure and visit blocks as needed Visits Visit_Number: 1 Exposure_Number: 1 Target_Name: HD93521 Config: HRS Opmode: ACQ Aperture: 2.0 Sp_Element: MIRROR-N2 Time_Per_Exposure: 1.8S Optional_Parameters: LOCATE=EXTENDED Special_Requirements: ONBOARD ACQ FOR 2 Comments: STEP-TIME = 0.2 SEC Exposure_Number: 2 Target_Name: HD93521 Config: HRS Opmode: ACQ/PEAKUP Aperture: 0.25 Sp_Element: MIRROR-A2 Time_Per_Exposure: 5.0S Special_Requirements: ONBOARD ACQ FOR 3-7 Comments: STEP-TIME = 0.2 SEC Exposure_Number: 3 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-B Wavelength: 2261.1 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 544S Exposure_Number: 4 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-B Wavelength: 2799.4 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF, COMB=TWO Number_of_Iterations: 1 Time_Per_Exposure: 489.6S Exposure_Number: 5 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-B Wavelength: 2853.0 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 544S Exposure_Number: 6 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-B Wavelength: 1808.3 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF Number_of_Iterations: 2 Time_Per_Exposure: 761.6S Exposure_Number: 7 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-A Wavelength: 1241.2 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF Number_of_Iterations: 2 Time_Per_Exposure: 1196.8S Visit_Number: 51 Visit_Comments: HOPR repeat of Visit 1 Exposure_Number: 1 Target_Name: HD93521 Config: HRS Opmode: ACQ Aperture: 2.0 Sp_Element: MIRROR-A2 Time_Per_Exposure: 1.8S Optional_Parameters: Special_Requirements: ONBOARD ACQ FOR 2 Comments: STEP-TIME = 0.2 SEC Exposure_Number: 2 Target_Name: HD93521 Config: HRS Opmode: ACQ/PEAKUP Aperture: 0.25 Sp_Element: MIRROR-A2 Time_Per_Exposure: 5.0S Special_Requirements: ONBOARD ACQ FOR 3-7 Comments: STEP-TIME = 0.2 SEC Exposure_Number: 3 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-B Wavelength: 2261.1 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 326.4S Exposure_Number: 4 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-B Wavelength: 2799.4 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 435.2S Exposure_Number: 5 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-B Wavelength: 2853.0 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 435.2S Exposure_Number: 6 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-B Wavelength: 1808.3 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF Number_of_Iterations: 1 Time_Per_Exposure: 1305.6S Exposure_Number: 7 Target_Name: HD93521 Config: HRS Opmode: ACCUM Aperture: 0.25 Sp_Element: ECH-A Wavelength: 1241.2 Optional_Parameters: FP-SPLIT=DSFOUR, STEP-PATT=DEF Number_of_Iterations: 2 Time_Per_Exposure: 1196.8S ! ! ***** DATA DISTRIBUTION FOLLOWS ***** ! Data_Distribution ! Defaults indicated; change if desired Medium: 8MM ! 8MM or 6250BPI or 1600BPI Blocking_Factor: 10 ! 10 or 1 ! Only astronomers with very old 9- ! track tape drives should consider ! a blocking factor of 1 Ship_To: PI_address ! Ship_Via: UPS ! UPS (2-day) or OVERNIGHT ! Overnight shipping done at PI expense Recipient_Email: