6415( 7) - 10/22/96 09:08 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6415 Version: 7 Check-in Date: 21-Oct-1996 17:10:33 1.Proposal Title: Thermal Pressures of the Local Interstellar Medium ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle INTERSTELLAR MEDIUM GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: EDWARD B. JENKINS PRINCETON UNIVERSITY OBSERVATORY ------------------------------------------------------------------------------------ 6. Abstract The thermal pressure nkT of the interstellar medium is a fundamental quantity whose average value and fluctuations tell us much about key physical processes. Theorists place heavy reliance on good measurements of this pressure to construct global theories or explain specific phenomena. The observations proposed here seek to measure this pressure inside H I clouds in a very local part of the Milky Way, with the promise of expanding into a more distant survey in later HST observing cycles. Our method relies on measuring absorption features produced by C I and Si II in different levels of fine-structure excitation that are populated by collisions with neutral atoms and electrons. Observations of the soft x-ray background indicate that our solar system is located near the middle of a bubble of hot gas (T=10^6K) that has a diameter somewhere between 100 and 200 pc. By measuring the pressure of the ambient medium just outside the boundary of this region, we can help to differentiate between two models for the Local Bubble: (1) it is bounded by an expanding shock produced by an explosion some 10^5 yr ago, or (2) it is a pressure- confined region that is about 10^7 yr old. ------------------------------------------------------------------------------------ Observations Description ------------------------ 1. Selected Targets Special care was exercised to find background stars that would satisfy the program's objectives and yield results that should be easy to interpret. Potential targets were selected from a recently produced compilation of stars that show interstellar Na I D-line absorption [Welsh, et al. 1994, ApJ 437, 211]. Before being considered for this program, the listed candidate stars had to pass the following tests: 1. 10^11 < N(Na I) < 10^12 cm^-2 (insures that there is enough material to give measurable lines, but not so much that we might be sampling gas well beyond the edge of the Local Bubble 2. d <= 100 pc. (again insures that we are not sampling too far out) 3. F_1565 > 10^-10 erg cm^ -2 s^-1 A^-1 (to get sufficient signal-to-noise in 2 orbits) 4. v sin i >~ 50 km s^-1 (insures that the stellar continuum is well behaved) 5. No evidence that the star has a circumstellar disk or shell, such as an "e" in the spectral classification (in the Bright Star Catalog) or excess long-wavelength IRAS fluxes relative to that in the 12 micron band. Short-period binaries were also rejected, since they could have interaction gas streams. 6. No detection of diffuse IR emission near the target [Gaustad & VanBuren 1993, Pub. A.S.P., 105, 1127], signifying the presence of dust grains being heated by the star. Screening out stars that might be embedded in relatively dense clouds (test \\#6) should help to insure that we are not sampling a medium being acted upon by the target star. Stars finally accepted for this program are listed in Table 1. TABLE 1: Target Stars Star l b d log N(H) F(1565) V Sp v sin i (deg)(deg) (pc) (erg cm^-2 (km s^-1) s^-1 A^-1) gamma Ori 197 +16 93 19.4 1.5e-8 1.64 B2III 59 kappa Vel 276 -4 102 20.2 5.8e-9 2.50 B2IV-V 49 lambda Lup 327 +11 104 19.6 1.4e-9 4.05 B3V 166 delta Cyg 79 +10 50 19.7 4.9e-10 2.92 B9.5III 149 alpha Del 60 -15 125 19.4 3.8e-10 3.77 B9IV 162 Notes: log N(H) estimated fro the observed N(Na I) and converted to N(H) using the empirical relationship of Ferlet, Vidal-Madjar & Gry 1985, ApJ, 298, 838. Values of F(1565) taken from Thompson, et al. 1978, "Catalog of Stellar Ultraviolet Fluxes, (Science Research Council) 2. Lines to be Observed Absorption lines listed in Table 2 will be observed in 3 separate wavelength intervals (discussed in Section 3 below). TABLE 2: Absorption Features & their Significance to the Program S I 1262.860, 1807.311 0.515, 2.239 N(S I)/N(S II) gives n_e/Gamma S II 1250.6,1253.2,1259.5 0.834,1.135,1.311 Indicates N(H I): S undepl. C I 1260.736 1.696 Lowest excitation level of C I C I* 1260.9,1261.0,1261.122 1.219,1.094,1.316 Middle excitation level of C I C I** 1261.4,1261.6 1.094,1.571 Highest excitation lev. of C I Si II 1260.4,1808.0 3.16,0.576 lambda1808 should be unsat. Si II* 1264.7 3.058 ratio to Si II gives n_e T^-.5 Fe II 1260.5 1.499 Not essential; may be saturatd 3. Details of the Program All 5 stars will be observed with a single program that takes 2 orbits to complete in each case. The declinations of all targets are low enough that we'll have only 52-57 minutes of unocculted time in each orbit. All targets must be acquired with the A2 mirror because they're so bright. To save time, these targets will be set in the LSA with only nominal accuracy, with the end result they will have spectra with a slightly lower resolution (by about 10-20%). After GS and target acquisition for the LSA on the first orbit, all of the remaining unocculted time (1196.8 sec with STEP-PATT=7 + 4 min OH) will be directed to obtaining an FP-SPLIT exposure over the wavelength interval 1803- 1813 A with the GHRS Ech-B grating. During occultation, GHRS will be switched from Side 2 to Side 1. After the 6-minute re-acquisition of the guide stars, the second orbit will employ Ech-A, again with FP-SPLIT and STEP-PATT=7, to obtain a 761.6 sec exposure over the interval 1250-1256 A and a 1523.2 sec exposure over 1259-1265 A. Lines in the former wavelength interval are strong, hence the S/N requirements are not as great as for the latter interval that contains the most important, and weakest, lines. Count rates during the ACCUM exposures will range from about 3 events s^-1 diode^-1 for Alpha Del at 1253A to about 1500 events s^-1 diode^-1 for gamma Ori. For the integration times given above, we expect the lowest S/N (for alpha Del) to equal 180, 50 and 80 in the three respective passbands. Other stars, of course, will give better S/N because their count rates will be higher. The weakest line that will probably be registered in this program is the 1265 A line of Si II*. If a partially ionized medium were to have n_e = 0.1 cm^-3, a value that is enough to modify an interpretation of the C I excitation, we would expect to find W_Lambda = 0.7 mA, assuming a typical depletion of Si in the low-density ISM. Again, for the faintest star at this wavelength in the survey (Alpha Del), this line would be detected at the 3Sigma level if the absorption feature were spread over 6 km s^ -1 (i.e., 2 diodes). An unusual situation is presented by lambda Lup, which turns out to be a visual binary where the primary and secondary have both a small magnitude difference and a small separation. The images of both stars will pass through the LSA. To minimize the confusion during acquisition and a loss of resolution for the spectrum, it is important to have the GHRS detector y axes nearly aligned with the line joining the two stars. For the epoch 1996.75, the secondary will be at a position angle = 204.4 degrees and an angular separation of 0.296 arc sec from the primary. An ORIENT special requirement has been specified for the visit to lambda Lup. No wavelength calibration exposures beyond the ususal SPYBAL exposures should be necessary for this program. Real Time Justification ----------------------- None. None. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6415( 7) - 10/22/96 09:08 - [ 1] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD35468 STAR,B0-B2 III-I RA=5H 25M 7.80S +/- 0.02S, 2000 V=+18 V = 1.636+/-0.007 DEC=+6D 20' 58.6" +/- 0.2" TYPE=B2III B-V = -0.224+/-0.008 E(B-V) = 0.02+/-0.02 F(1565)=1.52+/-0.05e-8 Comments: Coordinates are from Hipparcos INCA Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 -0.0006 -0.014 0.0 2 HD81188 STAR,B0-B2 V-IV RA=9H 22M 6.82S +/- 0.02S, DEC=- 2000 V=+22 V = 2.49+/-0.005 55D 0' 38.6" +/- 0.2" TYPE=B2IV-V B-V = -0.183+/-0.010 E(B-V) = 0.04+/-0.02 F(1565)=5.85+/-0.1e-9 Comments: Coordinates are from Hipparcos INCA Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 -0.0009 0.0009 0.0 3 HD133955 STAR,B3-B5 V-IV RA=15H 8M 50.63S +/- 0.02S, DEC= 2000 V=+10 V = 4.055+/-0.004 -45D 16' 47.3" +/- 0.2" TYPE=B3V B-V = -0.162+/-0.002 E(B-V) = 0.04+/-0.02 F(1565)=1.36+/-0.1e-9 Comments: VISUAL BINARY: DELTA M = 0.2 MAG. SECONDARY LOCATED AT THETA = 204.4D AND A DISTANCE OF 0.296" FROM THE PRIMARY. MAGNITUDES AND FLUXES FOR SUM OF BOTH STARS. Coordinates are from Hipparcos INCA Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 -0.0013 -0.018 0.0 4 HD186882 STAR,B6-B9.5 III-I RA=19H 44M 58.57S +/- 0.02S, 2000 V=-20 V = 2.86+/-0.031 DEC=+45D 7' 50.3" +/- 0.2" TYPE=B9.5IV+F1V B-V = -0.002+/-0.003 E(B-V) = 0.03+/-0.02 F(1565)=4.91+/-0.1e-10 Comments: Coordinates are from Hipparcos INCA Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0052 0.037 0.0 5 HD196867 STAR,B6-B9.5 V-IV RA=20H 39M 38.3S +/- 0.02S, 2000 V=-3 V = 3.771+/-0.004 DEC=+15D 54' 43.3" +/- 0.2" TYPE=B9IV B-V = -0.057+/-0.005 E(B-V) = 0.00+/-0.02 F(1565)=3.80+/-0.1e-10 Comments: Coordinates are from Hipparcos INCA Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0046 -0.002 0.0 6415( 7) - 10/22/96 09:08 - [ 2] 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 HD35468 HRS ACQ 2.0 MIRROR-A2 1 1.8 S ONBOARD ACQ FOR 2-4 Comments: STEP-TIME=0.2S PRED. COUNT RATE = 3.4E5 C/S (INCL. DEAD TIME CORR.) ------------------------------------------------------------------------------------------------------------------------------------ 2 HD35468 HRS ACCUM 2.0 ECH-B 1808 FP-SPLIT=STD 1 1088 S ------------------------------------------------------------------------------------------------------------------------------------ 3 HD35468 HRS ACCUM 2.0 ECH-A 1253 FP-SPLIT=STD 1 761.6 S ------------------------------------------------------------------------------------------------------------------------------------ 4 HD35468 HRS ACCUM 2.0 ECH-A 1262.5 FP-SPLIT=STD 2 761.6 S ------------------------------------------------------------------------------------------------------------------------------------ 6415( 7) - 10/22/96 09:08 - [ 3] Visit: 02 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 HD81188 HRS ACQ 2.0 MIRROR-A2 1 1.8 S ONBOARD ACQ FOR 2-4 Comments: STEP-TIME=0.2S PRED. COUNT RATE = 2.8E5 C/S (INCL. DEAD TIME CORR.) ------------------------------------------------------------------------------------------------------------------------------------ 2 HD81188 HRS ACCUM 2.0 ECH-B 1808 FP-SPLIT=STD 1 1305.6 ------------------------------------------------------------------------------------------------------------------------------------ 3 HD81188 HRS ACCUM 2.0 ECH-A 1253 FP-SPLIT=STD 1 761.6 S ------------------------------------------------------------------------------------------------------------------------------------ 4 HD81188 HRS ACCUM 2.0 ECH-A 1262.5 FP-SPLIT=STD 2 870.4 S ------------------------------------------------------------------------------------------------------------------------------------ 6415( 7) - 10/22/96 09:08 - [ 4] Visit: 03 Visit Requirements: ORIENT 54D TO 84D On Hold Comments: Additional Comments: ORIENTATION REQ. IS TO PUT GHRS DETECTOR Y AXIS ALONG LINE JOINING PRIMARY AND SECONDARY OF THIS VISUAL BINARY WITH A COMPUTED THETA = 204.4D, SEPARATION = 0.296" AND DELTA M = 0.3 MAG. NOTE THAT 180D FROM SPECIFIED ORIENTATION IS JUST AS GOOD. (BUT RPS2 INDICATES THAT IT'S UNSCHEDULABLE Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HD133955 HRS ACQ 2.0 MIRROR-A2 1 1.8 S ONBOARD ACQ FOR 2-4 Comments: STEP-TIME=0.2S PRED. COUNT RATE = 1.1E5 C/S (INCL. DEAD TIME CORR.) ------------------------------------------------------------------------------------------------------------------------------------ 2 HD133955 HRS ACCUM 2.0 ECH-B 1808 FP-SPLIT=STD 1 1196.8 ------------------------------------------------------------------------------------------------------------------------------------ 3 HD133955 HRS ACCUM 2.0 ECH-A 1253 FP-SPLIT=STD 1 652.8 S ------------------------------------------------------------------------------------------------------------------------------------ 4 HD133955 HRS ACCUM 2.0 ECH-A 1262.5 FP-SPLIT=STD 2 870.4 S ------------------------------------------------------------------------------------------------------------------------------------ 6415( 7) - 10/22/96 09:08 - [ 5] Visit: 04 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 HD186882 HRS ACQ 2.0 MIRROR-A2 1 1.8 S ONBOARD ACQ FOR 2-4 Comments: STEP-TIME=0.2S PRED. COUNT RATE = 6.9E4 C/S (INCL. DEAD TIME CORR.) ------------------------------------------------------------------------------------------------------------------------------------ 2 HD186882 HRS ACCUM 2.0 ECH-B 1808 FP-SPLIT=STD 1 1196.8 ------------------------------------------------------------------------------------------------------------------------------------ 3 HD186882 HRS ACCUM 2.0 ECH-A 1253 FP-SPLIT=STD 1 652.8 S ------------------------------------------------------------------------------------------------------------------------------------ 4 HD186882 HRS ACCUM 2.0 ECH-A 1559.5 FP-SPLIT=STD 2 870.4 S ------------------------------------------------------------------------------------------------------------------------------------ 6415( 7) - 10/22/96 09:08 - [ 6] Visit: 05 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 HD196867 HRS ACQ 2.0 MIRROR-A2 1 1.8 S ONBOARD ACQ FOR 2-4 Comments: STEP-TIME=0.2S PRED. COUNT RATE = 3.8E4 C/S (INCL. DEAD TIME CORR.) ------------------------------------------------------------------------------------------------------------------------------------ 2 HD196867 HRS ACCUM 2.0 ECH-B 1808 FP-SPLIT=STD 1 1088 S ------------------------------------------------------------------------------------------------------------------------------------ 3 HD196867 HRS ACCUM 2.0 ECH-A 1253 FP-SPLIT=STD 1 761.6 S ------------------------------------------------------------------------------------------------------------------------------------ 4 HD196867 HRS ACCUM 2.0 ECH-A 1262.5 FP-SPLIT=STD 2 761.6 S ------------------------------------------------------------------------------------------------------------------------------------ 6415( 7) - 10/22/96 09:08 - [ 7] Summary Form for Proposal 6415 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FP-SPLIT=STD ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category INTERSTELLAR MEDIUM ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2-4 ORIENT 54D TO 84D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-A2 ECH-B ECH-A ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD35468 HD81188 HD133955 HD186882 HD196867 ------------------------------------------------------------------------------------------------------------------------------------