6655( 3) - 02/04/96 17:05 - [ 1]
PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only
ID: 6655
Version: 3
Check-in Date: 04-Feb-1996
17:02:42
1.Proposal Title:
SEARCH FOR INTERSTELLAR CH_2 IN THE SPECTRUM OF HD 154368
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2. Scientific Category 3. Proposal For 4. Cycle
INTERSTELLAR MEDIUM GO 6
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5. Investigators
Contact?
PI: Andrew M. Smith The Goddard Space Flight Center
CoI: Frederick Bruhweiler The Catholic University of America N
CoI: Cheng-Hsuan Lyu STX-Hughes N
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6. Abstract
We propose to search for the important interstellar molecule, CH_2, which is
predicted to be very abundant in theoretical models of gas phase chemistry in
diffuse and translucent clouds, but which has not yet been detected. Our goal is to
detect the absorption band spectrum in the wavelength interval extending from 139 nm
to 142 nm to both establish the presence of CH_2 and to clarify details in the
possible reaction channels leading to the abundant and often observed molecule, CH.
The target star is HD 154368 (O9.5Iab; 6.1 m_V) which exhibits a well observed
interstellar molecular spectrum. IUE observations show a locally smooth continuum at
the locations of the CH_2 bands, and indicate that significant conclusions can be
made with an exposure time of 258 minutes.
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Observations Description
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We will obtain a medium resolution spectrum of HD 154368 with the GHRS using
the G160M grating. The spectrum will be centered on the diode array so that the
CO 4,0 vibrational band at 141.9nm and the CH_2 band at 139.68nm will be
recorded simultaneously. Since the interstellar spectrum is well observed at
optical and radio wavelengths we will only observe the UV spectrum in this one
band. The CO band will be saturated, but a velocity dispersion determined from
radio measurements together with the band equivalent width will permit us to
derive a CO column density, which can be used to compare our results with other
observations. We estimate that use of the N2 acqusition mirror will yield a
count rate per pixel of 289,600 counts/second. (The star is bright at V=6.1,
but it is heavily reddened with E(B-V)=0.82). A 0.2 second integration time
yields a count per pixel of 57,920. Thus, a standard target acqusition time of
11 minutes is in order. We plan to use the large aperture, the FP-SPLIT=STD,
STEP-PATT=5, COMB=4 options, and the same initial carrousel position for each
exposure. After reconstituting the spectrum from all the FP-SPLIT subspectra
the spectra will be re-binned in a 2 to 1 or even larger ratio. This is done to
increase the S/N in a situation where intrinsic resolution can be sacrificed to
a limited extent. The 141.45nm feature we are trying to detect is about 0.05nm
wide and is thus oversampled by approximately 10 pixels. Tomographic techniques
developed by our group (Lyu, Bruhweiler, and Smith 1995) will be used to reduce
the effects of fixed pattern noise in our data. Taking into account the guide
star aquisition (7.5 min, 6 min), the target acquisition (11 min) and the ACCUM
mode overhead (4 min) leads to an 8 orbit visit in order to obtain 258 minutes
of on-target observing time. Since we will observe the 4,0 band of the CO A-X
system and the S I resonance line at 140.15 nm we do not need a WAVECAL. These
interstellar features will provide adequate fiducials to align the subexposures
of the FT-SPLIT procedure. We will have to use the GHRS capability to pause an
observation across orbits to obtain our exposure time objective within the 8
orbit limit. High signal-to-noise is crucial. This implies a need for the best
possible counting statistics. Hence, minimum impact from the SAA is highly
desirable.
Real Time Justification
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There are no special requirements for this observation. No supporting
observations are required.
Calibration Justification
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Special calibrations are not required for this observation.
Additional Comments
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Data Distribution
Media: 8MM
Blocking Factor: 10
Ship To: PI_Address
Ship Via: UPS
Email: ,
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TARGET LIST a) Fixed Targets
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Tar| Target | Target | Target |Coord | Radial | Flux data
No | Name | Description | Position |Eqnx | Vel. |
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1 HD154368 STAR,Supergiant O RA=17H 06M 28.40S +/- 0.02S, 2000 V=+13 V = 6.13+/-0.03
DEC=-35D 27' 03.9" +/- 0.2" TYPE=O9.5IAB
B-V = +0.48+/-0.03
E(B-V) = 0.82+/-0.04
F(1414)=8.0+/-0.8e-12
Comments: Flux from IUE observation, SWP 41365
Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec)
2000 0.0007 -0.002 0.0
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Visit: 01
Visit Requirements: <none>
On Hold Comments: <none>
Additional Comments: Assume one visit, eight contiguous orbits. The following program occupies
eight full orbits with twelve 1740.8s exposures and one 544.0s exposure. The
last exposure consists of five FT-SPLIT cycles each 108.8s in duration. The
available time in the final (eighth) orbit is 0.8s short of including a 6th
FT-SPLIT cycle. It is hoped that by creative scheduling a 6th FT-SPLIT cycle
can be included in the final exposure.
Exposures
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Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special
Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements
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1 HD154368 HRS ACQ 2.0 MIRROR-N2 BRIGHT=RETURN,S 1 1.8S ONBOARD ACQ FOR 2-5
EARCH-SIZE=3
Comments: STEP-TIME = 0.2 S
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2 HD154368 HRS ACCUM 2.0 G160M 1408.0 FP- 6 1740.8S
SPLIT=STD,STEP-
PATT=5,COMB=FOU
R
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3 HD154368 HRS ACCUM 2.0 G160M 1408.0 FP- 1 1740.8S
SPLIT=STD,STEP-
PATT=5,COMB=FOU
R,SPYBAL=YES
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4 HD154368 HRS ACCUM 2.0 G160M 1408.0 FP- 5 1740.8S
SPLIT=STD,STEP-
PATT=5,COMB=FOU
R
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5 HD154368 HRS ACCUM 2.0 G160M 1408.0 FP- 1 544.0S
SPLIT=STD,STEP-
PATT=5,COMB=FOU
R
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Summary Form for Proposal 6655
Item Used in this proposal
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Configurations HRS
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Opmodes ACQ ACCUM
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Optional Parameters BRIGHT=RETURN SEARCH-SIZE=3 COMB=FOUR SPYBAL=YES STEP-PATT=5 FP-SPLIT=STD
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Proposal Category GO
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Scientific Category INTERSTELLAR MEDIUM
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Special Requirements ONBOARD ACQ FOR 2-5
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Spectral Elements MIRROR-N2 G160M
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Target Names HD154368
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