6116(  9) - 07/31/95 14:43  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      6116
                                                       Version: 9
                                                       Check-in Date: **********

1.Proposal Title:
A search for the optical counterpart of the triple pulsar system PSR B1620-26 in M4
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2. Scientific Category   3. Proposal For  4. Cycle
Hot Stars                   GO               5
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5. Investigators
                                                                                     Contact?
    PI: Charles Bailyn                    Yale University
   CoI: Terrence M. Girard                Yale University                               N
   CoI: Frederic A. Rasio                 Institute for Advanced Study                  N
   CoI: Brian Yanny                       Fermi National Accelerator Laboratory         N

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6. Abstract

The accurate optical identification of the millisecond pulsar B1620-26 in the
globular cluster M4 (NGC6121) is crucial for determining the nature of this unique
hierarchical triple system.  Ground based imaging has revealed a potential
condidate, which may well be blended and/or a chance superposition.  We propose a
short WF/PC II observation which will unambiguously demonstrate whether an optical
counterpart exists, and if so, determine its magnitude and color.  The results will
be of great interest both for issues relating to the origin of pulsars, and in terms
of dynamical processes in globular clusters.
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                                                6116(  9) - 07/31/95 14:43  - [  2]

  Observations Description
  ------------------------

     We wish to detect or rule out the presense of a V=23th magnitude object without
     overly saturating nearby stars (1sec point 5 to 5'', corresponding to 30 to 100
     pixels from the target), which have magnitude 16. A 16th magnitude star
     saturates in the F555W filter (hereafter ``V'') in about 10 seconds (based on
     the exposure time calculations given in the WFPC-2 Instrument Handbook and on
     previous WFPC-2 images of the globular cluster M30, Yanny et al 1994b).  To
     reach V=23 with S/N=10, requires a total exposure of about 120 secs.
     Additionally, to determine the color of the object, which if a star, will be
     quite red, we request I (F814W) exposures of the same field. The sensitivity of
     the WFPC-2 in the I filter is about half that of the V band and so we require
     similar exposure times for objects with colors expected to have V-I~ 1.
     Several short exposures in F555W and F814W will be used to model the PSF and
     subtract the wings of the target's bright neighboring stars.  Then a series of
     longer exposure in both filters will be used to obtain the full required S/N
     for faint objects. Thus we request three 10 sec and three 40 second exposures
     in each filter, for a total exposure time of 5 minutes.  With acquisition time
     of 12 minutes, and overhead of 3* 12=36  minutes, all the exposures can be
     completed in one 53 minute orbit.

  Real Time Justification
  -----------------------

     In order to avoid bleeding and diffraction spikes from a very bright saturated
     V=13.0 mag star onto the target 5 arcseconds distant, we request that the roll
     angle of the spacecraft be such that the orientation of the y-axis of the WFPC-
     2/PC CCD with respect to the east-west axis be less than 25 degrees. New
     photographic plates are scheduled to be taken with the double astrograph of the
     Yale Southern Observatory at El Leoncito Argentina next spring.  This plate
     material will serve to tie the optical images to a standard system without the
     need for proper motion corrections. We also hope to obtain improved ground
     based CCD images (deeper and better seeing) when the WIYN telescope comes on
     line next spring. With these new data, we expect to be able to tie the PC data
     requested here to the FK5 astrometric system with an accuracy better than 0sec
     point1.  Thus the dominant systematic error in locating the pulsar position on
     the PC image will be the accuracy of the transformation between the solar
     system ephemeris used to determine the pulsar timing positions, and the FK5
     system.  This error is estimated to be ~ 0sec point1 (Standish, 1990).

  Calibration Justification
  -------------------------


  Additional Comments
  -------------------

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                                                6116(  9) - 07/31/95 14:43  - [  3]

  Data Distribution

  Media:           8MM
  Blocking Factor: 10
  Ship To:         PI_Address
  Ship Via:        UPS
  Email:                                                              ,
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                                                6116(  9) - 07/31/95 14:43  - [  4]

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   PSR-B1620-26 STAR, pulsar            RA=16H 23M 38.22S +/- 0.1S, DEC= 2000              V=21 +/- 2
                                         -26D 31' 53.7" +/- 0.2", PLATE-
                                         ID=02J6

                                                6116(  9) - 07/31/95 14:43  - [  5]

Visit: 01
     Visit Requirements:  ORIENT 18D TO 72D
     On Hold Comments:    <none>
     Additional Comments: The orientation requirement is to avoid bleeding from a 13th magnitude star
                          located 5" due north of the target.  Also, we are informed by our scientific
                          contact (Howard Lanning) that the diagnostic relating to overfull tape
                          recorders can be waived for orbits containing up to 14 WFPC2 images, since a
                          second tape recorder was installed during the refurbishment mission.


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        PSR-B1620-  WFPC2  IMAGE   PC1     F555W                            1   40S
         26
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2        PSR-B1620-  WFPC2  IMAGE   PC1     F814W                            1   40S
         26
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3        PSR-B1620-  WFPC2  IMAGE   PC1     F555W                            1   3S
         26
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4        PSR-B1620-  WFPC2  IMAGE   PC1     F555W                            1   3S
         26
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5        PSR-B1620-  WFPC2  IMAGE   PC1     F555W                            1   3S
         26
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6        PSR-B1620-  WFPC2  IMAGE   PC1     F555W                            1   40S
         26
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7        PSR-B1620-  WFPC2  IMAGE   PC1     F555W                            1   120S
         26
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11       PSR-B1620-  WFPC2  IMAGE   PC1     F814W                            1   120S
         26
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12       PSR-B1620-  WFPC2  IMAGE   PC1     F814W                            1   40S
         26
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13       PSR-B1620-  WFPC2  IMAGE   PC1     F814W                            1   3S
         26
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14       PSR-B1620-  WFPC2  IMAGE   PC1     F814W                            1   3S
         26
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15       PSR-B1620-  WFPC2  IMAGE   PC1     F814W                            1   3S
         26
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16       PSR-B1620-  WFPC2  IMAGE   PC1     F814W                            1   40S
         26
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17       PSR-B1620-  WFPC2  IMAGE   PC1     F555W                            1   40S
         26
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                                                6116(  9) - 07/31/95 14:43  - [  6]

            Summary Form for Proposal  6116

  Item                         Used in this proposal
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  Configurations               WFPC2
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  Opmodes                      IMAGE
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  Proposal Category            GO
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  Scientific Category          Hot Stars
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  Special Requirements         ORIENT 18D TO 72D
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  Spectral Elements            F555W F814W
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  Target Names                 PSR-B1620-26
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