! Hubble Space Telescope Cycle 6 (1996) Phase II Proposal Template ! $Id: 6797,v 5.1 1997/04/22 19:14:07 pepsa Exp $ ! ! Refer to the HST Phase II Proposal Instructions to fill this out ! ! Anything after a "!" is ignored, and may be deleted ! ! All keywords with multiple entries are comma delimited except the ! Visit_Requirements and Special_Requirements keywords which can be ! delimited with carriage returns or semi-colons, but not commas ! ! For help call your Program Coordinator: Karla Peterson ! Phone: 410-338-4774 , E-mail: peterson@stsci.edu ! Proposal_Information ! Section 4 Title: The Very-Low-Mass Star Content of the Globular Cluster NGC 6397 Proposal_Category: GO Scientific_Category: STELLAR POPULATIONS Cycle: 6 Investigators PI_name: Ivan R. King PI_Institution: University of California CoI_Name: Adrienne M. Cool CoI_Institution: San Francisco State University Contact: ! Y or N (designate at most one contact) CoI_Name: Jay Anderson CoI_Institution: University of California Contact: ! Y or N (designate at most one contact) CoI_Name: William van Altena CoI_Institution: YaleUniversity Contact: ! Y or N (designate at most one contact) CoI_Name: Giampaolo Piotto CoI_Institution: Universita di Padova Contact: ! Y or N (designate at most one contact) Abstract: ! Free format text (please update) Very few constraints exist on the population of very-low-mass stars in globular clusters. Observations with HST have come close to reaching the hydrogen-burning limit at the lower end of the main sequence in the globular cluster NGC 6397, the cluster with lowest distance modulus. Determination of the faintest part of the luminosity function is limited, however, by serious field-star contamination. This proposal is to take second-epoch images for determination of proper motions of individual faint stars. Tests of the positional accuracy that we now achieve show that over the time baseline that will be available by Cycle 6, separation of cluster members from field stars will be close to perfect. This will allow us to extend the measurable main-sequence luminosity function two magnitudes beyond its present limit, to I ~eq 25, or V ~eq 28, which corresponds to an absolute-magnitude of M_V ~eq 15.7. This will allow us to come closer than anyone ever has to the hydrogen-burning limit at the bottom of the main sequence. Questions ! Free format text (please update) Observing_Description: The second-epoch data would be taken with the F814W filter only, which is the filter for which our first-epoch material reaches the faintest stars. Additional observations in a second filter are not needed, as the existing V data already allow us to distinguish white dwarfs from red dwarfs for all stars detected in I. Since the majority of the stars in the first epoch were covered by 3 orbits' worth of exposure time, we propose 3 orbits again for the second epoch, in order to achieve the same astrometric accuracy. The area covered by our 1994 images is irregular, due to a series of shifts and two orientations, and covers an area equivalent to ~ 1.8 WFPC2 fields. Two pointings will be required to insure coverage of at least 85\% of the area that our first-epoch observations encompass, irrespective of the spacecraft orientation. Given that the cluster LF may be falling, it is important to cover a maximum of the area of the existing observations, so that membership can be assessed for as many stars as possible. The exposures in each orbit will be divided into one short (80s) and three long exposures. Considering overhead times and the 57-minute visibility time for this cluster, the long exposures will each be >= 12 min long. The short exposure will be used to correct saturated centers of the many bright stars in the images, a step that is absolutely essential in deriving the accurate PSFs that are needed for the astrometry. Each orbit will have a CR-split, but the 3 orbits at each pointing will be dithered with respect to each other, to combat the undersampling of the WFC. We know from our previous observations that we can do accurate astrometry down to I=25 (as demonstrated in Fig.\ 3). To maximize the time baseline, the observations would best be made late in Cycle 6. However, the project is feasible if carried out at any time in Cycle 6. Real_Time_Justification: None. Calibration_Justification: ! Move appropriate text from Real_Time_J ustification Additional_Comments: Fixed_Targets ! Section 5.1 Target_Number: 1 Target_Name: NGC6397-1 Alternate_Names: Description: STELLAR CLUSTER, Globular Cluster Position: RA=17H 41M 00.83S +/- 0.1S, ! Most common specification format DEC=-53D 43' 55" +/- 1" ! RA=0H 0M 0.00S +/- 0S, ! DEC=0D 0' 0.0" +/- 0", ! PLATE-ID=0000 Equinox: J2000 RV_or_Z: RA_PM: ! Units are seconds of time per year Dec_PM: ! Units are seconds of arc per year Epoch: Annual_Parallax: Flux: V=5.25 B-V=0.73 ! Include at least V and B-V Comments: Target_Number: 2 Target_Name: NGC6397-2 Alternate_Names: Description: STELLAR CLUSTER, Globular Cluster Position: RA=17H 41M 00.41S +/- 0.1S, ! Most common specification format DEC=-53D 42' 29" +/- 1" ! RA=0H 0M 0.00S +/- 0S, ! DEC=0D 0' 0.0" +/- 0", ! PLATE-ID=0000 Equinox: J2000 RV_or_Z: RA_PM: ! Units are seconds of time per year Dec_PM: ! Units are seconds of arc per year Epoch: Annual_Parallax: Flux: V=5.25 B-V=0.73 ! Include at least V and B-V Comments: ! This is a template for a single visit containing a single exposure ! Repeat exposure and visit blocks as needed Visits ! Section 6 Visit_Number: 1 Visit_Requirements: ORIENT 345.7D TO 345.9D ! Section 7.1 AFTER 1-JAN-97 ! Uncomment or copy visit level special requirements needed ! Most of these requirements (including ORIENT) will limit scheduling ! PCS MODE [Fine | Gyro] ! GUIDing TOLerance ! DROP TO GYRO IF NECESSARY [NO REACQuisition] ! ORIENTation TO ! ORIENTation TO FROM ! ORIENTation TO FROM NOMINAL ! SAME ORIENTation AS ! CVZ ! PARallel ! SCHEDulability ! AFTER [BY [TO ]] ! AFTER ! BEFORE ! BETWEEN AND ! GROUP WITHIN