Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 3851c Report Date: 09-May-96:18:53 Version: ********** Check-in Date: ********** 1.Proposal Title: DEEP SEARCH FOR CVS AND COMPACT BINARIES IN THE COLLAPSED CLOSE GLOBULAR NGC6397 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID STELLAR POPULATIONS GO 1 Long Term yrs 3851 Sub Category GLOBULAR CLUSTERS ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Prof Jonathan E. Grindlay HARVARD UNIVERSITY USA 617-495-7204 PROF ------------------------------------------------------------------------------------ 7. Abstract We propose to obtain deep H-ALPHA and R exposures with the PC on the closest core- collapsed globular cluster, NGC 6397. This will allow the most sensitive search to date for cataclysmic variables (CVs) and other compact binaries in the core of this dynamically evolved cluster. The search is sensitive to CVs with increasingly faint luminosities, which are distinguished by increasingly strong H-ALPHA emission, and thus will provide the most definitive test yet for the possible deficit of CVs in globulars. Comparison of these images with archived HST uv images that we have recently received, with our ground-based H-ALPHA and uv images (for regions outside the core), and with our ROSAT x-ray images will allow the long-standing question of the nature of the population of low luminosity x-ray sources in globulars (CVs \?) to be probed in unprecedented detail. The search will also be sensitive to the populations and distributions of blue subdwarfs (H-ALPHA absorbers, and possibly WD- stellar merger products) and millisecond pulsar emission nebulae, so that models of binary formation and evolution in globulars will be tested. ------------------------------------------------------------------------------------ 8. Scientific Key Words: GLOBULAR CLUSTERS ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 5.43 par: 0 10. Num targs pri: 3 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: WF/PC ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 3851c PI: Prof Jonathan E. Grindlay Proposal Title: DEEP SEARCH FOR CVS AND COMPACT BINARIES IN THE COLLAPSED CLOSE GLOBULAR NGC6397 ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Prof Jonathan E. Grindlay HARVARD UNIVERSITY U.S.A. Charles D. Bailyn YALE UNIVERSITY U.S.A. Paul J. Callanan HARVARD UNIVERSITY U.S.A. Haldan N. Cohn INDIANA UNIVERSITY U.S.A. Adrienne M. Cool HARVARD UNIVERSITY U.S.A. Phyllis M. Lugger INDIANA UNIVERSITY U.S.A. ------------------------------------------------------------------------------------ 3. Description of proposed observations. The proposed observations are direct imaging with the Planetary Camera (PC) of the core of the nearby globular cluster NGC 6397. The entire observation will be centered on the cluster core center for maximum sensitivity. Two filters will be used: the F656N filter for images in H-ALPHA and the F675W filter for comparison continuum images in the (approximate) R band. Based on our preliminary program on the more distant cluster NGC 6752, we need a ratio of at least 10:1 in favor of the narrow band H-ALPHA filter. The total H-ALPHA exposure time to achieve the desired sensitivity limit of absolute magnitude Mv = 11 in the cluster is about 18 ksec. In order to keep cosmic rays manageable, and to minimize stellar saturation, this total exposure will be obtained in 1000 sec integrations. The R integration times will be 100 sec, with several 10 sec exposures to remove saturated stars. The narrow band-pass of the F656N filter means that short exposures will not be necessary. Thus the total program will need about 19 ksec of total integration time, or just under 10 hours of spacecraft time. To minimize uncertainties due to flat fielding in the H-ALPHA filter as well as imperfect bias subtraction we shall divide our total number of exposures into 3 groups. For each group, we shall offset point the telescope by about 4 arcsec, so that the total observation is distributed over three distinct regions on the CCD: thus differences in flat fielding or residual Newton's rings in the stacked H- ALPHA images will be removable. ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. HST is absolutely essential for this project. Even with the degraded psf of the telescope and conditions of crowded-field stellar photometry (the NGC 1850 tests of Holtzman et al 1991), the 0.1 arcsec (radius) cores of the stellar images contain enough of the total light (15%) that they can be photometered to about 0.1 mag down to 21st mag (cf. Holtzman et al 1991). This is the apparent magnitude limit needed to reach absolute mag Mv = 9 in NGC 6397. Even at Mv = 11 in NGC 6397, the NGC 1850 photometry results of Holtzman et al would suggest errors of less than 0.3 mag, or enough for a threshold "detection" (3-4 sigma) of a CV with equivalent width of about 30 angstroms. Thus HST can carry out the color-excess photometry to sufficient accuracy to detect CVs with Mv = 9-11 and H -ALPHA equivalent width below 30 angstroms in the core of NGC 6397 in the exposure times requested. For comparison, the extensive ground-based H-ALPHA vs. narrow band R photometry (as well as UBV) we have carried out with the 4m-PFCCD at CTIO (cf. Grindlay et al 1991) will only reach limits of Mv = 9 (for the same H-ALPHA equivalent width) and this only beyond 3-4 core radii where stellar crowding does not severely affect our deep 1 arcsec seeing images. The proposed program for NGC 6397 will be 5 magnitudes deeper (i.e. from Mv = 6 to Mv = 11) than our similar observations obtained on the globular cluster NGC 6752 in Cycle 1 (Program P2555). The proximity of NGC 6397 (factor of 2 closer than NGC 6752) makes it the very best globular for a very deep CV search. Page 3 ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. In order to ensure that the multiple ROSAT error circles are well placed on two of the CCDs in the HST/PC, we are requesting a specific detector orientation (with a +/- 10 degree tolerance). ------------------------------------------------------------------------------------ 6. Description of special calibration exposures. In order to limit the effect of read noise on our H-ALPHA images, we are requesting that our 1000s exposures not be divided into shorter exposures. We are confident that with the 3 sets of 6 exposures we are planning, we will be able to remove cosmic rays effectively. Our experience with our 1300s H-ALPHA exposures of NGC6752 with the HST/PC support this view. ------------------------------------------------------------------------------------ 7. Data reduction and analysis plans. We propose to reduce these frames using standard reduction techniques and STScI tools and packages. We shall employ standard IRAF tools and packages we have used extensively in analysis of our gound-based CTIO H-ALPHA vs. R data on globulars. We shall analyze our data by a variety of techniques. First, we shall employ the ratio or colormap techniques we have developed (Bailyn et al 1988, Cederbloom et al 1991) on our CTIO and CFHT data. Second, we shall use DAOPHOT for stellar photometry. Again, we have considerable experience with this and will coordinate our efforts with both P. Stetson and STScI (e.g. the modifications mentioned by Holtzman et al 1991) for optimum choices of psf parameter fitting. Further, we shall develop new techniques of analysis of stellar image shape and sharpness to be combined with DAOPHOT analysis. Finally, we have developed techniques to simulate HST PC images of clusters in order to test and refine our analysis procedures. We have recently received archived FOC frames (uv filters) on NGC 6397. These data will be extremely important for final analysis of the proposed investigation (cf. question 8) since uv excess colors and H-ALPHA emission are the hallmark of CVs vs. other H-ALPHA objects likely to be found (e.g. planetaries or possibly millisecond pulsar nebulae). ------------------------------------------------------------------------------------ 8. Additional comments or special requests. This proposed observation will be supported by our continuing analysis of our ground-based H-ALPHA vs. narrow-band R images (CTIO/PF-CCD) of NGC 6397. As mentioned in question 4 above, these data are sensitive to about Mv = 9 and H- ALPHA equivalent width 30-40 angstrom at radii OUTSIDE about 3-4 core radii in the cluster, or at about 30 arcsec (i.e. the edge of the PC field) given the approximate core radius recently derived by Auriere et al (1991). Thus the CTIO data will complement (though not be as sensitive as) the HST data, particularly at the edge of the field. By using the HST data to derive stellar positions (e.g. with DAOPHOT's FIND routine), however, the CTIO data can be searched deeper than the overlapping stellar images would otherwise allow. This was pointed out in our proposal by Bailyn et al for use of archival HST/FOC images on NGC 6397. By comparing the HST/PC H-ALPHA images with the CTIO H-ALPHA images, we can not only gain additional sensitivity (at least in the outer parts of the core) for the CTIO data, but also search for large amplitude variability such as could be expected if dwarf novae are present in the cluster CV population. Page 4 ------------------------------------------------------------------------------------ 9. Description of previous HST work. The PI and the Co-I's for this proposal are currently recalibrating the data from their Cycle 1 observation of the globular cluster NGC 6752. These data were obtained in Program P2555 entitled "Cataclysmic Variables and Millisecond Pulsars in Globular Cluster Cusps". The initial calibration suffered from the use of an incorrect bias frame. Our recalibration is using a new bias frame together with the latest flat available. Preliminary colormap and DAOPHOT analysis of the data, prior to recalibration, revealed no obvious CV candidates down to an approximate limiting magnitude of about Mv = 6. However this result is extremely preliminary and requires considerable and careful full analysis, which we will perform following the recalibration. ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). Each institution will supply already-obtained computing and data handling facilities to process these extensive data. Sun Workstations will be used at Harvard and at Indiana, and VAX workstations at Yale. Each instituion has full IRAF and DAOPHOT capabilities. Graduate students will work with the PI and Co-I's on reducing and analyzing fully these data. These students will be partially supported by other research funds. ------------------------------------------------------------------------------------ 11. Address Information Name: JONATHAN E. GRINDLAY Category: PI Institution: HARVARD COLLEGE OBSERVATORY Address: CENTER FOR ASTROPHYSICS 60 GARDEN STREET City: CAMBRIDGE State: MA Zip Code: 02138 Country: U.S.A. Telephone: 617-495-7204 Telex (or e-mail): 921428 ------------------------------------------------------------------------------------ TARGET LIST a) Fixed Targets ID = 3851c [ 5] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data No | Name | Description | Position |Eqnx | Vel. |Prblm|REF| | | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6397-POS1 C,201 RA = 17H 40M 41.74S +/- J2000 1 V = 21 0.06S, DEC = -53D 40' 24.94" +/- 0.50" ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC6397-POS2 C,201 RA = 17H 40M 41.51S +/- J2000 1 V = 21 0.06S, DEC = -53D 40' 24.94" +/- 0.50" ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC6397-POS3 C,201 RA = 17H 40M 41.74S +/- J2000 1 V = 21 0.06S, DEC = -53D 40' 28.94" +/- 0.50" ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 3851c [ 6] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6397- PC IMAGE PC6-FIX F656N PRE-FLASH=YES 6 1000S 1 1 ORIENT 80.0D +/- POS1 CR-SPLIT=NO 10.0D CYCLE 2 /1-9 ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC6397- PC IMAGE PC6-FIX F675W PRE-FLASH=YES 2 10S 1 1 ORIENT 80.0D +/- POS1 10.0D ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC6397- PC IMAGE PC6-FIX F675W PRE-FLASH=YES 5 100S 1 1 ORIENT 80.0D +/- POS1 10.0D ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC6397- PC IMAGE PC6-FIX F656N PRE-FLASH=YES 6 1000S 1 1 ORIENT 80.0D +/- POS2 CR-SPLIT=NO 10.0D ------------------------------------------------------------------------------------------------------------------------------------ 5 NGC6397- PC IMAGE PC6-FIX F675W PRE-FLASH=YES 2 10S 1 1 ORIENT 80.0D +/- POS2 10.0D ------------------------------------------------------------------------------------------------------------------------------------ 6 NGC6397- PC IMAGE PC6-FIX F675W PRE-FLASH=YES 5 100S 1 1 ORIENT 80.0D +/- POS2 10.0D ------------------------------------------------------------------------------------------------------------------------------------ 7 NGC6397- PC IMAGE PC6-FIX F656N PRE-FLASH=YES 6 1000S 1 1 ORIENT 80.0D +/- POS3 CR-SPLIT=NO 10.0D ------------------------------------------------------------------------------------------------------------------------------------ 8 NGC6397- PC IMAGE PC6-FIX F675W PRE-FLASH=YES 1 10S 1 1 ORIENT 80.0D +/- POS3 10.0D ------------------------------------------------------------------------------------------------------------------------------------ 9 NGC6397- PC IMAGE PC6-FIX F675W PRE-FLASH=YES 5 100S 1 1 ORIENT 80.0D +/- POS3 10.0D ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 3851c [ 7] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations PC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters PRE-FLASH CR-SPLIT ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 5.43 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category STELLAR POPULATIONS ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category GLOBULAR CLUSTERS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 80.0D +/- 10.0D; CYCLE 2 /1-9 ORIENT 80.0D +/- 10.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F656N F675W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6397-POS1 NGC6397-POS2 NGC6397-POS3 ------------------------------------------------------------------------------------------------------------------------------------