Photometry and Astrometry of SIM Planetquest Globular

Cluster Targets T. M. Girard (Yale), A. Sarajedini (U. Florida), B. Chaboyer (Dartmouth)

Table 1. Target ClustersOVERVIEW

Our SIM Planetquest key project† will obtain accurate parallaxes to a number of Population II objects resulting in a significant improvement in the calibration of the Pop-II distance scale and greatly reducing the uncertainty in the estimated ages of the oldest stars in our galaxy. In addition to field-star targets, we will obtain parallaxes with an accuracy of 5 microarcseconds for a sample of 21 relatively nearby globular clusters. The distances to these globular clusters will be determined to an accuracy of 1 to 5%. SIM Planetquest will observe 5 - 10 bright giant-branch or horizontal-branch stars in each globular cluster. To maximize the efficiency of our SIM Planetquest observations, we are obtaining wield field-of-view photometry and astrometry of our target globular clusters. Images have been obtained with Mosaic CCD cameras on the CTIO 4-m (36 x 36 arcmin field of view) and WIYN 0.9-m on Kitt Peak (59 x 59 arcmin).

To date, observations of 19 globular clusters have been obtained. Photometry of all stars in the field of view is obtained in the standard BV system and used to compile a list of potential target stars for SIM Planetquest. An astrometric solution is obtained using the UCAC2 catalog, leading to positions with a precision of 25 to 40 milli-arcseconds for well-measured stars. These positions help facilitate multi-fiber spectroscopic observations of potential target stars. The spectroscopic monitoring will assure that the stars are cluster members of the expected luminosity class and, importantly, to identify troublesome binaries among the candidates. Our photometry allows us to select candidate stars. Accurate astrometry is necessary for efficient throughput while performing the multi-object fiber spectroscopy.___________________________________________________________________________________________

† http://www.dartmouth.edu/~sim/

NGC Name R (kpc) VHB[Fe/H]

104 47 Tuc 4.3 14.06 -0.76288 --- 8.1 15.30 -1.24362 --- 8.3 15.43 -1.16

3201 --- 5.1 14.80 -1.484590 M68 10.1 15.68 -2.065139 ω Cen 5.1 14.53 -1.625272 M3 10.0 15.65 -1.575904 M5 7.3 15.07 -1.296205 M13 7.0 14.90 -1.546218 M12 4.7 14.60 -1.486341 M92 8.1 15.10 -2.296352 --- 5.6 15.13 -0.706362 --- 7.5 15.34 -1.066397 --- 2.2 12.87 -1.956541 --- 7.4 15.30 -1.836652 --- 9.4 15.85 -0.856723 --- 8.6 15.50 -1.126752 --- 3.9 13.70 -1.556809 M55 5.3 14.40 -1.816838 M71 3.8 14.44 -0.737099 M30 7.9 15.10 -2.12

AN OBSERVATIONAL EXAMPLE: M3

Photometric reductions are performed using DAOPHOT and standard mosaic techniques, each chip being calibrated separately. Calibration standards for each cluster are culled from the literature. Figure 1 shows a sample observation, that of M3 (NGC 5272) taken with the Mosaic camera on the KPNO 0.9-m telescope. From three dithered pointings and observations in B and V filters, the CMD’s shown in Figure 2 were constructed.

Astrometric reductions were made based on the DAOPHOT centers, corrected for the Mosaic camera’s chip-to-chip offsets/orientations and for telescope (cubic) distortion, and then placed on the system of the ICRS via UCAC2 reference stars. Originally, the chip-to-chip relative positions were adopted from the WCS information stored in the headers of the FITS files, provided by the NOAO observing pipeline for the Mosaic camera. However, taking advantage of the three different pointings of our M3 observations, as shown in Figure 3, it was possible to make an internal check on the derived positions. In Figure 4, top panels, we show the differences in α,δ’s derived independently from two different pointings, and assuming the pipeline (Davis) chip-to-chip relative positions. The variations and discontinuities across chip boundaries were unsatisfactory. Fortunately, Platais et al. 2002 have made a more detailed investigation into the Mosaic camera’s geometry. A re-reduction of the M3 frames using the Platais et al. chip-to-chip relative positions yields superior astrometric results, as illustrated in the bottom panels of Figure 4. We suggest that whenever optimal astrometric precision is desired while reducing Mosaic-camera data, the Platais et al. chip constants should be employed.___________________________________________________________________________________________

Davis, L. E. 1998, in ASP Conf. Ser. 145, Astronomical Data Analysis Software and Systems VII, ed. R. Albrecht, R. N. Hook, & H. A. Bushouse (San Francisco: ASP), 184

Platais, I., et al. 2002, Astronomical Journal 124, 601 Figure 1. Mosaic CCD frame of M3 taken with the KPNO 0.9-m. The field of view is 59 x 59 arcmin. Candidate giant-branch and horizontal-branch stars are preferentially selected from the outer portions of the cluster. This minimizes the crowdedness and risk of superposition of a fainter second star into the SIM Planetquest observing pupil.

Figure 2. BV color magnitude diagrams of stars in four areas, annuli with various radii, around the center of M3. The annuli are also indicated in Figure 1. As can be seen, candidate giant-branch and horizontal-branch stars can be identified well away from the crowded cluster core.

PHOTOMETRY ASTROMETRY

Figure 3. Superimposed DAOPHOT detections from three different pointings within our M3 observation set. The exposures are offset from one another by 10 to 15 arcmin. Spurious detections along the borders of the chips have been included to better show the positions of the chip gaps. 1 pixel = 0.423 arcsec.

Figure 4. Differences in celestial coordinates derived from two of the telescope pointings shown in Figure 3, as a function of x-position in the Mosaic field of view. In the top panels, the raw CCD positions were transformed using the default Davis 1998 WCS chip-to-chip transformation coefficients provided in the FITS file headers. In the bottom panels, the chip coefficients given by Platais et al. 2002 were used instead. The improvement obtained by use of the Platais et al. coefficients is readily apparent.