Imaging Asteroidal Companions with the Hubble Space Telescope (HST)

1. Asteroid Date Observed (2001) R (AU) D (AU) Phase (deg) Dataset name 45 Eugenia 22 June 2.58 1.67 12.3 u62h27 87 Sylvia 23 Feb. 3.76 2.80 4.47 u62h35 107 Camilla 1 Mar. 3.27 2.29 2.97 u62h39 Imaging Asteroidal Companions with the Hubble Space Telescope (HST) A. Storrs, K. Makhoul (Towson Univ.), E. Wells (CSC/STScI), M. Gaffey (Univ. of N. Dakota), F. Vilas (NASA HQ), R. Landis (JPL), C. Wood (Univ. of N. Dakota), and B. Zellner (Georgia Southern Univ.) Implications: How could a companion have a different color than its primary? The process of “space weathering” (see, e.g., Madey et al. 2001) can alter silicate surfaces exposed to the interplanetary environment. The longer the surface is exposed, the more red it becomes. We suggest that , as they present larger targets, resurfacing events are more common for the primary objects than for the companions. Thus while the surfaces of 87 Sylvia and its companion have roughly the same exposure age, the surfaces of the companions to 45 Eugenia and 107 Camilla are significantly older than those of the primary asteroids. 45 Eugenia 87 Sylvia 107 Camilla Introduction: We present visible and near-IR images of main-belt asteroids 45 Eugenia, 87 Sylvia, and 107 Camilla. These images show not only the primary object but also companion objects (satellites). We present reconstructions of these images and photometric information on the relative reflectance of the companions and the primaries. While the companions of 45 Eugenia and 107 Camilla are noticeably redder than the main body in the visible region, the companion of 87 Sylvia has substantially the same color. We hypothesize that a red color difference may be due to the companion having an older surface than the primary object. This implies that the surface of 87 Sylvia is much older than the other two, and/or that 45 Eugenia and 107 Camilla have been resurfaced more recently than their companions. Note that our original DPS abstract was based on incorrect photometric reductions. The companions are redder than the primaries, not bluer. The pairs of images shown above are representative of the data for each asteroid. The left image is the result of standard HST “pipeline” processing, stretched to show the companion objects (circled). Thus the primary object is hidden by its scattered light halo. The right image of each pair is the result of running a MISTRAL restoration on the data, as described below. Although restored at enhanced spatial resolution, the images displayed at the same scale. Note that while 87 Sylvia is round, both 45 Eugenia and 107 Camilla are noticeably oblong after restoration. Each image is 1.5 arcsec square, and north is 68o CCW from straight up for 45 Eugenia, 152o CCW for 87 Sylvia, and 13o CCW for 107 Camilla. Future Work: The use of HST imaging to detect companions and to constrain the ages relative to their primary objects is promising. We hope to pursue this in future HST programs. Brightness of companion objects compared to their primary asteroids, in magnitudes. Note that fainter values are toward the bottom. Thus while the companion to 87 Sylvia exhibits the same colors as the primary body within the limits of the photometry, the companions of both 45 Eugenia and 107 Camilla are noticably more red than the primary bodies. This result is significant to several standard deviations, and possible interpretations are presented below. Companions are only weakly detected if at all in the 0.95 mm and 1.04 mm bands, these points should be regarded as upper limits. Acknowledgements: Support for this work provided by NASA through grant GO-8559 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Thecompanion to 107 Camilla: WFPC-2 images of 107 Camilla were the first to show the companion. Adaptive optics (AO) observations of this asteroid have not been able to confirm its existence (W. Merline, pvt. comm.) even though it appears only 6.5 mag. fainter than the primary. Note that we detect (45)(1) Petit-Prince (discovered by the AO technique) at 7.5 magnitudes fainter than 45 Eugenia, at 0.44 mm wavelength. We include (right) all of our images of 107 Camilla, showing the multiple detections of the companion with the visible and blue filters. The figure shows the primary on the left, and the same image stretched to show the companion on the right. The sequence runs (top to bottom) F439W, F671N, F953N, F1042M, F791W, and F791W (overexposed), over a total of 15 minutes. The companion is visible only in the deepest exposures. Image Restoration: Normal astronomical deconvolution processes do not work well on extended objects with sharp brightness variations, such as asteroids. These deconvolution processes will tend to over-enhance the edges of such sources, and so here we have used the MISTRAL routine (Conan et al. 2000) to avoid this problem. WFPC-2 images of the asteroids were restored with a theoretical (TinyTim, Krist 1993) PSF. The resultant images have a four times resolution improvement over the unrestored images. References: Conan, J.-M., T. Fusco, L. Mugnier, F. Marchis, C. Roddier, F. Roddier 2000. Deconvolution of Adaptive Optics Images: From Theory to Practice. in Adaptive Optical Systems Technology (P. Wizinowich, Ed.), pp 913-924. SPIE, Bellingham. Holtzman, J.A. 1995. The Photometric Performance and Calibration of WFPC2. PASP 107, pp. 1065-1093 Krist, J. 1993. The Tiny Tim User’s Manual, Space Telescope Science Institute Madey, T.E., R.E. Johnson, and T.M. Orlando 2001. Far-out surface science: radiation-induced surface processes in the solar system. Surface Science 500, pp. 838-858 TheImages: WFPC images of main belt asteroids were obtained as part of the HST asteroid snapshot survey, program 8559 (PI Storrs). A sequence of images in the planetary camera of the WFPC-2 was designed to fit into short segments of the HST schedule that could not accommodate regular HST observations. Note that the CCD sensitivity drops at long wavelengths—the most sensitive filter is the F791W so an extra exposure with this filter ended each sequence, in which the primary was deliberately saturated to enhance any faint satellite objects. Photometry: To measure brightness in WFPC images, we integrated the flux in concentric square apertures surrounding each object. The background flux can be determined trivially by dividing the flux difference between the outer and inner boxes by the difference in their areas. This process will remove both zero-order (constant) background as well as first-order background (e.g. scattered light from a nearby bright source). Note that some charge is lost in the CCD readout process-- the flux must be corrected for charge-transfer efficiency (CTE) (Holtzman et al. 1995). The primary source of error in the photometric reduction process is the aperture correction– small apertures reduce background noise but don’t measure all the flux. The error in this correction is less than 0.1 mag.