Mineralogical Mappinig of Asteroid 4 Vesta

1
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.
The companion 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 Users 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
The Images 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 wavelengthsthe 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 dont
measure all the flux. The error in this
correction is less than 0.1 mag.