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Adaptive Optics Observations of Saturn's Ring Plane Crossing in August 1995
Icarus 143, 299–307 (2000) doi:10.1006/icar.1999.6251, available online at http://www.idealibrary.com on
Adaptive Optics Observations of Saturn’s Ring Plane Crossing in August 1995 F. Roddier and C. Roddier Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822 E-mail: [email protected]
A. Brahic Observatoire de Paris and Universit´e de Paris VII, 92195 Meudon, France
C. Dumas Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS 183-501, Pasadena, California 91109–8099
and J. E. Graves, M. J. Northcott, and T. Owen Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822 Received June 25, 1999; revised September 27, 1999
thanks to both the Hubble Space Telescope (HST) operating at visible wavelengths (Bosh and Rivkin 1996, Nicholson et al. 1996, Bosh et al. 1997) and adaptive optics (AO) systems operating in the near infrared (Sicardy et al. 1995, Roddier et al. 1996a,b). We report here on observations made with the University of Hawaii (UH) 13-actuator AO system at the Canada– France–Hawaii Telescope (CFHT) on Mauna Kea.
Adaptive optics (0.1500 resolution) infrared images of the rings and satellites of Saturn were obtained in August 1995 as the Earth was crossing the ring plane. Twelve clumps were detected in the F ring, including HST S5 and S7 objects. For the first time H magnitudes were obtained for Prometheus, Pandora, Telesto, and Calypso, and J magnitudes for Epimetheus, Janus, Mimas, Telesto, and Helene. c 2000 Academic Press °
Key Words: instrumention, adaptive optics; planetary rings; Saturn; satellites of Saturn; surfaces, satellites.
2. OBSERVATIONS
1. INTRODUCTION
At alternate intervals of 13.75 and 15.75 years, the Sun crosses Saturn’s ring plane. Within a few months, the Earth also crosses the ring plane either once or three times. Such events have been regularly observed since it first perplexed Galileo in 1612. As the rings disappear, satellites that are too faint to be observed at other times become detectable. Many were discovered on such occasions: Janus and Epimetheus in 1966 (Dollfus 1968, Fountain and Larson 1978), and Helene, Telesto, and Calypso in 1979–1980 (Lecacheux et al. 1980, Reitsema et al. 1980). Since that time four additional satellites were discovered from data taken by Voyager 1: Prometheus, Pandora, Atlas, and Pan. The last Sun crossing occurred on November 19, 1995, and the Earth crossed the ring plane on May 22, 1995, August 10, 1995, and February 11, 1996. For the first time it was possible to observe these events with a much higher angular resolution,
Unlike conventional AO systems developed for defense application, the UH AO system is based on the concept of wavefront curvature sensing and compensation (Roddier 1988) and was specifically developed at the Institute for Astronomy (IfA) for astronomical observations (Roddier et al. 1991). A small (30-mmdiameter) image of the telescope entrance aperture is formed on a 13-actuator deformable mirror which compensates for wavefront aberrations. The mirror—called bimorph—consists of two piezoelectric wafers glued together. The correction rate (1.2 kHz) is sufficiently high to compensate wavefront distortions introduced by the atmosphere. Near-infrared (1–2.5 µm) images are recorded with a 1024 × 1024-pixel HgCdTe infrared camera developed by Hodapp et al. (1995). At the f/35 CFHT focus, the pixel size is 000 .035, i.e., 223 km on Saturn at the time of these observations. It gives us a field-of-view of 3600 × 3600 . Immediately before the camera, light shortward of 1 µm is reflected toward a wavefront sensor. An array of 13 photon-counting avalanche photodiodes detect any unbalanced illumination between
299 0019-1035/00 $35.00 c 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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TABLE I Data List Date (UT)
Time (UT)
Frame No.
Number of exposures
Exposure time (s)
Filter
Ansa
Guide source
9 10 10 10 10 10 10 11 11 11 11 11 11 11 12 12 12 12 12 12 12 12
14:45–15:15 11:03–11:48 11:49–12:08 12:10–12:25 12:26–12:56 14:33–15:07 15:08–15:19 11:23–11:49 11:51–11:58 12:09–12:16 13:26–13:34 13:37–13:47 13:49–13:58 15:15–15:17 11:26–11:41 11:46–12:08 12:13–12:23 12:38–12:46 12:49–12:52 12:53–13:03 13:23–13:46 15:06–15:32
217-243A 282-318B 319-340B 341-355B 356-389B 458-490B 491-499B 269-278C 279-283C 289-293C 319-323C 324-329C 330-334C 365-366C 161-180D 181-205D 206-220D 221-226D 227-229D 230-244D 262-282D 324-347D
27 37 22 15 34 33 9 10 5 5 5 6 5 2 20 25 15 6 3 15 21 24
15 15 15 15 15 15 30 60 60 60 60 60 60 60 30 30 30 60 30 10 30 30
H H J J H H H J H J H J H J H J H J J J H H
East East East West West West West West West West East East East East East East East West West West West West
Dione Dione Dione Tethys Tethys Tethys Tethys Dione Dione Dione Tethys Tethys Tethys Tethys Dione Dione Dione Tethys Tethys Tethys Tethys Tethys
oppositely defocused images of a guide source. The output signals relate to local errors in the wavefront curvature, and are used to drive the deformable mirror through a computer. A remotely controlled offset mirror allows the telescope to be pointed anywhere up to a maximum distance of 2000 away from the guide source, a distance that was determined to be small enough for the quality of the compensation not to be significantly affected. The observations presented here were made in August 1995, the most favorable period for ground-based observations. The AO system was mounted at the f/35 Cassegrain focus of the CFHT. At this focus, the effective telescope aperture is 3.35 m. Either Dione or Tethys was used as a guide source. The offset mirror was used to record images of the nearest ring side at different positions on the camera. Saturn itself was carefully left outside the field-of-view to avoid saturation of the detector array. Observations were made during two nights before the crossing and two nights after. The crossing itself occurred during daytime, at dawn after the second night. Data used for this work are listed in Table I.
3. DATA PROCESSING
Figure 1 is an example of an image taken under good seeing conditions. This 15-s exposure (frame No. 362) of the west ansa was taken in the H band on August 10 at 12:29:17 UT (midexposure time), that is, about 8 h before the crossing. It is displayed here with different intensity levels. Images on the left are not
saturated, and show the full dynamic range. Images on the right have been adjusted to better show the lowest brightness features. From top to bottom one sees three different steps in the image processing. Photometric cross sections taken along the vertical line drawn on the right-side images are shown in Fig. 2. Images on top of Fig. 1 were obtained after standard flatfielding and sky background subtraction. Although Saturn (on the left side) was kept outside of the field-of-view, light scattered from the bright planet produces a high background with intensity decreasing from left to right. Compared with J-band images, the problem is less severe in the H band where Saturn is darker because of the methane absorption in its atmosphere. To estimate this background we made local least-squares fits with two-dimensional first- and second-degree polynomials over small windows on each sides of the ring. The polynomials are orthogonal over each window and are generated by an SVD routine (Press et al. 1990). The windows are at the top and bottom ends of the domain delimited in the upper right image of Fig. 1. Discontinuities in the domain outline are due to the finite pixel size. The windows form almost a square of 10 × 10 pixels, and are moved along the ring at an angle of 3.8◦ with the horizontal axis. These scans form two bands parallel to the ring, on each side of it. The gap with the ring in the middle is 30 pixels wide. Along the bands, successive windows overlap each other by half their width, and each background estimate is the average of the values obtained from two overlapping windows. Slightly larger windows were used for the frames taken after the crossing when the rings were brighter.
ADAPTIVE OPTICS OBSERVATIONS OF SATURN’S RING PLANE CROSSING
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FIG. 1. H-band image of the west ansa obtained on August 10 at 12:29:17 UT which is about 8 h before the crossing. On the right-side frames, the intensity scale has been saturated to better show the low-intensity levels. Epimetheus is visible on the right side of each frame, slightly below the ring plane. Top frames show the flat-fielded and sky-background-subtracted data. Middle frames show data after subtraction of the light scattered by Saturn. Bottom frames show deconvolved images. Right-side frames show the boxes used for our photometric analysis.
Middle images in Fig. 1 show the ring after subtraction of the above estimated polynomials over the whole domain outlined in the upper right image. Light scattered by Saturn has mostly disappeared. The 20-pixel-long and 2-pixel-wide box shown in the middle right image was used to make photometric scans along the ring to study the ring photometry (results will be presented in a forthcoming paper). Because adaptive optics only partially compensates the effect of atmospheric turbulence, an image of a point source consists of a narrow core surrounded by a halo of light scattered by the residual wavefront errors. The halo can be removed by deconvolution, enhancing the contrast of the images. For this operation, we used images of satellites recorded on the same frames as a measure of the point-spread function (PSF). Data from August 9 were deconvolved with Dione. Data from August 10 were de-
convolved first with Janus (east ansa), then with Mimas and Enceladus (west ansa). Whenever the satellite image used as a PSF was close to the ring, a ring image from a different exposure was subtracted from the satellite image to avoid contamination (see procedure in Section 5). The bottom images in Fig. 1 show the result of the deconvolution. The angular resolution limit of these images is fairly uniform along the ansa and of the order of 0.15 arcsec or 960 km at Saturn. Small-scale brightness fluctuations are now clearly seen along the ring. As we shall see most of these fluctuations are produced by real ring features. The 14-pixel-long and 2-pixel-wide box shown in the bottom right image was used to make the radial photometric scans presented here. The illumination in the rings was integrated over the box area, and the box was translated along the ring by 1 pixel at each time as with a microdensitometer slit. Figure 3 shows
FIG. 2. Photometric cross sections of the rings taken along the vertical line shown in Fig. 1. Left profile is for the top frame. Right profile is for the bottom frame. The intensity scale is in camera ADUs (analog-to-digital units) and shows the intensity level in each pixel along the cross section.
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FIG. 3. Radial photometric profiles of the middle frames (solid line) and bottom frames (dotted line) in Fig. 1. Distance to the center of Saturn is indicated in arcseconds (bottom scale) and in kilometers (upper scale). Intensities on the left are expressed as the height (in km) of an equivalent perfect Lambert diffuser (vertically integrated I /F). Intensities on the right show the ADU values integrated along pixel-wide cross sections such as that shown in Fig. 2.
the radial-scan photometric profiles corresponding to the middle image (full line) and bottom image (dotted line) in Fig. 1. Distance to the center of Saturn is indicated in arcseconds (bottom scale) and in kilometers (upper scale). Intensities are expressed as the height (in km) of an equivalent perfect Lambert diffuser (vertically integrated I /F). Conversion from magnitudes to reflectivities was done as described in Section 5. 4. THE DETECTION OF MOVING OBJECTS
A number of intensity peaks in Fig. 3 have been identified with satellites such as Pandora (on the left) and Epimetheus (on
the right). A fainter peak was identified with 1995 S5, an object discovered with the Hubble Space Telescope (Nicholson et al. 1995) and hereafter referred to as S5. For all other bodies detected during the 1995 ring-plane crossing, we drop the leading “1995” in a similar manner. Intensity peaks such as those produced by Pandora or S5 can be distinguished from the ring structure because they move from one frame to another. The following procedure was found to be quite effective in detecting such moving objects. It consists of creating two-dimensional displays of the ring brightness as a function of both time and position along the ring. Figure 4 is an example of such a display covering data taken on August 10 from
FIG. 4. Ring brightness as a function of distance from Saturn and time. This time sequence shows data taken on August 10 from 12:15 to 12:56 (UT). Each line displays the intensity in a profile such as that shown in Fig. 3. Time increases quasi-monotonically from bottom to top. Vertical bands show the fixed ring structure, whereas slanted lines are produced by moving objects. Note the eclipse of Epimetheus through the shadow of the A ring.
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12:15 to 12:56 (UT). Each line in this diagram displays the intensity of a photometric profile (such as that shown in Fig. 3) obtained from a single 15-s exposure. Since exposures were taken at fairly regular 1-min intervals, time increases quasi-monotonically from bottom to top. Two different kinds of patterns can be seen in Fig. 4: vertical bands corresponding to the fixed ring structure, and slanted lines produced by moving objects. We still see here the side of the rings opposite the Sun. Whereas most of the rings are opaque and appear dark, light scattered through the Cassini division and the C ring makes them appear bright, and produces the bright vertical bands seen in Fig. 4. One can also note the bright edge extending beyond the tip of the A ring, due to light scattered by the F ring. Moving objects can be identified by their orbital velocity (given by the slope of the slanted line) as well as their orbital phase. Four previously known objects have been identified in Fig. 4. These are Pandora, Epimetheus, S5, and Mimas. The detection of Pandora and S5 clearly demonstrates the remarkable increase in the sensitivity of ground-based observations due to adaptive optics (see also Sicardy et al. 1995). Figure 4 shows a temporary disappearance of Epimetheus, which we identified as due to an eclipse through the shadow of the A ring. A photometric analysis of this eclipse will be presented in a forthcoming paper. In addition to the traces left by the four above-mentioned objects, Fig. 4 also shows a number of fainter traces that we believe to be produced by newly discovered objects. The velocity of S5 as well as those of the newly detected fainter objects is fully consistent with Keplerian orbits at the distance of the F ring (Roddier et al. 1996b, Roddier 1997). We therefore believe they are clumps in the F ring similar to those detected by Voyager (Schowalter 1997). A small area centered on Mimas was used for the PSF estimation. The very faint line observed outside the F ring, below and parallel to the Mimas track, is an artifact at the edge of this area produced by deconvolution. To make the comparison easier between various observations, one can display the intensity as a function of the longitude along the F ring instead of distance to Saturn. Figure 5 shows such a display. Here we have assumed a Keplerian motion at a distance of 140,500 km from the center of Saturn and used the longitude at epoch 1995, August 10.5 TDT (at Saturn) as a horizontal coordinate. Longitudes are measured from the ascending node of Saturn’s equator on the Earth’s J2000.0 equator. In this display, any object with the above Keplerian motion (i.e., in or near the F ring) appears as fixed, and produces a bright vertical line. Objects moving at different rates produce slanted or curved lines. Note that a given distance to Saturn corresponds to two possible longitudes depending on whether it is on the front side or on the back side of the rings. Since we have plotted both solutions, each object has produced two traces, one corresponding to the forward motion and one to the backward motion. For objects in the F ring, one trace is a vertical line, the other one is a slanted line. The four wide horizontal strips are from our adaptive optics observations. From bottom to top, the first strip shows results from the AO data in the first line of Table I, the second
strip from data in lines 2 and 3, the third strip from data in lines 4 and 5, and the fourth strip from data in lines 6 and 7. AO data taken after the crossing time have not been included here because the rings were too bright to allow the detection of faint objects. The thinner strips on top of Fig. 5 are from observations made with the Planetary Camera (PC) on the HST (Nicholson et al. 1996). We have resampled them to our own standard of 000 .035 per pixel and processed them in the same way as our own data. These observations span from 13:37 to 23:40 on August 10. The early part of these PC observations coincides in time with the last AO observations displayed on the top large strip, but observations were made on the opposite ansa. A number of objects are identified on the top, especially the three clumps S5, S6, and S7 announced by the HST team (Nicholson et al. 1995). Both S5 and S7 are clearly seen in our AO data as in the HST data. We have no AO data at the location of S6. Of the objects identified by the May 1995 HST team (Bosh and Rivkin 1996), the only one that would appear as a vertical line on this plot is S3, which has been tentatively identified with S6 (Nicholson et al. 1996). Unfortunately, these fall in a region for which we have no data. Another prominent clump is observed at a longitude of about 317◦ . It was announced as S9 (Roddier et al. 1996a). It is harder to see in the HST images where it was later identified (McGhee et al. 1997). Another prominent clump is also seen near 46◦ . It was not announced with S9 because at that time our images had not yet been deconvolved and the detection was considered marginal. It is clear that many fainter clumps can be seen in Fig. 5. They have been announced in a subsequent IAU circular (Roddier et al. 1996b). Their estimated longitudes (as defined above) are indicated in Table II. The uncertainty on these numbers is ±1◦ . We have also estimated the clump sizes by comparing their brightness with that of S5. Table II shows the radii derived under the assumption that they all have the same albedo and that S5 has a radius of 26 km (Nicholson et al. 1995). It is interesting to note that S5 does not appear in our last set of data (fourth wide strip from bottom on Fig. 5), that is, past its maximum elongation. During the first part of the sequence, light scattered by Enceladus may impede the observation. However, S5 should become visible at the end of the sequence apparently moving back toward Saturn, but it does not. The same phenomenon can be seen in data taken by the Wide Field Planetary Camera (WFPC) on HST (Nicholson et al. 1996). S5 should be visible in the fourth image of the second set of HST data (second
TABLE II Longitude and Size of the Clumps Name
S9
S11 S12 S13 S14 S15 S16 S17 S18 S19
Longitude (deg)a 317 302 320 330 Radius (km) 16 12 10 12 a
46 16
Longitude is on August 10.5 TDT (at Saturn).
105 114 116 118 120 12 10 10 10 10
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FIG. 5. Time sequences showing the ring brightness as a function of time and longitude. From bottom to top, the four wide horizontal strips show results from the AO data listed respectively in lines 1, 2–3, 4–5, and 6–7 of Table I. The thinner strips on top of them are from observations made with the Planetary Camera on board the Hubble Space Telescope. In this representation, moving objects produce two tracks (see text). For those orbiting at a selected distance of 140,500 km from the center of Saturn, one of the tracks is a straight vertical line. The abscissa of the line on the horizontal axis is the object longitude on August 10.5 TDT (at Saturn). One notes a large number of faint objects orbiting near this distance. These include objects S5, S6, and S7 announced by Nicholson et al. (1995), as well as the other S-numbered objects announced by our team (Roddier et al. 1996a,b).
dotted line from top of Fig. 5 in Nicholson and colleagues’ paper), but it is not despite the good image quality. Similarly, S7 should later be visible at the same location in the sixth set of data (sixth dotted line from top of Nicholson and colleagues’ paper Fig. 5), but again it has disappeared. Olkin and Bosh (1996) have shown evidence that the F ring is inclined. Since these clumps are believed to be part of the F ring, their disappearance brings further evidence that the remote side of the west F-ring ansa may have been occulted by the main rings (Bosh, private communication). One may wonder why ground-based adaptive optics observations have been more efficient than HST observations in detecting clumps in the F ring. In both cases, the angular resolution and the noise level are similar. Part of the answer may be in the exposure time: 15 s for our observations, compared with 100 s for the HST observations. The apparent clump motion being on the order of 000 .1 per minute, the resulting blur may have lowered the signal-to-noise ratio in the HST images. The most important factor is probably the much larger number of frames taken from the ground as shown by the respective widths of the horizontal bands in Fig. 5.
5. THE PHOTOMETRY OF SATURN’S SATELLITES
Each night images of UKIRT faint standard stars FS2, FS4, and FS35 were taken and used as primary references (Casali and Hawarden 1992). We attempted to use the IRAF aperture photometry software, but for our data we found it more appropriate to fit the photometric profiles of the reference with those of the measured object along both the x and y directions. Profiles along the 45◦ and 135◦ directions were also used to center the object and the reference. Error bars were determined by the uncertainty on the fit. Since none of the nights was absolutely photometric and the seeing was variable, we found it necessary to use secondary standards, a technique particularly useful when doing photometry with AO images. Dione and Tethys were used for this purpose. However, except for the first night (August 9 UT), both of them were saturated, and the fit was limited to the wings of the profiles. The procedure was tested on the nonsaturated images and was found to give the same results within the given error bars. First, we compared the brightness of the primary standard with that of the secondary on frames taken as close in time as
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TABLE III J and H Band Photometry of Small Satellites Satellite
Filter
Magnitude
aS (km2 )
Orbital phase
Date, time (UT)
Prometheus Pandora Pandora Epimetheus Epimetheus Epimetheus Epimetheus Janus Janus Janus Janus Janus Janus Janus Janus Mimas Mimas Mimas Mimas Telesto Telesto Telesto Telesto Calypso Helene Helene
H H H J J H H J J J H H H H H J J H H J H H H H J H
15.3 ± 0.15 15.2 ± 0.1 15.35 ± 0.1 14.45 ± 0.05 14.45 ± 0.05 14.65 ± 0.1 14.75 ± 0.05 13.7 ± 0.1 13.9 ± 0.1 13.9 ± 0.1 13.7 ± 0.05 13.8 ± 0.05 13.8 ± 0.03 13.8 ± 0.03 13.8 ± 0.03 11.4 ± 0.1 11.7 ± 0.2 11.5 ± 0.03 11.45 ± 0.03 17.2 ± 0.3 17.5 ± 0.2 17.6 ± 0.4 17.4 ± 0.4 18.7 ± 0.4 16.7 ± 0.1 16.65 ± 0.1
2,000 ± 550 2,190 ± 400 1,910 ± 350 5,840 ± 540 5,840 ± 540 3,640 ± 670 3,620 ± 330 11,600 ± 2,100 9,690 ± 1,800 9,690 ± 1,800 8,720 ± 800 7,950 ± 730 7,950 ± 440 7,950 ± 440 7,950 ± 440 96,900 ± 18,000 73,500 ± 27,000 66,200 ± 37,00 69,300 ± 3,800 464 ± 260 263 ± 97 240 ± 180 289 ± 220 87 ± 66 735 ± 140 576 ± 110
73◦ (E) 67◦ (E) 270◦ (W) 292◦ (W) 260◦ (W) 298◦ (W) 270◦ (W) 145◦ (E) 287◦ (W) 94◦ (E) 53◦ (E) 132◦ (E) 216◦ (W) 296◦ (W) 90◦ (E) 297◦ (W) 300◦ (W) 307◦ (W) 305◦ (W) 306◦ (W) 309◦ (W) 145◦ (E) 148◦ (E) 225◦ (W) 215◦ (W) 217◦ (W)
Aug. 10, 11:30 Aug. 9, 15:00 Aug. 10, 12:40 Aug. 10, 12:20 Aug. 12, 12:50 Aug. 10, 12:30 Aug. 12, 13:30 Aug. 10, 12:00 Aug. 11, 11:40 Aug. 12, 12:00 Aug. 9, 15:00 Aug. 10, 11:30 Aug. 10, 15:00 Aug. 11, 12:00 Aug. 12, 11:30 Aug. 10, 12:20 Aug. 11, 11:40 Aug. 10, 12:30 Aug. 11, 12:00 Aug. 10, 12:20 Aug. 10, 12:40 Aug. 11, 13:30 Aug. 11, 13:50 Aug. 12, 15:15 Aug. 10, 12:20 Aug. 10, 12:40
possible. Then, we compared the brightness of the primary with that of the secondary on the object frame. This gave us a correction for the fluctuation in sky transparency. In doing this, we assumed that the brightness of Dione or Tethys was independent of its orbital position. It introduces an error that we have estimated by using the photometric curves of Franz and Millis (1975), and by assuming that the brightness variations on the orbit were the same in the infrared as in the visible. The error was found to be small and within the indicated error bars. Since several objects under study are quite faint, we averaged as many images of them as possible. However, when the image of a faint object is not well compensated, including it in the average leads to an underestimate of the flux. Therefore, we averaged only the best images. Background subtraction was made very carefully. Whenever the satellite was close to the ring, we subtracted a ring contribution estimated from a different image taken under similar atmospheric conditions (for instance, having similar values for the flux of a bright satellite). When the ring was very close, we used three different estimates. Errors are then dominated by the uncertainty of this contribution, and the error bar was taken as the root mean square (rms) variation over the three different estimates. We also cross-checked our results by comparing the profiles of two different satellites. As a result, the error bars are quite different from one night to the other or even from time to time during the same night. Maximum accuracy
was obtained when the satellite was observed nearly at the same time as the reference star and as far as possible from the ring. Our photometric results are given in Table III. They are consistent with those of Bauer et al. (1997) which were obtained from data taken at the IRTF during the same ring-plane crossing, but with no adaptive optics. Despite differences in the orbital phases, all our values fall within their errors bars. However, one can see from Table III that our error bars are substantially smaller. For the first time we were able to measure the J magnitudes of Epimetheus, Janus, Mimas, Telesto, and Helene and the H magnitudes of Prometheus, Pandora, Telesto, and Calypso. In those cases where both J and H magnitudes are available, it is possible to derive the J–H color index, which provides some crude information about surface composition. These small inner satellites have always been suspected to be composed of water ice, in part because of their high albedos (Morrison et al. 1986). The value of J–H is 0.0 within the errors of observation for Janus, Mimas, and Helene, while Epimetheus has J–H = −0.30 ± 0.1. The Telesto magnitudes were not determined well enough to give a definitive result, but also suggest a slightly negative value of J–H. All of these colors are consistent with a surface composition of water ice [e.g., J–H (Rhea) = −0.05, J–H (Dione) = −0.20] but exclude rocky surfaces [e.g., J–H (Ceres) = +0.31, J–H (Vesta) = +0.17] or organic compounds (e.g., Iapetus’ leading hemisphere has a
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TABLE IV Albedos Name
Visiblea
Prometheus Pandora Epimetheus Janus Mimas Telesto Calypso Helene
0.50 0.50 0.50 0.50 0.77 0.60 0.90 0.60
a
J band
H band
0.55 ± 0.12 0.39 ± 0.17 0.78 ± 0.10 0.90 ± 0.46
0.24 ± 0.40 0.34 ± 0.30 0.32 ± 0.12 0.30 ± 0.14 0.55 ± 0.03 0.60 ± 0.39 0.30 ± 0.54 0.72 ± 0.10
0.91 ± 0.1
Visible values are from Burns (1986).
J–H color ∼ = +0.4) (reference colors compiled by Tokunaga 1999). Table III also shows the equivalent area aS (in km2 ) of a perfect Lambert diffuser obtained from the expression aS = π
r12r22 (m sun −m sat )/2.5 10 , r02
where r0 is the heliocentric distance of the Earth, r1 is the heliocentric distance of Saturn, and r2 is the geocentric distance of Saturn. At the time of the observations, these distances were respectively 1.014, 9.619, and 8.784 AU. The quantity m Sun − m Sat is the difference in stellar magnitude between the Sun and the satellite. Assuming that the visible magnitude of the Sun is −26.74 (Allen 1973) and using the colors compiled by Tokunaga (1999), we took m Sun = −27.86 in the J band, and m Sun = −28.17 in the H band. The dimensions of the satellites were taken from Voyager’s measurements (Burns 1986) to estimate the cross-section area S and derive values for the satellite albedos a. These values are listed in Table IV. Unfortunately, satellites smaller than Mimas are often characterized by their irregular shape, adding uncertainty in the cross section at the time of the observations. The error bars given in Table IV have been obtained by adding quadratically the error bar due to photometric errors (Table III) and that due to the uncertainty in the cross section. Given the error bars, no correction was made for the solar phase angle which was close to 3.5◦ at the time of the observations. Table IV also lists the visible geometric albedos derived from the Voyager images recorded at 0.47 µm (Burns 1986).
FIG. 6. Plot of five satellite albedos in the V, J, and H bands. The very large error bars of Telesto are not shown for better clarity (see Table IV).
ADAPTIVE OPTICS OBSERVATIONS OF SATURN’S RING PLANE CROSSING
Figure 6 is a plot of these albedos as a function of wavelength for the five cases were we have visible, J, and H values. Although the errors bars are very large, all the satellites reported in the figure have decrasing albedos between J and H bands which agree with a surface dominated by water ice. Nevertheless, Helene and Telesto display a rather peculiar maximum at J band that is not visible for the other three objects. It is important to consider this result with caution since their J -band albedo has been estimated from only one photometric measurement and the error bar for Telesto is too large to derive any conclusion for this particular object. Therefore, if we consider the case of Helene only, the J-band maximum displayed by its albedo, if it turns out to be real, would have direct implications on the nature of its surface material. Helene’s surface, although dominated by water ice, would be coated in addition by a more complex material responsible for the measured reddening between V and J bands. This interesting result requires further confirmation to be fully validated. 6. SUMMARY
High-angular-resolution infrared images of the rings and satellites of Saturn were obtained in August 1995 as the Earth was crossing the ring plane. By using adaptive optics on a 3.35-m telescope aperture, we were able to obtain images with an optical quality comparable to that of the HST (0.1500 resolution in the H band). Twelve objects were discovered orbiting Saturn with a Keplerian motion close to that of the F ring. Two of them were identified as objects S5 and S7 first announced by the HST team (Nicholson et al. 1995). These objects are all believed to be clumps in the F ring, similar to those observed by the Voyager spacecraft. Infrared H magnitudes were obtained for the first time for Prometheus, Pandora, Telesto, and Calypso, as were J magnitudes for Epimetheus, Janus, Mimas, Telesto, and Helene. These results demonstrate the effectiveness of ground-based observations with adaptive optics. ACKNOWLEDGMENTS The UH AO system was built under NSF Grant AST 93-19004. The processing and analysis of the data were supported on NASA Grants NAGW 4935 and NAG 5-3731. We also acknowledge the use of Mark Showalter’s ephemeris generator written under the auspices of the Planetary Data System’s Rings Node, a very convenient tool.
REFERENCES Allen, C. W. 1973. Astrophysical Quantities, 3rd ed., p. 162. Athlone Press, London. Bauer, J., J. J. Lissauer, and M. Simon 1997. Edge-on observations of Saturn’s E and G rings in the near-IR. Icarus 125, 440–445. Bosh, A. S., and A. S. Rivkin 1996. Observations of Saturn’s inner satellites during the May 1995 ring-plane crossing. Science 272, 518–521.
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Bosh, A. S., A. S. Rivkin, J. W. Percival, M.-J. Taylor, and G. Wayne van Citters 1997. Saturn ring-plane crossing, May 1995: Pole precession and ring thickness. Icarus 129, 555–561. Burns, J. A. 1986. Some background about satellites. In Satellites (J. A. Burns and M. S. Matthews, Eds.), pp. 14–15. Univ. of Arizona Press, Tucson. Casali, M., and T. Hawarden 1992. A set of faint standards for UKIRT. JCMTUKIRT Newslett. 4, 33–35. Dollfus, A. 1968 La d´ecouverte du 10e satellite de Saturne. L’Astronomie 82, 253–262. [In French] Fountain, J. W., and S. M. Larson 1978. Saturn’s ring and nearby faint satellites. Icarus 36, 92–106. Franz, O. G., and R. L. Millis 1975. Photometry of Dione, Tethys, and Enceladus on the UBV system. Icarus 24, 433–442. Hodapp, K.-W., J. L. Hora, D. N. B. Hall, L. L. Cowie, M. Metzger, E. M. Irwin, T. J. Keller, K. Vural, L. J. Kozlowski, and W. E. Kleinhans 1995. Astronomical characterization results of 1024 × 1024 HgCdTe Hawaii detector array. SPIE Proc. 2475, 8–14. Lecacheux, J., P. Laques, L. Vapillon, A. Auge, and R. Despiau 1980. A new satellite of Saturn: Dione B. Icarus 43, 111–115. McGhee, C. A., P. D. Nicholson, and R. G. French 1997. HST observations of small saturnian satellites and F ring clumps. Paper presented at the Wellesley Workshop, July 25–26, 1997. [Abstracts edited by M. Showalter, P. Nicholson, and D. French.] Morrison, D., T. Owen, and L. A. Soderblom 1986. The satellites of Saturn. In Satellites (J. A. Burns and M. S. Matthews, Eds.), pp. 764–801. Univ. of Arizona Press, Tucson. Nicholson, P. D., C. A. McGhee, M. R. Showalter, L. Dones, R. G. French, S. M. Larson, J. J. Lissauer, P. O. Seitzer, B. Sicardy, and G. E. Danielson 1995. Satellites of Saturn. In IAU Circular No. 6243. Nicholson, P. D., M. R. Schowalter, L. Dones, R. G. French, S. M. Larson, J. J. Lissauer, C. A. McGhee, P. Seitzer, B. Sicardy, and G. E. Danielson 1996. Observations of Saturn’s ring-plane crossing in August and November 1995. Science 272, 509–515. Olkin, C. B., and A. S. Bosh 1996. The inclination of the F ring. Bull. Am. Astron. Soc. 28, 1125. Press, W. P., B. P. Flannery, S. A. Teukolsky, and W. T. Vetterling 1990. Numerical Recipes in C, p. 60. Cambridge Univ. Press. Cambridge. Reitsema, H. J., B. A. Smith, and S. M. Larson 1980. A new Satellite near Dione’s L4 point. Icarus 43, 116–119. Roddier, F. 1988. Curvature sensing and compensation: A new concept in adaptive optics. Appl. Opt. 27, 1223–1225. Roddier, F., A. Brahic, C. Dumas, C. Ferrari, J. E. Graves, M. J. Northcott, T. Owen, L. Perret, L., C. Roddier, and P. Thebault 1996a. Satellites of Saturn. In IAU Circular No. 6407. Roddier, F., M. J. Northcott, and J. E. Graves 1991. A simple low-order adaptive optics system for near-infrared applications. Publ. Astron. Soc. Pac. 103, 131– 149. Roddier, C. 1997. Observations du syst`eme solaire avec l’optique adaptive. C.R. Acad. Sci. (Paris) 325(IIb), 109–114. [In French] Roddier, C., F. Roddier, A. Brahic, J. E. Graves, M. J. Northcott, and T. Owen 1996b. Satellites of Saturn. In IAU Circular No. 6515. Schowalter, M. R. 1997. Dynamics of clumps in Saturn’s F ring. Bull. Am. Astron. Soc. 29, 999. Sicardy, B., J.-L. Beuzit, F. Poulet, and P. Prado 1995. Satellites of Saturn. In IAU Circular No. 6269. Tokunaga, A. T. 1999. Infrared astronomy. In Astrophysical Quantities (A. Cox, Ed.), 4th ed. In press.
Adaptive Optics Observations of Saturn’s Ring Plane Crossing in August 1995 F. Roddier and C. Roddier Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822 E-mail: [email protected]
A. Brahic Observatoire de Paris and Universit´e de Paris VII, 92195 Meudon, France
C. Dumas Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS 183-501, Pasadena, California 91109–8099
and J. E. Graves, M. J. Northcott, and T. Owen Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822 Received June 25, 1999; revised September 27, 1999
thanks to both the Hubble Space Telescope (HST) operating at visible wavelengths (Bosh and Rivkin 1996, Nicholson et al. 1996, Bosh et al. 1997) and adaptive optics (AO) systems operating in the near infrared (Sicardy et al. 1995, Roddier et al. 1996a,b). We report here on observations made with the University of Hawaii (UH) 13-actuator AO system at the Canada– France–Hawaii Telescope (CFHT) on Mauna Kea.
Adaptive optics (0.1500 resolution) infrared images of the rings and satellites of Saturn were obtained in August 1995 as the Earth was crossing the ring plane. Twelve clumps were detected in the F ring, including HST S5 and S7 objects. For the first time H magnitudes were obtained for Prometheus, Pandora, Telesto, and Calypso, and J magnitudes for Epimetheus, Janus, Mimas, Telesto, and Helene. c 2000 Academic Press °
Key Words: instrumention, adaptive optics; planetary rings; Saturn; satellites of Saturn; surfaces, satellites.
2. OBSERVATIONS
1. INTRODUCTION
At alternate intervals of 13.75 and 15.75 years, the Sun crosses Saturn’s ring plane. Within a few months, the Earth also crosses the ring plane either once or three times. Such events have been regularly observed since it first perplexed Galileo in 1612. As the rings disappear, satellites that are too faint to be observed at other times become detectable. Many were discovered on such occasions: Janus and Epimetheus in 1966 (Dollfus 1968, Fountain and Larson 1978), and Helene, Telesto, and Calypso in 1979–1980 (Lecacheux et al. 1980, Reitsema et al. 1980). Since that time four additional satellites were discovered from data taken by Voyager 1: Prometheus, Pandora, Atlas, and Pan. The last Sun crossing occurred on November 19, 1995, and the Earth crossed the ring plane on May 22, 1995, August 10, 1995, and February 11, 1996. For the first time it was possible to observe these events with a much higher angular resolution,
Unlike conventional AO systems developed for defense application, the UH AO system is based on the concept of wavefront curvature sensing and compensation (Roddier 1988) and was specifically developed at the Institute for Astronomy (IfA) for astronomical observations (Roddier et al. 1991). A small (30-mmdiameter) image of the telescope entrance aperture is formed on a 13-actuator deformable mirror which compensates for wavefront aberrations. The mirror—called bimorph—consists of two piezoelectric wafers glued together. The correction rate (1.2 kHz) is sufficiently high to compensate wavefront distortions introduced by the atmosphere. Near-infrared (1–2.5 µm) images are recorded with a 1024 × 1024-pixel HgCdTe infrared camera developed by Hodapp et al. (1995). At the f/35 CFHT focus, the pixel size is 000 .035, i.e., 223 km on Saturn at the time of these observations. It gives us a field-of-view of 3600 × 3600 . Immediately before the camera, light shortward of 1 µm is reflected toward a wavefront sensor. An array of 13 photon-counting avalanche photodiodes detect any unbalanced illumination between
299 0019-1035/00 $35.00 c 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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TABLE I Data List Date (UT)
Time (UT)
Frame No.
Number of exposures
Exposure time (s)
Filter
Ansa
Guide source
9 10 10 10 10 10 10 11 11 11 11 11 11 11 12 12 12 12 12 12 12 12
14:45–15:15 11:03–11:48 11:49–12:08 12:10–12:25 12:26–12:56 14:33–15:07 15:08–15:19 11:23–11:49 11:51–11:58 12:09–12:16 13:26–13:34 13:37–13:47 13:49–13:58 15:15–15:17 11:26–11:41 11:46–12:08 12:13–12:23 12:38–12:46 12:49–12:52 12:53–13:03 13:23–13:46 15:06–15:32
217-243A 282-318B 319-340B 341-355B 356-389B 458-490B 491-499B 269-278C 279-283C 289-293C 319-323C 324-329C 330-334C 365-366C 161-180D 181-205D 206-220D 221-226D 227-229D 230-244D 262-282D 324-347D
27 37 22 15 34 33 9 10 5 5 5 6 5 2 20 25 15 6 3 15 21 24
15 15 15 15 15 15 30 60 60 60 60 60 60 60 30 30 30 60 30 10 30 30
H H J J H H H J H J H J H J H J H J J J H H
East East East West West West West West West West East East East East East East East West West West West West
Dione Dione Dione Tethys Tethys Tethys Tethys Dione Dione Dione Tethys Tethys Tethys Tethys Dione Dione Dione Tethys Tethys Tethys Tethys Tethys
oppositely defocused images of a guide source. The output signals relate to local errors in the wavefront curvature, and are used to drive the deformable mirror through a computer. A remotely controlled offset mirror allows the telescope to be pointed anywhere up to a maximum distance of 2000 away from the guide source, a distance that was determined to be small enough for the quality of the compensation not to be significantly affected. The observations presented here were made in August 1995, the most favorable period for ground-based observations. The AO system was mounted at the f/35 Cassegrain focus of the CFHT. At this focus, the effective telescope aperture is 3.35 m. Either Dione or Tethys was used as a guide source. The offset mirror was used to record images of the nearest ring side at different positions on the camera. Saturn itself was carefully left outside the field-of-view to avoid saturation of the detector array. Observations were made during two nights before the crossing and two nights after. The crossing itself occurred during daytime, at dawn after the second night. Data used for this work are listed in Table I.
3. DATA PROCESSING
Figure 1 is an example of an image taken under good seeing conditions. This 15-s exposure (frame No. 362) of the west ansa was taken in the H band on August 10 at 12:29:17 UT (midexposure time), that is, about 8 h before the crossing. It is displayed here with different intensity levels. Images on the left are not
saturated, and show the full dynamic range. Images on the right have been adjusted to better show the lowest brightness features. From top to bottom one sees three different steps in the image processing. Photometric cross sections taken along the vertical line drawn on the right-side images are shown in Fig. 2. Images on top of Fig. 1 were obtained after standard flatfielding and sky background subtraction. Although Saturn (on the left side) was kept outside of the field-of-view, light scattered from the bright planet produces a high background with intensity decreasing from left to right. Compared with J-band images, the problem is less severe in the H band where Saturn is darker because of the methane absorption in its atmosphere. To estimate this background we made local least-squares fits with two-dimensional first- and second-degree polynomials over small windows on each sides of the ring. The polynomials are orthogonal over each window and are generated by an SVD routine (Press et al. 1990). The windows are at the top and bottom ends of the domain delimited in the upper right image of Fig. 1. Discontinuities in the domain outline are due to the finite pixel size. The windows form almost a square of 10 × 10 pixels, and are moved along the ring at an angle of 3.8◦ with the horizontal axis. These scans form two bands parallel to the ring, on each side of it. The gap with the ring in the middle is 30 pixels wide. Along the bands, successive windows overlap each other by half their width, and each background estimate is the average of the values obtained from two overlapping windows. Slightly larger windows were used for the frames taken after the crossing when the rings were brighter.
ADAPTIVE OPTICS OBSERVATIONS OF SATURN’S RING PLANE CROSSING
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FIG. 1. H-band image of the west ansa obtained on August 10 at 12:29:17 UT which is about 8 h before the crossing. On the right-side frames, the intensity scale has been saturated to better show the low-intensity levels. Epimetheus is visible on the right side of each frame, slightly below the ring plane. Top frames show the flat-fielded and sky-background-subtracted data. Middle frames show data after subtraction of the light scattered by Saturn. Bottom frames show deconvolved images. Right-side frames show the boxes used for our photometric analysis.
Middle images in Fig. 1 show the ring after subtraction of the above estimated polynomials over the whole domain outlined in the upper right image. Light scattered by Saturn has mostly disappeared. The 20-pixel-long and 2-pixel-wide box shown in the middle right image was used to make photometric scans along the ring to study the ring photometry (results will be presented in a forthcoming paper). Because adaptive optics only partially compensates the effect of atmospheric turbulence, an image of a point source consists of a narrow core surrounded by a halo of light scattered by the residual wavefront errors. The halo can be removed by deconvolution, enhancing the contrast of the images. For this operation, we used images of satellites recorded on the same frames as a measure of the point-spread function (PSF). Data from August 9 were deconvolved with Dione. Data from August 10 were de-
convolved first with Janus (east ansa), then with Mimas and Enceladus (west ansa). Whenever the satellite image used as a PSF was close to the ring, a ring image from a different exposure was subtracted from the satellite image to avoid contamination (see procedure in Section 5). The bottom images in Fig. 1 show the result of the deconvolution. The angular resolution limit of these images is fairly uniform along the ansa and of the order of 0.15 arcsec or 960 km at Saturn. Small-scale brightness fluctuations are now clearly seen along the ring. As we shall see most of these fluctuations are produced by real ring features. The 14-pixel-long and 2-pixel-wide box shown in the bottom right image was used to make the radial photometric scans presented here. The illumination in the rings was integrated over the box area, and the box was translated along the ring by 1 pixel at each time as with a microdensitometer slit. Figure 3 shows
FIG. 2. Photometric cross sections of the rings taken along the vertical line shown in Fig. 1. Left profile is for the top frame. Right profile is for the bottom frame. The intensity scale is in camera ADUs (analog-to-digital units) and shows the intensity level in each pixel along the cross section.
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FIG. 3. Radial photometric profiles of the middle frames (solid line) and bottom frames (dotted line) in Fig. 1. Distance to the center of Saturn is indicated in arcseconds (bottom scale) and in kilometers (upper scale). Intensities on the left are expressed as the height (in km) of an equivalent perfect Lambert diffuser (vertically integrated I /F). Intensities on the right show the ADU values integrated along pixel-wide cross sections such as that shown in Fig. 2.
the radial-scan photometric profiles corresponding to the middle image (full line) and bottom image (dotted line) in Fig. 1. Distance to the center of Saturn is indicated in arcseconds (bottom scale) and in kilometers (upper scale). Intensities are expressed as the height (in km) of an equivalent perfect Lambert diffuser (vertically integrated I /F). Conversion from magnitudes to reflectivities was done as described in Section 5. 4. THE DETECTION OF MOVING OBJECTS
A number of intensity peaks in Fig. 3 have been identified with satellites such as Pandora (on the left) and Epimetheus (on
the right). A fainter peak was identified with 1995 S5, an object discovered with the Hubble Space Telescope (Nicholson et al. 1995) and hereafter referred to as S5. For all other bodies detected during the 1995 ring-plane crossing, we drop the leading “1995” in a similar manner. Intensity peaks such as those produced by Pandora or S5 can be distinguished from the ring structure because they move from one frame to another. The following procedure was found to be quite effective in detecting such moving objects. It consists of creating two-dimensional displays of the ring brightness as a function of both time and position along the ring. Figure 4 is an example of such a display covering data taken on August 10 from
FIG. 4. Ring brightness as a function of distance from Saturn and time. This time sequence shows data taken on August 10 from 12:15 to 12:56 (UT). Each line displays the intensity in a profile such as that shown in Fig. 3. Time increases quasi-monotonically from bottom to top. Vertical bands show the fixed ring structure, whereas slanted lines are produced by moving objects. Note the eclipse of Epimetheus through the shadow of the A ring.
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12:15 to 12:56 (UT). Each line in this diagram displays the intensity of a photometric profile (such as that shown in Fig. 3) obtained from a single 15-s exposure. Since exposures were taken at fairly regular 1-min intervals, time increases quasi-monotonically from bottom to top. Two different kinds of patterns can be seen in Fig. 4: vertical bands corresponding to the fixed ring structure, and slanted lines produced by moving objects. We still see here the side of the rings opposite the Sun. Whereas most of the rings are opaque and appear dark, light scattered through the Cassini division and the C ring makes them appear bright, and produces the bright vertical bands seen in Fig. 4. One can also note the bright edge extending beyond the tip of the A ring, due to light scattered by the F ring. Moving objects can be identified by their orbital velocity (given by the slope of the slanted line) as well as their orbital phase. Four previously known objects have been identified in Fig. 4. These are Pandora, Epimetheus, S5, and Mimas. The detection of Pandora and S5 clearly demonstrates the remarkable increase in the sensitivity of ground-based observations due to adaptive optics (see also Sicardy et al. 1995). Figure 4 shows a temporary disappearance of Epimetheus, which we identified as due to an eclipse through the shadow of the A ring. A photometric analysis of this eclipse will be presented in a forthcoming paper. In addition to the traces left by the four above-mentioned objects, Fig. 4 also shows a number of fainter traces that we believe to be produced by newly discovered objects. The velocity of S5 as well as those of the newly detected fainter objects is fully consistent with Keplerian orbits at the distance of the F ring (Roddier et al. 1996b, Roddier 1997). We therefore believe they are clumps in the F ring similar to those detected by Voyager (Schowalter 1997). A small area centered on Mimas was used for the PSF estimation. The very faint line observed outside the F ring, below and parallel to the Mimas track, is an artifact at the edge of this area produced by deconvolution. To make the comparison easier between various observations, one can display the intensity as a function of the longitude along the F ring instead of distance to Saturn. Figure 5 shows such a display. Here we have assumed a Keplerian motion at a distance of 140,500 km from the center of Saturn and used the longitude at epoch 1995, August 10.5 TDT (at Saturn) as a horizontal coordinate. Longitudes are measured from the ascending node of Saturn’s equator on the Earth’s J2000.0 equator. In this display, any object with the above Keplerian motion (i.e., in or near the F ring) appears as fixed, and produces a bright vertical line. Objects moving at different rates produce slanted or curved lines. Note that a given distance to Saturn corresponds to two possible longitudes depending on whether it is on the front side or on the back side of the rings. Since we have plotted both solutions, each object has produced two traces, one corresponding to the forward motion and one to the backward motion. For objects in the F ring, one trace is a vertical line, the other one is a slanted line. The four wide horizontal strips are from our adaptive optics observations. From bottom to top, the first strip shows results from the AO data in the first line of Table I, the second
strip from data in lines 2 and 3, the third strip from data in lines 4 and 5, and the fourth strip from data in lines 6 and 7. AO data taken after the crossing time have not been included here because the rings were too bright to allow the detection of faint objects. The thinner strips on top of Fig. 5 are from observations made with the Planetary Camera (PC) on the HST (Nicholson et al. 1996). We have resampled them to our own standard of 000 .035 per pixel and processed them in the same way as our own data. These observations span from 13:37 to 23:40 on August 10. The early part of these PC observations coincides in time with the last AO observations displayed on the top large strip, but observations were made on the opposite ansa. A number of objects are identified on the top, especially the three clumps S5, S6, and S7 announced by the HST team (Nicholson et al. 1995). Both S5 and S7 are clearly seen in our AO data as in the HST data. We have no AO data at the location of S6. Of the objects identified by the May 1995 HST team (Bosh and Rivkin 1996), the only one that would appear as a vertical line on this plot is S3, which has been tentatively identified with S6 (Nicholson et al. 1996). Unfortunately, these fall in a region for which we have no data. Another prominent clump is observed at a longitude of about 317◦ . It was announced as S9 (Roddier et al. 1996a). It is harder to see in the HST images where it was later identified (McGhee et al. 1997). Another prominent clump is also seen near 46◦ . It was not announced with S9 because at that time our images had not yet been deconvolved and the detection was considered marginal. It is clear that many fainter clumps can be seen in Fig. 5. They have been announced in a subsequent IAU circular (Roddier et al. 1996b). Their estimated longitudes (as defined above) are indicated in Table II. The uncertainty on these numbers is ±1◦ . We have also estimated the clump sizes by comparing their brightness with that of S5. Table II shows the radii derived under the assumption that they all have the same albedo and that S5 has a radius of 26 km (Nicholson et al. 1995). It is interesting to note that S5 does not appear in our last set of data (fourth wide strip from bottom on Fig. 5), that is, past its maximum elongation. During the first part of the sequence, light scattered by Enceladus may impede the observation. However, S5 should become visible at the end of the sequence apparently moving back toward Saturn, but it does not. The same phenomenon can be seen in data taken by the Wide Field Planetary Camera (WFPC) on HST (Nicholson et al. 1996). S5 should be visible in the fourth image of the second set of HST data (second
TABLE II Longitude and Size of the Clumps Name
S9
S11 S12 S13 S14 S15 S16 S17 S18 S19
Longitude (deg)a 317 302 320 330 Radius (km) 16 12 10 12 a
46 16
Longitude is on August 10.5 TDT (at Saturn).
105 114 116 118 120 12 10 10 10 10
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FIG. 5. Time sequences showing the ring brightness as a function of time and longitude. From bottom to top, the four wide horizontal strips show results from the AO data listed respectively in lines 1, 2–3, 4–5, and 6–7 of Table I. The thinner strips on top of them are from observations made with the Planetary Camera on board the Hubble Space Telescope. In this representation, moving objects produce two tracks (see text). For those orbiting at a selected distance of 140,500 km from the center of Saturn, one of the tracks is a straight vertical line. The abscissa of the line on the horizontal axis is the object longitude on August 10.5 TDT (at Saturn). One notes a large number of faint objects orbiting near this distance. These include objects S5, S6, and S7 announced by Nicholson et al. (1995), as well as the other S-numbered objects announced by our team (Roddier et al. 1996a,b).
dotted line from top of Fig. 5 in Nicholson and colleagues’ paper), but it is not despite the good image quality. Similarly, S7 should later be visible at the same location in the sixth set of data (sixth dotted line from top of Nicholson and colleagues’ paper Fig. 5), but again it has disappeared. Olkin and Bosh (1996) have shown evidence that the F ring is inclined. Since these clumps are believed to be part of the F ring, their disappearance brings further evidence that the remote side of the west F-ring ansa may have been occulted by the main rings (Bosh, private communication). One may wonder why ground-based adaptive optics observations have been more efficient than HST observations in detecting clumps in the F ring. In both cases, the angular resolution and the noise level are similar. Part of the answer may be in the exposure time: 15 s for our observations, compared with 100 s for the HST observations. The apparent clump motion being on the order of 000 .1 per minute, the resulting blur may have lowered the signal-to-noise ratio in the HST images. The most important factor is probably the much larger number of frames taken from the ground as shown by the respective widths of the horizontal bands in Fig. 5.
5. THE PHOTOMETRY OF SATURN’S SATELLITES
Each night images of UKIRT faint standard stars FS2, FS4, and FS35 were taken and used as primary references (Casali and Hawarden 1992). We attempted to use the IRAF aperture photometry software, but for our data we found it more appropriate to fit the photometric profiles of the reference with those of the measured object along both the x and y directions. Profiles along the 45◦ and 135◦ directions were also used to center the object and the reference. Error bars were determined by the uncertainty on the fit. Since none of the nights was absolutely photometric and the seeing was variable, we found it necessary to use secondary standards, a technique particularly useful when doing photometry with AO images. Dione and Tethys were used for this purpose. However, except for the first night (August 9 UT), both of them were saturated, and the fit was limited to the wings of the profiles. The procedure was tested on the nonsaturated images and was found to give the same results within the given error bars. First, we compared the brightness of the primary standard with that of the secondary on frames taken as close in time as
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TABLE III J and H Band Photometry of Small Satellites Satellite
Filter
Magnitude
aS (km2 )
Orbital phase
Date, time (UT)
Prometheus Pandora Pandora Epimetheus Epimetheus Epimetheus Epimetheus Janus Janus Janus Janus Janus Janus Janus Janus Mimas Mimas Mimas Mimas Telesto Telesto Telesto Telesto Calypso Helene Helene
H H H J J H H J J J H H H H H J J H H J H H H H J H
15.3 ± 0.15 15.2 ± 0.1 15.35 ± 0.1 14.45 ± 0.05 14.45 ± 0.05 14.65 ± 0.1 14.75 ± 0.05 13.7 ± 0.1 13.9 ± 0.1 13.9 ± 0.1 13.7 ± 0.05 13.8 ± 0.05 13.8 ± 0.03 13.8 ± 0.03 13.8 ± 0.03 11.4 ± 0.1 11.7 ± 0.2 11.5 ± 0.03 11.45 ± 0.03 17.2 ± 0.3 17.5 ± 0.2 17.6 ± 0.4 17.4 ± 0.4 18.7 ± 0.4 16.7 ± 0.1 16.65 ± 0.1
2,000 ± 550 2,190 ± 400 1,910 ± 350 5,840 ± 540 5,840 ± 540 3,640 ± 670 3,620 ± 330 11,600 ± 2,100 9,690 ± 1,800 9,690 ± 1,800 8,720 ± 800 7,950 ± 730 7,950 ± 440 7,950 ± 440 7,950 ± 440 96,900 ± 18,000 73,500 ± 27,000 66,200 ± 37,00 69,300 ± 3,800 464 ± 260 263 ± 97 240 ± 180 289 ± 220 87 ± 66 735 ± 140 576 ± 110
73◦ (E) 67◦ (E) 270◦ (W) 292◦ (W) 260◦ (W) 298◦ (W) 270◦ (W) 145◦ (E) 287◦ (W) 94◦ (E) 53◦ (E) 132◦ (E) 216◦ (W) 296◦ (W) 90◦ (E) 297◦ (W) 300◦ (W) 307◦ (W) 305◦ (W) 306◦ (W) 309◦ (W) 145◦ (E) 148◦ (E) 225◦ (W) 215◦ (W) 217◦ (W)
Aug. 10, 11:30 Aug. 9, 15:00 Aug. 10, 12:40 Aug. 10, 12:20 Aug. 12, 12:50 Aug. 10, 12:30 Aug. 12, 13:30 Aug. 10, 12:00 Aug. 11, 11:40 Aug. 12, 12:00 Aug. 9, 15:00 Aug. 10, 11:30 Aug. 10, 15:00 Aug. 11, 12:00 Aug. 12, 11:30 Aug. 10, 12:20 Aug. 11, 11:40 Aug. 10, 12:30 Aug. 11, 12:00 Aug. 10, 12:20 Aug. 10, 12:40 Aug. 11, 13:30 Aug. 11, 13:50 Aug. 12, 15:15 Aug. 10, 12:20 Aug. 10, 12:40
possible. Then, we compared the brightness of the primary with that of the secondary on the object frame. This gave us a correction for the fluctuation in sky transparency. In doing this, we assumed that the brightness of Dione or Tethys was independent of its orbital position. It introduces an error that we have estimated by using the photometric curves of Franz and Millis (1975), and by assuming that the brightness variations on the orbit were the same in the infrared as in the visible. The error was found to be small and within the indicated error bars. Since several objects under study are quite faint, we averaged as many images of them as possible. However, when the image of a faint object is not well compensated, including it in the average leads to an underestimate of the flux. Therefore, we averaged only the best images. Background subtraction was made very carefully. Whenever the satellite was close to the ring, we subtracted a ring contribution estimated from a different image taken under similar atmospheric conditions (for instance, having similar values for the flux of a bright satellite). When the ring was very close, we used three different estimates. Errors are then dominated by the uncertainty of this contribution, and the error bar was taken as the root mean square (rms) variation over the three different estimates. We also cross-checked our results by comparing the profiles of two different satellites. As a result, the error bars are quite different from one night to the other or even from time to time during the same night. Maximum accuracy
was obtained when the satellite was observed nearly at the same time as the reference star and as far as possible from the ring. Our photometric results are given in Table III. They are consistent with those of Bauer et al. (1997) which were obtained from data taken at the IRTF during the same ring-plane crossing, but with no adaptive optics. Despite differences in the orbital phases, all our values fall within their errors bars. However, one can see from Table III that our error bars are substantially smaller. For the first time we were able to measure the J magnitudes of Epimetheus, Janus, Mimas, Telesto, and Helene and the H magnitudes of Prometheus, Pandora, Telesto, and Calypso. In those cases where both J and H magnitudes are available, it is possible to derive the J–H color index, which provides some crude information about surface composition. These small inner satellites have always been suspected to be composed of water ice, in part because of their high albedos (Morrison et al. 1986). The value of J–H is 0.0 within the errors of observation for Janus, Mimas, and Helene, while Epimetheus has J–H = −0.30 ± 0.1. The Telesto magnitudes were not determined well enough to give a definitive result, but also suggest a slightly negative value of J–H. All of these colors are consistent with a surface composition of water ice [e.g., J–H (Rhea) = −0.05, J–H (Dione) = −0.20] but exclude rocky surfaces [e.g., J–H (Ceres) = +0.31, J–H (Vesta) = +0.17] or organic compounds (e.g., Iapetus’ leading hemisphere has a
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TABLE IV Albedos Name
Visiblea
Prometheus Pandora Epimetheus Janus Mimas Telesto Calypso Helene
0.50 0.50 0.50 0.50 0.77 0.60 0.90 0.60
a
J band
H band
0.55 ± 0.12 0.39 ± 0.17 0.78 ± 0.10 0.90 ± 0.46
0.24 ± 0.40 0.34 ± 0.30 0.32 ± 0.12 0.30 ± 0.14 0.55 ± 0.03 0.60 ± 0.39 0.30 ± 0.54 0.72 ± 0.10
0.91 ± 0.1
Visible values are from Burns (1986).
J–H color ∼ = +0.4) (reference colors compiled by Tokunaga 1999). Table III also shows the equivalent area aS (in km2 ) of a perfect Lambert diffuser obtained from the expression aS = π
r12r22 (m sun −m sat )/2.5 10 , r02
where r0 is the heliocentric distance of the Earth, r1 is the heliocentric distance of Saturn, and r2 is the geocentric distance of Saturn. At the time of the observations, these distances were respectively 1.014, 9.619, and 8.784 AU. The quantity m Sun − m Sat is the difference in stellar magnitude between the Sun and the satellite. Assuming that the visible magnitude of the Sun is −26.74 (Allen 1973) and using the colors compiled by Tokunaga (1999), we took m Sun = −27.86 in the J band, and m Sun = −28.17 in the H band. The dimensions of the satellites were taken from Voyager’s measurements (Burns 1986) to estimate the cross-section area S and derive values for the satellite albedos a. These values are listed in Table IV. Unfortunately, satellites smaller than Mimas are often characterized by their irregular shape, adding uncertainty in the cross section at the time of the observations. The error bars given in Table IV have been obtained by adding quadratically the error bar due to photometric errors (Table III) and that due to the uncertainty in the cross section. Given the error bars, no correction was made for the solar phase angle which was close to 3.5◦ at the time of the observations. Table IV also lists the visible geometric albedos derived from the Voyager images recorded at 0.47 µm (Burns 1986).
FIG. 6. Plot of five satellite albedos in the V, J, and H bands. The very large error bars of Telesto are not shown for better clarity (see Table IV).
ADAPTIVE OPTICS OBSERVATIONS OF SATURN’S RING PLANE CROSSING
Figure 6 is a plot of these albedos as a function of wavelength for the five cases were we have visible, J, and H values. Although the errors bars are very large, all the satellites reported in the figure have decrasing albedos between J and H bands which agree with a surface dominated by water ice. Nevertheless, Helene and Telesto display a rather peculiar maximum at J band that is not visible for the other three objects. It is important to consider this result with caution since their J -band albedo has been estimated from only one photometric measurement and the error bar for Telesto is too large to derive any conclusion for this particular object. Therefore, if we consider the case of Helene only, the J-band maximum displayed by its albedo, if it turns out to be real, would have direct implications on the nature of its surface material. Helene’s surface, although dominated by water ice, would be coated in addition by a more complex material responsible for the measured reddening between V and J bands. This interesting result requires further confirmation to be fully validated. 6. SUMMARY
High-angular-resolution infrared images of the rings and satellites of Saturn were obtained in August 1995 as the Earth was crossing the ring plane. By using adaptive optics on a 3.35-m telescope aperture, we were able to obtain images with an optical quality comparable to that of the HST (0.1500 resolution in the H band). Twelve objects were discovered orbiting Saturn with a Keplerian motion close to that of the F ring. Two of them were identified as objects S5 and S7 first announced by the HST team (Nicholson et al. 1995). These objects are all believed to be clumps in the F ring, similar to those observed by the Voyager spacecraft. Infrared H magnitudes were obtained for the first time for Prometheus, Pandora, Telesto, and Calypso, as were J magnitudes for Epimetheus, Janus, Mimas, Telesto, and Helene. These results demonstrate the effectiveness of ground-based observations with adaptive optics. ACKNOWLEDGMENTS The UH AO system was built under NSF Grant AST 93-19004. The processing and analysis of the data were supported on NASA Grants NAGW 4935 and NAG 5-3731. We also acknowledge the use of Mark Showalter’s ephemeris generator written under the auspices of the Planetary Data System’s Rings Node, a very convenient tool.
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