- Email: [email protected]
The Discovery of Uranus XIX, XX, and XXI
Icarus 147, 320–324 (2000) doi:10.1006/icar.2000.6463, available online at http://www.idealibrary.com on
NOTE The Discovery of Uranus XIX, XX, and XXI B. Gladman1,2 Observatoire de la Cˆote d’Azur, BP4229, 06304 Nice Cedex 4, France E-mail: [email protected]
JJ Kavelaars1 Department of Physics and Astronomy, ABB-241, McMaster University, Hamilton, Ontario, L8S 4M1, Canada
M. Holman1 Harvard–Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138
J-M. Petit1 and H. Scholl Observatoire de la Cˆote d’Azur, BP4229, 06304 Nice Cedex 4, France
and P. Nicholson2 and J.A. Burns2 Cornell University, Ithaca, New York 14853 Received May 3, 2000 will thus provide valuable clues about the late stages of formation of the giant planets. The introduction of mosaic CCD cameras on 4-m class telescopes has allowed satellite searches to be extended to previously unattainable levels of sky coverage and magnitude depth. A cursory 3-h search of the environs around Uranus at the 5-m Hale telescope in 1997 yielded the first two irregular moons of Uranus, Caliban (U XVI) and Sycorax (U XVII) (see Gladman et al. 1998). Up to that time, Uranus was the only giant planet lacking such moons. A full search of the stable regions surrounding a giant planet is a daunting task, because in principle stable satellite orbits exist out to distances of order the Hill sphere radius RH of the planet (H´enon 1970), given by
Motivated by the discovery of the first two irregular satellites of Uranus in 1997, our team has conducted a search covering approximately 90% of the dynamically stable region around Uranus. We have discovered three additional objects moving at rates consistent with satellite orbital motion. At the end of 1999, the available observations are of sufficient quality to almost guarantee that these three new objects are bound to the planet and confirm that Uranus is host to a system of numerous, but faint and small, irregular satellites. We report a preliminary negative result in a similar search near Neptune. °c 2000 Academic Press Key Words: satellites; Uranus; Neptune.
µ R H = ap
1. Introduction. Because of their orbital distances and often inclined/ eccentric orbits, irregular satellites are usually thought to be objects captured into orbit around a forming planet early in the Solar System’s history. Although the capture processes are poorly understood, gas drag is often favored (Pollack et al. 1979). In this process a small body on heliocentric orbit is “decelerated” during a passage through a circumplanetary disk or “gas cocoon” and trapped in a bound orbit about a planet. The distribution of irregular satellite orbits 1 Visiting astronomer, Canada–France–Hawaii Telescope; operated by the National Research Council of Canada, le Centre National de la Recherche Scientifique de France, and the University of Hawaii. 2 Visiting astronomer, Palomar Observatory; observations made as part of a continuing collaborative agreement between the California Institute of Technology and Cornell University.
¶1/3 (1)
where m p and ap are the planet’s mass and semimajor axis about the Sun, the latter with mass M¯ . Theoretically (H´enon 1970) stable orbits for retrograde satellites exist to somewhat larger radii than RH , although no known examples exist in the Solar System. When viewed from the Sun, the angular radius of the planetary Hill sphere is 2 = RH /ap and thus the apparent size of the stable region depends only on the mass of the planet; when seen from the Earth there is a small correction of order 1/ap , where ap is in AU. For Uranus viewed at opposition, the Hill sphere subtends an angular radius of 2U ' 0.024 radians = 1.4◦ . Covering this area of more than 6 square degrees with a CCD detector was formerly a prohibitively time-consuming task on a large-aperture telescope. A moderateto large-aperture telescope is necessary since previous photographic searches (reviewed in Gladman et al. 1998) had shown that no irregular satellites brighter than 20th magnitude existed. Sycorax and Caliban have R-band magnitudes of 20.4 and 21.9, respectively.
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FIG. 1. The observed positions of S/1999 U 1, U 2, and U 3 relative to Uranus (at the origin of this figure) in 1999, on July 17 (CFHT), Aug. 9 (Palomar), Sept. 2 (KPNO 4-m), and Oct. 15 (KPNO 4-m) (UT dates). S/1999 U 2 was not observed in October due to its faintness. Arrows indicate the sense of motion of each irregular satellite about Uranus. For comparison, points at 10-day intervals for Caliban and Sycorax (generated from their now well-known orbits) between July 15 and Oct 15/99 are shown. Caliban and Sycorax were observed and measured at their predicted locations on all of the above dates. The projected orbit of Oberon, the outermost uranian regular satellite, is shown for reference. The inset shows the arrangement of CFH12K fields (each of 350 × 280 ) relative to the planet (the heavy dot). The dotted lines on the main panel correspond to the field edges of pointings 5, 6, 7, and 8 from the inset, as imaged during discvory observations in July 1999. Thus, Sycorax, S/1999 U 1, and S/1999 U 2 were all imaged on pointing 5. North is up and East is left. Note the differing x and y scales. 2. Observations. The development of mosaic CCD cameras on moderateaperture telescopes has produced an order-of-magnitude improvement in deep, wide-field imaging capabilities. We were granted time in July 1999 on the CFHT 3.5-m telescope on Mauna Kea, equipped with the CFH12K detector, an 8,000 × 12,000 pixel mosaic of 12 2K × 4K thinned CCDs (Cuillandre et al. 1999). At CFHT prime focus the camera has a pixel scale of 0.200 /pixel. During July 1999 only 10 of the CCDs were science-quality, resulting in a usable field of view 2 '6 of ∼350 × 280 ' 0.27 square degrees. This rendered a search of the π 2U square degrees of the uranian Hill sphere practical. To reduce scattered-light problems, we avoided the region within 30 of right ascension or declination of the planet and chose 24 fields as shown in Fig. 1 (inset), almost covering the uranian Hill sphere. Uranian satellites on circular orbits move at angular rates of at most (due to projection effects) ∼0.2200 /h at 100 from the planet, this rate scaling as the inverse square root of the separation. Thus the motion of satellites relative to the planet is small and very difficult to detect during a single night; rather, satellites “follow” the planet across the sky at fixed offsets each night. Imaging on multiple nights is required even to determine the sense of orbital motion about the planet; follow-up observations over months or years are subsequently required to accurately determine the orbital elements of the objects. Our observational strategy consisted of cycling three times among four pointings using 8-min exposures. This exposure time was the maximum allowed to prevent trailing losses on uranian satellites which, at a sidereal rate of 600 /h at
opposition, would move 1 FWHM in 10 min. in 100 seeing. This exposure time should reach m R ' 24 at CFHT in 100 seeing. With a roughly 2-min readout overhead, obtaining three images of four pointings consumed 2 h of telescope time. The three frames of a single field were thus spaced at 40-min intervals, producing ∼400 of motion between frames. This motion is excellent for automated detection codes, of which our group had two available: one based on wavelet analysis developed by H. Scholl (Scholl et al. 1999) and the other written by M. Holman based on the public domain object-classification software SExtractor (Bertin and Arnouts 1996) and coordinate transformation routines available in the WCSTools package (Mink 1999). Both codes were run on all the processed frames (debiased and flattened with standard techniques) in real time as the images came off the telescope. Real-time analysis was critical since it allowed us to avoid reimaging pointings relative to the planet which did not contain satellite candidates, almost doubling our sky coverage. If data reduction had occurred after the run it would have been necessary to image all of the pointings on two nights in order to have multinight detections of any satellite candidates. We found that using two automated codes was critical, because the two software packages sometimes found different objects (due to how “objects” are classified in each code). We had abundant tests of the code available in that we discovered roughly 40 Kuiper Belt objects moving <400 /h (these results will be reported elsewhere). It is clear that the two codes together complemented each other to provide a more complete survey than either operating alone would have
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done. The use of an automated detection code was forced by the data volume from the mosaic camera (∼25 Gbyte/night). We have not attempted to search the full 100-Gbyte data set by manual blinking. A major problem for satellite searches is that re-searching the entire Hill sphere would be necessary if, for example, only the northern half of it is covered in a given observing season. Because orbital periods vary from years to decades, one cannot simply image just the southern half (using the previous example) of the Hill sphere one year after the northern search because satellites could very well have rotated into the northern half in the intervening time. It is therefore imperative that the entire stable region be searched in a single observing season. In July 1999, we successfully imaged the first 23 of our 24 planned mosaic fields (Fig. 1 inset), with position 24 being missed due to time lost to equipment problems. All data were obtained under seeing conditions between 00 0.700 and 1.2 , roughly the norm for CFHT. Because fields without satellite candidates were not reimaged on a second night, and because we used a classical three-frame moving-object detection algorithm (where one requires that the object be found by the automated code in all three frames), objects which passed in front of a star or galaxy on one of the three frames were missed. By examining the images, we calculate that 3% of the field area was covered with stationary objects sufficiently bright (>3σ ) so that they are effectively eliminated from the search on any single frame. Because objects move several FWHM between frames, this problem repeats for every exposure and so ∼9% of the field is lost. The chip gaps and “bad” regions of the chips contribute only ∼1%, and thus we estimate that our search was ∼90% efficient to a 50% “completeness magnitude” of m R = 24.3 ± 0.3, covering approximately an annulus surrounding the planet with inner/outer radii of ∼30 /750 , respectively (see Fig. 1). The magnitude limit of m R = 24.3 ± 0.3 has been determined by implanting false objects in a subset of the data frames and observing where the automated codes dropped to 50% detection efficiency; this characterization of the data will be improved in our forthcoming thorough reanalysis for the Kuiper Belt objects. 3. Results. Our observations identified three satellite candidates. All were moving (within astrometric errors) at a velocity identical to that of the planet on any given night, and yet all were much faster than the numerous and more distant Kuiper Belt objects. The first two satellites candidates were discovered on our first night of observation, and because they were identified that same night, we were able to reimage them 24 and 72 h later, establishing their uranocentric motion. These data were of sufficient quality, and supported by the lack of any other objects moving at anywhere near the uranian rate, for these objects to be designated “Probable New Satellites of Uranus,” S/1999 U 1 and S/1999 U 2 by B. Marsden in IAU telegram 7230 on July 31, 1999. Their discovery magnitudes of m R = 23.3 and 24.3 correspond to diameters of 30 and 20 km assuming an albedo of 0.07 (see Gladman et al. 1998). Object S/1999 U 3 was not found in real time at the telescope, due to blending of its image with a bright star; M. Holman identified it 2 weeks later after re-searching the data set with different values of the detection parameters in his code. Its discovery was reported in IAU Circular 7248 after observations were obtained on multiple nights at the Palomar 5-m telescope (see below). The discovery magnitude of m R = 23.2 implies a nominal diameter of ∼30 km. High-precision astrometry of the candidates was conducted using plate solutions computed using the United States Naval Observatory A2.0 catalogue. The residuals of the plate solution fit were usually on the order of 0.300 , but multiple solutions using different astrometric software packages and/or different samples of reference stars gave scatters of 0.4–0.500 for a given image. We estimate a 1σ astrometric error of ±0.400 for the majority of our astrometric positions for the discovery and subsequent recovery observations, with exceptions due to poor seeing or crowding noted in Table I. We also conducted an additional survey, nearly identical to that discribed above, but centered on Neptune, during our July 1999 observing run. We searched all 24 of the offset fields (Fig. 1 inset) covering >90% of the dynamically stable region around Neptune. As for Uranus, we estimate a 50% completeness limit of m R ∼ 24. This survey resulted in the detection of numerous Kuiper belt objects and one centaur but no new neptunian satellites. A more careful reanalysis of this data set (that is, not under real-time pressure at altitude) is underway.
4. Preliminary orbital geometry. After the July discovery all satellite candidates were tracked in every dark run over the following 3-month period. In August 1999 all three candidates were imaged at the Palomar 5-m Hale telescope by P. Nicholson, B. Gladman, and J.A. Burns. On September 2 and 3, 1999, they were again all recovered at the KPNO 4-m Mayall telescope by D. Davis, B. Gladman, and C. Neese. Lastly, in mid-October the brightest two candidates (S/1999 U 1 and U 3) were reimaged from the KPNO 4-m by M. Holman and J. Kavelaars. The resulting astrometry is reported in Table I. One position for each satellite relative to the planet during each of these observing runs is shown in Fig. 1. In all three cases orbital curvature can be seen. Although curvature does not prove that the object is on a bound planetocentric orbit, several independent orbital calculations (B. Marsden in IAU Circular 7385; B. Gray, private communication; P. Nicholson; R. Jacobson, private communication) show that reasonable planetocentric solutions exist that fit the astrometry for each of the putative satellites. Using the data up to the end of 1999, it is not possible to rule out the possibility that the objects are centaurs. However, the surface density of centaurs brighter than m R = 24.2 is estimated to be only ∼0.5/square degree (Jewitt et al. 1996), taking into account all heliocentric distances. Thus, in our Uranus search of ∼6 square degrees, we should have found three centaurs. But such centaurs could be at any heliocentric distance, whereas the three detected objects were all at exactly the distance of Uranus, and all lay within ∼1 square degree. This is a potentially troubling coincidence for the hypothesis that any one of S/1999 U 1–U 3 is a centaur. The additional facts that all three candidates closely follow the planet over a 3-month orbital arc, with planetocentric motions consistent with bound orbits at their projected distances, lends strong support to the satellite hypothesis. The 1999 observations are comparable in quality (in terms of length of arc and sampling) to those of Caliban and Sycorax when they were lost behind the Sun after their discovery apparition (Jacobson 1999). We thus feel that it is exceedingly likely that all of these objects will be confirmed as satellites upon their recovery in the summer of 2000. During our search we found no other objects with motions in the range expected for centaurs; this is somewhat disturbing but only 2σ deviant from the reported sky density. Not much can yet be said about the orbital geometries of these new satellites. Even the direct or retrograde sense of revolution (using the heliocentric motion of Uranus as a “prograde” reference) is difficult to establish because of ambiguous projection effects. From an examination of the offsets relative to the planet, we can make the following tentative observations. It appears as though S/1999 U 1 was discovered near apocenter, in a geometry similar to that of Sycorax during summer 1999 (Fig. 1). The slow motion and lack of strong deceleration of S/1999 U 3 as it moved away from the planet implies that it was likely far from the planet in the foreground or background, in a moderately eccentric orbit (e ∼ 0.5). S/1999 U 2 can be fit by a nearly circular retrograde orbit similar to that of Caliban, but the observed arc is insufficient to prove that the eccentricity is small, and a prograde solution of moderate eccentricity is possible. The orbital periods of U 1 and U 3 may lie in the range 7–11 years. The only certain orbital constraint that can be set on the new objects is a forbidden orbital inclination (I ) range relative to the uranian orbital plane. This constraint is due to the “Kozai effect” in which solar perturbations cause coupled eccentricity/inclination oscillations in the orbital elements of the satellites. These either produce mild variations as in the case of Sycorax (Gladman et al. 1998) or for large enough inclinations and semimajor axes can drive the pericenters into the planet, eliminating the satellite (Kinoshita and Nakai 1991). This effect depends on sin2 I , and so is “symmetric” with respect to prograde or retrograde orbits. Assumuing an initially circular orbit for S/1999 U 1 at its current projected separation of 940 RU excludes orbital inclinations in the range 85◦ < I < 95◦ , based on a semianalytic calculation of the perturbation Hamiltonian valid for large e and I (A. Morbidelli, private communication, 2000). None of the proposed orbital fits violate this limited constraint. 5. Discussion. Our observational program has proven that Uranus is surrounded by an unexpectedly rich system of irregular satellites, albeit they are smaller than those of Jupiter. We think it likely that satellites even smaller than these await discovery. The orbital characteristics of these moons will provide constraints for those wishing to reexamine theories of satellite capture
NOTE
TABLE I
Notes. 1—Kavelaars, Gladman, Holman, Petit (CFHT); 2—Nicholson, Gladman, Burns (Palomar 5-m); 3—Davis, Gladman, Neese (KPNO 4-m); 4—Holman, Kavelaars (KPNO 4-m); astrometric uncertainty α ± 0.03 s, δ ± 0.400 except ∗ for which α ± 0.07 s, δ ± 100 .
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(cf. Pollack et al. 1979, Burns 1986). Although firm orbits for the 1999 candidates are not yet available, it is clear that the range of orbital distances of the uranian irregulars is large. Caliban’s pericenter is at 257 planetary radii (RU = 25 560 km), or 0.095 RH , while S/1999 U 1’s minimum apocentric distance (bounded only from below because of projection effects) is >940 RU (>0.347 RH ). In contrast, Jupiter’s irregular satellite system is about a factor of 2 more distant, occupying an annulus of roughly 0.17–0.61 jovian Hill spheres. Gas drag capture, which may seem plausible for the distant jovian irregulars (Saha and Tremaine 1993), is more problematic for a small satellite such as Caliban, which must be dragged down to a nearly circular orbit very deep inside the uranian gravity well and yet not fall all the way to the planet (see Peale 1999 for a recent discussion). It will be particularly interesting to determine if the uranian irregulars are grouped in the same way as the prograde and retrograde groups of Jupiter, a fact often ascribed to collisional breakup after the capture event(s). Clearly there is great promise that a more complete inventory of the irregular satellite systems will provide valuable clues about the physical conditions and/or processes occuring during the final stages of the formation of the giant planets (e.g., Parisi and Brunini 1997). 6. Note added in Proof. On May 27–29 2000, KPNO 4-m observations by Holman, Gladman, and Kavelaars recovered S/1999 U 3. On Jun 28–29 2000, NOT 2.5-m observations by Holman et al. measured S/1999 U 1 and U 3. The initial recoveries were 1–20 from ephemeris calculations by the abovementioned workers. Recovery of the much fainter S/1999 U 2 will be attempted in summer 2000. See http://www.obs-nice.fr/fr/gladman/urhome.html for up-todate information.
REFERENCES Bertin, E., and S. Arnouts 1996. SExtractor: Software for source extraction. Astron. Astrophys. 117, 393–404. Burns, J. A. 1986. Some background about satellites. In Satellites (J. A. Burns and M. S. Matthews, Eds.), pp. 1–37. Univ. of Arizona Press, Tucson.
Cuillandre, J.-C., G. Luppino, B. Starr, and S. Isani 1999. CFH12K: Optimizing 12 MIT/LL CCID20 CCDs for a direct imaging application. In Optical Detectors for Astronomy, Proc. of 4th ESO CCD Workshop (P. Amico and J. W. Beletic, Eds.). Kluwer Academic, Dordrecht/Norwell, MA. Gladman, B., P. Nicholson, J. A. Burns, J. J. Kavelaars, B. Marsden, G. Williams, and W. Offut 1998. Discovery of two distant irregular moons of Uranus. Nature 392, 897–899. H´enon, M. 1970. Numerical exploration of the restricted problem. VI. Hill’s case: Non-periodic orbits. Astron. Astrophys. 9, 24–36. Jacobson, R. 1999. The determination and long-term integration of the orbits of Caliban and Sycorax. Bull. Am. Astron. Soc. 31, 1224. Jewitt, D. G., J. Luu, and J. Chen 1996. The Mauna Kea–Cerro Tololo Kuiper Belt and Centaur survey. Astron. J. 112, 1225–1238. Kinoshita, H., and H. Nakai 1991. Secular perturbations of fictitious satellites of Uranus. Celest. Mech. Dynam. Astron. 52, 293–303. Mink, D. J. 1999. WCSTools: An image astrometry toolkit. In Astronomical Data Analysis Software and Systems VIII (D. Mehringer, R. Plante, and D. Roberts, Eds.), A.S.P. Conference Series, Vol. 172, pp. 498–501. Parisi, M., and A. Brunini 1997. Constraints to Uranus’ great collision, II. Planet. Space Sci. 45, 181–187. Peale, S. 1999. Origin and evolution of the natural satellites. Ann. Rev. Astron. Astrophys. 37, 533–602. Pollack, J. B., J. A. Burns, M. E. Tauber 1979. Gas drag in primordial circumplanetary envelopes—A mechanism for satellite capture. Icarus 37, 587– 611. Saha, P., and S. Tremaine 1993. The orbits of the retrograde Jovian satellites. Icarus 106, 549–561. Scholl, H., A. Bijaoui, and R. Savalle 1999. Automatic detection of solar system bodies based on wavelet filtering. In Treasuring Hunting in Astronomical Plates (P. Kroll, C. la Dous, and H.-J. Braeuer, Eds.), Acta Hitsor. Astron. 6, 188–190.
NOTE The Discovery of Uranus XIX, XX, and XXI B. Gladman1,2 Observatoire de la Cˆote d’Azur, BP4229, 06304 Nice Cedex 4, France E-mail: [email protected]
JJ Kavelaars1 Department of Physics and Astronomy, ABB-241, McMaster University, Hamilton, Ontario, L8S 4M1, Canada
M. Holman1 Harvard–Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138
J-M. Petit1 and H. Scholl Observatoire de la Cˆote d’Azur, BP4229, 06304 Nice Cedex 4, France
and P. Nicholson2 and J.A. Burns2 Cornell University, Ithaca, New York 14853 Received May 3, 2000 will thus provide valuable clues about the late stages of formation of the giant planets. The introduction of mosaic CCD cameras on 4-m class telescopes has allowed satellite searches to be extended to previously unattainable levels of sky coverage and magnitude depth. A cursory 3-h search of the environs around Uranus at the 5-m Hale telescope in 1997 yielded the first two irregular moons of Uranus, Caliban (U XVI) and Sycorax (U XVII) (see Gladman et al. 1998). Up to that time, Uranus was the only giant planet lacking such moons. A full search of the stable regions surrounding a giant planet is a daunting task, because in principle stable satellite orbits exist out to distances of order the Hill sphere radius RH of the planet (H´enon 1970), given by
Motivated by the discovery of the first two irregular satellites of Uranus in 1997, our team has conducted a search covering approximately 90% of the dynamically stable region around Uranus. We have discovered three additional objects moving at rates consistent with satellite orbital motion. At the end of 1999, the available observations are of sufficient quality to almost guarantee that these three new objects are bound to the planet and confirm that Uranus is host to a system of numerous, but faint and small, irregular satellites. We report a preliminary negative result in a similar search near Neptune. °c 2000 Academic Press Key Words: satellites; Uranus; Neptune.
µ R H = ap
1. Introduction. Because of their orbital distances and often inclined/ eccentric orbits, irregular satellites are usually thought to be objects captured into orbit around a forming planet early in the Solar System’s history. Although the capture processes are poorly understood, gas drag is often favored (Pollack et al. 1979). In this process a small body on heliocentric orbit is “decelerated” during a passage through a circumplanetary disk or “gas cocoon” and trapped in a bound orbit about a planet. The distribution of irregular satellite orbits 1 Visiting astronomer, Canada–France–Hawaii Telescope; operated by the National Research Council of Canada, le Centre National de la Recherche Scientifique de France, and the University of Hawaii. 2 Visiting astronomer, Palomar Observatory; observations made as part of a continuing collaborative agreement between the California Institute of Technology and Cornell University.
¶1/3 (1)
where m p and ap are the planet’s mass and semimajor axis about the Sun, the latter with mass M¯ . Theoretically (H´enon 1970) stable orbits for retrograde satellites exist to somewhat larger radii than RH , although no known examples exist in the Solar System. When viewed from the Sun, the angular radius of the planetary Hill sphere is 2 = RH /ap and thus the apparent size of the stable region depends only on the mass of the planet; when seen from the Earth there is a small correction of order 1/ap , where ap is in AU. For Uranus viewed at opposition, the Hill sphere subtends an angular radius of 2U ' 0.024 radians = 1.4◦ . Covering this area of more than 6 square degrees with a CCD detector was formerly a prohibitively time-consuming task on a large-aperture telescope. A moderateto large-aperture telescope is necessary since previous photographic searches (reviewed in Gladman et al. 1998) had shown that no irregular satellites brighter than 20th magnitude existed. Sycorax and Caliban have R-band magnitudes of 20.4 and 21.9, respectively.
320 0019-1035/00 $35.00 c 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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FIG. 1. The observed positions of S/1999 U 1, U 2, and U 3 relative to Uranus (at the origin of this figure) in 1999, on July 17 (CFHT), Aug. 9 (Palomar), Sept. 2 (KPNO 4-m), and Oct. 15 (KPNO 4-m) (UT dates). S/1999 U 2 was not observed in October due to its faintness. Arrows indicate the sense of motion of each irregular satellite about Uranus. For comparison, points at 10-day intervals for Caliban and Sycorax (generated from their now well-known orbits) between July 15 and Oct 15/99 are shown. Caliban and Sycorax were observed and measured at their predicted locations on all of the above dates. The projected orbit of Oberon, the outermost uranian regular satellite, is shown for reference. The inset shows the arrangement of CFH12K fields (each of 350 × 280 ) relative to the planet (the heavy dot). The dotted lines on the main panel correspond to the field edges of pointings 5, 6, 7, and 8 from the inset, as imaged during discvory observations in July 1999. Thus, Sycorax, S/1999 U 1, and S/1999 U 2 were all imaged on pointing 5. North is up and East is left. Note the differing x and y scales. 2. Observations. The development of mosaic CCD cameras on moderateaperture telescopes has produced an order-of-magnitude improvement in deep, wide-field imaging capabilities. We were granted time in July 1999 on the CFHT 3.5-m telescope on Mauna Kea, equipped with the CFH12K detector, an 8,000 × 12,000 pixel mosaic of 12 2K × 4K thinned CCDs (Cuillandre et al. 1999). At CFHT prime focus the camera has a pixel scale of 0.200 /pixel. During July 1999 only 10 of the CCDs were science-quality, resulting in a usable field of view 2 '6 of ∼350 × 280 ' 0.27 square degrees. This rendered a search of the π 2U square degrees of the uranian Hill sphere practical. To reduce scattered-light problems, we avoided the region within 30 of right ascension or declination of the planet and chose 24 fields as shown in Fig. 1 (inset), almost covering the uranian Hill sphere. Uranian satellites on circular orbits move at angular rates of at most (due to projection effects) ∼0.2200 /h at 100 from the planet, this rate scaling as the inverse square root of the separation. Thus the motion of satellites relative to the planet is small and very difficult to detect during a single night; rather, satellites “follow” the planet across the sky at fixed offsets each night. Imaging on multiple nights is required even to determine the sense of orbital motion about the planet; follow-up observations over months or years are subsequently required to accurately determine the orbital elements of the objects. Our observational strategy consisted of cycling three times among four pointings using 8-min exposures. This exposure time was the maximum allowed to prevent trailing losses on uranian satellites which, at a sidereal rate of 600 /h at
opposition, would move 1 FWHM in 10 min. in 100 seeing. This exposure time should reach m R ' 24 at CFHT in 100 seeing. With a roughly 2-min readout overhead, obtaining three images of four pointings consumed 2 h of telescope time. The three frames of a single field were thus spaced at 40-min intervals, producing ∼400 of motion between frames. This motion is excellent for automated detection codes, of which our group had two available: one based on wavelet analysis developed by H. Scholl (Scholl et al. 1999) and the other written by M. Holman based on the public domain object-classification software SExtractor (Bertin and Arnouts 1996) and coordinate transformation routines available in the WCSTools package (Mink 1999). Both codes were run on all the processed frames (debiased and flattened with standard techniques) in real time as the images came off the telescope. Real-time analysis was critical since it allowed us to avoid reimaging pointings relative to the planet which did not contain satellite candidates, almost doubling our sky coverage. If data reduction had occurred after the run it would have been necessary to image all of the pointings on two nights in order to have multinight detections of any satellite candidates. We found that using two automated codes was critical, because the two software packages sometimes found different objects (due to how “objects” are classified in each code). We had abundant tests of the code available in that we discovered roughly 40 Kuiper Belt objects moving <400 /h (these results will be reported elsewhere). It is clear that the two codes together complemented each other to provide a more complete survey than either operating alone would have
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done. The use of an automated detection code was forced by the data volume from the mosaic camera (∼25 Gbyte/night). We have not attempted to search the full 100-Gbyte data set by manual blinking. A major problem for satellite searches is that re-searching the entire Hill sphere would be necessary if, for example, only the northern half of it is covered in a given observing season. Because orbital periods vary from years to decades, one cannot simply image just the southern half (using the previous example) of the Hill sphere one year after the northern search because satellites could very well have rotated into the northern half in the intervening time. It is therefore imperative that the entire stable region be searched in a single observing season. In July 1999, we successfully imaged the first 23 of our 24 planned mosaic fields (Fig. 1 inset), with position 24 being missed due to time lost to equipment problems. All data were obtained under seeing conditions between 00 0.700 and 1.2 , roughly the norm for CFHT. Because fields without satellite candidates were not reimaged on a second night, and because we used a classical three-frame moving-object detection algorithm (where one requires that the object be found by the automated code in all three frames), objects which passed in front of a star or galaxy on one of the three frames were missed. By examining the images, we calculate that 3% of the field area was covered with stationary objects sufficiently bright (>3σ ) so that they are effectively eliminated from the search on any single frame. Because objects move several FWHM between frames, this problem repeats for every exposure and so ∼9% of the field is lost. The chip gaps and “bad” regions of the chips contribute only ∼1%, and thus we estimate that our search was ∼90% efficient to a 50% “completeness magnitude” of m R = 24.3 ± 0.3, covering approximately an annulus surrounding the planet with inner/outer radii of ∼30 /750 , respectively (see Fig. 1). The magnitude limit of m R = 24.3 ± 0.3 has been determined by implanting false objects in a subset of the data frames and observing where the automated codes dropped to 50% detection efficiency; this characterization of the data will be improved in our forthcoming thorough reanalysis for the Kuiper Belt objects. 3. Results. Our observations identified three satellite candidates. All were moving (within astrometric errors) at a velocity identical to that of the planet on any given night, and yet all were much faster than the numerous and more distant Kuiper Belt objects. The first two satellites candidates were discovered on our first night of observation, and because they were identified that same night, we were able to reimage them 24 and 72 h later, establishing their uranocentric motion. These data were of sufficient quality, and supported by the lack of any other objects moving at anywhere near the uranian rate, for these objects to be designated “Probable New Satellites of Uranus,” S/1999 U 1 and S/1999 U 2 by B. Marsden in IAU telegram 7230 on July 31, 1999. Their discovery magnitudes of m R = 23.3 and 24.3 correspond to diameters of 30 and 20 km assuming an albedo of 0.07 (see Gladman et al. 1998). Object S/1999 U 3 was not found in real time at the telescope, due to blending of its image with a bright star; M. Holman identified it 2 weeks later after re-searching the data set with different values of the detection parameters in his code. Its discovery was reported in IAU Circular 7248 after observations were obtained on multiple nights at the Palomar 5-m telescope (see below). The discovery magnitude of m R = 23.2 implies a nominal diameter of ∼30 km. High-precision astrometry of the candidates was conducted using plate solutions computed using the United States Naval Observatory A2.0 catalogue. The residuals of the plate solution fit were usually on the order of 0.300 , but multiple solutions using different astrometric software packages and/or different samples of reference stars gave scatters of 0.4–0.500 for a given image. We estimate a 1σ astrometric error of ±0.400 for the majority of our astrometric positions for the discovery and subsequent recovery observations, with exceptions due to poor seeing or crowding noted in Table I. We also conducted an additional survey, nearly identical to that discribed above, but centered on Neptune, during our July 1999 observing run. We searched all 24 of the offset fields (Fig. 1 inset) covering >90% of the dynamically stable region around Neptune. As for Uranus, we estimate a 50% completeness limit of m R ∼ 24. This survey resulted in the detection of numerous Kuiper belt objects and one centaur but no new neptunian satellites. A more careful reanalysis of this data set (that is, not under real-time pressure at altitude) is underway.
4. Preliminary orbital geometry. After the July discovery all satellite candidates were tracked in every dark run over the following 3-month period. In August 1999 all three candidates were imaged at the Palomar 5-m Hale telescope by P. Nicholson, B. Gladman, and J.A. Burns. On September 2 and 3, 1999, they were again all recovered at the KPNO 4-m Mayall telescope by D. Davis, B. Gladman, and C. Neese. Lastly, in mid-October the brightest two candidates (S/1999 U 1 and U 3) were reimaged from the KPNO 4-m by M. Holman and J. Kavelaars. The resulting astrometry is reported in Table I. One position for each satellite relative to the planet during each of these observing runs is shown in Fig. 1. In all three cases orbital curvature can be seen. Although curvature does not prove that the object is on a bound planetocentric orbit, several independent orbital calculations (B. Marsden in IAU Circular 7385; B. Gray, private communication; P. Nicholson; R. Jacobson, private communication) show that reasonable planetocentric solutions exist that fit the astrometry for each of the putative satellites. Using the data up to the end of 1999, it is not possible to rule out the possibility that the objects are centaurs. However, the surface density of centaurs brighter than m R = 24.2 is estimated to be only ∼0.5/square degree (Jewitt et al. 1996), taking into account all heliocentric distances. Thus, in our Uranus search of ∼6 square degrees, we should have found three centaurs. But such centaurs could be at any heliocentric distance, whereas the three detected objects were all at exactly the distance of Uranus, and all lay within ∼1 square degree. This is a potentially troubling coincidence for the hypothesis that any one of S/1999 U 1–U 3 is a centaur. The additional facts that all three candidates closely follow the planet over a 3-month orbital arc, with planetocentric motions consistent with bound orbits at their projected distances, lends strong support to the satellite hypothesis. The 1999 observations are comparable in quality (in terms of length of arc and sampling) to those of Caliban and Sycorax when they were lost behind the Sun after their discovery apparition (Jacobson 1999). We thus feel that it is exceedingly likely that all of these objects will be confirmed as satellites upon their recovery in the summer of 2000. During our search we found no other objects with motions in the range expected for centaurs; this is somewhat disturbing but only 2σ deviant from the reported sky density. Not much can yet be said about the orbital geometries of these new satellites. Even the direct or retrograde sense of revolution (using the heliocentric motion of Uranus as a “prograde” reference) is difficult to establish because of ambiguous projection effects. From an examination of the offsets relative to the planet, we can make the following tentative observations. It appears as though S/1999 U 1 was discovered near apocenter, in a geometry similar to that of Sycorax during summer 1999 (Fig. 1). The slow motion and lack of strong deceleration of S/1999 U 3 as it moved away from the planet implies that it was likely far from the planet in the foreground or background, in a moderately eccentric orbit (e ∼ 0.5). S/1999 U 2 can be fit by a nearly circular retrograde orbit similar to that of Caliban, but the observed arc is insufficient to prove that the eccentricity is small, and a prograde solution of moderate eccentricity is possible. The orbital periods of U 1 and U 3 may lie in the range 7–11 years. The only certain orbital constraint that can be set on the new objects is a forbidden orbital inclination (I ) range relative to the uranian orbital plane. This constraint is due to the “Kozai effect” in which solar perturbations cause coupled eccentricity/inclination oscillations in the orbital elements of the satellites. These either produce mild variations as in the case of Sycorax (Gladman et al. 1998) or for large enough inclinations and semimajor axes can drive the pericenters into the planet, eliminating the satellite (Kinoshita and Nakai 1991). This effect depends on sin2 I , and so is “symmetric” with respect to prograde or retrograde orbits. Assumuing an initially circular orbit for S/1999 U 1 at its current projected separation of 940 RU excludes orbital inclinations in the range 85◦ < I < 95◦ , based on a semianalytic calculation of the perturbation Hamiltonian valid for large e and I (A. Morbidelli, private communication, 2000). None of the proposed orbital fits violate this limited constraint. 5. Discussion. Our observational program has proven that Uranus is surrounded by an unexpectedly rich system of irregular satellites, albeit they are smaller than those of Jupiter. We think it likely that satellites even smaller than these await discovery. The orbital characteristics of these moons will provide constraints for those wishing to reexamine theories of satellite capture
NOTE
TABLE I
Notes. 1—Kavelaars, Gladman, Holman, Petit (CFHT); 2—Nicholson, Gladman, Burns (Palomar 5-m); 3—Davis, Gladman, Neese (KPNO 4-m); 4—Holman, Kavelaars (KPNO 4-m); astrometric uncertainty α ± 0.03 s, δ ± 0.400 except ∗ for which α ± 0.07 s, δ ± 100 .
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(cf. Pollack et al. 1979, Burns 1986). Although firm orbits for the 1999 candidates are not yet available, it is clear that the range of orbital distances of the uranian irregulars is large. Caliban’s pericenter is at 257 planetary radii (RU = 25 560 km), or 0.095 RH , while S/1999 U 1’s minimum apocentric distance (bounded only from below because of projection effects) is >940 RU (>0.347 RH ). In contrast, Jupiter’s irregular satellite system is about a factor of 2 more distant, occupying an annulus of roughly 0.17–0.61 jovian Hill spheres. Gas drag capture, which may seem plausible for the distant jovian irregulars (Saha and Tremaine 1993), is more problematic for a small satellite such as Caliban, which must be dragged down to a nearly circular orbit very deep inside the uranian gravity well and yet not fall all the way to the planet (see Peale 1999 for a recent discussion). It will be particularly interesting to determine if the uranian irregulars are grouped in the same way as the prograde and retrograde groups of Jupiter, a fact often ascribed to collisional breakup after the capture event(s). Clearly there is great promise that a more complete inventory of the irregular satellite systems will provide valuable clues about the physical conditions and/or processes occuring during the final stages of the formation of the giant planets (e.g., Parisi and Brunini 1997). 6. Note added in Proof. On May 27–29 2000, KPNO 4-m observations by Holman, Gladman, and Kavelaars recovered S/1999 U 3. On Jun 28–29 2000, NOT 2.5-m observations by Holman et al. measured S/1999 U 1 and U 3. The initial recoveries were 1–20 from ephemeris calculations by the abovementioned workers. Recovery of the much fainter S/1999 U 2 will be attempted in summer 2000. See http://www.obs-nice.fr/fr/gladman/urhome.html for up-todate information.
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