Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/pasa/15_2/brown/paper.pdf Äàòà èçìåíåíèÿ: Mon Jul 27 05:20:24 1998 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 04:19:40 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: star trail

Publ. Astron. Soc. Aust., 1998, 15, 176­8
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A Search for Bright Kuip er Belt Ob jects
Michael J. I. Brown and R. L. Webster
School of Physics, University of Melbourne, Parkville, Vic. 3052, Australia mbrown@physics.unimelb.edu.au Received 1997 September 2, accepted 1998 April 20

Abstract: Since 1992, 60 large Kuiper Belt ob jects have been detected by ground-based telescopes. Previous surveys which have detected ob jects have searched approximately 60 and detected ob jects with magnitudes 20 · 6 < mR < 25 · 0. However, the luminosity function of brighter Kuiper Belt ob jects is not well determined. The detection of brighter ob jects would improve our ability to determine the Kuiper Belt ob jects' surface composition and provide constraints on the population statistics of different formation mechanisms. This paper describes a survey of 12 · 0 of sky near the ecliptic to a limiting magnitude of mR 21. A slow moving candidate was detected near the magnitude limit of the survey. Keywords: comets: general--solar system: general

1 Introduction Since the discovery of 1992QB1 (Jewitt & Luu 1993), a total of 60 large ( > 50 km radius) but dim (mR > 20 · 6) ob jects have been found with orbits beyond Neptune (Marsden 1997). It is thought that these ob jects are members of a belt of ob jects beyond Neptune (the Kuiper Belt) hypothesised by Edgeworth (1949) and Kuiper (1951). Previous published surveys for Kuiper Belt ob jects are summarised in Table 1. Successful surveys for Kuiper Belt ob jects have covered a total area of 60 near the ecliptic (Jewitt & Luu 1995; Williams et al. 1995; Irwin, Tremaine & Zytkow 1995; Jewitt, Luu & Chen 1996; Jewitt Luu & Trujillo 1998). The ob jects detected by ground-based searches have had magnitudes 20 · 6 < mR < 25 · 0 with the ma jority of ob jects having magnitudes 22 < mR < 24. Several surveys have covered larger areas of sky to brighter limiting magnitudes, but these surveys have not detected any Kuiper Belt ob jects. The size and magnitude distribution of Kuiper Belt ob jects brighter than mR 21 is therefore not well constrained. Ideally one would like to continue previously successful surveys until larger ob jects are detected. However, all the successful surveys so far have used 1 · 5 m to 4 m telescopes at good sites that are heavily used by other astronomers. In this search, the compromise solution was to use a 1 m telescope with a large field of view. The disadvantage with using a smaller telescope is that the limiting magnitude is significantly brighter than that expected on a larger telescope. While shallow surveys of large areas of sky have not detected Kuiper Belt ob jects, several have
Astronomical Society of Australia 1998

detected ob jects in orbits between Jupiter and Neptune (Centaurs). Only seven Centaurs have been detected which have well-determined orbits (Marsden 1997). Centaurs and Kuiper Belt ob jects have a slow apparent motion compared to main-belt asteroids and they are often described as slow moving ob jects (SMOs).
Table 1. Surveys for Kuiper Belt ob jects The validity of the statistical detection of 29 ob jects by Cochran et al. (1995) is debatable (Jewitt, Luu & Chen 1996; Brown, Kulkarni & Liggett 1997) and it has not been included in the analysis of the sky surface density presented in this paper. The mR limit for Kowal (1989) is that given in Irwin, Tremaine & Zytkow (1995), while the mR limit for Levison & Duncan (1990) is that given by Levison (private communication). Only 50% of the Kuiper Belt ob jects at the magnitude limit of Jewitt, Luu & Trujillo (1998) could be detected by that survey. A detailed discussion of previous surveys for Kuiper Belt ob jects can be found in Brown (1997). Another 22 Kuiper Belt ob jects have been detected (Marsden 1997) by surveys that have not been published in peer reviewed journals Reference Tombaugh (1961) Kowal (1989) Luu & Jewitt (1988) Irwin et al. (1995) Levison & Duncan (1990) Present work Williams et al. (1995) Jewitt et al. (1998) Jewitt et al. (1996) Irwin et al. (1995) Luu & Jewitt (1988) Jewitt et al. (1996) Jewitt & Luu (1995) Cochran et al. (1995) mR limit 16 18 19 20 21 21 22 22 23 23 24 24 24 28 · · · · · · · · · · · · · · 8 5 5 0 0 0 0 5 2 5 0 2 8 6 Area ( ) 1530 6400 297 50 4·9 12 · 0 0·5 51 · 5 4·4 0·7 0 · 34 3·9 1·2 0 · 0011 Detections 0 0 0 0 0 0 1 13 3 2 0 12 7 29?

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2 Observations Four observing runs between April 1995 and April 1997 used the Mount Stromlo and Siding Spring Observatories 40-inch telescope to search for Kuiper Belt ob jects. The first three observing runs used a 2250 â 1152 CCD with a focal reducer, the Low Dispersion Survey Spectrograph (Colless et al. 1990), to increase the size of the field. The resulting pixel scale was 2 · 1 per pixel, however the entire CCD was not illuminated; so the field was 0 · 44 . The 2 · 1 pixel scale results in undersampling of stellar images so analysis techniques such as difference maps (Irwin, Tremaine & Zytkow 1995) could not be used. The final observing run used a 2048 â 2048 thinned Tektronix CCD with a pixel scale of 0 · 6 per pixel and a field of view of 0 · 11 . All observing runs used R-band filters as this is the most efficient band because of the solar to red ob ject colours (Luu 1994; Luu & Jewitt 1996) and the CCD wavelength response. The apparent motion (in /hr) near opposition of an ob ject orbiting at distance R, where R 1 au, is given by d dt 148 · 5 au â cos , R (1)

ob ject was not a member of the Kuiper Belt. It is possible that this ob ject is a Centaur in an eccentric and inclined orbit but it is also possible that it is closer to Earth (Marsden, private communication). Astrometry for the candidate is given in Table 2.
Table 2. UT date 25/4/95 25/4/95 25/4/95 Time 11 08 13 38 16 36 Candidate astrometry RA (J2000) ±0 · 7 14 02 16 · 99 14 02 15 · 78 14 02 14 · 45 Dec (J2000) ±0 · 7 -12 47 27 · 4 -12 47 14 · 5 -12 47 00 · 7

where is the angle from opposition. For an ob ject just beyond Neptune, this results in an apparent motion of 3 /hr near opposition. This motion is detectable in a single night's observations so, where possible, all observations were done in a single night in dark time. This reduced problems caused by changing weather conditions and allowed for easier image analysis. A minimum of three sets of 3 â 400 s integrations of each target field were made at intervals of at least 2 hours. Integrations longer than half an hour were not possible due to trailing loss caused by the apparent motion of SMOs across the sky. Images were also taken of Landolt (1992) and Graham (1982) standards to provide photometric calibration. All images were bias subtracted, flat fielded and blinked to search for candidates. Blinking was used along with less time consuming automated techniques as fainter candidates can be detected by eye (Jewitt and Luu 1995; Levison, private communication; Jewitt, private communication). Images (Irwin 1985), an image detection program, was used to detect ob jects and our own software was then used to select ob jects moving with a regular rate of motion. While not as efficient as blinking, this allowed candidates to be detected at the telescope, allowing rapid follow-up observations. Full details of the observation strategy and data reduction are presented in Brown (1997). An mR 21 SMO candidate was identified in three images taken on 25 April 1995. The 8 · 3 ± 0 · 3 /hr apparent motion of the candidate indicated that this

A second mR 21 SMO candidate was identified in three images taken on 24 April 1995. The candidate's apparent motion of 1 · 8 /hr corresponds to a distance of 55 au. Observations of the same field in April 1997 with the 2048 â 2048 CCD in good seeing detected a mR 21 star near the position where the candidate was detected and it is assumed that the candidate was produced by the star and noise. Magnitude limits were determined by adding artificial SMOs to sets of images at random positions. The artificial SMOs were then detected by blinking and with automated techniques. The limiting magnitude of the 2250 â 1152 CCD images was mR 21. The higher quantum efficiency and lower read-out noise of the 2048 â 2048 CCD resulted 21 · 5. Deeper in a limiting magnitude of mR limiting magnitudes were achieved for a small number of images where seeing was below 1 · 5 . The capability of the search to detect SMOs was also tested by recovering 2060 Chiron (mR 16), 1995 GO (mR 19) and 1995 DW2 (mR 21 · 2).
Table 3. m 18 19 21 22 23 24 24
R

Sky surface density of Kuiper Belt ob jects Area ( ) 6800 393 69 41 10 5 1 Detections 0 0 2 7 16 20 7 <6 · <1 · 3· 0· 1· 3· 5· deg-
2

· · · · · · ·

5 5 0 0 2 2 8

· · · ·

6 2 1 2

8â10-4 1â10-2 3 0±2 · 8 â10 ·7 0 19±0 · 11 · 10 0·4 6±0 · 4 1 9±1 · 1 ·1 3 8±2 · 3 ·9

-2

3 Luminosity Function The luminosity function of the Kuiper Belt has, until recently, not been well constrained near mR 21. This is due to the small areas covered by most surveys to limiting magnitudes mR > 21. Using the data from this survey and previous work, new constraints were determined on the luminosity function of the Kuiper Belt near mR 21. Table 3 shows the constraints on the sky surface density of Kuiper Belt ob jects using data from this work, Kowal (1989), Luu and Jewitt (1988), Levison and Duncan (1990), Jewitt and Luu (1995), Irwin, Tremaine & Zytkow


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(1995), Jewitt, Luu & Chen (1996) and Jewitt, Luu & Trujillo (1998). Upper limits and error margins were calculated with Poisson statistics. Upper limits are 99% confidence limits and error margins have 68% confidence. It should be noted that the sky surface density does not always equal the number of detections divided by the survey area due to the decrease of detection efficiency with magnitude. In particular, Jewitt et al. (1998) has an 80% detection efficiency at mR 21, but this decreases to 60% by mR 22. Pluto has been excluded from the analysis as it is 8 magnitudes brighter than any known Kuiper Belt ob ject. As Table 3 includes data from surveys with different limiting magnitudes, the number of ob jects detected does not always increase with limiting magnitude. Figure 1 is plot of the sky surface density of Kuiper Belt ob jects near the ecliptic. The detections of bright Kuiper Belt ob jects by Jewitt et al. (1998) indicate that a break in the luminosity of the Kuiper Belt is not required unless the photographic survey by Kowal (1989) had a high detection efficiency to the limiting magnitude stated by Kowal or Irwin et al. (1995). A CCD survey of 100 would provide a much improved estimate of the population of bright Kuiper Belt ob jects and possibly detect a break in the luminosity function.

4 Conclusions A search of 12 · 0 near the ecliptic to a limiting magnitude of mR 21 has been conducted for bright Kuiper Belt ob jects. This search and other surveys constrain the number of Kuiper Belt ob jects brighter 3 than our limiting magnitude to 3 · 0 ±2 · 8 â10-2 per ·7 . Surveys with the planned 8k â 8k CCD on the Siding Spring 40-inch telescope should provide an improved estimate of the population of bright Kuiper Belt ob jects and further constrain the bright end of the luminosity function. Acknowledgments The authors would like to thank Chris Fluke, Michael Hicks and Thomas Irving for their assistance during observing with the Mount Stromlo and Siding Spring Observatories 40-inch telescope. We would also like to thank the Mount Stromlo and Siding Spring Observatories Time Allocation Committee for allocating several weeks of dark time on the 40-inch telescope for this survey. References
Brown, M. E., Kulkarni, S. R., & Liggett, T. J. 1997, ApJ, 490, L119 Brown, M. J. I. 1997, MSc thesis, University of Melbourne Cochran, A. L., Levison, H. F., Stern, S. A., & Duncan, M. J. 1995, ApJ, 455, 342 Colless, M., Ellis, R. S., Taylor, K., & Hook, R. N. 1990, MNRAS, 244, 408 Edgeworth, K. 1949, MNRAS, 109, 600 Graham, J. 1982, PASP, 94, 244 Irwin, M. 1985, MNRAS, 214, 575 Irwin, M., Tremaine, S., & Zytkow, A. 1995, AJ, 110, 3082 Jewitt, D., & Luu, J. 1993, Nature, 362, 730 Jewitt, D., & Luu, J. 1995, AJ, 109, 1867 Jewitt, D., Luu, J., & Chen, J. 1996, AJ, 112, 1225 Jewitt, D., Luu, J., & Trujillo, C. 1998, AJ, 115, 2125 Kowal, C. 1989, Icarus, 77, 118 Kuiper, G. P. 1951, in Astrophysics, ed. J. A. Hynek (New York: McGraw­Hill), p. 357 Landolt, A. 1992, AJ, 104, 340 Levison, H. F., & Duncan, M. J. 1990, AJ, 100, 1669 Luu, J. 1994, in Asteroids, Comets, Meteors 1993, ed. A. Milani et al. (Amsterdam: Kluwer), p. 31 Luu, J., & Jewitt, D. 1988, AJ, 95, 1256 Luu, J., & Jewitt, D. 1996, AJ, 111, 499 Marsden, B. G. 1997, Minor Planet Electronic Circular V25 Tombaugh, C. W. 1961, in Planets and Satellites, ed. G. P. Kuiper & B. M. Middlehurst (University of Chicago Press), p. 12 Williams, I., O'Ceallaigh, D., Fitzsimmons, A., & Marsden, B. G. 1995, Icarus, 116, 180

Figure 1--Sky surface density of Kuiper Belt ob jects.