Near-nucleus photometry of comets using archived NEAT data

Icarus 188 (2007) 457–467 www.elsevier.com/locate/icarus

Near-nucleus photometry of comets using archived NEAT data Michael D. Hicks a,∗ , Raymond J. Bambery b , Kenneth J. Lawrence a , Priya Kollipara c a Jet Propulsion Laboratory, JPL MS 183-501, 4800 Oak Grove Drive, Pasadena, CA 91109, USA b Jet Propulsion Laboratory, JPL MS 168-527, 4800 Oak Grove Drive, Pasadena, CA 91109, USA c Department of Astronomy, California Institute of Technology, 105-24 Caltech, 1201 East California Blvd., Pasadena, CA 91125, USA

Received 3 May 2006; revised 13 September 2006 Available online 30 January 2007

Abstract Though optimized to discover and track fast moving Near-Earth Objects (NEOs), the Near-Earth Asteroid Tracking (NEAT) survey dataset can be mined to obtain information on the comet population observed serendipitously during the asteroid survey. We have completed analysis of over 400 CCD images of comets obtained during the autonomous operations of two 1.2-m telescopes: the first on the summit of Haleakala on the Hawaiian island of Maui and the second on Palomar Mountain in southern California. Photometric calibrations of each frame were derived using background catalog stars and the near-nucleus comet photometry measured. We measured dust production and normalized magnitudes for the coma and nucleus in order to explore cometary activity and comet size–frequency distributions. Our data over an approximately two-year time frame (2001 August–2003 February) include 52 comets: 12 periodic, 19 numbered, and 21 non-periodic, obtained over a wide range of viewing geometries and helio/geocentric distances. Nuclear magnitudes were estimated for a subset of comets observed. We found that for low-activity comets (Afρ < 100 cm) our model gave reasonable estimates for nuclear size and magnitude. The slope of the cumulative luminosity function of our sample of low-activity comets was 0.33 ± 0.04, consistent with the slope we measured for the Jupiter-family cometary nuclei collected by Fernández et al. [Fernández, J.A., Tancredi, G., Rickman, H., Licandro, J., 1999. Astron. Astrophys. 392, 327–340] of 0.38 ± 0.02. Our slopes of the cumulative size distribution α = 1.50 ± 0.08 agree well with the slopes measured by Whitman et al. [Whitman, K., Morbidelli, A., Jedicke, R., 2006. Icarus 183, 101–114], Meech et al. [Meech, K.J., Hainaut, O.R., Marsden, B.G., 2004. Icarus 170, 463–491], Lowry et al. [Lowry, S.C., Fitzsimmons, A., Collander-Brown, S., 2003. Astron. Astrophys. 397, 329–343], and Weissman and Lowry [Weissman, P.R., Lowry, S.C., 2003. Lunar Planet. Sci. 34. Abstract 34]. © 2007 Elsevier Inc. All rights reserved. Keywords: Comets; Photometry

1. Introduction: Physical studies of comets help us to understand conditions in the outer Solar System at the late stage of planetary formation, as well as providing a record of subsequent collisional evolution (Weidenschilling, 1997). NEAT (Near-Earth Asteroid Tracking) is a NASA funded program based at the Jet Propulsion Laboratory and designed to discover Near-Earth Objects (NEOs). In this paper we sought to mine the NEAT archives for short and long period comets collected during the course of the NEO survey. The majority of our comet observations were made near perihelion, where reflected light from the dust coma * Corresponding author.

E-mail address: [email protected] (M.D. Hicks). 0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2006.12.023

obscures any signal from the bare nucleus. For these comets we sought to compile activity curves as a function of heliocentric distance. Knowledge of the dust production is critical for any potential spacecraft flyby or rendezvous target. For a select number of low-activity comets we were able to subtract coma contamination and measure nuclear sizes directly, enabling us to explore size and luminosity distribution functions for the comet population. Physical evolution models, by both collision and sublimation, are critically constrained by observed size– frequency distributions. The NEAT program utilizes two nearly autonomous 1.2-m telescopes: the first on the summit Haleakala on the Hawaiian island of Maui and the second on Palomar Mountain in southern California. Though NEAT shares both facilities with other users, scheduling was such that typically at least one telescope

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was scanning the sky each night for NEAT, limited mainly by weather. The Maui Space Surveillance Site (MSSS) 1.2-m telescope was made available to NEAT through the cooperation of the United States Air Force (USAF) Space Command. The NEAT team provided the 15 micron pixel 4K×4K CCD camera and control computers. The MSSS camera was thermoelectrically cooled and its images unfiltered, with a field of view of 2.6 square degrees. On a clear night with good seeing, the limiting magnitude of the MSSS system was typically V ∼ 19.5 in a 20 s exposure. A previous incarnation of the NEAT utilizing the 1-m USAF GEODSS telescope is described by Pravdo et al. (1999), with the operations and software nearly identical to the MSSS system used in this study. The NEAT/Palomar instrument consisted of 3 identical thermoelectrically cooled front illuminated 4K×4K CCDs mounted at the prime focus of the Oschin telescope in a North– South linear array, spaced such that they tiled the sky efficiently with staggered pointings. The NEAT/Palomar instrument was capable of reaching V ∼ 20.5 in a 60 s exposure under good sky conditions. With a plate scale of 1 arcsec pixel−1 , each 3-array image covered 3.7 square degrees of sky. To search for moving objects each pointing was visited three times at 15–30 min intervals, with exposure times of 60 s for NEAT/Palomar and 20 s for NEAT/MSSS (in order to maximize sky coverage). The data presented in this paper cover a two-year time span, 2001 August–2003 February, during which the NEAT/MSSS and the NEAT/Palomar 3-array camera concurrently operational. In early 2003, the Oschin 3-array camera was removed and the Palomar-QUEST large-area camera (Rabinowitz et al., 2003) was installed. The QUEST camera is a LN2 112-CCD mosaic covering 9.5 square degrees. NEAT utilizes this camera in stare mode for approximately 12 nights per lunation. For consistency between the NEAT/Palomar and NEAT/MSSS datasets, we chose to restrict this study to the earlier 3-array camera. The NEAT program is unique in that it archives all image data and makes it publicly available through the web-based interface SKYMORPH (http://skys.gsfc.nasa.gov/ skymorph/skymorph.html), where it is regularly used for asteroid pre-discoveries, supernova searches, and high-proper motion surveys (Wood-Vasey et al., 2003; Teegarden et al., 2003). As outlined below, we bypassed SKYMORPH and worked directly from the raw data, as the images we needed were available directly from the JPL NEAT hard-drives. 2. Photometric data reductions The normal NEAT reduction programs are designed to identify moving objects against a backdrop of stars. Each of the three CCD images in a given triplet was dark-subtracted and point sources extracted. A plate solution for each image was measured and stationary objects filtered out. The remaining objects are checked for linear motion between adjacent frames and automatically giving each a NEAT specific label which encodes the date/time when the moving object was first observed. For NEAT/MSSS objects this preliminary designation label contains 6 characters and for NEAT/Palomar an additional A, B, or C was appended to denote the CCDs in the 3-array camera.

Fig. 1. Contour plot of 67P/Churyumov–Gerasimenko. This comet represents an example of a moderately active comet.

The astrometry is submitted to the Minor Planet Center, who in turn linked up the positions with known and new asteroids and comets and emailed the identifications back to the NEAT team. We used the designations provided by the Minor Planet center to identify the known and newly discovered comets in our archive of submitted NEAT astrometry. The NEAT reduction logs provided a record of the image file names, dark frames and approximate pixel coordinates associated with each comet detection. The full 4K×4K fits images were de-archived, dark subtracted, shifted to the comet centroids, and cropped to 512×512 subarrays. Fig. 1 illustrates a typical comet image after this process. In every case, some amount of coma was apparent by eye in the dark-subtracted images. Over 140 triplets containing observations of comets were identified in the archived NEAT data. Table 1 summarizes our photometric results; listing the comet name, the UT date, the heliocentric and geocentric distances and solar phase angle. Each image in the triplet was analyzed separately with results compared and averaged. It was often necessary to reject spurious measurements, as when the comet passed near a background star. The IRAF astronomical reduction software package was used in the standard fashion to extract centroids and instrumental magnitudes for the comet and all point sources with a greater than 2 − σ peak above the sky background, using a photometric aperture of 8 pixel radius and 8–20 pixel annular sky windows. Both NEAT cameras observed though an extreme range of weather conditions, often variable on minute time-scales. We used the instrumental magnitudes of the background stars as on-chip calibrators to provide reasonably accurate (10–20%) absolute photometry for our comets. With the WCS toolkit and USNO-A2 stellar catalog (Mink, 2002), we extracted Rband magnitudes for the background sources, compared with

Photometry of NEAT comets

459

Table 1 Observational geometry, photometric magnitudes, and model results Object name

Date of observation

r (AU)

 (AU)

ϕ (◦ )

δR (mag)

Htot (mag)

Hnuc (mag)

Non-periodic comets 5.3 18.4 ± 0.1 1.8 18.6 ± 0.2 7.5 19.5 ± 0.1

R (mag)

Afρ (cm)

1.4 1.5 1.5

10.0 10.1 10.6

11.4 11.6 12.0

680 ± 84 650 ± 93 350 ± 47

C/1999 T2 C/1999 T2 C/1999 T2

2002 03 10 2002 04 05 2002 05 11

5.32 5.49 5.75

4.43 4.50 5.02

C/2000 CT54 C/2000 CT54 C/2000 CT54

2002 10 28 2002 10 28 2002 11 20

5.50 5.50 5.65

4.57 4.45 4.95

4.2 4.2 7.6

18.6 ± 0.2 18.6 ± 0.2 19.0 ± 0.1

1.1 1.0 1.0

10.5 10.5 10.6

11.5 11.5 11.6

440 ± 65 440 ± 70 340 ± 31

C/2000 K2 C/2000 K2 C/2000 K2

2001 08 26 2001 09 03 2002 01 02

4.10 4.17 5.10

3.51 3.47 4.85

12.4 11.1 10.9

17.6 ± 0.1 17.5 ± 0.2 18.8 ± 0.1

1.0 1.1 1.0

10.5 10.4 10.6

11.4 11.3 11.5

490 ± 55 560 ± 94 330 ± 34

C/2001 B2 C/2001 B2 C/2001 B2 C/2001 B2 C/2001 B2 C/2001 B2 C/2001 B2 C/2001 B2 C/2001 B2

2001 04 24 2002 01 01 2002 01 07 2002 01 26 2002 02 05 2002 02 24 2002 03 05 2002 03 13 2002 04 12

5.58 6.39 6.42 6.50 6.54 6.62 6.66 6.69 6.82

4.92 5.82 5.75 5.60 5.58 5.64 5.72 5.81 6.34

8.2 7.4 6.8 3.8 2.4 1.4 3.0 4.3 7.7

17.6 ± 0.1 17.6 ± 0.2 17.2 ± 0.2 17.9 ± 0.1 17.8 ± 0.1 18.0 ± 0.1 18.4 ± 0.1 18.2 ± 0.1 18.4 ± 0.2

1.4 1.1 0.9 1.2 1.4 1.3 1.4 1.3 1.4

8.9 8.5 8.3 8.8 8.6 8.9 9.0 8.8 8.7

10.2 9.5 9.1 9.9 9.9 10.1 10.4 10.1 10.0

1700 ± 150 2020 ± 310 2510 ± 350 1750 ± 190 2200 ± 230 1650 ± 150 1400 ± 180 1580 ± 120 1590 ± 270

C/2001 G1 C/2001 G1 C/2001 G1

2002 04 04 2002 04 05 2002 04 18

8.30 8.30 8.31

7.31 7.31 7.32

1.0 0.9 1.2

18.6 ± 0.1 18.8 ± 0.1 18.8 ± 0.1

1.0 1.3 1.1

8.7 8.6 8.8

9.7 9.9 9.9

1510 ± 210 1600 ± 180 1390 ± 190

C/2001 HT50 C/2001 HT50 C/2001 HT50 C/2001 HT50 C/2001 HT50 C/2001 HT50 C/2001 HT50

2002 02 08 2002 02 08 2002 02 11 2002 02 16 2002 02 20 2002 03 10 2002 03 15

5.61 5.61 5.59 5.56 5.53 5.40 5.36

4.83 4.83 4.77 4.69 4.63 4.43 4.41

6.5 6.5 6.1 5.3 4.7 3.1 3.4

17.5 ± 0.1 17.5 ± 0.1 17.2 ± 0.1 17.3 ± 0.1 17.3 ± 0.1 17.3 ± 0.1 17.2 ± 0.1

1.2 1.2 1.1 1.4 1.3 1.6 1.3

9.1 9.1 8.9 8.7 8.9 8.8 9.0

10.2 10.2 9.9 10.1 10.1 10.3 10.2

1510 ± 110 1520 ± 160 1810 ± 160 2100 ± 190 1910 ± 150 2270 ± 270 1900 ± 230

C/2001 K5 C/2001 K5 C/2001 K5 C/2001 K5

2001 05 28 2001 06 20 2001 07 12 2002 06 28

6.40 6.28 6.20 5.24

5.39 5.45 5.63 4.53

2.1 5.7 8.2 8.5

16.6 ± 0.2 17.2 ± 0.1 16.6 ± 0.1 15.4 ± 0.1

0.5 0.5 0.3 0.7

8.3 9.0 8.4 7.6

8.8 9.4 8.7 8.2

2820 ± 400 1480 ± 170 2220 ± 170 5810 ± 600

C/2001 M10 C/2001 M10 C/2001 M10

2001 06 30 2001 08 11 2002 07 26

5.30 5.31 5.92

4.37 4.34 5.15

4.9 3.6 7.0

19.1 ± 0.2 18.9 ± 0.1 19.3 ± 0.5

1.0 0.8 0.8

11.2 11.3 10.9

12.1 12.0 11.7

240 ± 47 230 ± 31 250 ± 120

C/2001 O2 C/2001 O2 C/2001 O2 C/2001 Q1

2001 08 02 2001 08 16 2001 08 24 2001 08 18

6.90 6.98 7.02 5.84

5.98 5.99 6.03 4.85

3.8 2.2 1.9 2.5

19.9 ± 0.2 19.1 ± 0.4 19.8 ± 0.1 19.6 ± 0.2

2.0 1.5 1.7 1.6

9.7 9.5 9.9 10.7

11.7 10.9 11.6 12.2

680 ± 120 870 ± 340 580 ± 77 350 ± 48

C/2001 T4 C/2001 T4 C/2001 T4

2002 10 29 2002 11 21 2002 11 22

8.48 8.59 8.59

7.60 7.70 7.71

1.0 3.0 3.1

19.6 ± 0.1 19.7 ± 0.2 20.0 ± 0.2

0.9 0.9 1.0

9.7 9.6 9.8

10.5 10.5 10.8

590 ± 59 580 ± 84 500 ± 83

C/2002 A2 C/2002 A2

2002 01 17 2002 02 04

4.72 4.73

3.76 3.88

3.2 6.7

18.8 ± 0.1 18.8 ± 0.2

0.6 0.4

11.9 12.0

12.5 12.3

140 ± 17 120 ± 18

C/2002 A3 C/2002 A3 C/2002 A3

2003 01 17 2003 01 28 2003 02 19

5.53 5.57 5.63

4.62 4.59 4.66

4.2 2.0 2.5

18.7 ± 0.1 18.6 ± 0.1 18.3 ± 0.2

1.1 1.0 1.1

10.5 10.5 10.1

11.5 11.5 11.2

440 ± 39 440 ± 54 650 ± 110

C/2002 C2 C/2002 C2

2002 11 15 2003 01 13

3.85 4.15

2.98 3.85

8.2 13.4

18.1 ± 0.2 19.2 ± 0.2

0.8 0.8

11.9 12.2

12.6 12.7

180 ± 28 99 ± 19

C/2002 J4

2002 05 14

5.68

4.72

3.5

18.9 ± 0.2

1.4

10.3

11.6

510 ± 75

C/2002 K2

2002 06 28

5.39

4.63

7.4

19.2 ± 0.2

0.8

11.3

12.0

200 ± 30

C/2002 K4

2002 07 07

2.76

1.95

15.4

18.2 ± 0.3

1.3

13.0

14.1

86 ± 25 (continued on next page)

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M.D. Hicks et al. / Icarus 188 (2007) 457–467

Table 1 (continued)  (AU)

ϕ (◦ )

R (mag)

δR (mag)

Htot (mag)

Hnuc (mag)

4.34 4.33 4.32 4.25 4.20 4.16 4.15 4.08

3.39 3.35 3.33 3.31 3.40 3.56 3.59 3.99

3.9 2.7 1.8 4.8 8.8 11.6 12.0 13.9

17.2 ± 0.1 17.3 ± 0.1 16.9 ± 0.1 17.4 ± 0.1 17.4 ± 0.1 17.1 ± 0.2 17.4 ± 0.1 17.6 ± 0.1

0.5 0.5 0.5 0.8 0.7 0.7 0.6 0.5

10.8 10.9 10.6 10.7 10.8 10.4 10.8 10.9

11.3 11.4 11.0 11.5 11.3 10.9 11.2 11.1

450 ± 46 400 ± 37 570 ± 65 480 ± 39 420 ± 51 540 ± 96 380 ± 43 310 ± 36

2002 12 03

1.90

0.93

7.7

15.8 ± 0.1

1.1

13.3

14.3

160 ± 17

C/2003 A2

2003 01 27

11.52

10.53

0.5

19.7 ± 0.2

1.1

8.1

9.2

1810 ± 270

C/2003 E1 C/2003 E1

2003 01 13 2003 03 22

4.15 4.19

3.85 3.30

13.4 6.8

19.2 ± 0.2 19.1 ± 0.2

0.4 0.4

12.5 12.9

12.8 13.2

70 ± 15 65 ± 12

2P/Encke

2001 08 24

3.59

2.58

Numbered comets 1.1 20.1 ± 0.2

0.8

14.5

15.2

21 ± 4

22P/Kopff 22P/Kopff

2002 01 14 2002 04 11

3.18 2.70

2.72 1.76

16.8 9.9

19.2 ± 0.2 17.2 ± 0.2

0.8 1.0

13.5 12.7

14.1 13.5

39 ± 6 140 ± 25

30P/Rein 1 30P/Rein 1 30P/Rein 1

2003 01 29 2003 02 03 2003 03 01

1.90 1.91 1.97

1.05 1.03 2.17

19.8 17.7 13.8

15.9 ± 0.2 16.0 ± 0.1 15.2 ± 0.2

1.1 1.1 1.1

13.0 13.2 10.8

13.8 14.0 11.7

150 ± 25 140 ± 17 640 ± 120

31P/Sc-Wa 2 31P/Sc-Wa 2 31P/Sc-Wa 2 31P/Sc-Wa 2 31P/Sc-Wa 2 31P/Sc-Wa 2 31P/Sc-Wa 2

2002 01 09 2002 02 03 2002 02 05 2002 03 04 2002 03 12 2003 03 25 2003 04 02

3.41 3.41 3.41 3.41 3.42 3.78 3.81

2.53 2.42 2.42 2.52 2.60 2.86 2.84

8.5 0.8 0.7 8.3 10.9 6.9 3.9

18.3 ± 0.1 17.3 ± 0.2 17.8 ± 0.1 18.7 ± 0.1 19.7 ± 0.1 19.5 ± 0.2 19.3 ± 0.2

0.9 0.7 1.0 1.0 1.9 1.3 0.7

12.5 12.0 12.2 12.8 12.9 12.9 13.4

13.3 12.7 13.2 13.8 14.6 14.1 14.0

110 ± 12 220 ± 32 170 ± 24 87 ± 12 80 ± 8 71 ± 11 50 ± 7

36P/Whipple 36P/Whipple

2002 07 12 2002 07 21

3.53 3.51

2.69 2.60

10.8 8.7

20.1 ± 0.3 20.0 ± 0.2

0.5 0.8

14.6 14.3

14.9 15.0

16 ± 4 21 ± 5

53P/Van B 53P/Van B 53P/Van B

2002 02 24 2002 02 25 2002 03 12

4.75 4.74 4.67

3.80 3.79 3.68

4.0 3.8 0.7

19.5 ± 0.2 19.4 ± 0.2 19.3 ± 0.1

1.0 0.7 0.5

12.2 12.3 12.5

13.1 13.0 13.1

110 ± 16 97 ± 18 86 ± 10

57P/duT-N-D 57P/duT-N-D 57P/duT-N-D 57P/duT-N-D 57P/duT-N-D 57P/duT-N-D

2002 06 02 2002 07 01 2002 07 13 2002 07 21 2002 09 12 2002 11 06

1.81 1.75 1.74 1.73 1.77 1.94

0.96 0.77 0.73 0.72 0.93 1.53

24.0 12.9 6.9 3.9 24.6 30.3

17.4 ± 0.1 16.8 ± 0.1 16.6 ± 0.1 16.1 ± 0.2 16.8 ± 0.1 18.3 ± 0.1

1.2 1.5 1.5 1.3 1.3 1.3

14.6 14.4 14.4 14.2 14.1 14.1

15.5 15.8 15.9 15.5 15.0 15.0

37 ± 5 61 ± 7 71 ± 8 91 ± 14 60 ± 6 32 ± 3

65P/Gunn

2002 04 01

3.32

2.43

9.2

16.6 ± 0.1

1.6

10.4

11.8

850 ± 100

66P/du Toit

2003 04 08

2.18

1.19

4.6

18.8 ± 0.2

0.4

16.3

16.6

8±2

67P/Ch-Ge 67P/Ch-Ge

2003 02 05 2003 04 09

2.25 2.71

1.40 1.86

16.3 13.8

17.2 ± 0.4 18.4 ± 0.3

1.4 1.5

13.1 13.2

14.2 14.5

110 ± 39 81 ± 19

74P/Sm-Ch 74P/Sm-Ch 74P/Sm-Ch 74P/Sm-Ch

2002 05 18 2002 05 19 2002 05 30 2002 06 25

3.89 3.90 3.91 3.94

2.88 2.89 2.92 3.13

0.9 1.0 3.8 10.0

17.3 ± 0.1 17.6 ± 0.1 17.5 ± 0.1 17.9 ± 0.1

1.4 1.7 1.2 1.5

10.7 10.6 10.9 10.7

12.1 12.3 12.1 12.1

610 ± 70 660 ± 75 460 ± 52 480 ± 64

81P/Wild 2

2002 12 15

2.92

1.97

6.3

16.7 ± 0.1

1.1

11.7

12.7

310 ± 34

92P/Sanguin 92P/Sanguin 92P/Sanguin 92P/Sanguin 92P/Sanguin 92P/Sanguin

2002 06 06 2002 07 08 2002 07 26 2002 11 03 2002 12 16 2002 12 31

2.12 1.97 1.91 1.86 2.00 2.07

1.54 1.17 0.99 1.17 1.72 1.94

26.7 23.7 19.2 28.3 29.4 28.1

19.3 ± 0.2 18.0 ± 0.1 17.4 ± 0.1 16.2 ± 0.1 17.6 ± 0.1 17.9 ± 0.9

1.1 1.3 1.4 1.2 1.4 1.4

15.3 14.5 14.4 12.9 13.0 13.1

16.0 15.4 15.5 13.7 14.0 14.1

11 ± 2 34 ± 3 45 ± 6 130 ± 11 79 ± 6 70 ± 69

110P/Hart 3

2002 04 08

3.26

2.32

7.1

19.7 ± 0.2

0.6

14.7

15.1

18 ± 4

Object name

Date of observation

C/2002 R3 C/2002 R3 C/2002 R3 C/2002 R3 C/2002 R3 C/2002 R3 C/2002 R3 C/2002 R3

2002 10 28 2002 11 01 2002 11 04 2002 11 23 2002 12 06 2002 12 19 2002 12 21 2003 01 15

C/2002 V1

r (AU)

Afρ (cm)

Photometry of NEAT comets

461

Table 1 (continued) r (AU)

 (AU)

ϕ (◦ )

Object name

Date of observation

R (mag)

δR (mag)

Htot (mag)

Hnuc (mag)

118P/S-L4 118P/S-L4

2002 09 08 2002 09 29

2.98 2.89

2.02 1.89

6.8 2.8

18.3 ± 0.2 18.3 ± 0.1

0.5 0.8

13.8 13.7

14.2 14.5

44 ± 7 56 ± 5

126P/IRAS

2003 02 04

9.51

8.77

4.1

17.9 ± 0.1

0.1

8.2

8.2

1910 ± 220

151P/Helin

2001 11 17

2.57

1.91

19.2

19.8 ± 0.2

2.0

14.1

15.7

32 ± 6

152P/He-La 152P/He-La

2002 05 07 2002 05 12

3.34 3.33

2.40 2.40

7.2 8.5

17.8 ± 0.1 18.0 ± 0.1

0.7 0.8

12.5 12.6

13.1 13.3

120 ± 11 110 ± 11

155P/Shoe 3

2002 12 06

1.82

1.14

28.8

16.3 ± 0.1

1.2

13.1

13.9

110 ± 12

P/2000 Y3

2002 02 05

4.38

3.40

Periodic comets 1.9 18.9 ± 0.1

0.7

12.2

13.0

120 ± 16

P/2001 T3 P/2001 T3

2001 11 20 2002 01 02

2.58 2.52

1.76 2.16

15.0 22.6

18.3 ± 0.2 18.9 ± 0.1

1.2 1.4

13.6 13.4

14.5 14.5

57 ± 11 48 ± 6

P/2001 TU80 P/2001 TU80 P/2001 TU80 P/2001 TU80

2001 11 17 2001 11 19 2002 01 07 2002 02 04

1.94 1.94 1.95 1.99

1.34 1.32 0.99 1.02

28.0 27.7 10.0 6.5

18.7 ± 0.1 18.5 ± 0.1 16.4 ± 0.1 17.0 ± 0.9

1.2 1.2 1.2 1.1

15.1 14.8 13.6 14.2

15.8 15.6 14.7 15.3

16 ± 2 20 ± 2 110 ± 13 61 ± 53

P/2001 Q2

2001 11 18

1.36

1.27

44.1

18.6 ± 0.1

1.7

15.1

16.1

13 ± 1

P/2001 Q6 P/2001 Q6

2001 08 28 2001 09 04

1.70 1.65

1.19 1.08

35.8 36.1

18.8 ± 0.2 18.6 ± 0.1

1.0 1.1

15.7 15.6

16.2 16.2

9±1 10 ± 1

P/2001 R1

2001 11 18

1.70

1.43

35.5

19.3 ± 0.2

1.5

15.3

16.3

11 ± 2

P/2001 X2 P/2001 X2 P/2001 X2 P/2001 X2

2002 01 05 2002 01 16 2002 02 04 2002 02 06

2.76 2.59 2.62 2.62

1.61 1.61 1.67 1.69

5.7 1.4 7.9 8.7

19.6 ± 0.3 18.6 ± 0.2 19.7 ± 0.2 19.6 ± 0.2

0.8 0.4 1.1 0.8

15.4 15.0 15.3 15.5

16.2 15.4 16.2 16.2

13 ± 3 20 ± 4 14 ± 3 11 ± 2

P/2001 YX127 P/2001 YX127 P/2001 YX127

2003 02 02 2003 02 19 2003 03 13

3.43 3.43 3.43

2.47 2.46 2.56

4.5 3.7 9.4

19.3 ± 0.2 19.1 ± 0.2 19.5 ± 0.2

0.6 0.6 0.7

13.9 13.8 14.0

14.5 14.4 14.5

34 ± 6 39 ± 7 30 ± 6

P/2002 JN16 P/2002 JN16

2002 06 02 2002 06 03

1.86 1.86

0.87 0.87

10.8 11.4

17.4 ± 0.2 17.5 ± 0.1

0.7 0.6

15.5 15.6

16.1 16.1

20 ± 3 19 ± 2

P/2002 T5 P/2002 T5 P/2002 T5 P/2002 T5 P/2002 T5

2002 11 05 2002 11 14 2002 12 12 2003 01 14 2003 01 24

4.15 4.14 4.09 4.04 4.03

3.36 3.31 3.28 3.49 3.58

9.1 8.4 8.8 12.4 13.2

17.6 ± 0.2 17.5 ± 0.1 17.0 ± 0.1 17.8 ± 0.1 18.0 ± 0.2

0.5 0.9 0.6 1.0 1.0

11.2 10.8 10.7 10.9 11.0

11.6 11.6 11.1 11.7 11.9

290 ± 48 420 ± 38 480 ± 57 360 ± 41 300 ± 48

P/2002 T6 P/2002 T6 P/2002 T6 P/2002 T6 P/2002 T6 P/2002 T6

2002 11 03 2002 11 11 2002 12 08 2002 12 10 2003 01 28 2003 02 10

3.75 3.73 3.66 3.65 3.54 3.52

2.82 2.86 3.07 3.09 3.67 3.83

6.1 8.1 13.5 13.8 21.0 14.7

18.9 ± 0.2 19.3 ± 0.2 18.5 ± 0.2 18.6 ± 0.1 18.2 ± 0.2 17.9 ± 0.1

0.1 0.2 0.2 0.2 0.4 0.4

13.5 13.9 12.9 13.0 11.9 11.6

13.6 14.0 12.9 13.0 12.0 11.8

42 ± 7 28 ± 5 65 ± 12 59 ± 6 120 ± 18 160 ± 19

P/2002 X2

2002 11 05

2.70

1.78

9.9

18.9 ± 0.2

0.9

14.5

15.2

27 ± 5

P/2003 A1 P/2003 A1 P/2003 A1 P/2003 A1

2003 01 28 2003 02 04 2003 02 07 2003 02 11

1.92 1.92 1.92 1.92

1.93 2.01 2.01 2.09

29.7 29.0 28.9 28.1

18.3 ± 0.1 18.3 ± 0.1 17.8 ± 0.1 18.4 ± 0.2

0.7 1.1 0.8 0.9

14.3 13.8 13.6 14.1

14.5 14.5 14.0 14.6

22 ± 2 33 ± 4 39 ± 5 26 ± 5

the instrumental photometry and computed an appropriate photometric offset for each NEAT image. We applied the photometric correction determined by Mark Kidger of R(Landolt) = 1.031 × R(USNO-A2) − 0.417 (Ref.: http://www.iac.es/galeria/ mrk/comets/USNO_Landolt.htm). Fig. 2 illustrates this calibration for a representative NEAT image. Though both NEAT cameras were unfiltered, the peak in CCD response closely

Afρ (cm)

matched the R filter (0.62 µm). Analysis of selected standard stars (Landolt, 1992) observed serendipitously by NEAT verified our procedure of on-chip calibration. We took care to assure that our reported R magnitudes are within the photometric errors listed in Table 1. The cometary fluxes Fdust were used to generate the well known measure of dust production Afρ (A’Hearn et al., 1984),

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M.D. Hicks et al. / Icarus 188 (2007) 457–467

Fig. 2. Example calibration curve. A separate calibration was determined for each image using the R magnitudes of the point sources listed in the Digitized Sky Survey.

where A is the wavelength dependent effective albedo of the dust grains, f is the filling factor or ratio of the cross section of dust grains to the total field of view, and ρ is the projected radius of the photometric aperture. Assuming a steady-state ρ −1 dust profile, Afρ is aperture independent and allows for comparison of cometary activity measured at differing times and observational geometries. From conservation of reflected sunlight, the product Afρ can be found to be Afρ = 42 r 2 ρ −1 Fdust /Fsolar , where r is the heliocentric distance in AU, and  and ρ are the geocentric distance and aperture radius in centimeters. The ratio of coma to solar flux was computed assuming an R magnitude of the Sun Rsolar = −27.10 (Fitzimmons, personal communication). The Afρ were averaged over the triplet and then multiplied by 1.5 to correct for the dust contamination in the photometric sky annulus. We assumed a ρ −1 dust column density in order to compute the amount of coma subtracted in the sky annulus. The mean Afρ values for each triplet are listed in Table 1. 3. Cometary coma/size fitting model All comets observed by NEAT exhibited dust production, therefor a coma model was necessary to provide an estimate of nuclear size. We employed a coma fitting model as described by Lowry (2001). For each exposure we constructed a pointsource PSF by combining a weighted average of the PSFs of all non-saturated stars in the image. By using background stars in each image in the triplet we correct for seeing changes, image jitter, optical distortions, etc., that can significantly vary during the 45-min timespan spent to collect each triplet. Object motion

Fig. 3. Efficacy of our PSF fitting model to determine coma contamination. A stellar PSF (shown by open symbols) obtained from averaging background stars was used to fit the innermost pixels of the comet (shown by filled symbols). The circular symbols represent a profile taken along the north to south column of the image while the diamond symbols illustrate the profile taken along the west to east row. The coma was assumed to dominate the skirt. This provides an lower-limit only for moderately active comets, as shown in the upper panel. The lower panel illustrates to model results on the fast-moving near-Earth Asteroid 2100 Ra-Shalom. The model correctly measured negligible coma.

was negligible due to the short exposures (20–60 s). The stellar PSF was fitted using a weighted average of the innermost pixels to the comet PSF, as shown in Fig. 3 for the well-defined Comet 67P/Churyumov–Gerasimenko. In the uppermost panel of Fig. 3 the skirt of the coma (filled symbols) stands clearly above the background sky (open symbols). This difference was used to model coma contribution. We tested our model on the triplet of moderately fast moving near-Earth Asteroid 2100 RaShalom, as shown in the bottom panel of Fig. 3. The stellar PSF matched the moving asteroid extremely well and the model measured negligible coma. For each comet, the measured coma was averaged over the triplet and the ratio between the nuclear contribution and the total flux is given as R in Table 1. Observational geometry and the total/nuclear flux estimates were used to derive the absolute magnitudes of comet, Htotal , and the absolute magnitudes of the nucleus, Hnuc , also listed in Table 1. We assumed an R-band geometric albedo AR = 0.05 in order to compute nuclear sizes. For cometary observations dominated by coma, we stress that the nuclear size estimates are upper limits only.

Photometry of NEAT comets

4. Discussion The R magnitudes of all observed comets as a function of heliocentric distance are plotted in Fig. 4. The numbered periodic comets, dominated by Jupiter-family comets JFCs, and the newly discovered periodic comets span a range of heliocentric distance from 1 < r < 5 AU. There is a slight trend in increasing brightness at smaller heliocentric distances. The nonperiodic, or dynamically new, comets where often observed near perihelion and span a much greater range in heliocentric distances, from 1.5 < r < 11.5 AU. No comets were observed fainter than R ∼ 20.1 mag, which represented our best detection limits. A more typical nightly detection limit was on the order of R ∼ 19.5−19.0 mag. The NEAT system balances sky coverage and detection limit in order to optimize the primary goal of near-Earth asteroid discovery. NEAT’s relatively bright detection limit precludes its use to survey bare inactive distant Jupiter-family comet nuclei, which tend to have observed R magnitudes from 21 to 24 mag at aphelion. As shown in Fig. 4, there is an apparent deficit of comets fainter that 18 at heliocentric distances between 1 and 2 AU, consistent with a lack of faint (therefor small) JFCs. This effect has been noted by many observers and may be evidence that small comets entering the inner Solar System undergo relatively rapid disintegration (Fernández et al., 1999). It is unlikely to be the result of observational bias, since a detection limit of R ∼ 19 mag is significantly fainter than the observered comets in this heliocentric distance range. The normalized coma brightness, corrected for heliocentric and geocentric distance and solar phase angle, is plotted in Fig. 5 as a function of heliocentric distance. The periodic and non-periodic comets occupy partially overlapping regions at heliocentric distances less than 7 AU. The distant and highly

Fig. 4. Total comet brightness as a function of heliocentric distance.

463

active comets, C/2001 G1 and C/2003 A2, at 8.5 and 11.5 AU, exhibit activity presumably powered by the sublimation of an ice more volatile than water. Our data covers a two-year time span and a large fraction of our comets were imaged multiple times over their apparition. The measurements of dust production and coma brightness can be used to construct activity curves for a significant number of comets. Fig. 6 illustrates dust production as a function of time and heliocentric distance for two comets: C/2002 R3 and P/2002 T6. In each case the images collected span approximately one hundred days. The Afρ and normalized coma brightness track one another quite well. Though the heliocentric distances for both comets decrease slightly by ∼10%, the dust production of C/2002 R3 remains constant while the dust production of P/2002 T6 increases by a factor of 4. This rapid brightening may be the result of active areas moving into and out of solar illumination. The ongoing NEAT survey can be used to build an atlas of dust production of a large number of comets over several apparitions and provide data useful for the determination of comet pole position, latitudes of active areas, etc., as support for future spacecraft missions. Our simple PSF-fitting model provides a lower limit for coma contamination for highly active comets. We sought to explore under which conditions our model would give reasonable estimates for nuclear sizes. Recently, there has been much work by Lamy and colleagues (Lamy et al., 2001, 2002, etc.) by which JFC nuclear sizes were directly measured using HST imaging. These estimates have proven to be quite accurate. The size estimated for 19P/Borrelly (Lamy et al., 1998) agreed well with the dimensions determined by the Deep Space 1 flyby (Soderblom et al., 2004). We were fortunate that several of the NEAT observed comets have well established nuclear

Fig. 5. Normalized coma brightness as a function of heliocentric distance. The trend to brighter comets at larger heliocentric distance is an observational bias effect.

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M.D. Hicks et al. / Icarus 188 (2007) 457–467

Fig. 6. Partial lightcurves as a function of time and heliocentric distance for selected comets. Multiple imaging throughout an apparition allows for time-variable dust production rates to be measured for a large number of comets.

sizes. Plotted in Fig. 7 are our size estimates in comparison to the Lamy et al. (2004) values as a function of dust production. It is clear that for Afρ > ∼100 cm consistently overestimates the nuclear size by up to a factor of four, while there is typically good agreement between our model diameters and those measured by other observes at low dust production levels. We selected only the low-activity comets with Afρ < 100 cm in our sample for size–frequency studies. Using their own observations and previously published data, Fernández et al. (1999) analyzed a large number of Jupiterfamily comets with perihelion distance q < 2 AU and determined the cumulative distribution of absolute nuclear magnitude. They found that the luminosity function was linear to approximately HV ∼ 16, after which the distribution progressively flattens presumably due to the incompleteness of discoveries at small sizes. The Fernández et al. data is plotted as the middle curve in Fig. 8, after shifted by 4 magnitudes to the right for greater clarity. We note that the Fernández et al. data lists absolute magnitudes though the V filter while the NEAT survey was measured in R. Assuming that both the NEAT and Fernández surveys are equally complete at large sizes, we can use the

difference in the intercepts of the cumulative luminosity distribution of the two surveys as illustrated in Fig. 8 to measure the mean V − R of the comet population. The slight offset between the Fernández and NEAT datasets before shifting was measured and the mean V −R = 0.44±0.03, assuming no systematic offset between the Landolt and USNO-A2 R filters. This color is consistent with the previously measured moderately red colors of V − R = 0.440 ± 0.124 (Meech et al., 2004). Fernández et al. (1999) measured a luminosity function with a power-law slope of 0.54, giving a cumulative mass distribution function steeper than small main-belt asteroids and NearEarth Asteroids but consistent with the Kuiper-belt population. The steep power law determined by Fernández et al. (1999) may be the result of the restricted range they choose to fit their data. Our data gave a more modest slope of 0.33 ± 0.04 as measured over a wider range of nuclear magnitudes. We determined the cumulative luminosity function of the Fernández et al. data and measured a more shallow slope of 0.38 ± 0.02. Though we sample extremely different size scales, our cumulative luminosity functions are inconsistent with the power law slope exhibited by the Kuiper-belt objects, as illustrated in Fig. 8.

Photometry of NEAT comets

Fig. 7. Measured and model diameters as a function of dust production.

465

Fig. 9. Cumulative size distribution of the NEAT comets. The NEAT comets exhibit a power-law slope of 1.50 ± 0.08, consistent with recent values published by Whitman et al. (2006), Meech et al. (2004), Lowry et al. (2003), and Weissman and Lowry (2003).

tional data compiled from other observers (Meech et al., 2004). Objects listed in the Meech survey are given in Table 2, along with source references. Meech et al. measured a cumulative size −α where the distribution function of the form N (> RN ) ∝ RN value of α is determined by finding the linear fit in log–log space. For objects in the 1 to 10 km range, they found a slope α = 1.45 ± 0.05. We plot our cumulative size distribution in Fig. 9, along with our best fit slope of α = 1.50 ± 0.08. Table 3 lists the cumulative size index for several authors. With the exception of Fernández et al. (1999), the observed slopes are in good agreement. Intensive modeling of the cumulative size distribution by Meech et al. (2004) have lead them to determine that the JF comets represent a collisionally relaxed population, with a differential power law index of −3.5, truncated at small sub-km sizes. Our measured distribution is consistent with this conclusion. We have demonstrated that the NEAT survey can provide robust cometary nuclear size measurements even in the case of visible coma, given that we restrict ourselves to low-activity comets only. Fig. 8. Cumulative luminosity functions for NEAT comets, Jupiter-family comets (JFCs) as tabulated by Fernández et al. (1999), and trans-neptunian objects (TNOs). The NEAT comets exhibit a slope of 0.33 ± 0.04 which is compatible with the JFC slope of 0.38 ± 0.02.

Table 2 lists the perihelion distances and the estimated diameters of the low-activity comets in the NEAT survey. There is a weak trend of size with perihelion distance, though this is likely the result of discovery bias. Recently, Meech and colleagues published a compressive survey of properties of cometary nuclei, with 21 short and long term comets measured photometrically with the Keck and Hubble Space Telescopes, with addi-

5. Conclusions (1) Though optimized to discover and track fast-moving nearEarth objects, the archived NEAT data can be mined to obtain information on the cometary population observed serendipitously during the asteroid survey. (2) Using on-chip calibrator stars it was possible to obtain reasonably accurate (∼10%) R-band photometry and to measure dust production and coma magnitudes. A given comet was often observed several times throughout its apparition allowing for the construction of partial activity curves.

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M.D. Hicks et al. / Icarus 188 (2007) 457–467

Table 2 Model diameters of low-activity comets Name

q (AU)

Model diameter (km)

C/2002 K4 C/2003 E1 2P/Encke 22P/Kopff 31P/Schwassmann–Wachmann 2 36P/Whipple 53P/Van Biesbroeck 57P/du Toit–Neujmin–Delporte 66P/du Toit 67P/Churyumov–Gerasimenko 92P/Sanguin 110P/Hartley 3 118P/Shoemaker–Levy 4 151P/Helin P/2001 T3 P/2001 TU80 P/2001 Q2 P/2001 Q6 P/2001 R1 P/2001 X2 P/2001 YX127 P/2002 JN16 P/2002 T6 P/2002 X2 P/2003 A1

2.76 3.24 0.34 1.58 3.41 3.09 2.41 1.71 1.27 1.28 1.81 2.47 2.01 2.53 2.51 1.93 0.95 1.41 1.36 2.52 3.43 1.79 3.39 2.53 1.92

7.9 ± 1.1 11.9 ± 1.1 4.6 ± 0.5 7.9 ± 0.6 6.1 ± 0.6 5.3 ± 0.7 12.2 ± 0.9 3.4 ± 0.2 2.5 ± 0.3 6.5 ± 0.7 3.3 ± 0.3 4.8 ± 0.5 6.5 ± 0.3 3.6 ± 0.4 6.3 ± 0.6 3.6 ± 0.2 3.0 ± 0.1 3.0 ± 0.2 2.8 ± 0.3 2.9 ± 0.3 6.4 ± 0.6 3.1 ± 0.2 8.2 ± 0.7 4.6 ± 0.4 6.2 ± 0.5

a b c d e f

Other measurements (km)

Ref.

4.8 3.14 6.2 4.56 6.66

a

4 2.4 4.3 4.8 8.4

d a

b c a c

f e f

Fernández et al. (2000). Lamy et al. (2002). Meech et al. (2004). Lamy et al. (2003). Lowry et al. (2003). Lamy et al. (2004).

Table 3 Cumulative size distribution summary Reference

αa

This work Whitman et al. (2006) Tacredi et al. (2006) Meech et al. (2004) Lowry et al. (2003) Weissman and Lowry (2003) Weissman and Lowry (2001) Fernández et al. (1999)

1.50 ± 0.08 1.5 ± 0.3 2.70 ± 0.15 1.45 ± 0.05 1.6 ± 0.1 1.59 ± 0.03 1.40 ± 0.03 2.65 ± 0.25

the cometary size distribution matches that of the Kuiperbelt objects. A comparison between the NEAT and the Fernández et al. datasets gives a mean color for the cometary population of V − R = 0.44 ± 0.03. (5) Our cumulative size distribution agrees well with the slopes measured by Meech et al. (2004), Lowry et al. (2003), and Weissman and Lowry (2003), supporting the conclusion by Meech that the intrinsic distribution of comet nuclei represents a collisionally relaxed population truncated at very small sizes.

a Cumulative size index for N (> R ) ∝ R −α . N N

Acknowledgments (3) A simple PSF-fitting model was used to estimate coma contamination. We found that for low-activity comets (Afρ < 100 cm) our model gave reasonable estimates of nuclear size and magnitude. Twenty comets in our survey met our low-activity criteria and were used to explore the size– frequency distributions. The model diameters for active comets (Afρ > 100 cm) should be considered upper limits only. (4) The slope of cumulative luminosity function of our sample of comets was 0.33 ± 0.04, in good agreement with the slope we determined for the comets collected by Fernández et al. (1999) of 0.38 ± 0.02. Though we are forced to compare wildly different size scales we find no evidence that

The authors wish to thank our colleagues vital for the NEAT operations at both the Maui Space Surveillance Site and the Palomar Mountain Observatory, most notably: R. Thickston, P. Kervin, J. Africano, and R. Meada. We are grateful for the thoughtful reviews of A. Fitzimmons and G. Tacredi, whose comments and suggestions greatly aided this paper. The Digital Sky Survey was produced at the Space Telescope Science Institute under the US Government Grant NAG W-2166 (for a complete list of contributions to the DSS, please refer to the URL http://stdatu.stsci.edu/dss/dss_acknowledgments. html). This work was supported by the JPL Research and Technology Spontaneous Concepts Program.

Photometry of NEAT comets

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