Karen Jean Meech -- Comet Rotation

Karen Jean Meech -- Comet Rotation

VLT, Chile -- 11/2000

General Comet Rotation Characteristics

Not only is knowledge of the global physical properties of comets important for the understanding of the early solar system accretion models (i.e. how big did the nuclei grow), but the nucleus size, albedo and rotational periods are critical parameters which affect the solar energy distribution on the surface which will dictate the nature of a comet's activity. We have information about the nucleus properties of only a couple dozen comets, in part because when they are inactive and it is possible to observe the nucleus without contamination from the coma, the nucleus is far from the sun, hence faint, and requires large telescopes. Part of my research program is to study the physical properties of the nuclei. Various aspects of the program have been done in collaboration with Marc Buie (Lowell Observatory), Olivier Hainaut (Institute for Astronomy), Michael Belton, Nalin Samarasinha and Beatrice Mueller (NOAO), along with several others). Below is a summary of general nucleus properties from a paper which was presented as an invited paper on "Physical Properties of Comets" at the ACM 96 meeting in Versailles, France (submitted to Advances in Space Research).

Image	Comet	Rad [km]	Per [hr]	Lightcurve (click to enlarge)	References
	46P/Wirtanen 8/17/96 r=2.53 AU; UH 2.2m; [Meech, Bauer, Hainaut]	0.58	7.6		Meech, K. J. et al.. (1977). A&A, 326, 1268-1276.
	107P/ Wilson-Harrington 6/14/96 r=2.20 AU; UH 2.2m; [Meech, Kakazu, Hainaut]	2.0	6.1		Lightcurve: Osip, D. J. et al. (1995). Icarus 114, 423.
	3200 Phaethon 01/05/95 r=1.09 AU; UH 2.2m; [Meech, Hainaut]	2.6	3.604		>Meech, K. J. (1997). Invited review at ACM96; in press.
	22P/Kopff 01/04/92 r=4.73 AU; [Meech]	2.8, 1.8	12.91		Meech, K. J. et al., in preparation.
	31P/SW2 5/13/91 r=4.74 AU; CTIO 4m [Meech]	3.1	5.58		Lightcurve: Luu, J. & D. Jewitt (1992). AJ 104, 2243.
	2P/Encke 5/13/91 r=2.72 AU; CTIO 4m [Meech]	3.1	15.08		Jewitt & Meech '87. AJ 93, 1542; ('90) Luu & Jewitt Icarus 86, 69.
	49P/ Arend-Rigaux 1/6/92 r=1.28 AU; UH 2.2m [Meech]	5.2	13.56		Lightcurve: Millis et al. (1988). Ap.J. 324, 1194.
	10P/Tempel 2 1/6/92 r=4.60 AU; UH 2.2m [Meech]	5.9	8.876		Lightcurve: Mueller, B. & I. Ferrin (1996). Icarus
No image	P/1991 L3 Levy	8.2	8.34		Fitzsimmons, A. and I. P. Williams (1994). A&A 289, 304.
	28P/Neujmin 1 1/4/92 r=11.94 AU; UH 2.2m [Meech]	10.4	12.67		Jewitt, D. and K. Meech (1988). Ap.J. 328,974.
	C/1995 O1 Hale-Bopp 9/16/96 r=3.06 AU; CFHT 3.6m [Meech, Bauer, Hainaut]	22.5	11.47	No lightcurve available	Int. Astron. Circulars No. 6560, 6583, 6587.
	2060 Chiron 2/19/93 r=9.31 AU; UH 2.2m [Meech]	90	5.9178		Lightcurve: Bus, S. et al. (1989). Icarus 77, 223; Icarus 104, 234.

Summary of Recent work on 46P/Wirtanen

In collaboration with post-doctoral associate Olivier Hainaut and graduate student James Bauer, we have been observing comet 46P/Wirtanen which is a target of the ESA Rosetta mission. The mission will be studying the activity of the comet from beyond a distance of 3 AU as it approaches perihelion, and for the mission planning, as much as possible needs to be known about the comet in advance. We have recently submitted a paper to Astronomy and Astrophysics discussing the observations we obtained during 3 observing runs in 1996 June, 1996 August and 1996 November using the UH 2.2m telescope. Below is a summary of the highlights of the observations. For a pre-prepint, please send mail to meech@ifa.hawaii.edu.

Onset of Activity - From a composite lightcurve made from our data and other published data, it is apparent that the activity on the comet began between 3.5-4.5 AU. In the images at the right, showing data from 1996 June, August and November, coma is clearly evident in the August data (r = 2.5 AU). The coma extended to greater than 18,000 km during August 1996 (middle image) and 25,000 km in November (bottom image).

Nucleus Rotation Period - We have observed a periodic variation in the light curve of 3.8 hours which we interpret as the brightness modulation caused by a nucleus rotating (see Figure) with a period near 7.6 hours.

Nucleus Axis Ratio and Size - Using the HST estimate of the nucleus radius (0.58 km) and the rate at which the surface brightness decreased in brightness (slopes of -1.6 and -1.4 in August and November, respectively), we were able to deduce the range of brightness variations that the comet would have if there were no coma. We found that the bare nucleus axis ratio should have been a:b = 1.7 for a low albedo nucleus.

Last modified: February 4, 2001
Karen Meech Institute for Astronomy 2680 Woodlawn Drive Honolulu, HI 96822 meech@ifa.hawaii.edu