C/2000 WM1 (LINEAR)

Lore & Background

The comet was discovered on 16 December 2000 by the LINEAR team, initially appearing as an asteroid with magnitude 17.8. It was later found in earlier LINEAR images from 16 and 18 November 2000 at magnitude 18. Timothy Bruce Spahr observed it with the 1.2-m reflector at the Smithsonian Astrophysical Observatory, noting a coma 10 arcseconds across and a faint tail 10–20 arcseconds long. The ephemeris predicted it would reach magnitude 4 in early January 2000, becoming naked-eye visible.

By late August 2001 it had brightened to magnitude 14, and by late September to 11.5. On 14 November its magnitude was 7.2 with a tail 1.1 degrees long visible in 10×50 binoculars. On 3 December it reached magnitude 5.3 with a coma 21 arcminutes across. It approached Earth on 2 December 2001 at 0.316 AU. After mid-December it became easier to observe from the southern hemisphere. Around perihelion it experienced an outburst: on 27 January 2002, five days after perihelion, it brightened to magnitude 4.6, and two days later to 2.8–3, with a tail several degrees long. It then faded, becoming visible from the northern hemisphere again in late March 2002 at magnitude 9–10. It was last observed on 8 August 2002 at magnitude 14.

Scientific results from near-infrared spectroscopy on 23–25 November 2001 showed the comet was depleted in methanol and carbon monoxide relative to water. CO and acetylene were considerably depleted compared to Oort cloud comets, while hydrogen cyanide, methane, and methanol were moderately depleted. Radio spectroscopy also indicated depletion in CO and hydrogen sulfide. This depletion suggests the comet formed closer to the Sun and was later ejected to the Oort cloud. Other spectrographic studies detected C2, C3, CN, NH2, water cation, and possibly C2 anion. The CN production rate was 2.43×10^26 molecules per second and C2 production rate 3.1×10^26 molecules per second on 2–4 December 2001, indicating relative enrichment in diatomic carbon. The HNC/HCN ratio was 0.09–0.19, similar to 153P/Ikeya-Zhang.

Reader's Guide

C/2000 WM1 (LINEAR) is significant for its unusual chemical composition, which challenges the assumption that all Oort cloud comets share a common formation environment. The depletion of CO, methanol, and other volatiles suggests it formed closer to the Sun than typical Oort cloud comets and was later scattered outward. This provides evidence for dynamical mixing in the early solar system. The comet also contributed to isotopic studies: the unambiguous detection of nitrogen-15 in its ultraviolet-visual spectrum, obtained with the UVES spectrograph on the Very Large Telescope in mid-March 2002, was only the second such detection after Comet Hale-Bopp. The 14N/15N ratio was 140±30, about half the terrestrial value, while the 12C/13C ratio was 115±20, close to the solar system standard of 89. The comet's outburst near perihelion, brightening from magnitude 6 to 2.8–3, and its symmetrical dust coma with a double-jet CN structure, add to its observational interest. Its water production rate on 13 December 2001 was 5.2×10^28 molecules per second, implying an active sublimation area of 10 km² and a nucleus diameter of at least 1.8 km.

Did You Know?

• The comet was discovered by LINEAR on 16 December 2000, initially appearing as an asteroid of magnitude 17.8.
• It experienced an outburst near perihelion, brightening from magnitude 6 to 2.8–3 within a few days.
• It was only the second comet in which nitrogen-15 was unambiguously detected, after Comet Hale-Bopp.