Sunday, 3 August 2014

Emissions from Comet C/2002 VQ94 (LINEAR).

C/2002 VQ94 (LINEAR) was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) team at the Massachusetts Institute of Technology on 11 November 2002. It was originally classified as a Damocloid Family Asteroid (asteroids with eccentric, comet-like orbits that average more than 8 AU from the Sun and come no closer than 5.2 AU from the Sun), but was later reclassified as a Long Period Comet. The designation 2002 VQ94 implies the 2367th asteroid (asteroid Q94) discovered in the first half of November 2002 (period 2002 V), while C/ implies a Long Period Comet and (LINEAR) indicates the discoverer.

C/2002 VQ94 (LINEAR) has a 2875 year orbital period and an elliptical orbit tilted at an angle of 70.4˚ to the plane of the Solar System, which takes it from 6.8 AU from the Sun (6.8 times as far from the Sun as Earth, and outside the orbit of Jupiter) to 397 AU from the Sun (397 times as far from the Sun as the Earth, more than 13 times as far from the Sun as Neptune and about 8 times as far from the Sun as the outer margin of the Kuiper Belt - but still on the inner fringes of the Oort Cloud).

The calculated orbit of C/2002 VQ94 (LINEAR). JPL Small Body Database Browser.

Cometary activity was first detected from C/2002 VQ94 (LINEAR) on 28 August 2003 by David Jewitt using the University of Hawaii 2.2-m Telescope, when it was 8.9 AU from the Sun (i.e. 8.9 times as far from the Sun as Earth is). Jewitt detected a distinct cometary halo (i.e. particles being shed from the bosy) and also calculated the comet to be 40.7 km in diameter. 

Follow up observations by Pavlo Korsun and Oleksandra Ivanova of the Main Astronomical Observatory of National Academy of Sciences of Ukraine and Viktor Afanasiev of the Special Astrophysical Observatory of the Russian Academy of Sciences using the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences on 9 March 2006 confirmed that C/2002 VQ94 (LINEAR) had a cometary halo, and were able analyse this spectrographically, detecting the molecules CN, C₃, CO+, and N₂+. 

The detection of molecular emissions from bodies this distant from the Sun has only been achieved for four comets previous to this; C/1961 R1 (Humason), 29P/Schwassmann–Wachmann 1, C/1995 O1 (Hale-Bopp) and the Centaur Chiron (expand), making this a significant discovery, leading to repeat observations of C/2002 VQ94 (LINEAR) 10 April 2007, when the comet was 7.33 AU from the Sun, and emissions of the ions CO+ and N₂+ were again detected, though the neutral molecules CN and C₃ were no longer detectable.

In a paper published on the arXiv database at Cornell University Library on 14 January 2014 and accepted for publication in the journal Icarus, Pavlo Korsun, Philippe Rousselot of the Observatoire des Sciences de l'Univers THETA at the University of Franche-Comté, Irina Kulyk of the Main Astronomical Observatory of National Academy of Sciences of Ukraine and Viktor Afanasiev and Oleksandra Ivanova describe the results of a series of follow-up observations of C/2002 VQ94 (LINEAR) using the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences in March 2008 and March 2009 and the 2.5-m Nordic Optical Telescope  at the Observatory del Roque de los Muchachos in June 2011 and July 2013.

In March 2008 C/2002 VQ94 (LINEAR) was 8.36 AU from the Sun. A cometary halo was still visible, and it was possible to detect the presence of CO+ and N₂+ spectrographically, and to determine the densities of these molecules within the coma (cloud of material around the comet), at 0.0572 grams per cubic meter for N₂+ and 0.00913 grams per cubic meter for CO+. In addition the comet was calculated to be producing dust (particulate solid material released by the sublimation of ice to gas; there is no liquid phase in a vacuum) at a rate of 10-20 kg per second. 

In March 2009 C/2002 VQ94 (LINEAR) was 9.86 AU from the Sun. A cometary halo was still clearly visible, but it was not possible to perform a spectrographic analysis of this. The comet was calculated to be producing dust at a rate of 4-6 kg per second.

In June 2011 C/2002 VQ94 (LINEAR) was 13.40 AU from the Sun. A cometary halo was still faintly visible, though spectrographic analysis of this was not possible. The comet was calculated to be producing dust at a rate of 3-5 kg per second.

In July 2013 C/2002 VQ94 (LINEAR) was 16.84 AU from the Sun. By this point all cometary activity appeared to have ceased.

Korsun et al. also produced a revised size estimate for C/2002 VQ94 (LINEAR), of approximately 48 km in diameter.

120"×120" extractions from the summarized images of VQ94. North, East, sunward direction, and scale bar are indicated. The vertical narrow box across the comet marks the position of the slit in the spectroscopic mode. Korsun et al. (2014).

CO+ and N₂+ are unusual in cometary emissions; they have only previously been detected from three other comets, C/1908 R1 (Morehouse), C/1961 R1 (Humason) and 29P/Schwassnann-Wachmann 1. As a group these four bodies have little in common; C/1908 R1 (Morehouse) was a hyperbolic comet (expand) which reached perihelion at 0.95 AU and had an orbit inclined at 140.2˚ to the plane of the Solar System, C/1961 R1 (Humason) has not been placed in any defined orbit class, it reached perihelion at 2.13 AU and has an orbit inclined at 153.3˚ to the plane of the Solar System and 29P/Schwassnann-Wachmann 1 is a Jupiter Family Comet (expand) with a perihelion of 5.74 AU and an orbit inclined at 9.4˚ to the plane of the Solar System.

The calculated orbit of C/1908 R1 (Morehouse). JPL Small Body Database Browser.

CO+ and N₂+ are likely to be derived from CO and N2 ices, which can only form at very low temperatures; laboratory experiments have suggested these ices can only accumulate at 25 K or less (-148˚C or less), and bodies containing them must have formed in a part of the Solar System where such temperatures occurred. Significantly three of the four comets recorded to have produced CO+ and N₂+ emissions are also members of the six known bodies that have produced detectable molecular emissions at distances of greater than 5 AU from the Sun.

See also…

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