The Great Christmas Comet of 2011, 2011 W3 (Lovejoy) |
The Great Christmas Comet of 2011, 2011 W3 (Lovejoy) |
Guest_Sunspot_* |
Dec 2 2011, 09:59 PM
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Guests |
http://sungrazer.nrl.navy.mil/index.php?p=.../birthday_comet
Possible very bright sungrazing comet coming mid December - Comet Lovejoy C/2011 W3 (Lovejoy) Information in the link above. |
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Jan 3 2012, 11:27 PM
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Member Group: Members Posts: 723 Joined: 13-June 04 Member No.: 82 |
Comet-ml post #19203 by Richard Miles:
QUOTE John Bortle wrote:
And I'm still very curious about the nature of the faint yet distinct "sheath" that is seen to envelope both the dust and gas tails of numerous Kreutz sungrazers post-T, well seen with 2011 W3, yet does not seem evident with regard to other very small "q" non-Kreutz comets. What is the nature of it? And in the case of the Great September Comet it not only surrounded the tail but was described to extend well sunward of the head! John - Allow me to follow through one possible interpretation based on my earlier description of the near-perihelion scenario: We talk about dust (i.e. refractory particles ejected from the nucleus) and gas (low-boiling point volatiles) but there has to be a third type of particle created in the case of a sungrazer nucleus. This material is in effect the "smoke' which is created when otherwise fairly refractory material is vaporized and is able to recondense to an extent. Two mechanisms generate this "smoke". One is created in the scenario I have already described. The temporary gas shell / bow shock feeds molecules of vaporised refractories into the comet's tail. As the gas pressure and temperature within the tail declines from Poiseuille conditions towards Knudsen conditions, a dynamic situation unfolds whereby many of these molecules collide and stick together building up extremely fine, sub-micron size refractory condensates - in essence a newly-created dust or "smoke" forms. A second mechanism can also contribute, i.e. sputtering of the surface of the nucleus by fast, highly energetic particles in the solar wind. Calculations would need to be done to see whether enough "smoke" could be created via sputtering of the surface in the time available - I somehow doubt that this can be the entire explanation. So taking these hypotheses into account, how can we explain the absence of the "sheath" in other very small "q" non-Kreutz comets. My interpretation here is that if the nucleus is too small it cannot build up a stable melt zone within the near-surface, so no temporary envelope of volatilized refractories and no bow-shock can form. Under such conditions, refractory materials will still volatilze but will not experience the conditions required for particles to seed and grow by molecular collision. Such refractory molecules fail to condense along with others, hence no "smoke" can form. This may explain the absence of the "sheath" seen in larger Kreutz sungrazing comets. Richard Miles BAA |
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