David - As a first approximation, I would say that your conclusions seem
quite reasonable. Certainly, something quite unusual occurred with Comet
Lovejoy over the course of its apparition. That its post-T survival, at least
to a degree, violates my perihelion survival "law" is most interesting,
although I would note (as my paper on the subject indicated) that a number of
small, periodic comets (like P/Encke, et al.) do so on a quite regular
basis. The situation being so, this suggests to me that Comet Lovejoy must have
experienced at least one previous perihelion passage as a totally
independent body (i.e. it was not a fragment formed a its immediate previous
perihelion passage) and thus had a fully formed and fairly dense overall
insulating layer over its entire surface.Such a "baked" surface which totally
shuts
down early might well also explain why Kreutz sungrazers tend to disappear
much sooner post-T (typical by about 1.5 AU post-T) than do other comets of
similar intrinsic brightness.

There is also the problem that although of a seemingly extremely faint
intrinsic brightness both pre and post-T, Comet Lovejoy still presented a
viable and distinct "head" post-perihelion. This while objects like the Great
Southern Comet of 1887, assumed to be much brighter intrinsically than 2011
W3, appeared to have survived only as huge tail apparently without any
head. Of course, the Kreutz sungrazing group's orbital orientation so strongly
favors visibility from the Southern Hemisphere that it has undoubtedly
limited the opportunities to watch the development of other examples of
seemingly faint members of this clan in the more distant past.

I find it equally interesting how one explains the long-enduring bright
streak extending from the position the nucleus should occupy to relatively
far out into the tail. And the fact that this feature seems to have evolved
surprisingly little since its sudden appearance. A few descriptions of the
Great September Comet of 1882 do make mention of a similar "streak" in that
comet's head, dotted with a number of brighter star-like nucleii, but that
feature seemed fairly short lived and was on a physical scale apparently
far smaller than that displayed by Comet Lovejoy. Are we perhaps seeing a
very long train of tens of thousand of ONLY tiny fragments with absolutely no
large survivers, distributed along the orbit by size/mass? But then how
could this form so suddenly and how could a coma persist without some viable
solid body evident at its focus? According to Rod, there is no evidence of
any independent surviving body down to 19th magnitude in that location.

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!

I unquestionably foresee a long and interesting future of papers
attempting to address the amazing sights we've seen over the course of the past
month!

J.Bortle




In a message dated 1/2/2012 11:49:46 P.M. Eastern Standard Time,
seargent@... writes:





Hi all,
Just a few ideas to put before the group.
As I wrote previously, I suspect that the initial intrinsic faintness of
this comet was not so much a function of the small size of the nucleus, but
of the presence of a surface crust of refractory material. If the nucleus
was about 500 metres diameter (as against the 100 - 200 as initially
estimated) and covered by an insulating crust, this might explain how it
survived
perihelion passage intact. If the insulating layer was blown off around
perihelion, this may even have formed a "sun umbrella" of particles that
shielded the freshly-exposed icy surface of the nucleus, rather as is thought
to
have happened to Seki-Lines in 1962 (analysis of the dust tail suggests
that this comet shut down for a few hours at perihelion - q = 0.03 AU - which
also helps to explain why there were no daylight sightings of this
intrinsically bright object). In the case of Lovejoy, a similar event may have
been a factor in preserving its existence. Once the meteoric cloud dispersed,
the co met burst into furious activity, however by then the worst of its
ordeal was already over.
The presence of an ion tail clearly indicated an active nucleus following
perihelion. However, as this has this has now disappeared, it may be that
ice-driven activity has ceased. This could mean that the nucleus has
disappeared, or run out of ice or (I think the most likely explanation) has had
the ice cooked out of the surface layers. In other words, the comet may by
now have built up a new insulating layer that is effectively keeping heat
from underlying ice.
Yet, the "head" appears to be persisting as if some dust continues to be
released. Just a speculative thought, but electrostatic repulsion caused by
solar radiation can levitate fine dust on the surface of the Moon (causing
the unexpected crepuscular rays seen by the Apollo astronauts) and is
thought responsible for the small flare experienced by Phaethon in 2009. With
respect to the latter, David Jewitt called Phaethon a "rock comet" - capable
of low-level activity even in the absence of ice - and suggested that this
process may even be responsible for the formation of the Geminid meteor
stream. For what it is worth, I suggest that the present weak activity of
Lovejoy could be due to this process lifting dust from what has again become a
totally encrusted nucleus.
All very speculative I know, but comments welcome.
Cheers,
David

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