Nick et al.,

29P is like no other comet we study.
It certainly does not produce jets that are long-lasting in the visible
spectrum - the far IR (>25 micron) could be different.
The illusion of a 'spiral' is therefore NOT caused by a jet, the direction
of which is modulated by the rotation of the nucleus - much of the
literature is wrong in making this assumption.
The more likely explanation is that the eruption is caused by a longitudinal
fissure that opens up progressively over an interval of several seconds.
When one end of the fissure first opens it sprays ejecta in a particular
direction propelled at the highest speed. Then as the fissure opens the
outflow starts to slow as the cavity pressure declines and depending on the
local shape of the crust, it is likely that the direction of teh slowing
ejecta also progressively changes - Hey presto! You end up with a spiral. We
also saw this in the case of P/2020 H2 (Vales).

The strong outbursts are caused by a sudden cryo-eruption of a volcanic
source of which there are several but they are not all active at the same
time - usually only one or two are.

There are also mini-outbursts (<1.0 mag amplitude) of which there are many
more than strong outbursts yet before 2014 they went unobserved by
astronomers - a big mistake!
These arise from a different type of vent on the nucleus - one that behaves
more like a cryo-geyser.

The Pac-Man shape is very common but only for strong outbursts.
BTW: Another misleading quote in the literature is that individual outbursts
can reach 11, 10 or even 9th magnitude (1946 01 24). In the last 25 years,
there has been no outburst brighter than 11.5R. People confuse Total
Magnitude (m1), so if you integrate a large coma then this can have been
produced by the cumulative contributions of several outbursts. Another
mistake has come about by incorrect photographic photometry bak in teh
1930's and 1940's.

Anyway back to Pacman. As we observe more distinctive strong outbursts, we
should be able to learn a lot more about the 29P system - I have yet to
pursue this as we need a good collection of results for strong,
well-observed outbursts before this can be done properly. But ... amateur
observations of the start and progression of an outburst as the brightness
rises to a maximum are helping enormously
I used to think that, like any standard comet, that the thermal stress on
the crust reached a maximum after local Noon on the nucleus. However 29P is
far from being a standard comet and the observational evidence for the
strong outbursts is showing that this is not the case.

29P has an ultra-slow rotation rate of 57-58 days: the exact value depends
on our solving the direction of rotation. I now think that the Pacman
feature is caused by a cryo-eruption of a source that has relatively newly
appeared over the terminator some 2-5 days prior to the eruption. Remember
that 29P is distant and so the phase angle is almost always <10 degrees.
When an eruption occurs fairly close to the terminator, as seen from the
Earth the initial direction of motion will be almost sideways-on. Strong
outbursts are mainly powered by ices / fluids volatilising when exposed to
the Sun's heat, the gas from which then inflates the cloud of dust. The
outer edge of the outburst coma can travel at 200-250 m/s because of the
fast-moving gas molecules. But because the nucleus is very large (60km or
larger) it will shield the expansion in the direction normal into the
nucleus and only that going in the opposite direction will develop into a
spherical shape.

Will stop now as it time to eat and the above is enough for anyone to digest
in a single sitting!
Richard

----- Original Message -----
From: "Nick James" <ndj@nickdjames.com>
To: <baa-comet@simplelists.com>
Sent: Monday, November 23, 2020 6:12 PM
Subject: Re: [BAA Comets] 29P early hours of 23rd November

> Richard,
>
> Thanks. Could you explain a bit more about the characteristic Pacman shape
> that we are seeing?
>
> Presumably the eruption is effectively an impulse and the material then
> expands almost symmetrically in a hemisphere above the surface. The coma
> expansion rate is 200 m/s or so which is an order of magnitude or more
> times the escape velocity so I would expect most of the material to travel
> away in straight lines so we would see an expanding hemisphere from some
> external angle. I don't see how that causes the shape we actually see.
>
> I know I can probably go and read your Icarus paper but a short summary
> would help!
>
> Nick
>
>
> On 23/11/2020 17:55, Richard Miles - rmiles.btee at btinternet.com (via
> baa-comet list) wrote:
>> Nick - Great image showing the discontinuity between the star-like
>> pseudonucleus and the ever-expanding visible coma.
>>
>> No joy with 2.0-m images due to bad weather.
>> However, provided you build the SNR as you have with a longish
>> time-series, your images can provide data almost as good given suitable
>> seeing and focus. This becomes increasingly the case as the coma expands
>> because amateurs have the luxury of sitting on a target for a long time
>> whereas access to the 2.0-m is only for a limited few minutes. At least
>> we got a good result with the FT South just 19 hours after the outburst
>> started - that's where the 2.0-m really scores well.
>
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