Just checked on JPL Horizons, comparing the direction of the linear features in the July and October outburst comae with the direction an ion tail or the dust trail would be expected at these two epochs. Turns out that neither fit the observed directions. So that means the direction has been imposed by the cryo-eruption but in the sense of a shadow of the expanding dust shell. (A bit like the Pac-Man feature we see sometimes when 29P has an energetic outburst). So that means the eruption is in the OPPOSITE direction to the line of symmetry through the linear features.

R.

-----Original Message-----
From: baa-comet@simplelists.com <baa-comet@simplelists.com>
Sent: Saturday, October 7, 2023 10:27 PM
To: baa-comet@simplelists.com
Subject: RE: [BAA Comets] RE: UPDATE: Comet 12P/Pons-Brooks: Time of outburst now more precise

Thanks Nick,

Yes - Closer to the Sun now at 3.5 au, therefore higher temperatures.
As the outburst coma is inflated by expanding gas, the gas is hotter and so the speeds of the dust particles are higher.

As this is the BAA's Comet Discussion group, I am sharing a couple of images of the two outbursts taken with the 2.0-m Faulkes Telescope North.
It is interesting to compare these not only because the similarities are striking but also to contrast any differences.

My thoughts are that the linear features are caused by the body of the nucleus shielding the expanding cloud. The shielding is less effective in the October outburst presumably 'cos the speeds are greater but why are there 'horns' that travel even faster? What speeds them up and forms the linear flows either side of the nucleus? These flows are even more marked this time?  Maybe they are caused by some peculiarly-shaped cryovolcanic vent with some sort of blockage causing material to be expelled with a weird flow pattern?

The inner halo is present in both, although the later one looks more translucent, a bit like an expanding shell of material. It is also slightly smaller compared with the size of the main outer coma. Where we see this with 29P it is because there have been 2 outbursts with a time gap between them. Maybe the nucleus has two different hypervolatiles, like CO and CO2, and they expand at different rates?

One very strange comet!

Clear skies,
Richard

-----Original Message-----
From: baa-comet@simplelists.com <baa-comet@simplelists.com> On Behalf Of Nick James
Sent: Saturday, October 7, 2023 9:16 PM
To: baa-comet@simplelists.com
Subject: Re: [BAA Comets] RE: UPDATE: Comet 12P/Pons-Brooks: Time of outburst now more precise

Richard,

I get an outer coma diameter expansion rate of 18 arcsec/day. At the current distance (3.06 au) that corresponds to around 230 m/s expansion velocity which is similar to last time (I got 200 m/s last time from images measured over 7 days). The comet was 3.9 au out at the time of the last major outburst.

Nick.

On 06/10/2023 00:53, rmiles.btee at btinternet.com (via baa-comet list)
wrote:
> Thanks Nick,
>
> Using the V band you are seeing more of the gas emission than what is
> visible this evening in the Sloan r' filter.
>
> B filter images may also be worthwhile.
>
> Alan Hale has caught the outburst in the later stages of its rise to
> maximum.
> He used a 'w' filter, which is almost like an unfiltered image.
> He just managed to avoid any significant saturation in 180-s exposures
> but it was a close-run thing!
>
> It'll be interesting to compare and contrast this event with the July
> 20 one.
> Given the changing distance, it now looks like this was actually
> slightly less intense than the former one.
>
> Clear skies,
> Richard

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