[BAA Comets] Post-perihelion fragmentation of 168P/Hergenrother

[Richard Miles]

Denis asked, "Do you think that the fragmentation we have witnessed was the 
cause of the brightness outburst seen over the past month? "

The first notification I saw of this perihelic outburst of 168P was from 
Reinder Bouma and Edwin van Dijk on September 22.  On October 22, i.e. 1 
month later, there was no discrete 'fragment' visible but 4 days after this 
date, lo and behold, a bright region appears to have separated from the 
nucleus as seen in the Faulkes image of October 26 (seeing 1.2").  That this 
was what I call a mini-comet was evident in the 3.5-m WIYN image obtained on 
October 30 at an effective seeing of probably 0.3-0.4" as you could see what 
looked like a small brightspot trailing a faint coma behind it.
[BTW: That was some incredible image using their new 'on-chip' method for 
improving resolution based on the new Orthogonal Transfer Array technology 
as well as tracking the comet during the exposures.  As you pointed out to 
comets-ml, they have removed this WIYN image from their blog probably 
because they are still only in the commissioning stage for that instrument. 
I wonder when will OTA CCDs be available to amateurs?]

Those are the facts.  What actually is going on at the comet is quite a 
different matter and is open to speculation.  What I'd like to draw folk's 
attention to is that none of these images actually show the comet nucleus or 
mini-comet nucleus.  That's because we generally see something mainly thanks 
to all the dust that is ejected into space from the physical nucleus or 
fragment(s).  Comet nuclei are very dark, so an object say 1 km across at 
the distance of 168P would appear about magnitude 18, an easy target for the 
HST to resolve but not for the amateur-size telescope when the solid body is 
surrounded by a bright coma.  So outbursts are generally associated with the 
release of dust ejected from the nucleus by gas pressure causing the surge 
in brightness.

We could speculate what has been happening to 168P:  Although dust and gas 
release has been dramatically enhanced in the past few weeks, the 'fragment' 
might have split off  a few weeks ago but may have taken a fair while to 
heat up and for volatiles and dust to be ejected so as to make it visible. 
Such a change might have happened around October 26 since at that point the 
'fragment' does not show any significant tail having formed.  That means it 
might have become activated and released material a few days earlier (say 
Oct 23-24) so by Oct 26 the dust surrounding it renders it visible.  By 
October 30, the 'fragment' will have faded as some of the dust is 
transferred into the tail forming the mini-comet'.  By November 2, more of 
the 'fragment' has disintegrated, fading in the process and streaming more 
dust and gas behind it.  Given such a scenario I'd expect this 'fragment' to 
disappear completely unless some large piece(s) of debris remain in which 
case a large scope will be needed for its detection.

By January 1, the comet will be twice as far away as it is now so we don't 
have a great deal of time.  Probably the best windows for observing will be 
Nov 8-19 and Dec 7-18.  At least it will be well placed for observers in the 
northern hemisphere.

Richard Miles
BAA

----- Original Message ----- 
From: "denis buczynski" <buczynski8166 at btinternet.com>
To: "BAA Comets discussion list" <comets-disc at britastro.org>
Sent: Friday, November 02, 2012 1:21 PM
Subject: Re: [BAA Comets] Post-perihelion fragmentation of 168P/Hergenrother


> Hi Richard,
> Thanks for the comments below. It is very beneficial to us to be able to 
> access the images taken by
> the Foulkes Telescopes for the schools programme which allow us to see in 
> detail the progression of
> this fragmentation process. As you say it is probably unlikely that 
> we(amateurs) will be able to
> image this fragment now that it is fading.I suppose it is not surprising 
> that the fading is
> occurring, partly due to the increasing distance from earth and also that 
> the fragment must have
> parted from the main body at its surface and will subsequently lose its 
> volatiles and material quite
> quickly without any replenishment. I see from the Foulkes image that there 
> is still quite a lot of
> material flowing down the tail ( as also seen in Nick James's image from 
> last night) so I suppose
> that there may be a chance of more large fragments appearing. However now 
> that perihelion passage is
> in the past now I suppose that there are not the same extreme forces 
> acting on the comet and that it
> will probably return to is quiescent state soon. Do you think that the 
> fragmentation we have
> witnessed was the cause of the brightness outburst seen over the past 
> month? This has been a
> fascinating comet to observe and has been so well placed in high northerly 
> declinations, we have
> been very lucky that so many observations have been possible. This comet 
> apparition reinforces the
> age old advice that comets are unpredictable objects and we should never 
> be surprised when one
> behaves unexpectedly.
> Denis Buczynski
>
> -----Original Message----- 
> From: Richard Miles
> Sent: Friday, November 02, 2012 12:31 PM
> To: BAA Comets discussion list
> Subject: Re: [BAA Comets] Post-perihelion fragmentation of 
> 168P/Hergenrother
>
> Thanks Denis for trying to detect the fragment.  It turns out that it has
> faded quite a bit and is still only about 3.3" from the main nucleus.
>
> This morning two schools/colleges (Queen's College and Dollar Academy)
> working with the Faulkes Team coordinated by Nick Howes each managed to
> image the comet continuously for about 50 minutes using the Faulkes
> Telescope North.  I downloaded their images and processed 19 of them using
> the same approach as previous.  (The first few images were contaminated by 
> a
> nearby star so I did not use those.)  The results can be seen at:
> http://www.britastro.org/~rmiles/Documents/168P_2012Nov02.png
>
> The 'fragment' is now sporting a small tail of its own and is really very
> faint compared to the main nucleus, most of the light from which has been
> removed by the rotational gradient image processing.
>
> For comparison the earlier images are available at:
> http://www.britastro.org/~rmiles/Documents/168P_20121022-26.png
>
> The apparent drift rate of the fragment relative to the main nucleus is 
> very
> slow.  This is in part caused by the increasing delta distance over recent
> days, so although the actual separation in km is increasing, the 
> mini-comet
> appears to remain hugging the parent body.  In 6 days the separation has
> maybe gone from 2.4" to 3.3".  Compounding the problem for amateur-size
> telescopes appears to be that the fragment has faded significantly so may 
> be
> starting to break-up - hard to tell at this stage.
>
> Richard Miles
> Director, Asteroids and Remote Planets Section
> British Astronomical Association
>
> ----- Original Message ----- 
> From: "denis buczynski" <buczynski8166 at btinternet.com>
> To: "BAA Comets discussion list" <comets-disc at britastro.org>
> Sent: Friday, November 02, 2012 12:06 PM
> Subject: Re: [BAA Comets] Post-perihelion fragmentation of 
> 168P/Hergenrother
>
>
>>I was also able to image this comet last night, in a dark sky (first with
>>moon out of sky for some
>> time) and took a set of 20x30s in r band filter to see if the reported
>> fragmentation was visible yet
>> at 2"/pixel resolution I use with my C14/ST9XE combination. I could not
>> detect any fragment with the
>> processing I used (stacking and log stretch). I wonder if a drift rate 
>> can
>> be calculated fro the
>> fragment from the large telescope images which would give us an 
>> indication
>> when it should be
>> possible for observers using the size of telescope we employ to image the
>> main fragment? I will
>> continue to image this active comet. I have placed all the images
>> submitted to the Comet Section on
>> the Comet Gallery on the BAA Website.
>> Direct links to the images I took last night are at:
>>
>> http://britastro.org/baa/index.php?view=detail&id=646&option=com_joomgallery&Itemid=200
>>
>>
>> http://britastro.org/baa/index.php?view=detail&id=645&option=com_joomgallery&Itemid=200
>>
>>
>> Denis Buczynski
>> Secretary Comet Section
>> British Astronomical Association
>>
>> -----Original Message----- 
>> From: Nick James
>> Sent: Friday, November 02, 2012 7:49 AM
>> To: BAA Comets discussion list
>> Subject: Re: [BAA Comets] Post-perihelion fragmentation of
>> 168P/Hergenrother
>>
>> I had a good opportunity to take some images of 168P last night in
>> transparent skies using the relatively small (C11) telescope in my back
>> garden. It is interesting to compare the results with those obtained
>> with much larger telescopes.
>>
>> The resulting image is here:
>>
>> http://www.nickdjames.com/Comets/168p_20121101_ndj.jpg
>>
>> FWHM was 3".5 which is not bad for here but doesn't really compare well
>> to the 1".5 or less using the bigger scopes on mountains. There's no
>> sign of the fragment (as expected at this resolution) but the rotational
>> gradient does show flow down the near tail.
>>
>> This is definitely a comet that is worth keeping under observation.
>>
>> Nick.
>
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