[BAA Comets] Q&A about the recent outburst of C/2012 X1 (LINEAR)

[Andrew Robertson]

An excellent write up Richard.

Many thanks,

Andrew 

-----Original Message-----
From: comets-disc-bounces at britastro.org
[mailto:comets-disc-bounces at britastro.org] On Behalf Of Richard Miles
Sent: 26 October 2013 16:08
To: BAA Comets discussion list
Subject: [BAA Comets] Q&A about the recent outburst of C/2012 X1 (LINEAR)

Denis Buczynski and Nick James have asked me a few questions about the very
recent outburst of Comet C/2012 X1 (LINEAR) and in particular the analysis
of structures in the inner coma. See the various images here:
http://britastro.org/baa/index.php?view=category&catid=99&option=com_joomgal
lery&Itemid=200

Well here are my answers for all to read (if you have the time that is!):


Q1.  Impressive how similar the images are despite the time separation. I
assume the spiral stucture is due to the nucleus rotating but the effects
are projected in some way so that we don't see anything much changing? 
(Nick)

A1.  No, not at all. The curves on the spiral outflows are definitely not a
result of the nucleus rotating! Earlier models always interpreted coma
models in terms of the gradual release of dust/debris/gas from active
regions (often termed jets). The rate of release would build up and then
slow down in time as the subsurface reservoir of volatiles becomes
exhausted. In the last 10-15 years, it has become apparent from observations
of comets notably 17P/Holmes and 29P/Schwassmann-Wachmann that there is
another type of outburst: one which starts with an explosive event releasing
dust/debris/cometary ices all of which is over within a matter of seconds. 
So in this second type of outburst, the rotation of the nucleus does not
have time to impress any rotational signature on the expanding material.


Q2.  Can you offer some explanation of what we are seeing in our images. 
Also, other images by different observers show the same detail. How do you
see the outburst deleveloping? (Denis)

A2.  C/2012 X1 exhibits characteristics in common with Holmes and 29P. The
shape of the outflows are governed by the way in which the explosive event
releases material into space. Pressure builds up in the subsurface capped by
a consolidated near-surface layer or plate. In the case of 29P and C/2012
X1, the force begins to lift the capping layer, which largely remains in
place (unlike Holmes where it was blown out into space). Debris/ices and
dust escape from around the edge of the capped surface region, momentarily
starting at one particular point but then quickly spreading around the
fissure, which continues to open up. It is this progression of the explosion
of material around the fissure which creates the spiral structure because
the velocity of escape is highest soon after the fissure starts to open
(when subsurface pressure is highest) but then this decreases/changes and
the different speeds of material ejected in different directions create the
curved outflows which can have the illusion of being spiral in nature like a
rotating catherine wheel. That part is all over and done with in a few
seconds.

However, the process which creates the expanding spherical shell of material
is different again! This results from processes happening in the first few
minutes after the explosion. Where a substantial quantity of dust and
associated ice are ejected into space propelled by escaping gas from the
subsurface, they suddenly become exposed to solar heating. The heat causes
the exposed ices to sublimate rapidly in a matter of minutes. During this
time period, the pressure of the gas around the dust and debris is
significant [higher than 10^(-9) or 10^(-8) bar]. In chemistry this is
termed the pressure region for thermomolecular flow - basically molecules of
gas are able to hit dust particles which are physically big (micron-sized or
smaller) but the pressure is too low for those molecules to hit other gas
molecules. So the outcome of this is that the dust is accelerated outwards
in all directions (isotropically) whilst the gas pressure stays high enough,
i.e. only a few minutes. Interestingly (again this is chemistry - kinetic
theory of gases) the velocity of the gas molecules is entirely governed by
their temperature and the mass of the molecule - they also span a range (the
so-called Boltzmann distribution). The dust is accelerated by the impacts of
the gas molecules and typically the outer edge of the expanding shell
reaches a speed not so different to the 'root-mean-square' speed of the gas
molecules, typically about 0.5 km/s. So we saw the spherical shells for both
17P and C/2012 X1 expanding at this speed. Interestingly, because the
nucleus of 29P is so large (several tens of kilometres across), the
inflatory shell of gas and dust can only accelerate outwards from one side
of the surface of the nucleus and so we see a characteristic 'hemishell' in
the case of 29P outbursts.

N.B. Only a fraction of the gas accelerates the dust. Most of the gas
molecules fail to hit a dust particle and so continues expanding faster than
the dust shell. That's why we saw the outer greenish-blue gas emission ahead
of the expanding dust shell of 17P/Holmes.

One other important feature to pick up on re. the outflows of the inner coma
is that here (and in 29P also) we see outflows in roughly mutually opposite
directions (projected on the sky). Of course it is not possible for a single
active region emitting a jet to direct the ject in more than one direction
at a time. However, where a capped region of the surface is lifted up by a
rapid pressure increase beneath it, material can esape from fissures around
the entire perimeter of the raised area and often the pressure is exerted in
roughly equal and opposite directions: hence the oppositely-directed
outflows.


Q3.  It seems that the expanding coma is becoming fainter and more diffuse. 
Will it just dissipate over time and leave us with a view of the pseudo
nucleus, or will there be continuing supply of material into the nucleus as
perihelion is approached?  (Denis)

A3.  Yes - since virtually all of the material we see was suddenly ejected
into space in a matter of a few minutes, there is no significant
replenishment taking place, at least during the first few days. That means
we see an ever increasing dilution of the coma. The velocities of the
outflows are more or less uniform at the start: that is before solar
radiation pressure starts to act on the particles gradually compressing the
sun-facing side of the spherical shell and slightly extending the other
side. Those particular changes take several days to become noticeable and
then gradually become more marked. Despite the sudden outburst, the rest of
the comet's nucleus will be outgassing and be feeding the inner coma with
dust in the usual way that comets do. So the rest of the approach to
perihelion should involve normal brightening. Unless there happens to be
another region in its subsurface where pressure is able to build, held down
by a surface capping region, in which case watch out for a second outburst!


Q4.  Where is a trailing tail of dust and gas from this material that has
been ejected into the coma.  (Denis)

A4.  The normal comet's tail is largely unaffected by this sudden outburst. 
The tail should gradually grow. Comet Holmes was rather different however. 
In the last few weeks I have discovered and downloaded some more Hubble
Space Telescope of Holmes and have been studying these. Already from amateur
images of 17P by Peter Birtwhistle, by myself and by Juan Lacruz, I have
found that the tail took some 6 or 7 days to re-establish itself. The HST
images, when analysed carefully, indicate that this nucleus takes many days
to rotate and the delay in the tail appearing was partly linked to its slow
rotation. More of that another time. I would urge observers to look for a
normal tail in the case of C/2012 X1.


Q5.  Comet Holmes did eventually produce a gas tail. Has anyone recorded
this comet (C/2012X 1) in colour yet, what is the spectral signature of  the
material in the expanding coma and how fast is it moving?  Fascinating
objects comets,eh?  (Denis)

A5. Haven't seen any colour images as yet but we should see similar
characteristics to 17P/Holmes: reddish inner dust coma and greenish outer
gas emission I think its for the professionals using very large scopes to
derive detailed spectra, etc. Whether anyone is using HST, I do not know but
that instrument is by far the best for resolving the very inner coma. The
Keck telescopes produce some marvellous near-infrared spectra of comets (see
work of researchers Neil Dello Russo and Cary Lisse, for example). The speed
of coma expansion, I expect, will be around 0.5 km/s for reasons explained
above.

For the last few years, I have been working on the reasons why the pressure
can suddenly build up beneath the surface of a cometary nucleus. But that's
another story!
btw: I am giving a talk on this whole subject to the Bristol Astronomical
Society next Friday (November 1, starting about 7.30pm).

Richard Miles
BAA


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