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

[Richard Miles]

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_joomgallery&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