Recent observations of comet C/2013 R1 Lovejoy showed the presence of characteristic structures in the vicinity of the jet fake comet's nucleus.
Such structures can be explained by the presence of collimated jets of gas and dust emitted from the surface of the comet nucleus.
This article attempts to interpret some observations in order to create a kinematic model of the comet's nucleus.
The present study is not a scientifically rigorous analysis of the phenomenon as it is based on the measurement of only two photographs, not perfectly astrometrically reducible, and on subjective assumptions on basic parameters such as the spatial position of the rotation axis, the ejection speed of the jets from the core and the composition of the comet dust.
It is therefore to be considered just "play with the parameters" attempting to reproduce the observations.
Nevertheless I think the results are significant since they succeed, in spite of the simplicity of the model, in reproducing the observations.
In the coming months we will be able to compare new observations with the predictions of the model to see if the chosen parameters are correct or, more likely, if need adjustment
The model used for the simulation is extremely simple:
- The comet nucleus is spherical , rotates around a fixed axis in space with constant period .
- From the core spray radial jets of dust at a constant speed , the jets are active on the illuminated hemisphere of the nucleus while they are turned off at night.
- The dust particles emitted from the nucleus are spherical , all the same size and with the same physical characteristics (density and albedo )
- Once emitted from the nucleus the dust particles are subject only to the force resulting from the radiation pressure of the sunlight : for a rigorous treatment should also take into account the gravity, but in the immediate vicinity of the cometary nucleus,it can be omitted .
- The jets behave as garden sprinklers that create spirals of dust expanding in space and deformed by the action of solar radiation .
- The coils are wrapped around a cone as wide as the colatitude of the jet and are much more narrow as the higher is the rotation speed of the nucleus or lower the ejection speed of the dust.
The simulations were performed with the software Comet written by me in QBASIC in 1997 for the comet Hale Bopp and still running under DOS.
The choice of parameters was carried out by trial and error in order to try to replicate the observations.
To clarify the model here are three illustrative examples: in all cases there is a single jet on the comet active at a latitude of 45 ° N, the only thing changing is the orientation in space of the rotation axis.
Example 1: axis pointing in the direction of the Earth (inclination 90 ° to the plane of the sky)
In this case we see the spirals expand into space. Because of the radiation pressure spirals are gathering in the direction of the Sun, and less frequently in the opposite direction.
Example 2: Axis with position angle of 45 ° and inclination with respect to the plane of the sky of 60 °
In this case the coils are observed almost along a the cone edge and the effect of overlapping spirals can create the illusion of fountain jets coming from the core.
Example 3: axis with position angle of 45 ° and lying in the plane of the sky
In this case one can observe the cone on which are the spirals are wrapped on. Since the gaze on the sides intercepts a greater amount of dust will have the illusion of almost radial jets which then bend away from the nucleus due to the radiation pressure.
I thank the authors for giving me the opportunity to experiment with their images.
Stefano Quaresima picture taken on November 30, 2013, with and processed with elaborate Larson Sekanina, clearly shows the presence of at least three jets in a fountain.
So I set the simulation in order to have three jets at different latitudes positioned so as to have the generating approximately pointing towards the Earth.
As mentioned above, the choice of parameters is completely arbitrary and dictated by the attempt to reproduce the observed image.
The parameter set that seems to play better the image is:
- Position angle of the rotation axis: 85 °
- Angle to the plane of the sky: -25 °
- (These parameters result in a rotation axis inclined at 37.67 ° with respect to the orbital plane and with an angle of 145.852 ° relative to the direction of the ascending node of the comet.)
- Latitude of jets: 52 ° N, 33 ° N, 18 ° S
- Jet power (arbitrary units used for the simulations pseudoreali): 35, 45, 55.
|Image by Stefano Quaresima
|Wide field simulation
The model was then made evolve until December 7 date of the picture by Giovanni Donatiello.
Using the same parameters obtained for the first image the following simulation was obtained.
|Image by Giuseppe Donatiello
Another picture of the comet structures of the comet was found on the Web at
Calculating the model for that date and rotating it to align roughly to the image shown on the website I got get the following simulations
Although this work is not to be considered scientifically rigorous, success in reproducing images of the comet leaves some hope that the chosen parameters are not too far from the actual values.
The only way to be sure is to wait for new images to be compared with the predictions of the model.