For those of you who have been following our news of recent comet observations and analysis by Richard Miles (BAA), he has provided us with an update on his observations of comet Vales.
The evolution of structure within the coma of P/2010 H2 (Vales) has been followed at 15, 20 and 25 days after its initial outburst on April 15 thanks to recent images taken with both the Faulkes Telescope North and South. The coma itself has now expanded and taken on a similar appearance to that of Comet 17P/Holmes some 4 weeks into its outburst. See for example:
http://www.observadores-cometas.com/cometas/17p/imagenes/17P-071123-VI-j77.jpg
In the case of Comet Vales, the nucleus is smaller than Comet Holmes' and the extent of the outburst is much less energetic. However, at least 3 discrete curved dust arcs can be seen in the latest rotational gradient-processed image of May 10. We can now see that the arcs are not jets continually emanating from the nucleus, as suspected earlier, since they have not exhibited any rotation in position angle with time. Instead, the material of which the arcs are composed appears to move away from the nucleus in a radial manner.
These features are characteristic of an outburst where material is ejected from discrete sites on the nucleus over a short time interval amounting to less than a few hours (i.e. short in duration compared to the typical rotation period of a comet nucleus). The shape of each arc depends on the ejection velocity vs. time profile. For the most obvious arc (about 230 deg p.a. at the nucleus), it is likely that the fastest-moving ejecta arose at the start of the outburst but that increasingly slower-moving ejecta were emitted from this most active site over a time-scale of minutes to a few hours thus creating the characteristic arc-like pattern. The next most significant arc (about 280 deg p.a. at the nucleus) is quite different in shape indicative of a more sustained ejection of fast-moving material at first and a more sudden decline later although its shape will also be affected by solar radiation pressure (see below).
Of course we are seeing these structures projected onto the sky in two dimensions whereas in actual fact the outflows are three-dimensional in nature and arise from a rotating nucleus, the rotational axis of which will be inclined to our line of sight by some unknown amount thus making a more exact interpretation difficult at this stage. There is also the complicating effect of the pressure of sunlight, and to a lesser extent the solar wind, which slowly accelerates the finest dust in the opposite direction to the Sun. The smaller the dust grains, the more they are perturbed by solar radiation pressure causing the outer part of the comet's coma to take on a distinctly asymmetric shape.
