Ordinary Meeting, 2009 March 25


Comet 17P/Holmes

Dr Miles opened by summarising the physical characteristics of comet 17P/Holmes, explaining that it was a short-period comet of the Jupiter family. At present, its orbital period was 6.9 years and its perihelion and aphelion distances from the Sun were 2.05 AU and 5.18 AU respectively; this meant that it usually lay between the orbits of Mars and Jupiter, never passing inside Mars' orbit but briefly passing outside Jupiter's orbit at each aphelion. However, its orbit had not been constant over the 117 years since its discovery in 1892. From time to time it passed sufficiently close to Jupiter at aphelion that its orbit could be substantially perturbed by Jupiter's gravitational influence, and this had happened three times since its discovery, in 1908, 1968 and 2004. In the course of these interactions, it had passed through a range of orbits with periods ranging between 6.9 years and 7.33 years.

The speaker went on to explain that the comet had been discovered by BAA member Edwin Holmes on 1892 November 6, a year after the Association's foundation. It had rapidly attracted considerable interest since its conspicuous brightness – visible to the naked eye at around mag. 6 – and position – close to M31 – meant that it could not have lain unnoticed in that state for more than a day or two, and it seemed certain that it must have undergone an exceptionally rapid outburst. Its evolution had been very well recorded over subsequent days and weeks; a series of photographs taken by Edward Barnard at the Lick Observatory showed a growing circular patch of nebulosity.1 The comet had faded rapidly over the following two months, but just as it had verged upon becoming lost, Barnard had recorded it in outburst once again on 1893 January 16, this time brightening to a more modest mag 8 and for only a short time. Shortly thereafter, it had faded beyond the reach of the telescopes of the time, and had only been rediscovered as a modest mag 16-17 object in the 1960s.

The comet had regained attention on 2009 October 24 following a report at 01:33 UT from Spanish amateur astronomer Juan Antonio Henriquez Santana, of the Observadores Cometas observing group, that it had been observed at tenth magnitude, whereas it had been no brighter than sixteenth magnitude in previous days. In subsequent hours, events had unfolded rapidly.2 The comet's outburst had been confirmed by Ramón Naves at 02:15 UT, and within 24 hours, the comet had become an easy naked-eye object at almost mag 2. The first recorded UK-based observations of the comet had been made by Denis Buczynski at 18:35 UT. At the time of the 2007 October meeting of the Association, on October 31, it had remained sufficiently bright that a number of members had had no difficulty in observing it without any optical aid from the pavement of Piccadilly in Central London.

Dr Miles then went on to describe the comet's observed morphology, explaining that aside from its nucleus, three distinct components were visible in the best images. Closest to the centre, a yellowish and almost circular inner halo was attributed to dust. Around this, a more widespread greenish-blue glow was apparent, with similarly circular shape, but less well-defined outer edge; this was attributed to water vapour. Finally, an ion tail could be seen extending from the coma in the anti-solar direction. By combining measurements of the observed rate of expansion of the comet's coma with the known distance of the comet, it was possible to infer that it was growing outwards at a speed of 500 m/s. This was suggestive that the coma was made up of material which had been thrown off the comet's nucleus with explosive violence.

A few days after the initial outburst, it had become apparent that there was not just one central brightness maximum within the comet's inner coma, which might be associated with the comet's nucleus, but also a second. Over subsequent days it had become further apparent that this second brightness maximum was moving relative to the centre of the coma at an estimated speed of 135 m/s.3 Subsequently, a larger number of small point-sources had been recorded within the comet's inner coma, travelling at a range of speeds and in a range of directions, but appearing to have emanated from a common source at around the time of the observed outburst; the speaker informally dubbed these mini-comets.

The Observadores Cometas observing group of which Juan Antonio Henriquez Santana was a member had gone on to monitor the comet's lightcurve for several months, seeing principally a steady decline in its brightness which was compatible with the expansion of a spherical shell of material. However, regular small outbursts, each by a little less than a magnitude, were also observed every 45 days. If this period was to be interpreted as being associated with the comet's rotation period, then 17P/Holmes was an unusually slow rotator: comets normally had rotation periods which were only a few hours. The speaker conjectured that the slow rotation of 17P/Holmes might have played a part in its outburst, perhaps because it would lead to exceptionally large temperature differences between its day and night sides.

The speaker concluded his talk by discussing observations which he had made of the comet over recent months using the Faulkes Telescopes, going on to discuss possible physical mechanisms for the apparent explosion which had triggered its outburst. He explained that a popular theory in the professional literature was that a large mass of amorphous ice beneath the comet's surface had undergone a phase transition to form crystalline ice. However, he argued that this theory seemed unable to provide sufficient energy. He then argued that liquid water might exist beneath the surface of the comet, and that this might have become gradually converted to hydrogen peroxide as a result of interactions with cosmic rays. This hydrogen peroxide could then react explosively if brought into contact with metal-rich parts of the comet, producing large quantities of heat and oxygen gas, the latter perhaps being sufficient to blast the surface off the comet. These ideas will be written up more fully in a future Journal paper.

Following the applause, the President introduced the evening's second speaker, Mr Tom Boles, the Association's most prolific discoverer of supernovae, a former president of the Association, and one of its current Vice Presidents.




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