Did ISON Fragment?

One of our primary functions as coordinators of the Comet ISON Observing Campaign is to encourage and foster data sharing and discussion regarding comet C/2012 S1 (ISON). This brief blog post is to do just that.

There is some discussion over a report that has been made in which the authors describe “ coma wings [that] suggest the presence of two or more sub-nuclei” in Laplacian-filtered comet ISON images. This is a report that has been made by some respected scientists, and thus the CIOC team are definitely taking the report seriously. Indeed, as we have commented, comet ISON’s recent dramatic outburst could absolutely be the result of fragmentation of its nucleus. However, it could equally likely be the result of the expect increase in activity as it continues to approach the Sun. Both of these are possibilities that we have outlined out on this site.

We encourage more observations and analysis, but do urge caution with this latest report. The “wings” that the authors describe certainly could be the result of fragmentation, as so-called arclets are indeed correlated with fragmentation events. However, other possibilities exist:
  • 1. Their appearance is remarkably symmetrical, which leads us to be a little more skeptical that they could be the result of a fragmentation. Fragmentation of cometary nuclei is typically an asymmetrical event, often with fragments trailing in the comet's tail, as was the case with 73P/Schwassmann–Wachmann and C/1996 B2 (Hyakutake), for example.
  • 2. Prior to this outburst, several so-called “jets” had been reported in the area of activity that seems to be the source of these wings and thus it is possible that we are seeing enhanced dust emission due to jets in ISON’s nucleus.
  • 3. We could simply be seeing solar wind ion tail structures, perhaps after some solar wind interaction event, for example with a coronal mass ejection or a co-rotating interaction region, or some other solar wind density enhancement. Indeed, spacecraft observations certainly indicate such interactions to have happened (this will be the source of a bog post later this week), but whether or not that interaction caused this new feature is still unknown.
The CIOC Team supports and encourages reports such as this, and we certainly monitor them closely. Our motivation for this blog post is to urge this discussion to continue, and we strongly encourage further data analysis efforts and a community-wide sharing of information on this and related subjects.

For now we continue to monitor the situation, to see which one of these scenarios is occurring. If the comet has fragmented, we should see some dramatic changes in the next few days. Typically we expect a large increase in brightness followed then by a dramatic decrease in the comet's brightness in these situations. If instead jets are the cause, we expect the "wings" to persist and grow. If the behavior is driven by the solar wind then we should see continual changes as the solar wind changes. This is monitored by a number of spacecraft 24/7 and reliable solar wind models, so should be possible to verify.

Keep up-to-date on the latest ISON and sungrazing comet news via my @SungrazerComets Twitter feed. All opinions on there, and in my blog posts, are my own unless otherwise stated.