Comet C/2015 ER61 fragmentation event: some further data and some simple analysis

As you may have heard, comet C/2015 ER61 Panstarrs has recently experienced a nucleus splitting. We were among those who reported this event. Digging in our image archives, we have found evidence of that fragment in June 02 2017 images and could document the motion of the “b” component.

Comet C/2015 ER61 and its "b" component: 2, 17 and 21 June 2017

Comet C/2015 ER61 and its “b” component: 2, 17 and 21 June 2017

The splitting of comet C/2015 ER65 was announced on CBET 4409 (subscription required), issued on 24 June 2017 by the Central Bureau for Astronomical Telegrams, while on 18 June the Minor Planet Center issued the MPEC-M09, both reporting our independent observations.

We inspected all the images we have of this comet, finding the first evidence of fragment “b” on images taken on 02 June 2017. Images from May 26 do not show evidence of that secondary component. Using all the frames from each observing session (2, 17 and 21 June), we created three master frames, to be further processed.  All the images used were remotely taken with the 16″-f/3.75 Tenagra III (“Pearl”) robotic unit part of Tenagra Observatories in Arizona.

We managed to carefully apply an unsharp mask filter on the master frames, using the resulting images to make the animation above. The image scale is 2.4″/pixel. North is up, East on the left.

It is easy to see how the “b” separates from the parent nucleus. Angular distances from the main nucleus are 11″, 14″ and 15″ (+/- 1″) on June 2, 17 and 21, respectively. It is worth to remember that around April 4, 2017, comet C/2015 ER61 had an outburst. Assuming a simple scheme where the geometry does not change and a constant motion of fragment b, we can calculate that the fragment detached about 70 days before June 21, that is around the outburst time. Of course, this is a very (too) elementary approach, more data will help investigating in this direction.

We plan some further imaging to cover the evoltution of this event.

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