A repeating CN jet feature from 45P/Honda-Mrkos-Pajdušáková

Comet 45P/Honda-Mrkos-Pajdušáková had a close approach of 0.08 au to the Earth on February 11, 2017, presenting a unique opportunity to investigate its inner coma structure in high spatial and temporal resolution with ground-based observations. We observed 45P on the nights of 16 and 17 February, 2017 with the Bok 90-in telescope on Kitt Peak, using r', CN, C₂, OH, and BC filters. We detected typical curved jet features as well as a feature in CN emerging from the 45P nucleus with a periodicity close to the rotation period of the nucleus (7-7.5 hours, Lejoly & Howell, CBET 4357, 2017). We detect evidence for a similar feature in the C₂ filter. Radial profiles across the inner nucleus region of the images show CN moving away from the nucleus with a projected velocity close to that of gas expansion. This repeating CN structure may be the result of a diurnally active high intensity gas jet feature; the short period of active CN emission is consistent with a topographic feature hosting CN-bearing materials that is only briefly illuminated for a part of a diurnal cycle. Furthermore, this interpretation is consistent with the preliminary shape model of the nucleus (Howell et al. 2017), which exhibits topographic deviations from a spheroid, suggesting that 45P may host scarps and cliffs with volatile rich talus at their bases. Structures with sharp topographic relief may be the sources of diurnally active jet features when illuminated (typically only a fraction of a diurnal cycle). This repeating feature was not observed in other filters, including r' and BC (proxies for dust) or OH (a proxy for water), suggesting that CN releases from an active area, independently of sources of H₂O or dust.