Comet C/2015 ER61 (PANSTARRS) was discovered on 2015 March 14 by the PS1 survey. The object is on a long period comet orbit but is not dynamically new (a = 3224.1 AU, e = 0.99967, I = 6.1882 degrees) and will come to perihelion at 1.042 AU on 2017 May 9. At the time of its discovery, at a heliocentric distance of r=8.44 AU, the object appeared inactive. We considered it as a Manx object candidate (an object on long-period comet orbits which exhibit minimal or no activity even near the Sun; Meech et al. 2016) for follow-up observations to obtain its surface spectral reflectivity. Data were obtained of the nucleus with the Gemini North 8-m telescope through griz filters on 2015 June 12.3 UT at r=7.74 AU. The composite images showed weak activity in the form of a slightly extended PSF (FWHM=1.0 arcsec compared with 0.8 arcsec seeing), and a weak tail-like extension, 2 arcsec long at PA~120 degrees. Additional grizY data from Gemini were obtained on 2016 Feb. 4 at r=5.70 AU when the comet was much more active, allowing for a comparison of the surface and coma spectral reflectivities. The WISE observatory also detected C/2015 ER61 on 2015 Dec. 23 and 2016 May 25 at r=6.09 and r=4.64 AU, respectively. We will report on the nucleus characteristics for C/2015 ER61 based on the NEOWISE and Gemini observations. The NEOWISE observations will also be used to place limits on the amount of CO2 outgassing from the comet. Serendipitous observations were also collected with the VST on Paranal at 11 and 6.2 AU. We have used this data in addition to photometry obtained from the CFHT 3.6m and UH2.2m telescopes on Maunakea the HCT 2m telescope in India, and pre-discovery data from the PS1 survey to model the onset of activity. A strong indication for this activity to be driven by deeply-buried CO or CO2 is shown by preliminary modeling. C/2015 ER61 is therefore a rare case of a long-period comet discovered and characterized before the activity started, and whose sublimation onset was observed.References:Meech, K. J. et al (2016), Science Advances 2, 4, id. E1600038.
AAS/Division for Planetary Sciences Meeting Abstracts #48
- Pub Date:
- October 2016