Observations of Comet Ison (C/2012 S1) From Lowell Observatory

Abstract

We observed the dynamically new sungrazing comet ISON (C/2012 S1) extensively at Lowell Observatory throughout 2013 in order to characterize its behavior prior to perihelion. ISON had "typical" abundances for an Oort Cloud comet. Its dust production, as measured by r Af , remained nearly constant during the apparition but its CN gas production increased by ∼50 ×. The minimum active area necessary to support observed water production rates exceeded the likely surface area of the nucleus and suggests a population of icy grains in the coma. Together with the flattening of the dust radial profile over time, this is consistant with ejection of a large quantity of slow moving dust and icy grains in the coma at large heliocentric distance. The dust morphology was dominated by the tail, but a faint sunward dust fan was detected in March, April, May, and September. We imaged multiple gas species in September, October, and November. All gas species were more extended than the dust coma, although only CN had sufficient signal-to-noise for detailed morphological study. Excess CN signal was observed in the sunward hemisphere in September and early October. In November the excess CN signal was in the tailward hemisphere and two faint CN features appeared approximately orthogonal to the tail with position angles varying by about ±20°from night to night. Using numerical modeling, we best reproduced the orientation and shape of these features as well as the bulk brightness with a pole oriented approximately toward the Sun and a single source located within ∼35°of the equator. Variations in position angle and relative brightness of the CN features from night to night suggest a rotation period shorter than 24 hr. The production rates and coma morphology suggest a nucleus that was active over nearly its entire sunward facing hemisphere in September and October but which underwent a significant mass loss event, potentially including fragmentation, shortly before November 1. Significant subsequent mass loss likely continued at the same site over subsequent days/weeks and may have catastrophically weakened the nucleus prior to perihelion.

The Main Belt Comets and ice in the Solar System

“…Assuming a system could be scaled up directly, a photometer on a 10 m class telescope would provide a factor of ∼ 100 improvement over existing facilities, e.g., the 1.1 m telescope at Lowell Observatory used by D. Schleicher. Knight and Schleicher (2015) detected CN in 30 min on comet C/2012 S1 ISON at r = 4.55 au with a production rate of Q(CN) ∼ 10 24 mol s −1 , near the limit for this instrument. This implies a hypothetical detection capability of 10 21 − 10 22 mol s −1 when accounting for the larger mirror, longer integrations, and an MBC at smaller r and .…”

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

“…It was soon determined to be a sungrazing comet with a perihelion distance of 0.01244 AU (2.7 R ) to be reached on 2013 November 28. Due to an unprecedented lead time before perihelion, ISON became the subject of a broad and global observing campaign (http://www.isoncampaign.org/) with numerous ground-and space-based observatories making observations (e.g., Meech et al 2013;O'Rourke et al 2013;Bonev et al 2014;Cordiner et al 2014;Hines et al 2014;Shinnaka et al 2014;Knight and Schleicher 2015;Schmidt et al 2015). A fortuitous route through the inner solar system took the comet relatively close to Mars and Mercury, enabling observations from planetary spacecraft operating at those planets, before passing through the fields of view of the STEREO and SOHO solar observatories.…”

“…As most observations leading to these size estimates occurred beyond Mercury's orbit, and Comet ISON was entering the inner solar system for the first time, the size estimates do not inform our specific understanding of near-Sun comets. Tentative measurements of ISON's rotation period seem to indicate that it was < 24 hours (Lamy et al 2014;Knight and Schleicher 2015;Santos-Sanz et al 2015). ISON is suspected to have broken up multiple times as it approached the Sun, evidenced by several changes in activity levels (Meech et al 2013;Opitom et al 2013).…”

On the Rotation Period and Shape of the Hyperbolic Asteroid 1I/‘Oumuamua (2017 U1) from Its Lightcurve

“…We obtained 30 consecutive 300 s observations, dithering every three frames, along with a single short image at the beginning (30 s) and at the end (3 s) of the sequence in which the stars were minimally trailed. All images have been processed by applying our standard image processing procedures (e.g., Knight & Schleicher 2015) to remove the bias and perform flat-field corrections; we used images of a diffusely illuminated spot on the inside of the dome to construct the flat-field images.…”