PRELIMINARY ANALYSIS OF SOHO/STEREO OBSERVATIONS OF SUNGRAZING COMET ISON (C/2012 S1) AROUND PERIHELION

Abstract

We present photometric and morphological analysis of the behavior of sungrazing comet C/2012 S1 ISON in SOHO and STEREO images around its perihelion on 2013 November 28.779 UT. ISON brightened gradually November 20-26 with a superimposed outburst on November 21.3-23.5. The slope of brightening changed about November 26.7 and was significantly steeper in SOHO's orange and clear filter images until November 27.9 when it began to flatten out, reaching a peak about November 28.1 (r H ≈17R ), then fading before brightening again from November 28.6 (r H ≈5R ) until disappearing behind the occulting disc. ISON brightened continuously as it approached perihelion while visible in all other telescopes/filters. The central condensation disappeared about November 28.5 and the leading edge became progressively more elongated until perihelion. These photometric and morphological behaviors are reminiscent of the tens of meter sized Kreutz comets regularly observed by SOHO and STEREO and strongly suggest that the nucleus of ISON was destroyed prior to perihelion. This is much too small to support published gas production rates and implies significant mass loss and/or disruption in the days and weeks leading up to perihelion. No central condensation was seen post-perihelion. The post-perihelion lightcurve was nearly identical in all telescopes/filters and fell slightly steeper than r −2 H . This implies that the brightness was dominated by reflected solar continuum off of remnant dust in the coma/tail and that any remaining active nucleus was <10 m in radius.

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

“…Significant mass loss was not likely triggered by tidal forces, which should not have become significant until the day of perihelion (Knight & Walsh 2013). The recognition of this extreme mass loss helps explain why ISONʼs brightness and morphological behavior in STEREO and Solar and Heliospheric Observatory (SOHO) images in the days before perihelion (Knight & Battams 2014) was akin to that of the small ( < R 50 N m) Kreutz sungrazing comets regularly observed by those telescopes being destroyed as they approach perihelion (Knight et al 2010). While ISONʼs brightness and gas production rates at larger heliocentric distances implied it was large enough to survive insolation despite its small perihelion distance (Knight & Walsh 2013), the largest remaining fragment as it entered the SOHO fields of view was likely quite a bit smaller.…”

“…These observations were intended to establish a baseline for comparison with its composition and behavior after perihelion. Since ISON did not survive perihelion (e.g., Knight & Battams 2014;Sekanina & Kracht 2014) this paper represents the totality of our observations. In addition to its destruction near perihelion, ISON was fainter throughout most of its apparition than early expectations, so community data were somewhat limited.…”

Catastrophic Disruption of Comet Ison

“…Comet ISONʼs brightness briefly leveled off but then gradually increased through 0.17 au. Inside 0.16 au, comet ISON dramatically brightened, peaking in brightness at 0.051 au just prior to perihelion (Knight & Battams 2014). After perihelion, all spacecraft data showed a steep and accelerating fading.…”

“…With a perihelion distance of 0.0125 au (∼2.7 solar radii (R e )), it was predicted to become exceedingly bright, initiating many observing campaigns. The large number of optical observations from ground-and spacebased telescopes made it possible to have a complete view of the mass, size distribution, and evolution of the (sub)micronsized dust in the coma of comet ISON (Knight & Battams 2014;Samarasinha et al 2015). A comprehensive analysis of the light curve revealed that comet ISONʼs intrinsic brightness evolved in cycles of activity between heliocentric distances of 9.4 and 0.7 au, with the duration of the cycles progressively shortening (Sekanina & Kracht 2014).…”

Short-term variability of comet C/2012 S1 (ISON) at 4.8 AU from the Sun

“…This very close approach suggested the possibility that the comet Table 2 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/575/A52 could disintegrate near its perihelion, which actually happened: ISON's nucleus did not survive the close approach to the Sun and it totally vanished (Sekanina & Kracht 2014). At the first moment after the perihelion passage, the nucleus -or at least chunks of particles from the nucleus -seemed to have survived, but finally the brightness of ISON faded dramatically in a short period of time (Knight & Battams 2014;Combi et al 2013;Moreno et al 2014). Numerous amateur and professional astronomers tried to recover the comet some days after the perihelion; however, the results were negative (Sako et al 2014).…”