Title: Dynamic Sublimation Pressure and the Catastrophic Breakup of Comet ISON Author: Jordan K. Steckloff, Brandon C. Johnson, Timothy Bowling, H. Jay Melosh, David Minton, Carey M. Lisse, Karl Battams Previously proposed mechanisms have difficulty explaining the disruption of Comet C/2012 S1 (ISON) as it approached the Sun. We describe a novel cometary disruption mechanism whereby comet nuclei fragment and disperse through dynamic sublimation pressure, which induces differential stresses within the interior of the nucleus. When these differential stresses exceed its material strength, the nucleus breaks into fragments. We model the sublimation process thermodynamically and propose that it is responsible for the disruption of Comet ISON. We estimate the bulk unconfined crushing strength of Comet ISON's nucleus and the resulting fragments to be 0.5 Pa and 1-9 Pa respectively, assuming typical Jupiter Family Comet (JFC) albedos. However, if Comet ISON has an albedo similar to Pluto, this strength estimate drops to 0.2 Pa for the intact nucleus and 0.6-4 Pa for its fragments. Regardless of assumed albedo, these are similar to previous strength estimates of JFCs. This suggests that, if Comet ISON is representative of dynamically new comets, then low bulk strength is a primordial property of some comet nuclei, and not due to thermal processing during migration into the Jupiter Family. Read more (3553kb, PDF)
Title: Short-term variability of comet C/2012 S1 (ISON) at 4.8 AU from the SunAuthors: Pablo Santos-Sanz, José Luis Ortiz, Nicolás Morales, Rene Duffard, Francisco Pozuelos, Fernando Moreno, Estela Fernández-Valenzuela (Instituto de Astrofísica de Andalucía-CSIC)
We observed comet C/2012 S1 (ISON) during six nights in February 2013 when it was at 4.8 AU from the sun. At this distance and time the comet was not very active and it was theoretically possible to detect photometric variations likely due to the rotation of the cometary nucleus. The goal of this work is to obtain differential photometry of the comet inner coma using different aperture radii in order to derive a possible rotational period. Large field of view images were obtained with a 4k x 4k CCD at the f/3 0.77m telescope of La Hita Observatory in Spain. Aperture photometry was performed in order to get relative magnitude variation versus time. Using calibrated star fields we also obtained ISON's R-magnitudes versus time. We applied a Lomb-Scargle periodogram analysis to get possible periodicities for the observed brightness variations, directly related with the rotation of the cometary nucleus. The comet light curve obtained is very shallow, with a peak-to-peak amplitude of 0.03 ± 0.02 mag. A tentative synodic rotational period (single-peaked) of 14.4 ± 1.2 hours for ISON's nucleus is obtained from our analysis, but there are other possibilities. We studied the possible effect of the seeing variations in the obtained periodicities during the same night, and from night to night. These seeing variations had no effect on the derived periodicity. We discuss and interpret all possible solutions for the rotational period of ISON's nucleus.
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Title: Observations of Comet ISON (C/2012 S1) from Lowell Observatory Author: Matthew M. Knight, David G. Schleicher We observed dynamically new sungrazing comet ISON (C/2012 S1) extensively at Lowell Observatory throughout 2013 in order to characterise its behavior prior to perihelion. ISON had "typical" abundances for an Oort Cloud comet. Its dust production, as measured by Afrho, remained nearly constant during the apparition but its CN gas production increased by ~50x. 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 consistent 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. 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 degrees from night to night. Using numerical modelling, we best reproduced the orientation and shape of these features as well as the bulk brightness with a pole oriented approximately towards the Sun and a single source located within ~35 degrees of the equator. 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 mass loss likely continued at the same site over subsequent days/weeks and may have catastrophically weakened the nucleus prior to perihelion. Read more (1280kb, PDF)
Title: Disintegration of Comet C/2012 S1 (ISON) Shortly Before Perihelion: Evidence from Independent Data Sets Author: Zdenek Sekanina, Rainer Kracht As an Oort Cloud object with a record small perihelion distance of 2.7 Rsun and discovered more than a year before its encounter with the Sun, comet C/2012 S1 is a subject of considerable scientific interest. Its activity along the orbit's inbound leg evolved through a series of cycles. Two remarkable events preserved in SOHO's and/or STEREO's near-perihelion images of its tail were an early massive production of gravel at heliocentric distances of up to ~100 AU(!), evidently by the annealing of amorphous water ice on and near the nucleus' surface; and, about a week before perihelion, a rapid series of powerful explosions, from the comet's interior, of water with dust at extremely high rates, causing precipitous fragmentation of the nucleus, shattering it into a vast number of sublimating boulders, and ending up, a few days later, with a major, sudden drop in gas emission. The disintegration of the comet was completed by about 3.5 hours before perihelion, at a heliocentric distance of 5.2 Rsun, when C/2012 S1 ceased to exist as an active comet. The orbital motion in this period of time was subjected to progressively increasing outgassing-driven perturbations. A comprehensive orbital analysis results in successfully fitting the comet's observed motion from 2011 to ~7 hours before perihelion. Read more (1186kb, PDF)
Title: Molecular observations of comets C/2012 S1 (ISON) and C/2013 R1 (Lovejoy): HNC/HCN ratios and upper limits to PH3 Author: M. Agundez, N. Biver, P. Santos-Sanz, D. Bockelee-Morvan, R. Moreno We present molecular observations carried out with the IRAM 30m telescope at wavelengths around 1.15 mm towards the Oort cloud comets C/2012 S1 (ISON) and C/2013 R1 (Lovejoy) when they were at 0.6 and 1 AU, respectively, from the Sun. We detect HCN, HNC, and CH3OH in both comets, together with the ion HCO+ in comet ISON and a few weak unidentified lines in comet Lovejoy, one of which could be tentatively assigned to methylamine (CH3NH2). The monitoring of the HCN J = 3-2 line showed a tenfold enhancement in comet ISON on November 14.4 UT due to an outburst of activity whose exact origin is unknown, although it could be related to some break up of the nucleus. The set of CH3OH lines observed was used to derive the kinetic temperature in the coma, 90 K in comet ISON and 60 K in comet Lovejoy. The HNC/HCN ratios derived, 0.18 in ISON and 0.05 in Lovejoy, are comparable to those found in most previous comets and are consistent with an enhancement of HNC as the comet approaches the Sun. Phosphine (PH3) was also searched for unsuccessfully in both comets through its fundamental 1-0 transition, and 3sigma upper limits corresponding to PH3/H2O ratios 4-10 times above the solar P/O elemental ratio could be derived. Read more (135kb, PDF)
Subaru Telescope Detects Rare Form of Nitrogen in Comet ISON
A team of astronomers, led by Ph.D. candidate Yoshiharu Shinnaka and Professor Hideyo Kawakita, both from Kyoto Sangyo University, successfully observed the Comet ISON during its bright outburst in the middle of November 2013. Subaru Telescope's High Dispersion Spectrograph (HDS) detected two forms of nitrogen--14NH2 and 15NH2--in the comet. This is the first time that astronomers have reported a clear detection of the relatively rare isotope 15NH2 in a single comet and also measured the relative abundance of two different forms of nitrogen ("nitrogen isotopic ratio") of cometary ammonia (NH3). Their results support the hypothesis that there were two distinct reservoirs of nitrogen the massive, dense cloud ("solar nebula") from which our Solar System may have formed and evolved.Read more
A team of astronomers, led by Ph.D. candidate Yoshiharu Shinnaka and Professor Hideyo Kawakita, both from Kyoto Sangyo University, successfully observed the Comet ISON during its bright outburst in the middle of November 2013. Subaru Telescope's High Dispersion Spectrograph (HDS) detected two forms of nitrogen--14NH2 and 15NH2--in the comet. This is the first time that astronomers have reported a clear detection of the relatively rare isotope 15NH2 in a single comet and also measured the relative abundance of two different forms of nitrogen ("nitrogen isotopic ratio") of cometary ammonia (NH3). Their results support the hypothesis that there were two distinct reservoirs of nitrogen the massive, dense cloud ("solar nebula") from which our Solar System may have formed and evolved.
Zomet C/2012 S1 (ISON) makes its closest approach to the Earth (0.429 AU) on the 27th December 2013
Comet Ison 'vaporised' in Sun pass
Comet Ison seems to have been destroyed in its encounter with the Sun. Astronomers saw the giant ball of ice and dust disappear behind the star, but then fail to emerge as expected.Read more
Comet Ison seems to have been destroyed in its encounter with the Sun. Astronomers saw the giant ball of ice and dust disappear behind the star, but then fail to emerge as expected.
Comet C/2012 S1 (ISON) is at perihelion (0.013 AU) on the 28th November 2013.