On The Wings of ISON

Blog written by Dr. Padma A. Yanamandra-Fisher, Senior Research Scientist, Space Science Institute, Boulder, CO and Admin of Facebook group, CIOC_ISON.
Posted on her behalf by Karl Battams.

The recent excitement over the “ comet wings” or “arclets” seen in images of C/ISON has brought attention to, again, the discussion of possible disintegration of the comet’s nucleus. Plausible as this sounds, especially based on anecdotal evidence, there still remain several major unknowns as far as I know: (i) rotation (period) of the nucleus and (ii) tensile strength of the nucleus.

The lack of high-resolution direct observations of the nucleus; the existence of possible jets; the similarities of the two outbursts, from the symmetric nature of the current outburst (on the 19th November) and the previous one (from 13/14th Nov), to the increased production rates of various species during their growth phases on similar timescales indicate that:
  • (i) trapped gas in the nucleus may have been liberated and formed a coma, with very little release of dust, thereby giving rise to a symmetric expanding coma shell and increased brightness; or
  • (ii) fragmentation may have occurred but the size of the fragments and their velocity is small/low that they hover near the main nucleus - in which case, the observed symmetry would be hard to explain as no reports of the detection of the fragments have be made; and
  • (iii) a possible periodic phenomenon not identified yet. Professional and amateur astronomers’ reports indicate that the increased brightness of the comet in the past couple of days range from Levy’s 4.2 to Nevski's report of 3.9, an increase of approximately 1.0 from the reported magnitude of 5.0 from three days prior, when the first outburst seemed to subside.
The wings from the first outburst faded as the dust/ion tail became elongated and started to map the solar wind field, probably crossing the HCS and so we see structure reminiscent of the famous disconnection events seen in comets Halley (1986) and Hyakutake (1996). The reported visible spectra from several amateur astronomers, indicate a very gassy comet with little/no dust in the continuum, with C2, OI, possibly Na and many NH2 lines are present- and not much CN; we need professional astronomers with visible spectra to confirm these detections. Finally, the thermal spectra obtained from SUBARU/COMICS and NASA/IRTF/BASS indicate a low dust continuum, mostly amorphous silicate, implying not much fresh dust.

These independent lines of observations can be explained by several scenarios: (a) a slow- to non-rotator, gradually being warmed with increased insolation, with the possibility that the night-side of the comet is throwing off pieces of its crust and releasing trapped volatiles; (b) existence of possible jet(s), based on the similarity of increased production rates of various daughter species and brightness increases of the past two outbursts; (c) occurrence of nuclear fragmentation – this is the least likely scenario as no fragments have been observed so far nor reported. Scenarios (a) and (b) will require continued observations during the critical week leading to perihelion to determine the nature of these outbursts. Will they provide evidence for determination of the rotation of the nucleus and/or the tensile strength of the nucleus? The combined collaboration of professional and amateur observations, and one of the goals of the CIOC via FB group CIOC_ISON, is important for the characterization of C/ISON in the next few days as C/ISON heads to perihelion.

Comet ISON as imaged on November 21, 2013 by Juanjo Gonzalez (Oveido, Spain)

State of the Comet: 21 November 2013: T-7 days (as posted in the CIOC_ISON group)

The first image reported for 21 November 2013 comes from Juanjo Gonzalez, of Oviedo, Spain (opposite). The comet seems to have a very distinct central core and the "coma wings" seen following the outburst of 13/14 November, 2013. The intriguing feature is the narrowing of the main tail - a DE (disconnection event) in the making? The similarities in the physical nature of the outburst is remarkable and therefore, continued observations are important. The solar amateur community is joining in the campaign too. C/ISON seems to be in good health, although I cannot ascertain if the outburst is still going on or if the fade phase of the outburst has started. Any reports of production rates will be appreciated.

The bottom line: C/ISON is putting on a show and I urge all amateur astronomers and any professional astronomers with access to observatories to observe the comet. Visual observations, sketches and blogs are equally important to record as well.