Hi all.

Yesterday morning ((Jan. 1 UT) the sky was clear though possibly not quite as transparent as previous morning, but the tail appeared very faint (only clear with averted vision) and could be traced for "only" 26 degrees. But this morning (Jan 2 UT), conditions appeared worse, with lots of high cloud and the head hidden behind a thick patch of cloud. Yet, remarkably, the tail was much clearer, obvious (albeit faint) with direct vision and traced for some 42 degrees - until lost in the Milky Way near the border of Vela. And this time I double checked the figures and tripple checked the calculation!

Cheers,
David

Hi all,

Just a few ideas to put before the group.

As I wrote previously, I suspect that the initial intrinsic faintness of this comet was not so much a function of the small size of the nucleus, but of the presence of a surface crust of refractory material. If the nucleus was about 500 metres diameter (as against the 100 - 200 as initially estimated) and covered by an insulating crust, this might explain how it survived perihelion passage intact. If the insulating layer was blown off around perihelion, this may even have formed a "sun umbrella" of particles that shielded the freshly-exposed icy surface of the nucleus, rather as is thought to have happened to Seki-Lines in 1962 (analysis of the dust tail suggests that this comet shut down for a few hours at perihelion - q = 0.03 AU - which also helps to explain why there were no daylight sightings of this intrinsically bright object). In the case of Lovejoy, a similar event may have been a factor in preserving its existence. Once the meteoric cloud dispersed, the comet burst into furious activity, however by then the worst of its ordeal was already over.

The presence of an ion tail clearly indicated an active nucleus following perihelion. However, as this has this has now disappeared, it may be that ice-driven activity has ceased. This could mean that the nucleus has disappeared, or run out of ice or (I think the most likely explanation) has had the ice cooked out of the surface layers. In other words, the comet may by now have built up a new insulating layer that is effectively keeping heat from underlying ice.

Yet, the "head" appears to be persisting as if some dust continues to be released. Just a speculative thought, but electrostatic repulsion caused by solar radiation can levitate fine dust on the surface of the Moon (causing the unexpected crepuscular rays seen by the Apollo astronauts) and is thought responsible for the small flare experienced by Phaethon in 2009. With respect to the latter, David Jewitt called Phaethon a "rock comet" - capable of low-level activity even in the absence of ice - and suggested that this process may even be responsible for the formation of the Geminid meteor stream. For what it is worth, I suggest that the present weak activity of Lovejoy could be due to this process lifting dust from what has again become a totally encrusted nucleus.

All very speculative I know, but comments welcome.

Cheers,
David

David - As a first approximation, I would say that your conclusions seem quite reasonable. Certainly, something quite unusual occurred with Comet Lovejoy over the course of its apparition. That its post-T survival, at least to a degree, violates my perihelion survival "law" is most interesting, although I would note (as my paper on the subject indicated) that a number of small, periodic comets (like P/Encke, et al.) do so on a quite regular basis. The situation being so, this suggests to me that Comet Lovejoy must have experienced at least one previous perihelion passage as a totally independent body (i.e. it was not a fragment formed a its immediate previous perihelion passage) and thus had a fully formed and fairly dense overall insulating layer over its entire surface.Such a "baked" surface which totally shuts down early might well also explain why Kreutz sungrazers tend to disappear much sooner post-T (typical by about 1.5 AU post-T) than do other comets of similar intrinsic brightness.

There is also the problem that although of a seemingly extremely faint intrinsic brightness both pre and post-T, Comet Lovejoy still presented a viable and distinct "head" post-perihelion. This while objects like the Great Southern Comet of 1887, assumed to be much brighter intrinsically than 2011 W3, appeared to have survived only as huge tail apparently without any head. Of course, the Kreutz sungrazing group's orbital orientation so strongly favors visibility from the Southern Hemisphere that it has undoubtedly limited the opportunities to watch the development of other examples of seemingly faint members of this clan in the more distant past.

I find it equally interesting how one explains the long-enduring bright streak extending from the position the nucleus should occupy to relatively far out into the tail. And the fact that this feature seems to have evolved surprisingly little since its sudden appearance. A few descriptions of the Great September Comet of 1882 do make mention of a similar "streak" in that comet's head, dotted with a number of brighter star-like nucleii, but that feature seemed fairly short lived and was on a physical scale apparently far smaller than that displayed by Comet Lovejoy. Are we perhaps seeing a very long train of tens of thousand of ONLY tiny fragments with absolutely no large survivers, distributed along the orbit by size/mass? But then how could this form so suddenly and how could a coma persist without some viable solid body evident at its focus? According to Rod, there is no evidence of any independent surviving body down to 19th magnitude in that location.

And I'm still very curious about the nature of the faint yet distinct "sheath" that is seen to envelope both the dust and gas tails of numerous Kreutz sungrazers post-T, well seen with 2011 W3, yet does not seem evident with regard to other very small "q" non-Kreutz comets. What is the nature of it? And in the case of the Great September Comet it not only surrounded the tail but was described to extend well sunward of the head!

I unquestionably foresee a long and interesting future of papers attempting to address the amazing sights we've seen over the course of the past month!

J.Bortle

David Sargeant wrote:

As I wrote previously, I suspect that the initial intrinsic faintness of this comet was not so much a function of the small size of the nucleus, but of the presence of a surface crust of refractory material. If the nucleus was about 500 metres diameter (as against the 100 - 200 as initially estimated) and covered by an insulating crust, this might explain how it survived perihelion passage intact. If the insulating layer was blown off around perihelion, this may even have formed a "sun umbrella" of particles that shielded the freshly-exposed icy surface of the nucleus, rather as is thought to have happened to Seki-Lines in 1962 (analysis of the dust tail suggests that this comet shut down for a few hours at perihelion - q = 0.03 AU - which also helps to explain why there were no daylight sightings of this intrinsically bright object). In the case of Lovejoy, a similar event may have been a factor in preserving its existence. Once the meteoric cloud dispersed, the comet burst into furious activity, however by then the worst of its ordeal was already over.

I agree that a temporary surface crust of material can form, but not as you have envisaged here.

One key factor here is that time near closest approach is relatively short - i.e. the nucleus remained within 5 solar radii of the barycenter for about 6 h, and 2 solar radii for just 1.5 h. You have to consider both the solar electromagnetic radiation flux and also the flux of high energy baryons / charged particles, typically protons travelling at speeds of ~500 km/s. The initial effect of these is to strip away any "umbrella of dust particles" leaving the bare nucleus exposed to the 'onslaught' from the Sun.

A more likely scenario may involve a Leidenfrost-type phenomenon. Here's how I see it:

A large fraction of the near-surface material within the nucleus is likely to melt. The surface tension between the melt and any residual solids provides significant mechanical strength, more especially if most of the refractory solids are in the 1-1000 micron size range. This process temporarily inhibits physical break-up. Now if you assume a large fraction of the incident energy (electromagnetic radiation and particle kinetic energy) is absorbed by the surface, this will cause a proportion of the molten material to vaporize - but how much depends on the latent heat of vaporization of the material and the time-scale involved. The vapour boiled off from the melt is in effect a thin gaseous atmosphere, which will increase in pressure until a temporary bow-shock front develops. It is this bow-shock effect which may act as the "umbrella" - a "parapluie" could be more descriptive a word for this. If the gas pressure behind the bow-shock reaches a sufficient magnitude (Poiseuille conditions of P and T arise), particulates will also be entrained in the gas flow. Given this scenario, the surface of the nucleus can be shielded to a degree by two processes; (a) partial deflection of the intense oncoming solar wind by the bow-shock, and ( b ) particulates suspended in the temporary gas layer absorb some of the e-m radiation and re-radiate it back into space. Overall this creates a type of Leidenfrost effect and a temporary pseudo-steady state enabling the nucleus to survive perihelion passage.

Remember, although H2O ice is an important constituent, as the thermal regime evolves to higher and higher temperatures, different materials which are normally solid will each begin to melt and play a significant role.

What will be important now is to characterise the nature of any remaining particulates close to the centre of any debris field using large ground-based telescopes or the HST. Let's hope such observations are successful.

The fate of the nucleus depends on what happened post-perihelion. Sufficient time has passed such that, given the very large thermal gradients, significant heat conduction to the central region of the nucleus would have occurred. You then have a complex situation in which solids melt, liquids vaporize and internal gas pressures develop leading to gradual disintegration of the nucleus. What debris remains will be to an extent an assay of the more refractory material from deep within the original nucleus.

Richard Miles
BAA

Fully dark adapted and the comet higher, the best I could do was trace the tail to 30 deg from the head. However this was only ~25 degrees of tail to the naked eye with averted vision, not including about 5 degrees of tail nearest the head that was invisible. Above the false cross it was possibly present, but occasional glimpses could just as easily have been stars. To direct vision perhaps over 10 degrees was weakly visible, centered around 10 degrees from the head.

Cheers, Rob

John Bortle wrote:

And I'm still very curious about the nature of the faint yet distinct "sheath" that is seen to envelope both the dust and gas tails of numerous Kreutz sungrazers post-T, well seen with 2011 W3, yet does not seem evident with regard to other very small "q" non-Kreutz comets. What is the nature of it? And in the case of the Great September Comet it not only surrounded the tail but was described to extend well sunward of the head!

John - Allow me to follow through one possible interpretation based on my earlier description of the near-perihelion scenario:

We talk about dust (i.e. refractory particles ejected from the nucleus) and gas (low-boiling point volatiles) but there has to be a third type of particle created in the case of a sungrazer nucleus. This material is in effect the "smoke' which is created when otherwise fairly refractory material is vaporized and is able to recondense to an extent.

Two mechanisms generate this "smoke". One is created in the scenario I have already described. The temporary gas shell / bow shock feeds molecules of vaporised refractories into the comet's tail. As the gas pressure and temperature within the tail declines from Poiseuille conditions towards Knudsen conditions, a dynamic situation unfolds whereby many of these molecules collide and stick together building up extremely fine, sub-micron size refractory condensates - in essence a newly-created dust or "smoke" forms.

A second mechanism can also contribute, i.e. sputtering of the surface of the nucleus by fast, highly energetic particles in the solar wind. Calculations would need to be done to see whether enough "smoke" could be created via sputtering of the surface in the time available - I somehow doubt that this can be the entire explanation.

So taking these hypotheses into account, how can we explain the absence of the "sheath" in other very small "q" non-Kreutz comets. My interpretation here is that if the nucleus is too small it cannot build up a stable melt zone within the near-surface, so no temporary envelope of volatilized refractories and no bow-shock can form. Under such conditions, refractory materials will still volatilze but will not experience the conditions required for particles to seed and grow by molecular collision. Such refractory molecules fail to condense along with others, hence no "smoke" can form. This may explain the absence of the "sheath" seen in larger Kreutz sungrazing comets.

Richard Miles
BAA

Hi John and all,

John - I agree that the comet most probably has made a previous perihelion passage as an independent body, but it may not necessarily have been an independent COMET. Although unlikely, it is possible that it may have broken away as a secondary nucleus while the parent was still moving toward its previous perihelion. The situation would then be a little like the Marsden sunskirters C/2004 V9 and V10. Sekanina has shown that V10 broke away from V9 about 3 months prior to its previous perihelion passage, at which the split comet was observed as C/1999 J1 (albeit not resolved into two nuclei in the SOHO images). Interestingly, the circumstances of this disruption meant that the smaller V10 (although discovered after V9) reached perihelion before V9; the opposite of what happens when comets split at or shortly after their previous perihelion passage. IF something similar has happened with W3, it is likely that the primary nucleus is still on its way and probably not very far away!

Please, nobody read this as a prediction of another bright sungrazer in the near future. There is no real reason to think that W3 did break away from a larger object just prior to its previous perihelion, only that this remains one possibility. But it might be wise to monitor the inward path of the Kreutz group just in case...!

Cheers,
david

I posted a report in IIS a little while ago regarding my trip over the mountains to beat our prevailing winds. A superb sky resulted in my detecting the tail out to Alpha Volantis at 14.00 U.T., length = 33.9 degrees. Once again beyond that I am very uncertain due to the brightening Milkyway in that vicinity of the False Cross .

I found it relatively easy to see. The brightest section was where it passed through the brighter stars of Apus and Chamaeleon. I was even sure that I could see that part before the moon set.The darklanes in that region were easy to detect with the naked-eye, usually a good sign of transparency.

Hello all,

I observed comet C/2011 W3 Lovejoy this morning near the vicinity of Castlemaine, Victoria.
I have uploaded a photo taken 2012 Jan 4.69UT to my website:

http://members.westnet.com.au/mmatti/sc.htm

Approximately 10 degrees of tail was observed with the unaided eye using averted vision, extending just beyond Alpha Apus.
The intensity has dropped off substantially since my last observation on Dec 30.71, and is now a very difficult naked eye target.
Through 8x40mm Binoculars, the tail extends through to Beta Chameleonis for a total of 20 degrees.
Photographically the tail length is about 30 degrees, passing near Beta Carinae.
Interestingly the brightest section of tail now appears to be at the head of the comet, wheras previously it was some distance along the tail.
I estimated the head section to be about magnitude 6.5
I could offer an explanation - the the head section is now closest to Earth, in a few days time.
Comments are welcome.

cheers,
Michael Mattiazzo
Castlemaine Vic.

David, once again your suppositions re 2011 W3 are certainly within the realm of reasonable possibility. In fact, I recall reading a paper by Sekanina in which he suggests just such a possible scenario for 1970 K1, W-O-B. For reasons I no longer recall, Sekanina felt that a major secondary component closely associated with W-O-B could possibly have been trailing it by only a few months. He gave the time of potential perihelion passage as about late July of 1970, when the comet would have been hopelessly hidden from discovery in daylight (unless spotted in the daytime at T). Of course, as you say, this does not in any way directly infer such a situation might be true for 2011 W3.

In my mind this possibility also arises concerning the 'apparent' spate of sungrazers in the late 18th century. How many additional such objects might have appeared then during the annual intervals when Kreutz sungrazers might only have been seen from the Southern Hemisphere and went totally unreported? Lots of interesting conjecture is possible in this area with our knowledge of the sungrazer family so incomplete!

J.Bortle

Animation I prepared from images by Justin Tilbrook, South Australia, 04-05 Jan 2012 UT (05-06 Jan local time), posted with permission. Nice deep subs, tail visible right to edge of frames. 38 degrees for 4 Jan, 35.5 degrees for 5 Jan and extending out of frame. Times not given, between 16:30 and 17:00 each morning.

Animated GIF Here

Cloudy/hazy last two nights here so I will now have to wait till late this coming week.

Cheers -

Rob Kaufman
Bright, Victoria, Australia

It cleared unexpectedly here just on dark. The moon had been up for 5 minutes (9.25 hrs U.T.) and evening twilight was still dwindling, but the Mag Clouds and the comet were in the darkest part of the sky and culminating at about 55 degrees high for me.

I could detect it as a persistent, elongated haze seemingly attached to the Bar of the LMC at the Tarantula nebula end. I could just see it with direct vision a few times. The cleanest view I had was at 9.36 U.T. by which time I was reasonably dark adapted and the comet's part of the sky was still relatively black. I traced the tail to 8 degrees before it merged with the LMC. Binoculars did not improve the view. Transparency was not great for most of the time with clouds trying to form in that area, but they moved on and leaving the constant glow behind. It reminded me of some very faint spindle galaxies that I have observed over the years.

From a Southern hemisphere perspective, I feel the comet should be classified as a "Great".

At it's peak around Christmas time, the comet sported a tail over 30 degrees in length, and was easierly visibly in the morning sky

I will never forget how it looked over the SE horizon.

As mentioned, the comet has now faded greatly, and is well below naked eye visibility.

I could not detect it in my 70mm binoculars from an relatively dark sky a couple of days ago, but I did read a report yesterday, where it was detected in small binoculars from a loction with an exceptionally dark sky.

I would agree with your comments about Comet 2011 W3 being classed as a 'great' comet. The weather didn't treat me kindly from my location in Canberra, Australia but I did manage to see it one morning and the tail was easily visible and was just over 20 degrees long. I can't tell you the date I saw it offhand. It was an unusually comet to in that it is the only bright comet I have seen that didn't have an apparent head. To the unaided eye, it just faded away towards where the head should have been.

My two cents worth. If you take a non-astronomer and tell them there's a comet in the sky, and they can see it without assistance, it qualifies (IMO) as a "Great Comet". Hale Bopp, when it finally came far enough south to be seen in the evening twilight in Canberra, failed that test with my neighbour (though it was quite good). But it was obviously a Great Comet. OTOH, my brother saw C/2011 W3 Lovejoy without any prompting.

It's very annoying when you know there's a bright comet in the wrong hemisphere :-(

My two bob's worth on the 'greatness' (I thought it was a done deal?) - anyone who saw this gigantic comet sprawling along the Milky Way around Christmas time would have no doubts. The really good recent comets such as 17P pale into insignificance beside this spectacle. That Lovejoy could well produce science on an unparalleled scale because of our unprecedented ability to observe it through perihelion passage is just icing on the comet cake!

Was Lovejoy a "Great Comet" YES! Definitely! I never got to see Hale-Bopp under dark skies, but photographs i have seen certainly leave no doubt about it. Even when P1 McNaught rolled around our Southern Skies in January 2007, i reckon more was written in magazines about 17P/Holmes later that year (which was also a grand sight!) than P1 at the start of 2007!

The most exciting part of W3 Lovejoy for me was the first time i saw it, like a headlight beam over the horizon, Michael Mattiazzo was on the receiving end of a very excited phone call from me that morning!

After flicking through David Seargents book "The Greatest Comets In History" there was a drawing or painting or something of the Great Comet of 1880, which reminded me of this comet, the way it's angled and the "beam" look it had about it, it's like you're taking a trip back in time, incredible!

I eagerly await further findings from W3 in future, it was a lesson learned!

It is always interesting to mull over the question of whether or not this, or that, comet is indeed a 'Great Comet'. The actual justification for assigning the title is rather more ambiguous than most think, as the meaning of this appellation is less well defined than most believe.

Although most often used to designate some truly extraordinary, brilliant, long-tailed comet, prior to about 1850 the name was occasionally applied simply to some moderately bright comet that happened to appear in a given year. In other instances some truly outstanding objects were initially widely known by the name of their discoverer, only gaining the prefix 'Great Comet' after their apparition ended. The name has also often been affixed to bright comets that have appeared suddenly out of the twilight and were spotted by so many that no specific first discoverer(s) could be determined and it was easier to do so.

However, the more accepted modern interpretation seems mainly to center around comets of extreme brilliance and possessing long bright tails. Even so, any critical determination is still dubious enough that various authors can differ in their lists of 20th/21st century examples and particularly in their pecking order.

From my own viewpoint I regard Comet Lovejoy as probably making the 'Great Comet' cut, but only just barely. I would tend to group it along with the Great Southern Comets of 1880 and 1887 as a marginal member of the clan. The two earlier comets gained the title mostly by virtue of their impressive tails and sudden appearance, as their heads were never seen as very bright in a reasonably dark sky (the 1887 object even lacked any head!). In fact, these three objects are likely the 'faintest' in terms of coma brightness among all of the Great Comets in history.

J.Bortle

I have no doubt that Lovejoy will go down in history as a Great Comet, but we still must put things into their right perspective. Together with the Great Comets of 1880 and 1887, this was one of the intrinsically faintest of the major sungrazers and, like them, achieved "Greatness" principally because of very good observing geometry and the fact that all three passed on the "Earth side" of the Sun. I cannot agree with some statements (not on this list) that Lovejoy was probably not much smaller than Ikeya-Seki. I-S passed on the far side of the Sun (except for a brief period only hours before perihelion) and was seen more head on and relatively distant. Lovejoy, on the other hand, was viewed more or less "broadside" and passed closer to Earth than any other sungrazer - just about as close as a Kreutz can come. If the circumstances of the two were swapped, Lovejoy would have been close to the naked-eye limit at the end of October 1965 when Ikeya-Seki was at its most spectacular. Conversely, had Ikeya-Seki been the comet that passed perihelion last December, it would have shone at around mag 0 or -1 on Christmas morning and would still be visible naked eye with possibly 60 degrees of tail. This is not said to detract in any way from the spectacle of Comet Lovejoy. As observed under the circumstances that actually prevailed and not these hypothetical ones, the apparent difference between these two was much less than their intrinsic difference might imply.

Regards,
David

In this work, a novel approach to explain the survival of sungrazing comets within the Roche limit is presented. It is shown that the reaction force caused by the sublimation of the icy constituents can prevent tidal splitting of cometary nuclei, even if the tensile strength of the material is low. Furthermore, this approach is used to estimate the maximum size of the nucleus of comet C/2011 W3 (Lovejoy) during perihelion.

The size of the nucleus of comet C/2011 W3 (Lovejoy) can be assessed following Knight et al. (2010) who investigated the light curves of Kreutz group comets during their approach towards the Sun. The brightness of these comets peaks around a heliocentric distance of 10 to 12 Rsun. A comet with a radius of 4m shows a brightness of 8 mag at 12 Rsun. The peak brightness of comet Lovejoy was estimated around -4 mag (Karl Battams, NRL (2012)). This converts to a radius of ~1 km for the nucleus of comet C/2011 W3 (Lovejoy).

Due to the outgassing of the icy constituents, the maximum radius of sungrazing comets able to survive within the Roche limit is relatively large. However, if the effective gas production decreases, the outgassing force decreases and, therewith, the maximum radius of the nucleus able to survive within the Roche limit also decreases. Thus, cometary nuclei with low tensile strength can only survive within the Roche limit if they are active. Members of the Kreutz group comets like comet C/2011 W3 (Lovejoy) are probably young fragments of a big progenitor comet (Sekanina and Chodas, 2002) and are, thus, active.

Two very big Kreutz group comets, 1882 II and 1963 V were observed within the Roche limit of the Sun (R1882II = 30.7 km and R1963V = 13.7 km; Knight et al., 2010). The perihelion distances were rp;1882II = 1:67 Rsun and rp;1963V = 1:09 Rsun, respectively. Comet 1882 II had broken into at least five fragments (Gill, 1883). This obersvation is in agreement with our model, because the estimated radius of comet 1882 II was bigger than the derived maximum radius for the survival of sungrazing comets within the Roche limit. The survival of comet 1963 V can be explained with our model within the error of our model and the error of the size estimation.