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The Great Christmas Comet of 2011, 2011 W3 (Lovejoy)
Paolo
post Jan 19 2012, 07:35 PM
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speaking of sungrazer comets, tomorrow's "Science" has a paper on one of them:
Destruction of Sun-Grazing Comet C/2011 N3 (SOHO) Within the Low Solar Corona
see also The Final Flight of a Sun-Diving Comet
of course, both require a subscription

edit: even if you don't have a subscription, be sure to check the supplementary material for the paper and download the two awesome videos!


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Mongo
post Jan 19 2012, 09:30 PM
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Hello all,

On post #68 of this thread I referenced a "Great Comet rating system" that I had first used here at the time of 2007 McNaught. While better than nothing, that system was far from perfect, full of arbitrary cutoffs and (probably) incorrect weightings for various comet attributes, as well as some more serious issues.

Is there interest here in putting together a more logical rating system for comets visible by naked eye from Earth, starting with the basic "ground rules" to be followed in setting up the system, followed by determining what comet attributes will be considered and what points will be awarded for a given magnitude of that attribute?

There are surely interested people here who could contribute to such an effort, if I do get a positive response. If we do decide to go ahead, I will probably start a new thread for the effort.
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ngunn
post Jan 19 2012, 11:01 PM
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I would suggest two ratings: the time integral of the of the apparent magnitude of the comet and the time integral of its absolute magnitude. That would be enough, I think, if those quantities could be determined from the observations. I'm sure we could do that now but maybe the historical data are insufficient.
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Mongo
post Jan 19 2012, 11:47 PM
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I had been thinking along similar lines, although I had also been thinking about separating each comet's rating into a "peak" rating (the rating for the brightness + tail length + possibly other attributes (visible all night, multiple tails, etc.) on the particular day that the computed daily rating is at its greatest) and a "career" rating, summing the individual daily ratings into one rating, which would result in higher numbers for a longer term of naked-eye visibility.

The problems for this approach would be: one, this would involve a lot of work finding the daily measurements and entering them into a spreadsheet (one row for each day of visibility), and two, the information would simply not be available for earlier Great Comets. It should still be possible for the more recent comets, which presumably are the ones of most interest.

This is definitely one area that needs to be improved; the current system combines the tail brightness from the day that the brightness is at its maximum, with the tail length from the day when the visible tail length is at its maximum, as if they were on the same day, which is clearly incorrect (plus adding points for a daylight comet, which would be from yet a third day). I think that computing a rating for each day is the way to go, if the data is known.

I do think that ideally, we should use more than just the apparent and absolute magnitudes of the comet, since the impressiveness of the tail as seen from a dark-sky site does matter.
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nprev
post Jan 21 2012, 07:03 AM
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All good things must come to an end, and that aphorism is most apropos for Sun-grazing comets.

As a coda for this thread, here's a shot taken by one of my stepdaughter's fellow travelers in Chile at the end of December.

Farewell, Lovejoy, and thanks.
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Mongo
post Mar 9 2012, 03:04 AM
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Science papers about Lovejoy are starting to appear:

A note on the survival of the sungrazing comet C/2011 W3 (Lovejoy) within the Roche limit

QUOTE
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.


QUOTE
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).


QUOTE
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.
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Guest_Sunspot_*
post Mar 10 2012, 01:45 AM
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Looks like we may have another bright sungrazer on March the 16th
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Mongo
post May 29 2012, 02:07 AM
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Comet C/2011 W3 (Lovejoy): Orbit Determination, Outbursts, Disintegration of Nucleus, Dust-Tail Morphology, and Relationship to New Cluster of Bright Sungrazers

We describe the physical and orbital properties of C/2011 W3. After surviving perihelion, the comet underwent major changes (permanent loss of nuclear condensation, formation of spine tail). The process of disintegration culminated with an outburst on December 17.6 (T+1.6 d) and this delayed response is inconsistent with the rubble pile model. Probable cause was thermal stress from the heat pulse into the nucleus after perihelion, which could also produce fragmentation of sungrazers far from the Sun. The spine tail was a synchronic feature, made up of dust released at <30 m/s. Since the nucleus would have been located on the synchrone, we computed the astrometric positions of the missing nucleus as the coordinates of the points of intersection of the spine tail's axis with lines of forced orbital-period variation, derived from orbital solutions based on preperihelion astrometry from the ground. The resulting osculating orbital period was 698+/-2 years, which proves that C/2011 W3 is the first major member of the predicted new, 21st-century cluster of bright Kreutz-system sungrazers. The spine tail's tip contained dust 1-2 mm in diameter. The bizarre appearance of the dust tail in images taken hours after perihelion with coronagraphs on SOHO and STEREO is readily understood. The disconnection of the comet's head from the preperihelion tail and the apparent activity attenuation near perihelion are both caused by sublimation of all dust at heliocentric distances smaller than ~1.8 solar radii. The tail's brightness is strongly affected by forward scattering of sunlight by dust. The longest-imaged grains had a radiation-pressure parameter beta ~ 0.6, probably submicron-sized silicate grains. The place of C/2011 W3 within the hierarchy of the Kreutz system and its genealogy via a 14th century parent suggest that it is indirectly related to the celebrated sungrazer X/1106 C1

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Paolo
post Jun 6 2013, 07:13 PM
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comet Lovejoy in tomorrow's Science:

Probing the Solar Magnetic Field with a Sun-Grazing Comet
while the paper is behind the paywall, you can see this video


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I'm one of the most durable and fervent advocates of space exploration, but my take is that we could do it robotically at far less cost and far greater quantity and quality of results.

James Van Allen
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