poniedziałek, 2 grudnia 2013

Ashes to ashes, dust to dust.

Ostatnie zdjęcie truchła komety C/2012 S1 ISON.
Kometa C/2012 S1 (ISON) ostatecznie dokonała żywota. Po pobycie w obłoku Oorta prze ok. 4.5 mld lat zrządzeniem losu i praw fizyki została skierowana ku Słońcu. dla nas życiodajnej gwizdy, dla komet będącej zabójcą. Zapowiadało się piękne widowisko, choć od pewnego czasu dochodziły już pewne informacje o tym, że kometa zachowuje się w sposób wskazujący na to, iż może ulec dezintegracji podczas przelotu przez peryhelium. Mimo nadziei wielu miłośników astronomii stało się to co było do przewidzenia. Kometa rozpadła się. Jeszcze następnego dnia pozostawał cień szansy na to, że coś z komety zostało. Jednak nadzieje okazały się płonne. Obecnie kometa to tylko kupka rzopraszającego się w przestrzeni pyłu i gazu. Ocenia się jej jasność na ok 8-9 magnitudo. Nie ma zatem żadnych szans aby cokolwiek zaobserwować. Chmurka prawdopodobnie do czasu dotarcia blisko Ziemi ulegnie zupełnemu rozproszeniu. No cóż, wiele komet tak kończy a nam pozostaje tylko czekać na pojawienie się innej komety - może nawet bardziej efektownej.Ciekawy telegram CBET:

"Electronic Telegram No. 3731
Central Bureau for Astronomical Telegrams
CBAT Director: Daniel W. E. Green; Hoffman Lab 209; Harvard University;
20 Oxford St.; Cambridge, MA 02138; U.S.A.
e-mail:cbatiau@... (alternatecbat@...)
Prepared using the Tamkin Foundation Computer Network

The comet's nucleus apparently disrupted near perihelion, with the comet's head fading from perhaps a peak brightness of visual mag -2 some hours before perihelion to well below mag +1 before perihelion. M. Knight, Lowell Observatory, finds that the comet peaked around visual mag -2.0 around Nov. 28.1 UT, adding that the brightest feature in the coma of the comet faded steadily after perihelion from about mag 3.1 in a 95"-radius aperture when the comet first appeared from behind the SOHO coronagraph occulting disk on Nov. 28.92 to about mag 6.5 on Nov. 29.98. K. Battams, Naval Research Laboratory, writes that, based on the most recent LASCO C3 images (Nov. 30.912 UT), there is no visible nucleus or central condensation; what remains is very diffuse, largely
transparent to background stars, and fading; it appears that basically a cloud of dust remains from the nucleus. S. Nakano, Sumoto, Japan, writes that he measured the comet's total magnitude in a 27' photometric aperture from the SOHO C3 camera images to be as follows: Nov. 29.383, 0.5; 29.755, 1.4; 30.013, 2.0; 30.496, 3.0; 30.883, 5.4.

Z. Sekanina, Jet Propulsion Laboratory, reports that, from the position of the northeastern boundary of the comet's fan-shaped tail in three images taken with the C3 coronagraph onboard the SOHO spacecraft between 0.7 and 1.9 days after perihelion (Nov. 29.46 to 30.66 UT), he finds that the comet's production of dust terminated about 3 hours before perihelion. Although this result is preliminary, it is unlikely to be significantly in error, because the position angles of a perihelion emission are off in the three images by 14-22 deg, and those of post-perihelion emissions still more. The peak radiation-pressure accelerations derived from the tail boundary's angular lengths
(estimated at 1.8-2.5 deg) are about 0.1-0.2 the solar gravitational acceleration, implying the presence of micron-sized particles. The estimated time of terminated activity is consistent with the absence of any feature that could be interpreted as a condensation around an active nucleus in the 20 or so images taken with the C2 coronagraph on Nov. 28.8-29.0 UT (0.8 to 5.4 hr after perihelion) and with the appearance of a very sharp tip (replacing a rounded head) at the comet's sunward end in the C2 images starting about 4 hr before perihelion and continuing until its disappearance behind the occulting disc around Nov. 28.74 UT (or some 50 minutes before perihelion). The time of terminated activity is here interpreted as the end of nuclear fragmentation, a process that is likely to have begun shortly before a sudden surge of brightness that peaked nearly 12 hr prior to perihelion. Fine dust particles released before perihelion moved in hyperbolic orbits with perihelion distances greater than is the comet's, thus helping some of them survive. The post-perihelion tail's southern, sunward-pointing boundary consists of dust ejected during the pre-perihelion brightening. However, the
streamer of massive grains ejected at extremely large heliocentric distances, so prominently seen trailing the nucleus along the orbit before perihelion (cf.CBET 3722), completely disappeared. The dust located inside the fan, between both boundaries, was released in intervening times, mostly during the last two days before perihelion.The strong forward-scattering effect (phase angles near 120-130 deg) has tempered the rate of post-perihelion fading of the comet, but the merciless inverse-square power law of increasing heliocentric distance is necessarily the dominant factor in the comet's forthcoming gradual disappearance.

H. Boehnhardt, J. B. Vincent, C. Chifu, B. Inhester, N. Oklay, B.Podlipnik, C. Snodgrass, and C. Tubiana, Max Planck Institute for Solar System Research, Katlenburg-Lindau, reports that two diffuse tail structures were analyzed in post-perihelion images obtained by the LASCO-C3 corongraph onboard the SOHO spacecraft between Nov. 29.60 and 29.81 UT. The southward tail extended toward p.a. about 167 deg to about 0.4 deg distance from the central brightness peak. The eastward tail had an approximate position angle of 68 deg and extended to at least 1.2 deg distance. By Finson-Probstein simulations, the eastward tail can best be interpretated as being caused by a dust release about 1 hr around perihelion. The maximum beta value in the eastward tail reaches values up to 1.5, typical for graphite or metallic grains of about 0.1 micron radius. No indications are found for a continuation of the release of similar dust after 2 hr post-perihelion. The shorter southward tail may be a relict of heavier grains released about 1-2 days before perihelion passage. Diffuse cometary material is noticeable in the p.a. range covered by the two dust tails. The match of the synchrone pattern for the eastward tail is not optimal, which may indicate secondary effects to the dust grains involved.

NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
superseded by text appearing later in the printed IAU Circulars.

(C) Copyright 2013 CBAT
2013 December 1 (CBET 3731) Daniel W. E. Green

oraz post z grupy comets-ml:

"(...) The temperatures near perihelion were exceeding 2500°C in hours after perihelion they were dropping but for long time they were higher then 1000°C and now are still in range of hundreds celsius degrees. In near vacuum environment not only volatiles and water are sublimating, but most of minerals and even iron during most close approach. Other thing is heat stress, the minerals were spending over 4 billion years in constant temperatures under 90 K, so when you heat this material on high temperatures I named before, they will be subject to strong heat stress, tension and will be extensively cracking, therefore producing new dust. I am barely finding any physical law that would explain dormancy of such body so near to Sun (except alien ships). About disintegrating comets, there are some basic signs on which we determine that comet no longer exist. By terms of "disintegrating" or "demise" the scientific meaning is not that comet would simply disappear for good, but that there is no longer active solid nucleus with mass in significant amount compared to original nucleus. So yes there can be debris cloud with even larger particles with size in meters or tens of meters. An example, when shuttle Columbia disintegrated in atmosphere, finding larger debris doesn’t mean that shuttle is still alive and working.

Signs of comet demise can be listed like this:
1. loosing a central condensation - observed directly before reaching perihelion and all time after perihelion
2. changing inner coma shape to nearly "cigar shape" - observed directly before reaching perihelion and all time after perihelion
3. shutting down of all cometary activity
- HCN emissions from IRAM are still negative, comet have no gas tail
- photometric behaviour corresponds to inactive dust cloud
- modelling dust distribution in coma shows that dust production shut downs 2 hours after perihelion passage

so ISON fulfill all signs of comet that disintegrated and there are others supporting things as absolute magnitude, which is after passing perihelion 15 times smaller then average value of this comet when it was living. In such high temperatures we observing thousands of small, tens of meters sized sungrazers as active until they sublimate for good. We may think about a dormant body in case of comets which are more distant from Sun but not in case of sungrazers in such high temperatures.

Best regards,
Jakub Cerny

Zapamiętajmy zatem C/2013 S1 ISON jako kometę, która wzbudziła tak wiele emocji i oczekiwań.

Animacje z sondy STEREO:

Na animacji widać początek rozpadu komety oraz jej pojaśnienie po przejściu przez peryhelium - moim zdaniem spowodowane dostaniem się resztek w strumień cząstek CME.
Film, który stworzyłem z fotografii SOHO LASCO C3:

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