[Prev][Next][Index][Thread]
(meteorobs) C/1998 K5 LINEAR
The first mail didn't get through so here I go again...
Comet C/1998 K5 was the subject of a debate on #namn somewhere in February 1999. Because at the time I didn't find any conclusive data about the comet I hypothesised, big time (.
Anyway, this is what I came up with ( note ? I haven't done any research about K5 since this was written, so if someone is really in the field of comets, this might seem pointless):
HISTORY OF C/1998 K5 LINEAR:
On May 26 C/1998 K5 LINEAR was discovered, The comet was reported to be of magnitude 18, but was later revised to magnitude 16. It was completely stellar - an asteroid, or so the discoveres thought. The first hint of cometary nature of this object came from its highly eccentric orbit. Later, the discoverers made longer exposures of the comet and found a hint of a coma and a very faint 0.5' tail (IAUC 6923).
But with a great surprise additional observations yielded an object not far from Earth ( 0.3 A.U.). At this time the coma was smaller than 1000 km!! During June, comet brightened from magnitude 15 to magntude 13. Later the brightness continued to increase, but not nearly as rapidly, until end of August, when it reached 12.3 magnitude (a diagram can be found at
http://www.tu-chemnitzdot de/~mmey/fgk/C1998K5/sele98k5.gif ).
From the diagram it can clearly be seen that the comet's brightness didn't change much in July and August. K5 passed perihelion on 17.45 July, 0,96 A.E. from the Sun.
>From the discovery, until second half of August, the degree of condensation remained 9, stellar. Later, some observers reported minute coma, about 0.3' visually. These reports start 40 days after perihelion passage - at the time of brightness behaviour change.
DIFFERENT EXPLANATIONS:
Some suspect that this (brightness behaviour change) may be a result of only one active region on the whole nucleus - the spike tail. It was weak during Earth passage, but became more active as time progressed. The activity reached it's maximum 40 days
after perihelion passage. Thereafter it decreased slowly with the comet-sun distance increasing. So in this scenario coma observations prior to mid-August would have been abservations of a tail/jet emerging from an active region, an interpretation confirmed with photographs. Until year's end the coma diameter reached 10, - 15,000 km, stil very condensed (DC 8 - 7 ).
In another scenario, this was K5's last perihelion passage - it is essentially a bare comet nucleus, a rock. In this scenario, the spike tail is interpreted as the last bits and pieces of volatile material being scrapped from the nucleus by solar wind, but does not explain the later development of coma that well.
The third scenario is that this is a mini comet (which have been a subect of debate in the thread 'Tunguska bolide & Beta Taurids'), a very small nucleus, which should perhaps be put in a class of it's own.
So I went through some data, comparing the comet with other comets. The first thing that bothers me is scenario no.3. K5's absolute magnitude (15.3 ) in my opinion does not neccessarily mean that the nucleus is small. Let's take a look at some other comets. Ok, C/1995 O1 Hale - Bopp's nucleus is around 40 km in diameter and it's H0 is -0.9 (or something similar, negative). Comet Halley's nucleus is 16 km at it's greatest and it's H0 is +4.4 mag. This would imply that K5's nucleus is very small. Which is not the case if we take a look at some other comets. Take C/1996 B2 Hyakutake and 55P/Tempel ? Tuttle. Hyakutake's nucleus is only about 3 km at it's greatest, Tempel - Tuttles slightly larger, 3.5 km at it's greatest. This would mean their absolute magnitudes are very similar. It turns out that Hyakutake's H0 is +4.4, which is equal to Halley's, but Halley's nucleus is almost 6 times
as big, with the surface area xxx times as big!! T-T's absolute magnitude on the other hand is +8.3 mag.
This actually means that K5's nucleus might be quite big. So calling it a mini comet is perhaps not the best thing to do. So what counts is also it's activity. K5's nucleus was obviusly not very active. This opens two possible scenarios. In Whipple's model of a cometary nucleus the volatile interior is coated with a crust of darker material which is composed of complex organic molecules and some other stuff (dunno what, but it's dark). What is important it that the comet's activity originates from areas where this crust is thin or absent ( as the comet rounds the sun several times this crust is depleted and much of the comet's volatile material is vaporised ).
Low activity of K5's nucleus either means it's a very old nucleus stripped of almost all volatile material, with a small patch of such material left or it is a fres nucleus, straight from the Oorth cloud, with only one, very small active region, which should get larger with next perihelion passages.
Clear skies!
Jure Atanackov
To UNSUBSCRIBE from the 'meteorobs' email list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html