Geoscience Reference
In-Depth Information
Table 1.
Derived sungrazer parameters.
Comet name
Q
H
2
O
(kg/s)
r
(m)
h
(R
)
C/1996 Y1
20.0
3.4
6.80
C/2000 C6
71.8
3.0
3.26
140
5.8
4.56
34.6
3.4
5.71
10.5
2.5
6.36
820
a
C/2001 C2
20.3
3.60
58
.
9
a
7.8
4.98
28
.
5
b
5.4
4.98
a
Main nucleus.
b
Subfragment.
apparent inconsistency may be removed by introducing in the sungrazer
erosion model the presence of one or more companions traveling with the
main nucleus: if these subfragments have a lower erosion rate, they can
survive longer than the main nucleus becoming visible very close to the
Sun.
13
Another interesting result shown in Table 1 is the observation of
sudden increases in the
Q
H
2
O
values as the comets approach the Sun: these
may be ascribed to a fragmentation of the comet nucleus which increases
the surface exposed to the solar flux (even if there are other alternative
explanations for this phenomenon
4
).
Table 1 has different entries for the two fragments of the C/2001 C2
sungrazer. As we anticipated in Sec. 2, this comet shows two Ly
α
tails at
4.98 R
and a single tail at 3.60 R
: a possible interpretation
5
is that at
4.98 R
we are observing the outgassing from two fragments. By applying
the technique described above to the two Ly
α
intensity versus time profiles,
we derived an estimate for the radii of both fragments. At 3.60 R
the
subfragment probably disappears because it completely sublimates between
these two heights (as we verified starting from the estimated radius of 5.4 m
and integrating over the cometary orbit the rate of change in radius
14
caused
by the erosion process
5
). Again, the sudden increase in the estimated radius
at 3.60 R
indicates that at this heliocentric distance the main nucleus
is going through a complete disruption due to either or both thermal or
gravitational stresses. This explains also why we did not observe the comet
laterinUVCSdataat2.20R
: the comet never reached this heliocentric
distance.
Fragmentation processes are believed to play a key role in the origin and
evolution of sungrazing comets: first, the occasional secondary brightenings
observed in some sungrazer lightcurves (i.e., the variation of the sungrazer
