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magnitude with heliocentric distance) have been explained by introducing
one or more nearby companions traveling with the main nucleus.
14
Second,
from a comparison between the orbital parameters of different comets, it
has been demonstrated
15
that many sungrazers arriving in pairs or triplets
originated via fragmentation events from a single sungrazer far from the
Sun. More recently, an intriguing scenario has been published involving
splitting both close and far to the Sun
16
: this could explain the genera-
tion, via runway fragmentation, of the observed sungrazers from a single
progenitor. The UVCS instrument has a spatial resolution higher than the
LASCO resolution, hence is able to resolve objects which are unresolved
in the LASCO data. The UVCS observations of sungrazers can strongly
improve our knowledge on this subject, as confirmed by the presence of
subfragments inferred for the C/2000 C6 and directly observed for the first
time for the C/2001 C2 sungrazers.
Another important result from the observations of the C/2001 C2 sun-
grazer arises from the detection, in the Ly
α
intensity versus time profile
of its main tail at 4.98 R
, of a very slow exponential decay. By fitting
the observed curve, the derived local electron density
n
e
was lower than
expected. This led the authors
5
to hypotesize the presence of a background
Ly
α
emission superposed onto an exponential decay: after the subtraction
of this background, the fit to the exponential Ly
α
decay gave a more reliable
n
e
value. This additional emission has been ascribed to neutral H atoms
formed by the interaction between the products from the cometary dust
photodissociation and the coronal protons. The idea arises from the asso-
ciation that has been made between the heliocentric distance of 4-5 R
at
which the cometary Ly
α
emission peaks and the sublimation rate of pyrox-
ene dust grains which peaks at about the same height.
17
In principle, it is
possible that pyroxene dust grains act as agent to neutralize coronal pro-
tons creating an additional number of neutrals and the C/2001 C2 sungrazer
has been observed at 4.98 R
, hence around this height. In this scenario, we
expect a mixing between the Ly
α
emission formed by the charge transfer
between cometary neutrals and coronal protons and the Ly
α
emission from
the charge transfer between coronal protons and pyroxene grains. Given
the cross sections
σ
for the charge trasfer processes between protons and
O or Si atoms from the photodissociation of SiO
2
molecules and assuming
a typical bulk density and radius for the pyroxene grains,
17
the authors
5
derived an expression for the expected number of H neutrals produced
by the charge transfer as a function of the unknown dust number density.
Equating this formula to the value of the additional H atoms estimated from
