A direct way to observe the photodissociation of water vapor in a cometary coma is to detect the OH prompt emission. This emission is shifted of delta lambda = 4 nm with respect to the OH 309 nm fluorescence band. The extended data set obtained with the three-channel spectrometer on-board Vega 2 reveals at short distance of the nucleus (i.e., less than 600 km) an excess of emission on the right wing of the OH band which may be interpreted as being mainly due to prompt emission

Clairemidi, Jacques

Moreels, Guy

Rousselot, Phillippe

Vernotte, F.

English

NASA Technical Reports Server

Asteroids, Comets, Meteors 1991, pp. 525-528
Lunar and Planetary Institute, Houston, 1992 .._//,._ _. _ _) 525
EVOLUTION OF NEAR UV HALLEY'S SPECTRUM IN THE INNER COMA
P. Rousselot, J. Clairemidi, F. Vemotte, G. Moreels
Observatoire de Besanqon, BP 1615
25010 Besanqon Cedex, France
Abstract
A direct way to observe the photodissociation of water vapor in a cometary coma consists to
detect the OH prompt emission. This emission is shifted of _.=4 nm with respect to the OH 309 nm
fluorescence band. The extended data set obtained with the three-channel spectrometer on-board Vega
2 reveals at short distance of the nucleus (i.e. less than 600 km) an excess of emission on the right wing
of the OH band which may be interpreted as being mainly due to prompt emission.
The three-channel spectrometer embarked on Vega 2 transmitted approximately 3600 spectra
during the encounter day on March 9, 1986.
The spectra cover the range 275 to 710 nm. They were obtained in a spectro-imagery mode, since
the instrument scanned a rectangular field of view of 2xl.5 °. During the last half-hour before
encounter, this field was restricted to a 2° line scanned in a see-saw motion. Taking advantage of the
zoom effect which results of the approach motion, monochromatic images of the inner coma were
constructed at selected wavelengths (Clairemidi eta/.; 1990 a,b). They show the progressive increase
of the emissions when the projected distance nucleus-optical axis decreases from 38 000 to 420 kin. The
distance between the spacecraft and the nucleus decreases then from 516 000 to 8 030 kin.
The measured spectrum consists of molecular fluorescence bands superposed on a solar dust-
scattered continuum. This solar component is carefully subtracted from the measurement signal in
adjusting an absolute solar spectrum, taken from Labs eta/. (1987) convoluted with the instrument
response function, which has a FWHM of 7 nm.
A careful study of the OH emission in the inner coma is of special interest to obtain a direct
record of H20 photolysis. When a water molecule escapes from the nucleus, it is rapidly
photodissociated under the following mechanism:
H20 + hVl --_ H + OH(A2Z +) 0.1<136 nm)
Then the electronically excited OH releases a photon:
OH(A2Z +) ---) OH(X2Hi) + hv 2 (_.2_309 nm)
The emission of the photon called hv2, at the scale of the inner coma, occurs at the same place
as where the H20 molecule is photolysed. The lifetime of OH(A2Z +) is estimated to 0.83x10 -6 s and
the ejection velocity of H20 to 1 km.s -1 (probably higher than the ejection velocity of OH with
respect to H20). As a consequence, the hv2 photon is emitted within a distance of about 1 mm from the
place where H20 photodissociation occurs.
This means that the observation of hv2 photons is a straightforward way to show evidence of
H20 photodissociation. Since the distance of the nucleus to the optical axis is as small as 400 km, the
detection of the OH prompt emission in the three-channel spectra can reasonably be expected.
Theoretical spectrum calculation
A difficulty arises when trying to detect the OH prompt emission because the OH molecules
emit fluorescence radiation between the same x2rIi and A2Z + states (Budzien and Feldman, 1991). This
emission is dominant in the near UV, especially at 309 nm, which corresponds to the (0,0) transition.
The fluorescence emission is superposed on the prompt emission which makes its detection difficult.
However, a small difference exists, which is due to a temperature difference of the rotational levels.
Let us consider both processes:
i) H20 photolysis and prompt emission (fig. 1): in the present calculation, we have taken into
account the effect of the solar Ly a only, because this line is responsible for 86 to 93% of the water
https://ntrs.nasa.gov/search.jsp?R=19930010046 2020-03-17T07:27:45+00:00Z
526 Asteroids, Comets, Meteors 1991
vapor photodissociation, depending upon solar activity (Festou, 1981). The rotational quantum number
of OH(A2_ +) can reach J'=23 in the v'--0 and J'=19 in the v'=l states.
ii)Resonance fluorescence excitation (fig. 2_: here we have used the rotational population
calculated in full detail in Schleicher's thesis (1983) for the case when the heliocentric component of
the velocity is equal to +24 km.s -1. It may be noted that the only first four rotational levels of the
A2X +, v'=0 and v'=l states are significantly populated,
g|vll ltn|thE00 300 320 (,,,,, N_wS IengthE00 300 320 (,,,,)
Fig. 1. Emission spectrum of OH (A2I; + --_=
X2FIi) due to H20 photolysts. This spectrum
is calculated with the relative population
distribution of the rotational levels given by
Carrington (1964) for v'--1 and by Sirnons and
Smith (1984) for v'--0. The branching ratio
P(v'=l)/P(v'=0) is taken in Okabe (1980), and
the wavelengths and rotational-transition
probabilities of the lines are taken in Dieke
and Crosswhite (1962). The convolution is
calculated with a FWHM of 7 nm.
Fig. 2. Emission spectrum of OH
(A2I_+--+X21"Ii) excited by resonance
fluorescence. The relative population
distribution of the rotational levels are taken
in Schleicher (1983) for a radial velocity
v=+24 km.s -1. The wavelengths and
rotational-transition probabilities of the
lines are taken in Dieke and Crosswhite
(1962). The convolution is calculated with a
FWHM of 7nm.
A close examination of both figures shows a difference in the wavelengths of maximum
intensity, even after convolution: about 313 nm for prompt emission, and 309 nm for fluorescence. In
order to know whether OH prompt emission is detectable or not it is necessary to compute the intensity
ratio p of the two types of emission.We usehere the formulae givenby Bertaux (1986):
p = ¢(H20)/e(OH) = (g(H20)/g(OH)) (n(H20)/n(OH))
(under optically thin conditions)
where: g(H20) and g(OH) are the excitation rates, in photon.s'l.molecule -I -
n(H20) and n(OH) are the number of molecules along the line of sight
g(H20)= cr_-1
q and x are, respectively, the fraction of OH radicals which are produced in the A2_; +
state, and the lifetime of the water molecules against photodissociation by solar photons, both at the
wavelength of Ly c_.
The folIowlng values are obtained:
i) x,-1.49x105 s at 0.83 AU (sun-comet distance on March 9, 1986), according to Lewis eta/.
(1983). Consequently, if we take q equal to 7.5%, the quantum efficiency is:
g(H20)=5.5x10 -7 photon.s'l.molecule "1 at 0.83 AU (if Ly 0_ is responsible for 90% of the process).
Asteroids, Comets, Meteors 1991 527
ii) g(OH)=l.67x10 -3 photon.s-l.molecule -1 according to Schleicher (1983). In this calculation
we suppose that g(OH) is proportional to r-2, where r is the distance nucleus-sun in AU.
Consequently we obtain: p=3_3x10 -4 (n(H20)/n(OH))
It is more difficult to estimate the ratio n(H20)/n(OH) with accuracy. However, since H20 is
the major source of OH molecules, we shall examine spectra taken as close as possible of the nucleus.
TKS Observations
Figure 3 shows two spectra obtained around 309 nm which show the (0,0) transition of OH
(A2X + -- X21-Ii) and the much weaker (1,1) transition. The intensities of these two emission bands are
normalized, in order to compare their features. They correspond to impact parameters of, respectively,
538 and 3631 km.
I II _ Noi se
i
32o n m
Fig. 3. Spectra obtained with TKS. The
intensities are normalized in order to compare
the features. The impact parameter p of the
spectrum (A) is 538 km and of the spectrum (B)
is 3631 km. Intensity units are
KiloRayleighs/nm. The noise level, which is
equal to about 4 KR/nm, is shown for A and B.
EXCESS OF EMISSION
, MPT ,
308 318 328 H.v.i.ngth
nm
Fig. 4. Theoretical visualisation of OH
prompt emission. The weaker spectrum is due
only to a resonance fluorescence mechanism
and the other is increased by 7.5 % of OH
prompt emission. The intensities of the two
spectra are normalized in order to compare
their features. The convolutions are made
with an instrument response function having a
FWHM of 7nm.
An excess of emission may clearly be seen on the right wing, which can be modelized (Fig. 4)
with a spectrum of pure fluorescence and a spectrum mixed with 7.5% of prompt emission. This
spectrum includes the (0,0) and (1,1) bands.
This modelization implies a ratio n(H20)/n(OH) of nearly 230. Effectively, we have:
p=O,075=3.3x10"4(n(H20)/n(OH)) =_ n(H20)/n(OH)-230 at 538 km of the nucleus
This ratio is appreciably higher than the value obtained with Haser's classical model (1950)
which gives n(H20)/n(OH)=40-45. It may also be compared with the value of about 180 for comet
IRAS-Araki-Alcock, that can be inferred from the calculation of Budzien and Feldman (1991, table II,
exposure LWR 15914).
Therefore, the study of the radial distribution of OH radicals shows a slight decrease of the
integrated intensity of this radical near the nucleus, which is not predicted by Haser's model (which
overestimates appreciably the integrated OH quantity in this region). This decrease is may be due to
an absorption of the solar photons by the inner coma (mainly the H20 molecules).
528 Asteroids, Comets, Meteors 1991
The true ratio n(H20)/n(OH) is consequently probably as high as 100-150, which gives the
best agreement with the spectra obtained by TKS.
Conclusion
The spectra obtained with TKS in the inner coma of Halley's comet, on March 9, 1986, show
that:
i) The intensity of the OH (A2_ + --->X2FIi) emission bands stops to increase when the distance
to the nucleus decreases, below 2000 km. : :_ : :
ii) The-e_ssion shows an excess of intensity on the right wing of the (0,0) band, only in the
vicinity of the nucleus (i.e. within a few hundred km of the nucleus).
The emission excess on the right wing can be partially explained in introducing an OH prompt
emission due to the photolysis of water.
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Evolution of near UV Halley's spectrum in the inner coma

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