Results of observations of comet Bradfield using the Haystack telescope are summarized with emphasis on the detection of the 1.35 cm emission line of water in the comet. The excitation of water and methyl cyanide in comets is briefly considered

Clark, T.

Donn, B.

Jackson, W. M.

English

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^76-21065
RADIO DETECTION OF H2O IN COMET BRADFIELD (1974b)
W. M. Jackson, T. Clark and B. Donn
The present authors (Clark et al, 1971) previously
attempted to detect the 1.35 cm microwave line of water
in Comet Bennett (19691) using the 26 m Maryland Point
radio telescope of the Naval Research Laboratory. An
upper limit for the H?0 column density of 10 ' molec/cm
was obtained assuming the rotational levels were in thermal
equilibrium. A more recent search in Comet Kohoutek
using the 37 m radio telescope of the Haystack Observatory
with a low noise traveling wave maser preamplifier was also
unsuccessful. In this comet the identification of HpO in
the visible spectrum gave strong support to the idea of
water as a major parent molecule. The radio detection of
HCN (Heubner et al, 1974) and C1UCH (Uhlich and Conklin,
1974) in Comet Kohoutek suggested that the attempt to detect
HpO should be repeated in a suitable comet.
The present paper discusses the successful detection
of HpO in Comet Bradfield (1974b) using the Haystack tele-
scope when the heliocentric and geocentric•distances were
1.22 A.U. and 1.15 A.U. respectively. At 1.35 cm this
antenna has a main-beam half-width of 1.5 arc min. and an
efficiency of 0.33. The traveling wave maser was used with
a 100 channel autocorrelator operating at a 667 KHz band-
width. The average system temperature for these observations
was 158°K. An observing sequence of 10 minutes on the comet
272
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and 10 minutes off was employed. Each off source scan was
taken along the azimuth evaluation path on the sky that the
comet would traverse ten minutes later. This procedure
compensated for gross atmospheric and instrumental effects
and sky background. Some on source runs were made with
the antenna intentionally displaced one half beam width on
the sunward or tailward side of the comet. Cometary
positions for pointing purposes were based on ephemerides supplied
by B. Marsden.
The results are summarized in Figure 1 for runs taken
when pointing at the comet position and in Figure 2 for
runs with the telescope pointed one half beam width on the
sun or tailward side. All twelve 20 minute on-off scans on
the comet show some signal excess at the -0.82 km/sec feature
which we identify as the H-O transition. It is therefore
unlikely that this feature is an artifact. Figure 2 also
shows the H?0 feature although the signal is weaker. Because
of reduced integration time the noise is worse.
This data supplies strong evidence for the detection of
the 1.35 cm (22.2 GHz) emission line of water in Comet
Bradfield with a peak antenna temperature of 0.15 K and a
FWHM of OA km/sec. The -0.82 km/sec line shift from
Marsden's ephemeris is several orders of magnitude larger
than any possible errors in the calculations. The decrease
in signal by ~ 1/3 when the telescope is shifted 1/2 beam
width from the predicted position of the comet indicates an
intrinsic source size of ^  10-15 arc sec.
There are serious difficulties with the interpretation
and further analysis of these results. Reliable column
273
densities and in turn water production rates, Q, can be
readily obtained only if the rotational levels are in thermo-
dynamic equilibrium with a B°ltzmann distribution over
rotational states. This is questionnable in the case of
comets as the density is low near the nucleus and falls off
rapidly. The treatment of Clark et al'(l97l) based on the
equilibrium assumption yields an average column density
<N> of 2 x 10 molec/cm . A temperature of 240°K calculated
from the measured FWHM was used.
Heubner and Snyder (197°) have derived a relationship
between <N> and Q:
Where A = geocentric distance, v = expansion velocity, @ =
half power beam width and r = radius of molecular cloud.
All of these are known or may be reliably estimated except
r . They defined r as VT where T is the photodissociation
lifetime. Be rtaux et al (197*0 obtained a value of approx-
4
imately JO hours which makes r = 5.7 x 10 km. Equation (1)
po
yields a water production rate of 3.4 x 10 y molec/ster sec.
A comparison with other derivations of production rates of
parent molecules is given in Table 1. It is seen that our
value is an order of magnitude larger except for the CH-CN
result. The two other water results are based on ultra-
violet observations of H and OH fragments. As the photo-
chemistry of water and the optical excitation of the fragments
are well understood, it seems likely that these results are
substantially correct. HCN is detected by the J = 1-0
274
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275
transition and no excitation problem occurs. Methyl cyanide
is detected by emission from excited state comparable to that
of water and a similar, as yet unknown, excitation process
occurs.
A more careful analysis of the excitation of water and
methyl cyanide in comets is required and preliminary con-
siderations are now given. The upper level of the 6-^ g - 5?^
HO transition lies ^ Tcm" above the ground rotational
level. In order to maintain thermodynamic equilibrium,
collosional pumping rates of all levels must be larger than
the radiative decay rates. The principal mode of radiative
decay of the 6,g level is via the 6^ - 5Q(- transition with
a lifetime of about 1 sec (Buhl et al. , 1969).
To a first approximation the steady state condition is
given by equating radiative and collisional lifetimes.
The approximation neglects the details of the system in
~which there are numerous rotational levels with radiative
and collisional transitions among them. It also cancels
Boltzmann factors for rotational and translational popula-
tions. We write for the lifetime T of collisional processes:
\*r
•[_ _
T = (van) < 1 sec. (2)
c v
where v is the thermal' velocity.
As the principle constituent is assumed to be water,
rotational excitation will be governed by a dipole-dipole
potential. This leads to cross-sections for rotational
Op
excitation of the order of 1000A (Levine and Bernstein,
276
With v = 10 cm/sec, the water density must be greater than
Q -^2.5 x 10 cm" . The fluid dynamic model of comets (Jackson
and Bonn, 1966) predicts densities of this order at 1600 km
from the nucleus for water production rates of 10 -^ molec/sec
ster. This radius for the cloud is a factor of ^0 less than
that used to calculate the water production rate from
Equation (1). The assumption of thermodynamic equilibrium
leads to an inconsistency and production rates for molecules
detected in excited rotational states may not be calculated
on such an assumption.
Some other mechanism of rotational excitation than
molecular collisions is required. Recent 'calculations by
Itikawa (1972) have yielded cross-sections for rotational
excitation of water by 0.01 ev electrons of the order of
-1^ 3 210 ' cm . Even this large cross-section would require an
electron density comparable to the particle density. An
anomalous excitation of the 6,g - 5p, water transition is
observed in the interstellar water masers requiring a non-
equilibrium excitation mechanism. A suggestive process is
infra-red pumping as described by Litvak (1972). Such a
scheme may also work for comets. The cometary system is
known in considerable detail and a comprehensive analysis
for cometary H?0 and CH^CN emission would be very valuable.
The more favorable observing conditions with Comet
Bradfield compared to Comet Kohoutek appear to account for
the detection of water in Bradfield for the first time.
Comet Bradfield was nearly circumpolar (6=87°) at the time
of our observations. Hence we were able to observe for
277
longer times without suffering extensive atmosphe ric atten-
uation. Comet Halley will also be favorably located and will
be a good candidate for further water observations.
278
0.2
 T
COMET BRADFIELD
-o.\-L
VELOCITY (KM/SEC)
Figure 1. 1. 35 cm (22.2 GHz) water transition. Sum of
all scans centered on comet position.
COMET BRADFIELD
-O.l-1-
VELOCITY (KM/SEC)
Figure 2. 1. 35 cm (22.2 GHz) water transition. Sum of scans dis-
placed 1/2 beam width in sun or tail direction.
279
REFERENCES
1. Bertaux, J.L., Blamont, J.E., and Festou, M., 1974.
Astron. and Astrophys., 25, 415.
2. Buhl, D., Snyder, L., Schwartz, P. R., and Barrett, A.M.,
1969, Ap. J., 158, L97.
3. Carruthers, G.R., Opal, C.B., Page, T.L., Meier, R.R.,
and Prinz, D.K., 1974, Icarus, 23, 526.
4. Clark, T.A., Donn,.B., Jackson, W., Sullivan, W.T., and
Vanderberg, N., 1971, Astron. J., 76, 6l4.
5. Heubner, W.F., and Snyder, L.F., 1970, Astron. J., 75, 759.
6. Heubner, W.F., Snyder, L.S., and Buhl, D., 1974, Icarus,
23, 580.
7. Itikawa, Y., 1972, J. Phy. Soc. Japan, 32, 217.
8. Jackson, W.M., and Donn, B., 1966, Colloque Int. Astrophys.,
Liege, 1965, p..133.
9. Levine, R.D., and Bernstein, R.B., 1974, "Molecular
Reaction Dynamics", Oxford University Press,
p. 152.
10. Litvak, M.M., 1972, in Molecules in the Galactic
Environment, ed. M.A. Gordon and L.E. Snyder
(New York, Wiley), p. 267.
11. Snyder, L.E., 1975, These Proceedings.
12. Uhlich, B.L. and Conklin, E.K., 1974, Nature, 248, 121.
280


Radio detection of H2O in comet Bradfield (1974b)

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