The rate of evolution of CO from Comet Kohoutex is presented. The observations were made approximately two months after perihelion. The rate of evolution of CO at that time was apparently less than that of CH3CN observed before perihelion

Geballe, T. R.

Greenberg, L. T.

Lacy, J. H.

Rank, D. M.

Townes, C. H.

Wollman, E. R.

English

NASA Technical Reports Server

AN UPPER LIMIT ON THE EVOLUTION OF CARBON MONOXIDE
FROM COMET KOHOUTEK *
E.R. Wollman,t T.R. Geballe,t L.T. Greenbergt
J.H. Lacy**t and C.H. Townest
Department of Physics, University of California, Berkeley
94720 (415) 642-5604
and
D.M. Rank t
Lick Observatory, Board of Studies in Astronomy
and Astrophysics, University of California, Santa Cruz
95060 (408) 429-2277
Received
ABSTRACT
An upper limit on the rate of evolution of CO from Comet
Kohoutek has been obtained from a search for resonant scattering
of sunlight near 4.74. The observations were made approximately
two months after perihelion. The rate of evolution of CO at that
time was apparently less than that of CH3CN observed before
perihelion.
(NASA-CR-13850 1) AN UPPER LIMIT ON THE . .4...
.VOLToN.. r ' V ~UNOOXIDE 1ROi COMET
KOHOUTEK (California Univ.) 6 p HC l
CSCL 038 Unclas
G3/30 40788
*Work supported in part by NASA Grants NGR-05"r-598, NGR-
05-003-452 and NGL-05-003-272
**National Science Foundation Fellow
Guest Observer, Mauna Kea Observatory, Institute for Astronomy,
University of Hawaii.
https://ntrs.nasa.gov/search.jsp?R=19740018184 2020-03-23T08:52:20+00:00Z
2Under favorable conditions, and with a'very sensitive
spectrometer, infrared resonant scattering of sunlight should
allow a determination of the rate of evolution of CO from a
bright comet (Biermann, 1973a). Such a determination was
attempted for Comet Kohoutek 1973f. Optimum observing condi-
tions were not obtained and CO not detected, but the limited
measurements which could be made provide an upper limit on the
CO evolution rate approximately two months after perihelion.
Observations were made on February 23, 1974 at the Mauna
Kea Observatory's 88" telescope using a tandem scanning Fabry-
Perot interferometer (Holtz, 1971). The field of view was 9.5"
by 14" and was approximately centered on the comet's nucleus.
A spectral region near 4.7p including the 1-0 P2 and P3 funda-
mental rotation-vibration transitions of 12C 160 was scanned with
,a resolution of 0.1 cm-1. The comet's radial velocity of 47
km s- 1 with respect to the earth, giving a doppler shift of
0.33 cm- 1 , was sufficient to move the expected positions of the
cometary lines well outside the half-power points of the terres-
trial CO line absorption.
The sensitivity of the spectrometer system was determined
by measurement of the signal from W Hya in the same wavelength
region. If a represents the r.m.s. noise fluctuation, any
signal larger than 30 should have been easily observed. With
this criterion, the lack of an observed signal sets an upper
3limit of 1.0 x 10-18 W cm- 2 for the flux in either CO line at
the top of the earth's atmosphere. Since the rate of resonant
scattering of 4.7p radiation by a CO molecule at 1.5 A.U. from
the sun is approximately 6 x 10 - 5 s- 1 , the above limit for the
flux corresponds to an upper limit of 2.5 x 1033 for the total
number of CO molecules within the field of view in a single
rotation-vibration state. For an assumed temperature of 2500 K,
the J=3 level contains about 7% of all CO molecules present.
Thus, the upper limit on the total amount of CO in the field
of view is 3.5 x 1034
Assuming a model in which CO is produced in a region small
compared to the observed field of view, the density of CO is
given by n(CO) = 1given by , where 4fQ is the total production
r
rate, r the distance from the source, and 1 is the mean speed
V . For a molecular lifetime T, the CO cloud will have a
radius r = VT. The total number of molecules within a circu-
lar field of radius p centered on the comet is approximately
2( Q ) p if P<ro, which is almost certainly the case. At a
distance of 1.5 A.U., the rectangular field of view has the
area of a circular field of radius p = 7.3 x 10 cm. Hence
the observed limit on the flux corresponds to Q/Y (CO).< 2x102 4
-1 -1
cm ster Biermann (1973a) has estimated that Q/9 for CO
in bright comets is 1023±1.
Ulich and Conklin (1974) have.reported. a detection of
CH CN in Comet Kohoutek by its microwave emission and concluded
that the total number of methyl cyanide molecules within their
antenna beam was -.9 x 1034. The observations- were made in
early December, before perihelion. They estimate the CH3CN
cloud radius to be ~-10 cm, which is considerably smaller than
the size of the microwave antenna beam. In that case,
N 24 cm-1 -1Q/V (CH 3 CN) 7.2 x 10 cm ster
Scloud
If the CH 3CN cloud is larger than the estimated 10 cm, a lower
limit on Q/3 is given by 4N .. The effective radius of the
beam
beam used by Ulich and Conklin is - 4 x 109 cm, so that
Q/V (CH CN) > 1.8 x 1024 cm-1 ster - 1
.This is essentially equal to the above upper limit for CO,
Q/1 (CO) < 2 x 1024 cm- 1 ster - 1
It is surprising that the rate of evolution from the comet
found for CH CN is equal to or greater than the rate of evolution
of CO. However, it must be noted that CH CN was detected before
perihelion and that subsequent search after perihelion failed to
detect this molecule. Observations which set the present upper
limit on CO were made only after perihelion. Hence, it is
possible that the Comet lost most of its volatile gases such
as CO in the course of its trajectory before and during peri-
5helion, (cf. Biermann, 1973b) or that the more volatile gases
were already depleted long before its recent approach to the
sun.
6REFERENCES"
Biermann, L. (1973a), To be published.
Biermann, L. (1973b), Nature 246, 400.
Holtz, J.Z. (1971), Unpublished, University of California,
Berkeley.
Ulich and Conklin (1974), Nature 248, 121.


An upper limit on the evolution of carbon monoxide from Comet Kohoutek

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