Several rotational transitions of HCN, H2S, H2CO, and CH3OH were detected in comets P/Brorsen-Metcalf 1989 X, Austin (1989c1) and Levy (1990c) with the Institute for Millimeter Radioastronomy (IRAM) 30-m radio telescope. This allows us to determine the production rates of these molecules and to probe the physical conditions of the coma

Bockelee-Morvan, D.

Colom, P.

Crovisier, Jacques

Despois, D.

Paubert, G.

English

NASA Technical Reports Server

q
Asteroids, Comets, Meteors 1991, pp. 133-136 "J...-_' (_ C) /
Lunar and Planetary Institute, Houston, 1992
OBSERVATIONS OF COMETARY PARENT MOLECULES // "
WITH THE IRAM RADIO TELESCOPE.
P. Coloml), D. Despois2), G. Paubert 3) D. Bockelde-Morvanl), J. Crovisier 1)
1) Observatoire de Paris, Section de Meudon, F-92195 Meudon, France
2) Observatoire de Bordeaux, BP 89, Avenue Pierre Sdmirot, F-33270 Bordeaux, France
3) IRAM, Avenida Divina Pastora, 7, N.C., E-18012 Granada, France
133
Abstract
Several rotational transitions of HCN, H2S, H2CO and CH3OH were detected in comets P/Brorsen-
Metcalf 1989 X, Austin (1989cl) and Levy (1990c) with the IRAM 30-m radio telescope. This
allows us to determine the production rates of these molecules and to probe the physical conditions
of the coma.
OBSERVATIONS
Comets P/Brorsen-Metcalf 1989 X, Austin (1989cl) and Levy (1990c) were observed on
September 2-7 1989, May 21-25 and August 26-31 1990, respectively, with the IRAM (Institut de
Radio Astronomie MiUimdtrique) 30-m radio telescope at Pico Veleta (Spain). Three SIS mixer
receivers were used simultaneously (85-115, 130-170 and 209-270 GHz). The spectrometers
consisted in two banks of 128x100 kHz channels, two banks of 512xl MHz channels, and an AOS
(Acousto Optical Spectrometer) of 864 channels with a 505 MHz bandwidth.
RESULTS
The results concerning the detected species are summarized in the table. More details were (or will
be) published by Bockelde-Morvan et al. (1990, 1991), Colom et al. (1990, 1992) and Crovisier et
al. (1990, 1991).
HCN, H2CO and H2S production rates were derived from the observed line intensities using models
treating the evolution of the excitation conditions from the collision dominated region (inner coma,
collisions with H20, s = 10 -14 cm 2, Tkin = 50 K) to the radiation dominated region (outer coma, IR
excitation of the vibrational bands by the Sun). For CH3OH, we assume LTE and used a rotational
temperature of 30 K, in agreement with the observed relative line intensities. For the density
distribution we assumed isotropic outflow from the nucleus at constant velocity (0.8 km s 1) and
took into account the molecular lifetime against photodissociation.
Hydrogen cyanide
The J(1-0) 89 GHz and J(3-2) 266 GHz rotational transitions of HCN were marginally detected in
comet P/Brorsen-Metcalf, whereas clear detections were obtained in comet Austin (1989cl) and
Levy (1990c). HCN seems to be more abundant by at least a factor of two in periodic comets
(P/Halley, P/Brorsen-Metcalf) than in non periodic comets (Wilson, Austin, Levy). This suggests a
chemical difference between periodic and new comets. The very low upper limit obtained on the
relative abundance of HC3N (5x10 -_) shows that it is not the major lacking source of CN radicals.
Formaldehyde
The observations of the H2CO 312-211 transition at 226 GHz in comet P/Brorsen-Metcalfgave only
a marginal detection (S/N = 4). The 226 GHz line was easily detected in comet Austin (S/N = 10)
and in comet Levy (S/N = 8). Observations of the 515-414, 212-1 ll, 303-202, 322-221 and 32t-22o
https://ntrs.nasa.gov/search.jsp?R=19930009954 2020-03-17T07:33:36+00:00Z
134 Asteroids, Comets, Meteors 1991
lines were negative, in agreement with excitation models (Bockel6e-Morvan and Crovisier 1992).
Production rates inferred in the assumption of release from the nucleus show that formaldehyde is a
minor component of the nucleus with an abundance relative to water which ranges from 4x10 -4 in
Levy to 3x10 -3 in P/Brorsen-Metcalf (Colom et al. 1992). These abundances are at|east an order of
magnitude less than the Vega IKS value for P/Halley (4%; Combes et al. 1988).
Hydrogen sulfide
The observation of H2S 110-101 at 169 GHz in comet Austin led to the first detection of hydrogen
sulfide in a comet. In addition to the 169 GHz ortho line, the 220-211 para line of H2S at 217 GHz
was detected in comet Levy. H2S is a minor component, with a relative abundance of 2x10 -3. The
other sulfur-bearing molecules observed (SO2, OCS, H2CS) are less abundant than hydrogen
sulfide. (Crovisier et al. 1991.)
Production rates and abundances.
Comet and molecule Date Q a) Q/Q[H20] b)
[s-q
P/Brorsen-Metcalf (1989 X )
HCN J(1-0)
H2CO 312-211
89/09/04-07 4.5x1026 1.8x10 -3
89/09/134-07 7.6x 1026 3.0x 10-3
Austin (1989cl)
HCN J(1-0) 90/05/'23 2.0x 1025 5.0x 10-4
H2CO 312-211 90/05/21-25 4.6x 1025 1.1 x 10-3
H2S 1 lo-lol 90/05/24-25 1.1xl026 2.7x10 -3
CH3OH (3,0)-(2,0)A 90/05/25 2.0x 1026 5.0x 10.2
Levy (1990c)
HCN J(1-0) 90/08/29 6.6x1025 2.6x10 -4
H2CO 312-211 90/08/'26-30 1.0x 1026 4.0× 10 -4
H2S 110-101 90/08/30-31 5.0x1026 2.0x10 3
C H3OH (3,0)-(2,0)A 90/08/27 1.8x 1027 7.2x 10 -3
HC3N J(24-23 ) 90/08/27 < 1.2x 1025 < 5.0K 10 -5
SO2 717-606 90/08/29 < 6.0x 1026 < 2.5x 10 -3
OCS J(18-17) 90/08/'28 < 5.0x1026 < 2.0x10 -3
H2CS 414-313 90/08/28 < 2.5x1026 < 1.0xl0 -3
a Assuming a parent distribution.
b Q[H20] from OH 18-cm observations: 2.5x1029 s-1
4.0x1028 s -1 for Austin.
for P/Brorsen-Metcalf and Levy,
Asteroids, Comets, Meteors 1991 135
, _ _.. , _..z
._ 5 5
JJ
165040 165060 11550e0 t6StO0
Rent frequency (MHI)
, _ , z _ r
A
v
1651_ 165_ 2184_
I ' ' _w! '
v
The spectra of methanol (CH3OH) observed with the IRAM 30-m radio telescope in comet Levy
(1990c). The upper panels show spectra observed with a 1-MHz spectral resolution. The lower
panel shows parts of the spectra around 97 GHz and 145 GHz observed with a 100-kHz resolution.
136 Asteroids, Comets, Meteors 1991
Methanol
CH3OH was detected in comet Austin through its J(2-1) AK = 0 transitions at 97 GHz and its J(3-
2) AK = 0 transitions at 145 GHz. It was the first detection of methanol in a solar system body. A
dozen of CH3OH lines were detected in comet Levy, as is shown in the Figuy¢, Methanol is a
substantial component of the nucleus, with a relative abundance of the order of 1% in comets Austin
and Levy.
Other molecules.
Limitson many other interesting lines of potential parent molecules were also Obtained, either
during dedicated searches or serendipitously. Here is a preliminary list (some of the corresponding
limits on the production rates are given in the table[_the other ones are presentl _ under evaluation):
Hydrocarbons: CH3CCH (propyne; several _es at85,3 GHz); c-C3H2 (cyC|opropenylidene: many
lines).
OH species: HDO (deuterated water: 312-221 line at 225.897 GHz).
CHO species: many lines of HCOOH (formic acid), CH3CHO (acetaldehyde), C2H5OH (ethanol).
Nitrogen compounds: HC3N (cyanoacetylene: 24-23 at 218.325 GHz); CH3NH2 (methylamine:
lines around 85.4 and 88.6 GHz); CH2NH (methanimine: 110-0oo at 225.5 GHz); HNCO
(isocyanic acid); NH2CHO (formamide).
Sulfur compounds: SO2, OCS, H2CS (see Crovisier et al. 1991).
Re-feri_fices
Bockel6e-Morvan D., Colom P., Crovisier J., Despois D., and Paubert G. (1991) Microwave
detection of hydrogen sulphide and methanol in comet Austin (1989cl). Nature 350, 318-320:
Bockel6e-Morvan D, and Crovisier J. (1992) Formaldehyde in comets: II. Excitation of rotational
lines. Astron. Astrophys. (in press).
Bockel6e-Morvan D., Crovisier J., Colom P, _spois I), and Paubert G. 0990). Observatiohs of
parent molecules in comets P/Brorsen-Metcalf (1989o), Austin (1989cl) and Levy (ig_)_at
millimetre wavelengths: HCN, H2S, H2CO and CH3OH. In Formation of Stars and Planets and
the Evoluiion of the Solar System. ESA SP-315, 243-248.
Colom P., Crovisier J., Bockel6e-Morvan D., Despois D., and Paubert G. (1992). Formaldehyde
in comets: I. Microwave observations of P/Brorsen-Metcalf 1989 X, Austin (1989cl) and Levy
(1990c). Astron. Astrophys. (in press).
=
Colom P., Despois D., Bockel6e-Morvan D., Crovisier J., and Paubert G. (1990). Millimeter
observations of comets P/Brorsen-Metcalf (19890) and Austin (1989c 1) with the IRAM 30-m
radio telescope. In Workshop on Observations of Recent Comets. Eds W.F. Huebner et al., pp.
80-85.
Combes et al. (1988) The 2.5 to 12 microns spectrum of comet Halley from the IKS-VEGA
experiment. Icarus 76,404-436, - .... _........
Crovisier J., Bockel6e-Morvan D., Colom P_, Despois DII and Paubert G. (1990). Observations of
arent molecules in comet Austin (1989cl) at millimeter wavelengths: HCN, H2S, H2CO,
H3OH. Bull. Amer. Astron. Soc. 22, 1089.
Crovisier J., Despois D., Bockel6e-Morvan D, Colom P., and Paubert G. (1991) Mit:rowa_,e
observations of hydrogen sulfide and searches for other sulfur compounds in comets Austin
(1989cl) and Levy (1990c). Icarus 93, 246-258. .....


Observations of cometary parent molecules with the IRAM radio telescope

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