High resolution 2.8-4.0 micron spectra of the 'new' comet Austin 1989c1, taken on 15-16 May 1990 confirm the presence of the broad emission features around 3.4 and 3.52 micron seen in a number of bright comets and ascribed to organic material. Both the 3.4 micron band strength and the 3.52/3.36 micron flux ratios are among the largest so far observed. The data are consistent with the relationship between band strength and water production rate that was recently derived. Excess emission at 3.28 and 3.6 micron cannot be unambiguously identified as features due to the poor signal-to-noise ratio

Brooke, T.

Davies, J. K.

Geballe, T. R.

Green, S. F.

Tokunaga, A. T.

English

NASA Technical Reports Server

Asteroids, Comets, Meteors 1991, pp. 21 !-214
Lunar and Planetary Institute, Houston, 1992
A STRONG 3.4tim EMISSION FEATURE IN COMET
N u ¢ '16
AUSTIN 1989cl
1
S.F. Green 1, J.K. Davies 2, T.R. Geballe 3, T. Brooke 4 & A.T. Tokunaga 4.
1Unit for Space Sciences, Physics Laboratory, University of Kent at Canterbury, Kent, CT2 7NR
2Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ
3Joint Astronomy Centre, 665 Komohana St., Hilo, Hawaii.
4Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, 46822 Hawaii.
ABSTRACT
High resolution 2.8-4.0_tm spectra of the "new" comet Austin 1989cl, taken on May 15 & 16 1990 confirm the
presence of the broad emission features around 3.4 and 3.521_m seen in a number of bright comets and ascribed to
organic material. Both the 3.4_tm band strength and the 3.52/3.3611m flux ratios are among the largest so far
observed. The dam are consistent with the relationship between band strength and water production rate derived by
Brooke et al (Astron. J., 101,268 1991). Excess emission at 3.28 and 3.61.tin cannot be unambiguously identified
as features due to the poor signal-to-noise ratio.
3.4ttm EMISSION IN COMETS
Our knowledge of the composition of comets, believed to be the most primitive material in the solar system, and
perhaps retaining relatively unprocessed interstellar grains, has advanced in recent years through direct sampling by
spacecraft and the consequent intensive observing campaigns. A 3.41xm infrared emission band first detected in
Comet P/Halley (Wickramasinghe & Allen, 1986; Knacke et al 1986; Danks et al, 1987; Baas et al, 1986; and
Tokunaga et al, 1987) together with infrared spectral features observed by the Vega I IKS experiment (Combes et al,
1986; Moroz et al, 1987) have been interpreted as superpositions of C-H band stretching vibrations in gaseous or
solid hydrocarbons. The presence of a substantial organic component in small (10-19-10-15kg) cometary grains was
confirmed by the Giotto and Vega mass spectrometers, PIA and PUMA (Kissel et al, 1986a; 1986b). The 3.41ma
emission feature has been observed in several, but not all, bright comets over the last few years with detections for
Comet Wilson 19861 (Allen & Wickramasinghe 1987; Brooke et al, 1989), Bradfield 1987s (Brooke et al, 1990),
Austin 1989cl (this paper) and Levy 1990c (Davies et al, 1991) but not for Encke (Gehrz et al, 1989), West 1975n
(Oishi et al, 1978), Iras-Araki-Alcock 1983d 0-Ianner et al, 1985) or pre-perihelion in Halley (Gehrz et al, 1989).
The observed emission bands exhibit considerable structure presumably due to differences in the C-H band strength
and frequencies in different molecules. For example, 3.41xm emission is characteristic of the C-H stretching
frequency of saturated hydrocarbons (Bellamy, 1975) which shortens to 3.31am for unsaturated hydrocarbons. The
specific molecules responsible for the range of observed features are not known, and it is not certain if they are in the
gaseous or solid (or _) phases. The features have been observed above both the scattered solar continuum and the
grain thermal emission continuum, dependent on heliocentric distance, for comet Halley. Since the observed feature
near 3.41am can be produced by almost any molecule containing CH3 and CH2 clusters, the identification of parent
molecules will depend not only on detailed structure within the spectral region but the presence or absence of features
in other spectral regions, and the provision of a plausible emission mechanism. The major features at 3.28 and
3.36wn are interpreted by Encrenaz et al (1987) as due to resonance (fluorescence) scattering of solar infrared radiation
by unsaturated and saturated hydrocarbon gas molecules respectively. However, this requires hydrocarbon abundances
of - 30% of that of H20, and high resolution spectra of comet Halley (Drapatz et al, 1987) showed no prominent
line structure typical of gas-phase emission (Chyba et al, 1989) while Brooke et al (1991b) found that the methane
abundance is less than 0.0034 of that of water in comet Levy 1990c. Much lower abundances are required by Chyba
et al for thermal emission from sub-micron organic grains. Predicted features at longer wavelengths which are not
observed are masked by thermal emission from the larger, non-carbonaceous grain population in their two-component
model. A third mechanism, requiring molecular production of only 0.15% of water, involves UV excited infrared
fluorescence from very small grains (large molecules) (Baas et al, 1986).
Emission features around 3.521.tmand near 3.61_mhave been aUributed to formaldehyde (Knacke et al, 1986; Danks
et al, 1986; Moroz et al, 1987) although Brooke et al (1989) found a very poor fit between the observations of
comets Halley and Wilson and their predicted model spectrum for formaldehyde. They suggest that poly-
oxymethylene (POM, paraformaldehyde) could contribute to the observed emission but saturated and unsaturated
hydrocarbons are also required. Although the 3.521xrn feature is observed in all comets exhibiting infrared emission
in this region, the 3.6tam band is not always seen. Tokunaga et al (1987) observed an unidentified emission band at
2.8-2.91am in comet Halley, later seen in comets Wilson (Brooke et al, 1989) and Bradfield (Brooke et al, 1990).
This has been tentatively assigned to the long wavelength component of I-I20 gas emission bands (Bockel_-Morvan
& Crovisier, 1989). Other, less pronounced, features may be present in some dam.
PRE"C_DIi'_ P:,_E _LA[_}:_ NOT FILMED
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212 Asteroids, Comets, Meteors I991
Brooke et al, (1991a; hereafter BTK) conclude, from ratios of the 3.41am feature and continuum with water production
rate, that organics are present in all comets at comparable abundances with respect to water, although there are
potentially significant differences in details of their spectra. They deduced that the 3.41.tm band strengths were
correlated with water production rate rather than dust production.
After its discovery in December 1989 at a heliocentric distance of 2.4A.U., comet Austin 1989cl did not brighten as
expected but still reached a total visual magnitude -4 in April 1990 after perihelion passage at 0.35A.U., providing
an ideal opportunity for detailed spectroscopic study of a "new" comet. IUE spectra taken during the period 7-13 May
were characteristic of a non-dusty comet (Festou et al, 1990).
? ..... _===
OBSERVATIONS AND DATA REDUCTION
Observations of comet Austin were made with the cooled grating spectrometer CGS2 on the UK Infrared Telescope
(UKIRT) on Mauna Kea, Hawaii during service observations on 15 and 16 May i990 when the comet was at a
heliocentric distance of 0.97A.U. Service observations attempted on 3, 4 and 14 May were prevented by cloud which
was also present to a lesser extent on the two nights when spectra were obtained. Multiple scans were made in the
wavelength ranges 2.8-3.65ttm and 3.15-3.91am on the 15th and in the longer range only on the 16th. Sampling
was at 1/4 resolution element steps (0.0022lain at a wavelength of 3.4ttm) to allow for complete sampling despite
the loss of 2 of the 7 elements in the detector array. Sample time was 1.5s per point with an aperture of 5
arcseconds centred on the peak signal. Details are in Table 1. The stars BS8130 and BS8430 (F2IV, mag 2.60 at
3.451.tm with blackbody temperature 6800K and F5V, mag 2.59 at 3.81am with blackbody temperature 6400K
respectively) were used for flux calibration. Corrections for the thin cloud attenuation were made by normalising
scans to those with the maximum average signal. Uncertainties in the flux__calibration are estimated at -25%.
Despite the generally poor quality of the raw data, the presence of emissio n in the 3.41J.rn region is cpnfiym_esfl and_
some structure is evident_= In' order to improve the signal-to-noise ratio, each spectrum has been gauss_n Iil-tered
with a profile having - _I-IM-i/2 a resoh]tion element (0.0041.tin). These are shown in figure 1. The "Combined"
spectrum has been produced from a weighted mean of the three datasets, normalised to the flux for the 16th May over
the region of overlap. The "features" apparent around 3.2 and at 3.3 llam are due to incorrect atmospheric correction
for strong telluric absorption lines.
TABLE 1. Observations.
Date 15 May 15 May 16 May
Wavelength range (gm) 2.8- 3.65 3.15-3.9 3.15-3.9
Mid-observation time (UT) 14:01 14:57 13:39
Number of scans 7 6 10
Mean airmass 1.37 1.16 1.43
Heliocentric distance (Aid) 0.96 0.96 0.98
Geocentric distance (AU) 0.32 0.32 0.30
Calibration star BS8130 BS8430 BS8130
DISCUSSION
The broad 3.41xm emission band seen in other bright comets is Confirmed in comet Austin, together with the band
centred on 3.52pm. The slowly rising continuum is consistent with grain th_al emission w hic_h will be dominant
at these wavelengths for a heliocentric distance Of less than 1A.U. Possible emissi0n at ~3.621am is present in all::
three _tra but data longward of 3.521am are too noisy to positively conf'm'n any structure present. Although
atmospheric water absorption is high below 2.851xm the data indicate the edge of a bright emission feature peaking
shortward of this cut-off. No 3.28ttrn feature is apparent above the noise in these spectra although there is emission
at that wavelength in excess of the continuum
Using an interpolation of the continuum (between 2.9-3.2 and 3.6-3.9_tm) the 3.361am flux to continuum ratio is
3.5+0.4. This value is higher than for most comets observed at the same heliocentric distance but consistent with
the results of BTK who find high feature-to-continuum ratios for comets with low dust-to-gas ratios. Following
their procedure, the continuum flux at 3.361am and the continuum-subtracted integrated flux (between 3.31 and
3.561am) have been corrected to an effective instrument aperture projected radius of 980km and geocentric distance of
1A.U., for c0mp__i_n_ wi___r_esu!ts from Other comets. The corrected continuum flux F*cont = (0.18+0.05)x10 -14
W m "2 l.tm -1 and integrated 3.41am flux I'3.4 = (0.8_.2)x10 _15 W m -2, Thewater pr_@fl-d-educed from
IUE spectra (Festou et al, 1990) was 1.1xl029 .........s -1 on 9 May_..........and s,mdar"-_ .....on 13....May.:-_-Allowing:_.......... _--:_foruncertainties......................of
50% in this value for 15/16 May,
F*cont / QH20 = 0.16 :t: 0.09 (10 -14 W m "2 gm'l/1029 s"l)
Asteroids, Comets, Meteors 1991 213
F3A/QH20 = 0.7 + 0.4 (10 -15 W m-2/1029 s -1)
These values are consistent with the results of BTK who found that the 3.41am band strength is better correlated with
water production rates rather than with dust production rate. The relative strength of the 3.52rtm feature to the broad
emission -3.4pm appears higher for comet Austin than any other comet. The ratio of continuum subtracted fluxes,
F3.52/F3.36 = 0.5+0.2, is twice the typical value for other comets. Interpretation of such observations is difficult
because of the uncertainty in continuum determination and relative strengths of scattered and thermal continua at
different heliocentric distances. It is clear, however, that real differences are present in the structure of these features
since BTK find the 3.521am peak strongest for the dust-poor comets Okazaki-Levy-Rudenko 1989r and P/Brorsen-
Metcalf, despite the fact that they were observed at small heliocentric distances where the thermal emission
continuum dominates and might be expected to mask the longer wavelength feature to a greater extent. The FWHM
of the broad feature in comet Austin is -0.151am, comparable with the range 0.12-0.18 found for other comets. No
detailed interpretation of the shape is possible due to the poor signal-to-noise.
Comet Austin displayed the characteristics of a "new" comet both dynamically and physically (high activity at large
heliocentric distance pre-perihelion but with a subsequent low heliocentric brightness power law exponent, n<3).
There appears to be no obvious
correlation between age of the
comet and the presence (or
absence) of particular features
in the 3 to 4rtm region.
Although the 3.41.tm emission
may be ubiquitous (previous
non-detections in comets West;
Encke and Iras-Araki-Alcock
being explainable by low
signal to noise) real
compositional differences in
the organic content of comets
may be present. In order to
understand fully the structure
and composition of the
emitting material, high
resolution observations of a
number of comets, both 'old'
and 'new', at different
heliocentric distances, are
required. These will permit a
combination of detailed
modelling to fit the range of
observed features, and an
understanding of the evolution
of cometary material as it
'ages'.
Figure 1. CGS2 spectra of
comet Austin 1989cl. Data
have been filtered with a
gaussian profile with
a=0.002ttm (i.e FWHM-1/2
resolution element). Spectrum
d is a normalised weighted
mean of all the data. Error
bars are standard deviation of
counts from co-added scans.
Acknowledgements
3:
qt
.¢
4 D
2 -
0 --
4 D
2-
0P
2.8
c) 1339 UT 16 May 1990 h
C I
d) Combined data _-_'_u_
I I I I I I I I I I I
3.0 3.2 3.4 3.6 3.8
WAVELENGTH (.urn)
UKIRT is operated by the Royal Observatory Edinburgh on behalf of the Science and Engineering Research Council.
We thank all UKIRT staff involved in the service observing programme. Data reduction was performed partly on the
Starlink computer network.
21 4 Asteroids, Comets, Meteors 1991
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A strong 3.4 micron emission feature in comet Austin 1989c1

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