On the carbon and nitrogen isotope abundance ratios in comet Halley

Adv. Space Ret. Vol. 9. No. 3. pp. (3)151—(3)155. 1989 Printed in Great Britain. All rights reserved.

0273—1177/89 $0.00 + .50 Copyright © 1989 COSPAR

ON THE CARBON AND NITROGEN ISOTOPE ABUNDANCE RATIOS IN COMET HALLEY S. Wyckoff and E. Lindholm Department of Physics, Arizona State University, Tempe, Arizona, 85287, U.S.A.

ABSTRACT 13C14N have been resolved for the first time in ground-based spectra of Emission lines attributable to comet Halley. An analysis of the spectrum using six 13C14N lines results in a carbon isotope abundance ratio, ‘2C/’3C = 63t~,and a lower limit 14N/’5N> 200. The carbon isotope ratio is nearly 3o~less than the bulk solar system ratio, 89. The limit on the nitrogen isotope ratio is consistent with the bulk solar system ratio, 250. The carbon isotope ratio in the comet may be explained by selective fractionation enhancement of 13C in the parent of the CN molecule, or by a depletion of 12C relative to ‘3C. INTRODUCTION The first report of the measurement of the carbon isotope abundance ratio in a comet was by Bobrovnikoff /1/ from observations ofthe 1910 apparition of comet Halley. He estimated that comet Halley had a solar system carbon isotope ratio. Several reports of carbon isotope ratios have since been reported in comets and have recently been summarized by Van~sek/2/ and Wyckoff et al. /3/. The comet measurements vary from ‘2C/’3C ~-.~60to 115, but have been subject to criticism /4/ due to difficulties arising from blending with other extraneous cometary features. For example several claims have been made of the measurement of the (1,0) ‘2C ‘3C Swan band head near 4744 A in comets. Lambert and Danks /4/ have shown that the (0, 14, 0) band of NH 2 overlies the carbon isotopic feature. Since12C the13C C2/NH2 (1,0) varies band from head comet4744 near to comet, A, correction and thefor NH2 theband contaminating is usually NH considerably stronger than the 2 band becomes a difficult procedure which Lambert and Danks /4/ suggest has not yet been accomplished successfully in a comet. Unquestionably the most reliable method of determining the carbon isotope abundance ratio using spectroscopic techniques is to 3CN features was recently identify and reported for measure the first time ijolated in a isotopic comet line /3/. orHere bandwefeatures. wish to discuss Identification the identification of ‘ of the carbon and nitrogen isotopic features and present a preliminary analysis of the carbon isotope ratio in comet Halley. Further discussion of this analysis is reported elsewhere /5/. OBSERVATIONS Spectra of comet Halley were obtained 4 April 1986 (UT) with the Mount Stromlo Observatory 1.9-m telescope and coudé echelle spectrograph using a photon counting CCD detector. The wavelength region covered 3860.5 - 3876.5 A at a spectral resolution of 0.039 A (FWHM) and the angular extent of the slit length was 20 arcsec projected on the sky. Additional information regarding the observations and reductions has been discussed elsewhere /3, 5/. The comet was at a heliocentric distance, r = 1.22 AU and a geocentric distance, ~ = 0.48 AU at the time of the observations, which was about 20 days after the GIOTTO spacecraft transited the coma, and —~40days after perihelion. In a previous report /3/ a spectrum of scattered moonlight was used to correct the comet spectrum for light scattered by the coma dust. Here we use instead a solar spectrum with considerably higher signal-to-noise (S/N) ratio to correct the background scattered sunlight in the comet spectrum. In Figure 1 we show the entire (0,0) band of the CN B2E+ — X2E+ system at 0.3 A resolution in comet Halley. The spectrum was obtained by B. Peterson with the Mount Stromlo Observatory coudé spectrograph. Selected unresolved R-form branch spin doublets are labeled. Lines in the R branch out to a level N” = 29 were observed in comet Halley. The partially resolved rotational lines in the P-form branch can be seen in the figure. MaR 9:3—K

(3)15 1

(3)152

S. Wyckoff and E. Liadhotrn

3850

Fig. 1.

3855

3860

3865 3870 3875 WAV!LENCTH (A)

3880

3885

3890

Partially resolved rotational structure showing the R and P branches of the CN (0,0) band in comet Halley. (Spectral resolution, ~A 0.3 A).

In Figures 2 - 5 we show a portion of the R-branch of the CN (0,0) band at a. spectral resolution of 0.039 A (FWHM) in comet Halley after correcting for a —~70%contribution by scattered sunlight to the comet continuum flux. The remaining pseudo-continuum in the figures is attributable to the confluence of cometary line blends due to the finite resolution of the spectrograph. The spectra have been smoothed with a Gaussian function of three pixels full width at half maximum. The final rmj noise level in the 12C’4N lines is —~0.i%the intensity of the strong R(8) ‘2C14N line continuum region between the strong at 3868.4 A.

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-04 3664.8 0.039 3665 3665.5A of part 3565 of the3666.5 High resolution (~A A) VAVU.V4GTH~ spectrum R branch of the CN 13C’4N R lines. Other potential (0,0) band in comet Halley showing the resolved contributing species to the comet spectrum are indicated by vertical bars at the bottom of the figure. Synthesized CN spectra are shown for two abundance ratios, the solar system value, 89, and the best fit to the comet spectrum, 12C/13C = 65.

In Figures 2 - 5 we indicate with vertical bars below the comet spectrum various features which could potentially contribute weak emission lines to the comet spectrum. The P-branch lines ofthe (1,1) ‘2C14N band overlie the spectrum as indicated in the figures. Rotational perturbations between the upper level A and B states in CN give rise to extra 12C14N lines the positions of which are also indicated in figures 2-5. Additional details of the line identifications are given in /5/. In Table 1 we list the wavelengths of the CN lines. The calculated spectrum of the ‘2C’4N and 13C14N R-branch rotational lines is shown superimposed on the observed spectrum in the figures for two assumed abundance ratios, solar system (89) and the best fit to the comet spectrum (65). The positions of the ‘2C15N lines are indicated below the comet spectrum in figures 2-5.

C and N Isotope Abundance Ratios -‘a

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High resolution (A.)~ ‘-~ 0.039 A) spectrum of part of the R branch of the CN 3C’4N R lines. Other potential (0,0) band in comet Halley showing the resolved ‘ contributing species to the comet spectrum are indicated by vertical bars at the bottom of the figure. Synthesized CN spectra are shown for two abundance ratios, the solar system value, 89, and the best fit to the comet spectrum, 12C/’3C = 65.

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Fig. 4.

High resolution (z~.X ‘— 0.039 A) spectrum of part 3C’4N of the R lines. R branch Otherofpotential the CN contributing (0,0) band in species comet to Halley the comet showingspectrum the resolved are indicated ‘ by vertical bars at the bottom of the figure. Synthesized CN spectra are shown for two abundance ratios, the solar system value, 89, and the best fit to the comet spectrum, ‘2C/13C = 65. CC

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High resolution (~)~-~ 0.039 A) spectrum of part of the R branch of the CN (0,0) band in comet Halley showing the resolved ‘3C14N R lines. Other potential contributing species to the comet spectrum are indicated by vertical bars at the bottom of the figure. Synthesized CN spectra are shown for two abundance ratios, the solar system value, 89, and the best fit to the comet spectrum, ‘2C/13C = 65.

S. Wyckoff and E. Lindholm

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TABLE 1 - Wavelengths of CN Isotope Lines in 2E4 — X2E~(0,0) Band R-form Branch B N”

‘2C14N° ..\(A) air

13C14N°° .\(A) air

‘2C15N ~ .X(A) air

0 1 2 3 4 5 6 7 8 9 10 11 12

3874.607 3873.999 3873.369 3872.718 3872.053 3871.366 3870.667 3869.927 3869.181 3868.413 3867.625 3866.819 3865.999

3874.763 3874.180 3873.577 3872.954 3872.316 3871.658 3870.990 3870.280 3869.565 3868.829 3868.075 3867.303 3866.517

3874.722 3874.132 3873.523 3872.891 3872.247 3871.582 3870.905 3870.188 3869.464 3868.720 3867.957 3867.177 3866.381

Laboratory positions (Engleman 1974) Computed positions using rn(13C) = 13.00335482, molecular constants of /6/ and formulae of /7/. ~ Computed positions using rn(15N)

=

15.00010895, m(’4N)

=

14.00307400.

ISOTOPIC ABUNDANCE RATIOS The CN molecule is excited by fluorescent scattering of solar radiation /8/. The fluorescence efficiencies for the normal and low abundance isotopes of CN have been calculated by Zucconi and Festou /9/. We have shown /3/ that the lines in the CN (0,0) band are optically thin. Therefore the ratio of the column densities, ~ is related to the ratio of the fluorescence efficiencies, ~, and the observed line intensity ratio, by N gj N~— gI~ where the subscript i refers to the less abundant isotope. The isotope abundance ratio was determined using the method discussed previously (Wyckoff et a!. 1988), except here we fit the computed spectrum to the R(8), R(7) R(6), R(5), R(4) and R(3) lines using weights 3:1:1:2:1:1, respectively. The weights were set by consideration of the lack of potential line blends and the g-factors. The isotopic abundance ratio best fit was found to be N(12C) —63~~ N(13C) which agrees with the ratio found previously from only three lines /3/ namely, 65 ±9. Inspection of Figures 2 - 5 reveals that the R(12), R(11), R(10), R(9) and R(5) 12C15N lines are in clear spectral regions. There are weak cometary features near, but none convincingly aligned, with the R(12), R(11) and R(10) lines. The 12C15N R(9) line lies in a relatively clear spectral region, free from any known overlapping lines, and is coincident with a weak cometary feature at 3868.72 A. The R(5) 12C’5N line on the other hand is indistinguishable from the comet continuum level. The predicted intensity ratio of the 12C’5N R(9) and R(5) lines is = 5 /9/. Thus the R(5) 12C15N line could be below the detection level of the comet spectrum and the feature observed at 3868.72 A could be the R(9) line of ~2C15N,in which case an isotope abundance ratio 14N/’5N = 40 would be indicated. However, we are very reluctant to identify the ‘2C15N molecule (or any other molecule) in comet Halley based on one weak rotational line. We consider it far more likely that the comet feature observed at 3868.72 A is due to a species other than CN. Thus we derive a 2o upper limit for the nitrogen isotope abundance ratio, 14N/15N > 200 (5’5N < +36%) in comet Halley.

C and

N

Isotope Abundance

Ratios

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DISCUSSION Because the production rates of HCN and CN are comparable, it has been argued that HCN is the likely parent of CN /10, 3/. CN was observed to have a jet-like structure in low surface brightness images of comet Halley /11/. Therefore the jet material must be considered an additional parent source of the observed CN. Estimates have been made /11/ that 10-50% of the CN in comet Halley was released in jets. The source material in the jets is believed to be an organic volatile which sublimates slowly, possibly identified with the CHON particles /12/. Thus the parent of CN is probably a mixture of HCN and an organic component of CHON particles. On the other hand, since the HCN and CN production rates observed in comet Halley are comparable, a case can be made that the bulk of the parent of CN is HCN. In this case the cometary HCN would have a carbon isotope ratio 3o- lower than the bulk solar system value. Chemical fractionation at interstellar 2C/’3C ratio on or outer solar nebula temperatures and densities could have significantly altered the ‘ time scales of 106 years. However, the lifetime of the solar nebula was probably 106—7 years. Therefore there may not have been time for significant fractionation of the carbon isotopes to have taken place. If the carbon isotope ratio observed in the comet indicates an underabundance of ‘2C relative to the rest of the solar system, then inhomogeneous mixing of 12C-enriched material in the outer primitive solar nebula could explain our result. A suggestion has been made /13/ that the solar system formed in an OB association containing several supernovae which splattered the local environment with 12Cenriched, nuclear processed material. This scenario accounts for the oxygen anomaly as well as the 26Al enrichments found in meteorites /13/. The comet carbon isotope ratio would be consistent with this theory if there had been incomplete mixing of the supernova ejecta with the primitive solar nebula. Finally, we mention a further possible interpretation of the 12C/13C ratio measured in comet Halley, namely, that it is a captured interstellar comet with a completely different chemical evolutionary history from the rest of the solar system. This is the simplest interpretation of our observations. Two-body capture probabilities for interstellar comets /14, 15/ indicate that capture could be considered a plausible origin hypothesis for at least a small fraction of observed comets. REFERENCES 1.

Bobrovnikoff, N. 1930 Pub. Astron. Soc. Pacific, 42, 117.

2.

Van~sek,V. 1987 in Astrochemistry eds. M. S. Vardya and S. P. Tarafdar (IAU Symp.) p. 461.

3.

Wyckoff, S., Lindholdm, E., Wehinger, P., Peterson, B., Zucconi, 3., Festou, M. 1988, Astrophys. J., (in press).

4.

Lambert, D.L. and Danks, A.C. 1983, Astrophys. J., 268, 428.

5.

Wyckoff, S. and Lindholm, E. 1989, Icarus, (in press).

6.

Huber, K.P. and Herzberg, G., 1979, Constants of Diatomic Molecules, (New York: D. Van Nostrand Reinhold).

7.

Herzberg, C. 1963, Spectra of Diatomic Molecules, (New York: D. Van Nostrand).

8.

Swings, P. 1941, Lick- Obs. Bulletin, 19, 131.

9.

Zucconi, 3. and Festou, M. 1986,

10.

Schloerb, P.D. et al. 1986, Proc. 20th ESLAB Symposium on the Exploration of Halley’s

11.

Comet, 5P.250, 1, Eds. B. Battrick, E.J. Rolfe, and R. Reinhard, (Paris: ESA), 577. A’Hearn, M. ci al. 1986, Nature, 324, 649.

12.

Kissel ci al. 1986, Nature, 321, 280.

13.

Olive and Schramm, D. 1982, Astroplsys. J., 257, 276.

14.

Whipple, F.L. 1975, Astron. J..

15.

Napier, W. 1988, private communication.

Asiron.

A.strophys., 158, 382.