P. A. Wehinger

Carbon Isotope Abundances in Comets

Rotational lines of 13C14N have been identified in high-resolution (λ/Δλ ~ 60,000) echelle spectra of the CN B2Σ+-X2Σ+ (0-0) band in three comets. The 12C/13C abundance ratios determined using a full fluorescence excitation model for comets Levy (C/1990 K1), Austin (C/1989 X1), and Okazaki-Levy-Rudenko (C/1989 XIX) are 90 ± 10, 85 ± 20, and 93 ± 20, respectively, consistent with the solar system ratio, 90. A lower limit for the nitrogen isotope ratio, 12C14N/12C15N 200, found for comet Levy is consistent with previous comet measurements and the solar system value, 272. The mean CN carbon isotope ratio in the five comets measured to date is 12C14N/12C15N = 90 ± 10, and the mean for the three molecular species C2, HCN, and CN measured in nine comets is 101 ± 15. Consistency of the cometary carbon isotope ratios with the bulk solar system value indicates (1) chemical homogeneity in the outer protosolar nebula, (2) minimal isotopic fractionation in the protosolar precursor molecular cloud, and (3) that comets formed coevally with the solar system. The 14% difference between the solar system (90) and the present solar neighborhood interstellar 12C/13C ratio (77 ± 7) may be indicative of significant Galactic 13C enrichment over the past 4.6 Gyr. However, even though models can match to within a factor of 2 the solar system abundances, including the carbon isotope ratio, other evidence suggests that simple homogeneous Galactic evolution models may not be adequate to explain detailed stellar and interstellar abundances in the Galaxy.

Abundances in comet Halley at the time of the spacecraft encounters

Spectrophotometric observations of Comet Halley obtained in March 1986 at the time of the spacecraft encounters with the comet are described, and the column densities derived from the integrated emission band fluxes are presented. The scale lengths are determined from the spatial profiles of the observed species. Production rates are derived and the parent identities and abundance ratios of the studied species are discussed. The data indicate that the abundance of the primordial condensate NH3 comprises about 0.2 percent of the total volatile fraction of the comet nucleus. The ratio of production rates Q(NH2)/Q(H2O) = 0.003 indicates that Q(NH3)/Q(H2O) is roughly 0.3 percent, which is nearly 10 times lower than the value determined from Giotto ion mass spectrometer data. The production rate Q(CH)/Q(H2O) of 0.007 and the CH spatial profile indicate that CH cannot derive entirely from direct photodissociation of CH4.

The C-12/C-13 abundance ratio in Comet Halley

The individual (C-13)N rotational lines in Comet Halley are resolved using high-resolution spectra of the CN B2Sigma(+)-X2Sigma(+) (0,0) band. The observe C-12/C-13 abundance ratio excludes a site of origin for the comet near Uranus and Neptune and suggests a condensation environment quite distinct from other solar system bodies. Two theories are presented for the origin of Comet Halley. One theory suggest that the comet originated 4.5 Gyr ago in an inner Oort cloud at a heliocentric distance greater than 100 AU where chemical fractionation led to the C-13 enrichment in the CN parent molecule prior to condensation of the comet nucleus. According to the other, more plausible theory, the comet nucleus condensed relatively recently from the interstellar medium which has become enriches in C-13 and was subsequently gravitationally captured by the solar system. 107 refs.

The cometary flourescence spectrum of cyanogen: A model

We present a model to compute flourescence spectra of the stable isotopes of CN in the comae of comets. The effects of collisions are included and shown to be significant as a mechanism secondary to the flourescence process in the calculations. Our computed spectra are compared with high-resolution spectra of the B2 sigma(+)-X2 sigma(+) (0-0) band for comets Halley, Austin, and Levy. This comparison shows that collisonal effects are observable in the CN spectra of relatively high production rate comets, such as Halley and Levy. Moreover, the model confirms that the overall structure of the CN spectra in comets results predominantly from pure flourescence governed by the Swings effect. The primary goal of our model is to provide a methodology to compute accurate rotational line flourescence efficiencies for the stable isotopes of CN: (12)C(14)N, (13)C(14)N, and (12)C(15)N. The flourescence efficiencies are required to convert observed isotope line intensity ratios to isotope abundance ratios of carbons and nitrogen in comets.

Phase-resolved Spectroscopic Imaging of the Crab Pulsar

High time resolution spectroscopic imaging of the Crab pulsar across the optical wave band has been undertaken utilizing the 4 m Mayall telescope at Kitt Peak National Observatory. The 33.51 ms period of the pulsar was split into 723.7 time slices of 46.3 μs and a spectrum within each slice acquired. These data have allowed analysis of both the light curve as a function of wavelength and the spectrum as a function of phase position. Wavelength-dependent changes in phase position for both emission peaks have been found, and these appear to be uncorrelated. Related to these changes are phase position-dependent variations in the spectral index, and these have been measured, the shapes being in agreement with previously published data. Also related are energy-dependent changes in the FWHMs. In addition, we find, in support of previously published data, that the fluence of Peak 2 relative to Peak 1 decreases with increasing wavelength. These results then challenge the validity of currently existing models for pulsar emission.

Identifications and spectra of infrared objects

Thirty-six infrared objects have been identified from direct two-color image-tube photographs. Slit spectra in the 5800 A to 8800 A region have been obtained of each object. The sample consists of 35 M-type stars, one carbon star, and no S-type stars. Most of the M stars are probably semiregular and Mira-type variables. Key words: infrared objects - image-tube photographs - stellar spectra