Susan Wyckoff

The 12C/13C Isotope Gradient Derived from Millimeter Transitions of CN: The Case for Galactic Chemical Evolution

New measurements of 12 C/ 13 C ratios in Galactic molecular clouds have been conducted using the N ¼ 1 ! 0 transition of the CN radical. This species is unique in that it has extensive hyperfine structure that can be accurately used to correct for line saturation effects. Combined with the past observations of Savage and coworkers, the ratios derived from CN are the most extensive data set to date for molecular clouds, and they include sources that lie in the range of 0.09‐16.41 kpc in distance from the Galactic center (DGC). The ratios derived from CN indicate a gradient with Galactic distance of 12 C/ 13 C ¼ 6:01DGC þ 12:28. This gradient agrees rather closely with those derived from measurements of CO and H2CO. The least-squares fit to all data points for the three molecules is 12 C/ 13 C ¼ 6:21DGC þ 18:71. CO, CN, and H2CO are synthesized from quite varied reactions, and any 13 C fractionation must follow different pathways for these three species. The relatively good agreement between the 12 C/ 13 C ratios of the three molecules, as well as their lack of correlation with gas kinetic temperature, suggests that chemical fractionation and isotope-selective photodissociation both do not play a substantial role in influencing such ratios. Therefore, the 12 C/ 13 C gradient found in the Galaxy is a true indicator of Galactic chemical evolution. The apparent discrepancy between the solar system ( 12 C/ 13 C ¼ 89) and local interstellar medium values ( 12 C/ 13 C � 68) of this ratio may be a result of 13 C enrichment since the formation of the solar system, as predicted by recent models.

Galactic 12C/13C Ratios from Millimeter-Wave Observations of Interstellar CN

The N = 1 → 0 transitions of 12CN and 13CN(X 2Σ+) at 113.5 and 108.8 GHz, respectively, have been observed in a sample of 13 Galactic molecular clouds using the Kitt Peak 12 m radio telescope. The objects studied include the Galactic center [SgrB2(OH)], sources in the solar neighborhood such as Orion A and NGC 2024, and various other clouds with and without star formation. Hyperfine structure, arising from the nitrogen nuclear spin, was resolved in the spectra of both species, enabling an accurate determination of the opacity in 12CN. From these measurements, estimates of the 12C/13C isotope ratio were obtained. These values fall in the range 12C/13C ~ 20-70 and exhibit a noticeable gradient with distance from the Galactic center. In general, the ratios obtained from CN are very similar to those determined from millimeter observations of CO but are consistently lower than those derived from H2CO. If chemical fractionation is occurring in CN, it is comparable to that in CO. The highest 12C/13C ratios (65 ± 12 and 70 ± 11) were obtained toward two known photon-dominated regions (the Orion Bar and NGC 2024); CN in these two sources may be undergoing some isotope-selective photodissociation. The 12C/13C ratio of 43 ± 7, found in Orion A, is similar to that determined from optical observations of CN toward a nearby source, ζ Ophiuchi, but significantly lower than the average value found in comets (90 ± 10). This difference suggests that substantial enrichment of 13C has occurred during the past 4.6 Gyr in the solar neighborhood.

A survey of N2H+ in dense clouds: Implications for interstellar nitrogen and ion-molecule chemistry

Spectra of the N 2 H + J=1→0, J=3→2 and 15 NNH + and N 15 NH + J=1→0 rotational transitions have been obtained toward a sample of star-forming and cold dark clouds in the Galaxy. Toward the star-forming regions, line profiles are relatively narrow (typically 1-5 km/s) and show no evidence of line wings, in contrast to the spectra of HCO + . The apparent absence of N 2 H + in hot, shocked gas suggests that this ion may be a selective tracer of extended, quiescent material

Cyanide chemistry in comet Hale-Bopp (C/1995 O1)

Observations of comet Hale-Bopp (C/1995 O1) have been carried out near perihelion (1997 March) at millimeter wavelengths using the NRAO 12 m telescope. The J=1-->0, 2-->1, and 3-->2 lines of HCN at 88, 177, and 265 GHz were measured in the comet as well as the J=3-->2 lines of H13CN, HC15N, and HNC. The N=2-->1 transition of CN near 226 GHz was also detected, and an upper limit was obtained for the J=2-->1 line of HCNH+. From the measurements, column densities and production rates have been estimated. A column density ratio of [HCN]/[HNC] = 7+/-1 was observed near perihelion, while it was found that [HCN]/[HCNH+] greater, similar 1. The production rates at perihelion for HCN and CN were estimated to be Q(HCN) approximately 1x1028 s-1 and Q(CN) approximately 2.6x1027 s-1, respectively, resulting in a ratio of [HCN]/[CN] approximately 3. Consequently, HCN is sufficiently abundant to be the parent molecule of CN in Hale-Bopp, and HCNH+ could be a source of HNC. Finally, carbon and nitrogen isotope ratios of 12C/13C = 109+/-22 and 14N/15N = 330+/-98 were obtained from HCN measurements, in agreement with previous values obtained from J=4-->3 data. Such ratios suggest that comet Hale-Bopp formed coevally with the solar system.

Nitrogen abundance in Comet Halley

Data on the nitrogen-containing compounds that observed spectroscopically in the coma of Comet Halley are summarized, and the elemental abundance of nitrogen in the Comet Halley nucleus is derived. It is found that 90 percent of elemental nitrogen is in the dust fraction of the coma, while in the gas fraction, most of the nitrogen is contained in NH3 and CN. The elemental nitrogen abundance in the ice component of the nucleus was found to be deficient by a factor of about 75, relative to the solar photosphere, indicating that the chemical partitioning of N2 into NH3 and other nitrogen compounds during the evolution of the solar nebula cannot account completely for the low abundance ratio N2/NH3 = 0.1, observed in the comet. It is suggested that the low N2/NH3 ratio in Comet Halley may be explained simply by physical fractionation and/or thermal diffusion. 88 refs.

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.

Ammonia abundances in four comets

NH2 emission band strengths were measured in four comets and the NH2 column densities were determined in order to measure the ammonia content of the comets. The mean ammonia/water abundance ratio derived for the four comets is found to be 0.13 + or - 0.06 percent, with no significant variation among the comets. The uniformity of this abundance attests to a remarkable degree of chemical homogeneity over large scales in the comet-forming region of the primordial solar nebula, and contrasts with the CO abundance variations found previously in comets. The N2 and NH3 abundances indicate a condensation temperature in the range 20-160 K, consistent with virtually all comet formation hypotheses. 64 refs.

NH2 fluorescence efficiencies and the NH3 abundance in Comet Halley

If NH3 is the dominant source of the NH2 observed in comet spectra, then the NH3 abundance can in principle be accurately determined. Fluorescence efficiencies for the (0, v-prime/2/, O) to (0, 0, 0) progression of NH2 bands are calculated for NH2 bands likely to be observed in the 4500-8200-A region of comets. The results differ from previous determinations of the NH2 band fluorescence efficiencies by factors in the range 1.4-5.9, leading t6o significant changes in previously reported NH2 production rates in comets. A recalculation of the NH3/H2O abundance ratio in Comet Halley gives about (0.5 + or - 0.2) percent in better agreement with the Giotto ion-mass-spectrometer results of Allen et al (1987).

Formaldehyde in Comets C/1995 O1 (Hale-Bopp), C/2002 T7 (LINEAR), and C/2001 Q4 (NEAT): Investigating the Cometary Origin of H2CO

Observations offormaldehyde (H2CO) have been conducted toward comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT), and C/2002 T7 (LINEAR) using the Arizona Radio Observatory (ARO) 12 m telescope at 1.2 and 2 mm. Aperture synthesis maps of H2CO at 3 mm were made using the Berkeley-Illinois-Maryland Association (BIMA) interferometer toward comet Hale-Bopp. These data indicate that the production rate of H2CO is � 3:7 ; 10 28 s � 1 at � 1 AU in comet Hale-Bopp, using a simple Monte Carlo model, if a nuclear origin for the molecule is assumed. However, maps of H2CO in Hale-Bopp, in comparison with CO, show an extended distribution (rs � 15,000 km) with small-scale structure oriented roughly along the comet-Sun direction. This result suggests a source of H2CO other than the comet nucleus. The extended source of formaldehyde is probably grains composed of a mixture of silicates and organicmaterial.Theproductionrate forH2CO increasestoQ � 1:4 ; 10 29 s � 1 assuming suchanextended grain source. This value implies a Q/Q(H2O) � 1:4%, which is similar to the production rate ratio of Q/Q(H2O) � 4% derived from in situ measurements of H2CO in comet Halley. Production rates for H2CO toward comets C/2002 T7 (LINEAR) and C/2001 Q4 (NEAT) are 1:4 ; 10 27 and 5:6 ; 10 26 s � 1 , respectively, modeled using the extended grain source. The spectra of H2CO measured toward comet C/2002 T7 (LINEAR) show evidence for a second velocity component, most likely arising from comet fragmentation. Subject headingg astrobiology — comets: individual (Hale-Bopp (C/1995 O1), NEAT (C/2001 Q4), LINEAR (C/2002 T7)) — radio lines: solar system — techniques: interferometric

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.