Richard A. Linke

The bell laboratories H I survey

We present a galactic survey which to date consists of 73,000 positions covering −5° < l < 122°, −1° < b < 1° , observed in the J=1 → 0 line of 13CO to an rms noise level of 0.15 K in 0.68 km s−1 channels, using the 7 m antenna at Crawford Hill. It is shown that the internal velocity dispersions of molecular clouds tend to vary inversely with galactocentric radius.

Isotopic abundance variations in interstellar HCN

A study of the relative abundances of the rare isotopic species H/sup 12/C/sup 15/N and H/sup 13/C/sup 14/N has been made in six dense molecule clouds through observations of the 3.5 mm wavelength rotational transitions. The data indicate that in all sources the (HC/sup 15/N)/(H/sup 13/CN) integrated line-intensity ratio is smaller than that implied by terrestrial isotopic abundances. The excitation of HCN is discussed with special attention to the conditions which permit accurate abundance determination even when optical depths are large. We make a quantitative estimate of the line saturation and find this to be a small effect for the present measurements. There appears to be at least a factor of 6 between the average ratio for the sources in the galactic disk and that for two sources associated with the galactic center. This result is discussed in terms of increased nuclear processing in the galactic center region.

Isotope abundances in interstellar molecular clouds

The J=1..-->..J=0 transition of carbon monoxide is used to study the abundance ratios of carbon and oxygen isotopes in dense molecular clouds. Very high-quality spectra were obtained of /sup 12/C/sup 16/O (CO), /sup 13/C/sup 16/C(/sup 13/CO), and /sup 12/C/sup 18/O(C/sup 18/O) in 14 regions distributed throughout the galactic plane. Effects of line saturation are carefully examined, and it is found that the /sup 13/CO species often produces a slightly saturated line. When saturation is properly taken into account, the data from all the regions cluster about a single value of approx.14 for the /sup 13/C/sup 16/O//sup 12/C/sup 18/O abundance ratio. This value is significantly different from the corresponding terrestrial ratio of 5.6, and the difference probably results from chemical evolution having occurred in the Galaxy since the birth of the solar system. The results of additional isotope studies which indicate that the value of /sup 12/C//sup 13/C is approx.40 as compared with the terrestrial value of 89 are presented. (AIP)

Deuterium in the Galaxy

A survey of millimeter-wave line emission from the deuterated and carbon-13 isotopes of interstellar hydrocyanic acid (DCN and H/sup 13/CN) has been carried out. The results indicate that the (DCN)/(HCN) abundance ratio is about 100 times the local (i.e., within approx. 200 pc of the Sun) (D)/(H) ratio over a large portion of the Galaxy, with relatively little source-to-source variation. The (DCN)/(H/sup 13/CN) abundance ratio in Sgr B appears to be markedly lower than in the other sources. We associate this apparent deuterium deficiency with a greater amount of stellar processing of the material near the galactic center, such processing evidently leads to a net destruction of deuterium. This result seems to eliminate stellar activity as a principal source of observed deuterium. (AIP)

Detection of H/sup 15/NN/sup +/ and HN/sup 15/N/sup +/ in interstellar clouds

The nitrogen fifteen isotopes H/sup 15/NN/sup +/ and HN/sup 15/N/sup +/ have been detected for the first time in an interstellar cloud, DR 21 (OH), through obsevations of their J = 1--0 transitions near 90 GHz. These two isotopic species have nearly equal abundances as predicted by ion molecule formation of HN/sup +//sub 2/. Comparison with the weakest component of HN/sup +//sub 2/(F = 1--0) yields an intensity ratio /sup 14/N//sup 15/N>170, consistent with the terrestrial /sup 14/N//sup 15/N isotope ratio (270). The N/sub 2/H/sup +//HCO/sup +/ abundance ratio, which is important for comparison with theoretical chemical models, has been calculated from the H/sup 15/NN/sup +//HC/sup 18/O/sup +/ ratio and, after scaling by the terrestrial nitrogen and oxygen isotope ratios, found to be approx.0.9. This value suggests that in DR 21 (OH) nitrogen could be mostly in the form of N/sub 2/ in the gas phase, while only 10% to 15% of carbon is in the form of gaseous CO. In four other molecular clouds, only upper limits could be established for the H/sup 15/NN/sup +/ and HN/sup 15/N/sup +/ emission.

Observations of /sup 14/N//sup 15/N in the galactic disk

A new survey has been made of HC/sup 15/N and H/sup 13/CN which includes 16 giant molecular clouds and one circumstellar cloud of expelled material. The observed double isotope ratio (/sup 12/ C//sup 13/C) x (/sup 15/N//sup 14/N) is shown to be a good tracer of equilbrium CNO processing: smaller values are observed from more highly processed material. Using the results of recent measurements of the /sup 12/C/ /sup 13/C ratio, we determine that the /sup 15/N/ /sup 14/ N ratio shows a small difference between the galactic center region and the rest of the galactic disk but that /sup 15/N/ /sup 14/N has no appreciable radial gradient throughtout the outer Galaxy. This behavior, together with the observed circumstellar abundances, can best be understood in terms of galactic enrichment models in which /sup 15/N is produced by explosive CN burning of hydrogen in novae, and /sup 14/N by equilibrium CN processing in stars.

Detection of deuterated formaldehyde in interstellar clouds

Deuterated formaldehyde, HDCO, has been observed for the first time in interstellar clouds by the detection of emission from its J/sub K/-1,K+1=2/sub 02/..-->..1/sub 01/ transition at 128.81291 GHz Comparison of these data with line strengths of the millimeter emission lines of H/sub 2/CO implies that deuterium fractionation of formaldehyde is comparable to that found for other deuterated species. This indicates that formaldehyde is probably formed by gas-phase ion-molecule reactions rather than on grains. A comparison of the HDCO and H/sub 2/CO spectra for two dark clouds suggests that the centimeter and millimeter wavelength lines of H/sub 2/CO arise from different regions within the clouds.

The abundance ratio (/sup 17/O)/(/sup 18/O) in dense interstellar clouds

We have measured the interstellar (/sup 17/O)/(/sup 18/O) isotope abundance ratio in eight giant molecule clouds using the J=1..-->..J=0 transition of carbon monoxide at approx.2.7 mm. The average interstellar ratio derived from these data is 0.24, a value which is significantly higher than the corresponding terrestrial abundance ratio of 0.186. There is weak evidence of source-to-source variation of the abundance ratio, but the maximum deviation from the mean value of 0.24 is 40 percent. An increase in the (/sup 17/O)/(/sup 18/O) interstellar abundance ratio since the time of formation of the solar system is consistent with current knowledge of nucleosynthesis. (AIP)

Detection of Deuterated Formaldehyde in Interstellar Clouds

Deuterated formaldehyde has been detected for the first time in interstellar clouds; the observed ratio HDCO/H 2 CO implies formation by gas phase chemistry.