Peter G. Wannier

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.

Warm H I Halos around molecular clouds

Observations of H i have been made with the 305 m Arecibo dish toward the isolated edges of several molecular clouds. Comparison with 1.3 and 2.6 mm CO line emission indicates the existence of H i halos extending 0.5 to several pc beyond the CO boundaries. Because the halos appear in emission against the approx.100 K galactic H i background, the gas is evidently warmer than much of the interstellar Hi gas. The observed column densities and transition zone thicknesses agree well with steady state calculations equating the formation rate of H/sub 2/ on grains to its destruction by UV radiation.

The Perseus B5 Molecular Cloud Halo: Measurements of Pressure, Temperature, and Composition*

We present GHRS observations of interstellar C I and CO toward three stars located in the direction of the Perseus region near the B5 molecular cloud and discuss these together with published results from a fourth sight line. The column densities of the three fine-structure levels of C I are used together with ground-based optical data and published ultraviolet data to yield several properties of the gas along the stellar sight lines. The selected stars probe gas located in the neutral envelope surrounding the dense molecular cloud. The gas pressure is tightly constrained by our observations, yielding a value of ≈2200 K cm−3 in the vicinity of B5, somewhat higher than in the surrounding interstellar medium. We also present estimates of the kinetic temperature, based on observations of H2 and C2, yielding 20-60 K in the vicinity of B5. Together with the pressure determination, this implies a local gas density of 35-100 cm−3, very close to that which would be implied by the observed H2 column density assuming a filling factor of unity. This result provides additional support to the notion that the gas is part of an extensive, uniform gaseous halo surrounding the B5 cloud. The combination of temperature and pressure suggests that the diffuse gas may be part of a general outflow from the dense molecular cloud.

CO and CS in the Orion Nebula

The reported observations were made with an 11-m radio telescope. The CO map considered has a peak near the position of the infrared continuum source. There is little correlation between the CO contours and any optical structure. The CO contours show a pronounced north-south orientation. CS was observed as a representative example of a stable molecule with a higher dipole moment than CO. Questions regarding the structure of Orion A are discussed, giving attention to various models.

Warm neutral halos around molecular clouds. III. Interpretation of H I and CO J = 1-0 data

A study is presented of the physical characteristics of the transition region separating the molecular and the atomic medium in interstellar clouds, based on H I 21 cm and CO J = 1-0 data. Strong evidence is found for general heating of the surface layers of molecular clouds, and for the existence of large, heated neutral atomic halos around the molecular clouds. The heating of the molecular cloud surface is manifested through the general existence of limb brightening in the CO emission, with a characteristic depth of 1.7 pc, dependent on the Galactic latitude. The heated H I halos have a characteristic depth of 4.7 pc and a maximal extent of about 10 pc.

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)

CN abundance variations in the shell of IRC +10216

First observations of the N = 2-1 line of CN at 226.9 GHz are reported. High-resolution maps of IRC+10216 and OMC-1 show that the peak in both sources is compact but resolved in our 30 beam. In IRC+10216 the emission extends approx.70 at the line center velocity. A model for CN excitation and the absence of infrared vibration--rotation absorption lines suggest the presence of a source of CN molecules in the outer circumstellar shell. Examination of chemical, expansion, and photodissociation time scales in the outer shell leads to the identification of HCN photodissociation by the interstellar radiation field as a plausible source. This model explains the large discrepancy in the CN abundance predicted by chemical equilibrium models of the shell and confirms the importance of photodissociation in the chemistry of circumstellar shells. Maps and spectra were also made in OMC-1 and OMC-2. In OMC-1, CN emission lies along a narrow ridge, peaking in the BN/KL region.

SO and SO2 in mass-loss envelopes of red giants: probes of nonequilibrium circumstellar chemistry and mass-loss rates.

We have made a search for SO in the mass-loss envelopes of 24 mostly oxygen-rich red giants, using the 8(8)-7(7) (344.3 GHz), 6(5)-5(4) (219.9 GHz), 4(3)-3(2) (138.2 GHz), and 3(2)-2(1) (99.3 GHz) transitions. One or more of these transitions were detected toward 14 envelopes, including those of the supergiants VY CMa and IRC +10420. The SO envelope of IK Tau was measurably extended, and that of IRC +10420 marginally so, allowing us to determine the envelope sizes directly. The SiO (v = 0) J = 8-7, J = 5-4, and J = 3-2 lines at 347.3, 217.1, and 130.3 GHz were detected simultaneously with the 344.3, 219.9, and 138.2 GHz SO lines, respectively. From our SO data, as well as from SO2 data published elsewhere (for a few sources), we derive the circumstellar SO and SO2 abundances and compare them with theoretical predictions. We find that the photospheric abundances of SO and SO2 are insignificant, and that SO and SO2 must be produced in the circumstellar envelopes. However, the (average) SO abundances are larger than even the peak values predicted by nonequilibrium circumstellar chemistry models. The large SO abundances imply that S is not heavily depleted into grains in these envelopes, and further suggest that (i) the stellar mass-loss rates in these objects are substantially larger than accepted values, (ii) the circumstellar H/H2 abundance ratio is quite small, and (iii) the reaction of O and SH dominates over the reaction of S and OH in the production of SO. In the peculiar bipolar nebula OH 231.8+4.2, the combined SO and SO2 abundance exceeds the cosmic S abundance, if the mass-loss rate in this object is smaller than or equal to l0(-4) M solar yr-1. In IK Tau the data can only be fitted with models in which the SO is distributed in a hollow shell, as predicted by the nonequilibrium chemistry models. In another three, which have no measurable OH, the discovery of SO is difficult to explain by such models.

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)

Warm neutral halos around molecular clouds. V - OH (1665 and 1667 MHz) observations

Ten strip maps of 1665- and 1667-MHz OH emission, traversing the outer boundaries of five molecular clouds, were made. The OH emission is found to be significantly extended relative to CO, from which it inferred that OH is to be found in abundance in the shell of partly atomic, partly molecular gas surrounding the dense molecular clouds. The fractional OH abundance is calculated using existing H I and CO observations, and detailed source models which include a complete chemistry network and a radiative transfer code. It is concluded that the extended OH is formed, not by the exothermic reaction of O with H3(+), but by the endothermic reaction, H(+) + O yields H + O(+).