B. E. Turner

DEUTERATED MOLECULES IN TRANSLUCENT AND DARK CLOUDS

We present observations of 10 deuterated molecular species in the dark clouds TMC-1, L183, and the translucent object CB 17, as well as a subset of species in and other objects. With sensitive TMCNH 3 observations of the J \ 1¨0 and 2¨1 transitions of DCN, and DNC, we have been able to derive N 2 D‘, molecular constants that include the complex nuclear quadrupole hyper—ne splitting in these species, which is essential to determine accurate abundances. The spectroscopic results have required, in turn, new radiative transport techniques to handle the hyper—ne eUects. Our abundance determinations also utilize sensitive observations of secondary isotopomers involving 13C, 18O, and 15N. Compared with earlier molecular D/H ratios in the literature, these innovations have resulted in radically diUerent values in some cases in TMC-1 and in TMC-1; DCN/HCN in CB 17), (N 2 D‘/N 2 H‘ TMCNH 3 ;N H 2 D/NH 3 and important modi—cations in others in TMC-1). The new techniques usually produce (C 3 HD/C 3 H 2 deuteration ratios lower than those obtained earlier by simpler methods. Thus, in addition to the special cases noted above, our results are generally lower than previous ones by factors of typically 2. We also —nd that deuteration occurs only in regions of high density, while nondeuterated species generally reside at lower densities. A recently proposed model of translucent clouds as low-density objects containing embedded small, high-density fragments explains the observations. To study the chemistry of deuterated species, we have used the New Standard Model, modi—ed to include all monodeuterated species, and now containing 9930 reactions and 610 species. Our models explore the dependence of the molecular D/H ratios upon temperature, density, ionization rate, extinction, epoch, and elemental abundances. Within the uncertainties, we —nd agreement between observed and modeled ratios for nearly all species in nearly all sources. Our results generally agree with those of Roberts & Millar in a recent, similar study. We —nd signi—cantly higher ratios in L183 than in TMC, and intermediate values in CB 17. With our lower values in general, however, we believe that L183 is ii normal ˇˇ for a cold dark cloud, CB 17 is typical of a slightly warmer translucent object, and the TMC region is perhaps underdeuterated in general, certainly strongly so in the case of and These N 2 H‘ NH 3 . ii anomalous ˇˇ cases have no plausible single explanation in terms of gas-phase chemistry at this time. Grain processes are implicated. . .. . .. . ..

The Physics and Chemistry of Small Translucent Molecular Clouds. VIII. HCN and HNC

We have conducted a survey of HCN and HNC (two rotational transitions each) in our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores whose structures and chemistry have been studied earlier in this series. Both species are seen in all 38 objects. HCNH+ has been searched in three objects. These results are modeled in terms of our previous hydrostatic equilibrium and n ~ r-α structures together with other chemical and physical properties derived earlier. A detailed program has been written to handle the complex radiative transfer of the hyperfine splitting (hfs) of HCN. It is shown that serious errors are made in deriving HCN abundances by methods that ignore the hfs. Both HCN and HNC abundances are high, typically 1(-8) in most sources. The chemically important ratio HCN/HNC is found to be ~2.5 if these species are spatially centrally peaked and ~6 if not. Both species abundances increase monotonically with increasing extinction in the 1.2-2.7 mag range (edge to center), thus displaying the same characteristic transition between diffuse and dense cloud chemistry as do most other species we have studied. HCN/HNC decreases with increasing extinction to a value of 1.3 at Av0 ~ 10, approaching the expected value of 1.0 for dense clouds. Two types of ion-molecule chemistry models have been carried out: a full model using the Standard Model rate file and comprising 409 species (by Lee and Herbst), and a simplified model comprising 21 nitrogen-bearing species for conditions relevant to translucent clouds. Good agreement between observations and chemistry models is achieved throughout the translucent extinction range. Important conclusions are that (1) neutral-neutral reactions such as N + CH2 dominate the chemistry of HCN; (2) low ion-polar reaction rates are strongly favored over high ones; (3) the reaction C+ + NH3 → H2NC+ → HNC is unimportant, thus largely uncoupling the CN and NH chemistries; (4) the ratio HCN/HNC is not a particularly important diagnostic of the CN chemistry; (5) model NH3 abundances are at least a factor 100 lower than observed in translucent clouds, even if the reaction N+H+3→NH+2 is permitted at Langevin rate.

Microwave Detection of Interstellar Vinyl Alcohol, CH2=CHOH

Vinyl alcohol (CH2=CHOH) has been detected in emission toward Sagittarius B2N by means of its millimeter-wave rotational transitions. The simplest enol, vinyl alcohol is an important intermediate in many organic chemistry reactions. All three stable isomers of the C2H4O family, vinyl alcohol, ethylene oxide (c-C2H4O), and acetaldehyde (CH3–CHO) have now been identified in the interstellar medium, as have the three members of the C2H4O2 isomeric group (Hollis, Lovas, and Jewell). These complex species cannot be produced in detected amounts by quiescent gas-phase chemistry models, and grain processes have long been envisioned. Our analysis of the abundances of the six species suggests that evaporation from grains of copious amounts of less complex species such as CH3OH and H2CO, followed by gas-phase reactions among the evaporated species, explain the high abundance of four of the species, while isomerization (on grains) of the less stable glycol aldehyde and vinyl alcohol species accounts for their lower abundances. The role of catalysis on grains remains unclear.

A SPECTRAL LINE SURVEY OF SELECTED 3 MILLIMETER BANDS TOWARD SAGITTARIUS B2(N-LMH) USING THE NATIONAL RADIO ASTRONOMY OBSERVATORY 12 METER RADIO TELESCOPE AND THE BERKELEY-ILLINOIS-MARYLAND ASSOCIATION ARRAY. I. THE OBSERVATIONAL DATA

We have initiated a spectral line survey, at a wavelength of 3 millimeters, toward the hot molecular core Sagittarius B2(N-LMH). This is the first spectral line survey of the Sgr B2(N) region utilizing data from both an interferometer (BIMA Array) and a single-element radio telescope (NRAO 12 meter). In this survey, covering 3.6 GHz in bandwidth, we detected 218 lines (97 identified molecular transitions, 1 recombination line, and 120 unidentified transitions). This yields a spectral line density (lines per 100 MHz) of 6.06, which is much larger than any previous 3 mm line survey. We also present maps from the BIMA Array that indicate that most highly saturated species (3 or more H atoms) are products of grain chemistry or warm gas phase chemistry. Due to the nature of this survey we are able to probe each spectral line on multiple spatial scales, yielding information that could not be obtained by either instrument alone.We have initiated a spectral line survey, at a wavelength of 3 mm, toward the hot molecular core Sgr B2(N-LMH). This is the first spectral line survey of the Sgr B2(N) region utilizing data from both an interferometer (Berkeley-Illinois-Maryland Association [BIMA] array) and a single-element radio telescope (NRAO 12 m). In this survey, covering 3.6 GHz in bandwidth, we detected 218 lines (97 identified molecular transitions, one recombination line, and 120 unidentified transitions). This yields a spectral line density (lines per 100 MHz) of 6.06, which is much larger than any previous 3 mm line survey. We also present maps from the BIMA array that indicate that most highly saturated species (three or more H atoms) are products of grain chemistry or warm gas-phase chemistry. Because of the nature of this survey, we are able to probe each spectral line on multiple spatial scales, yielding information that could not be obtained by either instrument alone.

A Common Gas-Phase Chemistry for Diffuse, Translucent, and Dense Clouds?

This paper consolidates and extends the results of a large observational and chemical modeling study of translucent clouds. Thirty-eight molecular species have been observed in an ensemble of 38 translucent objects, for which detailed self-consistent physical models have been constructed. These are used with microturbulent radiative transfer analyses to obtain reliable fractional abundances. The abundances are essentially the same in translucent clouds and cold dense clouds (TMC-1). We have also scaled the column densities of the 12 species studied by Liszt & Lucas in diffuse clouds by adsorption against point-continuum sources to yield fractional abundances. These in turn are found to be remarkably similar to those in translucent and dense quiescent clouds. Thus a global abundance pattern emerges, which holds over a range of hundredfold in density, and transcends the completely photon-dominated chemistry regime to the collision-dominated regime. We have developed comprehensive quiescent gas-phase chemical models, based on the New Standard Model (NSM) reaction data set. We have found a single set of parameters (elemental abundances, depletion factors, and other physical conditions) that can explain the abundances of 34 of the 38 species observed in translucent clouds, spanning the transition region 1 ≤ Av0 ≤ 5 mag. The NSM is insensitive to density in the translucent region. The similarity of abundances in translucent and dense clouds (TMC-1) is explained by the same chemical model with a slightly higher depletion factor for C and O. To explain the diffuse cloud abundances, we need to augment the NSM reaction set of 4,300 reactions with just three turbulence-driven, weakly endothermic reactions as analyzed by Spaans. These latter reactions lose effect as the density increases from the diffuse to the translucent regime, in just such a way that the diffuse-cloud abundances seamlessly join with the translucent abundances, thus replicating the constancy of abundances from diffuse conditions (200 cm-3) to dense cloud conditions (20,000 cm-3). It seems clear that purely low-temperature gas-phase chemistry can explain many more observations than previously recognized.

The Physics and Chemistry of Small Translucent Molecular Clouds. XI. Methanol

We have made a survey of CH3OH in 27 of our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores whose structures and chemistry have been studied earlier in this series. CH3OH is detected in 17 objects, favoring those with larger extinctions. The mean fractional abundance is 1(-8), but if the four highest abundance objects are omitted, the mean abundance is 3(-9), the same as in two cold dark clouds. Collision rates remain poorly known for CH3OH, but uncertainties in propensity rules are shown not to affect abundances more than 10%. The geometric component of the rates is uncertain by a factor of 2; hence, also, the abundances. The gas-phase chemistry is particularly simple, formation occurring only via the radiative association reaction CH -->+3+H -->2O?CH -->3OH -->+2+h? followed by electron recombination. We have verified the predictions of this simple model by using the full Standard Model of over 3000 reactions, with conditions suitable for translucent clouds. These gas-phase models predict abundances 4 orders of magnitude less than the observed abundances. We have examined grain surface chemistry in which accreted CO hydrogenates to CH3OH on the surface under the action of UV or cosmic rays and then desorbs in various ways, photodesorption dominating. Despite the uncertainties of the grain processes, they can easily explain the observed abundances and in fact imply much lower desorption efficiencies than are usually adopted. Methanol is of intermediate complexity, as are several other species we will study in the next papers, with the goal of testing the boundary between gas-phase and grain chemistry, the latter believed to be important for the most complex species.

Detection of Interstellar Acetone toward the Orion-KL Hot Core

We present the first detection of interstellar acetone [(CH3)2CO] toward the high-mass star-forming region Orion-KL and the first detection of vibrationally excited (CH3)2CO in the interstellar medium (ISM). Using the BIMA array, 28 emission features that can be assigned to 54 acetone transitions were detected. Furthermore, 37 of these transitions have not been previously observed in the ISM. The observations also show that the acetone emission is concentrated toward the hot core region of Orion-KL, contrary to the distribution of other large oxygen-bearing molecules. From our rotational temperature diagram, we find a beam-averaged (CH3)2CO column density of [2.0(0.3)-8.0(1.2)] × 1016 cm-2 and a rotational temperature of 176(48)-194(66) K.

Detection of interstellar H3O(+) - A confirming line

Emission at the frequency of the (J, K) = 3, 2 → 2, 2 line of para-H 3 O + at 365 GHz has been detected in OMC-1 and Sgr B2. Two lines of this key ion in the manufacture of water lie at frequencies accessible to Earth-bound telescopes; both have now been detected. Mapping shows that the emission extends over ∼1 pc in Sgr B2 but lies concentrated to the core (∼0.07 pc) in OMC-1. The source of the emission is identified as H 3 O + and the abundance X(H 3 O + ) found. Comparison with water column densities suggests that 1500 < [H 2 O]/[H 3 O + ] < 6000

The Physics and Chemistry of Small Translucent Molecular Clouds. X. SiO

We have made a survey of SiO in 29 of our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores the structures and chemistry of which have been studied earlier in this series. SiO is detected in six objects, favoring those with large products of the local radiation field IUV and the column density. The fractional abundance of SiO is 1 × 10-10 in these six objects, and 1 × 10-11 over the remaining 23 searched objects for which an average of the observations yielded a detection. These SiO abundances are intermediate between those found in energetic star-forming regions (~1 × 10-8) and upper limits of ~2 × 10-12 found in cold dense clouds. We have analyzed the gas-phase chemistry of SiO and find that the temperature-dependent reactions Si + (OH, O2) → SiO cannot explain the strong dependence of SiO abundance upon cloud temperature, contrary to the conclusion of Langer & Glassgold. We have analyzed the SiO abundance characteristics in terms of accretion on and photodesorption from grains. Cosmic-ray desorption is found to be negligible. The same analysis applied to many other species studied in the translucent cloud series points to a uniquely strong binding of Si atoms onto grains, unlike the situation for other elements (C, N, O, S). We find this by determining a photodesorption yield factor for each molecular species which is consistent with the observations of the many species in both translucent and cold dense clouds, which agrees with experimental values for C, N, O, and S species, and which is much smaller for Si than for the other elements. Special binding mechanisms for Si onto grains are discussed.

Detection of 13C Isotopomers of the Molecule HC7N

The