M. J. Travers

Detection of HC[TINF]11[/TINF]N in the Cold Dust Cloud TMC-1

Two consecutive rotational transitions of the long cyanopolyyne HC11N, J=39-38, and J=38-37, have been detected in the cold dust cloud TMC-1 at the frequencies expected from recent laboratory measurements by Travers et al. (1996), and at about the expected intensities. The astronomical lines have a mean radial velocity of 5.8(1) km/s, in good agreement with the shorter cyanopolyynes HC7N and HC9N observed in this very sharp-lined source [5.82(5) and 5.83(5) km/s, respectively]. The column density of HC11N is calculated to be 2.8x10^(11) cm^(-2). The abundance of the cyanopolyynes decreases smoothly with length to HC11N, the decrement from one to the next being about 6 for the longer carbon chains.

Microwave Spectra of 11 Polyyne Carbon Chains

A summary is given of the laboratory study of the rotational spectra of 11 recently detected carbon chain molecules. Classified according to their various end groups, these are the cyanopolyynes HC15N and HC17N; the isocyanopolyynes HC4NC and HC6NC; the methylcyanopolyynes CH3(C≡C)3CN, CH3(C≡C)4CN, and CH3(C≡C)5CN; and the methylpolyynes CH3(C≡C)4H, CH3(C≡C)5H, CH3(C≡C)6H, and CH3(C≡C)7H. Measured line frequencies and derived spectroscopic constants are given for each. The microwave laboratory astrophysics of the entire set is now complete in the sense that all the astronomically most interesting rotational transitions, including those with nitrogen quadrupole hyperfine structure, have now been directly measured or can be calculated from the derived constants to a small fraction of 1 km s-1 in equivalent radial velocity. All 11 carbon chains are candidates for astronomical discovery since they are closely related in structure and composition to ones that have already been discovered in space.

Observations of Long CnH Molecules in the Dust Cloud TMC-1

We report detection of the J = 15.5-14.5 and 17.5-16.5 transitions of C8H, a sensitive upper limit for the J=10.5-9.5 transition of C7H, and measurements of two low-frequency transitions of C6H in TMC-1. These results give new information on the relative abundances of long carbon-chain radicals in TMC-1 and confirm the recent laboratory-measured hyperfine-splitting constants of C6H. We compare our results with the recent early-time, gas-phase chemistry model of Herbst and Terzieva and with that of Millar and coworkers. We find the abundance ratios in the longer CnH chains decline much more rapidly than found for the longer HCnN chains. Although the decrease in fractional abundance with increasing chain length from C4H to C8H is reasonably well reproduced by the models of Herbst and Terzieva, C8H is observationally somewhat underabundant compared to these calculations. Although we searched for the J = 7.5-6.5 and J = 10.5-9.5 transitions of C7H, nearby interference in the case of the higher transition and possible confusion with nearby U-lines prevent us from claiming a detection. However, we are able to report an upper limit to the abundance of C7H in TMC-1 that is at least a factor of 20 below that of C5H. The steep fall-off in abundance for the longer CnH molecules, also found in IRC+10216 by Guelin and coworkers, suggests that the likelihood is small that a large fraction of carbon is locked up in long CnH chains in dense dark clouds like TMC-1. If long CnH chains are not present in high abundance in either dark clouds or the envelopes of carbon stars, the possibility that they are present in abundance in the diffuse gas also appears less likely. The cyanopolyynes, which have been detected at relatively high abundances to and including HC11N, may then be the most abundant carbon-chain molecules in the diffuse gas. These observations and our earlier observations of HC9N in TMC-1 have allowed us to estimate the line density at a level of T*A~1 mK in this dust cloud to be 0.9 lines MHz-1.

New carbon chains in the laboratory and in interstellar space

Twenty seven new carbon-chain molecules have been detected over the past two years with a Fourier-transform microwave (FTM) spectrometer, 13 of which are reported here for the first time. Of the 27, 11 are closed shell polyynes, 9 are free radicals, 2 are cumulene carbenes, and 5 are carbenes formed by substituting carbon chains for one of the hydrogen atoms of the three member carbene ring C3H2. All the astronomically interesting rotational transitions [including hyperfine structure (hfs) for the radicals] of the entire set have either been measured to high precision, or are readily calculated to comparable accuracy from the spectroscopic constants derived from the laboratory data. On the basis of this data, 4 of the chains (C7H, C8H, H2C6, and HC11N) have already been detected in astronomical sources and, with large radio telescopes under construction or the discovery of better astronomical sources, it is possible that nearly all can be found. Astronomical detection is aided by the apparent high polarity of all unsymmetrical chains. The sensitivity of the present liquid-nitrogen-cooled spectrometer is far from fundamental limits; an increase by one to two orders or magnitude is possible with liquid helium cooling and other refinements, and better precursor gases may be found. Many of the chains here probably have low-lying isomers, and ionized and radical variations, which may be detected by the present techniques. Carbon chains are hard to stretch but easy to bend: centrifugal distortion is well described by a classical model according to which all chains distort under rotation like classical thin rods with the same Youngs modulus: E=1.7×1013 dyn cm-2, larger than that of diamond; the longest two chains we have detected, HC15N and HC17N, have low frequency bending vibrations which lie within the range of existing high altitude radio telescopes. Finally, it is pointed out that the density of the chains at the limit of detection in our spectrometer, ca. 108 cm-3, is high by the standards of modern laser spectroscopy, so that optical detection of many should be possible.

Rotational spectra of the carbon chain free radicals C10H, C12H, C13H, and C14H

The four carbon chain radicals C10H, C12H, C13H, and C14H have been observed in a pulsed supersonic molecular beam with a Fourier transform microwave spectrometer. The radicals were produced in a discharge through a dilute diacetylene/neon mixture in the throat of a supersonic nozzle. All are linear with 2Π electronic ground states, and all except C14H have resolved lambda-type doubling. For each species at least ten rotational transitions, between 6 and 16 GHz, were measured in the lowest spin component, which is 2Π3/2 for C10H, C12H, and C14H, and 2Π1/2 for C13H. Only three spectroscopic constants in the standard Hamiltonian for a molecule in a 2Π state were required to reproduce the spectra to a few parts in 107: an effective rotational constant, a centrifugal distortion constant, and a lambda-type doubling constant. All of the chains here have abundances in the most intense part of the supersonic molecular beam of ⩾5×109 per gas pulse, which suggests that optical transitions of all four may be detecta...

THE ISOCYANOPOLYYNES HC4NC AND HC6NC : MICROWAVE SPECTRA AND AB INITIO CALCULATIONS

Rotational spectra of HC4NC and HC6NC, linear molecules of interest to interstellar cloud chemistry, were recorded by Fourier transform microwave spectroscopy, and the ground state rotational constants were determined to be 1401.18227(7) and 582.5203(1) MHz. Nitrogen quadrupole hyperfine structure could be observed for HC4NC. On the basis of coupled cluster calculations including connected triple substitutions accurate equilibrium structures (uncertainty in bond lengths ca. 0.0005 A) could be established for both species. The equilibrium dipole moments, predicted to be −3.25 and −3.49 D for HC4NC and HC6NC, respectively, exhibit large correlation effects of 30% and 33%.

The Excitation Temperatures of HC9N and Other Long Cyanopolyynes in TMC-1

We report observations of seven different transitions of HC9N between 9.8 and 23.3 GHz in the cold, dark cloud TMC-1. Because of its lower rotational constant, HC9N is expected to yield a rotational temperature closer to the kinetic temperature of the gas than the shorter cyanopolyynes. Assuming that HC9N in TMC-1 is optically thin, we obtain a rotational temperature Trot = 7.5-8.7 K and a corresponding column density of NL(HC9N) = (5.4-2.3) × 1012 cm-2, depending on the source size L assumed. The rate of radiative cooling is less efficient in the longer cyanopolyyne chains and is approximately proportional to (n + 1)-6, where n is the number of carbon atoms in the chain. In steady state this is balanced by the collisional rate of excitation. The rotational temperatures of the cyanopolyynes are found to increase with the number of heavy atoms over the range HC5N-HC9N, in reasonable agreement with calculations. Concomitantly, we find evidence that the longer cyanopolyynes are located in regions that become progressively smaller with chain length. We also report newly measured values for the rotational and centrifugal distortion constants of HC9N that improve the accuracy of the calculated millimeter wave transitions.

Laboratory Detection of the Ring-Chain Carbenes HC4N and HC6N

The highly polar ring-chain carbenes HC4N and HC6N, formed by substituting either CN or CCCN for a hydrogen atom in cyclopropenylidene (c-C3H2), were detected in a supersonic molecular beam with a Fourier transform microwave spectrometer. Seven a- and four b-type rotational transitions of HC4N and 11 a-type transitions of HC6N, each with resolved nitrogen nuclear quadrupole hyperfine structure, were measured between 6 and 21 GHz, yielding precise values for the three rotational constants, the leading centrifugal distortion constants, and the quadrupole coupling constants. Like the hydrocarbon carbenes C5H2, C7H2, and C9H2, both new molecules have a planar ring-chain structures and singlet electronic ground states. The strongest lines of HC4N can be detected with a signal-to-noise ratio exceeding 10 in a total integration time of less than 1 s, but the lines of HC6N were nearly 100 times weaker.

Laboratory Detection of the C9H Radical

The linear carbon chain radical C9H in its 2Π1/2 electronic ground state has been detected in a Fourier transform microwave spectrometer with a supersonic pulsed-jet discharge nozzle. The frequencies of 44 rotational transitions between 5.4 and 16.1 GHz have been measured to a few parts in 107, from which precise values for the rotational, centrifugal distortion, lambda doubling, and hyperfine constants have been determined. These allow calculation of the entire radio spectrum of the ground state of C9H to a fraction of 1 km s-1 in equivalent radial velocity.

Laboratory Detection of the C7H Radical

The linear carbon chain radical C7H has been detected in the same acetylene-argon discharge in which C8H was recently observed. Fifty rotational transitions, 23 in the ground 2Π1/2 state and 27 in the 2Π3/2 state (37 K higher in energy), were measured. Precise values for the rotational, fine structure, and lambda-doubling constants have been derived which allow calculation of the entire microwave and millimeter-wave spectrum to an accuracy of 0.2 km s-1 or better. With the recent discovery of C8H in space, C7H is an excellent candidate for astronomical detection. To aid future observations, the transition frequencies for the most interesting astronomical lines are tabulated.