C. A. Gottlieb

Laboratory and Astronomical Identification of the Negative Molecular Ion C6H

The negative molecular ion C6H- has been detected in the radio band in the laboratory and has been identified in the molecular envelope of IRC +10216 and in the dense molecular cloud TMC-1. The spectroscopic constants derived from laboratory measurements of 17 rotational lines between 8 and 187 GHz are identical to those derived from the astronomical data, establishing unambiguously that C6H- is the carrier of the series of lines with rotational constant 1377 MHz first observed by K. Kawaguchi et al. in IRC +10216. The column density of C6H- toward both sources is 1%-5% that of neutral C6H. These surprisingly high abundances for a negative ion imply that if other molecular anions are similarly abundant with respect to their neutral counterparts, they may be detectable both in the laboratory at high resolution and in interstellar molecular clouds.

Laboratory and astronomical identification of cyclopropenylidene, C3H2

Twenty-seven rotational lines of C/sub 3/H/sub 2/ have been identified in the laboratory or in astronomical sources, and the rotational and centrifugal distortion constants of this previously unobserved carbene ring determined to high accuracy. The assigned astronomical transitions include the strong, ubiquitous interstellar lines at 85338 MHz and 18343 MHz, which are the lowest lying transitions of ortho C/sub 3/H/sub 2/:2/sub 12/ ..-->.. 1/sub 01/ and 1/sub 10/ ..-->.. 1/sub 01/, respectively. Interstellar C/sub 3/H/sub 2/ can be rapidly formed by dissociative recombination of the very stable ion C/sub 3/H/sup +//sub 3/, which in turn can be produced from acetylene in only two steps. In standard molecular sources such as Ori A and Sgr B2, C/sub 3/H/sub 2/ is only moderately abundant, but in diffuse molecular clouds it may be one of the most abundant molecules. There is some radio spectroscopic evidence for two related molecules in Sgr B2 or TMC-1: ethynylmethylene HCCCH, a hypothetical carbon chain isomer, and cyclopropene, C/sub 3/H/sub 4/, a known, stable three-membered ring.

Laboratory and Astronomical Detection of the Negative Molecular Ion C3N

The negative molecular ion C3N− has been detected at millimeter wavelengths in a low-pressure laboratory discharge, and then with frequencies derived from the laboratory data in the molecular envelope of IRC+10216. Spectroscopic constants derived from laboratory measurements of 12 transitions between 97 and 378 GHz allow the rotational spectrum to be calculated well into the submillimeter-wave band to 0.03 km s−1 or better in equivalent radial velocity. Four transitions of C3N− were detected in IRC+10216 with the IRAM 30 m telescope at precisely the frequencies calculated from the laboratory measurements. The column density of C3N− is 0.5% that of C3N, or approximately 20 times greater than that of C4H− relative to C4H. The C3N− abundance in IRC+10216 is compared with a chemical model calculation by Petrie & Herbst. An upper limit in TMC-1 for C3N− relative to C3N (<0.8%) and a limit for C4H− relative to C4H (<0.004%) that is 5 times lower than that found in IRC+10216, were obtained from observations with the NRAO 100 m Green Bank Telescope (GBT). The fairly high concentration of C3N− achieved in the laboratory implies that other molecular anions containing the CN group may be within reach.

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.

Detection of the carbon chain negative ion C8H- in TMC-1

The negative molecular ion C8H- has been detected in the Galactic molecular source TMC-1. Four rotational transitions have been observed in the centimeter-wave band with the NRAO 100 m Green Bank Telescope (GBT) at precisely the frequencies calculated from the recent laboratory spectroscopy of this large carbon chain anion. C8H- is about 5% as abundant as C8H, or somewhat more than C6H- relative to C6H (1.6%). Improved values of the column densities of C6H- and C6H, and an upper limit for the abundance of the smaller carbon chain C4H- of 0.014% with respect to C4H, have also been determined.

Astonomical and laboratory detection of the SiC radical

Laboratory and space observations of a number of mm-wave rotation lines in the previously unobserved 3Pi electronic ground state of the SiC radical are discussed. Laboratory-derived frequencies, accurate to better than 0.1 ppm, are used to obtain a highly precise determination of the fine structure, rotational, centrifugal distortion, and Lambda-doubling constants of the SiC ground state. It is found that SiC is appreciably extended toward IRC+10216, with a diameter of at least 54 arcsec. 11 refs.

Astronomical identification of CN-, the smallest observed molecular anion

We present the first astronomical detection of a diatomic negative ion, the cyanide anion CN-, as well as quantum mechanical calculations of the excitation of this anion through collisions with para-H2. CN- is identified through the observation of the J = 2-1 and J = 3-2 rotational transitions in the C-star envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles indicate that CN-, like the large anion C6H-, is formed in the outer regions of the envelope. Chemical and excitation model calculations suggest that this species forms from the reaction of large carbon anions with N atoms, rather than from the radiative attachment of an electron to CN, as is the case for large molecular anions. The unexpectedly large abundance derived for CN-, 0.25 % relative to CN, makes likely its detection in other astronomical sources. A parallel search for the small anion C2H- remains so far unconclusive, despite the previous tentative identification of the J = 1-0 rotational transition. The abundance of C2H- in IRC +10216 is found to be vanishingly small, < 0.0014 % relative to C2H.

Eight New Carbon Chain Molecules

A summary is given of the laboratory measurements of the rotational spectra of eight new carbon chains: the cyanopolyynes HC11N and HC13N; the carbon chain radicals C7H, C8H, C9H, and C11H; and the cumulene carbenes H2C5 and H2C6. Measured line frequencies and derived spectroscopic constants are listed for all eight, and rest frequencies for the astronomically important transitions that are not easily calculated are tabulated. With our laboratory measurements, four of the new carbon chains have already been detected in at least one astronomical source; the remaining four should be detectable with existing large centimeter- or millimeter-wave telescopes.

Astronomical detection of H2CCC

H2CCC, an isomer of the widely distributed interstellar ring C3H2, has been detected in TMC-1 and possibly IRC + 10216 with the IRAM 30 m telescope, following a recent laboratory determination of the rotational spectrum of this new type of highly polar carbon chain. The rotational temperature of H2CCC in TMC-1, like that of other highly polar molecules in this source, is very low: 4-6 K; the column density is also fairly low: (2.5 + or - 0.5) x 10 to the 12th/sq cm, slightly more than 1 percent that of the cyclic isomer. 16 refs.

Laboratory detection of the C3N an C4H free radicals

The millimeter-wave spectra of the linear carbon chain free radicals C3N and C4H, first identified in IRC + 10216 and hitherto observed only in a few astronomical sources, have been detected with a Zeeman-modulated spectrometer in laboratory glow discharges through low pressure flowing mixtures of N2 + HC3N and He + HCCH, respectively. Four successive rotational transitions between 168 and 198 GHz have been measured for C3N, and five rotational transitions between 143 and 200 GHz for C4H; each is a well-resolved spin doublet owing to the unpaired electron present in both species. Precise values for the rotational, centrifugal distortion, and spin doubling constants have been obtained, which, with hyperfine constants derived from observations of the lower rotational transitions in the astronomical source TMC 1, allow all the rotational transitions of C3N and C4H at frequencies less than 300 GHz to be calculated to an absolute accuracy exceeding 1 ppm.