M. Guelin

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.

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.

Three new ''nonterrestrial'' molecules

Eight new interstellar lines have been detected from three molecules not previously observed spectroscopically in space or in the laboratory. One is a linear or nearly linear molecule with microwave constants B/sub O/ = 21,337.15 +- 0.06 MHz, D/sub O/ = 21.4 +- 1.5 kHz. This is the thioformyl ion HCS/sup +/, first identified because B/sub O/ and D/sub O/ are close to those calculated, and now confirmed by laboratory detection of one of our lines (Gudeman et al.). The second molecule, also linear or nearly so, has microwave constants B/sub O/ = 10,691.406 +- 0.043 MHz, D/sub O/ = 1.84 +- 0.91 kHz close to those expected for the isoelectronic systems HOCO/sup +/ and HOCN; a choice between the two cannot be made on the basis of the available astronomical data. The existence of a third molecule is deduced from an unidentified line at 85,338 MHz that has been found in many sources, is fairly intense in several, and may be self-absorbed in Sgr B2. No hfs or harmonically related lines to reveal the identity of this molecule have been detected.

Detection of C5N− and Vibrationally Excited C6H in IRC +10216*

We report the detection in the envelope of the C-rich star IRC +10216 of four series of lines with harmonically related frequencies: B1389, B1390, B1394 and B1401. The four series must arise from linear molecules with mass and size close to those of C6H and C5N. Three of the series have half-integer rotational quantum numbers; we assign them to the 2Delta and 2Sigma vibronic states of C6H in its lowest (v_11) bending mode. The fourth series, B1389, has integer J with no evidence of fine or hyperfine structure; it has a rotational constant of 1388.860(2) MHz and a centrifugal distortion constant of 33(1) Hz; it is almost certainly the C5N- anion.

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.

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.

Detection of MgCn in IRC + 10216: A new metal-bearing free radical

A new metal-containing molecule, MgCN, has been detected toward the late-type star IRC + 10216, using the NRAO 12 m and IRAM 30 m telescopes. The N = 11 approaches 10, 10 approaches 9, and 9 approaches 8 transtions of this species which has a (sup 2)Sigma(sup +) ground state, have been observed in the outer envelope of this object at 3 mm. For the N = 11 approaches 10 transitions, the two spin-rotation components are clearly resolved and conclusively identify this new radical. These measurements imply a column of density for MgCN of N(sub tot) approximately 10(exp 12)/sq cm in the outer shell, which corresponds to a fractional abundance of f approximately 7x10(exp -10). This molecule, the metastable isomer of MgNC, is the third metal-bearing species thus far identified in the outer shell of IRC + 10216, and its detection implies a ratio of MgNC/MgCN approximately 22/1. MgCN may be formed through a reaction scheme involving magnesium and HNC or CN, both prominent outer shell molecules, or through synthesis on grains.

Molecular Gas in the Andromeda Galaxy

M 31, the closest large spiral galaxy to our own, is the best object for studying molecular clouds and their relation to the spiral structure. As one of the astronomical objects with the best known distance (0.78 ± 0.02 Mpc), it is also one of the best places where to estimate molecular clouds masses through the Virial Theorem.

Search for anions in molecular sources : C4H-detection in L1527

Based on observations carried out with the IRAM 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).-- Accepted for publication in Astronomy & Astrophysics Letters.

Molecules at z = 0.89 - A 4-mm-rest-frame absorption-line survey toward PKS 1830−211

We present the results of a 7 mm spectral survey of molecular absorption lines originating in the disk of a z = 0.89 spiral galaxy located in front of the quasar PKS 1830-211. Our survey was performed with the Australia Telescope Compact Array and covers the frequency interval 30-50 GHz, corresponding to the rest-frame frequency interval 57-94 GHz. A total of 28 different species, plus 8 isotopic variants, were detected toward the south-west absorption region, located about 2 kpc from the center of the z = 0.89 galaxy, which therefore has the largest number of detected molecular species of any extragalactic object so far. The results of our rotation diagram analysis show that the rotation temperatures are close to the cosmic microwave background temperature of 5.14 K that we expect to measure at z = 0.89, whereas the kinetic temperature is one order of magnitude higher, indicating that the gas is subthermally excited. The molecular fractional abundances are found to be in-between those in typical Galactic diffuse and translucent clouds, and clearly deviate from those observed in the dark cloud TMC 1 or in the Galactic center giant molecular cloud Sgr B2. The isotopic ratios of carbon, nitrogen, oxygen, and silicon deviate significantly from the solar values, which can be linked to the young age of the z = 0.89 galaxy and a release of nucleosynthesis products dominated by massive stars. Toward the north-east absorption region, where the extinction and column density of gas is roughly one order of magnitude lower than toward the SW absorption region, only a handful of molecules are detected. Their relative abundances are comparable to those in Galactic diffuse clouds. We also report the discovery of several new absorption components, with velocities spanning between -300 and +170 km s(-1). Finally, the line centroids of several species (e. g., CH3OH, NH3) are found to be significantly offset from the average velocity. If caused by a variation in the proton-to-electron mass ratio mu with redshift, these offsets yield an upper limit vertical bar Delta mu/mu vertical bar \textless 4x10(-6), which takes into account the kinematical noise produced by the velocity dispersion measured from a large number of molecular species.