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Brücken schlagen: Durch Kombination von hochauflösender Molekülspektroskopie im cm‐ und mm‐Wellenbereich mit quantenchemischen Coupled‐Cluster‐Rechnungen auf hohem Theorieniveau wurde die Struktur von HPSi ermittelt. Seine verbrückte Struktur (siehe Bild; berechnete Werte in Klammern) unterscheidet sich in bemerkenswerter Weise von den linearen Strukturen der C‐ und N‐Analoga wie HCN/HNC und HNSi.

A spectral-line survey of IRC+10216 in the 345 GHz band has been undertaken with the Submillimeter Array. Although not yet completed, it has already yielded a fairly large sample of narrow molecular emission lines with line widths indicating expansion velocities of [?] 4 km s-1, less than three times the well known value of the terminal expansion velocity (14.5 km s-1) of the outer envelope. Five of these narrow lines have now been identified as rotational transitions in vibrationally excited states of previously detected molecules: the v = 1, J = 17-16, and J = 19-18 lines of Si34S and 29SiS and the v = 2, J = 7-6 line of CS. Maps of these lines show that the emission is confined to a region within [?] 60 AU of the star, indicating that the narrow-line emission is probing the region of dust formation where the stellar wind is still being accelerated.

Five rotational lines of the C$_8$H$^-$ion were observed in the circumstellar envelope of IRC $+$10216 with the Nobeyama 45 m telescope. An improved value of the column density of the C$_8$H radical yielded a [C$_8$H$^-$]$/$[C$_8$H] ratio of 37%—nearly 4 times larger than the [C$_6$H$^-$]$/$[C$_6$H] ratio (8.6%) and nearly 1500 times the [C$_4$H$^-$]$/$[C$_4$H] ratio (0.024%), which may indicate more efficient formation of longer carbon chain anions. The excitation temperature of C$_8$H$^-$(16$\;\pm\;$2 K) derived here is somewhat lower than that of the two smaller anions C$_6$H$^-$(32 K) and C$_4$H$^-$(23 K) in IRC $+$10216.

The effectiveness of three molecular hydrogen gas tracers--the CH 2P1/2, J = 1/2, F = 1-1 transition, the CO(1-0) transition, and the color excess, E(B-V)--is evaluated for the translucent molecular cloud MBM 16. The CH emission line and E(B-V) are well correlated with each other but not with the CO(1-0) emission line. It is likely that in translucent molecular clouds CO is not a linear tracer of the molecular hydrogen column density because, in these objects, the CO/H2 ratio is undergoing rapid fluctuations as a function of the molecular hydrogen column density. For translucent and diffuse molecular clouds, and possibly for the translucent envelopes of giant molecular clouds, CH observations should be used to calibrate the CO to H2 conversion factor.

The interstellar gas and the gas ejected into space by cool stars are now known to contain a rich collection of molecules. Most of these are organic compounds, some of which are familiar to the terrestrial chemist and can be found in a standard chemical stockroom, but many are entirely new molecules which have been detected and identified in space for the first time. Most of the nearly 100 polyatomic molecules so far detected in astronomical sources by radio telescopes are highly unsaturated carbon chains, a structure which is explosively unstable at even moderate density, and therefore difficult to study spectroscopically on Earth. For this reason laboratory detection has until quite recently lagged well behind the astronomical discovery of many carbon chains. The application of Fourier transform microwave and laser cavity ringdown absorption spectroscopy to supersonic molecular beams has largely overcome this difficulty, yielding in this laboratory during the past four years more than 75 new reactive molecules, including long carbon chains, chains attached to rings, silicon–carbon rings, and several protonated molecular ions. All or nearly all of these are candidates for astronomical discovery because they are highly polar and are close in structure and composition to molecules which have already been discovered in space. Seven have already been found with large radio telescopes, six as a result of the laboratory work described here. These discoveries include the largest interstellar molecule HC₁₁N, the largest ring, SiC₃, and the largest positive ion, HC₃NH⁺. One of our most interesting recent laboratory discoveries is the laser detection of a strong molecular band at λ4429, the wavelength of the strongest and best known diffuse interstellar band.

To determine the distribution of molecular gas at high Galactic latitudes, nearly one-half of the southern Galactic hemisphere at b [?] -30deg was surveyed in the CO (1-0) transition at 115 GHz. The sampling was done on a locally Cartesian grid with 1deg (true angle) spacing in Galactic longitude and latitude. Of the 11,478 points in the grid, the 4982 that rise above an elevation of 30deg in Cambridge, Massachusetts, the site of the 1.2 m millimeter-wave telescope used for the survey, were observed to an rms of Timg1.gif ~ 0.1 K. We found 144 distinct CO (1-0) emission lines along 133 lines of sight. Of these detections, 58 are new and 75 are associated with 26 previously cataloged high-latitude molecular clouds situated within the survey boundaries. The surface filling factor of molecular gas is 0.03, a factor of 10 greater than that found in the northern Galactic hemisphere at b [?] +30deg. On the assumption that the CO/H2 ratio [N(H2)/W(CO)] is given by 1.0 x 1020 cm-2 (K km s-1)-1, the mass surface density of molecular gas in the region of the survey is 0.09 M pc-2. The Gaussian scale height of the 133 detections is ~100 pc, consistent with that determined for lower latitude dark clouds.

The conversion factor, XCO, between the velocity-integrated CO(1-0) antenna temperature, W(CO), and the H2 column density, N(H2), is determined for 32 positions in two translucent high-latitude molecular clouds, MBM 40 and MBM 16. XCO is calculated using CH observations of the 2P1/2 F = 1-1 hyperfine transition to infer N(H2). The latter quantity, divided by W(CO) yields XCO for various positions across the clouds. We observed 24 positions in MBM 16, and XCO values were derived in the range (1.6-17.3) x 1020 cm-2 (K km s-1)-1, with a mean value of 7.6 x 1020. Eleven lines of sight were sampled in MBM 40 yielding values of XCO in the range (0.7-9.7) x 1020, with a mean value of 2.6 x 1020. An inverse relationship between XCO and W(CO) may exist, suggesting that the variation in XCO for these two translucent clouds arises from varying CO abundances. This paper also reports the existence of a broad component in the CH spectra observed throughout much of MBM 16. This component possibly originates in the disturbed outer regions of the cloud where the gas is not gravitationally bound to the core of the cloud. It is unclear how sensitive the CO rotational transitions are to this component, but it is likely that the CO/H2 ratio in this broad-line gas is less than 10-5. However, if the CH/H2 ratio is the same for the gas in the extended wings as it is for the typical cloud gas, then up to 40% of the cloud mass could be contained in this difficult to trace molecular component.

The conversion factor, XCO, between the velocity-integrated CO(1-0) antenna temperature, W(CO), and the H2column density, N(H2), is determined for 32 positions in two translucent high-latitude molecular clouds, MBM 40 and MBM 16. XCOis calculated using CH observations of the 2?1/2F= 1-1 hyperfine transition to infer N(H2). The latter quantity, divided by W(CO) yields XCOfor various positions across the clouds. We observed 24 positions in MBM 16, and XCOvalues were derived in the range (1.6-17.3) × 1020cm-2(K km s-1)-1, with a mean value of 7.6 × 1020. Eleven lines of sight were sampled in MBM 40 yielding values of XCOin the range (0.7-9.7) × 1020, with a mean value of 2.6 × 1020. An inverse relationship between XCOand W(CO) may exist, suggesting that the variation in XCOfor these two translucent clouds arises from varying CO abundances. This paper also reports the existence of a broad component in the CH spectra observed throughout much of MBM 16. This component possibly originates in the disturbed outer regions of the cloud where the gas is not gravitationally bound to the core of the cloud. It is unclear how sensitive the CO rotational transitions are to this component, but it is likely that the CO/H2ratio in this broad-line gas is less than 10-5. However, if the CH/H2ratio is the same for the gas in the extended wings as it is for the typical cloud gas, then up to 40% of the cloud mass could be contained in this difficult to trace molecular component.

We surveyed the northern Galactic hemisphere (NGH) at b? 30° in the CO (1-0) emission line to determine the surface filling factor of molecular gas at high Galactic latitudes and to search for heretofore unknown molecular clouds. The NGH was sampled on a locally Cartesian grid with 1° (true-angle) spacing in Galactic longitude and latitude. Of the 11,478 points in our grid, we observed all 10,562 positions that rise to an elevation above 30° in Cambridge, MA, the site of the 1.2 m millimeter-wave telescope that was used for the survey. Only 26 lines of sight showed CO emission. Monte Carlo simulations based on our sampling grid and with cloud sizes, in a uniform distribution, ranging from 0 to 2 deg2suggest that the survey is ~70% complete. Power-law distributions yield fractional completenesses that are typically a factor of 2 lower. The surface filling factor, corrected for the incompleteness of our sampling grid, is 0.004-0.008, depending on which cloud size distribution is used. These values are substantially lower than what is found in the southern Galactic hemisphere at b? -30°. Adopting as the CO to H2conversion ratio NH2/WCO= 2.5 × 1020cm-2(K km s-1)-1, the mass surface density of molecular gas in the north ranges from 0.015 ± 0.009 to 0.035 ± 0.020 M?pc-2. With the exception of four fairly significant aggregations of clouds (the complexes associated with the Polaris flare, Ursa Major, Draco, and L134), and a handful of isolated cloudlets, the northern Galactic hemisphere at b? 30° is found to be largely free of molecular gas.

Raup and Sepkoski1have presented evidence from marine fossils for a 26-Myr periodicity in the occurrence of mass extinctions. Using the same data Rampino and Stothers2obtained a different period, 30±1 Myr, which agrees with the 33±3-Myr half-period for the vertical oscillation of the Solar System about the plane of the galaxy3. To explain this agreement they suggest2that encounters with molecular clouds perturb the Sun's family of comets, causing many to enter the inner Solar System where one or more collide with the Earth; the cloud encounter rate is modulated at twice the oscillation frequency, because the number density of clouds peaks at the galactic plane at the midpoint of the solar oscillation crossed by the Solar System twice per period. Notwithstanding an apparent objection to this that the most recent extinctions are not in phase with the solar oscillation4, their model, given its stochastic nature, can accommodate a few events with large phase discrepancies. The degree of modulation is crucial: it depends on the scale height of the population of molecular clouds relative to the amplitude of the solar motion and tends to zero if this ratio is large and encounters are entirely random. Here we present data from CO surveys of molecular clouds both within and beyond the solar circle, which permit explicit calculation of the strength of the modulation. The cloud layer near the Sun is too extended and, as a consequence, the modulation of cloud encounters is too weak for a statistically significant period to be extracted from the nine extinctions analysed by Rampino and Stothers.

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