DENSE GAS IN NEARBY GALAXIES .9. A SURVEY FOR HNC

Ten detections and five tentative detections of hydrogen isocyanide (HNC) J=1-0 emission are reported from a survey including sixteen galaxies. Full maps are presented for the nuclear regions of NGC 253 and IC 342, partial maps for Maffei 2, M 82, and M 83. Toward IC 342, the HNC and HCO+ distributions differ from those observed in 12CO, 13CO, HCN, CS, and NH3. This is likely a consequence of the density structure. Relative HNC abundances are with 10(-10)-10(-9) much smaller than those measured in nearby dark clouds and appear to be slightly smaller than those in regions of massive star formation of the Galactic disk. This is consistent with the presence of dense warm gas or a frequent occurrence of shocks in the nuclear regions of the galaxies observed. As in prominent Galactic star forming regions, 3 mm HNC line emission tends to be weaker than the corresponding emission from HCN and HCO+. Toward Arp 220, however, the 3 mm HNC/HCN line intensity ratio is > 1. HNC/HCO+, HNC/CO, and HNC to 20 cm radio continuum luminosity ratios are also particularly large. A possible interpretation is the presence of cool quiescent gas outside the central region which contains the starburst. In the other ultraluminous galaxy observed, NGC 6240, X(HNC) 10 smaller than in Arp 220, demonstrating that the molecular composition in ultraluminous galaxies is far from being uniform.

THE EFFECT OF RAPID NEUTRAL-NEUTRAL REACTIONS ON CHEMICAL-MODELS OF DENSE INTERSTELLAR CLOUDS

Recent experiments on rapid neutral-neutral reactions involving the radical CN at low temperature and the neutral C atom at room temperature suggest that atom-neutral and radical-neutral reactions may be generally more rapid at low temperature than hitherto thought. We have included a variety of rapid neutral-neutral reactions in our gas-phase chemical models of quiescent, dense interstellar clouds. We find the calculated abundances of many molecules to be greatly changed from previous values. In particular, the peak 'early-time' abundances of organic molecules are reduced.

DENSE GAS IN NEARBY GALAXIES .6. A LARGE C-12/C-13 RATIO IN A NUCLEAR STARBURST ENVIRONMENT

Toward the starburst nucleus of NGC 253, C-12/C-13 line intensity ratios from six carbon bearing molecules (CO, CN, CS, HCN, HCO+, and HNC) are used to confine the possible range of carbon and oxygen isotope ratios. A detailed analysis yields C-12/C-13 approximately 40 and O-16/O-18 approximately 200. Also reported are first detections of (CS)-C-13 and of the 0(0) - 1(-1) E line of methanol (CH3OH) in an extragalactic source.

PHOSPHORUS CHEMISTRY IN DENSE INTERSTELLAR CLOUDS

I have used recent laboratory studies on the reactions of the phosphorus hydride ions, PH(n)+ (n = 0-4) to construct a new model of phosphorus chemistry in interstellar clouds. I find that the non-detection of PN in cold, dark clouds in consistent with the chemical models only if the depletion of phosphorus in large, approximately 10(4) in TMC-1. Although the laboratory studies indicate that organo-phosphorus chains C(n)P can be formed, this large depletion precludes the detection of any phosphorus-bearing moleclues in cold clouds. However, in warm clouds associated with star formation, the depletion of phosphorus may be reduced. In this case one can reproduce the PN abundance toward Orion KL with a depletion factor of about 300. Interestingly, if the organo-phosphorus species are not destroyed by O atoms, I predict fractional abundances in Ori KL of between 10(-11) and 10(-10) for C(n)P (n = 2-4) and HCCP.

Emission lines of Fe XI in the 257-407 A wavelength region observed in solar spectra from EIS/Hinode and SERTS

Theoretical emission-line ratios involving Fe xi transitions in the 257-407 A wavelength range are derived using fully relativistic calculations of radiative rates and electron impact excitation cross-sections. These are subsequently compared with both long wavelength channel Extreme-Ultraviolet Imaging Spectrometer (EIS) spectra from the Hinode satellite (covering 245-291 A) and first-order observations (similar to 235-449 A) obtained by the Solar Extreme-ultraviolet Research Telescope and Spectrograph (SERTS). The 266.39, 266.60 and 276.36 A lines of Fe xi are detected in two EIS spectra, confirming earlier identifications of these features, and 276.36 A is found to provide an electron density (N-e) diagnostic when ratioed against the 257.55 A transition. Agreement between theory and observation is found to be generally good for the SERTS data sets, with discrepancies normally being due to known line blends, while the 257.55 A feature is detected for the first time in SERTS spectra. The most useful Fe xi electron density diagnostic is found to be the 308.54/352.67 intensity ratio, which varies by a factor of 8.4 between N-e = 108 and 1011 cm-3, while showing little temperature sensitivity. However, the 349.04/352.67 ratio potentially provides a superior diagnostic, as it involves lines which are closer in wavelength, and varies by a factor of 14.7 between N-e = 108 and 1011 cm-3. Unfortunately, the 349.04 A line is relatively weak, and also blended with the second-order Fe x 174.52 A feature, unless the first-order instrument response is enhanced.