Why HOC+ is detectable in interstellar clouds: the rate of the reaction between HOC+ and H2

The recent confirmation by Ziurys and Apponi of the detection of HOC+ toward Sgr B2 (OH), and their identification of the ion in Orion-KL and several other sources show that HOC+ is far more abundant than predicted by previous ion-molecule models. In these models, the reaction HOC(+) + H2 --> HCO(+) + H2 is assumed to rapidly destroy HOC+, based on the results of a prior calculation. We have recalculated the rate of this reaction as a function of temperature using a new ab initio potential surface and a phase space approach to the dynamics which includes tunneling. The newly calculated rate is small (< or = 1 x 10(-10) cm3 s-1) at temperatures under 100 K.     

Calculations concerning the HCO+/HOC+ abundance ratio in dense interstellar clouds

Calculations have been performed to determine the rate coefficients of several reactions involved in both the formation and depletion of interstellar HCO+ and HOC+. The abundance of HOC+ deduced from these calculations is consistent with the tentative identification of HOC+ in Sgr B2 by Woods et al. The large HCO+/HOC+ abundance ratio observed by Woods et al. is due at least in part to a more rapid formation rate for HCO+ and probably due as well as to a more rapid depletion rate for HOC+.     

The rate of the reaction between CN and C2H2 at interstellar temperatures

The rate coefficient for the important interstellar reaction between CN and C2H2 has been calculated as a function of temperature between 10 and 300 K. The potential surface for this reaction has been determined through ab initio quantum chemical techniques; the potential exhibits no barrier in the entrance channel but does show a small exit channel barrier, which lies below the energy of reactants. Phase-space calculations for the reaction dynamics, which take the exit channel barrier into account, show the same unusual temperature dependence as determined by experiment, in which the rate coefficient at first increases as the temperature is reduced below room temperature and then starts to decrease as the temperature drops below 50-100 K. The agreement between theory and experiment provides strong confirmation that the reaction occurs appreciably at cool interstellar temperatures.     

Predicting Effects of Temperature Increase on the Water Balance of Beech Forest – An Application of the ‘KAUSHA’ Model

The water balance model KAUSHA (Halldin, 1989) was applied to a 100-year-old beech (Fagus sylvatica L.) forest in northern Germany. Overall, a satisfying agreement between modelled evapotranspiration values and independent micrometeorological measurements (Bowen ratio energy balance method) could be observed, although for rainy days KAUSHA showed a tendency to overestimate evapotranspiration. The model was used to predict the effects of a climate warming on the water budgets of the forest. It is shown that a temperature increase of 2°C due to a rising CO₂ content of the atmosphere will not change the yearly totals of evapotranspiration significantly, but could have serious effects on the soil water balance during the vegetation period. Because under climate change conditions a higher amount of the available soil water has already been evaporated in winter and spring, soil water content will limit the transpiration of the trees from July to September much more strongly. Therefore, the yield of beech forest might also suffer from drought effects. It can be concluded that a better knowledge of the seasonal distribution of rainfall under climate change conditions is indispensable for predicting effects of rising temperatures and CO₂ concentrations on ecosystems.     

The possibility of nitrogen isotopic fractionation in interstellar clouds

The possibility of nitrogen isotopic fractionation owing to ion–molecule exchange reactions involving the most abundant N-containing species in dense interstellar clouds has been explored. We find that exchange reactions between N atoms and N-containing ions have most influence on the fractionation, although the extent of fractionation is too small to be readily detectable.     

LBT: Its Instrumentation and Key Science Projects

B25 The site testing for the Thirty Meter Telescope and its potential role in developing the shortterm forecasting of observing conditions B34 LUCIFER: a NIR Spectrograph and Imager for the LBT B41 PEPSI: the Potsdam Echelle Polarimetric and Spectroscopic Instrument for the Large Binocular Telescope B44 LUCIFER: High Redshift Science Cases B49 Feedback in galaxy cores: a LBT Key Science Project proposal B52 SERPIL/LIINUS: a design study for a Near‐Infrared Interferometric Integral Field Spectrometer for the LBT B64 The Construction of the Large Binocular Telescope B73 Operating the LUCIFER Instrument B85 M92 – a crucial testbed for atomic diffusion and mixing in stars B142 The Large Binocular Camera at LBT B148 LUCIFER and its Exposure‐Time‐Calculator B196 The Multi‐Object Double Spectrograph and Nulling Interferometer for the Large Binocular Telescope B199 The Laser Guide Star Facility for the LBT B231 The PEPSI “deep spectrum” project B236 Zeeman‐Doppler imaging from Stokes IQUV line profiles B257 D3Dnet: getting ready for MUSE, a 2nd Generation Instrument for the VLT     

A reanalysis of the HCO+/HOC+ abundance ratio in dense interstellar clouds

New theoretical and experimental results have prompted a reinvestigation of the HCO+/HOC+ abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC+ and H2, which was previously calculated by DeFrees, McLean, and Herbst to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC+ does indeed react efficiently with H2 in interstellar clouds, the calculated HCO+/HOC+ abundance ratio rises to substantially greater value under standard dense cloud conditions than in deduced via the tentative observation of HOC+ in Sgr B2.     

Invertebrates in salt lakes of the Bolivian Altiplano

Based upon a short reconnaissance (March 1991), the chemicalnature and principal invertebrate fauna of seven saline lakes on the Altiplano of southern Bolivia are discussed (Lago de Uru-uru, Pastos Grandes, Lago Ramiditas, Lago Hedionda, Lago Cañapa, Laguna Colorado, pool at L. Colorado). Salinities ranged between 4.4 and 156 g L^–1. The fauna was depauperate. Except at the lowest salinities (<5 gL^–1), rotifers were absent. At high salinities (>50 g L^–1), onlyArtemia, Boeckella poopoensis, ephydrids and a dolichopodid were present.     

Seismic sources for shallow investigations: A field comparison from Northern Germany

A field test of 14 seismic P-wave sources was designed to show the capabilities of several sources for shallow subsurface investigations in the quaternary landscapes of Northern Germany. The sources included accelerated dropped weights, shallow blasts, a shotgun using blank cartridges and controlled signal sources. Local changes in subsurface conditions showed a strong influence on the recorded data, so it proved necessary to record up to 120 shots per source in order to get representative results and to compare not only single shot records but stacked sections as well. Reducing the geophone spacing to 1 m, it became possible to trace very local features like fluctuations in travel times or in the recorded amplitudes. The amplitude spectra of the recorded traces indicated energy at frequencies up to 200 Hz and showed remarkable spatial changes due to different subsurface conditions near the receiver locations. Changes in signal form and amplitude due to compacting effects were visible in the first four impacts of strong sources.