A coupled,dynamical and chemical model for the prestellar core L1544: Comparison of modeled and observed C<Superscript>18</Superscript>O,HCO<Superscript>+</Superscript>, and CS emission spectra

We consider radiative transfer in C¹⁸O, HCO⁺, and CS molecular lines in a spherically symmetrical, coupled, dynamical and chemical model of a prestellar core whose evolution is determined by ambipolar diffusion. Theoretical and observed line profiles are compared for the well-studied core L1544, which may be a collapsing protostellar cloud. We study the relationship between the line shapes and model parameters. The structure of the envelope and kinematic parameters of the cloud are the most important factors determining the shape of the lines. Varying the input model parameters for the radiative transfer—the kinetic temperature and microturbulent velocity—within the limits imposed by observations does not result in any substantial variations of the line profiles. The comparison between the model and observed spectra indicates that L1544 displays a flattened structure, and is viewed at an oblique angle. A two-dimensional model is needed to reproduce this structure.     

Origin of composite dikes in the Gouldsboro granite, coastal Maine

Composite dikes, consisting of aphyric basaltic margins and phenocryst-rich rhyolitic interiors, cut the Gouldsboro granite of coastal Maine at many localities. Limited hybridization (exchange of crystals, commingling, and mixing) occurs in most of the dikes and indicates that the two magmas were contemporaneous with emplacement of rhyolitic magma following closely in time the initial emplacement of the basaltic dike. Petrographic characteristics and geochemistry indicate that the source of the rhyolite was resident magma in the Gouldsboro granite magma chamber. The composite dikes formed when basaltic dikes ruptured the Gouldsboro magma chamber, permitting partly crystallized magma from the margin of the chamber to flow outward into the center of the basaltic dikes. Field relations of similar composite dikes in other areas (e.g., Iceland, Scotland) are consistent with this model. A second type of composite dike (silicic margins with chilled basaltic pillows) commonly cuts mafic intrusions along the Maine coast and probably formed when a granitic dike ruptured an established chamber of mafic magma, permitting resident mafic magma to collapse downward into the still Liquid granitic dike. Most composite dikes have probably formed when a magma chamber was disrupted by a dike of contrasting magma rather than by tapping a stratified magma chamber.     

Fe-rich tholeiitic liquids and their cumulate products in the Pleasant Bay layered intrusion, coastal Maine

Fe-rich tholeiitic liquids are preserved as chilled pillows and as the chilled base of a 27 meter thick macrorhythmic layer in the Pleasant Bay mafic-silicic layered intrusion. The compositions of olivine (Fo₁) and plagioclase (An₁₃₋₈) in these extremely fine grained rocks suggest that they represent nearly end stage liquids that formed by fractionation of tholeiitic basalt. Their major element compositions (∼17.5 wt% FeO_T and 54 wt%SiO₂) closely resemble highly evolved glasses in the Loch Ba ring dike and some recent estimates of end-stage liquids related to the Skaergaard layered intrusion, and are consistent with recent experimental studies of tholeiite fractionation. Their trace element compositions are consistent with extensive earlier fractionation of plagioclase, olivine, clinopyroxene, ilmenite, magnetite and apatite. The mineral assemblage of the chilled rocks (olivine, clinopyroxene, quartz, ilmenite and magnetite), apatite saturation temperatures, and very low Fe³⁺/Fe²⁺indicate conditions of crystallization at temperatures of about 950 °C and f _{O 2} about two log units below FMQ. Cumulates that lie about 3 meters above the chilled base of the macrorhythmic layer contain cumulus plagioclase, olivine, clinopyroxene, ilmenite, apatite and zircon. This mineral assemblage and the Fe-Mg ratio in clinopyroxene cores suggest that this cumulate was in equilibrium with a liquid having a composition identical to that of the chilled margin which lies directly beneath it. The high FeO_T and low SiO₂ concentrations of this cumulate (23.3 and 45.8 wt%, respectively) are comparable to those in late stage cumulates of the Skaergaard and Kiglapait intrusions. This association of a chilled liquid and cumulate in the Pleasant Bay intrusion suggests that late stage liquids in tholeiitic layered intrusions may have been more SiO₂-rich than field-based models suggest and lends support to recent experimental studies of tholeiite fractionation at low f _O2 which indicate that saturation of an Fe-Ti oxide phase should cause FeO_T to decrease in the remaining liquid. Received: 17 January 1997 / Accepted: 10 June 1997     

The effect of the ionization rate on the chemical composition of dense cores of dark molecular clouds

The effect of variations in the rate of ionization of neutral chemical species by cosmic rays, ζ, on the abundances of some observed molecules in the dense cores of dark molecular clouds is studied. Changes in molecular abundances accompanying an increased (decreased) ionization rate have a single origin: the acceleration (deceleration) of processes that are affected directly or indirectlybychemical reactions with charged species. In addition to affecting the gas-phase chemistry, an increased cosmic-ray flux leads to the more efficient destruction of dust-grain mantles and also accelerates the freezing of some components onto dust. In particular, in a model with an increased ζ, the destruction of the volatile N₂ molecule by ionized helium leads to the rapid accumulation of nitrogen atoms in dust-phase ammonia, which has a higher desorption energythan N₂. As a result, the gas-phase abundance of NH₃ and N₂H⁺ decreases significantly. This mechanism can explain the unusual chemical structures of some dense globules, such as B68, where surprisingly low abundances of nitrogen-bearing molecules are observed together with a central drop in the NH₃ and N₂H⁺ column densities. Observations of clouds in HCN and HNC lines can discriminate between the two possible origins of the reduced NH3 and N₂H⁺ abundances: an increased cosmic-ray flux or N₂ freezing due to the higher desorption energy of this molecule.     

Holocene peat and carbon accumulation rates in the southern taiga of western Siberia

Although recent studies have recognized peatlands as a sink for atmospheric CO₂, little is known about the role of Siberian peatlands in the global carbon cycle. We have estimated the Holocene peat and carbon accumulation rate in the peatlands of the southern taiga and subtaiga zones of western Siberia. We explain the accumulation rates by calculating the average peat accumulation rate and the long-term apparent rate of carbon accumulation (LORCA) and by using the model of Clymo (1984, Philosophical Transactions of the Royal Society of London Series B 303, 605-654). At three key areas in the southern taiga and subtaiga zones we studied eight sites, at which the dry bulk density, ash content, and carbon content were measured every 10 cm. Age was established by radiocarbon dating. The average peat accumulation rate at the eight sites varied from 0.35 ± 0.03 to 1.13 ± 0.02 mm yr⁻¹ and the LORCA values of bogs and fens varied from 19.0 ± 1.1 to 69.0 ± 4.4 g C m⁻² yr⁻¹. The accumulation rates had different trends especially during the early Holocene, caused by variations in vegetation succession resulting in differences in peat and carbon accumulation rates. The indirect effects of climate change through local hydrology appeared to be more important than direct influences of changes in precipitation and temperature. River valley fens were more drained during wetter periods as a result of deeper river incision, while bogs became wetter. From our dry bulk density results and our age-depth profiles we conclude that compaction is negligible and decay was not a relevant factor for undrained peatlands. These results contribute to our understanding of the influence of peatlands on the global carbon cycle and their potential impact on global change.     

Searching for Added Value in Simulating Climate Extremes with a High-Resolution Regional Climate Model over Western Canada

We evaluate the capacity of a regional climate model to represent observed extreme temperature and precipitation events and also examine the impact of increased resolution, in an effort to identify added value in this respect. Two climate simulations of western Canada (WCan) were conducted with the Canadian Regional Climate Model (version 4) at 15 (CRCM15) and 45?km (CRCM45) horizontal resolution driven at the lateral boundaries by data from the European Centre for Medium-range Weather Forecasts (ECMWF) 40-year Reanalysis (ERA-40) for the period 1973–1995. The simulations were evaluated using the spline-interpolated dataset ANUSPLIN, a daily observational gridded surface temperature and precipitation product with a nominal resolution of approximately 10?km. We examine a range of climate extremes, comprising the 10th and 90th percentiles of daily maximum (TX) and minimum (TN) temperatures, the 90th percentile of daily precipitation (PR90), and the 27 core Climate Daily Extremes (CLIMDEX) indices.

Both simulations exhibit cold biases compared with observations over WCan, with the bias exacerbated at higher resolution, suggesting little added value for temperature overall. There are instances, however, of regional improvement in the spatial pattern of temperature extremes at the higher resolution of CRCM15 (e.g., the CLIMDEX index for the annual number of days when TX?>?25°C). The high-resolution simulations also reveal similarly localized features in precipitation (e.g., rain shadows) that are not resolved at the 45?km resolution. With regard to precipitation extremes, although both simulations generally display wet biases, CRCM15 features a reduced bias in PR90 in all seasons except winter. This improvement occurs despite the fact that spatial and interannual variability of PR90 in CRCM15 is significantly overestimated relative to both CRCM45 and ANUSPLIN. We posit that these characteristics are the result of demonstrable differences between corresponding topographical datasets used in the gridded observations and CRCM, the resulting errors propagated to physical variables tied to elevation and the beneficial effect of subsequent spatial averaging. Because topographical input is often discordant between simulations and gridded observations, it is argued that a limited form of spatial averaging may contribute added value beyond that which has already been noted in previous studies with respect to small-scale climate variability.     

The evolution of hydrocarbon dust grains in the interstellar medium and its influence on the infrared spectra of dust

Computations of the evolution of the distributions of the size and degree of aromatization of interstellar dust grains, destruction by radiation and collisions with gas particles, and fragmentation during collisions with other grains are presented. The results of these computations are used to model dust emission spectra. The evolution of an ensemble of dust particles sensitively depends on the initial size distribution of the grains. Radiation in the considered range of fluxes mainly aromatizes grains. With the exception of the smallest grains, it is mainly erosion during collisions with gas particles that leads to the destruction of grains. In the presence of particle velocities above 50 km/s, characteristic for shocks in supernova remnants, grains greater than 20 Å in size are absent. The IR emission spectrum changes appreciably during the evolution of the dust, and depends on the adopted characteristics of the grains, in particular, the energy of their C–Cbonds (E₀). Aromatic bands are not observed in the near-IR (2–15 μm) when E₀ is low, even when the medium characteristics are typical for the average interstellarmedium in our Galaxy; this indicates a preference for high E₀ values. The influence of the characteristics of the medium on the intensity ratios for the dust emission in various photometric bands is considered. The I_3.4/I_11.3 intensity ratio is most sensitive to the degree of aromatization of small grains. The I_3.3/I₇₀₊₁₆₀ ratio is a sensitive indicator of the contribution of aromatic grains to the total mass of dust.