Showing papers by "William D. Langer published in 1997"

Chemical Evolution in Preprotostellar and Protostellar Cores

Abstract: The chemistry of developing and collapsing low-mass protostellar cores is followed using a chemical code with a time-varying density. Two evolutionary scenarios are represented, gravitational collapse in the presence of magnetic fields and the slow core growth by accretion near equilibrium. The chemical code includes gas-phase reactions and depletion onto grains with both CO and H2O ice mantles. We find that various species will selectively deplete from the gas phase at times that correspond to the middle to late stages of dynamical evolution when the densities are highest. These depletions do not depend in detail on the dynamical solution and should exist for any centrally condensed density profile. Sulfur-bearing molecules are particularly sensitive to the density increase: CS, SO, and C2S show significant depletions both on a strongly bound water mantle and on the weakly bound CO-covered grain surface. In contrast, CO and HCO+ show large depletions only for an H2O grain mantle and remain in the gas phase for models with CO grain mantles. Two species, NH3 and N2H+, do not deplete from the gas phase for the densities considered in our models. We also find that for very high densities, nH2>106 cm-3, depletion becomes important for all molecules. The effects of coupling chemistry and dynamics on the resulting physical evolution are discussed. We compare our results with current high-resolution observations of preprotostellar cores and to more evolved objects and suggest that ratios of the abundances of few species can be used in concert with our models as sensitive discriminators between different stages of core and star formation.

The Dynamics of Low-Mass Molecular Clouds in External Radiation Fields

Abstract: We present the results of three-dimensional hydrodynamic calculations of the evolution of low-mass molecular clouds, performed using the numerical method of smoothed particle hydrodynamics. The clouds that we consider are subject to heating by the interstellar radiation field and by cosmic rays. They are able to cool through molecular line emission (primarily CO and its isotopes) and by emission from the fine structure lines of C+ and O I. We also include gas-dust thermal coupling in our models. A simplified chemical network is incorporated that models the conversion between C+ and CO, where the chemical balance is determined by the local flux of dissociating radiation. Calculations are performed for initially uniform density clouds, with masses in the range M = 100-400 M☉, sizes in the range R = 1.7-3.4 pc, with the initial number density in all cases being n = 100 cm-3. We performed calculations for clouds with different geometrical shapes: spherical, prolate, and oblate. Additionally, we considered the effects of an anisotropic radiation field on the cloud evolution. These are the main results: 1. Clouds that are initially Jeans stable are able to collapse because of the coupling between the dynamical and thermal evolution. This collapse results in core-halo structure where we have a cold, dense, CO core surrounded by a warmer, tenuous, C+ envelope. 2. A pressure gradient is set up in the clouds by the attenuation of the UV radiation field. When a cloud is anisotropically heated, this pressure gradient leads to the formation of a highly flattened cloud core when it collapses. 3. The combined thermal and dynamical evolution of the prolate and oblate clouds leads to the formation of highly elongated or flattened structures. These structures are able to fragment, typically with four to eight subcondensations forming, which have masses in the range 3-7.5 M☉.

First astronomical detection of the cumulene carbon chain molecule H2C6 in TMC-1.

Abstract: The cumulene carbenes are important components of hydrocarbon chemistry in low-mass star-forming cores. Here we report the first astronomical detection of the long-chain cumulene carbene H2C6 in the interstellar cloud TMC-1, from observations of two of its rotational transitions: J(K,K') = 7(1,7) --> 6(1,6) at 18.8 GHz and 8(1,8) --> 7(1,7) at 21.5 GHz, using NASA's Deep Space Network 70 m antenna at Goldstone, California. In addition we also observed the shorter cumulene carbene H2C4 at the same position. The fractional abundance of H2C6 relative to H2 is about 4.7 x 10(-11) and that of H2C4 is about 4.1 x 10(-9). The abundance of H2C6 is in fairly good agreement with gas-phase chemical models for young molecular cloud cores, but the abundance of H2C4 is significantly larger than predicted.

First Astronomical Detection of the Cumulene Carbon Chain Molecule H_2C_6 in TMC1

Abstract: The cumulene carbenes are important components of hydrocarbon chemistry in low mass star forming cores. Here we report the first astronomical detection of the long chain cumulene carbene H_2C_6 in the interstellar cloud TMC1, from observations of two of its rotational transitions: $J_{K,K'} = 7_{1,7} \rightarrow 6_{1,6}$ at 18.8 GHz and $8_{1,8} \rightarrow 7_{1,7}$ at 21.5 GHz, using NASA's Deep Space Network 70 m antenna at Goldstone, California. In addition we also observed the shorter cumulene carbene, H_2C_4 at the same position. The fractional abundance of H_2C_6 relative to H_2 is about $4.7 \times 10^{-11}$ and H_2C_4 is about $1.1 \times 10^{-9}$. The abundance of H_2C_6 is in fairly good agreement with gas phase chemical models for young molecular cloud cores, but the abundance of H_2C_4 is significantly larger than predicted.

The Chemical Composition and Evolution of Giant Molecular Cloud Cores: A Comparison of Observation and Theory

Abstract: We present the results of an observational and theoretical study of the chemical composition and evolution of three giant cloud cores in Orion A, M17, and Cepheus A. This study is the culmination of a chemical survey of 32 transitions of 20 different molecules and isotopic variants in these cloud cores. Using these data, combined with observationally derived physical conditions, chemical abundances were calculated for several positions in each cloud. A global analysis of the molecular abundances shows that, although abundance differences exist, the chemical composition of giant cloud cores is remarkably homogeneous. This agreement suggests that the chemical evolution of the individual giant cloud cores is not unique. The molecular abundances of giant cloud cores are also systematically lower than those observed in the more quiescent dark cloud core TMC-1. A one-dimensional chemical model is presented that examines internal chemical structure induced by a radiation field enhanced by a factor of 103-105 above the normal interstellar radiation field. This model integrates the abundances of the various species as a function of depth, producing column densities that can be compared with observations. The one-dimensional model is unable to reproduce the abundances of many molecules for any single time. Two assumptions have been investigated to improve the agreement between theory and observations. These are adding clumps and raising the initial C/O ratio. We find that the inclusion of clumps in the chemical model can reproduce the abundance of C and C+. However, because of the greater weight placed on the photon-dominated region within smaller clumps, clumps have a detrimental effect on reproducing the abundances of other species. Models with a range of C/O ratios are also compared with the measured abundances. Good agreement between this model and the observations at two positions with disparate physical properties is found for early times (t ~ 105 yr) and for C/O increased to ~0.8. We suggest that one possible interpretation of these results is that the cores are dynamically evolving objects. Either giant cloud cores are intrinsically young objects or the dense material is effectively young by virtue of a complex interchange of material between the clumps and the interclump medium. We suggest that the CS/SO ratio can be used to probe the evolutionary state of and the initial C/O ratio in dense molecular clouds.

Physical conditions in quiescent dark cloud cores determined from multitransition observations of ccs

Abstract: We have studied three transitions of the CCS molecule to determine physical conditions in L1498 and TMCD10D, two narrow-line dense cores in the Taurus region.

Galactic Carbon Monoxide Isotope Ratios

Abstract: For twenty-five years, almost since the first detection of CO, isotopic abundance ratios of molecules in interstellar clouds (ISC) have been measured by radio techniques. These observations have been used to: 1) determine Galactic chemical evolution (nucleosynthesis, star formation rate, injection and mixing of gas in the interstellar medium); 2) interpret physical conditions in ISCs; and, 3) test models of interstellar chemistry. The carbon isotopic species are important because 12 and 13 carbon trace primary and secondary nucleosynthesis, respectively, and carbon molecules are pervasive in ISCs. To appreciate the difficulty of these measurements, keep in mind that we are looking for small differences by usual astronomical standards, about a factor of two across the Galaxy and 25 percent locally.