Chemistry Of Interstellar Clouds Research Articles

Low‐Temperature Experiments on the Formation of Deuterated C3H+3

Manydeuteratedmoleculeshavebeendiscoveredininter-andcircumstellarregions.Insomecases,theobserved abundances can be explained with simple thermodynamic models; often, however, isotope enrichment is more complicated. This has been seen recently in detailed low-temperature experiments performed for the fundamental systemsH þ =H2D þ andCH þ =CHn� 1D þ .An unsolvedproblem is toexplainthelarge abundanceofC3H2andlarger hydrocarbons and their deuterated variants observed in cold, dark interstellar clouds. In this work a variabletemperature22poletrapisutilizedforcloselyscrutinizingvariousion-moleculereactionsthatmaycontributetothe formation of C3H2D þ or C3HD þ and, via dissociative recombination, C3HD. The experimental study of the promising candidate C3H þ þ HD ! C3H2D þ þ H2, which has already been excluded by theory, corroborates that this exothermic H-D exchange does not occur at all. A careful analysis of the data reveals that the 15 K rate coefficient is smaller than 4 ; 10 � 16 cm 3 s � 1 . In contrast, quite efficient routes have been found in the lowtemperature experiments, starting with C þ and proceeding via deuterated C3H þ to C3H þ and C3H þ . Formation of C3D þ in C þ þ HD collisions is 6 times faster than assumed in astrochemical models (k ¼ 9:3 ; 10 � 10 cm 3 s � 1 ). Surprisingly, direct production of C3HD þ via radiative association has also been observed (kr ¼ 6:0 ; 10 � 11 cm 3 s � 1 ).ReactionsofpartlyorfullydeuteratedC3H þ þ H2 collisionsystemarestronglydependentontemperatureand thehydrogenorthotopararatio.Inaddition,itshowsverycomplicatedisotopeeffects.Forexample,inC3H þ þ HD collisions the formation of C3HD þ (k ¼ 4:6 ;10 � 10 cm 3 s � 1 ) dominates over the H-D exchange (k ¼ 5:6 ; 10 � 11 cm 3 s � 1 ) and radiative association C3H2D þ (kr ¼ 3:2 ;10 � 11 cm 3 s � 1 ). The reactions of C3H þ þ H2 are very slow for all isotope combinations. Although several questions remain open and more low-temperature experiments are needed, it is recommended that the new values are included in astrochemical databases, since they are fundamental to the correct description of the carbon chemistry in interstellar clouds.

Temperature variable ion trap studies of C3Hn+ with H2 and HD

Hydrogenation and deuteration of C3+, C3H+, C3H2+ in collisions with H2 and HD has been studied from room temperature down to 10 K using a 22-pole ion trap. Although exothermic, hydrogenation of C3+ is rather slow at room temperature but becomes faster with decreasing temperature. In addition to the increasing lifetime of the collision complex this behavior may be caused by the floppy structure of C3+ and the freezing of soft bending modes below 50 K. For C3(+) + HD it has been shown that production of C3D+ is slightly favored over C3H+ formation. The controversy over which products are really formed in C3H(+) + H2 collisions and deuterated variants has a long history. Previous and new ion trap results prove that formation of C3H2(+) + H is not endothermic but rather fast, in contradiction to erroneous conclusions from flow tube experiments and ab initio calculations. In addition the reaction shows a complicated isotope dependence, most probably caused by the influence of zero point energies in entrance and exit transition states. For example hydrogen abstraction with HD is faster than with H2 while radiative association is slower. The most surprising result has been obtained for C3H(+) + HD. Here C3HD+ formation is over one hundred times faster than C3H2+. In addition to the details of the potential energy surface it may be that in this case an H-HD exchange reaction takes place via an open-chain propargyl cation intermediate (H2CCCH+). Reactions of C3H2+ and C3H3+ with H2 are very slow but, due to the unique sensitivity of the trapping technique, significant rate coefficients have been determined. The presented results are of fundamental importance for understanding the energetics, structures and reaction dynamics of the deuterated variant of the C3Hn+ collision system. They indicate that the previous quantum chemical calculations are not accurate enough for understanding the low energy behavior of the Cn,Hm+ reaction systems. The laboratory experiments are of essential relevance for the carbon chemistry of dense interstellar clouds, both for formation of small hydrocarbons and deuterium fractionation.

Chemistry of Diffuse Clouds and Circumstellar Envelopes

AbstractWe present and review recent millimeter-wave observations with interferometers, in particular, with the Plateau de Bure interferometer, that stress the importance of high-frequency resolution for studying the chemistry of diffuse interstellar clouds and of post main-sequence circumstellar envelopes. The future impact of new instruments, such as the Atacama Large Millimeter and submillimeter Array (ALMA), is also investigated.

Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling

Molecular hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of molecular hydrogen in simulated astrophysical environments. Since the formation of molecular hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.

Dissociative Recombination of N2H+: Evidence for Fracture of the N—N Bond

Branching ratios and absolute cross sections have been measured for the dissociative recombination of N2H+ using the CRYRING ion storage ring. It has been found that the channel N2H+ + e- → N2 + H accounts for only 36% of the total reaction and that the branching into the other exoergic pathway, N2H+ + e- → NH + N, consequently amounts to 64%. The cross section of the reaction could be fitted by the expression σ = (2.4 ± 0.4) × 10-16E-1.04±0.02 cm2, which leads to a thermal reaction rate of k(T) = (1.0 ± 0.2) × 10-7(T/300)-0.51±0.02 cm3 s-1, in favorable agreement with previous flowing afterglow Langmuir probe measurements at room temperature, although our temperature dependence is very different. The implications of these measurements for the chemistry of interstellar clouds are discussed. A standard model calculation for a dark cloud predicts a slight increase of N2H+ in the dark clouds but a five- to sevenfold increase of the NH concentration as steady state is reached.

Interstellar H3+Line Absorption toward LkHα 101

We present a detection of three lines of the H ion in the near-infrared spectrum of the Herbig Be star LkHα 101. H is the principal initiator of gas-phase chemistry in interstellar clouds and can be used to constrain the ionization rate or the path length through interstellar material along the line of sight. Essentially all of the observed H column of (2.2 ± 0.3) × 1014 cm-2 toward LkHα 101 originates in the same dense, dark cloud; less than 1 mag of the ~11 total magnitudes of visual extinction is attributable to diffuse material. Constraints on the density [1 × 104 cm-3 < n(H2) < 4 × 104 cm-3], along with estimates of distances to the obscuring cloud, imply H2 ionization rates of 6.7 × 10-17 s-1 < ζ < 2.7 × 10-16 s-1. The nondetection of fluorescent H2 emission at the cloud surface implies that the obscuring dark cloud is located at least 20 pc from LkHα 101 and is therefore decoupled from its evolution. The confirmation of significant H in dense material addresses earlier concerns that H may originate in diffuse material surrounding dense clouds.

Chemical effects of H 2 formation excitation

The effects of the production on dust grain surfaces of molecular hydrogen in excited states have been investigated. On the assumption that all of the H2 formed on the surface of grains has a sufficient level of excitation too vercome the energy barriers in the formation reactions for the important OH and CH+ radicals, we consider the likely abundances of excited H2 (H2*), OH and CH+ in various situations. Two different models are employed; the first links the H2* abundance directly to that of H2 using a steady-state approximation, whilst the second considers the time-dependence of H2*. The second model is applied to gas that has been subjected to a strong isothermal shock (specifically, the shock-induced collapse of a diffuse cloud), which results in an extreme (high density, high atomic hydrogen abundance) environment. In general, it is found that the presence of the excited H2 has only marginal effects on the chemistry of interstellar clouds. However, in the isothermal shock model, the abundances of CH+ are significantly enhanced, but only on short timescales, whilst the effects on the OH abundances are smaller, but last longer. We conclude that other than in such exceptional environments there are no obvious chemical signatures of the formation of H2*.

The large millimeter telescope/el Gran Telescopio Milimétrico: a new instrument for astrobiology.

The Instituto Nacional de Astrofísica, Optica y Electrónica in Mexico and the University of Massachusetts in the U.S.A. are collaborating to build the world's largest radio telescope that operates at short millimeter wavelengths. This facility, known as the Large Millimeter Telescope (LMT) or el Gran Telescopio Milimétrico (GTM), is being sited at an altitude of 4600 m on Volcan Sierra Negra in the Mexican state of Puebla. The telescope will be a fully steerable dish with a diameter of 50 m and a surface consisting of 180 panels that are actively adjusted under computer control to correct for deformations due to gravity and temperature gradients. Instruments will include focal plane arrays to image both continuum and spectral line emission from celestial sources. The LMT/GTM will be an extremely powerful facility for studies encompassing almost all areas of astronomy, including astrobiology. In particular, the high sensitivity, angular resolution, and mapping speed will enable detailed investigations of the organic chemistry of interstellar molecular clouds, protoplanetary disks, and comets.

Observations of deuterated molecules with the large millimeter telescope

The characteristics of a new, very sensitive radio telescope currently under construction for use in the 1– 4 mm wavelength region, the large millimeter telescope (LMT), are described. The LMT with its planned instrumentation will provide an important new tool for studies of deuterated molecules, giving astronomers powerful probes of the linked chemistry and physics of interstellar molecular clouds and comets.

Low temperature rate coefficients for the reactions of CN and C 2H radicals with allene (CH 2CCH 2) and methyl acetylene (CH 3CCH)

Using a continuous flow CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flow) apparatus, rate coefficients have been measured for the reactions of the cyanogen (CN) and ethynyl (C 2H) radicals with allene (CH 2CCH 2) and methyl acetylene (CH 3CCH) at temperatures from 295 down to 15 K for the reactions of CN and down to 63 K for those of C 2H. All four reactions occur at rates close to the collision-determined limit. The results are compared with those obtained earlier for the reactions of other alkenes and alkynes, and, in the accompanying Letter by Vakhtin et al., with results for C 2H+CH 2CCH 2 and C 2H+CH 3CCH obtained at 103 K using a pulsed Laval apparatus. The implications of these latest results for the chemistry of interstellar clouds and planetary atmospheres are discussed.

The Si($^\mathsf3$PJ) + O$_\mathsf2$ reaction: A fast source of SiO at very low temperature; CRESU measurements and interstellar consequences

The rate coecient of the reaction Si( 3 PJ )+O 2! SiO +Oh as been measured at temperatures down to 15 Ku sing aC RESU (Cinetique de R eaction en Ecoulement Supersonique Uniforme) apparatus coupled with the PLP-LIF (Pulsed Laser Photolysis { Laser Induced Fluorescence) technique. The temperature dependence of the rate coecient is well tted using the expression: 1:7210 10 (T /300 K) 0:53 exp( 17 K=T )c m 3 molecule 1 s 1 in the temperature range 15{300 K. The silicon chemistry in interstellar clouds is reviewed and possible consequences of our study are stressed.

Searches for new interstellar molecules, including a tentative detection of aziridine and a possible detection of propenal

Rotational spectroscopy at millimeter wavelengths is a powerful means of investigating the chemistry of dense interstellar clouds. These regions can exhibit an interesting complement of gas phase molecules, including relatively complex organics. Here we report the tentative first astronomical detection of aziridine (ethylenimine), the possible detection of propenal (acrolein), and upper limits on the abundances of cyclopropenone, furan, hydroxyethanal (glycolaldehyde), thiohydroxylamine (NH 2SH), and ethenol (vinyl alcohol) in various interstellar clouds.

Rate coefficients for the reactions of C($\mathsf{^3}$P$\mathsf{{\it _J}}$) atoms with C2H2, C2H4, CH3C$ \mathsf{\equiv} $CH and H2C$ \mathsf{=} $C$ \mathsf{=} $CH2 at temperatures down to 15 K

Rate coefficients for the reactions of ground state carbon atoms, C(3 PJ ), with the four unsaturated hydrocarbons C2 H2 , C2 H4 , and have been measured at temperatures down to 15 K. The experiments have been performed in a continuous flow CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus using pulsed laser photolysis of C3 O2 to generate C(3 PJ ) atoms and laser-induced fluorescence in the vacuum ultraviolet to observe the kinetic decays of the atoms and hence determine the rate coefficients. All four reactions are found to occur at rates close to the collision-determined limit and the rates show at most a very mild dependence on temperature. It is argued that normally these reactions proceed along a reaction path with no barrier to form a strongly bound energised adduct, which subsequently eliminates an H atom to form the reaction products. The rates of the reactions of C(3 PJ ) atoms with larger unsaturated hydrocarbons are discussed. Finally, the relevance of the results for the chemistry of dense interstellar clouds is considered.

The astrochemistry of H 3 +

The reactions of the molecular ion H3+ are pivotal to the chemistry of dense interstellar clouds. Produced by the cosmic-ray ionization of molecular hydrogen, H3 + reacts with a variety of atoms an...

Laboratory Studies of Ion/Molecule Reactions of Fullerenes: Chemical Derivatization of Fullerenes within Dense Interstellar Clouds and Circumstellar Shells

We discuss recent experimental results for ion/molecule reactions of ionized and multiply-ionized fullerenes, and of derivatized fullerene ions, with molecules relevant to the chemistry of interstellar clouds and circumstellar envelopes These reactions were studied using a selected-ion flow tube (SIFT) at 294 ± 2 K in helium at a pressure of 0.35 ± 0.01 torr. The present study supplements an earlier discussion on aspects of interstellar fullerene ion chemistry explored by the same technique. Several implications are apparent for the chemical processing of fullerenes in various astrophysical environments. Triply charged fullerene ions, such as C603+, may be formed under conditions prevailing within dense IS clouds, but their abundance will be very low owing to the large number of loss processes identified for such species. Derivatization of fullerene ions under interstellar or circumstellar conditions is less probable for larger fullerenes than for fullerenes smaller than C60. Hydrogenation may severely impede the efficiency of fullerene ion association with polar molecules and small unsaturated molecules, but should not substantially affect the efficiency of addition of radicals or PAHs under these conditions. We discuss prospects for neutralization of ionized fullerene adducts. Four classes of adduct ions are described, differing in their structure and expected neutralization tendencies. Adducts of fullerene ions with interstellar isonitriles, with radicals, and with linear polycyclic aromatic hydrocarbons (PAHs; class 1) are most likely to form derivatized fullerenes on neutralization, while fullerene ion adducts of nitriles, most hydrocarbons (class 3), and nonlinear PAHs (class 4) are most likely to yield the bare fullerene cage upon neutralization. Adducts of ammonia (class 2) appear to have an intermediate probability of surviving neutralization with the functionalizing group(s) intact.

Radiative electron attachment to small linear carbon clusters and its significance for the chemistry of diffuse interstellar clouds

Recent spectroscopic studies of carbon chain anions in the gas phase, and more specifically of C 7 −, have brought insight into the long-standing mystery of the unexplained diffuse interstellar bands. Previously, negative ions had not been considered highly abundant in interstellar clouds, and the question of efficient mechanisms leading to their formation had not been investigated in great detail. This work presents a statistical calculation of the rate coefficients for radiative attachment of an electron to small linear carbon clusters containing 4–9 atoms. We conclude that for molecules with 6 or more C atoms, the attachment occurs on every collision at the low temperatures of diffuse interstellar clouds.

A Molecular Orbital Study of the \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\mathrm{HC}\,_{3}\mathrm{NH}\,^{+}+e^{-}$ \end{document} Dissociative Recombination and Its Role in the Production of Cyanoacetylene Isomers in Interstellar Clouds

The dissociative recombination reaction between HC3NH+ and electrons is an important process in the chemistry of interstellar clouds. Motivated by the evidence that HCN and its metastable isomer HNC are both formed via the dissociative recombination reaction of HCNH+, we have examined the potential energy surface for the dissociation of the transient HC3NH following the recombination between HC3NH+ and electrons. We find that HC3NH+ gains 151 kcal mol-1 energy by the neutralization; this energy is high enough to isomerize HC3NH to HCCNCH, as well as to dissociate a hydrogen atom from either end of the molecule. The present calculation suggests, therefore, that the metastable linear and near-linear isomers of HC3N, namely, HNC3, HCCNC, and HCNCC, can be formed via the dissociative recombination between HC3NH+ and electrons. Two of these isomers, HNC3 and HCCNC, have already been detected in the dark cloud TMC-1. The unobserved species HCNCC is energetically the least stable. Since interstellar HC3N molecules are synthesized predominantly via neutral-neutral reactions, the HC3NH+ + e- process serves mainly to produce the metastable isomers of HC3N. We have incorporated these reaction products, as well as other relevant ion-molecule and neutral-neutral reactions, into our new standard model of dense interstellar clouds. Our model results show that the observed abundances in TMC-1 of HC3N, its two detected metastable isomers, and the related species C3H, C3N, and HC3NH+ can, in the main, be reproduced.

The Chemistry of Star-Forming Regions

During the last quarter century, chemists have responded magnificently to the challenges raised by astronomers in their attempts to understand the variety of molecules detected in interstellar clouds. Observations have shown the chemistry of these regions to be surprisingly complex, and now more than 100 molecular species have been identified in interstellar and circumstellar regions of the Galaxy. The chemistry of interstellar clouds that gives rise to these molecules is now believed to be reasonably well understood (see section 2) in terms of a network of some thousands of binary reactions between several hundred species.1,2 Astronomers are now applying the techniques of astrochemistry to interpret observations of star-forming regions.3 These regions are much more complex in physical terms than quiescent interstellar clouds. In starforming regions, interstellar gas is being compressed, the force of gravity overcoming the resistance provided by gas pressure, magnetohydro-dynamic (MHD) turbulence, magnetic pressure, and rotation. The chemistry is not in steady state during this collapse, and can therefore be used as a tracer of the evolution of the collapse. In addition, the chemistry modifies and controls the collapse through the provision of molecular coolants of the gas, and by determining the fractional ionization in the gas. It is this ionization that affects the level of magnetic and turbulent support available to the cloud. Molecular rotational emissions at millimeter and submillimeter wavelengths are both the main cooling processes and the most effective probes of these regions. Interstellar gas in the Galaxy is observed to be distributed in an irregular fashion, in clouds of a range of sizes. Much of the mass is encompassed in so-called giant molecular clouds (GMCs) which range in mass from about 104 to about 106 solar masses (the solar mass is about 2 × 1030 kg), and have linear extents of several hundred light years (a light year, ly, is about 1 × 1016 m). The gas in GMCs is largely H2, but because that molecule has no dipole moment, the material is most effectively traced in the 1-0 rotational emission of CO, the next most abundant molecule (CO/H2 = 10-4 by number). Isotopomers of CO are also used. The CO emission identifies cold gas of number density ∼103 H2 molecules cm-3. A detailed study4 of one particular GMC, the Rosette molecular cloud (RMC), shows that it contains almost 2 × 105 solar masses of gas, extending over 100 ly. The gas in the RMC is fragmented into about 70 clumps with masses ranging from a few tens to a few thousands of solar masses. The clumps are embedded in a more tenuous medium, typically contain 102-103 H2 molecules cm-3, and are cool (j30 K). Observations show that clumps with larger column densities of CO (J1016 CO molecules cm-2) are more likely to contain embedded stars. Therefore, clumps satisfying this criterion are likely to be the sites of star formation in the RMC. Collapse of a clump leads to fragmentation and the formation of a cluster of dense cores. Carbon monoxide (12C16O) is not an effective tracer of gas in dense cores because the CO lines are optically thick and CO level populations are thermalized at lower densities. However, species of lower abundance than 12C16O can trace the dense gas in cores (J104 H2 molecules cm-3), and they include NH3, CN, H2CO, and CS. A typical core cluster5 is illustrated in Figure 1. It is a contour map in intensity of 1-0 rotational emission from the minor isotopic species 12C18O. This core cluster contains cores which may evolve to form new stars. Several stars have already formed and are detected as infrared sources (IRS 1-4). A primary goal of astrophysics is the detailed study of the collapse of a dense core to form a star. It is also important to gain an understanding of how gravity overcomes the various resistances to collapse, and how a young star interacts with its environment through the stellar winds and jets that develop at the earliest stages of the star’s existence. The answers to these questions are certainly contained in the emissions from the molecules and dust present in the collapsing core. Identifying a collapsing core is observationally difficult. The indicators should be molecular lines that are broadened by the infalling velocities. In this Account, we describe how the search for the infall signature has led to a recognition that the interaction of gas and dust in the infalling gas produces profound changes to the chemistry and physics of star-forming regions. This interaction is poorly understood, and the nature of the star-forming process will

The isocyanopolyynes HC4NC and HC6NC: Microwave spectra and ab initio calculations

Rotational spectra of HC4NC and HC6NC, linear molecules of interest to interstellar cloud chemistry, were recorded by Fourier transform microwave spectroscopy, and the ground state rotational constants were determined to be 1401.18227(7) and 582.5203(1) MHz. Nitrogen quadrupole hyperfine structure could be observed for HC4NC. On the basis of coupled cluster calculations including connected triple substitutions accurate equilibrium structures (uncertainty in bond lengths ca. 0.0005 Å) could be established for both species. The equilibrium dipole moments, predicted to be −3.25 and −3.49 D for HC4NC and HC6NC, respectively, exhibit large correlation effects of 30% and 33%.

Some Interstellar Reactions Involving Electrons and Neutral Species: Attachment and Isomerization

While the role of neutral-neutral reactions in the chemistry of interstellar clouds is now recognized to be an important one, the role of negative ions is still regarded as minimal. In this paper, we present two possible exceptions to this view. First, we show that the radiative attachment reaction between electrons and the radical C3N to form the negative ion C3N- is sufficiently rapid to produce an abundance of the negative ion that is possibly large enough to be detected in the interstellar medium. Second, we show that electron-catalyzed isomerization of a metastable isomer M to its stable form S (M + e → M- → transition state → S- → S + e) is facile for conversion of the metastable isomer l-C3H into its stable counterpart c-C3H.