Cold Molecular Clouds Research Articles

Organic Globules in the Tagish Lake Meteorite: Remnants of the Protosolar Disk

Coordinated transmission electron microscopy and isotopic measurements of organic globules in the Tagish Lake meteorite shows that they have elevated ratios of nitrogen-15 to nitrogen-14 (1.2 to 2 times terrestrial) and of deuterium to hydrogen (2.5 to 9 times terrestrial). These isotopic anomalies are indicative of mass fractionation during chemical reactions at extremely low temperatures (10 to 20 kelvin), characteristic of cold molecular clouds and the outer protosolar disk. The globules probably originated as organic ice coatings on preexisting grains that were photochemically processed into refractory organic matter. The globules resemble cometary carbon, hydrogen, oxygen, and nitrogen (CHON) particles, suggesting that such grains were important constituents of the solar system starting materials.

A low fraction of nitrogen in molecular form in a dark cloud

Nitrogen is the fifth most abundant element in the Universe. In the interstellar medium, it has been thought to be mostly molecular (N2). However, N2 has no observable rotational or vibrational transitions, so its abundance in the interstellar medium remains poorly known. In comets, the N2 abundance is very low, while the elemental nitrogen abundance is deficient with respect to the solar value. Moreover, large nitrogen isotopic anomalies are observed in meteorites and interstellar dust particles. Here we report the N2H+ (and by inference the N2) abundance inside a cold dark molecular cloud. We find that only a small fraction of nitrogen in the gas phase is molecular, with most of it being atomic. Because the compositions of comets probably reflect those of dark clouds, this result explains the low N2 abundance in comets. We argue that the elemental nitrogen abundance deficiency in comets can be understood if the atomic oxygen abundance is lower than predicted by present chemical models. Furthermore, the lack of molecular nitrogen in molecular clouds explains the nitrogen anomalies in meteorites and interstellar dust particles, as nitrogen fractionation is enhanced if gaseous nitrogen is atomic.

A viscous heating mechanism for the hot plasma in the Galactic center region

In addition to lines originating in a soft phase at ~0.8 keV and to cold molecular clouds, the X-ray spectra from the Galactic center region also exhibit properties similar to those of a diffuse, thin, very hot plasma at 8 keV on a scale of hundreds of parsecs. This phase is surprising for more than one reason. First, such a hot plasma should not be bound to the Galactic plane and the power needed to sustain the escaping matter would be higher then any known source. Second, there is no known mechanism able to heat the plasma to more than a few keV. Recently we have suggested that, hydrogen having escaped, the hot plasma could be a helium plasma, heavy enough to be gravitationally confined. In this case, the required power is much more reasonable. We present here a possible heating mechanism which taps the gravitational energy of the molecular clouds. We note that the 8 keV plasma is highly viscous and we show how viscous friction of molecular clouds flowing within the hot phase can dissipate energy in the gas and heat it. We detail the MHD wake of a spherical cloud by considering the different MHD waves the cloud can excite. We find that most of the energy is dissipated by the damping of Alfvenic perturbations in two possible manners, namely by non-linear effects and by a large scale curvature of the field lines. We find that the total dissipation rate depends on the field strength. For fields $B\lesssim 200~\mu$G both mechanisms produce power comparable to or higher than the radiative losses; for strong fields $B\gtrsim1$ mG, only the curvature damping can balance the X-ray emission and requires a radius of curvature $R_\mathrm{c}\lesssim 100$ pc; whereas for intermediate fields, the total dissipation is more than one order of magnitude smaller, requiring a higher accretion rate. We note that the plasma parameters may be optimal to make the dissipation most efficient, suggesting a self-regulation mechanism. The loss of kinetic and gravitational energy also causes accretion of the clouds and may have significant action on the gas dynamics in this region between the large scale, bar dominated flow and the central accretion to the massive black hole.

The turbulent interstellar medium

An overview is presented of the main properties of the interstellar medium. Evidence is summarized that the interstellar medium is highly turbulent, driven on different length scales by various energetic processes. Large-scale turbulence determines the formation of structures like filaments and shells in the diffuse interstellar medium. It also regulates the formation of dense, cold molecular clouds. Molecular clouds are now believed to be transient objects that form on timescales of order 10 7 yrs in regions where HI gas is compressed and cools. Supersonic turbulence in the compressed HI slab is generated by a combination of hydrodynamical instabilities, coupled with cooling. Turbulent dissipation is compensated by the kinetic energy input of the inflow. Molecular hydrogen eventually forms when the surface density in the slab reaches a threshold value of ∼ 10 21 cm −2 at which point further cooling triggers the onset of star formation by gravitational collapse. A few Myrs later, the newly formed stars and resulting supernovae will disperse their molecular surrounding and generate new expanding shells that drive again turbulence in the diffuse gas and trigger the formation of a next generation of cold clouds. Although a consistent scenario of interstellar medium dynamics and star formation is emerging many details are still unclear and require more detailed work on microphysical processes as well as a better understanding of supersonic, compressible turbulence. To cite this article: A. Burkert, C. R. Physique 7 (2006).

Pathways to Molecular Complexity

Pathways leading to the formation of complex organic molecules are described. Gas-phase processes that may build large carbon-chain species in cold molecular clouds are summarised. Catalytic reactions on grain surfaces can lead to a large variety of organic species, and model calculations of mantle formation by atom additions to multiply-bonded molecules are presented. The subsequent desorption of these mixed molecular ices can initiate a distinctive organic chemistry via ion-molecule pathways. The predictions of this theory are briefly compared with observations to show how possible organic formation pathways in the interstellar medium may be constrained.

Modelling galaxies with a 3d multi-phase ISM

We present a new particle code for modelling the evolution of galaxies. The code is based on a multi-phase description for the interstellar medium (ISM). We include star formation (SF), stellar feedback by massive stars and planetary nebulae, phase transitions, and interactions between gas clouds and ambient diffuse gas, namely condensation, evaporation, drag, and energy dissipation. The last is realised by radiative cooling and inelastic cloud-cloud collisions. We present new schemes for SF and stellar feedback that include a consistent calculation of the star-formation efficiency (SFE) based on ISM properties, as well as a detailed redistribution of the feedback energy into the different ISM phases. As a first test we show a model of the evolution of a present day Milky-Way-type galaxy. Though the model exhibits a quasi-stationary behaviour in global properties like mass fractions or surface densities, the evolution of the ISM is strongly variable locally depending on the local SF and stellar feedback. We start only with two distinct phases, but a three-phase ISM is formed soon and consists of cold molecular clouds, a warm gas disk, and a hot gaseous halo. Hot gas is also found in bubbles in the disk accompanied by type II supernovae explosions. The volume-filling factor of the hot gas in the disk is ~. The mass spectrum of the clouds follows a power-law with an index of . The star-formation rate (SFR) is ~ on average, decreasing slowly with time due to gas consumption. In order to maintain a constant SFR, gas replenishment, e.g. by infall, of the order is required. Our model is in fair agreement with Kennicutt's (1998, ApJ, 498, 541) SF law including the cut-off at ~. Models with a constant SFE, i.e. no feedback on the SF, fail to reproduce Kennicutt's law. We performed a parameter study varying the particle resolution, feedback energy, cloud radius, SF time scale, and metallicity. In most these cases the evolution of the model galaxy was not significantly different to our reference model. Increasing the feedback energy by a factor of lowers the SF rate by ~, while decreasing the metallicity by a factor of ~100 increases the mass fraction of the hot gas from about 10% to 30%.

Very Cold and Massive Cores near ISOSS J18364−0221: Implications for the Initial Conditions of High‐Mass Star Formation

We report the discovery of two very cold and massive molecular cloud cores in the region ISOSS J18364-0221. The object has been identified by a systematic search for very early evolutionary stages of high-mass stars using the 170 μm ISOPHOT Serendipity Survey (ISOSS). Submillimeter continuum and molecular line measurements reveal two compact cores within this region. The first core has a temperature of 16.5 K, shows signs of ongoing infall and outflows, has no near- or mid-infrared counterpart, and is massive enough (M ~ 75 M☉) to form at least one O star with an associated cluster. It is therefore considered a candidate for a genuine high-mass protostar and a high-mass analog to the Class 0 objects. The second core has an average gas and dust temperature of only ~12 K and a mass of M ~ 280 M☉. Its temperature and level of turbulence are below the values found for massive cores so far, and we suggest that this represents the initial conditions from which high-mass star formation occurs.

Reaction dynamics of carbon-bearing radicals in circumstellar envelopes of carbon stars

Crossed molecular beams experiments on dicarbon molecules, C2(X1sigmag+/a3piu), with unsaturated hydrocarbons acetylene (C2H2(X1sigmag+), ethylene (C2H4(X1Ag)), methylacetylene (CH3CCH(X1A1)), and allene (H2CCCH2 (X1A1)) were carried out at 18 collision energies between 10.6 and 50.3 kJ mol(-1) utilizing a universal crossed beams machine to untangle the reaction dynamics forming hydrogen deficient hydrocarbon radicals in circumstellar envelopes of carbons stars and in cold molecular clouds. We find that all reactions proceed without the entrance barrier through indirect (complex forming) scattering dynamics. Each bimolecular collision is initiated by an addition of the dicarbon molecule to the pi bond of the unsaturated hydrocarbon molecule yielding initially acyclic (triplet) and three- or four-membered cyclic collision complexes (triplet and singlet surface). On the singlet surface, the cyclic structures isomerize to form eventually diacetylene (HCCCCH; C2/C2H2), butatriene (H2CCCCH2; C2/C2H4), methyldiacetylene (CH3CCCCH; C2/CH3CCH), and pentatetraene (H2CCCCCH2; C2/H2CCCH2) intermediates. The latter were found to decompose via atomic hydrogen loss yielding the buta-1,3-diynyl [C4H(X2sigma+) HCCCC], 1-butene-3-yne-2-yl [i-C4H3(X2A') H2CCCCH], penta-2,4-diynyl-1 [C5H3(X2B1) HCCCCCH2], and penta-1,4-diynyl-3 radical [C5H3(X2B1) HCCCHCCH] under single collision conditions. The underlying characteristics of these dicarbon versus atomic hydrogen replacement pathways (indirect scattering dynamics; no entrance barrier; isomerization barriers below the energy of the separated reactants; exoergic reactions) suggest the enormous potential of the dicarbon plus unsaturated hydrocarbon reaction class to form highly hydrogen-deficient carbonaceous molecules in cold molecular clouds and in circumstellar envelopes of carbon stars. The studies therefore present an important advancement in establishing a comprehensive database of reaction intermediates and products involved in bimolecular collisions of dicarbon molecules with unsaturated hydrocarbons which can be utilized in refined astrochemical models and also in future searches of hitherto unidentified interstellar molecules. Implications of these experiments to understand related combustion processes are also addressed.

A crossed beams study of the reaction of carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X1A′)—investigating the formation of cyano propargyl radicals

The chemical dynamics of the reaction of ground state carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X 1A'), were examined under single collision conditions at collision energies of 29.9 and 43.9 kJ mol(-1) using the crossed molecular beams approach. The experimental studies were combined with electronic structure calculations on the triplet C4H3N potential energy surface (H. F. Su, R. I. Kaiser, A. H. H. Chang, J. Chem. Phys., 2005, 122, 074320). Our investigations suggest that the reaction follows indirect scattering dynamics via addition of the carbon atom to the carbon-carbon double bond of the vinyl cyanide molecule yielding a cyano cyclopropylidene collision complex. The latter undergoes ring opening to form cis/trans triplet cyano allene which fragments predominantly to the 1-cyano propargyl radical via tight exit transition states; the 3-cyano propargyl isomer was inferred to be formed at least a factor of two less; also, no molecular hydrogen elimination channel was observed experimentally. These results are in agreement with the computational studies predicting solely the existence of a carbon versus hydrogen atom exchange pathway and the dominance of the 1-cyano propargyl radical product. The discovery of the cyano propargyl radical in the reaction of atomic carbon with vinyl cyanide under single collision conditions implies that this molecule can be an important reaction intermediate in combustion flames and also in extraterrestrial environments (cold molecular clouds, circumstellar envelopes of carbon stars) which could lead to the formation of cyano benzene (C6H5CN) upon reaction with a propargyl radical.

Laboratory Studies on the Formation of Three C2H4O Isomers—Acetaldehyde (CH3CHO), Ethylene Oxide (c‐C2H4O), and Vinyl Alcohol (CH2CHOH)—in Interstellar and Cometary Ices

Laboratory experiments were conducted to unravel synthetic routes to form three C2H4O isomers—acetaldehyde (CH3CHO),ethyleneoxide(c-C2H4O),andvinylalcohol(CH2CHOH)—inextraterrestrialicesviaelectronicenergy transfer processes initiated by electrons in the track of MeV ion trajectories. Here we present the results of electron irradiation on a 2:1 mixture of carbon dioxide (CO2) and ethylene (C2H4). Our studies suggest that suprathermal oxygen atoms can add to the carbon-carbonbond of an ethylene molecule to form initially an oxirene diradical (addition to one carbon atom) and the cyclic ethylene oxide molecule (addition to two carbon atoms) at 10 K. The oxirenediradicalcanundergoa(1,2)-Hshifttotheacetaldehydemolecule.Boththeethyleneoxideandtheacetaldehyde isomerscanbestabilizedinthesurroundingicematrix.Toaminoramount,suprathermaloxygenatomscaninsertinto a carbon-hydrogen bond of the ethylene molecule, forming vinyl alcohol. Once these isomers have been synthesized inside the ice layers of the coated grains in cold molecular clouds, the newly formed molecules can sublime as the cloudreaches thehot molecular corestage. These laboratory investigations helpto explainastronomicalobservations by Nummelin et al. and Ikeda et al. toward massive star-forming regions and hot cores, where observed fractional abundances of these isomers are higher than can be accounted for by gas-phase reactions alone. Similar synthetic routes could help explain the formation of acetaldehyde and ethylene oxide in comet C/1995 O1 (Hale-Bopp) and also suggest a presence of both isomers in Titan's atmosphere. Subject headingg astrobiology — astrochemistry — comets:general — ISM:molecules — methods:laboratory — molecular processes — planets and satellites: individual (Titan)

A Hot Helium Plasma in the Galactic Center Region

Recent X-ray observations by the space mission Chandra confirmed the astonishing evidence of a diffuse, hot, thermal plasma at a temperature of ~9 × 107 K (~8 keV) that was found by previous surveys to extend over a few hundred parsecs in the Galactic center region. This plasma coexists with the usual components of the interstellar medium, such as cold molecular clouds and a soft (0.8 keV) component produced by supernova remnants, and its origin remains uncertain. First, simple calculations using a mean sound speed for a hydrogen-dominated plasma have suggested that it should not be gravitationally bound, and thus it requires a huge energy source to heat it in less than the escape time. Second, an astrophysical mechanism must be found to generate such a high temperature. No known source has been identified to fulfill both requirements. Here we address the energetics problem and show that the hot component could actually be a gravitationally confined helium plasma. We illustrate the new prospects that this opens up by discussing the origin of this gas and by suggesting possible heating mechanisms.

On the Nature of Feedback Heating in Cooling Flow Clusters

We study the feedback between heating and cooling of the intracluster medium (ICM) in cooling flow (CF) galaxies and clusters. We adopt the popular view that the heating is due to an active galactic nucleus (AGN), i.e., a central black hole accreting mass and launching jets and/or winds. We propose that the feedback occurs within the entire cool inner region (r 5-30 kpc), where the nonlinear overdense blobs of gas with a density contrast ρ/ρa 2 cool fast and are removed from the ICM before experiencing the next major AGN heating event. We term this scenario feedback. Some of these blobs cool and sink toward the central black hole, while others might form stars and cold molecular clouds. We derive the conditions under which the dense blobs formed by perturbations might cool to low temperatures (T < 104 K) and feed the black hole. The main conditions are found to be as follows. (1) An overdense blob must be prevented from reaching an equilibrium position in the ICM; therefore, it has to cool fast, and the density profile of the ambient gas should be shallow. (2) Nonlinear perturbations are required; they might have chiefly formed by previous AGN activity. (3) The cooling time of these nonlinear perturbations should be short relative to a few times the typical interval between successive AGN outbursts. (4) In order not to evaporate the blobs, thermal conduction around the blobs should be suppressed by a factor 10-3.

Supernova Olivine from Cometary Dust

An interplanetary dust particle contains a submicrometer crystalline silicate aggregate of probable supernova origin. The grain has a pronounced enrichment in 18O/16O (13 times the solar value) and depletions in 17O/16O (one-third solar) and 29Si/28Si (<0.8 times solar), indicative of formation from a type II supernova. The aggregate contains olivine (forsterite 83) grains <100 nanometers in size, with microstructures that are consistent with minimal thermal alteration. This unusually iron-rich olivine grain could have formed by equilibrium condensation from cooling supernova ejecta if several different nucleosynthetic zones mixed in the proper proportions. The supernova grain is also partially encased in nitrogen-15-rich organic matter that likely formed in a presolar cold molecular cloud.

A Combined Experimental and Computational Investigation on the Synthesis of Acetaldehyde [CH3CHO(X1A′)] in Interstellar Ices

The synthetic routes to form acetaldehyde (CH3CHO(X 1 A 0 )) in extraterrestrial ices were investigated experi- mentally in a contamination-free ultrahigh vacuum scattering machine. Binary ice mixtures of carbon monoxide (CO(X 1 � + )) and methane (CH4(X 1 A1)) were condensed at 10 K onto a silver monocrystal and irradiated with 5 keV electrons to mimic the electronic energytransfer processes initiated by MeV cosmic-ray particle-induced � -electrons in the ''ultratrack'' of MeV ion trajectories; the carbon monoxide-methane ices served as model compounds to simulate neighboring COCH4 molecules in astrophysical ices, as present in cold molecular clouds and in cometary matter. Upon completion of the high-energy processing, the ice samples sublimed during the heating phase to 293 K, thus releasing the remaining reactants as well as the newly formed molecules into the gas phase. The experiment was monitoredonlineandinsituviaaFouriertransforminfrared(FTIR)spectrometerinabsorption-reflection-absorption mode (solid state) and a quadrupole mass spectrometer (gas phase). Our investigations were combined with electronic structure calculations. At 10 K, the primary reaction step involved the cleavage of the carbon-hydrogen bond of the methane moleculevia an electronic energy transfer process from the impinging electron tothe methanemolecule to form a methyl radical (CH3(X 2 A 00 )) plus a hydrogen atom (H( 2 S1/2)). The H atom contains the excess energy in the form of translational motion; suprathermal hydrogen atoms can add to the carbon-oxygen triple bond of the carbon monoxide molecule, overcoming the entrance barrier, to yield the formyl radical (HCO(X 2 A 0 )). Depending on the reactant geometry inside the matrix cage, the formyl radical recombined barrierlessly with the neighboring methyl radical inside the ices at 10 K. Upon warming of the ice sample, the acetaldehyde molecules sublime into the gas phase. This process mimics the sublimation of molecules from the grain mantles into the gas phase upon the transition ofthemolecularcloudtothehotmolecularcorephase.Thismechanismtoformacetaldehydeinsideinterstellarices (cold molecular clouds; 10 K) upon high-energy processing, followed by a radical-radical recombination and sub- limation in the hot core phase (molecular cores; few 100 K), presents a compelling route to account for high fractional abundances of acetaldehyde of a few times 10 � 9 toward star-forming regions, as compared to abundances of only some 10 � 10 in the cold cloud TMC-1, where solely gas-phase reactions are supposed to synthesize acetaldehyde. Subject headingg astrobiology — astrochemistry — comets:general — ISM:molecules — methods:laboratory — molecular processes

A theoretical study for the reaction of vinyl cyanide C2H3CN(XA′1) with the ground state carbon atom C(P3) in cold molecular clouds

The reaction of the ground state atomic carbon, C(3P), with simple unsaturated nitrile, C2H3CN(X1A' (vinyl cyanide), is investigated theoretically to explore the probable routes for the formation of carbon-nitrogen-bearing species in extraterrestrial environments particularly of ultralow temperature. Five collision complexes without entrance barrier as a result of the carbon atom addition to the pi systems of C2H3CN are characterized. The B3YLP/6-311G(d,p) level of theory is utilized in obtaining the optimized geometries, harmonic frequencies, and energies of the intermediates, transition states, and products along the isomerization and dissociation pathways of each collision complex. Subsequently, with the facilitation of computed RRKM rate constants at collision energy of 0-10 kcal/mol, the most probable paths for each collision complexes are determined, of which the CCSD(T)/6-311G(d,p) energies are calculated. The major products predicted are exclusively due to the hydrogen atom dissociations, while the products of H2, CN, and CH2 decompositions are found negligible. Among many possible H-elimination products, cyano propargyl (p4) and 3-cyano propargyl (p5) are the most probable, in which p5 can be formed via two intermediates, cyano allene (i8) and cyano vinylmethylene (i6), while p4 is yielded from i8. The study suggests this class of reaction is an important route to the synthesis of unsaturated nitriles at the temperature as low as 10 K, and the results are valuable for future chemical models of interstellar clouds.

Star Formation in the Galaxy: An Observational Overview

The problems of star and planet formation are among the most important challenges facing modern astrophysical research. Stars and their accompanying planetary systems are continuously being formed in the Galaxy enabling direct observation and investigation of the star forming process. However, stars form invisibly deep within cold and dark molecular clouds. Observations of these stellar birth sites at infrared and millimeter wavelengths from space and the ground have resulted in considerable progress toward a physical understanding of stellar origins. In this contribution I will review the empirical basis for our current understanding of the process of star formation with an emphasis on the origin of low mass (sunlike) stars.

Millimeter dust emission from an SMC cold molecular cloud

Millimeter dust emission from an SMC cold molecular cloud

Fire and Ice: Spitzer Infrared Spectrograph (IRS) Mid‐Infrared Spectroscopy of IRAS F00183−7111

We report the detection of strong absorption and weak emission features in the 4-27 μm Spitzer Infrared Spectrograph (IRS) spectrum of the distant ultraluminous infrared galaxy IRAS F00183-7111 (z = 0.327). The absorption features of CO2 and CO gas, water ice, hydrocarbons, and silicates are indicative of a strongly obscured (A9.6 ≥ 5.4; AV ≥ 90) and complex line of sight through both the hot diffuse interstellar medium and shielded cold molecular clouds toward the nuclear power source. From the profile of the 4.67 μm CO fundamental vibration mode, we deduce that the absorbing gas is dense (n ~ 106 cm-3) and warm (720 K) and has a CO column density of ~1019.5 cm-2, equivalent to NH ~ 1023.5 cm-2. The high temperature and density, as well as the small inferred size (<0.03 pc), locates this absorbing gas close to the power source of this region. Weak emission features of molecular hydrogen, polycyclic aromatic hydrocarbons (PAHs), and Ne+, likely associated with star formation, are detected against the 9.7 μm silicate feature, indicating an origin away from the absorbing region. Based on the 11.2 μm PAH flux, we estimate the star formation component to be responsible for up to 30% of the IR luminosity of the system. While our mid-infrared spectrum shows no telltale signs of active galactic nucleus (AGN) activity, the similarities to the mid-infrared spectra of deeply obscured sources (e.g., NGC 4418) and AGN hot dust (e.g., NGC 1068), as well as evidence from other wavelength regions, suggest that the power source hiding behind the optically thick dust screen may well be a buried AGN.

Resetting chemical clocks of hot cores based on S-bearing molecules

We report a theoretical study of sulphur chemistry, as applied to hot cores, where S-bearing molecular ratios have been previously proposed and used as chemical clocks. As in previous models, we follow the S-bearing molecular composition after the injection of grain mantle components into the gas phase. For this study, we developed a time-dependent chemical model with up-to-date reaction rate coefficients. We ran several cases, using different realistic chemical compositions for the grain mantles and for the gas prior to mantle evaporation. The modeling shows that S-bearing molecular ratios depend very critically on the gas temperature and density, the abundance of atomic oxygen, and, most importantly, on the form of sulphur injected in the gas phase, which is very poorly known. Consequently, ratios of S-bearing molecules cannot be easily used as chemical clocks. However, detailed observations and careful modeling of both physical and chemical structure can give hints on the source age and constrain the mantle composition (i.e. the form of sulphur in cold molecular clouds) and, thus, help to solve the mystery of the sulphur depletion. We analyse in detail the cases of Orion and IRAS16293-2422. The comparison of the available observations with our model suggests that the majority of sulphur released from the mantles is mainly in, or soon converted into, atomic form.

The nature of molecular cloud material in interplanetary dust

Eight interplanetary dust particles (IDPs) exhibiting a wide range of H and N isotopic anomalies have been studied by transmission electron microscopy, x-ray absorption near-edge structure spectroscopy, and Fourier-transform infrared spectroscopy. These anomalies are believed to have originated during chemical reactions in a cold molecular cloud that was the precursor to the Solar System. The chemical and mineralogical studies reported here thus constitute direct studies of preserved molecular cloud materials. The H and N isotopic anomalies are hosted by different hydrocarbons that reside in the abundant carbonaceous matrix of the IDPs. Infrared measurements constrain the major deuterium (D) host in the D-enriched IDPs to thermally labile aliphatic hydrocarbon groups attached to macromolecular material. Much of the large variation observed in D/H in this suite of IDPs reflects the variable loss of this labile component during atmospheric entry heating. IDPs with elevated 15N/ 14N ratios contain N in the form of amine (-NH 2) functional groups that are likely attached to other molecules such as aromatic hydrocarbons. The host of the N isotopic anomalies is not as readily lost during entry heating as the D-rich material. Infrared analysis shows that while the organic matter in primitive anhydrous IDPs is similar to that observed in acid residues of primitive chondritic meteorites, the measured aromatic:aliphatic ratio is markedly lower in the IDPs.