Interstellar Regions Research Articles

Isomerization and fragmentation of acetonitrile upon interaction with N(4S) atoms: the chemistry of nitrogen in dense molecular clouds

We experimentally show that the reaction between ground state nitrogen atoms N(4S) and acetonitrile CH3CN can lead to two distinct chemical pathways that are both thermally activated at very low temperatures. First is CH3CN isomerization which produces CH3NC and H2CCNH. Second is CH3CN decomposition which produces HNC and CH3CNH+CN− fragments, with the possible release of H2. Our results reveal that the mobility of N(4S)-atoms is stimulated in the 3–11 K temperature range, and that its subsequent encounter with one acetonitrile molecule is sufficient for the aforementioned reactions to occur without the need for additional energy to be supplied to the CH3CN + N(4S) system. These findings shed more light on the nitrogen chemistry that can possibly take place in dense molecular clouds, which until now was thought to only involve high-energy processes and therefore be unlikely to occur in such cold and dark interstellar regions. The reaction pathways we propose in this study have very important astrochemical implications, as it was shown recently that the atomic nitrogen might be more abundant, in many interstellar icy grain mantles, than previously thought. Also, these reaction pathways can now be considered within dense molecular clouds, and possibly affect the branching ratios for N-bearing molecules computed in astrochemical modelling.

Photodesorption and physical properties of CO ice as a function of temperature

Context. Ice photodesorption has been the topic of recent studies that aim to interpret the abundances of gas-phase molecules, in particular CO, toward cold interstellar regions. But little is known about the effect of the ice’s physical properties on the photodesorption rate. The linear decrease observed in the photodesorption rate, as a function of increasing CO ice deposition temperature, was provisionally attributed to a more compact CO ice structure.Aims. The goal of this work is to monitor the physical properties of solid CO as a function of ice deposition temperature. Then, we evaluate the possible link between the structure of ice and the ice’s photodesorption rate.Methods. Infrared spectroscopy is an efficient tool to monitor the structural evolution of pure ices during warm-up or irradiation. The infrared absorption bands of molecular ice components observed toward various space environments allow for the detection of H2 O, CO, CO2 , CH3 OH, NH3 , etc. Typically, a pure ice that is composed of one of these species displays significant changes in their mid-infrared band profiles as a result of warm-up. But, at most, only very subtle changes appear in the narrow CO ice infrared absorption band as the result of warm-up. We, therefore, also used vacuum-ultraviolet spectroscopy of CO ice to monitor the effect of temperature in the physical properties of the ice. Finally, temperature-programmed desorption and photo-desorption experiments for different CO ice deposition temperatures were performed.Results. Mid-infrared and vacuum-ultraviolet spectroscopy showed that warm-up of CO ice that is deposited at 8 K did not lead to structural changes. Only CO ice samples deposited at temperatures above 20 K displayed different spectroscopic properties compared to lower deposition temperatures. The observed gradual and linear drop in the photodesorption rate of CO ice, as a function of increasing ice deposition temperature in the 7 to 20 K range, is, therefore, not due to a gradual re-structuring toward a more compact and crystalline ice, which is only triggered above 20 K and increases for higher deposition temperatures.Conclusions. We suggest that this decrease of the photodesorption rate is related to the disorder of CO dipole molecules within the amorphous or glassy state, which influences the necessary transfer of photon energy from the first excited molecule to the desorbing molecule on the ice surface. The photodesorption yield of CO deposited at 20 K is about four times lower than at 7 K. Dust models should adopt CO photodesorption yields that are compatible with the thermal history of the cloud.

Potential formation of three pyrimidine bases in interstellar regions

Work on the chemical evolution of pre-biotic molecules remains incomplete since the major obstacle is the lack of adequate knowledge of rate coefficients of various reactions which take place in interstellar conditions. In this work, we study the possibility of forming three pyrimidine bases, namely, cytosine, uracil and thymine in interstellar regions. Our study reveals that the synthesis of uracil from cytosine and water is quite impossible under interstellar circumstances. For the synthesis of thymine, reaction between uracil and $:\mathrm{CH}_{2}$ is investigated. Since no other relevant pathways for the formation of uracil and thymine were available in the literature, we consider a large gas-grain chemical network to study the chemical evolution of cytosine in gas and ice phases. Our modeling result shows that cytosine would be produced in cold, dense interstellar conditions. However, presence of cytosine is yet to be established. We propose that a new molecule, namely, $\mathrm{C}_{4}\mathrm{N}_{3}\mathrm{OH}_{5}$ could be observable in the interstellar region. $\mathrm{C}_{4}\mathrm{N}_{3}\mathrm{OH}_{5}$ is a precursor (Z isomer of cytosine) of cytosine and far more abundant than cytosine. We hope that observation of this precursor molecule would enable us to estimate the abundance of cytosine in interstellar regions. We also carry out quantum chemical calculations to find out the vibrational as well as rotational transitions of this precursor molecule along with three pyrimidine bases.

MAGNETIC FLUX TUBE INTERCHANGE AT THE HELIOPAUSE

The magnetic field measured by Voyager 1 prior to its heliocliff encounter on 2012.65 showed an unexpectedly complex transition from the primarily azimuthal inner-heliosheath field to the draped interstellar field tilted by some 20° to the nominal azimuthal direction. Most prominent were two regions of enhanced magnetic field strength depleted in energetic charged particles of heliospheric origin. These regions were interpreted as magnetic flux tubes connected to the outer heliosheath that provided a path for the particles to escape. Despite large increases in strength, the field's direction did not change appreciably at the boundaries of these flux tubes. Rather, the field's direction changed gradually over several months prior to the heliocliff crossing. It is shown theoretically that the heliopause, as a pressure equilibrium layer, can become unstable to interchange of magnetic fields between the inner and the outer heliosheaths. The curvature of magnetic field lines and the anti-sunward gradient in plasma kinetic pressure provide conditions favorable for an interchange. Magnetic shear between the heliosheath and the interstellar fields reduces the growth rates, but does not fully stabilize the heliopause against perturbations propagating in the latitudinal direction. The instability could create a transition layer permeated by magnetic flux tubes, oriented parallel to each other and alternately connected to the heliosheath or the interstellar regions.

RARE ISOTOPIC SPECIES OF SULFUR MONOXIDE: THE ROTATIONAL SPECTRUM IN THE THz REGION

Many sulphur-bearing species have been detected in different astronomical environments and have allowed to derive important information about the chemical and physical composition of interstellar regions. In particular, these species have also been showed to trace and probe hot-core environment time evolution. Among the most prominent sulphur-bearing molecules, SO, sulphur monoxide radical, is one of the more ubiquitous and abundant, observed also in its isotopic substituted species such as $^{34}$SO and S$^{18}$O. Due to the importance of this simple diatomic system and to face the challenge of modern radioastronomical facilities, an extension to THz range of the rare isotopologues of sulphur monoxide has been performed. High-resolution rotational molecular spectroscopy has been employed to extend the available dataset of four isotopic species, SO, $^{34}$SO, S$^{17}$O, and S$^{18}$O up to the 1.5 THz region. The frequency coverage and the spectral resolution of our measurements allowed a better constraint of the molecular constants of the four species considered, focusing especially for the two oxygen substituted isotopologues. Our measurements were also employed in an isotopically invariant fit including all available pure rotational and ro-vibrational transitions for all SO isotopologues, thus enabling accurate predictions for rotational transitions at higher frequencies. Comparison with recent works performed on the same system are also provided, showing the quality of our experiment and the improvement of the datasets for all the species here considered. Transition frequencies for this system can now be used with confidence by the astronomical community well into the THz spectral region.

HNC IN PROTOPLANETARY DISKS

The distributions and abundances of small organics in protoplanetary disks are potentially powerful probes of disk physics and chemistry. HNC is a common probe of dense interstellar regions and the target of this study. We use the Submillimeter Array (SMA) to observe HNC 3–2 toward the protoplanetary disks around the T Tauri star TW Hya and the Herbig Ae star HD 163296. HNC is detected toward both disks, constituting the first spatially resolved observations of HNC in disks. We also present SMA observations of HCN 3–2 and IRAM 30 m observations of HCN and HNC 1–0 toward HD 163296. The disk-averaged HNC/HCN emission ratio is 0.1–0.2 toward both disks. Toward TW Hya, the HNC emission is confined to a ring. The varying HNC abundance in the TW Hya disk demonstrates that HNC chemistry is strongly linked to the disk physical structure. In particular, the inner rim of the HNC ring can be explained by efficient destruction of HNC at elevated temperatures, similar to what is observed in the ISM. However, to realize the full potential of HNC as a disk tracer requires a combination of high SNR spatially resolved observations of HNC and HCN and disk-specific HNC chemical modeling.

Molecular hydrogen emission in the interstellar medium of the Large Magellanic Cloud

We present the detection and analysis of molecular hydrogen emission towards ten interstellar regions in the Large Magellanic Cloud. We examined low-resolution infrared spectral maps of 12 regions obtained with the Spitzer infrared spectrograph (IRS). The pure rotational 0–0 transitions of H2 at 28.2 and 17.1 μm are detected in the IRS spectra for 10 regions. The higher level transitions are mostly upper limit measurements except for three regions, where a 3σ detection threshold is achieved for lines at 12.2 and 8.6 μm. The excitation diagrams of the detected H2 transitions are used to determine the warm H2 gas column density and temperature. The single-temperature fits through the lower transition lines give temperatures in the range 86–137 K. The bulk of the excited H2 gas is found at these temperatures and contributes ∼5–17 per cent to the total gas mass. We find a tight correlation of the H2 surface brightness with polycyclic aromatic hydrocarbon and total infrared emission, which is a clear indication of photoelectric heating in photodissociation regions. We find the excitation of H2 by this process is equally efficient in both atomic- and molecular-dominated regions. We also present the correlation of the warm H2 physical conditions with dust properties. The warm H2 mass fraction and excitation temperature show positive correlations with the average starlight intensity, again supporting H2 excitation in photodissociation regions.

Low-temperature surface formation of NH3 and HNCO: hydrogenation of nitrogen atoms in CO-rich interstellar ice analogues

Solid state astrochemical reaction pathways have the potential to link the formation of small nitrogen-bearing species, like NH3 and HNCO, and prebiotic molecules, specifically amino acids. To date, the chemical origin of such small nitrogen containing species is still not well understood, despite the fact that ammonia is an abundant constituent of interstellar ices toward young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of the present work is to experimentally investigate the formation routes of NH3 and HNCO through non-energetic surface reactions in interstellar ice analogues under fully controlled laboratory conditions and at astrochemically relevant temperatures. This study focuses on the formation of NH3 and HNCO in CO-rich (non-polar) interstellar ices that simulate the CO freeze-out stage in dark interstellar cloud regions, well before thermal and energetic processing start to become relevant. We demonstrate and discuss the surface formation of solid HNCO through the interaction of CO molecules with NH radicals - one of the intermediates in the formation of solid NH3 upon sequential hydrogenation of N atoms. The importance of HNCO for astrobiology is discussed.

MHD flows at astropauses and in astrotails

Abstract. The geometrical shapes and the physical properties of stellar wind – interstellar medium interaction regions form an important stage for studying stellar winds and their embedded magnetic fields as well as cosmic ray modulation. Our goal is to provide a proper representation and classification of counter-flow configurations and counter-flow interfaces in the frame of fluid theory. In addition we calculate flows and large-scale electromagnetic fields based on which the large-scale dynamics and its role as possible background for particle acceleration, e.g., in the form of anomalous cosmic rays, can be studied. We find that for the definition of the boundaries, which are determining the astropause shape, the number and location of magnetic null points and stagnation points is essential. Multiple separatrices can exist, forming a highly complex environment for the interstellar and stellar plasma. Furthermore, the formation of extended tail structures occur naturally, and their stretched field and streamlines provide surroundings and mechanisms for the acceleration of particles by field-aligned electric fields.

STELLAR METALLICITY OF THE EXTENDED DISK AND DISTANCE OF THE SPIRAL GALAXY NGC 3621

Low resolution ESO VLT/FORS spectra of blue supergiant stars are analyzed to determine stellar metallicities (based on elements such as Fe, Ti, Mg) in the extended disk of the spiral galaxy NGC3621. Mildly subsolar metallicity (-0.30 dex) is found for the outer objects beyond 7 kpc independent of galactocentric radius and compatible with the absence of a metallicity gradient confirming the results of a recent investigation of interstellar medium HII region gas oxygen abundances. The stellar metallicities are slightly higher than those from the HII regions when based on measurements of the weak forbidden auroral oxygen line at 4363 AE but lower than the ones obtained with the R23 strong line method. It is shown that the present level of metallicity in the extended disk cannot be the result of chemical evolution over the age of the disk with the present rate of in situ star formation. Additional mechanisms must be involved. In addition to metallicity, stellar effective temperatures, gravities, interstellar reddening, and bolometric magnitudes are determined. After application of individual reddening corrections for each target the flux-weighted gravity-luminosity relationship of blue supergiant stars is used to obtain a distance modulus of 29.07+/-0.09 mag (distance D=6.52+/-0.28 Mpc). This new distance is discussed in relation to Cepheid and tip of the red giant branch distances.

Radiation Processing of Formamide and Formamide:Water Ices on Silicate Grain Analogue

Lyman-α (121.6 nm) photon and 1 keV electron-beam irradiation of pure HCONH2 (FA) ice and H2O:HCONH2 ice mixtures on high-surface-area SiO2 nanoparticles have been investigated with FT-IR spectroscopy and temperature programmed desorption (TPD). Lyman-α photolysis of pure amorphous FA ice grown at 70 K and crystalline FA ice produced by annealing to 165 K gives spectral signatures between 2120 and 2195 cm(-1) that we assign primarily to OCN(-) and CO. The OCN(-) and CO yields are ∼25% less abundant for crystalline FA ice. Photon and electron processing also produces H2 that is released from the ice between ∼90 and 140 K. A decrease in the H2 TPD peak is seen for irradiated crystalline HCONH2 ice. Lyman-α photolysis of H2O:HCONH2 mixed ices increases OCN(-) and CO production, suggesting a catalytic role of H2O. Also, for pure FA, 1 keV electron irradiation slightly increases the yield of OCN(-), while CO decarboxylation is selectively prevented. CO is also not produced in H2O:HCONH2 ices upon electron irradiation. Dissociative ionization, direct dissociative excitation, and dissociative electron attachment (DEA) channels are accessible in the Lyman-α (121.6 nm) photon and 1 keV electron-beam energy range. DEA energetically favors OCN(-) and H(-) formation, with the latter leading to H2 formation. The FA fragment product identities, yields, and branching ratios are considerably different relative to the gas phase and depend upon the radiation type, ice structure, and the presence of SiO2 nanoparticles. The latter may increase ion-electron recombination and radical recombination rates. The main products observed suggest very different condensed-phase dissociation channels from those reported for gas-phase dissociation. Formation of ions/products from FA is not negligible upon Lyman-α photolysis or electron irradiation, both of which could process ices in interstellar regions as well as in Titan's atmosphere.

RADIATION DAMAGE AND ASSOCIATED PHASE CHANGE EFFECT ON PHOTODESORPTION RATES FROM ICES—LYα STUDIES OF THE SURFACE BEHAVIOR OF CO2(ICE)

Photodesorption from a crystalline film of CO{sub 2}(ice) at 75 K has been studied using Lyα (10.2 eV) radiation. We combine quantitative mass spectrometric studies of gases evolved and transmission IR studies of species trapped in the ice. Direct CO desorption is observed from the primary CO{sub 2} photodissociation process, which occurs promptly for CO{sub 2} molecules located on the outermost surface of the ice (Process I). As the fluence of Lyα radiation increases to ∼5.5 × 10{sup 17} photons cm{sup –2}, extensive damage to the crystalline ice occurs and photo-produced CO molecules from deeper regions (Process II) are found to desorb at a rapidly increasing rate, which becomes two orders of magnitude greater than Process I. It is postulated that deep radiation damage to produce an extensive amorphous phase of CO{sub 2} occurs in the 50 nm ice film and that CO (and CO{sub 2}) diffusive transport is strongly enhanced in the amorphous phase. Photodesorption in Process II is a combination of electronic and thermally activated processes. Radiation damage in crystalline CO{sub 2} ice has been monitored by its effects on the vibrational line shapes of CO{sub 2}(ice). Here the crystalline-to-amorphous phase transition has been correlated with the occurrencemore » of efficient molecular transport over long distances through the amorphous phase of CO{sub 2}(ice). Future studies of the composition of the interstellar region, generated by photodesorption from ice layers on grains, will have to consider the significant effects of radiation damage on photodesorption rates.« less

IONIZATION IN ATMOSPHERES OF BROWN DWARFS AND EXTRASOLAR PLANETS. IV. THE EFFECT OF COSMIC RAYS

Cosmic rays provide an important source for free electrons in the Earth's atmosphere and also in dense interstellar regions where they produce a prevailing background ionization. We utilize a Monte Carlo cosmic ray transport model for particle energies of 1 MeV < E < 1 GeV, and an analytic cosmic ray transport model for particle energies of 1 GeV < E < 1 TeV in order to investigate the cosmic ray enhancement of free electrons in substellar atmospheres of free-floating objects. The cosmic ray calculations are applied to Drift-Phoenix model atmospheres of an example brown dwarf with effective temperature Teff = 1500 K, and two example giant gas planets (Teff = 1000 K, 1500 K). For the model brown dwarf atmosphere, the electron fraction is enhanced significantly by cosmic rays when the pressure pgas < 10^-2 bar. Our example giant gas planet atmosphere suggests that the cosmic ray enhancement extends to 10^-4 - 10^-2 bar, depending on the effective temperature. For the model atmosphere of the example giant gas planet considered here (Teff = 1000 K), cosmic rays bring the degree of ionization to fe ~ 10^-8 when pgas < 10^-8 bar, suggesting that this part of the atmosphere may behave as a weakly ionized plasma. Although cosmic rays enhance the degree of ionization by over three orders of magnitude in the upper atmosphere, the effect is not likely to be significant enough for sustained coupling of the magnetic field to the gas.

Interstellar C&amp;lt;sub&amp;gt;60&amp;lt;/sub&amp;gt;

Fullerenes are cage-like molecules of pure carbon, such as C60, C70, C76, and C84. C60, also known as buckminster- lrene, is the most stable fullerene and has a soccer-ball like structure. The presence of fullerenes in space has been suggested and observationally explored since their first synthesis in the laboratory in 1985 by Harry Kroto and his colleagues which earned them the 1996 Nobel prize in chemistry. C60 (as well as C70) has recently been detected in reflection nebulae, post-asymptotic giant branch (AGB) stars, pre-planetary nebulae, planetary nebulae, Herbig Ae/Be stars, and young stellar objects through their characteristic infrared emission bands. The formation of C60 in interstellar and circumstellar environments is not firmly established. Experimental studies have shown that C60 can be made by gas-phase condensation (e.g. through vaporization of graphite) in a hydrogen-poor environment. In view of the simultaneous detection of C60 and polycyclic aromatic hydrocarbon (PAH) molecules in hydrogen-rich interstellar and circumstellar regions, it has also been suggested that C60 could be generated by the decomposition of hydrogenated amorphous carbon, or the destruction of PAHs, both induced by shocks and/or ultraviolet photoprocessing. The phase (gas or solid) and excitation mechanism of C60 in interstellar and circumstellar conditions are also hotly debated in the literature. One model suggests that C60 is attached to dust and emits in solid-phase at the equilibrium temperature of the dust. Another model suggests that C60 is stochastically excited by ultraviolet photons and emits in the gas-phase. We prefer the latter model as in interstellar and circumstellar conditions the energy content of a C60 molecule is often smaller than the energy of a single starlight photon and C60 is expected to undergo stochastical heating.

Analogs of solid nanoparticles as precursors of aromatic hydrocarbons

Context. Aromatic =CH and C=C vibrational bands have been observed within shocked interstellar regions, indicating the presence of aromatic emission carriers such as PAHs, which may have been created from adjacent molecular cloud material by interaction with a shock front.Aims. We investigate the evolution of the aromatic =CH and C=C vibrational modes at 3.3 and 6.2 μ m wavelength in heated HAC materials, PAHs and mixed PAHs and HACs, respectively, aiming at an explanation of the evolution of carbonaceous dust grains in the shocked regions.Methods. Materials used in these analogs (HAC and PAH materials) were prepared by the laser ablation and the laser pyrolysis methods, respectively. The transmission electron microscopy (TEM) in high-resolution mode was used as an analytical technique to characterize the aromatic layers in HACs. Spectroscopic analysis was prformed in the mid-IR range.Results. A remarkable destruction of aliphatic structures in HACs has been observed with the thermal processing, while aromatic structures become dominating by increasing the diameters of the graphene layers. The aromatic bands at 3.3 and 6.2 μ m, observed in the laboratory spectra of PAHs and of the combination of the PAHs and HAC materials, are also clearly observed in the spectrum of the heated HACs. These bands agree with those of aromatic bands observed in astronomical observations. Conclusions. Aromatization of HACs could be a pre-stage in the decomposition process of hydrocarbons that form PAH-clusters in such hot interstellar medium.

Ab initio study on the potential energy surfaces of NCO2+

Charged molecular species have special roles in upper atmosphere and interstellar regions. An ab initio calculation for NCO2+ electronic structures by cc-pVQZ/CASSCF/MRCI method is presented. The potential energy surfaces of low-lying states of NCO2+ have been explored. The ground state X 2Π and the lowest excited state a 4Σ− are metastable with a major collinear dissociation channel into N+ (X 3Pg)+CO+ (X 2Σ+). For the excited states from 3 to 8eV the pre-dissociation processes are dominant while the states above 8eV are repulsive. Several crossings, avoided crossings and conical intersections for NCO2+ have also been assigned.

ORTHO-PARA SELECTION RULES IN THE GAS-PHASE CHEMISTRY OF INTERSTELLAR AMMONIA

The ortho–para chemistry of ammonia in the cold interstellar medium is investigated using a gas-phase chemical network. Branching ratios for the primary reaction chain involved in the formation and destruction of ortho- and para-NH3 were derived using angular momentum rules based on the conservation of the nuclear spin. We show that the "anomalous" ortho-to-para ratio of ammonia (∼0.7) observed in various interstellar regions is in fact consistent with nuclear spin selection rules in a para-enriched H2 gas. This ratio is found to be independent of temperature in the range 5–30 K. We also predict an ortho-to-para ratio of ∼2.3 for NH2. We conclude that a low ortho-to-para ratio of H2 naturally drives the ortho-to-para ratios of nitrogen hydrides below the statistical values.

Prebiotic Matter in Space

AbstractA significant number of molecules that are used in contemporary biochemistry on Earth are found in interstellar and circumstellar regions as well as solar system environments. In particular small solar system bodies hold clues to processes that formed our solar system. Comets, asteroids, and meteorite delivered extraterrestrial material during the heavy bombardment phase ~3.9 billion years ago to the young planets, a process that made carbonaceous material available to the early Earth. In-depth understanding of the organic reservoir in different space environments as well as data on the stability of organic and prebiotic material in solar system environments are vital to assess and quantify the extraterrestrial contribution of prebiotic sources available to the young Earth.

Surviving the hole

The interstellar region within the few central parsecs around the super-massive black hole, Sgr A* at the very Galactic center is composed by a number of overlapping molecular structures which are subject to one of the most hostile physical environments in the Galaxy. We present high resolution (4"x3"~0.16x0.11 pc) interferometric observations of CN, 13CN, H2CO, SiO, c-C3H2 and HC3N emission at 1.3 mm towards the central ~4 pc of the Galactic center region. Strong differences are observed in the distribution of the different molecules. The UV resistant species CN, the only species tracing all previously identified circumnuclear disk (CND) structures, is mostly concentrated in optically thick clumps in the rotating filaments around Sgr A*. H2CO emission traces a shell-like structure that we interpret as the expansion of Sgr A East against the 50 km/s and 20 km/s giant molecular clouds (GMCs). We derive isotopic ratios 12C/13C~15-45 across most of the CND region. The densest molecular material, traced by SiO and HC3N, is located in the southern CND. The observed c-C3H2/HC3N ratio observed in the region is more than an order of magnitude lower than in Galactic PDRs. Toward the central region only CN was detected in absorption. Apart from the known narrow line-of-sight absorptions, a 90 km/s wide optically thick spectral feature is observed. We find evidences of an even wider (>100 km/s) absorption feature. Around 70-75% of the gas mass, concentrated in just the 27% densest molecular clumps, is associated with rotating structures and show evidences of association with each of the arcs of ionized gas in the mini-spiral structure. Chemical differentiation has been proven to be a powerful tool to disentangle the many overlapping molecular components in this crowded and heavily obscured region.

Can cytosine, thymine and uracil be formed in interstellar regions? A theoretical study

This theoretical study investigates possible synthetic routes to cytosine, uracil and thymine in the gas phase from precursor molecules that have been detected in interstellar media. Studies at the CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level of theory suggest that: The reactions between :CCCNH and :CCCO with monosolvated urea may constitute viable interstellar syntheses of cytosine and uracil. No low energy equilibration between cytosine and uracil has been demonstrated. The interaction of :CH(2) with the 5 C-H bond of uracil may form thymine in an energetically favourable reaction, but competing reactions where :CH(2) reacts with double bonds and other CH and NH bonds of uracil, reduce the effectiveness of this synthesis. The reaction between the hydrated propional enolate anion and isocyanic acid may produce thymine, in a reaction sequence where ΔG(reaction)(298 K) is -22 kJ mol(-1) and the maximum energy requirement (barrier to the first transition state) is only 47 kJ mol(-1).