Gas chemistry is typically characterized comparing integrated intensities of different molecular species. With the advent of ALMA, chemistry is now resolved not only in space but also in velocity. When observed at high spectral resolution, most of the clouds, filaments, and cores in the solar neighbourhood show a complex internal structure with multiple velocity components. This intrinsic complexity should be disentangled in order to correctly interpret their chemical properties. We present the first results of the FIVE+ analysis technique combining both state-of-the-art structure reconstruction (2D), kinematic decomposition (+1D), and chemical analysis (+1D) in large molecular datasets.

The search for complex organic molecules (COMs) in the ISM has revealed chemical species of ever greater complexity. This search relies heavily on the progress made in the laboratory to characterize the rotational spectra of these molecules. Observationally, the advent of ALMA with its high angular resolution and sensitivity has allowed to reduce the spectral confusion and detect low-abundance molecules that could not be probed before. We present results of the EMoCA survey conducted with ALMA toward the star-forming region Sgr B2(N). This spectral line survey aims at deciphering the molecular content of Sgr B2(N) in order to test the predictions of astrochemical models and gain insight into the chemical processes at work in the ISM. We report on the tentative detection of N-methylformamide, on deuterated COMs, and on the detection of a branched alkyl molecule. Prospects for probing molecular complexity in the ISM even further are discussed at the end.

Isocyanic acid (HNCO) is a simple molecule containing the four main atoms essential for life and can be considered as a prebiotic molecule. To model the HNCO emission in the IRAS16293-2422 class 0 low-mass protostar, we used a new set of HNCO collisional coefficients with ortho-H2 and para-H2, computed from a set of rotational excitation quenching rates between HNCO and H2 based on a novel potential energy surface for the rigid molecules interactions. We present here the HNCO Potential Energy Surface used to compute this new set of collisional coefficients and the result of the IRAS16293-2422 HNCO spectrum modelling using them.

A key parameter governing the secular evolution of protoplanetary disks is their outer radius. In this paper, the feedback of realistic dust grain size distributions onto the gas emission is investigated. Models predict that the difference of dust and gas extents as traced by CO is primarily caused by differences in the optical depth of lines vs continuum. The main effect of radial drift is the sharp decrease in the intensity profile at the outer edge. The gas radial extent can easily range within a factor of 2 for models with different turbulence. A combination of grain growth and vertical settling leads to thermal de-coupling between gas and dust at intermediate scale-heights. A proper treatment of the gas thermal structure within dust gaps will be fundamental to disentangle surface density gaps from gas temperature gaps.

Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. Especially, saturated, hydrogen-rich molecules are formed through surface chemistry. Astrochemical models have developed over the decades to understand the molecular processes in the interstellar medium, taking into account grain surface chemistry. However, essential input information for gas-grain models, such as binding energies of molecules to the surface, have been derived experimentally only for a handful of species, leaving hundreds of species with highly uncertain estimates. Moreover, some fundamental processes are not well enough constrained to implement these into the models.

The proceedings gives three examples how computational chemistry techniques can help answer fundamental questions regarding grain surface chemistry.

Results are presented from our ongoing studies of Titan using ALMA during the period 2012-2015, including a confirmation of the previous detection of vinyl cyanide (C2H3CN), as well as the first spatial map for this species on Titan. Simultaneous mapping of HC3N, CH3CN and C2H5CN reveal characteristic abundance patterns for each species that provide insight into their individual photochemical lifetimes, and help inform our understanding of Titan’s unique, time-variable atmospheric chemistry and global circulation. A time-sequence of HC3N maps covering 38 months reveals a dramatic change in the distribution of this gas consistent with high-altitude photochemical production followed by advection towards the southern (winter) pole, combined with rapid loss in the north after Titan’s 2009 seasonal equinox. The 2015 C2H3CN and C2H5CN maps show abundance peaks in Titan’s southern hemisphere, similar to those observed for the short-lived HC3N molecule. The longer-lived CH3CN, on the other hand, remains more concentrated in the north.

Silicon carbide dust grains are ubiquitous in circumstellar envelopes around C-rich AGB stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. To date, only three molecules containing an Si–C bond have been identified to have significant abundances in C-rich AGB stars: SiC2, SiC, and Si2C. The ring molecule SiC2 has been observed in a handful of evolved stars, while SiC and Si2C have only been detected in the C-star envelope IRC +10216. We aim to study how widespread and abundant SiC2, SiC, and Si2C are in envelopes around C-rich AGB stars and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars.

Gas-phase methanol was recently detected in a protoplanetary disk for the first time with ALMA. The peak abundance and distribution of methanol observed in TW Hya differed from that predicted by chemical models. Here, the chemistry of methanol gas and ice is calculated using a physical model tailored for TW Hya with the aim to contrast the results with the recent detection in this source. New pathways for the formation of larger complex molecules (e.g., ethylene glycol) are included in an updated chemical model, as well as the fragmentation of methanol ice upon photodesorption. It is found that including fragmentation upon photodesorption improves the agreement between the peak abundance reached in the chemical models with that observed in TW Hya (∼10−11 with respect to H2); however, the model predicts that the peak in emission resides a factor of 2 − 3 farther out in the disk than the ALMA images. Reasons for the persistent differences in the gas-phase methanol distribution between models and the observations of TW Hya are discussed. These include the location of the ice reservoir which may coincide with the compact mm-dust disk (≲ 60 au) and sources of gas-phase methanol which have not yet been considered in models. The possibility of detecting larger molecules with ALMA is also explored. Calculations of the rotational spectra of complex molecules other than methanol using a parametric model constrained by the TW Hya observations suggest that the detection of individual emission lines of complex molecules with ALMA remains challenging. However, the signal-to-noise ratio can be enhanced via stacking of multiple transitions which have similar upper energy levels.

We present first results of a new heterodyne spectrometer dedicated to high-resolution spectroscopy of molecules of astrophysical importance. The spectrometer, based on a room-temperature heterodyne receiver, is sensitive to frequencies between 75 and 110 GHz with an instantaneous bandwidth of currently 2.5 GHz in a single sideband. The system performance, in particular the sensitivity and stability, is evaluated. Proof of concept of this spectrometer is demonstrated by recording the emission spectrum of methyl cyanide, CH3CN. Compared to state-of-the-art radio telescope receivers the instrument is less sensitive by about one order of magnitude. Nevertheless, the capability for absolute intensity measurements can be exploited in various experiments, in particular for the interpretation of the ever richer spectra in the ALMA era. The ease of operation at room-temperature allows for long time integration, the fast response time for integration in chirped pulse instruments or for recording time dependent signals. Future prospects as well as limitations of the receiver for the spectroscopy of complex organic molecules (COMs) are discussed.

In external galaxies, some galaxies have higher activities of star formation and central supermassive black holes. The interstellar medium in those galaxies can be heated by different mechanisms such as UV-heating, X-ray heating, cosmic-ray heating, and shock/mechanical heating. Chemical compositions can also be affected by those heating mechanisms. Observations of many molecular species in those nearby galaxies are now possible with the high sensitivity of Atacama Large Millimeter/sub-millimeter Array (ALMA). Here I cover different chemical models for those heating mechanisms. In addition, I present recent ALMA results of extragalactic astrochemistry including our results of a face-on galaxy M83 and an infrared-luminous merger NGC 3256.

The SuperMALT survey is observing 76 MALT90 clumps at different evolutionary stages (from pre-stellar or quiescent to HII) in high excitation molecular lines and key isotopomers using the Apex 12m telescope with an angular resolution of ∼20” and a velocity resolution of ∼0.1 km/s. The aim of this survey is to determine the physical, chemical, and kinematical properties of the gas within clumps as they evolve. Here we report some preliminary results based on observations of the J=3-2 & 4-3 lines of HNC, HCN, HCO+, N2H+ and of the J=3-2 line of the isotopologue H13CO+. We find that the morphologies and line profiles vary with the evolutionary stage of the clumps. The average line width increases from quiescent to HII clumps while line ratios show hint of chemical differences among the various evolutionary stages.

We have analyzed rotational spectral line emission of OCS, CH3OH, HCOOCH3, and H2CS observed toward the low-mass Class 0 protostar IRAS 16293–2422 (Source A and B) at a sub-arcsecond resolution with ALMA. Significant chemical differentiation is found at a 50 au scale. OCS is found to trace the infalling-rotating envelope, while COM distributions are concentrated around the inner part of the envelope. The kinematic structure in Source A is explained with a ballistic model, and the protostellar mass and the radius of the centrifugal barrier are evaluated to be ~0.75 M⊙ and ~50 au, respectively. This study has revealed that the centrifugal barrier plays a central role not only in the disk formation but also in the associated chemical evolution.

The detection of iso-propyl cyanide (i-C3H7CN) toward the Galactic Center hot-core source Sgr B2(N) (by Belloche et al. 2014) marked the first interstellar detection of an aliphatic molecule with a branched carbon-chain structure. Surprisingly, this branched form was found to have an almost equal abundance with its straight-chain homologue, normal-propyl cyanide (i:n = 0.40 ± 0.06). The detection of this first example of an interstellar molecule with a side-chain raises the question as to how prominent such structures may be in interstellar chemistry, and whether the large branched-to-straight chain ratio is maintained for even larger molecules.

Here are presented recently published models that simulate the chemistry occurring in Sgr B2(N) using a chemical network that explicitly includes the straight-chain and branched forms of propyl cyanide (normal/iso) and butyl cyanide (normal/iso/sec/tert), as well as butane (n/i) and pentane (n/i/neo). Formation is assumed to occur on dust-grain surfaces, but a full complement of destruction mechanisms is included both on the grains and in the gas phase.

The models suggest that branched structures become increasingly dominant as molecular sizes increase. In the case of butyl cyanide, the sec form is at least ∼2 times more abundant than the straight-chain normal form, and together the branched forms dominate normal-butyl cyanide by a factor of at least 3. The results for the larger alkanes suggest similarly large ratios of branched to straight-chain molecules. A key set of reactions in the surface/ice chemistry of interstellar nitriles is found to be the addition of the CN radical to unsaturated hydrocarbons, especially acetylene and ethene. The models also predict that the dominant, sec form of butyl cyanide reaches a peak abundance equal to that of n-propyl cyanide, albeit with a smaller emission radius. This makes s-C4H9CN a good candidate for detection. New ALMA observations to search for this molecule are ongoing.

Far-infrared spectroscopy reveals gas cooling and its underlying heating due to physical processes taking place in the surroundings of protostars. These processes are reflected in both the chemistry and excitation of abundant molecular species. Here, we present the Herschel-PACS far-IR spectroscopy of 90 embedded low-mass protostars from the WISH (van Dishoeck et al. 2011), DIGIT (Green et al. 2013), and WILL surveys (Mottram et al. 2017). The 5 × 5 spectra covering the ∼50″ × 50″ field-of-view include rotational transitions of CO, H2O, and OH lines, as well as fine-structure [O I] and [C II] in the ∼50-200 μm range. The CO rotational temperatures are typically ∼300 K, with some sources showing additional components with temperatures as high as ∼1000 K. The H2O / CO and H2O / OH flux ratios are low compared to stationary shock models, suggesting that UV photons may dissociate some H2O and decrease its abundance. Comparison to C shock models illuminated by UV photons show a good agreement between the line emission and the models for pre-shock densities of 105 cm−3 and UV fields 0.1-10 times the interstellar value. The far-infrared molecular and atomic lines are the unique diagnostic of shocks and UV fields in deeply-embedded sources.

A systematic study, using ion trap time-of-flight mass spectrometry, is presented for the photo-dissociation processes of Bisanthenquinone (Bq) cations, C28H12O2+, a ketone substituted Polycyclic Aromatic Hydrocarbon (PAH). The Bq cation fragments through sequential loss of the two neutral carbonyl (CO) units upon laser (626nm) irradiation, resulting in a PAH-like derivative C26H12+. Upon further irradiation, C26H12+ exhibits both stepwise dehydrogenation and C2/C2H2 loss fragmentation channels. Quantum chemistry calculations reveal a detailed picture for the first CO-loss, which involves a transition state with a barrier of ∼ 3.4 eV, which is lower than the energy required for the lowest H-loss pathway (∼ 5.0 eV). The barrier for the second CO-loss is higher (∼ 4.9 eV). The subsequent loss of this unit changes the Bq geometry from a planar to a bent one. It is concluded that the photodissociation mechanism of the substituted PAH cations studied here is site selective in the substituted subunit. This work also shows that an acetone substituted PAH cation is not photo-stable upon irradiation.

Recent progress in astrochemistry of low-mass protostellar sources is reviewed. In particular, we focus on disk formation processes and associated chemical changes at a 50 au scale, which is extensively being studied with ALMA. A small scale chemical differentiation sensitively reflects changes in physical conditions, and hence, it provides us with unique opportunities of chemical diagnostics of disk-forming regions. Complex physical and chemical pictures of disk formation revealed by observations are summarized, and future prospects are discussed.

Back-diffusion is the phenomenon by which random walkers revisit binding sites on a lattice. This phenomenon must occur on interstellar dust particles, slowing down dust-grain reactions, but it is not accounted for by standard rate-equation models. Microscopic kinetic Monte Carlo models have been used to investigate the effect of back-diffusion on reaction rates on interstellar dust grains. Grain morphology, size, and grain-surface coverage were varied and the effects of these variations on the magnitude of the back-diffusion effect were studied for the simple H+H reaction system. This back-diffusion effect is seen to reduce reaction rates by a maximum factor of ∼5 for the canonical grain of 106 binding sites. The resulting data were fit to logarithmic functions that can be used to reproduce the effects of back-diffusion in rate-equation models.

We describe the characteristics and the capabilities of the laboratory facility, COSmIC, that was developed at NASA Ames to generate, process and analyze interstellar, circumstellar and planetary analogs in the laboratory. COSmIC stands for ’Cosmic Simulation Chamber’ and is dedicated to the study of neutral and ionized molecules and nanoparticles under the low temperature and high vacuum conditions that are required to simulate various space environments such as diffuse interstellar clouds, circumstellar outflows and planetary atmospheres. Recent results obtained using COSmIC will be highlighted. In particular, the progress that has been achieved in the domain of the diffuse interstellar bands (DIBs) and in monitoring, in the laboratory, the formation of circumstellar dust grains and planetary atmosphere aerosols from their gas-phase molecular precursors. Plans for future laboratory experiments on interstellar and planetary molecules and grains will also be addressed, as well as the implications of the studies underway for astronomical observations and past and future space mission data analysis.