Herschel observations of EXtra-Ordinary Sources: Analysis of the HIFI 1.2 THz Wide Spectral Survey Toward Orion KL II. Chemical Implications

We present chemical implications arising from spectral models fit to the Herschel/HIFI spectral survey toward the Orion Kleinmann-Low nebula (Orion KL). We focus our discussion on the eight complex organics detected within the HIFI survey utilizing a novel technique to identify those molecules emitting in the hottest gas. In particular, we find the complex nitrogen bearing species CH$_{3}$CN, C$_{2}$H$_{3}$CN, C$_{2}$H$_{5}$CN, and NH$_{2}$CHO systematically trace hotter gas than the oxygen bearing organics CH$_{3}$OH, C$_{2}$H$_{5}$OH, CH$_{3}$OCH$_{3}$, and CH$_{3}$OCHO, which do not contain nitrogen. If these complex species form predominantly on grain surfaces, this may indicate N-bearing organics are more difficult to remove from grain surfaces than O-bearing species. Another possibility is that hot (T$_{\rm kin}$$\sim$300 K) gas phase chemistry naturally produces higher complex cyanide abundances while suppressing the formation of O-bearing complex organics. We compare our derived rotation temperatures and molecular abundances to chemical models, which include gas-phase and grain surface pathways. Abundances for a majority of the detected complex organics can be reproduced over timescales $\gtrsim$ 10$^{5}$ years, with several species being under predicted by less than 3$\sigma$. Derived rotation temperatures for most organics, furthermore, agree reasonably well with the predicted temperatures at peak abundance. We also find that sulfur bearing molecules which also contain oxygen (i.e. SO, SO$_{2}$, and OCS) tend to probe the hottest gas toward Orion KL indicating the formation pathways for these species are most efficient at high temperatures.Comment: 31 pages, 6 figures, 1 Table, accepted to the Astrophysical Journa

Search for interstellar methoxyacetonitrile and cyanoethanol: insights into coupling of cyano- to methanol and ammonia chemistry

As part of an effort to study gas-grain chemical models in star-forming regions as they relate to molecules containing cyanide (–C≡N) groups, we present here a search for the molecules 2-cyanoethanol (OHCH_2CH_2CN) and methoxyacetonitrile (CH_3OCH_2CN) in the galactic center region SgrB2. These species are structural isomers of each other and are targeted to investigate the cross-coupling of pathways emanating from the photolysis products of methanol and ammonia with pathways involving cyano-containing molecules. Methanol and ammonia ices are two of the main repositories of the elements C, O, and N in cold clouds and understanding their link to cyanide chemistry could give important insights into prebiotic molecular evolution. Neither species was positively detected, but the upper limits we determined allow comparison to the general patterns gleaned from chemical models. Our results indicate the need for an expansion of the model networks to better deal with cyanochemistry, in particular with respect to pathways including products of methanol photolysis. In addition to these results, the two main observational routes for detecting new interstellar molecules are discussed. One route is by decreasing detection limits at millimeter wavelength through spatial filtering with interferometric studies at the Atacama Large Millimeter Array (ALMA), and the second is by searching for intense torsional states at THz frequencies using the Herschel Space Observatory. 2-cyanoethanol and methoxyacetonitrile would both be good test beds for exploring the capabilities of ALMA and Herschel in the study of complex interstellar chemistry

Carbon Chemistry in Dense Molecular Clouds: Theory and Observational Constraints

For the most part, gas phase models of the chemistry of dense molecular clouds predict the abundances of simple species rather well. However, for larger molecules and even for small systems rich in carbon these models often fail spectacularly. We present a brief review of the basic assumptions and results of large scale modeling of the carbon chemistry in dense molecular clouds. Particular attention will be paid to the influence of the gas phase C/O ratio in molecular clouds, and the likely role grains play in maintaining this ratio as clouds evolve from initially diffuse objects to denser cores with associated stellar and planetary formation. Recent spectral line surveys at centimeter and millimeter wavelengths along with selected observations in the submillimeter have now produced an accurate "inventory" of the gas phase carbon budget in several different types of molecular clouds, though gaps in our knowledge clearly remain. The constraints these observations place on theoretical models of interstellar chemistry can be used to gain insights into why the models fail, and show also which neglected processes must be included in more complete analyses. Looking toward the future, larger molecules are especially difficult to study both experimentally and theoretically in such dense, cold regions, and some new methods are therefore outlined which may ultimately push the detectability of small carbon chains and rings to much heavier species

Detection of a hot core in the intermediate-mass (IM) Class 0 protostar NGC 7129--FIRS2

We report high angular resolution (~0.6''x0.5'' at 1.3mm) observations of the Class 0 intermediate-mass (IM) protostar NGC 7129--FIRS 2 using the PdBI. Our observations show the existence of a hot core at the position of the Class 0 object. This is, up to our knowledge, the first IM hot core detected so far. Interferometric maps of the region in the the continuum at 3mm and 1.3mm and in the CH3CN 5_k-->4_k, CH3OH 5_kk'-->4_kk', and D2CO 4_04-->3_03 lines are presented in this paper. Enhanced CH3CN and CH3OH abundances are measured towards the hot core (X(CH3CN)~7.010^{-9}, X(CH3OH)~3 10^{-8}-- a few 10^{-7}). While intense D2CO emission is detected towards the hot core, the N2D+ line has not been detected in our interferometric observations. The different behaviors of D2CO and N2D+ emissions suggest different formation mechanisms for the two species and different deuteration processes for H2CO and N2H+ (surface and gas-phase chemistry, respectively). Finally, the spectrum of the large bandwidth correlator show a forest of lines at the hot core position revealing that this object is extraordinarily rich in complex molecules.Comment: 14 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

An Experimental and Master Equation Investigation of Kinetics of the CH2OO+RCN Reactions (R = H, CH3, C2H5) and Their Atmospheric Relevance

We have performed direct kinetic measurements of the CH2OO + RCN reactions (R = H, CH3, C2H5) in the temperature range 233-360 K and pressure range 10-250 Torr using time-resolved UV-absorption spectroscopy. We have utilized a new photolytic precursor, chloroiodomethane (CH2ICl), whose photolysis at 193 nm in the presence of O2 produces CH2OO. Observed bimolecular rate coefficients for CH2OO + HCN, CH2OO + CH3CN, and CH2OO + C2H5CN reactions at 296 K are (2.22 +/- 0.65) x 10-14 cm3 molecule-1 s-1, (1.02 +/- 0.10) x 10-14 cm3 molecule-1 s-1, and (2.55 +/- 0.13) x 10-14 cm3 molecule-1 s-1, respectively, suggesting that reaction with CH2OO is a potential atmospheric degradation pathway for nitriles. All the reactions have negligible temperature and pressure dependence in the studied regions. Quantum chemical calculations (omega B97X-D/aug-cc-pVTZ optimization with CCSD(T)-F12a/VDZ-F12 electronic energy correction) of the CH2OO + RCN reactions indicate that the barrierless lowest-energy reaction path leads to a ring closure, resulting in the formation of a 1,2,4-dioxazole compound. Master equation modeling results suggest that following the ring closure, chemical activation in the case of CH2OO + HCN and CH2OO + CH3CN reactions leads to a rapid decomposition of 1,2,4-dioxazole into a CH2O + RNCO pair, or by a rearrangement, into a formyl amide (RC(O)NHC(O)H), followed by decomposition into CO and an imidic acid (RC(NH)OH). The 1,2,4-dioxazole, the CH2O + RNCO pair, and the CO + RC(NH)OH pair are atmospherically significant end products to varying degrees.Peer reviewe

Complex organic chemistry in high-mass star forming regions

The quest for interstellar complex organic molecules (COMs) lies at the heart of astrochemistry. One basic motivation is to figure out how the rich inventory of COMs found in meteorites and comets is connected to interstellar chemistry. While the number of COMs detected in the interstellar medium increases, we wish to understand whether chemical complexity is a natural outcome of interstellar chemistry, which degree of complexity can be reached, and when, where and under which conditions these molecules form and how their occurence evolves from interstellar clouds to planetary systems. Most interstellar COMs were first detected toward the dense and warm parts of high-mass star-forming regions, called hot molecular cores. In particular, Sagittarius B2 (Sgr B2), one of the most active star-forming regions in our Galaxy, is an excellent target to study the production of COMs under extreme physical conditions (high densities, strong radiation field, high cosmic-ray flux). In this thesis we take advantage of the high sensitivity of a new type of imaging spectral line survey made possible by the Atacama Large Millimeter/submillimeter Array (ALMA). It affords studies of the spatial structure and chemical content of active star-forming regions in Sgr B2 in unprecedented detail. We report the detection of three new hot cores in Sgr B2(N), one of Sgr B2’s main star-forming sites. In a detailed comparative study, we determine their chemical composition, density, mass, temperature, and spatial structure. We check for association with maser sources and ultra-compact HII regions, signposts of recent high-mass star formation, as well as outflows, to evaluate the evolutionary stage of the hot cores. In the second part of the thesis we analyze their physical evolution from the cold pre-stellar phase to the present time. We use results of previous radiation-magnetohydrodynamical simulations of high-mass star formation and stellar structure calculations combined with a radiative transfer model to derive the thermal history of the sources. We compute time-dependent chemical abundances using the astrochemical code MAGICKAL, focusing especially on selected COMs to investigate in detail the chemical reactions and processes involved in their formation under the extreme conditions that characterize Sgr B2(N). We compare the chemical model results to the abundances derived from the observations toward the hot cores and find that a cosmic-ray ionization rate 50 times higher than the solar neighborhood value best characterizes Sgr B2(N)’s environmental conditions. We are also able to constrain the range of dust temperatures reached during the earlier pre-stellar phase at which COMs form on dust-grain surfaces. We show that COMs still form efficiently with minimum dust temperatures as high as 15 K, but the current chemical composition of the hot cores excludes minimum temperatures higher than 25 K

Study of Chemical Processes in Titan Atmosphere Initiated by Discharge in Electrode Configuration Like Gliding Arc Discharge

Cílem disertační práce je studium plazmochemických procesů a syntéza organických sloučenin vznikajících v elektrickém výboji plynné směsi odpovídající složení atmosféry Saturnova největšího měsíce Titanu. Atmosféra Titanu je svým chemickým složením velmi podobná atmosféře Země v době jejího vzniku. Látky obsažení v atmosféře jsou převážně dusík a metan s malý obsahem organických látek. Dosud byla publikována celá řada prací zabývajících se jak teoretickým-modelovým výzkumem, tak i laboratorními experimenty, v poslední době je snaha o jejich propojení. Hlavním cílem práce je identifikace plynných organických, amino, imino a kyano sloučenin promocí různých analytických metod jako jsou PTR-MS, FTIR a GC-MS. K získání vybraných parametrů plazmatu pak bylo využito OES. Ke studiu generace plynných produktů a radikálů bylo využito výboje generovaného ve směsích N2-CH4 (podíl metanu byl 0,5 až 5 %) za atmosférického tlaku v průtočném režimu při celkových průtocích 50 až 200 sccm a výbojovém proudu v rozmezí 15 až 40 mA. Část měření se rovněž zaměřila na studium vlivu příměsí vodíku a oxidu uhličitého. První část výsledků měření předložených v této práci byla získána z OES za vybraných podmínek (složení plynné směsi, dodávaný výkon). V naměřených spektrech byly identifikovány první negativní a druhý pozitivní systém dusíku, fialový systém radikálu CN a Swanův systém C2. Kromě nich byly rovněž detekovány čáry vodíku H, H a ve spektru druhého řádu i čára atomárního uhlíku. Tato spektra pak byla využita pro výpočet rotačních a vibračních teplot plazmatu. In situ analýzy plynných produktů pomocí FTIR ukázaly ve všech experimentech přítomnost různých nitrilů a uhlovodíků. Jako hlavní produkty generované výbojem byly identifikovány HCN, C2H2, NH3, přičemž jejich koncentrace odpovídaly relaci HCN > NH3 > C2H2. Tato relace byla potvrzena v závislosti na různých experimentálních parametrech. Další část práce byla zaměřena na studium vlivu příměsi CO2 na reaktivitu výše zmíněných plynných směsí. Kromě hlavních produktů, zmíněných výše, byly navíc detekovány CO2 a CO a také některé složitější kyslíkaté organické sloučeniny. Ty ale nebylo možné přesněji určit kvůli velké komplexnosti změřených spekter a překryvu jednotlivých absorpčních pásů. V případě příměsi vodíku do reakční směsi se nepodařilo identifikovat žádné nové látky ani funkční skupiny. K detailnější analýze plynných produktů výboje bylo využito in situ PTR-MS. V tomto případě bylo detekováno velké množství molekulárních struktur obsahujících nitrilové (–CN), amino (–NH2, –NH– a –N CH3CN > C2H5CN. Mimo tyto sloučeniny byla detekována celá řada dalších uhlovodíků i nitrilů.The aim of this work is the study of plasma processes and the synthesis of organic compounds due to electric discharge generated in gas mixture corresponding to the composition of the atmosphere of Saturn's largest moon Titan. This study focuses on the mimic of Titan's atmosphere at atmospheric pressure and ambient laboratory temperature. The chemical composition of Titan's atmosphere is very similar to atmosphere of prehistoric Earth. Many articles have been published with theoretical model-research, and laboratory experiments are the pursuit of their interconnection. The main aim of thesis is the identification of synthesized gaseous organic, amino, imino and cyano compounds by use to various analytical methods such as the PTR-MS, FTIR and GC-MS. The OES and electric measurements were applied to the determination of selected electric discharge parameters. The gaseous products and radicals formed in an atmospheric discharge fed by different mixtures of N2:CH4 (0,5 up to 5 % of CH4) operated in a flowing regime at the total gas mixture flows from 50 to 200 sccm at different discharge currents from 15 up to 40 mA were determined. A part of experiments was carried out with admixtures of CO2 and hydrogen. This first part of results has been obtained using OES in dependence on the gas mixture composition and supplied power. The bands of the nitrogen second positive and the first negative systems, CN violet system and Swan system of C2 were recorded. Besides them, atomic lines H, H, and C (in the second order) were also observed. These spectra allowed calculation of rotational and vibrational temperatures. FTIR in situ analysis of the gaseous products showed presence of various nitrile compounds and hydrocarbons in all experiments. The HCN, C2H2, NH3 were the main products generated in our system. The dependences of their concentrations on various experimental parameters were measured. The other part of this work was devoted to estimate the influence of CO2 traces addition on the reactivity in the gaseous mixtures mentioned above. Besides the main products mentioned above, CO2 and CO were detected and also some more complicated oxygen molecules has been confirmed but not estimated because of FTIR spectra complexity. In the case of hydrogen traces addition into the reaction gas mixture, no other compounds were determined. Impurities of CO2 as well as hydrogen have a great positive influence on the production efficiency of the major generated compounds at all conditions. The more detailed gaseous products analyses were carried out using the in situ PTR-MS. A huge number of different molecular structures containing nitrile groups (–CN), amino groups (–NH2, –NH–, –N CH3CN > C2H5CN. Besides them, many other hydrocarbons and nitriles were detected. Presence of all compounds was studi

The early stages of massive star formation: tracing the physical and chemical conditions in hot cores

Molecules are essential to the formation of stars, by allowing radiation to escape the cloud and cooling to occur. Over 180 molecules have been detected in interstellar environments, ranging from comets to interstellar clouds. Their spectra are useful probes of the conditions in which these molecules form. Comparison of rest frequencies to observed frequencies can provide information about the velocity of gas and indicate physical structures. The density, temperature, and excitation conditions of gas can be determined directly from the spectra of molecules. Furthermore, by taking a chemical inventory of a particular object, one can gain an understanding of the chemical processes occurring within a cloud. The class of molecules known as complex molecules (>6 atoms), are of particular interest when probing the conditions in massive starforming environments, as they are observed to trace a more compact region than smaller molecules. This thesis details the work of my PhD, to explore how complex molecules can be used to trace the physical and chemical conditions in hot cores (HCs), one of the earliest stages of massive star formation. This work combines both the observations and chemical modelling of several different massive star-forming regions. We identify molecular transitions observed in the spectra of these regions, and calculate column densities and rotation temperatures of these molecules (Chapters 2 and 3). In Chapter 4, we chemically model the HCs, and perform a comparison between observational column densities and chemical modelling column densities. In Chapter 5, we look at the abundance ratio of three isomers, acetic acid, glycolaldehyde, and methyl formate, to ascertain whether this ratio can be used as an indicator of HC evolution. Finally, we explore the chemistry of the HC IRAS 17233–3606, to identify emission features in the spectra, and determine column densities and rotation temperatures of the detected molecules

Chemistry of the High-Mass Protostellar Molecular Clump IRAS 16562-3959

We present molecular line observations of the high-mass molecular clump IRAS 16562$-$3959 taken at 3 mm using the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.\!\!^{\prime\prime}7 angular resolution ($0.014$ pc spatial resolution). This clump hosts the actively accreting high-mass young stellar object (HMYSO) G345.4938+01.4677, associated with a hypercompact HII region. We identify and analyze emission lines from 22 molecular species (encompassing 34 isomers) and classify them into two groups, depending on their spatial distribution within the clump. One of these groups gathers shock tracers (e.g., SiO, SO, HNCO) and species formed in dust grains like methanol (CH$_3$OH), ethenone or ketene (H$_2$CCO), and acetaldehyde (CH$_3$CHO). The second group collects species resembling the dust continuum emission morphology and which are formed mainly in the gas-phase, like hydrocarbons (CCH, c-C$_3$H$_2$, CH$_3$CCH), cyanopolyynes (HC$_3$N and HC$_5$N) and cyanides (HCN and CH$_3$C$_3$N). Emission from complex organic molecules (COMs) like CH$_3$OH, propanenitrile (CH$_3$CH$_2$CN), and methoxymethane (CH$_3$OCH$_3$) arise from gas in the vicinity of a hot molecular core ($T\gtrsim100$ K) associated with the HMYSO. Other COMs such as propyne (CH$_3$CCH), acrylonitrile (CH$_2$CHCN), and acetaldehyde seem to better trace warm ($T\lesssim80$ K) dense gas. In addition, deuterated ammonia (NH$_2$D) is detected mostly in the outskirts of IRAS 16562$-$3959, associated with near-infrared dark globules, probably gaseous remnants of the clump's prestellar phase. The spatial distribution of molecules in IRAS 16562$-$3959 supports the view that in protostellar clumps, chemical tracers associated with different evolutionary stages --- starless to hot cores/HII regions --- exist coevally.Comment: 97 pages, Accepted in The Astrophysical Journal Supplement Series. Journal file version have better quality figure