Molecular Astrophysics最新文献

{"title":"On dust irradiation in planetary nebulae in the context of survivability of ices","authors":"Ararat Yeghikyan","doi":"10.1016/j.molap.2017.06.002","DOIUrl":"10.1016/j.molap.2017.06.002","url":null,"abstract":"<div><p><span>A large number of molecules are observed in planetary nebulae, including simple and, - the most common (H</span>₂, CO and OH), more complex (H₂O, SiO, HCN, HNC, HCO⁺<span><span><span>), and even the polycyclic aromatic hydrocarbons and </span>fullerenes containing a few dozen and more atoms. Water molecules are observed, as a rule, in the young objects, in the gas phase (water \"fountains\" and related water masers) and solid phase (emission of crystalline ice particles). On the other hand, the results of calculations by the Cloudy computer program, given in this paper, show that the abundance of water ice in planetary nebulae, other conditions being equal, depends on the ionization rate of hydrogen, which depends in turn on the flux of </span>energetic particles (protons and alpha particles) in the range of MeV energies and higher. Calculated water ice column densities reach values of up to </span><span><math><mrow><msup><mn>10</mn><mn>19</mn></msup><mo>−</mo><msup><mn>10</mn><mn>20</mn></msup><mspace></mspace><mi>c</mi><msup><mrow><mi>m</mi></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup></mrow></math></span> at the usual average ISM H₂ ionisation rate of <span><math><mrow><msup><mn>10</mn><mrow><mo>−</mo><mn>16</mn></mrow></msup><mspace></mspace><msup><mrow><mi>s</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math></span><span><span> and sharply decrease at rates that are a thousand times larger. The possibility of an increased flux of energetic particles in planetary nebulae under conditions of the standard interacting stellar winds<span> scenario is discussed, and it is concluded that the flux may locally exceed by 1–3 orders of magnitude that of galactic cosmic rays. This may have important implications for the </span></span>chemistry of complex compounds under conditions of planetary nebulae, in particular, for models of the origin of fullerenes.</span></p></div>","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"8 ","pages":"Pages 40-47"},"PeriodicalIF":0.0,"publicationDate":"2017-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.molap.2017.06.002","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77299512","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"PAH chemistry at eV internal energies. 1. H-shifted isomers","authors":"Georges Trinquier,&nbsp;Aude Simon,&nbsp;Mathias Rapacioli,&nbsp;Florent Xavier Gadéa","doi":"10.1016/j.molap.2017.02.001","DOIUrl":"https://doi.org/10.1016/j.molap.2017.02.001","url":null,"abstract":"<div><p><span><span><span>The PAH family of organic compounds (polycyclic aromatic hydrocarbons), involved in several fields of </span>chemistry<span>, has received particular attention in astrochemistry, where their </span></span>vibrational spectroscopy, thermodynamics, dynamics, and fragmentation properties are now abundantly documented. This survey aims at drawing trends for low spin-multiplicity surfaces of PAHs bearing internal energies in the range 1–10</span> <span><span>eV. It addresses some typical alternatives to the ground-state regular structures of PAHs, making explicit possible intramolecular rearrangements leading to high-lying minima. These isomerisations should be taken into consideration when addressing PAH processing in astrophysical conditions. The first part of this double-entry study focuses on the hydrogen-shifted forms, which bear both a </span>carbene<span> center and a saturated carbon. It rests upon DFT<span> calculations mainly performed on two emblematic PAH representatives, coronene and pyrene, in their neutral and mono- and multi-cationic states. Systematically searched for in neutral species, these H-shifted minima are lying 4–5</span></span></span> <!-->eV above the regular all-conjugated forms, and are separated by barriers of about 1<!--> <!-->eV. General hydrogen-shifting is found to be easier for cationic species as the relative energies of their H-shifted minima are 1–1.5<!--> <span>eV lower than those for neutral species. As much as possible, classical knowledge and concepts of organic chemistry such as aromaticity and Clar's rules are invoked for result interpretation.</span></p></div>","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"7 ","pages":"Pages 27-36"},"PeriodicalIF":0.0,"publicationDate":"2017-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.molap.2017.02.001","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91625162","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Laboratory spectroscopy and astronomical significance of the fully-benzenoid PAH triphenylene and its cation","authors":"V. Kofman, P. Sarre, R. Hibbins, I. T. Kate, H. Linnartz","doi":"10.1016/J.MOLAP.2017.04.002","DOIUrl":"https://doi.org/10.1016/J.MOLAP.2017.04.002","url":null,"abstract":"","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"50 1","pages":"19-26"},"PeriodicalIF":0.0,"publicationDate":"2017-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86949648","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"PAH chemistry at eV internal energies. 2. Ring alteration and dissociation","authors":"Georges Trinquier,&nbsp;Aude Simon,&nbsp;Mathias Rapacioli,&nbsp;Florent Xavier Gadéa","doi":"10.1016/j.molap.2017.02.002","DOIUrl":"https://doi.org/10.1016/j.molap.2017.02.002","url":null,"abstract":"<div><p><span><span>Recognized as important interstellar constituents, polycyclic aromatic hydrocarbons (PAHs) have been intensively studied in </span>astrochemistry<span> and their spectroscopy, thermodynamics, dynamics, and fragmentations are now amply documented. There exists typical alternatives to the ground-state regular planar structures of PAHs, as long as they bear internal energies in the range 1–10</span></span> <span>eV. Resulting from intramolecular rearrangements, such high-lying minima on the potential-energy surfaces should be taken into consideration in the studies of PAH processing in astrophysical conditions. Resting upon DFT calculations<span><span> mainly performed on two emblematic PAH representatives, coronene and pyrene, in their neutral and mono- and multi-cationic states, this second survey addresses the following alternatives: (1) opened forms containing </span>ethynyl<span><span> or 2-butynyl groups, (2) vinylidene isomers, in which </span>phenanthrene patterns are reorganized into dibenzofulvene ones, (3) “twisted” forms, where external CH</span></span></span><img>CH bonds can be partly twisted, and (4) bicyclobutane forms, in which the latter are integrated in saturated bicyclic forms. A few scenarios for elimination of fragments H, H₂, C₂H₂ and C₂H₄<span> are explored. As far as possible, familiar concepts of organic chemistry, such as aromaticity or Clar's rules, are invoked for interpretations.</span></p></div>","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"7 ","pages":"Pages 37-59"},"PeriodicalIF":0.0,"publicationDate":"2017-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.molap.2017.02.002","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91625185","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"How hydroxylation affects hydrogen adsorption and formation on nanosilicates","authors":"Boutheïna Kerkeni ,&nbsp;Marie-Christine Bacchus-Montabonel ,&nbsp;Stefan T. Bromley","doi":"10.1016/j.molap.2017.04.001","DOIUrl":"https://doi.org/10.1016/j.molap.2017.04.001","url":null,"abstract":"<div><p><span>Silicate dust constitutes one of the primary solid components of the Universe and is thought to be an essential enabler for complex chemistry<span> in a number of astronomical environments. Hydroxylated silicate nanoclusters (MgO)</span></span>_x(SiO₂)_y(H₂O)_z<span><span>, where strongly absorbed water molecules are dissociated on the silicate surface, are likely to be persistent in diffuse clouds. Such precursor species are thus also primary candidates as seeds for the formation and growth of icy dust grains in dense molecular clouds. Using density functional calculations we investigate the reactivity of hydroxylated </span>pyroxene nanoclusters (Mg</span>₄Si₄O₁₂)(H₂O)_N (N<!--> <span>= 1−4) towards hydrogen physisorption<span>, chemisorption and H</span></span>₂<span><span> formation. Our results show that increased hydroxylation leads to a significant reduction in the energy range for the physisorption and chemisorption of single H atoms, when compared to bare </span>silicate grains and bare bulk silicate surfaces. Subsequent chemisorption of a second H atom is, however, little affected by hydroxylation. The H</span>₂ reaction barrier for the recombination of two chemisorbed H atoms tends to follow a linear correlation with respect to the 2H_chem binding energy, suggestive of a general Brønsted–Evans–Polanyi relation for H₂<span> formation on silicate grains, independent of dust grain size<span>, composition and degree of hydroxylation.</span></span></p></div>","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"7 ","pages":"Pages 1-8"},"PeriodicalIF":0.0,"publicationDate":"2017-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.molap.2017.04.001","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91625163","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Laboratory spectroscopy and astronomical significance of the fully-benzenoid PAH triphenylene and its cation","authors":"V. Kofman ,&nbsp;P.J. Sarre ,&nbsp;R.E. Hibbins ,&nbsp;I.L. ten Kate ,&nbsp;H. Linnartz","doi":"10.1016/j.molap.2017.04.002","DOIUrl":"https://doi.org/10.1016/j.molap.2017.04.002","url":null,"abstract":"<div><p><span>Triphenylene (C</span>₁₈H₁₂<span><span>) is a highly symmetric polycyclic aromatic hydrocarbon (PAH) molecule with a ‘fully-benzenoid’ electronic structure. This confers a high chemical stability compared with PAHs of similar size. Although numerous infrared and UV-vis experimental spectroscopic and theoretical studies of a wide range PAHs in an astrophysical context have been conducted, triphenylene and its </span>radical cation<span> have received almost no attention. There exists a huge body of spectroscopic evidence for neutral and ionised PAHs in astrophysical sources, obtained principally through detection of infrared emission features that are characteristic of PAHs as a chemical class. However, it has so far not proved possible to identify spectroscopically a single isolated PAH in space, although PAHs including triphenylene have been detected mass spectrometrically in meteorites. In this work we focus on recording laboratory electronic spectra of neutral and ionised triphenylene between 220 and 780 nm, trapped in H</span></span>₂<span><span><span><span>O ice and solid argon at 12 K. The studies are motivated by the potential for spectroscopic astronomical detection of electronic </span>absorption spectra of PAHs in ice mantles on </span>interstellar grains as discussed by Linnartz (2014), and were performed also in a cold Ar matrix to provide guidance as to whether triphenylene (particularly in its singly positively ionised form) could be a viable candidate for any of the unidentified diffuse </span>interstellar absorption bands. Based on the argon-matrix experimental results, comparison is made with previously unpublished astronomical spectra near 400 nm which contain broad interstellar absorption features consistent with the predictions from the laboratory matrix spectra, thus providing motivation for the recording of gas-phase electronic spectra of the internally cold triphenylene cation.</span></p></div>","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"7 ","pages":"Pages 19-26"},"PeriodicalIF":0.0,"publicationDate":"2017-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.molap.2017.04.002","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91625161","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"How hydroxylation affects hydrogen adsorption and formation on nanosilicates","authors":"B. Kerkeni, M. Bacchus-Montabonel, S. Bromley","doi":"10.1016/J.MOLAP.2017.04.001","DOIUrl":"https://doi.org/10.1016/J.MOLAP.2017.04.001","url":null,"abstract":"","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"68 1","pages":"1-8"},"PeriodicalIF":0.0,"publicationDate":"2017-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81307594","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Dehydrogenation effects on the stability of aromatic units in polycyclic aromatic hydrocarbons in the interstellar medium: A computational study at finite temperature","authors":"P. Parneix ,&nbsp;A. Gamboa ,&nbsp;C. Falvo ,&nbsp;M.A. Bonnin ,&nbsp;T. Pino ,&nbsp;F. Calvo","doi":"10.1016/j.molap.2017.05.001","DOIUrl":"10.1016/j.molap.2017.05.001","url":null,"abstract":"<div><p><span>Isomerization<span><span><span>, ionization and fragmentation of molecular compounds in the interstellar medium can be triggered by </span>stellar radiation and cosmic rays. In the present contribution, we examine the propensity for isomerization and the relative stability of </span>aromatic rings<span> in the pyrene and coronene molecules at various degrees of </span></span></span>dehydrogenation<span><span> by means of molecular modeling. Using the AIREBO reactive force field and advanced Monte Carlo techniques such as the Wang–Landau method based on suitable order parameters, entire free-energy profiles describing the isomerization pathways and equilibrium properties were calculated as a function of temperature or total energy. We generally find that hydrogenation significantly stabilizes the fully polycyclic aromatic hydrocarbon (PAH) structure, even though local dehydrogenation next to an aromatic ring favors ring opening. The formation of pentagonal rings, a typical defect motif in the polycyclic carbon skeleton, is predicted to be actually competitive with the loss of a </span>hydrogen atom. Our investigation emphasizes the likely presence of defects in astrophysical PAHs, whose spectral features remain to be better characterized and understood.</span></p></div>","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"7 ","pages":"Pages 9-18"},"PeriodicalIF":0.0,"publicationDate":"2017-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.molap.2017.05.001","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"88963219","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Suggestion for search of cyclopropenone (c-C3H2O) in a cosmic object","authors":"M.K. Sharma ,&nbsp;M. Sharma ,&nbsp;S. Chandra","doi":"10.1016/j.molap.2016.11.001","DOIUrl":"10.1016/j.molap.2016.11.001","url":null,"abstract":"<div><p>Following Minimum Energy Principle, out of the three isomers of chemical formula C₃H₂<span>O, the cyclopropenone (</span><em>c</em>-C₃H₂<span>O) is the most stable and therefore may be the most abundant and easily detectable in a cosmic object. The cyclopropenone is detected in Sgr B2(N). Owing to half-spin of each of two hydrogen atoms, the </span><em>c</em>-C₃H₂<span>O has two distinct ortho and para species. Using the rotational and centrifugal distortion constants<span> along with the electric dipole moment, we have calculated energies of 100 rotational levels of each of the ortho and para species of </span></span><em>c</em>-C₃H₂O and the Einstein <em>A</em><span>-coefficients for radiative transitions between the levels. The values of Einstein </span><em>A</em><span>-coefficients along with the scaled values for collisional<span> rate coefficients are used for solving a set of statistical equilibrium equations<span> coupled with the equations of radiative transfer.</span></span></span></p><p><span><span>Brightness temperatures of seven </span>rotational transitions of each of the ortho and para species of </span><em>c</em>-C₃H₂O are investigated. Out of fourteen transitions, seven are found to show anomalous absorption and rest seven are found to show emission feature. We find that the transitions <span><math><mrow><msub><mn>1</mn><mn>10</mn></msub><mo>−</mo><msub><mn>1</mn><mn>11</mn></msub></mrow></math></span> (1.544  GHz) may play important role in identification of cyclopropenone in a cosmic object.</p></div>","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"6 ","pages":"Pages 1-8"},"PeriodicalIF":0.0,"publicationDate":"2017-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.molap.2016.11.001","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81835568","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Hydrogen atom mobility, kinetic isotope effects and tunneling on interstellar ices (Ih and ASW)","authors":"Bethmini Senevirathne ,&nbsp;Stefan Andersson ,&nbsp;Francois Dulieu ,&nbsp;Gunnar Nyman","doi":"10.1016/j.molap.2017.01.005","DOIUrl":"10.1016/j.molap.2017.01.005","url":null,"abstract":"<div><p>Transitions of a single H atom between local minima on the surfaces of crystalline ice (I_<em>h</em><span>) and amorphous solid<span><span> water (ASW) are studied theoretically in the temperature range 4–25 K. Binding energies, barrier heights, transition rate constants and the kinetic isotope effect (KIE) with and without tunneling are calculated. Harmonic </span>transition state theory is used to obtain the transition rate constants and tunneling is treated with the Wigner tunneling correction, Eckart barrier correction and harmonic quantum transition state theory (HQTST). The classical binding energies are smaller on I</span></span>_<em>h</em> (&lt;47 meV) than on ASW (&lt;89 meV). Also the classical barrier heights are smaller on I_<em>h</em> (&lt;14 meV) than on ASW (&lt;69 meV) and distributed over a range of energies, in line with previous experimental observations. Similarly the vibrationally adiabatic ground state (VAG) barrier heights are smaller on I_<em>h</em> (&lt; 7 meV) than on ASW (&lt;54 meV). The surface morphology strongly influences the well depths. Tunneling increases some of the transition rate constants substantially but has a much smaller effect on others. The average KIE for I_<em>h</em> is higher than for ASW for the same range of barrier heights.</p></div>","PeriodicalId":44164,"journal":{"name":"Molecular Astrophysics","volume":"6 ","pages":"Pages 59-69"},"PeriodicalIF":0.0,"publicationDate":"2017-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/j.molap.2017.01.005","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73248717","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}