L. Biennier, H. Sabbah, S. Klippenstein, V. Chandrasekaran, I. Sims, B. Rowe
The mechanisms of nucleation and growth of carbon dust particles in circumstellar envelopes of carbon-rich stars in the red giant and AGB phases of their evolution are poorly understood. It has been proposed that the transition of gas phase species to solid particles, is achieved by the formation of a critical nucleus composed of two PAHs held together by van der Waals forces. Some insights into the validity of the nucleation of PAH molecules in the envelope can be gained through the investigation of the thermodynamics of dimers, representing the first stage towards condensation. We have performed experiments to identify the temperature range over which small PAH clusters form in saturated uniform supersonic flows. The kinetics of the formation has also been investigated. The experimental data have been combined with theoretical calculations. We unambiguously demonstrate that the association of small PAHs such as pyrene (C16H10) is slower than the destruction of the dimer in warm and hot environments such as IRC +10216. Our findings challenge a formation model based on the phys- ical stacking of small PAH units in circumstellar shells of carbon rich stars.
{"title":"Insights into the Condensation of PAHs in the Envelope of IRC +10216","authors":"L. Biennier, H. Sabbah, S. Klippenstein, V. Chandrasekaran, I. Sims, B. Rowe","doi":"10.1051/EAS/1146020","DOIUrl":"https://doi.org/10.1051/EAS/1146020","url":null,"abstract":"The mechanisms of nucleation and growth of carbon dust particles in circumstellar envelopes of carbon-rich stars in the red giant and AGB phases of their evolution are poorly understood. It has been proposed that the transition of gas phase species to solid particles, is achieved by the formation of a critical nucleus composed of two PAHs held together by van der Waals forces. Some insights into the validity of the nucleation of PAH molecules in the envelope can be gained through the investigation of the thermodynamics of dimers, representing the first stage towards condensation. We have performed experiments to identify the temperature range over which small PAH clusters form in saturated uniform supersonic flows. The kinetics of the formation has also been investigated. The experimental data have been combined with theoretical calculations. We unambiguously demonstrate that the association of small PAHs such as pyrene (C16H10) is slower than the destruction of the dimer in warm and hot environments such as IRC +10216. Our findings challenge a formation model based on the phys- ical stacking of small PAH units in circumstellar shells of carbon rich stars.","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130233476","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}
Strong IR emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 μ m are a common characteristic of the interstellar medium of the Milky Way and nearby galaxies and out to redshifts of3. Here, we review the history of the PAH hypothesis which attributes these emission features to vibrational fluorescence of large (~50 carbon-atom Polycyclic Aromatic Hydrocarbon molecules pumped by ultraviolet photons from nearby stars or the average interstellar radiation field. Over the last 25 years, our insight in the characteristics of these molecules and their role in the Universe has greatly improved and the PAH hypothesis is alive and well; not in the least due a remarkable adaptability. Not surprisingly, the precise characteristics of these species remains to be defined.
{"title":"25 Years of PAH Hypothesis","authors":"A. Tielens","doi":"10.1051/EAS/1146001","DOIUrl":"https://doi.org/10.1051/EAS/1146001","url":null,"abstract":"Strong IR emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 μ m are a common characteristic of the interstellar medium of the Milky Way and nearby galaxies and out to redshifts of3. Here, we review the history of the PAH hypothesis which attributes these emission features to vibrational fluorescence of large (~50 carbon-atom Polycyclic Aromatic Hydrocarbon molecules pumped by ultraviolet photons from nearby stars or the average interstellar radiation field. Over the last 25 years, our insight in the characteristics of these molecules and their role in the Universe has greatly improved and the PAH hypothesis is alive and well; not in the least due a remarkable adaptability. Not surprisingly, the precise characteristics of these species remains to be defined.","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"42 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131919687","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}
Infrared absorption and emission features observed spectroscopically in our Galaxy allow to probe the composition of solid dust grains, their evolution and thus follow the cycling of matter in the Galaxy. Many observables do reveal the presence of large amounts of carbonaceous particles in space, other than the PAH-like emission lines. The carbonaceous materials observed include amorphous carbons, diamondoids showing in emission for a few specific sources, and the recently detected fullerenes. An important hydrogenated amorphous carbon component (HAC or a-C:H), traced by the 2940 cm-1 structured absorption feature is observed against Galactic background sources. Since the discovery of this feature in the early eighties (Allen ), the observation of a-C:H has been extended to the mid-infrared by space observatories, giving insight into additional associated features. They are also observed in external galaxies, showing the ubiquitous nature of these components. We will focus on astronomical observations of organic matter other than PAHs, amorphous carbons and associated laboratory dust analogues relevant to astrophysical applications.
{"title":"Observations of Interstellar Carbon Compounds","authors":"E. Dartois","doi":"10.1051/EAS/1146039","DOIUrl":"https://doi.org/10.1051/EAS/1146039","url":null,"abstract":"Infrared absorption and emission features observed spectroscopically in our Galaxy allow to probe the composition of solid dust grains, their evolution and thus follow the cycling of matter in the Galaxy. Many observables do reveal the presence of large amounts of carbonaceous particles in space, other than the PAH-like emission lines. The carbonaceous materials observed include amorphous carbons, diamondoids showing in emission for a few specific sources, and the recently detected fullerenes. An important hydrogenated amorphous carbon component (HAC or a-C:H), traced by the 2940 cm-1 structured absorption feature is observed against Galactic background sources. Since the discovery of this feature in the early eighties (Allen ), the observation of a-C:H has been extended to the mid-infrared by space observatories, giving insight into additional associated features. They are also observed in external galaxies, showing the ubiquitous nature of these components. We will focus on astronomical observations of organic matter other than PAHs, amorphous carbons and associated laboratory dust analogues relevant to astrophysical applications.","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128700999","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}
Pub Date : 2020-11-04DOI: 10.1051/978-2-7598-2482-3.fm
{"title":"Frontmatter","authors":"","doi":"10.1051/978-2-7598-2482-3.fm","DOIUrl":"https://doi.org/10.1051/978-2-7598-2482-3.fm","url":null,"abstract":"","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"25 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130483803","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}
Pub Date : 2020-11-04DOI: 10.1051/978-2-7598-2482-3.c052
{"title":"Astronomical Object Index","authors":"","doi":"10.1051/978-2-7598-2482-3.c052","DOIUrl":"https://doi.org/10.1051/978-2-7598-2482-3.c052","url":null,"abstract":"","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131295172","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}
Pub Date : 2020-11-04DOI: 10.1051/978-2-7598-2482-3-fm
{"title":"Frontmatter","authors":"","doi":"10.1051/978-2-7598-2482-3-fm","DOIUrl":"https://doi.org/10.1051/978-2-7598-2482-3-fm","url":null,"abstract":"","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"118 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132349872","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}
The diffuse interstellar bands (DIBs) have been known of since 1922, but their carrier molecules remain unidentified to this day. We present a brief history of DIB observations, followed by a list of constraints any suggested origin must face, and finally a preview of current research for ultraviolet DIBs using the Cosmic Origins Spectrograph on the Hubble Space Telescope . We conclude that PAHs are consistent with all the listed constraints, but that PAHs may not be the only molecular species responsible for the DIBs.
{"title":"The Diffuse Interstellar Bands in History and in the UV","authors":"T. Snow, J. D. Destree","doi":"10.1051/EAS/1146035","DOIUrl":"https://doi.org/10.1051/EAS/1146035","url":null,"abstract":"The diffuse interstellar bands (DIBs) have been known of since 1922, but their carrier\u0000 molecules remain unidentified to this day. We present a brief history of DIB observations,\u0000 followed by a list of constraints any suggested origin must face, and finally a preview of\u0000 current research for ultraviolet DIBs using the Cosmic Origins Spectrograph\u0000 on the Hubble Space Telescope . We conclude that PAHs are\u0000 consistent with all the listed constraints, but that PAHs may not be the only molecular\u0000 species responsible for the DIBs.","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"11 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133641860","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}
Aromatic carbon, in some form, has been an essential in- gredient by and large in all models of the extinction curve, since the original proposal to attribute the bump at 217.5 nm to "astronomical graphite". This aromatic carbon is most naturally identified, in up to date models, with a population of Polycyclic Aromatic Hydrocarbons (PAHs), free and/or clustered. In all models, this PAH population ac- counts for the far-UV nonlinear rise in the extinction curve, contributes to the bump and possibly part of the large set of unidentified, discrete absorption features in the visible (the Diffuse Interstellar Bands). We review the current state of our understanding of the contribution of PAHs to interstellar extinction, and what constraints can be imposed on the PAH population by fitting extinction models to observations. 1 Interstellar extinction before PAHs Interstellar extintion has a long, winding story. It dates as far back in time as 1774, with sir William Herschel noticing a region in the Scorpio constellation remarkably devoid of stars, which he called a "hole in the sky". The fact that such regions were a common occurrence in the Milky Way was clear in the early years of 1900, with the systematic observations of Barnard, as was stated in a textbook of the time (Clerke 1903), and it began to be hypothesised that they were not actually holes in the fabric of the sky, but instead might be "obscured" by some intervening material. For some decades, this hypothesis was the topic of some heated debates, some of which are on record in the proceedings of academic meetings of the time (Shapley & Curtis 1921). It was only about in 1930 that firm evidence of interstellar extinction became available (Trumpler 1930).
{"title":"Polycyclic Aromatic Hydrocarbons and the Extinction Curve","authors":"G. Mulas, G. Malloci, C. Joblin, C. Pestellini","doi":"10.1051/EAS/1146034","DOIUrl":"https://doi.org/10.1051/EAS/1146034","url":null,"abstract":"Aromatic carbon, in some form, has been an essential in- gredient by and large in all models of the extinction curve, since the original proposal to attribute the bump at 217.5 nm to \"astronomical graphite\". This aromatic carbon is most naturally identified, in up to date models, with a population of Polycyclic Aromatic Hydrocarbons (PAHs), free and/or clustered. In all models, this PAH population ac- counts for the far-UV nonlinear rise in the extinction curve, contributes to the bump and possibly part of the large set of unidentified, discrete absorption features in the visible (the Diffuse Interstellar Bands). We review the current state of our understanding of the contribution of PAHs to interstellar extinction, and what constraints can be imposed on the PAH population by fitting extinction models to observations. 1 Interstellar extinction before PAHs Interstellar extintion has a long, winding story. It dates as far back in time as 1774, with sir William Herschel noticing a region in the Scorpio constellation remarkably devoid of stars, which he called a \"hole in the sky\". The fact that such regions were a common occurrence in the Milky Way was clear in the early years of 1900, with the systematic observations of Barnard, as was stated in a textbook of the time (Clerke 1903), and it began to be hypothesised that they were not actually holes in the fabric of the sky, but instead might be \"obscured\" by some intervening material. For some decades, this hypothesis was the topic of some heated debates, some of which are on record in the proceedings of academic meetings of the time (Shapley & Curtis 1921). It was only about in 1930 that firm evidence of interstellar extinction became available (Trumpler 1930).","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125841025","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}
PAHs are traditionally thought to form around carbon-rich AGB stars, where the presence of inner-wind shocks and nucleation seeds can aid their formation. However, my recent work has shown that the formation of benzene – thought to be the rate-limiting step in the formation of larger PAHs – can be efficient in other environments, namely the dense tori around the evolved stars of pre-planetary nebulae and in the inner regions of the accretion discs around young stars. I will discuss the chemical pathways to the formation of benzene in these regions and the subsequent formation of larger PAHs.
{"title":"The Formation of Benzene in Dense Environments","authors":"P. Woods","doi":"10.1051/EAS/1146025","DOIUrl":"https://doi.org/10.1051/EAS/1146025","url":null,"abstract":"PAHs are traditionally thought to form around carbon-rich AGB stars, where the presence of inner-wind shocks and nucleation seeds can aid their formation. However, my recent work has shown that the formation of benzene – thought to be the rate-limiting step in the formation of larger PAHs – can be efficient in other environments, namely the dense tori around the evolved stars of pre-planetary nebulae and in the inner regions of the accretion discs around young stars. I will discuss the chemical pathways to the formation of benzene in these regions and the subsequent formation of larger PAHs.","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"24 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115236076","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}
The hypothesis that polyaromatic molecules are the carriers of the infrared interstellar emission bands has spurred the laboratory spectroscopy of this class of molecules. Here we will give an overview of the infrared spectroscopic methods that have been applied over the past two decades to investigate the IR spectra of PAHs, their ions and related species.
{"title":"Laboratory Infrared Spectroscopy of PAHs","authors":"J. Oomens","doi":"10.1051/EAS/1146007","DOIUrl":"https://doi.org/10.1051/EAS/1146007","url":null,"abstract":"The hypothesis that polyaromatic molecules are the carriers of the infrared interstellar emission bands has spurred the laboratory spectroscopy of this class of molecules. Here we will give an overview of the infrared spectroscopic methods that have been applied over the past two decades to investigate the IR spectra of PAHs, their ions and related species.","PeriodicalId":197011,"journal":{"name":"PAHs and the Universe","volume":"7 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-11-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132587996","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}
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