DOCSLIB.ORG

Theoretical Investigations of H2 Formation on Interstellar Silicates Surfaces

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Theoretical Investigations of H2 Formation on Interstellar Silicates Surfaces Theoretical Investigations of H2 Formation on Interstellar Silicates Surfaces Javier Navarro Ruiz Tesi Doctoral Estudi de Doctorat en Química Directors: Mariona Sodupe Roure Albert Rimola Gibert Departament de Química Facultat de Ciències 2015 Universitat Autònoma de Barcelona Departament de Química Unitat de Química Física Memòria presentada per aspirar al Grau de Doctor per Javier Navarro Ruiz, Javier Navarro Ruiz Vist i plau, Mariona Sodupe Roure Albert Rimola Gibert Bellaterra, 29 d’octubre de 2015 Als meus pares i germà Preface Among the molecules in space, H2 is one of the most relevant of the universe. It is the most abundant one in the interstellar medium and is a key intermediate for the formation of bigger molecules. Its formation is complex, but due to its inherent relevance understanding its interaction and its formation can be considered as a paradigm of the astrophysical process. The present thesis is structured in four chapters. Chapter 1 introduces the astrochemical framework in which the thesis is located, pointing out the presence of interstellar hydrogen in the interstellar medium and where it takes place, in the interstellar dust grains around. After presenting the goals this thesis aims, Chapter 2 overviews the general theoretical aspects behind it, such as electronic structure, density functional methods, solids modelling and tunnelling effects, providing finally the computational details entailed. Chapter 3 corresponds to results and discussion and is divided into different sections. Section 3.1 presents some physicochemical properties of the crystalline bulk structure and the corresponding surfaces of Mg2SiO4 forsterite and of the Fe-containing Mg1.5Fe0.5SiO4 olivine systems. Section 3.2 reports the adsorption of H atoms and their recombination to form a H2 molecule on the crystalline Mg2SiO4 forsterite (010) surface and Section 3.3 analyses the relevance of surface morphology by considering the H2 formation on the crystalline Mg2SiO4 forsterite (001) and (110) surfaces. Finally, Section 3.4 investigates the influence of Fe2+ atoms by modelling the physisorption/chemisorption of H atom on the Fe-containing (010) surface, subsequently taking place the formation of H2. Chapter 4 addresses the general conclusions of the present thesis and possible future perspectives, Chapter 5 includes the references cited and Appendix A and B supports the information given in Chapter 3. Additionally, at the beginning of this work, a list of abbreviations of some terms used throughout the text is appended. Acknowledgements Primer de tot m'agradaria donar les gràcies als meus directors, sense ells aquesta tesi doctoral no hauria estat possible. He tingut el privilegi d'haver estat tutelat per persones tan extraordinàries com la Prof. Mariona Sodupe i el Dr. Albert Rimola, sota els quals he après profundament i perfeccionat moltes de les tècniques computacionals que ara conec i que s'analitzen detalladament en aquesta tesis. Expressar el meu agraïment a l’Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) de la Generalitat de Catalunya i a la Universitat Autònoma de Barcelona (UAB) pel suport financer concedit gràcies al Programa d’Ajuts per a la Contractació de Personal Investigador Novell (FI-DGR, NÚM. EXPEDIENT: 2015FI_B 00953 i 2015FI_B2 00083). Gràcies també al meu grup de recerca “Computational Studies of (Bio)inorganic Systems” (GETAB Group) i a les subvencions que ha rebut a través dels projectes del Ministeri de Ciència i Innovació (CTQ2011-24847/BQU, CTQ2013-40347-ERC, CTQ2014-59544-P i CTQ2014-60119-P) i de la Generalitat de Catalunya (2009SGR-638 i 2014SGR-482), molts dels congressos als quals he pogut assistir com a doctorand van ser possibles. També estic agraït al Consorci de Serveis Universitaris de Catalunya (CSUC) pels recursos computacionals proporcionats per l'assoliment d'aquesta tesi. Agraeixo especialment a Angelos Michaelides el haver-me donat la oportunitat de pertànyer al seu grup de recerca “Interfaces: Catalytic and Environmental” (ICE Group) i el seu suport durant la meva estada predoctoral al London Centre for Nanotechnology de la University College London (UCL) i al Thomas Young Centre a Londres, Regne Unit. També vull agrair al nostre estimat col·laborador des de fa temps i coautor de diversos treballs plegats, el Prof. Piero Ugliengo de la Università degli Studi di Torino, les seves valuoses contribucions als diversos treballs presentats en aquesta tesi. Estic en deute amb els excel·lents doctorands i professors amb els què he compartit moltes estones entranyables i he treballat en els darrers anys, per les nombroses aportacions que aquí es presenten i per tot el què m’han ensenyat i, en general, a tot el personal de la Unitat de Química Física. Finalment, però no per això menys important, paraules de gratitud vers la meva família i amics, un pilar fonamental durant tots aquests anys, el seu recolzament ha estat molt gratificant. Javier Navarro Ruiz Bellaterra, Octubre del 2015 List of Abbreviations Abbreviation Meaning AGB Asymptotic Giant Branch AGN Active Galactic Nuclei AIMD Ab Initio Molecular Dynamics BSSE Basis Set Superposition Error CC Coupled Cluster method CI Configuration Interaction method CO Crystalline Orbital cr Cosmic Ray DFT Density Functional Theory DRC Distinguished Reaction Coordinate ECP Effective Core Potential EPM Electrostatic Potential Map ER Eley–Rideal FUSE Far Ultraviolet Spectroscopic Explorer GGA Generalized Gradient Approximation GPW Gaussian and Plane Waves method GTBF Gaussian-type Bloch Functions GTF Gaussian-type Functions HA Hot Atom HF Hartree–Fock HK Hohenberg–Kohn IR Infrared light ISM Interstellar Medium KS Kohn–Sham LCAO Linear Combination of Atomic Orbitals LDA Local Density Approximation LSDA Local Spin Density Approximation LH Langmuir–Hinshelwood MBPT Many Body Perturbation Theory PAH Polycyclic Aromatic Hydrocarbon List of Abbreviations PAW Plane Augmented Wave PDR Photon–Dominated Region PP Pseudopotential PW Plane Wave SCF Self–Consistent Field TPD Temperature–Programmed Desorption TST Transition State Theory UV Ultraviolet light VIS Visible light YSO Young Stellar Object ZPE Zero-Point Energy xii Contents 1. INTRODUCTION ........................................................................................................ 1 1.1. INTERSTELLAR MEDIUM ........................................................................................... 1 1.2. INTERSTELLAR HYDROGEN .................................................................................... 10 1.2.1. Relevance ....................................................................................................... 10 1.2.2. Spectroscopic Detection ................................................................................. 11 1.2.3. Formation ...................................................................................................... 14 1.2.4. Previous Astrochemical Modelling and Experimental Studies ...................... 17 1.2.5. Chemical Role of H2 ....................................................................................... 18 1.3. INTERSTELLAR DUST GRAINS ................................................................................ 20 1.3.1. Composition ................................................................................................... 21 1.3.2. Formation ...................................................................................................... 22 1.3.3. Olivines .......................................................................................................... 24 1.4. PREVIOUS THEORETICAL WORKS ........................................................................... 26 1.4.1. Formation, Structure and Modelling of Olivines ........................................... 26 1.4.2. Surface Molecular Hydrogen Formation ....................................................... 28 1.5. OBJECTIVES ............................................................................................................ 29 2. METHODOLOGY ..................................................................................................... 31 2.1. GENERAL CONCEPTS ON ELECTRONIC STRUCTURE CALCULATION ........................ 31 2.2. WAVE FUNCTION-BASED METHODS ...................................................................... 33 2.2.1. Hartree-Fock Approximation ......................................................................... 33 2.2.2. Post Hartree-Fock Methods ........................................................................... 33 2.3. DENSITY FUNCTIONAL METHODS .......................................................................... 36 2.3.1. The Foundations of DFT ................................................................................ 36 2.3.2. The Kohn–Sham Method ................................................................................ 37 2.3.3. Approximations to the Exchange–Correlation Potential ............................... 39 2.3.4. Some Difficult Cases for DFT ........................................................................ 42 2.4. MODELLING SOLIDS AND SURFACES ...................................................................... 43 2.4.1. Cluster Approach ........................................................................................... 43 2.4.2. Periodic Approach ......................................................................................... 44 2.4.2.1. The Bloch Theorem ...................................................................................
Load more

Recommended publications
  • Large Molecules in the Envelope Surrounding IRC+10216
    Mon. Not. R. Astron. Soc. 316, 195±203 (2000) Large molecules in the envelope surrounding IRC110216 T. J. Millar,1 E. Herbst2w and R. P. A. Bettens3 1Department of Physics, UMIST, PO Box 88, Manchester M60 1QD 2Departments of Physics and Astronomy, The Ohio State University, Columbus, OH 43210, USA 3Research School of Chemistry, Australian National University, ACT 0200, Australia Accepted 2000 February 22. Received 2000 February 21; in original form 2000 January 21 ABSTRACT A new chemical model of the circumstellar envelope surrounding the carbon-rich star IRC110216 is developed that includes carbon-containing molecules with up to 23 carbon atoms. The model consists of 3851 reactions involving 407 gas-phase species. Sizeable abundances of a variety of large molecules ± including carbon clusters, unsaturated hydro- carbons and cyanopolyynes ± have been calculated. Negative molecular ions of chemical 2 2 formulae Cn and CnH 7 # n # 23 exist in considerable abundance, with peak concen- trations at distances from the central star somewhat greater than their neutral counterparts. The negative ions might be detected in radio emission, or even in the optical absorption of background field stars. The calculated radial distributions of the carbon-chain CnH radicals are looked at carefully and compared with interferometric observations. Key words: molecular data ± molecular processes ± circumstellar matter ± stars: individual: IRC110216 ± ISM: molecules. synthesis of fullerenes in the laboratory through chains and rings 1 INTRODUCTION is well-known (von Helden, Notts & Bowers 1993; Hunter et al. The possible production of large molecules in assorted astronomi- 1994), the individual reactions have not been elucidated. Bettens cal environments is a problem of considerable interest.
    [Show full text]
  • Organics in the Solar System
    Organics in the Solar System Sun Kwok1,2 1 Laboratory for Space Research, The University of Hong Kong, Hong Kong, China 2 Department of Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Vancouver, B.C., Canada [email protected]; [email protected] ABSTRACT Complex organics are now commonly found in meteorites, comets, asteroids, planetary satellites, and interplanetary dust particles. The chemical composition and possible origin of these organics are presented. Specifically, we discuss the possible link between Solar System organics and the complex organics synthesized during the late stages of stellar evolution. Implications of extraterrestrial organics on the origin of life on Earth and the possibility of existence of primordial organics on Earth are also discussed. Subject headings: meteorites, organics, stellar evolution, origin of life arXiv:1901.04627v1 [astro-ph.EP] 15 Jan 2019 Invited review presented at the International Symposium on Lunar and Planetary Science, accepted for publication in Research in Astronomy and Astrophysics. –2– 1. Introduction In the traditional picture of the Solar System, planets, asteroids, comets and planetary satellites were formed from a well-mixed primordial nebula of chemically and isotopically uniform composition. The primordial solar nebula was believed to be initially composed of only atomic elements synthesized by previous generations of stars, and current Solar System objects later condensed out of this homogeneous gaseous nebula. Gas, ice, metals, and minerals were assumed to be the primarily constituents of planetary bodies (Suess 1965). Although the presence of organics in meteorites was hinted as early as the 19th century (Berzelius 1834), the first definite evidence for the presence of extraterrestrial organic matter in the Solar System was the discovery of paraffins in the Orgueil meteorites (Nagy et al.
    [Show full text]
  • Implications for Extraterrestrial Hydrocarbon Chemistry: Analysis Of
    The Astrophysical Journal, 889:3 (26pp), 2020 January 20 https://doi.org/10.3847/1538-4357/ab616c © 2020. The American Astronomical Society. All rights reserved. Implications for Extraterrestrial Hydrocarbon Chemistry: Analysis of Acetylene (C2H2) and D2-acetylene (C2D2) Ices Exposed to Ionizing Radiation via Ultraviolet–Visible Spectroscopy, Infrared Spectroscopy, and Reflectron Time-of-flight Mass Spectrometry Matthew J. Abplanalp1,2 and Ralf I. Kaiser1,2 1 W. M. Keck Research Laboratory in Astrochemistry, University of Hawaii at Manoa, Honolulu, HI 96822, USA 2 Department of Chemistry, University of Hawaii at Manoa, Honolulu, HI 96822, USA Received 2019 October 4; revised 2019 December 7; accepted 2019 December 10; published 2020 January 20 Abstract The processing of the simple hydrocarbon ice, acetylene (C2H2/C2D2), via energetic electrons, thus simulating the processes in the track of galactic cosmic-ray particles penetrating solid matter, was carried out in an ultrahigh vacuum surface apparatus. The chemical evolution of the ices was monitored online and in situ utilizing Fourier transform infrared spectroscopy (FTIR) and ultraviolet–visible spectroscopy and, during temperature programmed desorption, via a quadrupole mass spectrometer with an electron impact ionization source (EI-QMS) and a reflectron time-of-flight mass spectrometer utilizing single-photon photoionization (SPI-ReTOF-MS) along with resonance-enhanced multiphoton photoionization (REMPI-ReTOF-MS). The confirmation of previous in situ studies of ethylene ice irradiation
    [Show full text]
  • Interstellar Dust Within the Life Cycle of the Interstellar Medium K
    EPJ Web of Conferences 18, 03001 (2011) DOI: 10.1051/epjconf/20111803001 C Owned by the authors, published by EDP Sciences, 2011 Interstellar dust within the life cycle of the interstellar medium K. Demyk1,2,a 1Université de Toulouse, UPS-OMP, IRAP, Toulouse, France 2CNRS, IRAP, 9 Av. colonel Roche, BP. 44346, 31028 Toulouse Cedex 4, France Abstract. Cosmic dust is omnipresent in the Universe. Its presence influences the evolution of the astronomical objects which in turn modify its physical and chemical properties. The nature of cosmic dust, its intimate coupling with its environment, constitute a rich field of research based on observations, modelling and experimental work. This review presents the observations of the different components of interstellar dust and discusses their evolution during the life cycle of the interstellar medium. 1. INTRODUCTION Interstellar dust grains are found everywhere in the Universe: in the Solar System, around stars at all evolutionary stages, in interstellar clouds of all kind, in galaxies and in the intergalactic medium. Cosmic dust is intimately mixed with the gas-phase and represents about 1% of the gas (in mass) in our Galaxy. The interstellar extinction and the emission of diffuse interstellar clouds is reproduced by three dust components: a population of large grains, the BGs (Big Grains, ∼10–500 nm) made of silicate and a refractory mantle, a population of carbonaceous nanograins, the VSGs (Very Small Grains, 1–10 nm) and a population of macro-molecules the PAHs (Polycyclic Aromatic Hydrocarbons) [1]. These three components are more or less abundant in the diverse astrophysical environments reflecting the coupling of dust with the environment and its evolution according to the physical and dynamical conditions.
    [Show full text]
  • Laboratory Astrophysics: from Observations to Interpretation
    April 14th – 19th 2019 Jesus College Cambridge UK IAU Symposium 350 Laboratory Astrophysics: From Observations to Interpretation Poster design by: D. Benoit, A. Dawes, E. Sciamma-O’Brien & H. Fraser Scientific Organizing Committee: Local Organizing Committee: Farid Salama (Chair) ★ P. Barklem ★ H. Fraser ★ T. Henning H. Fraser (Chair) ★ D. Benoit ★ R Coster ★ A. Dawes ★ S. Gärtner ★ C. Joblin ★ S. Kwok ★ H. Linnartz ★ L. Mashonkina ★ T. Millar ★ D. Heard ★ S. Ioppolo ★ N. Mason ★ A. Meijer★ P. Rimmer ★ ★ O. Shalabiea★ G. Vidali ★ F. Wa n g ★ G. Del-Zanna E. Sciamma-O’Brien ★ F. Salama ★ C. Wa lsh ★ G. Del-Zanna For more information and to contact us: www.astrochemistry.org.uk/IAU_S350 [email protected] @iaus350labastro 2 Abstract Book Scheduley Sunday 14th April . Pg. 2 Monday 15th April . Pg. 3 Tuesday 16th April . Pg. 4 Wednesday 17th April . Pg. 5 Thursday 18th April . Pg. 6 Friday 19th April . Pg. 7 List of Posters . .Pg. 8 Abstracts of Talks . .Pg. 12 Abstracts of Posters . Pg. 83 yPlenary talks (40') are indicated with `P', review talks (30') with `R', and invited talks (15') with `I'. Schedule Sunday 14th April 14:00 - 17:00 REGISTRATION 18:00 - 19:00 WELCOME RECEPTION 19:30 DINNER BAR OPEN UNTIL 23:00 Back to Table of Contents 2 Monday 15th April 09:00 { 10:00 REGISTRATION 09:00 WELCOME by F. Salama (Chair of SOC) SESSION 1 CHAIR: F. Salama 09:15 E. van Dishoeck (P) Laboratory astrophysics: key to understanding the Universe From Diffuse Clouds to Protostars: Outstanding Questions about the Evolution of 10:00 A.
    [Show full text]
  • Imaging the Circumstellar Envelope of OH 26.5+0.6
    A&A 396, 581–587 (2002) Astronomy DOI: 10.1051/0004-6361:20021272 & c ESO 2002 Astrophysics Imaging the circumstellar envelope of OH 26.5+0.6 D. Fong1, K. Justtanont2,M.Meixner1, and M. T. Campbell1 1 Department of Astronomy, University of Illinois, 1002 W. Green Street Urbana, IL 61801, USA 2 SCFAB, Stockholm Observatory, Department of Astronomy, 106 91 Stockholm, Sweden Received 10 July 2002 / Accepted 28 August 2002 Abstract. Using the Berkeley-Illinois-Maryland-Association (BIMA) Millimeter Array, we were able to map the extreme OH/IR star, OH 26.5+0.6, in the 12CO J = 1–0 line transition. The CO emission is partially resolved with a deconvolved source size of 8:500 5:500. The spectrum shows that the blue-shifted emission is missing, most likely due to interstellar absorption. By × 6 1 modelling the infrared spectral energy distribution, we derive a dust mass loss rate of 1:9 10− M yr− .Fromthisweareable × to place an upper limit on the extent of the dusty envelope of 1016 cm while our BIMA map shows that the CO photodissociation radius extends out to about 7 1016 cm. To best fit the BIMA observations and the higher CO rotational transitions using our full radiative transfer code, we needed× to include a second, more tenuous AGB wind, outside the high density superwind to account for the observed flux. From our model, we conclude that up to 80% of the CO flux comes from the unresolved superwind. Key words. stars: AGB and post-AGB – stars: circumstellar matter – stars: individual : OH 26.5+0.6 – stars: late-type – stars: mass loss 1.
    [Show full text]
  • Habitable Environments in Late Stellar Evolution
    Habitable Environments in Late Stellar Evolution Conditions for Abiogenesis in the Planetary Systems of White Dwarfs Author: Dewy Peters Supervisor: Prof. dr. F.F.S. van der Tak A thesis submitted in fulfillment of the requirements for the degree BSc in Astronomy Faculty of Science and Engineering University of Groningen The Netherlands March 2021 Abstract With very high potential transit-depths and an absence of stellar flare activity, the planets of White Dwarfs (WDs) are some of the most promising in the search for detectable life. Whilst planets with Earth-like masses and radii have yet to be detected around WDs, there is considerable evidence from spectroscopic and photometric observations that both terrestrial and gas-giant planets are capable of surviving post-main sequence evolution and migrating into the WD phase. WDs are also capable of hosting stable Habitable Zones outside orbital distances at which Earth-mass plan- ets would be disintegrated by tidal forces. Whilst transition- ing to the WD Phase, a main-sequence star has to progress along the Asymptotic Giant Branch (AGB), whereby orbit- ing planets would be subjected to atmospheric erosion by its harsh stellar winds. As a trade-off, the Circumstellar En- velope (CSE) of an AGB star is found to be rich in organics and some of the simple molecules from which more complex prebiotic molecules such as amino acids and simple sugars can be synthesised. It is found that planets with initial or- bital distances equivalent to those of Saturn and the Kuiper Belt would be capable of accreting a mass between 1 and 20 times that of the Earth’s atmosphere from the CSE and as a result, could obtain much of the material necessary to sustain life after the AGB and also experience minimal at- mospheric erosion.
    [Show full text]
  • Molecular Dust Precursors in Envelopes of Oxygen-Rich AGB Stars
    Why Galaxies Care About AGB Stars: A Continuing Challenge through Cosmic Time Proceedings IAU Symposium No. 343, 2018 c 2018 International Astronomical Union F. Kerschbaum, H. Olofsson & M. Groenewegen, eds. DOI: 00.0000/X000000000000000X Molecular dust precursors in envelopes of oxygen-rich AGB stars and red supergiants Tomasz Kami´nski Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 Submillimeter Array Fellow, email: [email protected] Abstract. Condensation of circumstellar dust begins with formation of molecular clusters close to the stellar photosphere. These clusters are predicted to act as condensation cores at lower temperatures and allow efficient dust formation farther away from the star. Recent observations of metal oxides, such as AlO, AlOH, TiO, and TiO2, whose emission can be traced at high angular resolutions with ALMA, have allowed first observational studies of the condensation process in oxygen-rich stars. We are now in the era when depletion of gas-phase species into dust can be observed directly. I review the most recent observations that allow us to identify gas species involved in the formation of inorganic dust of AGB stars and red supergiants. I also discuss challenges we face in interpreting the observations, especially those related to non- equilibrium gas excitation and the high complexity of stellar atmospheres in the dust-formation zone. Keywords. stars: AGB and post-AGB; circumstellar matter; stars: mass loss; stars: winds, outflows; dust, extinction; ISM: molecules 1. Dust formation and seed particles in O-rich stars Galaxies certainly care about AGB stars | in the local Universe, galaxies owe huge amounts of dust to these unexhaustive factories of cosmic solids.
    [Show full text]
  • Arxiv:1803.01452V1 [Astro-Ph.EP] 5 Mar 2018
    The when and where of water in the history of the universe Karla de Souza Torres1, and Othon Cabo Winter2 1CEFET-MG, Curvelo, Brazil; E-mail: [email protected] 2UNESP, Grupo de Din^amica Orbital & Planetologia, Guaratinguet´a,Brazil E-mail: [email protected] Abstract It is undeniable that life as we know it depends on liquid water. It is difficult to imagine any biochemical machinery that does not require water. On Earth, life adapts to the most diverse environments and, once established, it is very resilient. Considering that water is a common compound in the Universe, it seems possible (maybe even likely) that one day we will find life elsewhere in the universe. In this study, we review the main aspects of water as an essential compound for life: when it appeared since the Big Bang, and where it spread throughout the diverse cosmic sites. Then, we describe the strong relation between water and life, as we know it. Keywords water; life; universe; H2O; astrobiology 1. Introduction. Why water is essential for life? It is well known that liquid water has played the essential and undeniable role in the emergence, development, and maintenance of life on Earth. Two thirds of the Earth's surface is covered by water, however fresh water is most valuable as a resource for animals and plants. Thus, sustain- ability of our planet's fresh water reserves is an important issue as population numbers increase. Water accounts for 75% of human body mass and is the major constituent of organism fluids. All these facts indicate that water is one of the most important elements for life on Earth.
    [Show full text]
  • Imaging the Circumstellar Dust Around AGB Stars with Polcor⋆
    A&A 531, A148 (2011) Astronomy DOI: 10.1051/0004-6361/201015964 & c ESO 2011 Astrophysics Imaging the circumstellar dust around AGB stars with PolCor S. Ramstedt1, M. Maercker1,2, G. Olofsson3,H.Olofsson3,4, and F. L. Schöier4 1 Argelander Institute for Astronomy, University of Bonn, 53121 Bonn, Germany e-mail: [email protected] 2 European Southern Observatory, Karl Schwarzschild Str. 2, Garching bei München, Germany 3 Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden 4 Onsala Space Observatory, Dept. of Radio and Space Science, Chalmers University of Technology, 43992 Onsala, Sweden Received 21 October 2010 / Accepted 16 May 2011 ABSTRACT Aims. The aim of this paper is to investigate how the new imaging Polarimeter and Coronograph (PolCor) at the Nordic Optical Telescope (NOT) can be used in the study of circumstellar structures around AGB stars. The purpose is to prepare for a study of a larger sample. Methods. We have observed two types of AGB stars using the PolCor instrument on the NOT: the binary S-type star W Aql and two carbon stars with detached shells, U Cam and DR Ser. The polarized light traces the dust distribution around the stars. From the polarimeter images the polarized intensity, the polarization degree, and the polarization angle over the images are calculated. The location and extent of dust structures are examined in the images. The total dust mass and the dust-to-gas ratios of the detached shells are also calculated. Results. The images of the circumstellar envelope of W Aql show what seems to be an elongated structure in the south-west direction.
    [Show full text]
  • CO Line Emission from Circumstellar Envelopes
    A&A 450, 167–179 (2006) Astronomy DOI: 10.1051/0004-6361:20053759 & c ESO 2006 Astrophysics CO line emission from circumstellar envelopes D. Teyssier1,2,, R. Hernandez3,V.Bujarrabal3, H. Yoshida4, and T. G. Phillips5 1 Departamento de Astrofísica Molecular e Infrarroja, IEM, CSIC, 28006 Madrid, Spain 2 Space Research Organization Netherlands, P.O. Box 800, 9700 AV Groningen, The Netherlands e-mail: [email protected] 3 Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain e-mail: [email protected] 4 Caltech Submillimeter Observatory, 111 Nowelo Street, Hilo, HI 96720, USA e-mail: [email protected] 5 California Institute of Technology, Pasadena, CA 91125, USA e-mail: [email protected] Received 4 July 2005 / Accepted 30 December 2005 ABSTRACT Aims. We present the results of a multi-transition CO observational program conducted on a sample of AGB and post-AGB stars envelopes. We have collected maps and single pointing observations of these envelopes in 5 rotational transitions ranging from J = 1–0 to J = 6–5, including in particular new observations of the CO line at 691 GHz at the CSO. The use of such a set of mm and submm CO line on stellar envelopes is rare and limited to the work of some authors on IRC+10216. Methods. Using a model for the CO emission of an AGB circumstellar envelope, in combination with a standard LVG approach, we have conducted a systematic modelling analysis using the whole set of CO data collected for a sample of 12 sources. We simultaneously fit all five transitions, taking into account the spatial information provided by the maps.
    [Show full text]
  • Molecular Evolution from AGB Stars to Planetary Nebulae
    The Molecular Universe Proceedings IAU Symposium No. 280, 2011 c International Astronomical Union 2011 Jos´e Cernicharo & Rafael Bachiller, eds. doi:10.1017/S1743921311024987 Molecular Evolution from AGB Stars to Planetary Nebulae Sun Kwok1 1 Faculty of Science, The University of Hong Kong, Hong Kong, China email: [email protected] Abstract. The late stages of stellar evolution from the Asymptotic Giant Branch (AGB) to planetary nebulae represent the most active phase of molecular synthesis in a star’s life. Over 60 molecular species, including inorganics, organics, radicals, chains, rings, and molecular ions have been detected in the circumstellar envelopes of evolved stars. Most interestingly, complex organic compounds of aromatic and aliphatic structures are synthesized over very short time intervals after the end of the AGB. Also appeared during the post-AGB evolution are the unidentified 21 and 30 µm emission features, which are believed to originate from carbonaceous compounds. The circumstellar environment is an ideal laboratory for the testing of theories of chemical synthesis. The distinct spectral behavior among AGB stars, proto-planetary nebulae (PPN), and planetary nebulae (PN) and the short evolutionary time scales that separate these stages pose severe constraints on models. In this paper, we will present an observational summary of the chemical synthesis in the late stages of stellar evolution, discuss chemical and physical processes at work, and speculate on the possible effects these chemical products have on the Galaxy and the Solar System. 1. Introduction An intermediate mass (1-8 M⊙) star undergoes H and He shell burning after having developed a C–O electron-degenerate core.
    [Show full text]