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Uva-DARE (Digital Academic Repository) UvA-DARE (Digital Academic Repository) Mass loss and dust formation around oxygen-rich evolved stars Kemper, F. Publication date 2002 Document Version Final published version Link to publication Citation for published version (APA): Kemper, F. (2002). Mass loss and dust formation around oxygen-rich evolved stars. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:07 Oct 2021 Mass loss and dust formation around oxygen-rich evolved stars Mass loss and dust formation around oxygen-rich evolved stars Massaverlies en stofvorming rond zuurstofrijke ge¨evolueerde sterren Academisch Proefschrift ter verkrijging van de graad van doctor aan de Universiteit van Amsterdam, op gezag van de Rector Magnificus prof.mr. P.F. van der Heijden, ten overstaan van een door het college voor promoties ingestelde commissie, in het openbaar te verdedigen in de Aula der Universiteit op woensdag 4 september 2002, te 12:00 uur door Francisca Kemper geboren te Velsen Promotiecommissie Promotores prof.dr. L.B.F.M. Waters prof.dr. A.G.G.M. Tielens Co-promotor dr. A. de Koter Overige leden prof.dr. M.J. Barlow prof.dr. H.J. Habing prof.dr. D. Frenkel prof.dr. T. de Jong prof.dr. M. van der Klis prof.dr. V. Icke dr. C. Dominik Sterrenkundig Instituut \Anton Pannekoek" Faculteit der Natuurwetenschappen Universiteit van Amsterdam ISBN 90-5776088-6 Omslagillustratie: Mandala { Nel Kemper. De mandala is een tibetaans symbool voor het universum en de eeuwigheid, en de plaats van de mens daarin. Tibetaanse mon- niken maken de mandala's van gekleurd zand, die ze vervolgens door de wind laten verwaaien als metafoor voor de vergankelijkheid van het bestaan. Contents 1 An introduction to astromineralogy 1 1.1 Why study dust? . 1 1.2 The life cycle of dust in the galaxy . 2 1.2.1 Dust formation . 2 1.2.2 Dust in the ISM . 3 1.2.3 Dust around young stars . 4 1.3 Formation of dust around evolved stars . 4 1.3.1 Dying stars . 4 1.3.2 Mass loss and outflow properties . 6 1.3.3 Theoretical condensation sequences . 8 1.3.4 Dust condensation and processing experiments . 9 1.4 Determining the dust composition . 10 1.4.1 Amorphous and crystalline silicates . 10 1.4.2 Mineralogy of evolved stars . 11 1.5 Towards an understanding of dust formation around evolved stars . 23 2 2.4 { 197 µm spectroscopy of OH/IR stars: The IR characteristics of circumstellar dust in O-rich environments 25 2.1 Introduction . 26 2.2 Observations . 26 2.2.1 SWS . 27 2.2.2 LWS . 28 2.2.3 Joining the SWS and LWS spectra . 29 2.3 Results . 29 2.3.1 Determination of the continuum . 31 2.3.2 Description of the spectrum . 32 2.3.3 Water Ice . 36 2.3.4 Silicates . 42 2.4 Conclusions . 47 i CONTENTS 3 Crystallinity versus mass-loss rate in Asymptotic Giant Branch stars 49 3.1 Introduction . 49 3.2 Modelling the circumstellar dust shell . 51 3.2.1 Dust optical constants . 52 3.3 Model results . 53 3.3.1 Varying the mass-loss rate . 53 3.3.2 Varying the degree of crystallinity . 57 3.3.3 A trend in crystallinity? . 58 3.4 Physical explanation and discussion . 62 3.4.1 How to search for crystalline silicates in Miras . 63 3.4.2 Implications for the dust formation and processing in the ISM 65 3.5 Summary . 66 4 Dust and the spectral energy distribution of the OH/IR star OH 127.8+0.0: Evidence for circumstellar metallic iron 69 4.1 Introduction . 69 4.2 Modelling the circumstellar environment . 71 4.2.1 Model assumptions and default grid . 71 4.2.2 Problems in fitting the SED of AGB stars . 73 4.3 Improvements to the spectral fit . 73 4.3.1 Metallic iron as a source of NIR opacity . 73 4.3.2 Non-spherical grains . 76 4.3.3 Water ice features in the far-infrared . 76 4.4 Results for OH 127.8+0.0 . 77 4.4.1 Stellar parameters and radiative transfer modelling . 77 4.4.2 Comparison with astronomical silicate . 79 4.5 A consistency check . 81 4.6 Discussion . 83 5 Detection of carbonates in dust shells around evolved stars 85 6 The mineral composition and spatial distribution of the dust ejecta of NGC 6302 91 6.1 Introduction . 91 6.2 The TIMMI2 observations . 93 6.2.1 N- and Q-band imaging . 93 6.2.2 N-band spectroscopy . 95 6.3 Analysis of the ISO spectrum . 96 6.3.1 The identification of carbonates . 98 6.3.2 Dust model fit . 100 6.4 Discussion . 105 6.4.1 Geometry and composition of the circumstellar dust shell . 105 6.4.2 The astronomical relevance of carbonates . 110 ii CONTENTS 6.5 Summary . 112 7 Mass loss and rotational CO emission from Asymptotic Giant Branch stars 113 7.1 Introduction . 113 7.2 Observations and data reduction . 115 7.2.1 Instrumental set-up . 115 7.2.2 The MPIfR/SRON 800 GHz receiver . 116 7.2.3 Observations and data reduction . 116 7.3 Physical conditions in the outflow: a model . 118 7.3.1 Description of the model . 118 7.3.2 Free parameters . 119 7.4 Analysis of the results . 121 7.4.1 A constant mass-loss rate? . 124 7.4.2 Exploring parameter space . 125 7.4.3 A representative case: WX Psc . 129 7.4.4 Possible explanations for the inconsistency . 132 7.5 Concluding remarks . 136 7.5.1 CO rotational transitions as mass-loss indicators . 136 7.5.2 Future work . 137 8 Future prospects 139 8.1 The life cycle of silicates . 139 8.2 The life cycle of carbonates . 140 8.3 The outflow properties of evolved stars . 141 A JCMT observations 143 Nederlandse samenvatting 153 Nawoord 159 Bibliography 161 iii CONTENTS iv Chapter 1 An introduction to astromineralogy 1.1 Why study dust? The first hints that the vast distances between the stars were not empty came at the beginning of the previous century, when stationary Ca ii absorption lines were detected in the spectroscopic binary δ Ori (Hartmann 1904). It became apparent that especially the dust present in interstellar space was obscuring the view towards astronomical objects and that this dilution caused overestimates of the distances (Trumpler 1930). The interstellar absorption due to dust was thus considered a nuisance. Consequently, studying the Interstellar Extinction Curve became important, just with the purpose to determine the extinction at visual and ultraviolet wavelengths to correct for it when observing distant objects. Gradually, studying the properties of dust gained a right of existence on its own. Besides accounting for interstellar extinction, there appeared various other reasons to study dust in astrophysics. First, about 30% of the total luminosity of the galaxy is thermal emission in the infrared, provided by dust grains which make up only about 1% of the total mass of the interstellar medium. Second, dust grains provide a surface on which molecules are formed. Especially the formation of H2 proceeds very efficiently on the surface of grains, when compared to gas phase reactions. Yet another reason to study dust is that it is important for the thermal balance in astrophysical processes, such as star formation. On the one hand, heating is provided by photoelectric processes on dust grains, and on the other hand, as said before, dust is an important catalytic agent for the formation of molecules, which in turn dominate the gas cooling. In star formation, dust grains are not only important for the heat balance of the protostellar nebula, but they are also the building blocks of planets, as they coagulate in the circumstellar disk present around young stars. Finally, dust dominates the opacity and hence the spectral appearance of dust-enshrouded objects, including evolved and young stars. Studying the composition of dust in various astrophysical environments provides information on the physical conditions in these environments. The work presented here can be placed in this last category of dust studies. The dust in the diffuse interstellar medium can be studied by means of the Inter- stellar Extinction Curve in the visual and ultraviolet, although determining the dust 1 Chapter 1 composition is difficult due to the lack of spectral features besides the 2200 A˚ fea- ture (e.g. Cardelli et al. 1989; Mathis 1990). Circumstellar dust can be studied by its extinction properties in many cases as well, as the intrinsic spectrum of the star is often well known. This is, however, not the case for AGB stars. Dust grains present in the vicinity of a star are heated by stellar radiation to temperatures in the range of 20 { 1800 K, depending on the exact circumstellar environment.
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