thesis - IRS, The Infrared Spectrograph

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160 English Summaryis called radiation and gives the star a colour. The colour of a star is related to its temperature.For instance, Rigel in the constellation Orion is very blue (and hot) while Betelgeuse is red(and cool). This concept is easier if we think of a wire that is being heated up. As it getshotter (radiation is stronger) the colour of the wire changes from red to yellow and finallyblue. Actually, the wire emits in all colours but the temperature makes it emit more in onespecific colour. We can heat the wire up more and the colour will change but our eye won’t beable to detect it. In the same way as the wire, the colour of the light of the star is related to itstemperature. Astronomers think of the light as a wave in which the distance between the crests(that is called wavelength) depends on the energy. Therefore one element will emit photonsat a certain wavelength (or a certain colour). From laboratory measurements we know at whatwavelength the elements emit. These wavelengths are normally very small and are measuredin units such as nanometers (1 nm = 0.000000001 meters). Since the light that we see is acombination of many wavelengths from different elements, how can we distinguish them?Sir Isaac Newton (1642-1727) solved this problem for us. He realised that the sunlight whenpassing through a glass prism was divided into many colours (those of the rainbow). Hededuced that the light from the Sun was a combination of all those colours. But what is theprism really doing? The prism is deviating the incoming light according to their wavelengthand therefore it is decomposed into what we call a spectrum. It is important to recall that thehuman eye is only sensitive to a small range of wavelengths (colours), that is, the eye canonly detect light from a small portion of the total spectrum. This portion is called the opticalspectrum (the colours of the rainbow). The total spectrum and its subdivisions is shown inthe right panel of Fig. 4. Stars not only emit in the visible but also at other wavelengthsinvisible to the human eye (such as infrared, ultraviolet...). For that reason astronomers havedeveloped instruments that are sensitive not only to the visible but also to those other parts ofthe spectrum that the eye cannot see to measure the light from the stars. These instruments(called spectrometers) are based upon the same principle as the glass prism and decomposethe light into its different colours.The spectra of planetary nebulae consists thus of the radiation emitted by atoms whichwere excited by recombination of electrons and ions or by collisions. Transitions will spana wide wavelength range from the X-rays all the way to the submillimeter. The analysis inthis thesis has been done using mainly the infrared part of the spectrum because many ionsare emitted in that part of the spectrum. The ultraviolet has also often been used and to a lessdegree the optical part of the spectrum.An example of the spectrum of Planetary Nebula NGC 6302 in a small portion of theinfrared-region is shown in Fig. 5. The x-axis represents the wavelength and the y-axis theintensity. For instance we see that there is a line (a peak) at ∼ 24 300 nm of Ne 4+ (an atom ofneon that has lost 4 electrons). We know then that in this nebula neon is present. The strength(height) of the line can give us estimates of how much Ne 4+ is present.Photo-dissociation regions around Planetary NebulaeMost of the hydrogen in a planetary nebula is ionized. The shell in which this happens iscalled Strömgren sphere. In practice, this is more complicated because some nebulae arenot spherical as can be seen in Fig. 3. Outside this shell there is still radiation but of lower
English Summary 161Figure 5–. Spectrum of a PlanetaryNebula. The peaks correspondto transitions such as theone seen in Case A of Fig. 4 fordifferent elements.energies that is not able to ionize the hydrogen. These neutral 8 regions that are governedby the radiation from the central star are called photo-dissociation regions. Actually, theterm photo-dissociation regions is more generic since these regions are found in other stellarenvironments such as star forming regions. The photo-dissociation regions are very importantbecause it appears that there is more neutral than ionized material in the nebula. In theseregions elements such as neutral oxygen (O 0 ), and singly ionized carbon and silicon (C + ,Si + ) can exist.Enrichment of the interstellar mediumWe have seen that the gas from which the stars are formed consist mostly of hydrogen.Through the ejection of their envelope, Planetary Nebulae return gas to the interstellarmedium from which future generations of stars will be formed. This gas still contains mostlyhydrogen but it also contains now some other elements, metals! In astronomy metals meansany element except hydrogen and helium (i.e. carbon, nitrogen, oxygen, neon, etc...). Wheredo these metals come from?The metals have been produced in the interiors of stars via the thermonuclear reactions.Although only the envelope is ejected the stars experience several mixing episodes in thecourse of evolution in which material from the interior is brought up to the surface. Theenvelope is then contaminated with products (helium and metals) from the thermonuclearburning that took place in the interior of the star. Therefore the gas that the PlanetaryNebulae return to the interstellar medium has been enriched with metals. This is veryimportant because it means that future generations of stars that will be formed out of thisenriched gas will have higher abundances 9 of metals. Therefore Planetary Nebulae drive thechemical evolution and spectral appearance of the galaxy. Super Novae also contribute to thechemical enrichment of the interstellar medium and are more massive but smaller in number.The nitrogen production in the Milky Way is probably due to Planetary Nebulae, heavier8 They are called neutral because the hydrogen is neutral, but ions that can be ionized by lower energy radiationthan hydrogen, can still be present.9 By abundances we mean the ratios of amounts of elements with respect to each other
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RIJKSUNIVERSITEIT GRONINGENPhysics
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To my parents
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Contents1 Introduction 11.1 Evoluti
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CONTENTSvii5 Probing AGB nucleosynt
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1IntroductionPLANETARY NEBULAE are
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1.1. Evolution of low and intermedi
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1.2. Planetary Nebulae 5Flash−Dri
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1.3. Physics in PNe 7their whole li
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1.3. Physics in PNe 95.35 eVO IIIλ
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1.4. Studying PNe in the infrared 1
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2The ISO-SWS Spectrum ofPlanetary N
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2.3. Data reduction 15Table 2.1-. C
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2.4. Line flux discussion 17Table 2
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2.5. The visual and the ultraviolet
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2.6. Analysis 21Table 2.5-. Electro
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24 CHAPTER 2: The ISO-SWS Spectrum
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3An ISO and IUE study of PlanetaryN
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3.3. Corrections to line fluxes 33[
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3.4. Line fluxes 35Table 3.1-. Comp
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Table 3.3-. IUE intensities, fluxes
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3.5. Physical parameters 393.4.3 Op
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3.5. Physical parameters 41Table 3.
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3.6. Chemical abundances 43Table 3.
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3.6. Chemical abundances 45Table 3.
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3.7. Conclusions 47Acknowledgements
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50 CHAPTER 4: Abundances of Planeta
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5Probing AGB nucleosynthesis viaacc
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5.1. Introduction 71about the synth
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Table 5.2-. Radius, Zanstra tempera
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5.3. Accurate abundances of ISO-obs
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5.3. Accurate abundances of ISO-obs
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5.4. Nucleosynthesis in low- and in
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5.5. Synthetic TP-AGB calculations
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5.6. Comparison between models and
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5.7. Summary and conclusions 103Fig
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5.7. Summary and conclusions 105A s
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6Physical Conditions in PDRs around
Page 117 and 118: 6.2. Observations 109are not promin
Page 119 and 120: 6.4. General parameters of the PNe
Page 121 and 122: 6.4. General parameters of the PNe
Page 123 and 124: Table 6.3-. Radius, Zanstra tempera
Page 125 and 126: 6.6. Lines fluxes 117Figure 6.2-. S
Page 127 and 128: 6.7. Physical conditions of the PDR
Page 135 and 136: 6.8. PDRs versus Shocks 127Figure 6
Page 137 and 138: 6.10. Atomic, ionized and molecular
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Page 141 and 142: 6.11. General discussion 133Table 6
Page 143 and 144: 6.11. General discussion 135Figure
Page 145 and 146: 6.12. Summary and Conclusions 137su
Page 147 and 148: 7Conclusions and future workTHE res
Page 149 and 150: 141• PNe with He/H > 0.14. The st
Page 151 and 152: 143HERSCHEL The launch of this sate
Page 153 and 154: Nederlandse SamenvattingSTERREN wor
Page 155 and 156: Nederlandse Samenvatting 147Figuur
Page 157 and 158: Nederlandse Samenvatting 149PHOTOIO
Page 159 and 160: Nederlandse Samenvatting 151Figuur
Page 161 and 162: Nederlandse Samenvatting 153infraro
Page 163 and 164: English SummarySTARS are born, live
Page 165 and 166: English Summary 157Figure 2-. Two e
Page 167: English Summary 159PHOTOIONIZATIONR
Page 171 and 172: English Summary 163all) stages of i
Page 173 and 174: BibliographyAcker A., Marcout J., O
Page 175 and 176: BIBLIOGRAPHY 167Henry R.B.C., Kwitt
Page 177 and 178: BIBLIOGRAPHY 169Pottasch S.R., Bern
Page 179 and 180: List of PublicationsPublications in
Page 181 and 182: AcknowledgementsI could easily writ
Page 183: Acknowledgements 175sense of humor,
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