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The Physical Structure of Magellanic Cloud H II Regions

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The Physical Structure of Magellanic Cloud H II Regions The physical structure of Magellanic Cloud H II regions ¢¡ £¢¤ ¥¢¦ On the cover : a false-colour image of the giant Large Magellanic Cloud H ii region 30 Doradus taken by the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope. The main power source of this H ii region is the incredibly rich stellar cluster R 136 seen slightly below the center of the image. Rijksuniversiteit Groningen The physical structure of Magellanic Cloud H II regions Proefschrift ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit Groningen op gezag van de Rector Magni¯cus, dr. F. Zwarts, in het openbaar te verdedigen op maandag 16 september 2002 om 14:15 uur door Ronald Vermeij geboren op 18 augustus 1971 te Velsen Promotor: Prof. dr. J.M. van der Hulst Beoordelingscommissie: Prof. dr. J.-P. Baluteau Prof. dr. R.C. Kennicutt Jr. Prof. dr. A.G.G.M. Tielens Contents Chapter 1 Introduction 7 1.1 The physics of starformation regions . 8 1.2 The diagnostic tools . 11 1.3 Thesis outline . 13 Chapter 2 The physical structure of Magellanic Cloud H ii regions : the data 15 2.1 Introduction . 15 2.2 Sample objects . 17 2.3 Data . 23 2.4 Discussion. 33 2.5 Summary . 38 Chapter 3 The physical structure of Magellanic Cloud H ii regions : elemental abundances 41 3.1 Introduction . 42 3.2 Data . 43 3.3 Analysis . 43 3.4 Discussion. 47 3.5 The R23 and S23(4) abundance indicators . 57 3.6 Nucleosynthetic aspects . 60 3.7 Summary . 62 Chapter 4 The physical structure of Magellanic Cloud H ii regions : the giant LMC H ii region 30 Doradus 67 4.1 Introduction . 68 4.2 Observations and data reduction . 68 4.3 Analysis . 79 4.4 Discussion. 83 4.5 The R23 and S23 abundance indicators . 93 4.6 The global spectrum . 94 4.7 Summary . 95 5 6 Chapter 5 The stellar content, metallicity and ionization structure of H ii regions 99 5.1 Introduction . 99 5.2 The sample . 100 5.3 Variations in the ionization structure . 103 5.4 Influence of the SEDs . 103 5.5 Influence of the metallicity . 108 5.6 The case of starburst galaxies . 110 5.7 Summary and conclusions . 111 Chapter 6 The PAH emission spectra of Large Magellanic Cloud H ii regions 115 6.1 Introduction . 116 6.2 Data . 117 6.3 Correlations . 121 6.4 Discussion. 123 6.5 Summary . 129 Chapter 7 Summary and outlook 133 7.1 Summary . 133 7.2 Future work . 135 List of publications 137 Nederlandse samenvatting 139 Chapter 1 Introduction The sites of the most recent formation of massive stars are quite easily identi¯ed by their prodigious emission in the red H® line. These patches of red light come in all shapes and sizes, ranging from nebulae spanning a signi¯cant fraction of a spiral arm powered by rich OB associations, all the way down to small nebulae containing only a few ionizing stars. Commonly refered to as H ii regions, the large body of ionized gas of which the starformation site is comprised provides us with a wealth of spectral information. The large number of spectral lines emitted by the ionized gas gives us valuable information about the local physical conditions, and it is therefore not surprising that H ii regions have always been a favourite tool with which to study the interstellar medium (ISM). A famous example of an H ii region is shown in Fig. 1.1. The spectrum of an H ii region can in the ¯rst place be used to constrain the physical structure of the nebula itself. This structure includes the variation in electron temperature and density through the nebula, but also its ionization structure. The distribution of the elements over the various ionization stages is directly related to the energy distribution of the local radiation ¯eld, which is a signature of the nature of the main ionizing sources of the nebula. In the case of very young H ii regions, the ionizing stars can not be observed directly, so clues about the stellar content of the nebula can only be obtained by these indirect means. This is often the only way in which some information can be obtained about the high-mass end of the initial-mass function. The most important piece of information provided by the spectra of H ii regions, however, is the chemical composition of the local ISM. Knowledge of the gas-phase abundances is important for constraining the nucleosynthetic and starformation history of the host galaxy; the sequence of generations of stars formed in the past is traced by the gradual build-up of heavy metals such as oxygen, and clues about the rates at which various elements have been synthesized are given by heavy element-to-oxygen ratios such as N/O. Large-scale abundance variations such as the radial abundance gradients often encountered in spiral galaxies reflect the bulk hydrodynamical flow of gas through a galaxy. We refer to Henry & Worthey (1999) for a nice review on the subject. 8 Chapter 1 : introduction Figure 1.1| The most famous example of an H ii region : the Orion nebula (M42). 1.1 The physics of starformation regions The physical structure of an H ii region mainly depends on three parameters. Possibly the most important of the three is the spectral-energy distribution (SED) of the main ionizing source(s) of the H ii region; the ionization structure throughout the nebula is mostly determined by it. The second parameter, the ionization parameter, is also important for the ionization balance. This parameter ties together the local gas and photon density and is basically the ratio of ionizing photons to gas particles. The third important ingredient shaping the physics of an H ii region is the chemical composition of the ionized gas, i.e. the metallicity. The metallicity regulates the cooling of the gas and hence determines the temperature structure of the nebula. In this Section, a description of the basic physics of an H ii region is given. Also, a sketch of the `layout' of an H ii region will be given that will serve as a template to be used throughout this thesis. For a very thorough discussion on the physics of photoionized astrophysical plasmas one should consult Osterbrock (1989). 1.1.1 Ionization structure The dominant physical mechanism in setting up the ionization structure of an H ii region is photoionization. With increasing distance from the ionizing source the radiation ¯eld steadily degrades, which leads to a more or less strati¯ed ionization structure. We can divide this structure in roughly three parts or ionization zones, each with its own set of dominant ionic species. The general distribution of ions throughout an H ii region is shown in Fig. 1.2. 1.1 The physics of starformation regions 9 +3 + S ++ ++ + N + N Ar He S 0 + S ++ + + + ++ H O Ne Ar Si H0 O ++ + ++ ++ O 0 S O 0 + Ne N He +3 + 0 + S S H Ne S 0 2 + ++ H + 0 +3 H O + 2 Ne + N + N S +3 ++ H C 2 ++ Ar S + S 0 + Ne ++ O H + N + + + H He S 0 ++ H O He + + ++ O C +3 ++ ++ Ar + + 0 Ar N O H Si N Figure 1.2| The ionization structure throughout an H ii region. The rough boundaries of the three principal ionization zones inside an H ii region are shown as dashed lines. The thick line designates the boundary of the H ii region. The dashed area is the photodissociation region. The dominant ionic species in every zone are given. Being the closest to the ionizing source, the core of the H ii region is dominated by highly ionized species such as O++, Ne++ and S+3. These are also the most highly ionized species likely to be encountered in a regular H ii region; not even the hottest main-sequence stars produce enough photons with energies in excess of 40 eV to give a signi¯cant amount of more highly ionized species. To give an idea of the energies involved, the ionization potential of O++ and He+ is » 54 eV. For Ne++ and S+3 this is » 41 eV and » 48 eV, respectively. Moving outwards, the photons with the highest energy become depleted, and a shift in the ionization balance occurs. The most important ions are now S++ and Ar++. Some traces of O+ and Ne+ also appear, but the O++ ion is still dominant. The relevant ionization potentials are now in the order of 35 eV. The most profound changes in the ionization structure occur near the edge of the H ii region where the photon supply is nearly exhausted. Over a relatively small fraction of the total radius of the H ii region, many ions that dominate the inner parts of the nebula recombine, and their dominant role is taken over by e.g. N+ and O+. Helium that has thusfar been singly ionized now also appears in neutral form. When eventually the energy of the remaining photons drops below 13.6 eV, even hydrogen becomes fully neutral, and we have arrived at the outer boundary of the H ii region. This boundary is refered to as the StrÄomgren radius. At the interface of the H ii region and the molecular cloud, we ¯nd the photodissoci- ation region (PDR). Photon energies here are lower than the threshold 13.6 eV, and the photons are only energetic enough to dissociate small molecules and to ionize atomic species such as neutral sulfur.
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