TRACING ABUNDANCES IN GALAXIES WITH THE SPITZER ...

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of these beautiful objects. Recent references on which this discussion about them is based are: Pottasch (1984), Osterbrock (1989), and Bernard-Salas (2003). The blown off outer layers of a star form the gas cloud of a planetary nebula. This low density (∼10 2 –10 4 cm −3 ) gas is illuminated by a low to intermediate mass (∼1–8 M⊙) hot (T∗ ∼ 3×10 4 –2×10 5 K) central star which is evolving quickly towards a white-dwarf. The nebula expands at ∼100 km sec −1 , and as it expands the density and emission decrease, so that they become unobservable in a few ten thousand years. Most PNe have a higher ionization level of elements than H II regions due to the higher temperatures of their central stars, but the lower- ionization PNe have similar spectra to H II regions. PNe can have many shapes and are observed in our Galaxy and nearby galaxies. They are concentrated in the Galactic plane and the center of the Galaxy. PNe have strong forbidden lines in their spectra, which may be employed to derive abundances, as discussed in §1.3.3. A series of dredge-up events that occur as the central star evolves bring the products of nucleosynthesis from the core of the star (such as helium, carbon, and nitrogen) to the surface of the star. Additionally, for stars more massive than ∼4–4.5 M⊙, hot bottom burning leads to the production of elements in nuclear processing at the bottom of the convective envelope of the star. Stellar winds then push the outer envelope of the star out into the interstellar medium. Measuring the abundances of these elements in the PN then gives information about the nucleosynthesis processes inside the star. However, the abundances of elements not affected by nucleosynthesis in the central star (such as argon, neon, and sulfur) give information about the initial composition of the cloud from which the star formed, and thus about the chemical evolution of the interstellar medium at the time that the star formed. Abundances from PNe across the Galaxy give information about the chemical evolution of the Galaxy as 11
a whole. Additionally, abundances from different types of PNe can be employed in order to probe different time scales during the evolution of the Galaxy. 1.2.2 H II Regions H II regions are gaseous nebulae excited by young massive main sequence stars. They derive their name from the fact that they contain mostly ionized hydrogen. (PNe contain mostly ionized hydrogen as well, but the name H II region is reserved for clouds of gas heated by young massive stars.) The central radiation source of H II regions is a hot (effective temperature T∗ ∼ 40000 K) high-mass star (M∗ � few M⊙) or set of stars. The nebulae contain ionized hydrogen, singly ionized helium, and mostly single or double ionization stages of other elements. The electron density is typically 10 to 100 cm −3 but may range up to several thousands. H II regions are observed across our Galaxy and in other nearby galaxies, and they are most common in spirals and irregulars. In spiral galaxies the H II regions are concentrated in the disk in the spiral arms, whereas in irregulars their distribution is not as well organized (Osterbrock, 1989). H II regions are young and therefore they trace the composition of the interstellar medium today. The abundance gradient of H II regions across the Galaxy was first measured by Shaver et al. (1983) using optical and radio recombination lines. They determined an oxygen abundance gradient of ∆log10(O/H) ∆RG = -0.07 ± 0.015 dex kpc −1 across the Disk (the unit dex refers to the log10 scale). Some more recent studies employ infrared lines in determining abundances across the Disk of the Galaxy (e.g Simpson et al., 1995). Additionally we can determine abundances of H II regions in external galaxies; Chapter 4 discusses some infrared derived abundances for H II regions across the galaxy M51. 12
Page 1 and 2: TRACING ABUNDANCES IN GALAXIES WITH
Page 3 and 4: TRACING ABUNDANCES IN GALAXIES WITH
Page 5 and 6: BIOGRAPHICAL SKETCH Shannon was bor
Page 7 and 8: ACKNOWLEDGEMENTS First and foremost
Page 9 and 10: thank my husband Ryan. The rest of
Page 11 and 12: 3 Chemical Abundances and Dust in P
Page 13 and 14: LIST OF FIGURES 1.1 Diagram of the
Page 15 and 16: ditionally we find that these nebul
Page 17 and 18: seed nuclei relative to the β deca
Page 19 and 20: shape (Carollo et al., 2007). These
Page 21 and 22: Boltzmann velocity distribution cha
Page 23: etween recapture and photoionizatio
Page 27 and 28: at the appropriate temperature and
Page 29 and 30: (a) 3.37 eV 1.40 eV 0.10 eV 0.04 eV
Page 31 and 32: (Iion), the radiative transition ra
Page 33 and 34: lines. For example, changing the ad
Page 35 and 36: Figure 1.4 Starburst99 synthetic sp
Page 37 and 38: Figure 1.5 Example of crystalline s
Page 39 and 40: H H H H H H H H H Anthanthrene C H
Page 41 and 42: year lifetime. Spitzer is in an Ear
Page 43 and 44: CHAPTER 2 THE SPITZER IRS INFRARED
Page 45 and 46: these elements than previous studie
Page 47 and 48: dust is seen in the spectrum. We se
Page 49 and 50: Figure 2.2 Close-ups of emission li
Page 51 and 52: 2.4 Optical and UV Data We compleme
Page 53 and 54: 2.5 Data Analysis Our goal is to ca
Page 55 and 56: 2.5.2 Electron Density We assume an
Page 57 and 58: Table 2.6 Electron temperatures ass
Page 59 and 60: O, N, and C. The helium ionic abund
Page 61 and 62: Barlow (1994) use UV lines for impo
Page 63 and 64: is a spherical nebula with a low ma
Page 65 and 66: CHAPTER 3 CHEMICAL ABUNDANCES AND D
Page 67 and 68: essential data on important ionizat
Page 69 and 70: are members of the Bulge, (Acker et
Page 71 and 72: Table 3.2 Properties of observed GB
Page 73 and 74: nod-averaged line fluxes. Uncertain
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Table 3.3 Multiplicative scaling fa
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Table 3.4 Observed infrared line fl
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3.5 Data Analysis 3.5.1 Abundances
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Table 3.6 Comparison of the derived
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from the most reliable line(s), and
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Table 3.7 Electron temperatures and
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Table 3.9 Ionic abundances Ion x a
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3.5.2 Crystalline Silicates Crystal
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argon, neon, and sulfur tend to be
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Figure 3.2 Continuum subtracted spe
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Figure 3.3 Profiles of PAH features
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mean argon and sulfur abundances ar
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of two smaller than that of Simpson
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towards the Galactic Center. On the
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Table 3.14 Parameters of linear fit
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models that show how a binary compa
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y an outer dense O-rich torus, indi
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CHAPTER 4 ABUNDANCES IN H II REGION
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expect their abundance ratio to rem
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We extract spectra processed throug
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Figure 4.2 IRS spectra of H II regi
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IRS slits from these WIRC observati
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S IV) we employ is equal to Te(O +2
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Figure 4.3 M51 H II region Ne III/N
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Figure 4.4 shows a plot of the (Ne
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James Webb Space Telescope (JWST);
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Bernard-Salas, J. & Tielens, A. G.
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Edgar, R. G., Nordhaus, J., Blackma
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Hummer, D. G., Berrington, K. A., E
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Matsuura, M., Zijlstra, A. A., Mols
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Pradhan, A. K., Montenegro, M., Nah
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Vassiliadis, E. & Wood, P. R. 1993,
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