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Table of contents
1 Introduction
1.1 The interstellar medium
1.1.1 Interstellar dust
1.1.2 Molecules in the ISM
1.1.3 The structure of the ISM
1.2 Photon-dominated Regions
1.3 Physical and chemical processes in PDRs
1.3.1 Penetration of far-UV radiation
1.3.2 Thermal balance
1.3.3 Chemistry
1.4 Aims and structure of this thesis
2 The Horsehead nebula as a template PDR
2.1 The Horsehead nebula
2.1.1 A dense PDR seen edge-on
2.1.2 Star formation in the Horsehead
2.1.3 Formation and evolution of the Horsehead
2.1.4 Two different environments less than 40” away
2.1.5 Chemistry in the Horsehead
2.2 Spectral line surveys
2.3 The Horsehead WHISPER line survey
I Simple molecules
3 CF+ Fluoromethylidynium
3.1 Introduction
3.1.1 Fluorine chemistry
3.1.2 CF+ and HF in the interstellar medium
3.1.3 The importance of C+
3.1.4 CF+ hyperfine structure
3.2 Detection of CF+ in the Horsehead
3.2.1 Results
4 C3H+ Propynylidynium
4.1 Introduction
4.1.1 Chemistry of small hydrocarbons
4.1.2 The abundance of hydrocarbons in PDRs
4.2 A new molecule in space: Tentative detection of l-C3H+ in the Horsehead
II Complex molecules
5 H2CO and CH3OH
5.1 Introduction
5.2 Formation of H2CO and CH3OH
5.2.1 Gas-phase chemistry
5.2.2 Grain surface chemistry
5.2.3 Photo-desorption into the gas-phase
5.3 H2CO and CH3OH as tracers of physical properties
5.4 Observations of H2CO and CH3OH in the Horsehead
5.4.1 Determination of column densities
5.4.2 Results
6 CH3CN, CH3NC and HC3N
6.1 Formation of CH3CN
6.1.1 Gas-phase chemistry
6.1.2 Grain surface chemistry
6.2 Nitriles in the Horsehead nebula
6.2.1 Results
7 HCOOH, CH2CO, CH3CO and CH3CCH
7.1 Introduction
7.2 Observations
7.3 Rotational temperatures and column densities
7.3.1 Ketene
7.3.2 Acetaldehyde
7.3.3 Propyne
7.3.4 Formic acid
7.4 Discussion and conclusions
8 Comparison with other environments
8.1 Introduction
8.2 Prototypical PDRs
8.2.1 Horsehead
8.2.2 NGC 7023
8.2.3 Orion Bar
8.2.4 Mon R2
8.3 Other environments
8.3.1 Diffuse medium
8.3.2 Dense core: L1498
8.3.3 Hot corino: IRAS 16293-2422
8.3.4 Hot core: Orion KL
8.3.5 Galactic center GMC: Sgr B2(N)
8.3.6 Starburst galaxy: M82
8.3.7 Active galactic nucleus (AGN): NGC 1068
8.3.8 High-z galaxy: FG0.89
8.4 Abundance ratios
8.4.1 HCO/H13CO+ and CO+/H13CO+
8.4.2 HCO+/HOC+
8.4.3 CN/HCN
8.4.4 HNC/HCN and HCN/HCO+
8.4.5 C2H/HCO+ and C3H2/HCO+
8.4.6 N2H+/H2CO
8.4.7 H2CO, HCOOH, CH2CO, CH3CHO and CH3CCH relative to CH3OH
8.5 Tracers of /n and density in PDRs
8.6 Abundance estimator
8.7 Summary
9 Conclusions and Perspectives
9.1 The Horsehead: a benchmark to chemical models
9.1.1 CF+: a tracer of C+ and a measure of the fluorine abundance
9.1.2 Detection of a new molecule in space, tentatively attributed to l-C3H+
9.1.3 Photo-desorption of dust grain ice mantles: H2CO and CH3OH
9.1.4 Nitrile molecules: CH3CN, CH3NC and HC3N
9.1.5 Complex molecules in PDRs
9.1.6 Chemical diagnostics
9.2 On the impact of instrumental progresses in radioastronomy on astrochemistry
A Rotational diagrams
Bibliography
