1.3 Historical overview of astrochemical discoveries

Astrochemistry's journey began with the discovery of simple molecules in space during the 1930s. Over time, scientists detected complex organic compounds, expanding our understanding of the universe's chemical makeup and potential for life.

Milestones in astrochemistry have shaped our view of the cosmos. From mapping galactic structures to uncovering chemical complexity in distant regions, these discoveries have revolutionized our knowledge of the universe's composition and evolution.

Milestones in Astrochemistry

Discovery of First Interstellar and Circumstellar Molecules

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• The discovery of the first interstellar molecules, CH (methylidyne) and CN (cyano radical), in the 1930s marked the birth of astrochemistry as a distinct field of study
• The detection of ammonia (NH3) in the interstellar medium in 1968 was a significant milestone as it was the first polyatomic molecule discovered in space
• The detection of the first circumstellar molecule, carbon monoxide (CO), around the red giant star IRC+10216 in 1975 expanded the scope of astrochemistry to include the study of molecules in the vicinity of stars
• The discovery of molecular oxygen (O2) in the Orion Nebula in 2011, after decades of searches, provided insights into the chemical processes and radiation fields in star-forming regions (H II regions)

Detection of Complex Organic Molecules and Potential Prebiotic Compounds

• The discovery of complex organic molecules, such as formaldehyde (H2CO) and methanol (CH3OH), in the 1970s suggested that the building blocks of life could form in space
• The detection of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium in the 1980s revealed the presence of large, complex molecules in space and their potential role in the formation of prebiotic compounds
• The detection of complex organic molecules, such as glycolaldehyde (CH2OHCHO) and ethylene glycol (HOCH2CH2OH), in protostellar cores and comets in the 2000s and 2010s suggested that the building blocks of life are widespread in the universe
• The detection of the first chiral molecule, propylene oxide (CH3CHCH2O), in the interstellar medium in 2016 provided insights into the potential origins of homochirality in biological systems on Earth (preference for one mirror-image form over another)

Discoveries in Astrochemistry

Mapping the Distribution and Structure of the Universe

• The detection of the 21 cm hydrogen line in 1951 allowed astronomers to map the distribution of neutral hydrogen throughout the Milky Way galaxy and beyond, revealing the large-scale structure of the universe
• The discovery of the cosmic microwave background (CMB) radiation in 1965 provided evidence for the Big Bang theory and helped establish the primordial abundance of light elements, such as hydrogen, helium, and lithium
• The detection of the first extragalactic molecules, hydroxyl (OH) and formaldehyde (H2CO), in absorption toward the quasar PKS 0528-250 in 1976 demonstrated that molecular gas is present in galaxies other than the Milky Way

Expanding the Understanding of Chemical Complexity in Space

• The detection of the first interstellar anion, the hexatriynyl anion (C6H-), in 2006 expanded our understanding of the chemical complexity and charge balance in the interstellar medium
• The discovery of molecular ions, such as HCO+ and N2H+, in the 1970s and 1980s revealed the importance of ion-molecule reactions in the chemistry of interstellar clouds
• The detection of complex organic molecules, such as acetonitrile (CH3CN) and ethyl cyanide (CH3CH2CN), in hot molecular cores in the 1980s and 1990s demonstrated the chemical richness of star-forming regions
• The discovery of fullerenes (C60 and C70) in the planetary nebula Tc 1 in 2010 showed that carbon-based molecules can form and survive in the harsh environments around evolved stars

Contributions to Astrochemistry

Pioneers in Astrochemistry

• Swings and Rosenfeld (1937) were the first to propose that the observed absorption features in the spectra of interstellar clouds could be attributed to molecular transitions, laying the foundation for the field of astrochemistry
• Townes and Schawlow (1955) developed the maser, which enabled the detection of microwave transitions of molecules in space, leading to the discovery of numerous interstellar molecules
• Klemperer (1970) and Herbst and Klemperer (1973) developed the first comprehensive models of gas-phase chemistry in interstellar clouds, which formed the basis for our understanding of the chemical processes in the interstellar medium

Advancements in Astrochemical Models and Theories

• Dalgarno and Black (1976) proposed the concept of cosmic-ray-driven chemistry, which explained the formation of molecules in the cold, dense regions of interstellar clouds where gas-phase reactions are inefficient
• Tielens and Hagen (1982) and Hasegawa, Herbst, and Leung (1992) developed models of grain-surface chemistry, which demonstrated the importance of dust grains as catalytic sites for the formation of complex molecules in the interstellar medium
• Ehrenfreund and Charnley (2000) and Herbst and van Dishoeck (2009) wrote influential review articles that synthesized the state of knowledge in astrochemistry, highlighting the progress made in understanding the chemical complexity of the universe and the challenges that lay ahead
• Wakelam et al. (2010) developed the KIDA (KInetic Database for Astrochemistry) database, which provides a comprehensive compilation of reaction rate coefficients and uncertainties for astrochemical models, facilitating the comparison and improvement of chemical networks