Abstract
Stellar systems are often formed through the collapse of dense molecular clouds which, in turn, return copious amounts of atomic and molecular material to the interstellar medium. An in-depth understanding of chemical evolution during this cyclic interaction between the stars and the interstellar medium is at the heart of astrochemistry. Systematic chemical composition changes as interstellar clouds evolve from the diffuse stage to dense, quiescent molecular clouds to star-forming regions and proto-planetary disks further enrich the molecular diversity leading to the evolution of ever more complex molecules. In particular, the icy mantles formed on interstellar dust grains and their irradiation are thought to be the origin of many of the observed molecules, including those that are deemed to be “prebiotic”; that is those molecules necessary for the origin of life. This review will discuss both observational (e.g., ALMA, SOFIA, Herschel) and laboratory investigations using terahertz and far-IR (THz/F-IR) spectroscopy, as well as centimeter and millimeter spectroscopies, and the role that they play in contributing to our understanding of the formation of prebiotic molecules. Mid-IR spectroscopy has typically been the primary tool used in laboratory studies, particularly those concerned with interstellar ice analogues. However, THz/F-IR spectroscopy offers an additional and complementary approach in that it provides the ability to investigate intermolecular interactions compared to the intramolecular modes available in the mid-IR. THz/F-IR spectroscopy is still somewhat under-utilized, but with the additional capability it brings, its popularity is likely to significantly increase in the near future. This review will discuss the strengths and limitations of such methods, and will also provide some suggestions on future research areas that should be pursued in the coming decade exploiting both space-borne and laboratory facilities.
Highlights
Specialty section: This article was submitted to Astrochemistry, a section of the journal Frontiers in Astronomy and Space
Since the first evidence of interstellar molecular species arose in the late 1930s, the scientific community has come to recognize that the interstellar medium is home to a plethora of structurally diverse molecules, with different environments displaying their own characteristic chemistry
This icy mantle is typically composed of two zones (Figure 2): a lower polar layer which results from the deposition and synthesis of polar molecules such as H2O, NH3, and CH4; and an upper apolar layer characterized by the presence of CO and related molecules such as CO2 and CH3OH (Larsson et al, 2012; Öberg 2016)
Summary
The idea of active chemistry occurring within interstellar space is a relatively new one. These clouds are considered to be “stellar nurseries” in which localized gravitational collapse results in the formation of a proto-star, with further accretion of material occurring via magneto-hydrodynamic flows onto the growing star. At the low temperatures encountered within the interiors of dense interstellar clouds, many species are able to freeze-out onto the surfaces of the carbonaceous and silicate dust grains, forming an icy mantle This icy mantle is typically composed of two zones (Figure 2): a lower polar layer which results from the deposition and synthesis (via hydrogenation reactions) of polar molecules such as H2O, NH3, and CH4; and an upper apolar layer characterized by the presence of CO and related molecules such as CO2 and CH3OH (Larsson et al, 2012; Öberg 2016). Reviews on the chemistry occurring during the various other phases of the astrochemical cycle (Figure 1) are available elsewhere (van Dishoeck and Blake 1998; Williams and Hartquist 1999; Larsson et al, 2012; Geppert and Larsson 2013; van Dishoeck 2014)
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