More Like this

1. Remarkable Chemical Complexity in Planetary Nebulae: A Molecule and Dust Perspective

   https://doi.org/10.1017/S1743921324000358  

   Ziurys, L M; Schmidt, D R (December 2023, Proceedings of the International Astronomical Union)

   Abstract Despite model predictions, many planetary nebulae appear to have a relatively rich molecular content. Observational studies of over 30 such objects show the presence of a variety of gas-phase molecules, from simple species such as CN and CS, to more complex organics including H₂CO, HC₃N, c-C₃H₂, and CH₃CN. Other PNe contain fullerenes; carbonaceous and silicate dust features are also found. Molecular abundances also do not appear to vary with nebular age. Remnant material from the asymptotic giant branch appears to undergo chemical processing in the protoplanetary nebula phase and then is frozen out in planetary nebulae. PN ejecta are thus in part molecular in content and may account for the observation of complex molecules in diffuse clouds. 

   more » « less
2. A nanometric window on fullerene formation in the interstellar medium: Insights from molecular dynamics studies

   https://doi.org/10.1063/5.0069166  

   Thakur, Abhishek Kumar; Muralidharan, Krishna; Zega, Thomas J.; Ziurys, L. M. (April 2022, The Journal of Chemical Physics)

   Understanding the fundamental mechanisms that underlie the synthesis of fullerene molecules in the interstellar medium (ISM) and in the environments of astrophysical objects is an open question. In this regard, using classical molecular dynamics, we demonstrate the possibility of in situ formation of fullerene molecules, such as C 60 from graphite, which is known to occur in the ISM, in particular, circumstellar environments. Specifically, when graphite is subjected to thermal and mechanical stimuli that are typical of circumstellar shells, we find that the graphite sheet edges undergo significant restructuring and curling, leading to edge-induced interlayer-interactions and formation of mechanically strained five-membered-ring structural units. These units serve as precursors for the formation of fullerene structures, such as pristine and metastable C 60 molecules. The pathways leading to molecular C 60 formation consist of a series of steps that involve bond-breakage and subsequent local rearrangement of atoms, with the activation energy barriers of the rate-limiting step(s) being comparable to the energetics of Stone–Wales rearrangement reactions. The identified chemical pathways provide fundamental insights into the mechanisms that underlie C 60 formation. Moreover, they clearly demonstrate that top-down synthesis of C 60 from graphitic sources is a viable synthesis route at conditions pertaining to circumstellar matter. 

   more » « less
3. Synthetic Approaches to Complex Organic Molecules in the Cold Interstellar Medium

   https://doi.org/10.3389/fspas.2021.789428  

   Herbst, Eric; Garrod, Robin T. (January 2022, Frontiers in Astronomy and Space Sciences)

   The observation and synthesis of organic molecules in interstellar space is one of the most exciting and rapidly growing topics in astrochemistry. Spectroscopic observations especially with millimeter and submillimeter waves have resulted in the detection of more than 250 molecules in the interstellar clouds from which stars and planets are ultimately formed. In this review, we focus on the diverse suggestions made to explain the formation of Complex Organic Molecules (COMs) in the low-temperature interstellar medium. The dominant mechanisms at such low temperatures are still a matter of dispute, with both gas-phase and granular processes, occurring on and in ice mantles, thought to play a role. Granular mechanisms include both diffusive and nondiffusive processes. A granular explanation is strengthened by experiments at 10 K that indicate that the synthesis of large molecules on granular ice mantles under space-like conditions is exceedingly efficient, with and without external radiation. In addition, the bombardment of carbon-containing ice mantles in the laboratory by cosmic rays, which are mainly high-energy protons, can lead to organic species even at low temperatures. For processes on dust grains to be competitive at low temperatures, however, non-thermal desorption mechanisms must be invoked to explain why the organic molecules are detected in the gas phase. Although much remains to be learned, a better understanding of low-temperature organic syntheses in space will add both to our understanding of unusual chemical processes and the role of molecules in stellar evolution. 

   more » « less
4. Gas-phase formation of silicon monoxide via non-adiabatic reaction dynamics and its role as a building block of interstellar silicates

   https://doi.org/10.1039/D2CP02188A  

   He, Chao; Luo, Yuheng; Doddipatla, Srinivas; Yang, Zhenghai; Millar, Tom J.; Sun, Rui; Kaiser, Ralf I. (August 2022, Physical Chemistry Chemical Physics)

   Silicon monoxide (SiO) is classified as a key precursor and fundamental molecular building block to interstellar silicate nanoparticles, which play an essential role in the synthesis of molecular building blocks connected to the Origins of Life. In the cold interstellar medium, silicon monoxide is of critical importance in initiating a series of elementary chemical reactions leading to larger silicon oxides and eventually to silicates. To date, the fundamental formation mechanisms and chemical dynamics leading to gas phase silicon monoxide have remained largely elusive. Here, through a concerted effort between crossed molecular beam experiments and electronic structure calculations, it is revealed that instead of forming highly-stable silicon dioxide (SiO 2 ), silicon monoxide can be formed via a barrierless, exoergic, single-collision event between ground state molecular oxygen and atomic silicon involving non-adiabatic reaction dynamics through various intersystem crossings. Our research affords persuasive evidence for a likely source of highly rovibrationally excited silicon monoxide in cold molecular clouds thus initiating the complex chain of exoergic reactions leading ultimately to a population of silicates at low temperatures in our Galaxy. 

   more » « less
5. A chemical dynamics study on the gas phase formation of thioformaldehyde (H ₂ CS) and its thiohydroxycarbene isomer (HCSH)

   https://doi.org/10.1073/pnas.2004881117  

   Doddipatla, Srinivas; He, Chao; Kaiser, Ralf I.; Luo, Yuheng; Sun, Rui; Galimova, Galiya R.; Mebel, Alexander M.; Millar, Tom J. (September 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Complex organosulfur molecules are ubiquitous in interstellar molecular clouds, but their fundamental formation mechanisms have remained largely elusive. These processes are of critical importance in initiating a series of elementary chemical reactions, leading eventually to organosulfur molecules—among them potential precursors to iron-sulfide grains and to astrobiologically important molecules, such as the amino acid cysteine. Here, we reveal through laboratory experiments, electronic-structure theory, quasi-classical trajectory studies, and astrochemical modeling that the organosulfur chemistry can be initiated in star-forming regions via the elementary gas-phase reaction of methylidyne radicals with hydrogen sulfide, leading to thioformaldehyde (H 2 CS) and its thiohydroxycarbene isomer (HCSH). The facile route to two of the simplest organosulfur molecules via a single-collision event affords persuasive evidence for a likely source of organosulfur molecules in star-forming regions. These fundamental reaction mechanisms are valuable to facilitate an understanding of the origin and evolution of the molecular universe and, in particular, of sulfur in our Galaxy. 

   more » « less