Abstract
Until a decade ago, the only anion observed to play a prominent role in astrophysics was H-. The bound-free transitions in H- dominate the visible opacity in stars with photospheric temperatures less than 7000 K, including the Sun. The H- anion is also believed to have been critical to the formation of molecular hydrogen in the very early evolution of the Universe. Once H2 formed, about 500 000 years after the Big Bang, the expanding gas was able to lose internal gravitational energy and collapse to form stellar objects and "protogalaxies", allowing the creation of heavier elements such as C, N, and O through nucleosynthesis. Although astronomers had considered some processes through which anions might form in interstellar clouds and circumstellar envelopes, including the important role that polycyclic aromatic hydrocarbons might play in this, it was the detection in 2006 of rotational line emission from C6H- that galvanized a systematic study of the abundance, distribution, and chemistry of anions in the interstellar medium. In 2007, the Cassini mission reported the unexpected detection of anions with mass-to-charge ratios of up to ∼10 000 in the upper atmosphere of Titan; this observation likewise instigated the study of fundamental chemical processes involving negative ions among planetary scientists. In this article, we review the observations of anions in interstellar clouds, circumstellar envelopes, Titan, and cometary comae. We then discuss a number of processes by which anions can be created and destroyed in these environments. The derivation of accurate rate coefficients for these processes is an essential input for the chemical kinetic modeling that is necessary to fully extract physics from the observational data. We discuss such models, along with their successes and failings, and finish with an outlook on the future.
Highlights
Calculations of chemical evolution in astrophysical environments typically employ chemical kinetics using a reaction network that describes the formation and destruction of each species
In the remainder of this section, we describe the compilation of networks for anion chemistry and the models adopted for the different sources in which molecular anions have been observed and/or where they are expected to be abundant: dark clouds, protostellar envelopes, photon-dominated regions (PDRs), circumstellar envelopes (CSEs) of evolved stars, planetary atmospheres, and cometary comae
Model II reproduced the relatively high C6H−/C6H ratio observed in L1527 (∼10%), and they suggested that this scenario is an alternative to the warm carbon-chain chemistry” (WCCC) postulated by Sakai et al.[35]
Summary
McCarthy et al.[13] reported the laboratory spectrum of C6H− and recognized that these transitions corresponded to a set of harmonically related unidentified lines in the spectrum of IRC+1021624 that had been suggested to be C6H− by Aoki.[25] In addition, McCarthy et al.[13] observed seven lines in IRC+10216 and two lines in the cold, dark cloud TMC-1, another rich source of carbon-chain molecules, and derived column densities and anion-to-neutral abundance ratios of 1−5% in IRC+10216 and ∼2.5% and an overall column density of 1011 cm−3 in TMC-1, consistent with the predictions of Millar et al.[12] for IRC+10216. Some of the molecular anions identified in space amu), fall within C4H− (49 tahmesue),mCas3sNr−an(g5e0s:amC2uH),−an(2d5Ca8mHu−),(C97Na−m(u2)[6]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
