Where have all the electrons gone? Laboratory and astronomical detection of molecular anions

Sandra Brünken, Harvard-Smithsonian Center for Astrophyics

The importance of negative ions (anions) in astronomy was demonstrated in 1939 
by Rupert Wildt who showed that H¯ is the major source of optical opacity in 
the solar atmosphere, and therefore the material which one mainly sees when 
looking at the sun and similar stars. It is remarkable that in the many years 
since, during which nearly 130 neutral molecules and 14 positive molecular ions 
have been found in a variety of astronomical sources, no molecular anion had 
been identified, owing in large part to the absence of laboratory rotational 
spectra. All that has now changed. Initiated by our recent laboratory 
identification of the large molecular anion C6H¯ as the carrier of a series of 
unidentified rotational emission lines in the circumstellar shell of the late 
carbon star IRC+10216, and its detection in the cold molecular cloud TMC-1, 
high resolution spectroscopy of molecular anions and searches for them in 
astronomical sources is now advancing at an astonishing pace. Five additional, 
closely related carbon chain anions were in the last year detected in our 
laboratory, and three of them have in the meantime also been detected with 
surprisingly high abundances in space. 
In this talk I will briefly review the spectroscopic techniques that we use to 
obtain rotational spectra of (under normal conditions mostly unstable) 
molecules, and how this data can be used to identify these molecules in 
astronomical sources. I will then describe our recent results on molecular 
anions in detail and outline how these new findings might improve our knowledge 
about the interstellar and circumstellar medium.