A new heavy anion in IRC+10216: Theory favors C10H- versus C9N-*

Continuing Q-band (31-50 GHz) integrations on IRC +10216 with the Yebes 40m telescope have now reached sub-millikelvin noise with hundreds of new lines arising above an average 3 sigma detection limit of 0.71mK (as low as 0.45mK in the lower frequency end). The recent discovery of the C7N- anion and the relatively large abundance of the HC9N member of the cyanopolyyne family opens the door for searching in the data for harmonically related series of lines belonging to singlet species, with intensities close to the detection limit, that could belong to heavier anions. One such series has been found with rotational quantum numbers from J = 52 - 51 up to J = 74 - 73, with at least 15 distinguishable features clearly detected as isolated or partly blended. There are two main candidates for the carriers of the series: C9N- and/or C10H-, for which our high-level-of-theory ab initio calculations predict a quite compatible rotational constant of similar to 300 MHz. In this paper we discuss, based on our ab initio calculations and also on chemical models, which of these two candidates is the most likely carrier. There is more evidence for the C10H- candidate. It would be, to date, the heaviest anion discovered in space through a series of detected individual lines.