The magnesium paradigm in IRC +10216: Discovery of MgC4H+, MgC3N+, MgC6H+, and MgC5N+

Abstract

We found four series of harmonically related lines in IRC +10216 with the Yebes 40 m and IRAM 30 m telescopes. The first series corresponds to a molecule with a rotational constant, B, of 1448.5994±0.0013 MHz and a distortion constant, D, of 63.45±1.15 Hz and covers upper quantum numbers from Ju = 11 up to 33 (B1449). The second series is fitted with B = 1446.9380±0.0098 MHz and D = 91±23 Hz and covers upper quantum numbers from Ju = 11 up to 17 (B1447). The third series is fitted with B = 598.7495±0.0011 MHz and D = 6.13±0.43 Hz and covers quantum numbers from Ju = 26 up to 41 (B599). Finally, the frequencies of the last series of lines can be reproduced with B = 594.3176±0.0026 MHz and D = 4.92±1.16 Hz (B594). The large values of D point toward four metal-bearing carriers. After exploring all plausible candidates containing Na, Al, Mg, and other metals, our ab initio calculations indicate that the cations MgC4H+, MgC3N+, MgC6H+, and MgC5N+ must be the carriers of B1449, B1447, B599, and B594, respectively. These cations could be formed by the radiative association of Mg+ with C4H, C3N, C6H, and C5N, respectively. We calculated the radiative association rate coefficient of Mg+ with C4H, C3N, C6H, and C5N and incorporated them in our chemical model. The results confirm that the Mg-bearing cations can be formed through these radiative association reactions in the outer layers of IRC +10216. This is the first time that cationic metal-bearing species have been found in space. These results provide a new paradigm on the reactivity of ionized metals with abundant radicals and open the door for further characterization of similar species in metal-rich astrophysical environments.