Laboratory and astronomical discovery of magnesium dicarbide, MgC$_2$

TL;DR

This paper reports the laboratory detection and astronomical identification of magnesium dicarbide (MgC$_2$) in the circumstellar envelope of IRC+10216, revealing its molecular structure, excitation conditions, and abundance relative to other magnesium-carbon molecules.

Contribution

It presents the first laboratory and astronomical detection of MgC$_2$, including its molecular structure and role in circumstellar chemistry, providing new insights into metal-bearing molecule formation.

Findings

MgC$_2$ has a T-shaped structure with ionic bonding.

Detected MgC$_2$ lines indicate a low rotational temperature of 6 K.

MgC$_2$ abundance relative to other magnesium-carbon molecules is established.

Abstract

We report the detection of magnesium dicarbide, MgC$_2$, in the laboratory at centimeter wavelengths and assign $^{24}$MgC$_2$, $^{25}$MgC$_2$, and $^{26}$MgC$_2$ to 14 unidentified lines in the radio spectrum of the circumstellar envelope of the evolved carbon star IRC+10216. The structure of MgC$_2$ is found to be T-shaped with a highly ionic bond between the metal atom and the C$_2$ unit, analogous to other dicarbides containing electropositive elements. A two-temperature excitation model of the MgC$_2$ emission lines observed in IRC+10216 yields a very low rotational temperature of $6\pm1$ K, a kinetic temperature of $22\pm13$ K, and a column density of $(1.0 \pm 0.3) \times 10^{12}$ cm$^{-2}$. The abundance of MgC$_2$ relative to the magnesium-carbon chains MgCCH, MgC$_4$H, and MgC$_6$H is $1{:}2{:}22{:}20$ and provides a new constraint on the sequential radiative association-dissociative recombination mechanisms implicated in the production of metal-bearing molecules in circumstellar environments.