Radiative association formation of AlO, a candidate seed of dust nucleation

ABSTRACT The dust formation in the stellar winds of Asymptotic Giants Branch stars or red supergiants requires the presence of seeds where silicates are predicted to grow. Aluminium monoxide is considered an excellent candidate for these seeds. We studied its formation by the radiative association of aluminium and oxygen ($^{16}$O/$^{18}$O) atoms using quantum-mechanical and semi-classical methods. Two different incoming channels are considered, namely those involving atomic Al in its ground $^2$P$_u$ state and either the ground state of atomic O, $^3$P$_g$, or its first electronically excited state, $^1$D$_g$. While there were previous determinations of the rate coefficients for the reaction involving O($^3$P$_g$), the radiative association reaction with O($^1$D$_g$) has been investigated for the first time. Numerous resonances and larger cross-sections, reflected in higher rate coefficients up to 2000 K, are observed with respect to the case of the O($^3$P$_g$) reaction at low and medium collision energies. We have also considered the reaction involving $^{18}$O to explore the kinetic isotope effect. A negligible increase in the calculated rate coefficients of the Al + $^{18}$O isotopic variant has been observed, thus implying that no chemical enrichment or depletion is caused by the kinetic isotope effect. The possible role of AlO formed by radiative association is discussed in the context of the chemistry of the circumstellar envelopes of Asymptotic Giants Branch stars and red supergiants. A possible explanation of the AlO emission bands is suggested.