On the effective temperature of AlC molecular lines in sunspot umbral spectra

Abstract

In the present work, a significant rotational lines of the $B^{4}\Sigma ^{-} \mbox{--} X^{4}\Sigma ^{ -} $ ( $0,0$ ) band system of the AlC molecule were identified in high resolution FTS sunspot umbral spectra in the region from 22,450 to 22,600 cm−1. Among the well-resolved identified lines, the rotational temperatures have been estimated to be 4018 K for photosphere and 3722 K for a hot umbra. Hence, the estimated effective rotational temperatures provide evidence for the possible presence of the AlC molecule in a sunspot umbra. The radiative transition parameters for $A^{4}\Pi ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ and $B^{4}\Sigma ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ systems of the AlC molecule have been estimated for experimentally known vibrational levels using the Rydberg–Klein–Rees (RKR) potential. The Franck-Condon (FC) factor of $B^{4}\Sigma ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ ( $0,0$ ) band system is most intense ( $q_{v'v''} = 0.741$ ). For the $B^{4}\Sigma ^{ -}$ state, the radiative lifetime of $v'=0$ level is found as 103.93 ns. The effective vibrational temperature of the $A^{4}\Pi ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ and $B^{4}\Sigma ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ band systems of the AlC molecule is 3981 K and 2706 K, respectively. Therefore, the effective vibrational temperature and radiative transition parameters help us to ascertain the possible presence of the AlC molecule in the solar atmosphere.