On the effective temperature of large sunspot umbra using AlH molecular lines

•The radiative transition parameters of the band help for determination of AlH molecule in solar atmosphere.•The AlH molecular rotational lines of A-X (0,0), (1,1) and (0,1) bands are identified in the sunspot umbral spectrum.•The effective vibrational and rotational temperature helps to ascertain the presence of AlH molecule in sunspot atmosphere. In this work, the radiative transition parameters including Franck–Condon (FC) factors, r-centroids, electronic transition moment, Einstein coefficients, band strength, oscillator strength, lifetime and effective vibrational temperature have been computed for A1Π−X1Σ+ and C1Σ+−A1Π systems of aluminium monohydride (AlH) molecule by the numerical integration method for the experimentally known vibrational levels using Rydberg – Klein – Rees (RKR) potential. The computed radiative transition parameters are tabulated. A significant number of well resolved AlH molecular rotational lines of A1Π−X1Σ+ (0,0), (1,1) and (0,1) bands are identified in the sunspot umbral spectrum and the estimated effective rotational temperature has been determined as 2315 K, 2155 K and 1838 K respectively. The effective vibrational temperatures of the A1Π−X1Σ+ and C1Σ+−A1Π systems are found to be 5682 K and 3472 K respectively. Hence, the radiative transition parameters and the effective temperatures confirm the presence of AlH molecule in sunspot spectra.