A study of weak molecular and atomic lines in the photospheric spectrum

An attempt is made to detect the lines of Mg²⁵H and Mg²⁶H in the photospheric spectrum, using calculated isotope shifts. From comparisons with the Mg²⁴H lines of the ² –² transition in the (0, 0) band the ratios Mg²⁵/Mg²⁴ = 0.12 ± 0.04 and Mg²⁶/Mg²⁴ = 0.12 ± 0.02 are derived. These are essentially the same as the terrestrial ratios. The profile of one line of Mgi confirms these values.The wavelengths of MgH and C₂ lines, when corrected for the gravitational red shift, indicate that macroturbulent (or streaming) velocities die out near log ₀ = – 1.0. From the equivalent widths of the MgH lines a rotational temperature of 5132 ± 200 K is obtained and compared to predictions from various model atmospheres. The band oscillator strength is found to be 0.024 ± 0.002, in serious disagreement with the single laboratory determination.The profiles of MgH, C₂, CN and some weak atomic lines are used to derive the variation of the radial component of microturbulence with optical depth, on the assumption of streaming velocities of 2.5–3.0 km/sec. A slow increase with increasing height in the photosphere is found, over the range –1.5 < log₀< 0.2.The center-limb variation of the equivalent widths of MgH and C₂ lines for a wholly inhomogeneous model is found to be the same as for a model which is homogeneous above log ₀ = - 1.0. With such a model as the latter, the center-limb variation of the profiles of the selected molecular and atomic lines is moderately-well reproduced by an anisotropic microturbulent velocity with a tangential component of 3 km/sec which seems to be constant with height over the range considered.