Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by Aura, Inc., under NASA contract NAS 5-2655; and on observations collected at the VLT/Kueyen telescope ESO, Paranal, Chile, programme ID 066.A-0212.

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$[X/Y] = (X/Y) - (X/Y)_\odot$ , where (X/Y) is the logarithmic value of the element ratio by number without reference to the solar value. Photospheric solar abundances $(X/Y)_\odot$ are taken from Grevesse & Sauval (1998) and from Holweger (2001).

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... L4P4

Since all H₂ transitions considered in the present paper arise from the ground electronic-vibrational state, we use a short notation like L4P4 which means L4-0 P(4) in the standard form.

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... levels

$t_{J=2} = 3.4\times10^{10}$ s, and $t_{J=3} = 2.1\times10^{9}$ s (Turner et al. 1977).

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