K-band spectroscopy of Be-star X-ray binaries

We present the first 2.0–2.5 μm spectra of four Be/X-ray binaries, at a resolution of λ/∆λ = 310–400. The spectra show strong hydrogen recombination lines, notably Brγ and, in most cases, the higher order Pfund series. The relative recombination strengths imply that Br γ has a large optical depth, whereas the Pfund lines have relative strengths close to case B and so may be optically thin. The He I 2.06-μm line is also in emission. Mg II emission lines are present, as are Na I lines; the latter are normally associated with emission-zone temperatures that are cooler than the conventional Be-star circumstellar disc. We conclude that optical effects are evident in H I line emission.