Continuum-subtracted first CO bandhead of 51 Oph normalised to the peak intensity (black dots). Coloured lines shown models with fixed values of T = 2400 K, N_CO = 4 × 10²¹ cm⁻², Δv = 15 km s⁻¹ and v_rot sini values of 100 km s⁻¹ (solid green line), 130 km s⁻¹ (solid red line), and 160 km s⁻¹ (solid blue line). The black vertical line in the top left corner of the peak of the bandhead has a lengths of twice the rms value measured atthe continuum.