A proposed new diagnostic for Herbig disc geometry: FWHM versus J of CO ro-vibrational lines

Aims. The CO ro-vibrational lines observed from Herbig groupII discs are often seen to be broad, while the same lines observed from groupI discs are often narrow. This difference is not well understood. In this paper we explore the underlying cause for this difference and provide a pathway for a better understanding of the geometry and structure of the inner discs around Herbig Ae/Be stars. Methods. High spectral resolution infrared spectra of CO ro-vibrational emission from six Herbig Ae/Be candidate stars were taken with the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) at the Very Large Telescope (VLT). From these spectra, we produce individual and co-added CO ro-vibrational line profiles. We investigate line profile shape differences, and we explore the full width at half maximum (FWHM) variations with J quantum number in the context of disc geometry. Furthermore, we put our new sources into the context of earlier observed sources to study a large sample. For comparison, we also investigate the FWHM variations with J of modelled CO ro-vibrational lines from two typical disc geometries produced with the thermochemical disc modelling code ProDiMo. Conclusions. The FWHM versus J is a potential new gas diagnostic for the inner disc with, for example, a constant FWHM versus J indicating the presence of a large gas hole or gap. Models and observations both indicate the potential of this new diagnostic. Our extended sample does not fully support the previous trend where groupI discs have CO ro-vibrational emission lines with small FWHM. Instead, our CO ro-vibrational data from a handful of sources indicates different inner disc geometries for the gas and dust of these sources.