A proposed new diagnostic for Herbig disc geometry: FWHM versus J of CO
  ro-vibrational lines
  Aims. CO ro-vibrational lines observed from Herbig group II discs are seen to
be often broad while the same lines observed from group I 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
CRIRES (CRyogenic high-resolution InfraRed Echelle Spectrograph) at the VLT
(Very Large Telescope). From these spectra, we produce individual and co-added
CO ro-vibrational line profiles. We investigate line profile shape differences,
and we explore the FWHM (Full Width Half Maximum) 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
thermo-chemical disc modelling code ProDiMo. Results. - abbreviated - Comparing
dust and gas inner disc geometries (inferred by SED and CO ro-vibrational
emission) for the expanded sample of observed Herbig discs, we find no clear
correspondence between the SED groups of the sources and their inner CO radius.
Conclusions. The FWHM versus J is a potential new gas diagnostic for the inner
disc with e.g. a constant FWHM versus J indicating the presence of a large gas
hole or gap. Both models and observations indicate the potential of this new
diagnostic. - abbreviated -
