Investigating the inner discs of Herbig Ae/Be stars with CO bandhead and Brγ emission

Abstract

Herbig Ae/Be stars lie in the mass range between low- and high-massyoung stars, and therefore offer a unique opportunity to observe anychanges in the formation processes that may occur across this boundary.This paper presents medium-resolution Very Large Telescope(VLT)/X-shooter spectra of six Herbig Ae/Be stars, drawn from a sampleof 91 targets, and high-resolution VLT/Cryogenic Infrared EchelleSpectrograph (CRIRES) spectra of five Herbig Ae/Be stars, chosen basedon the presence of CO first overtone bandhead emission in their spectra.The X-shooter survey reveals a low detection rate of CO first overtoneemission (7 per cent), consisting of objects mainly of spectral type B.A positive correlation is found between the strength of the CO v = 2-0and Brγ emission lines, despite their intrinsic linewidthssuggesting a separate kinematic origin. The high-resolution CRIRESspectra are modelled, and are well fitted under the assumption that theemission originates from small scale Keplerian discs, interior to thedust sublimation radius, but outside the corotation radius of thecentral stars. In addition, our findings are in very good agreement forthe one object where spatially resolved near-infrared interferometricstudies have also been performed. These results suggest that the HerbigAe/Be stars in question are in the process of gaining mass via discaccretion, and that modelling of high spectral resolution spectra isable to provide a reliable probe into the process of stellar accretionin young stars of intermediate to high masses.