The origin of hydrogen line emission for five Herbig Ae/Be stars spatially resolved by VLTI/AMBER spectro-interferometry

Abstract

To trace the accretion and outflow processes around YSOs, diagnostic spectral lines such as the BrG 2.166 micron line are widely used, although due to a lack of spatial resolution, the origin of the line emission is still unclear. Employing the AU-scale spatial resolution which can be achieved with infrared long-baseline interferometry, we aim to distinguish between theoretical models which associate the BrG line emission with mass infall or mass outflow processes. Using the VLTI/AMBER instrument, we spatially and spectrally (R=1500) resolved the inner environment of five Herbig Ae/Be stars (HD163296, HD104237, HD98922, MWC297, V921Sco) in the BrG emission line as well as in the adjacent continuum. All objects (except MWC297) show an increase of visibility within the BrG emission line, indicating that the BrG-emitting region in these objects is more compact than the dust sublimation radius. For HD98922, our quantitative analysis reveals that the line-emitting region is compact enough to be consistent with the magnetospheric accretion scenario. For HD163296, HD104237, MWC297, and V921Sco we identify a stellar wind or a disk wind as the most likely line-emitting mechanism. We search for general trends and find that the size of the BrG-emitting region does not seem to depend on the basic stellar parameters, but correlates with the H-alpha line profile shape. We find evidence for at least two distinct BrG line-formation mechanisms. Stars with a P-Cygni H-alpha line profile and a high mass-accretion rate seem to show particularly compact BrG-emitting regions (R_BrG/R_cont<0.2), while stars with a double-peaked or single-peaked H-alpha-line profile show a significantly more extended BrG-emitting region (0.6<R_BrG/R_cont<1.4), possibly tracing a stellar wind or a disk wind.