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

Context. Accretion and outflow processes are of fundamental importance for our understanding of the formation of stars and planetary systems. To trace these processes, diagnostic spectral lines such as the Br gamma 2.166 mu m line are widely used, although due to a lack of spatial resolution, the origin of the line emission is still unclear.

Aims. 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 Br gamma line emission with mass infall (magnetospheric accretion, gaseous inner disks) or mass outflow processes (stellar winds, X-winds, or disk winds).

Methods. Using the VLTI/AMBER instrument, we spatially and spectrally (lambda/Delta lambda = 1500) resolved the inner (less than or similar to 5 AU) environment of five Herbig Ae/Be stars (HD 163296, HD 104237, HD 98922, MWC 297, V921 Sco) in the Br. emission line as well as in the adjacent continuum. From the measured wavelength-dependent visibilities, we derive the characteristic size of the continuum and Br gamma line-emitting region. Additional information is provided by the closure phase, which we could measure both in the continuum wavelength regime (for four objects) as well as in the spectrally resolved Br. emission line (for one object). The spectro-interferometric data is supplemented by archival and new VLT/ISAAC spectroscopy.

Results. For all objects (except MWC297), we measure an increase of visibility within the Br gamma-emission line, indicating that the Br gamma-emitting region in these objects is more compact than the dust sublimation radius. For HD 98922, our quantitative analysis reveals that the line-emitting region is compact enough to be consistent with the magnetospheric accretion scenario. For HD 163296, HD104237, MWC297, and V921 Sco we identify an extended stellar wind or a disk wind as the most likely line-emitting mechanism. Since the stars in our sample cover a wide range of stellar parameters, we also search for general trends and find that the size of the Br gamma-emitting region does not seem to depend on the basic stellar parameters (such as the stellar luminosity), but correlates with spectroscopic properties, in particular with the Ha line profile shape.

Conclusions. By performing the first high-resolution spectro-interferometric survey on Herbig Ae/Be stars, we find evidence for at least two distinct Br. line-formation mechanisms. Most significant, stars with a P-Cygni Ha line profile and a high mass-accretion rate seem to show particularly compact Br gamma-emitting regions (R-Br gamma/R-cont < 0.2), while stars with a double-peaked or single-peaked H alpha-line profile show a significantly more extended Br gamma-emitting region (0.6 less than or similar to R-Br gamma/R-cont less than or similar to 1.4), possibly tracing a stellar wind or a disk wind.