AMBER/VLTI high spectral resolution observations of the Brγ emitting region in HD 98922. A compact disc wind launched from the inner disc region

Abstract

Context. High angular and spectral resolution observations can provide us with fundamental clues to the complex circumstellar structure of young stellar objects (YSOs) and to the physical processes taking place close to these sources. <BR /> Aims: We analyse the main physical parameters and the circumstellar environment of the young Herbig Be star HD 98922. <BR /> Methods: We present AMBER/VLTI high spectral resolution (R = 12 000) interferometric observations across the Brγ line, accompanied by UVES high-resolution spectroscopy and SINFONI-AO assisted near-infrared (NIR) integral field spectroscopic data. To interpret our observations, we develop a magneto-centrifugally driven disc-wind model. <BR /> Results: Our analysis of the UVES spectrum shows that HD 98922 is a young (~5 × 10<SUP>5</SUP> yr) Herbig Be star (SpT = B9V), located at a distance of 440± <SUP>60</SUP><SUB>50</SUB> pc, with a mass accretion rate (Ṁ<SUB>acc</SUB>) of ~(9 ± 3) × 10<SUP>-7</SUP> M<SUB>☉</SUB> yr<SUP>-1</SUP>. SINFONI K-band AO-assisted imaging shows a spatially resolved circumstellar disc-like region (~140 AU in diameter) with asymmetric brightness distribution. Our AMBER/VLTI UT observations indicate that the Brγ emitting region (ring-fit radius ~0.31 ± 0.04 AU) is smaller than the continuum emitting region (inner dust radius ~0.7 ± 0.2 AU), showing significant non-zero V-shaped differential phases (i.e. non S-shaped, as expected for a rotating disc). The value of the continuum-corrected pure Brγ line visibility at the longest baseline (89 m) is ~0.8 ± 0.1, i.e. the Brγ emitting region is partially resolved. Our modelling suggests that the observed Brγ line-emitting region mainly originates from a disc wind with a half opening angle of 30°, and with a mass-loss rate (Ṁ<SUB>w</SUB>) of ~2 × 10<SUP>-7</SUP> M<SUB>☉</SUB> yr<SUP>-1</SUP>. The observed V-shaped differential phases are reliably reproduced by combining a simple asymmetric continuum disc model with our Brγ disc-wind model. <BR /> Conclusions: In conclusion, the Brγ emission of HD 98922 can be modelled with a disc wind that is able to approximately reproduce all interferometric observations if we assume that the intensity distribution of the dust continuum disc is asymmetric. <P />Based on observations collected at the VLT (ESO Paranal, Chile) with programmes 075.C-0637(A), 083.C-0236(A-D), 090.C-0192(A), 090.C-0378(A) and 090.C-0371(A).Appendices are available in electronic form at <A href="http://www.aanda.org/10.1051/0004-6361/201526002/olm">http://www.aanda.org</A>