Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

Wind and disk interaction in MWC 297 The young stellar object MWC 297 is an embedded B1.5Ve
star exhibiting strong hydrogen emission lines and a strong near-infrared continuum excess. This object has
been observed by the AMBER/VLTI instrument on a 45m baseline in a region of the K spectral band centered
around the Brγ line. The object has been resolved in the continuum with a visibility of 0.50 +0.08
−0.10 in the Brγ
line, where the flux is about twice larger than in the continuum, with a visibility about twice smaller 0.33±0.06.
We applied a combined model that includes a geometrically thin accretion disk model consisting of gas and
dust and a stellar wind model. The hydrogen emission, about twice more resolved than the continuum, can be
reproduced by the emission of a latitude-dependent wind model above the disk surface. A picture is emerging
in which MWC 297 is surrounded by an equatorial flat disk, possibly still accreting, and an outflowing wind
which has a much higher velocity in the polar region than at the equator (Malbet et al. 2005, A&A, submitted).
These observations are the first to separate spatially and spectrally the disk radiation and the wind emission
within the inner 1 AU from the central sources. It shows the power of AMBER and many similar sources will
be observed and yield hopefully to a better understanding of the connection between the disk and the wind in
young stellar objects.
6.3.2 The star-disk interaction
The previous section dealt with the parts of the disk located typically at a few AU’s from the star (1-3 AU).
Moving further inward, the dust in the disk sublimates and only gas is left. At one point the disk material
moving inward because of accretion will encounter the top of the stellar magnetosphere. Whether the accretion
flow will keep going “equatorially” to form a boundary layer before reaching the photosphere or be channelled
by the magnetic field depends on the relative balance between “magnetic pressure” and “accretion pressure”.
Magnetospheric accretion, i.e. the magnetic channelling of the accretion flow from the inner disk onto the
star, is now believed to be a central process in young stars in that it regulates their angular momentum during
pre-main sequence evolution, diverts part of the accretion flow into a powerful ionized jet, and possibly halts the
inward migration of proto-Jupiters close to the young star. FOST’s involvement in the study of magnetospheric
accretion is mostly observational. However, to improve our capacity for models and interpretations, a PhD
thesis (N.Bessolaz) has been started jointly with SHERPAS in november 2004.
Magnetospheric accretion
Our work aims at deriving the structure of the magnetospheric cavity, the physical conditions at the inner disk
edge and in magnetic funnel flows (accretion columns), as well as constraining the temporal evolution of the
whole structure. With this goal in mind, we have organised and performed several international campaigns of
observations involving between 10-15 observatories world-wide and an even larger number of collaborators. By
monitoring the spectroscopic, photometric and polarimetric variability of young stars on a timescale of several
weeks, we characterize the dynamical processes (accretion, ejection, magnetic field evolution) involved in the
magnetospheric accretion process (Bouvier et al. 1999, A&A, 439, 619; Bouvier et al. 2003; Ménard et al.
2003).
We thus showed that the observed variations of the photometric and spectral diagnostics (luminosity, line
profiles an intensity, veiling, etc.) were broadly consistent with the magnetospheric accretion concept : all
diagnostics are modulated on a timescale of a week which reflects the rotation of the star and its magnetosphere
up to the inner disk edge. Hence, all accretion signatures (hot spot at the stellar surface, veiling, accretion
columns) vary in phase as the whole magnetospheric accretion structure rotate at the same angular velocity
than the star. The photospheric luminosity is modulated as well, with the same period but anti-phased. This
suggests that the inner disk edge is warped by the stellar magnetosphere as material is lifted up above the
disk plane to be loaded into magnetic funnel flows and thus periodically occults the central star. This further
indicate that the stellar magnetosphere is inclined relative to the stellar rotational (and disk) axis (Bouvier et
al. 2003).
While the observed variability can be broadly accounted for in the framework of the magnetospheric accretion
model, we additionally showed that the variability pattern changes over a timescale of several weeks. This
indicates that the magnetospheric structure itself reacts to the interaction with the disk perhaps in a cyclic-like
fashion, first inflating as the result of differential rotation between the star and the disk, then opening and
reconnecting to eventually return to its initial, roughly dipolar configuration. We have reported evidence for
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