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

et al. 2002; 2003). We have found that their spectrum is entirely featureless, showing that they are either
intermediate-mass early-type stars seen in scattered light due to heavy obscuration along our line-of-sight by
some dusty circumstellar material or lower mass objects entirely embedded in an optically thick envelope.
In the case of T Tau Sa, we have further obtained a high resolution (R∼30000) near-infrared spectrum that
clearly shows the presence of warm (∼400K) gaseous material on the line-of-sight to the central target; the
absence of kinematical signatures of infall or outflow clearly suggests that the object is an early-type star that
is surrounded by a compact, opaque edge-on disk that has not been spatially resolved so far. More recently,
VLT-NACO observations of another infrared companion system, WL 20 in Rho Ophiuchus (see Fig. 6.7, right
panel), has revealed a complex nebulosity that is most likely seen in scattered light, again suggesting that
the interpretation of these infrared companions in terms of embedded protostars is unlikely to be an adequate
framework.
In the near future, we plan on obtaining additional high angular resolution images in the near- and midinfrared
of these systems, as well as high spectral resolution spectra in order to identify photospheric features
and/or gaseous material located along our line of sight to these objects.
Binary stars and dynamical masses
The mass of a star is probably its most fundamental property. It is however extremely difficult to measure
directly and one usually relies on models (or mass-luminosity relations) to get an estimation. Of concern for
us, pre-main sequence evolutionary models are subject to significant uncertainties related to the complexity of
the various relevant physical processes. It is therefore necessary to determine model-independent masses for T
Tauri stars in order to calibrate the theoretical evolutionary tracks, in particular towards the low-mass end, as
masses for very low-mass stars and brown dwarfs are then used in other studies, such as that of the initial mass
function in star-forming regions.
Using speckle interferometric observations of young tight binary systems at Keck in collaboration with
Andrea Ghez (UCLA), we have monitored several tight T Tauri binary systems in order to determine the
orbital parameters and, eventually, the masses of these objects. Dynamical masses for several systems were
obtained (V773 Tau (Duchêne et al. 2003); DF Tau, TWA 5) with typical accuracies of 10-20%, where the
uncertainty in the distance to the system dominates the uncertainties in the Keplerian orbital fit. We have also
applied the same technique to the orbit of a field star-brown dwarf system, 2MASS0746425+2000321 (Bouy et
al. 2004), for which an accuracy of 5% was reached, this time helping to calibrate the relations valid for the
lower-main sequence.
As part of the AMBER consortium, we have obtained Guaranteed Observation Time to conduct the same
type of project on much tighter (