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

Multiplicity of protostars
Following our investigation of multiple systems among low-mass PMS and ZAMS stars aimed at constraining
the star formation process from the statistics of multiplicity (e.g., Duchêne, 1999, A&A, 341, 547), we have
recently focussed on the earliest stages of star formation by investigating the multiplicity of near-infrared
embedded sources in star forming regions. These embedded sources are the observable objects closest to the
protostellar stage. Duchêne & Bouvier, with Co-I André & Motte (Saclay), and Bontemps (Bordeaux) derived
the fraction of wide binaries among these sources from direct near-IR imaging at CFHT and ESO. An excess of
multiple systems compared to older low-mass stars in the field is found. This new result, for the youngest stellar
objects known, tends to confirm the ubiquity of fragmentation during molecular core collapse, thus leading to
multiple protostars which later decay through dynamical relaxation or are disrupted by gravitational encounters
in dense clusters (Duchêne et al. 2004).
Taking advantage of the unique capabilities of the near-infrared wavefront sensor of the NACO Adaptative
Optics system on VLT, we are now in the process of investigating the tightest of these protostellar systems,
down to a resolution of 10 AU. Preliminary results reveal a number of close systems with separations in this
range, see Fig. 6.7. A complete census of multiple protostellar systems over the range 10-1500 AU awaits the
completion of the VLT NACO runs. Meanwhile, we have started follow up spectroscopy of the faintest and
most embedded companions to protostars, with the aim to search for proto-brown dwarfs. In 2005,we will also
obtain the first VLTI-MIDI mid-infrared interferometric observations of these objects, in an attempt to detect
the tightest embedded multiple systems, thereby setting constraints on the fragmentation scenario for systems
just a few AU appart, and to determine the structure of the dusty envelopes surrounding these protostars.
L1689 IRS 5 (ophiuchus) SVS 20 (Serpens) WL 20 (Ophiuchus)
0.5"
Figure 6.7: Three examples of multiple protostellar systems observed with VLT/NACO. Note the strange
physionomy of the third companion of the WL 20 system, which is not an artefact but most likely an edge-on
disk. Image credit G. Duchêne and J.Bouvier
While our current results indicate a high frequency of multiplicity among protostars indeed, thus supporting
fragmentation during core collapse, we find no evidence for the existence of small-N unstable aggregates (N=5-
10) in the protostellar stage, contrary to what several numerical models of core collapse predict through multiple
fragmentation. This disagreement probably points to the simplified physics included so far in the numerical
models which, for instance, neglect magnetic field, use a poorly constrained equation of state, and do not take
into account feedback effects resulting from the strong wind of young protostars onto the collapsing cloud. Our
results regarding the end products of core collapse in several star forming regions thus provide some guidance
to improve numerical models.
On the nature of Infrared companions to T Tauri stars
Taking advantage of the high-angular resolution imaging and spectroscopic devices available on the VLT and
Keck telescopes, we have been studying for the last few years the nature of the so-called ”infrared companions”
to T Tauri stars. These objects are best exemplified by T Tau Sa, the optically undetectable companion to the
prototype young stellar object T Tau. The spectral energy distribution of these objects would classify them
as embedded, Class I protostars, but their close vicinity (a few tens to hundreds of AU) to more evolved T
Tauri stars suggests otherwise. Using the Keck telescope in collaboration with Andrea Ghez (UCLA), we have
obtained medium resolution near-infrared spectra of two such objects (T Tau Sa and V773 Tau D (Duchêne
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