Multiplicity among T Tauri stars in OB and T associations
1996 Astronomy & Astrophysics 307, 121
Abstract
We present first results of a survey for companions among X-ray
selected pre-main sequence stars, most of them being
weak-line T Tauri stars (WTTS). These T Tauri stars have been identified
in the course of optical follow-up observations of sources from the
ROSAT All Sky Survey associated with star forming regions.
The areas surveyed include the T associations of Chamaeleon and Lupus
as well as Upper Scorpius, the latter
being part of the Scorpius Centaurus OB association (Sco OB 2).
Using SUSI at the NTT under subarcsec seeing conditions we observed
195 T Tauri stars through a 1 mu ("Z") filter and identified companions to
31 of them (among these 12 subarcsec binaries).
Based on statistical arguments we conclude that almost all of them are
indeed physical (i.e. gravitationally bound) binary or multiple systems.
For 10 systems located in Upper Scorpius and Lupus, we additionally obtained
spatially resolved near-infrared photometry in the J, H, and K bands
with the MPIA 2.2m telescope at ESO, La Silla.
The near-infrared colours of the secondaries are consistent with those of
dwarfs and are
clearly distinct from those of late type giant stars.
Based on astrometric measurements of some binaries we show that the
components of these binaries are common proper motion pairs, very likely
in a gravitationally bound orbit around each other.
We find that the overall binary frequency among T Tauri stars in a range of
separations between 120 and 1800 AU is in
agreement with the binary frequency observed among main sequence stars
in the solar neighbourhood. However, we note that within individual regions
the spatial distribution of binaries -- within a distinct range of separation --
is non-uniform. In particular,
in Upper Scorpius, WTTS in the vicinity of early type stars seem to be almost
devoid of multiple systems, whereas in another area in Upper Scorpius half
of all
WTTS have a companion in a range of separation between 0.7" and 3.0".
Furthermore, we find no preponderance of systems with large brightness
differences between primary and companion stars (median Delta Z =
1.0 mag ... 1.5 mag).
We conclude that binarity is established very early in stellar evolution,
that the orbital parameters of wide binaries (a >= 120AU) remain
virtually unchanged during their pre-main sequence evolution, and that these
wide binaries were formed either through collisional fragmentation
or fragmentation of rotating filaments.
A copy of the article is available via NASA's
Astrophysics Data System .