Issue |
A&A
Volume 446, Number 3, February II 2006
|
|
---|---|---|
Page(s) | 1165 - 1176 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361:20053406 | |
Published online | 20 January 2006 |
Radial velocity survey for planets and brown dwarf companions to very young brown dwarfs and very low-mass stars in Chamaeleon I with UVES at the VLT
Leiden Observatory / Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, Netherlands e-mail: viki@strw.leidenuniv.nl
Received:
11
May
2005
Accepted:
6
September
2005
We present results of a radial velocity (RV) survey for planets and
brown dwarf (BD) companions to very young BDs and (very) low-mass stars
in the Cha I star-forming cloud.
Time-resolved high-resolution echelle spectra
of Cha Hα 1–8 and Cha Hα 12 (M6–M8), B34 (M5), CHXR 74 (M4.5),
and Sz 23 (M2.5) were taken with UVES at the VLT between 2000 and 2004.
The precision achieved for the relative RVs range between 40 and 670 m s-1 and is
sufficient to detect Jupiter mass planets around the targets. This is the first
RV survey of very young BDs. It probes multiplicity, which is a key parameter
for formation in an as yet unexplored domain, in terms of age, mass, and orbital separation.
We find that the subsample of ten BDs and
very low-mass stars (VLMSs, , spectral types M5-M8)
has constant RVs on time scales of 40 days and less.
For this group, estimates of upper limits for masses
of hypothetical companions range between 0.1 MJup and 1.5 MJup for assumed orbital separations of 0.1 AU.
This hints at a rather small multiplicity fraction for very young BDs/VLMSs, for
orbital separations of
0.1 AU and orbital periods of
40 days.
Furthermore, the non-variable objects
demonstrate the lack of any significant RV noise due to stellar activity
down to the precision necessary to detect giant planets.
Thus, very young BDs/VLMSs are suitable targets for
RV surveys for planets.
Three objects of the sample exhibit significant RV variations with peak-to-peak RV differences of 2-3 km s-1. For the highest mass object observed with UVES (Sz 23, ~0.3
),
the variations are on time scales of days, which might be explained by rotational modulation.
On the other hand, the BD candidate Cha Hα 8 (M6.5) and the low-mass star CHXR 74
(M4.5) both display significant RV variations on times scales of
150 days, while
they are both RV constant or show only much smaller amplitude variations
on time scales of days to weeks, i.e. of the rotation periods.
A suggested explanation for the detected RV variations of CHXR 74 and Cha Hα 8 is that they are caused by giant planets or very low-mass BDs of
at least a few Jupiter masses orbiting with periods of several months or longer.
Thus, the presented RV data indicate that orbital periods of companions to very young BDs
and (very) low-mass stars are possibly
several months or longer, and that orbital separations are
0.2 AU.
This parameter range has not been covered for all targets yet, but will
be probed by follow-up observations.
Furthermore, we show that
the scaled down equivalent to the BD desert found around solar-like stars would be
a giant planet desert around BD and VLMS primaries, if formed
by the same mechanism. The present data test its existence for the targets
in the limited separation range of the survey. So far,
no hints of companions in a “giant planet desert” have been found.
Key words: stars: low-mass, brown dwarfs / stars: pre-main sequence / binaries: spectroscopic / techniques: radial velocities / planetary systems
© ESO, 2006
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