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BROWN DWARFS AND ISOLATED
PLANETARY-MASS OBJECTS
Using data from a deep infrared survey of the young σ
Orionis open cluster, astronomers searched for low-mass
objects in the central region down to 6 Jupiter masses and
studied their mass distribution.
The survey was built from data taken with the Wide Field
Camera on the INT and ISAAC on VLT’s UT1 Antu. Deep
I- and J-band images of a 0.22deg 2 region close to the
cluster centre were obtained, reaching limiting magnitudes
of I~24.1 and J~21.8 respectively. The LIRIS infrared
spectrograph and imager on the WHT was also used to
obtain some deep images in the H and K s
bands. And
follow-up of candidates was performed using data from the
2MASS survey, the Spitzer Space Telescope, and other
telescopes.
The survey yielded 30 very young cluster low-mass
objects, among them brown dwarfs (BDs) and isolated
planetary-mass objects (IPMOs). By studying their mass
spectrum, or counting the number of objects per mass
interval, the astronomers found a rising mass spectrum in
the mass interval 0.11 to 0.006 solar masses implying that
IPMOs could be as frequent as our Sun.
Figure 15. Colour composition of the σ Orionis open cluster (North
is up and East to the left) from images taken using the Wide Field
Camera on the INT.
They found no direct evidence for the presence of an
opacity-mass limit for objects formed via fragmentation and
collapse of molecular clouds implying that BDs and IPMOs
probably form as an extension of the low-mass star
formation process. Any possible mass cut-off would lie
below 0.006 solar masses or 6 Jupiter masses.
Another interesting result of their research is that almost
half of the studied BDs harbour protoplanetary discs.
A LOW-MASS PRE-MAIN-SEQUENCE
ECLIPSING BINARY SYSTEM
Eclipsing binaries are a special class of spectroscopic
binaries, systems where the components are so close
together that they cannot be separated by imaging. In an
eclipsing binary, the system is edge-on as seen from the
Earth, so one star passes in front of the other every half
orbit, causing the total light emitted to dim in a periodic
fashion. By measuring the shape of these eclipses in a
light curve of the system, astronomers can derive the radii
of the stars in the binary.
The binary system JW 380, located in the very young Orion
Nebula Cluster (ONC), was identified using time-series
Figure 16. Mosaic of I-band images centred on σ Orionis taken
with the Wide Field Camera on the INT showing the location of
some of the low-mass objects. North is up and East to the left. The
size of each Wide Field Camera frame is about 11×22 arcmin 2 .
The background image is from DSS-2-IR.
imaging observations taken with the Wide Field Camera on
the INT. These observations were part of the Monitor
project, a large-scale survey searching for eclipsing binary
and transiting extrasolar planet systems in a number of
young open star clusters.
Follow-up spectroscopic observations obtained using the
FLAMES fibre-fed spectrograph on the VLT UT2 and the
NOAO Phoenix spectrograph on Gemini-South were used
to derive the radial velocities and hence component
masses of the stars, as well as to confirm the system's
membership in the ONC.
ING BIENNIAL R EPORT 2006–2007 • 23