Max Planck Institute for Astronomy - Annual Report 2005

22 II. Highlights
AB Dor C
AB Dor A
50 m�
Fig. II.2.2: The orbits of AB Dor A and C, determined from
astrometric data.
The supposed binary therefore actually is a quadruple
system composed of two close pairs orbiting one another
at a distance of about 135 AE. The orbit of AB Dor C
around A (Fig. II.2.2) allows the orbital parameters to be
determined and thus, for the first time, enabling the mass
of such a low-mass star to be calculated from astrometric
observations. AB Dor C gravitationally acts on its central
star while both stars are moving around a common center
of gravity. Such observations of the so-called reflex
motion had already been performed in previous years
with VLBI, and hipparCoS and could now be used to reconstruct
the orbit. As it turns out, AB Dor C is moving
on a highly elliptical orbit (e � 0.59), taking 11.75 years
to complete one period.
The known mass of AB Dor A of 0.865 solar masses
and the orbital parameters yielded a mass of 0.09 solar
masses for AB Dor C (corresponding to 93 Jupiter
masses). Thus the object is just above the upper limit of
0.07 solar masses for brown dwarfs. From the magnitudes
measured at three near-infrared wavelengths, a
spectral type of M 7 – M 9 could be derived for AB Dor
C. In order to determine the type more precisely, the astronomers
took a spectrum of the object using the naCo
adaptive optics spectrograph. The spectrum best matched
a spectral type of M 8 (Fig. II.2.3) which is assigned a
temperature of 2600 K. The luminosity of AB Dor C
was determined from the measured infrared fluxes to be
0.0018 solar luminosities.
Normalized flux
In this manner, the physical parameters of this lowmass
star were determined and could be compared to the
predictions made by theoretical models. Surprisingly,
the companion was cooler by 400 degrees and 2.5 times
fainter than expected for the given mass based on the
latest theoretical models (Fig. II.2.4). In other words:
current models yield masses that are only half as large
for such young, low-mass objects.
This significantly affects the interpretation of a number
of observations. Thus many young, low-mass objects
were found by infrared surveys in the area of the Orion
star-forming region. With the help of theoretical evolutionary
tracks, a majority of them had been classified as
brown dwarfs. According to the new findings obtained
for AB Dor C, many of them should now be considered
to be low-mass stars. The existence of so-called free
floating planetary objects should also now be considered
with caution. These low-luminosity objects also located
in the Orion Nebula had been classified as planets. If the
masses were underestimated here, too, by a factor of two,
these objects may be misidentified brown dwarfs.
This work illustrates the danger of using evolutionary
tracks for low-mass stars and brown dwarfs. In order
to obtain a reliable calibration of the mass-luminosityrelation,
more cases such as AB Dor C must be found.
However, such objects are extremely rare. Only one
percent of all stars have close low-mass companions, and
only one percent of all stars in the solar neighborhood are
young. Therefore, the fact that this measurement could be
performed at all can be considered a real stroke of luck.
A verification of these results with more objects would
be desirable.
Fig. II.2.3: Spectrum of AB Dor C in the near-infrared range.
1
0.8
0.6
0.4
2.0
Na I
2.1 2.2
Wavelength [�m]
CO
2.3 2.4