Max Planck Institute for Astronomy - Annual Report 2005

II.2 AB Doradus C: Young, Low-mass Star is Twice as Massive as Expected
Mass is one of the most fundamental parameters of
stellar evolution. It determines, among other things, the
luminosity, temperature and lifetime of a star. The determination
of mass therefore is one of the fundamental
tasks of astronomy. However, in general, astronomers
measure the luminosity of a star and then indirectly
infer the mass via a mass-luminosity relation. This
relation is well-established for evolved, massive stars,
but up to now could not be calibrated for young, lowmass
stars - in particular for brown dwarfs. Here one
has to rely completely on evolutionary models. Now, for
the first time, astronomers at the MPIA, together with
colleagues from Spain and the USA, have derived the
mass of a young, very low-mass star from astrometric
data for the given luminosity. Surprisingly, the value
turned out to be twice as high as was expected from
theory. This will have far-reaching consequences, e.g.,
for the hitherto assumed frequency of brown dwarfs
and planets in young stellar clusters.
During their formation, young brown dwarfs and
planets are hot and bright, and then cool down over the
course of time. Thus they are observed best during an
early evolutionary stage. One star that has previously
been searched for low-mass companions for a longer
period is AB Doradus. Its small distance from the sun of
14.94 pc (49 light years as measured by hipparCoS) and
its relatively well-known age of 50 million years make it
Fig. II.2.1: The discovery image of AB Doradus C, obtained with
the naCo-SDI camera built at the MPIA. Left: Image taken with
naCo at a wavelength of 1.625 µm. Right: The companion is
clearly visibly as a result of the differential effect of the SDI.
a
C
ideal for this kind of study. It is of spectral type K1 and
still in its pre-main-sequence stage. Since the early 1990s
this star is known to have a companion at a distance of 9
arcseconds (corresponding to 135 AU). The two bodies
were named AB Dor A and B.
Astronomers at the MPIA, together with colleagues
from Spain and the USA, have observed this binary using
a recent upgrade of the naCo infrared camera built at the
MPIA. The new instrument, called naCo Simultaneous
Differential Imager (naCo SDI), is extremely well-suited
for finding low-mass stars and brown dwarfs in the close
vicinity of a star. The naCo-SDI camera is equipped with
an adaptive optics system which removes the blurring
caused by atmospheric turbulences. The ancillary optics
SDI divides the light of an individual star into four identical
images at adjacent wavelengths inside and outside
the infrared methane absorption band that is typical for
low-mass objects. On suitably chosen differential images
of these four exposures, the bluish primary star with its
bright halo almost completely disappears, rendering the
low-mass, cool and reddish companion clearly visible.
Using this instrument on one of the 8m telescopes
of the Very Large Telescope (VLT), the astronomers
detected a companion separated only 0.07 arcsec (1 AU)
from AB Dor B. The two objects were then designated
as AB Dor Ba and Bb. At the same time, another previously
unknown companion was found at a distance of
only 0.156 arcsec (2.3 AE) from AB Dor A. This object,
called AB Dor C, is about one hundred times fainter in
the near infrared range than AB Dor A. Thus it is the
faintest companion object ever imaged so closely to a
star (Fig. II.2.1). Earlier attempts to detect AB Dor C
with the hubble Space Telescope failed, which clearly
emphasizes the superior performance of naCo-SDI.
b
* A
* A
0.�156 0.�156
C
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