A&A 365, 514-518 (2001)
DOI: 10.1051/0004-6361:20000051
E. W. Guenther1 - R. Neuhäuser2 - N. Huélamo2 - W. Brandner3 - J. Alves4
Send offprint request: E. Guenther,
1 - Thüringer Landessternwarte Tautenburg, Sternwarte 5,
07778 Tautenburg, Germany
2 -
MPI für extraterrestrische Physik, 85740 Garching, Germany
3 -
University of Hawaii, Institute for Astronomy, 2680 Woodlawn Dr.,
Honolulu, HI 96822, USA
4 -
European Southern Observatory, Karl-Schwarzschild-Straße 2,
85748 Garching, Germany
Received 4 August 2000 / Accepted 24 October 2000
Abstract
Up to now only four brown dwarf companions to normal stars have been
found and confirmed by both spectroscopy and proper motion (namely
Gl229B, G196-3 B, Gl570D, and
CoD
B). On the basis of an optical spectrum taken
with HST/STIS Lowrance et al. (2000) recently pointed out another
possible candidate companion. The companion candidate is located at a
distance of
from the A0-star HR7329,
which is considered as a member of a moving group of young stars in
Tucanae located at a distance of only
48 pc. In order to confirm
or disregard the companion nature of the candidate, we have determined
the proper motion of the brown dwarf candidate with an epoch difference of
1.8 years, and found that it is consistent with a co-moving companion of
HR7329. Additional to the proper motion measurement, we have
also taken an H-band spectrum using ISAAC on the ESO-VLT. From this
spectrum, we conclude that the companion candidate has spectral type M 7
to M 8, which is in agreement with the optical spectrum.
We thus conclude that HR7329B is most likely a brown dwarf
companion. The mass ratio of this pair (A0 to M 7-8, i.e.
100:1)
is the largest known among brown dwarf companions, which is relevant for
studying the formation of brown dwarfs as companions.
Key words: stars: binaries: visual - individual: HR 7329 - late-type - pre-main sequence
Author for correspondance: guenther@tls-tautenburg.de
Brown dwarfs are objects with masses below the stellar limit but with
masses above planets. They thus form a natural bridge between low-mass
stars and planets. Brown dwarfs are unable to sustain stable nuclear
fusion of hydrogen, but can burn deuterium until they are 107yrs old. The sub-stellar limit, i.e. the dividing line between stars
and brown dwarfs is the hydrogen burning mass, namely
,
depending on metallicity. Planets cannot even burn
deuterium and have masses below
.
However, the
important physical difference between brown dwarfs and planets seems to
be the way in which they form. One apparent difference between planets
and brown dwarfs is that the brown dwarfs are unlikely to be found as
close companions of stars (Latham et al. 1989; Mayor &
Queloz 1995; Marcy & Butler 1998; Zucker & Mazeh
2000). This is often referred to as the brown-dwarf desert. In
contrast to this, numerous free-floating brown dwarfs have been
discovered by direct imaging searches. Interestingly, some of the
free-floating objects seem to have masses below the deuterium burning
limit (Zapatero Osorio et al. 2000a; Zapatero Osorio et
al. 2000b). Since the masses of these objects seem to be
of the order of
or even lower, they are
often referred to as free-floating planets. On the other hand, they could
also be considered as extremely low-mass brown dwarfs. The dividing
line between brown dwarfs and planets is currently controversial and
poorly understood. The fact that planets in orbit around solar-mass
stars exist, whereas brown dwarfs are at least extremely rare, must
apparently be a consequence of the way in which these objects form. In
order to shed more light on to this problem, it is necessary to find out
how empty this brown-dwarf companion desert really is, and what the
properties of the brown-dwarfs that are found in orbit around a star are.
Can stars of all masses have brown dwarf companions? Do isolated and
companion brown dwarfs have the same mass function, or does the mass
function depend of the companion on the mass of the primary? Are the
orbital-characteristics of the brown-dwarf companions different from
those of planets?
Up to now, only very few brown dwarfs were found as companions to normal
stars: Gl229B (Nakajima et al. 1995;
Oppenheimer et al. 1995), G196-3B (Rebolo
et al. 1998), Gl570D (Burgasser et
al. 2000), and CoD
B (suggested as
sub-stellar companion candidate by Lowrance et al. 2000,
and confirmed as such by Neuhäuser et al. 2000b), all
of which are brown dwarfs confirmed as companions by both spectroscopy
and proper motion. Two more candidates were suggested, namely
GGTauBb (White et al. 1999) and
HR7329B (Lowrance et al. 2000, henceforth
L2K), the latter is investigated in this paper. In those two companion
candidates, optical spectra are taken, but proper motion was not yet
available. Brown dwarfs and L-dwarfs can have brown dwarf companions
themselves (Basri & Martín 1999; Martín et
al. 1999a). In view of the fact that brown dwarf companions
are so rare, it is advisable to ensure that these companions are really
brown dwarfs. Thus not only the spectral types have to be determined, but
it also has to be shown that the object is orbiting the primary (or at
least co-moving with it in case the orbital period is too long).
There is no direct imaging detection of an extra-solar planet, yet.
Previous candidates for ejected (Terebey et al. 1998), or
orbiting (Neuhäuser et al. 2000a) planets could not be
confirmed as cool planets by spectroscopy (Terebey et al.
2000; Neuhäuser et al. 2000c).
Like L2K, we also search for substellar companions to young nearby stars, where companions are still relatively bright and well-separated (see Neuhäuser et al. 2000a, 2000b). We use ground-based facilities, mainly the NTT on La Silla, the VLT on Cerro Paranal, and ALFA, the adaptive optics infrared imaging instrument on the Calar Alto 3.5 m.
By investigating archived HST NICMOS data, we also noticed the faint object near the presumably young star HR7329 (Sect. 2) and then performed follow-up infrared spectroscopy, which we present in Sect. 3. By comparing the relative positions of HR7329A and B in our new acquisition image with the two year old archived HST NICMOS image (published by L2K), we estimate the proper motion of HR7329B (Sect. 4). We discuss our findings in Sect. 5.
Simultaneously and independently, both Zuckerman & Webb
(2000) and Torres et al. (2000) presented
evidence for two nearby young moving groups called Tucanae cluster and
Horologium association, respectively. The probable and likely members
of the Tuc and HorA groups are both at 40 to 50 pc (mostly
measured by the Hipparcos satellite) with signatures for youth in their
spectra like H
emission and Lithium absorption lines. They show
similar radial velocities and proper motions and are located close to
each other. Hence, both groups may very well form one large moving
group altogether. The young star
Hor is probably also
located in HorA (Torres et al. 2000). Recently, Kürster et
al. (2000) found radial velocity variations of this star
that are consistent with the presence of a planet, and it may also have
a circumstellar disk.
By comparing the X-ray luminosity function of the Tuc members observed by ROSAT with those of the T Tauri stars in the Taurus and TWHya associations as well as with zero-age main-sequence stars of the Pleiades, Stelzer & Neuhäuser (2000) concluded that the Tuc stars have an age of 10 to 30 Myrs, i.e. young, but close to or already on the zero-age main-sequence.
The star HR7329 is listed as probable member of the Tucanae
group (Zuckerman & Webb 2000). It has a spectral type A0
and its radial velocity and proper motion are consistent with kinematic
membership. Most recently, L2K presented evidence for a brown dwarf
companion (called HR7329B), located
off the star HR7329A. The companion candidate
HR7329B was found to have a spectral type M 7 to M 8 in their
HST/STIS spectrum.
![]() |
Figure 1:
Our H-band spectrum of HR 7329 B compared with the young M 7
(average of Cha H![]() ![]() |
Open with DEXTER |
To confirm a companion candidate found by direct imaging as true companion, one needs to show not only that its spectral type is consistent with its colours and with the observed magnitude difference (assuming the same distance), but also that it is co-moving with its primary star. Orbital motion would be a final proof of companionship.
We obtained an H-band spectrum (
)
on 16th of April 2000
with the Infrared Spectrograph and Array Camera (ISAAC) at the European
Southern Observatory (ESO) 8.2 m telescope Antu, Unit Telescope No. 1
(UT1) of the Very Large Telescope (VLT). The spectrum consists of 20
co-added 60 s exposures through a
slit, aligned
neither along the position angle of the pair nor perpendicular to it,
but in between those two positions, so that the two objects are well
separated and that the flux from the companion candidate is several
times larger than the flux from the bright star. Darks, flats, and
standards were taken in the same night.
After standard data reduction, we modeled and subtracted the flux of
the bright star from the faint object's spectrum at each wavelength.
The final spectra of both HR7329A and B are shown in
Fig.1. The primary and HD188112 (van der
Bliek et al. 1996) were used to obtain a relative
flux-calibration. For very late-type objects, the shape of the
continuum in the near-infrared is sensitive to the spectral type
(Kirkpatrick 2000). Comparing our spectrum with spectra
of young M 7 and M 8-type dwarfs from the Cha I dark cloud (Comerón et al. 2000), we find that HR7329B
has spectral type M 7 to M 8. As pointed out by Martín et al.
(1999a) the
H-band index at
can also be used to determine the spectral types of very late M, and
L-stars. Using this index, we also derive a spectral type of M 7 to M 8
for HR7329B. These results agree well with the optical
spectrum taken with HST/STIS, presented by L2K.
HR7329B was first detected by L99 using HST NICMOS on 29
June 1998 in the F160W filter. The object was then located
east and
south
of HR7329A, corresponding to a separation of
and a position angle of
measured from north over east to south.
![]() |
Figure 2:
ISAAC acquisition image of HR7329A and B, the FWHM
is
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Open with DEXTER |
We detected HR7329B in our 2 s exposure ISAAC (
/pixel) acquisition image taken on the 22nd of April
2000 using two narrow band filters centered on
m (Br
)
and
m (with
and
m,
respectively). The seeing was
,
and both
objects are clearly detected and resolved. Star A is not saturated. The
magnitude difference between star A and object B in the acquisition
image (Fig.2) is 5.7 mag. The companion
candidate was located
east and
south of the bright star, corresponding to
and
,
consistent with the NICMOS image (L2K).
Using the infrared imager SofI (Son of Isaac) at the ESO 3.5 m New
Technology Telescope (NTT) on La Silla we took an additional image on
which both HR7329A and B are detected, and resolved,
despite
seeing conditions. This image was
taken on 17th of May 2000 with the H-band filter using the so-called "small
SofI field'', to obtain the highest angular resolution possible (
/pixel). The image was exposed for 10 min total
(
s in auto-jitter mode). Darks, flats, and standard
stars were taken in the same night, and we performed standard data
reduction using eclipse and MIDAS.
In the SofI image, HR7329B is located
east and
south of the
bright star, corresponding to
and
.
Although this is consistent with
the NICMOS and ISAAC images, the precision is much lower, due to
the very bad seeing. Hence, we disregard this image from further
analysis. This is not a big loss of information, since the maximum
epoch difference would not increase by much if we would include the SofI
image.
The offsets in right ascension and declination as well as the
separations and position angles observed for the HR7329A
and B pair with HST/NICMOS and VLT/ISAAC as given above are consistent
with each other within .
However, this does not prove object B
to be a companion of star A. Whether we can already show that the motion
of HR 7329 B relative to star A is inconsistent with B being an
unrelated field star, depends on the proper motion of HR 7329 A.
L2K argued that HR7329B is possibly co-moving with A, as B
was in the
HST/STIS slit when they took the
spectrum on 22nd of May 1999, using the offset determined from an image taken
about one year earlier. Using all, except the SofI image, we can now
investigate this point with much higher accuracy than before.
Table 1 gives proper motion of HR 7329 taken from four
different catalogues. For the rest of the paper, we will use the
Hipparcos data, because it has the highest accuracy.
Catalogue | designation |
![]() |
![]() |
in catalogue | [mas
![]() |
[mas
![]() |
|
Hipparcos | HIP 95261 |
![]() |
![]() |
PPM | PPM 347630 |
![]() |
![]() |
TRC | TRC 8765-2571 |
![]() |
![]() |
ACT | ACT 8765-2571 |
![]() |
![]() |
![]() |
Figure 3:
Motion of companion candidate B and star A: Plotted are
![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 4:
Another way to investigate whether B is a non moving background
object, or not is shown here. Plotted are just the two measured offsets
(HST and ISAAC) of object B relative to star A, without taking any
proper motion into account. If A and B form a common proper motion
pair, then the error ellipses should overlap, which they do. If B is a
background source that does not move, the error ellipses would be well
separated, given the relatively large proper motion of the
A-component. Using the known proper motion, we can estimate that the
probably for that is only ![]() ![]() |
Open with DEXTER |
Figure 3 shows the proper motion of the A and B
component. The position of the A-component on the 29th of June 1998 is
plotted at the origin (
), and the position on
the 22nd of April 2000 south-east of it is given by its proper motion. The
errors in the position of star A at the 2nd epoch is given by the error
of the Hipparcos proper motion. In addition, we plot the offsets of
object B relative to star A with errors given by the errors of the
measured offsets and the proper motion of star A. Clearly visible is
that object B moves in to the same direction as A. The amount of the
proper motion of B is within the errors, also consistent with the proper
motion of A. However, the error-ellipses of the HST and the ISAAC
observation do overlap. Thus formally, a non-moving background object
would be possible. In order to test this hypothesis we plotted in
Fig.4 the two measured offsets (HST and ISAAC)
of object B relative to star A, without taking any proper motion into
account. If A and B form a pair then the error ellipses should overlap
(ignoring orbital motion), which they do. If the object were a
non-moving background object, the position should within the errors
remain the same in Fig.3. While the errors are
large, this is clearly not the case. While we cannot completely
rule out that the companion candidate is a non-moving background
object, it is far more convincing that it is co-moving. From
Fig.3 we conclude that the proper motions of A and
B are similar, namely by 1
regarding their amount and by
2
regarding their direction. However, while this demonstrate
that both A and B are members of a co-moving group of objects, it does
not strictly demonstrate that B is orbiting around A. On the other
hand, the probability for HR 7329 A and B to be two non-bound Tuc
members being located that close together by chance, is very low.
As already discussed in L2K, the M 7 to M 8-type object
HR7329B would be located at a distance of 19 pc, if
it were a foreground main sequence dwarf. To find such a faint late-type
object by chance
off HR7329 is very
unlikely, the probability being
10-7 (L2K).
If the A and B-components were at 48 pc and 19 pc respectively, the relative parallactic motion would be -32.11 mas in right ascension (towards the east) and -8.65 mas in declination (towards the south) for the HST and ISAAC observations. The corresponding error-circles would then have a slightly larger overlap than in Fig.3, and the corresponding figure would be almost identical to Fig.3. That object B is an unrelated, non-moving background object is extremely unlikely, because it would have to be very young given its magnitude and spectral type, if it would be located behind the Tucanae cluster. A non-moving foreground object is also very unlikely. While a background object is strictly possible, it is less convincing than a co-moving object.
L2K derive an effective temperature of
K and a bolometric
luminosity of
for HR7329B
(at 48 pc) and compare its location in the HR diagram with th e
evolutionary tracks and isochrones of Burrows et al. (1997)
and Baraffe et al. (1998) to derive a mass range of
30 to 50
at an age of
20 to 30 Myrs. According to
the new Chabrier et al. (2000) models,
HR7329B has a mass of
20 to 40
.
Given
that the primary star HR7329A is an A0V star
(2.9
), this is a binary with very high mass ratio (
0.01).
The angular separation of
corresponds
to a projected physical separation of
AU at
48 pc
distance. This in turn corresponds to an orbital period of
years and orbital motion of
mas per year (for a
face-on circular orbit), only a factor of 2 smaller than the relative
astrometric precision achieved in the HST NICMOS and VLT ISAAC images
discussed here. We thus conclude that HR7329B is most
likely a brown dwarf companion of an 2.9
star with a mass of
only
.
The mass-ratio thus is the largest
known among brown dwarf companions. As noted by Martín et al.
(1999a) systems with very high mass ratios have large
(
AU) separations and systems with low mass ratios involving brown
dwarfs have typically smaller separations (
AU). With a
projected physical separation of
AU HR7329 seems
to be another example of this rule.
Acknowledgements
We thank the referee, Gibor Basri, for his insightful comments on the paper. We would also like to thank the ESO User Support group with F. Comerón for assistance, the ISAAC team with J.-G. Cuby and C. Lidman for their service mode observations, the NTT team with L. Vanci, E. Le Floc'h, G. Martin, and J. Miranda for support during the NTT observations, and F. Comerón for providing his H-band spectra of late-type dwarfs in electronic form, as well as G. Wuchterl and S. Frink for very useful discussion. RN wishes to acknowledge financial support from the the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) under grant number 50 OR 0003.