A&A 459, 669-678 (2006)
DOI: 10.1051/0004-6361:20065966
P. Kervella1 - F. Thévenin2 - V. Coudé du Foresto1 - F. Mignard2
1 - LESIA, UMR 8109, Observatoire de Paris-Meudon, 5 place Jules Janssen,
92195 Meudon Cedex, France
2 - Département Cassiopée, UMR 6202, Observatoire de la Côte
d'Azur, BP 4229, 06304 Nice Cedex 4, France
Received 4 July 2006 / Accepted 17 August 2006
Abstract
Context. Centauri is our closest stellar neighbor, at a distance of only 1.3 pc, and its two main components have spectral types comparable to the Sun. This is therefore a favorable target for an imaging search for extrasolar planets. Moreover, indications exist that the gravitational mass of
Cen B is higher than its modeled mass, the difference being consistent with a substellar companion of a few tens of Jupiter masses.
Aims. We searched for faint comoving companions to Cen B. As a secondary objective, we built a catalogue of the detected background sources.
Methods. We used the NACO adaptive optics system of the VLT in the J, H, and
bands to search for companions to
Cen B. This instrument allowed us to achieve a very high sensitivity to point-like sources, with a limiting magnitude of
at 7'' from the star. We complemented this data set with archival coronagraphic images from the HST-ACS instrument to obtain an accurate astrometric calibration.
Results. Over the observed area, we did not detect any comoving companion to Cen B down to an absolute magnitude of 19-20 in the H and
bands. However, we present a catalogue of 252 background objects within about 15'' of the star. This catalogue fills part of the large void area that surrounds
Cen in sky surveys due to the strong diffused light. We also present a model of the diffused light as a function of angular distance for the NACO instrument, that can be used to predict the background level for bright star observations.
Conclusions. According to recent numerical models, the limiting magnitude of our search sets the maximum mass of possible companions to 20-30 times Jupiter, between 7 and 20 AU from Cen B.
Key words: techniques: high angular resolution - stars: individual: Cen - stars: planetary systems - stars: solar neighbourhood - astronomical data bases: miscellaneous - infrared: stars
Our closest stellar neighbor, the Cen visual triple star (d = 1.34 pc),
is an extremely attractive target for an extra-solar planet search. The main
components,
Cen A (HD 128620) and B (HD 128621),
are G2V and K1V solar-like stars
(e.g. have solar-like asteroseismic oscillation frequencies),
while the third member is the red dwarf Proxima (M5.5V).
In all imaging planet searches, the main difficulty is in retrieving the
planetary signal in the bright point-spread function (hereafter PSF)
from the star. The proximity of
Cen is a clear advantage as it
allows a faint companion to be easily separated angularly from the star
itself down to orbital distances as close as a few astronomical units.
After a discussion of the potential for companions around
Cen
(Sect. 2), we present our adaptive optics observations (Sect. 3)
and the existing data from the HST archive (Sect. 4),
followed by the catalogue of the detected sources (Sect. 5)
and a discussion (Sect. 7).
Two factors led us to consider the possibility of a planetary mass companion
orbiting around Cen B:
the mass discrepancy between models and the dynamical mass of B on one hand,
and the existence of chaotically stable orbits at intermediate distances from the star
on the other hand.
Thévenin et al. (2002, hereafter T02) have proposed a model of Cen B
that reproduces well both the asteroseismic observables and the high-precision
radius measurement obtained using long-baseline interferometry
(Kervella et al. 2003; Bigot et al. 2006).
This model yields a mass of
for
Cen B, in agreement with the study by Guenther & Demarque (2000).
Simultaneously, Pourbaix et al. (2002, hereafter P02)
have measured the radial velocity of
this star with an overall precision of a few m s-1 and deduced a
dynamical mass of
.
The difference between the model mass and the dynamical mass of B reaches
Jupiter masses
(hereafter
).
No such difference is observed for
Cen A, for which the agreement
is excellent between the measured properties of
Cen A and
the model from T02, computed using the mass measured by P02.
In order to explain the 3
difference between the modeled and measured
mass of B, a possible scenario is that the implicit assumption made by P02
of a two-body system is incorrect due to the existence of
a companion orbiting
Cen B. We note that these authors
have introduced a correction of the radial velocity
of B, as they find an offset with respect to the data obtained by
Kamper & Wesselink (1978). This correction may mask the signature
of a long-period, low-mass companion orbiting B. The contribution
of Proxima to the radial velocity of the main pair is negligible (due to its large distance
from the A-B pair). Alternatively, a companion could also orbit
the A-B pair on a very long period orbit and currently be
located closer to B. Its gravitational contribution would make B appear heavier in the A-B interaction. This is however less
probable, as the mass of this companion would have to be significantly higher
than the proposed
to compensate for its increased distance.
In summary, an
brown dwarf (hereafter BD) within
10'' from
Cen B (or up to 50-100'' if orbiting around the pair) could be a viable explanation
for the mass discrepancy between P02 and T02.
This hypothesis is also favored by the fact that
the
Cen system is metal-rich and
Cen A
is Li-poor, as expected for stars hosting massive planets
(Santos et al. 2003; Israelian et al. 2004).
Table 1 lists the relevant physical properties of Cen A and B,
and the astrometric and orbital parameters of the pair
are given in Table 2.
The position of the barycenter is computed from the Hipparcos astrometric solution of
components A and B using the masses of
Table 2. This gives a perfect consistency
with the Hipparcos data, but with an accuracy limited by the poor
astrometric solution of the B component. Another solution would
have been to use the astrometric solution of component A and the
orbital elements of the system, whose uncertainty is hard to evaluate. The difference between the two
approaches is about 0.02 arcsec, so it can be used as a good
estimate of the uncertainty of the astrometric position of the
barycenter in the ICRS frame at epoch J 1991.25.
Table 1:
Properties of Cen A and B.
Table 2:
Astrometric parameters of Cen A-B.
Presently, at least 15 examples of extrasolar planets are known to orbit binary star members:
16 Cyg B, And,
Boo, etc. (Eggenberger et al. 2003;
Eggenberger et al. 2004; Mugrauer et al. 2005).
The 40 yrs period binary
Cep is also very likely the host of a 1.3
planet on a 1.8 AU orbit (Cochran et al. 2002).
Wiegert & Holman (1997) have identified how stable orbits
can be found within 2'' of
Cen B (interior planets)
or at distances of up to 50'' (exterior planets, orbiting the pair).
As a further incentive, it has been demonstrated that
the Kozai resonance (Holman et al. 1997; Innanen et al. 1997)
can prevent the ejection of a binary star companion
through chaotic variations in the excentricity of its orbit. This mechanism
is invoked by Mazeh et al. (1997) to explain the presence of the planet
around 16 Cyg B. High relative inclinations favor this mechanism, and
it can also be observed in the Solar system through the secular
perturbations introduced by Jupiter on asteroids (Kozai 1962).
Such a dynamical, chaotic
behavior could stabilize the orbit of a BD around
Cen B
beyond the maximum angular separation found by Wiegert & Holman (1997).
Recently, a hot Jupiter was detected around the primary star of the triple
system HD 188753 (Konacki et al. 2005).
The semi-major axis of the primary-secondary orbit is a = 12.3 AU,
only half of
Cen A-B (a = 23.7 AU).
Moreover, any angular separation can exist for a companion in
orbit around the
Cen pair. This is such a favorable target for
deep imaging of its environment that it stands out as an
important step in testing the results of these numerical simulations.
We have chosen to adapt our observation technique depending on the angular distance to the star.
Very close to the two stars, within a radius of about 20'', adaptive optics (subsequently AO) imaging allows us
to reach the highest sensitivity thanks to the concentration of the companion light within the Airy disk.
The contrast between the companion and the diffused light background is much more favorable
than for atmosphere-limited imaging.
At distances of more than 20'', the diffused light is less of a problem, and
classical (non-AO) imaging is the best solution.
Moreover, the degradation of the AO correction quality
at such large distances from the star would not bring a
significant improvement in the sensitivity.
We will present our wide-field imaging observations
of the environment of Cen in a forthcoming paper.
We thus observed the environment of Cen B using the
Nasmyth Adaptive Optics System (NAOS, Rousset et al. 2000;
Rousset et al. 2003) of the Very Large Telescope (VLT),
coupled to the CONICA infrared camera (Lenzen et al. 1998).
The combination of these two devices is abbreviated as NACO.
NAOS is equipped with a tip-tilt mirror and a deformable mirror controlled by
185 actuators, as well as two wavefront sensors: one for visible light and one for the infrared
domain. For our observations, we exclusively used the visible light wavefront
sensor. The detector is a
pixels ALADDIN InSb array.
As its name indicates, NACO is installed at the Nasmyth focus of the
Unit Telescope 4 (Yepun), the easternmost of the four 8 m telescopes of the VLT.
Our observations were obtained shortly after the recoating of the primary
mirror, which was executed in October 2003, in order to benefit from the best possible
uniformity in reflectivity.
This excellent state of the primary mirror coating allowed us to minimize the
PSF light leaks, and consequently to obtain the best sensitivity.
The NACO instrument offers two coronagraphic modes, based on a classical Lyot
coronagraph or an innovative four-quadrant phase mask (Rouan et al. 2000), but
due to the extreme brightness of
Cen, the rejection level was
insufficient for preventing the saturation of the detector.
As a consequence, we preferred to use the direct
imaging mode and keep the two stars outside the detector.
This was achieved simply by offsetting the NACO field of view.
The first series of observations were obtained between February 18 and
April 10, 2004. We obtained repeated short exposures of four fields arranged in
a cross around B and (accidentally) one field East of A using the S13 mode of CONICA
and JH broadband filters.
The pixel scale in this mode is
mas/pix (Masciadri et al. 2003),
giving a field of view of
.
This small scale results in an excellent sampling of the PSF, with
5 pix/PSF,
an important advantage when distinguishing the point-like sources
from the speckle cloud based on their dimension and shape.
For all fields, the AO reference star was
Cen B.
Table 3: Log of the first series of NACO images.
Table 4: Second series of NACO images.
Table 5: Third series of NACO images.
We repeated the same observations one year later in order to identify the proper-motion companions, using the
The processing was achieved using the IRAF package (v.2.12).
The images obtained on each night were dark-subtracted and flat-fielded with
standard infrared astronomical techniques. No sky subtraction was done, as the
inhomogeneous diffused light from Cen largely dominates
the sky background level, even in the
band (see Sect. 3.4).
We interpolated the bad pixels and mosaicked the dither pattern using
the bright sources visible in each field as references. The observed drift rate remains
below 30 mas h-1 for all fields (less than 2.5 pixels, or half the FWHM of the PSF).
This is a remarkable performance for such a large and massive instrument
and telescope configuration. For the northern field in the J band, no source
was detected in each individual frame, so the registration was achieved using the
offsets measured on the H and
band images. This procedure is
justified by the fact that the observed relative drifts were identical in the three bands.
In any case, no source was detected in the northern field in the combined J band image.
Bright artefacts ("ghosts'') are present in the images of the fields located to the west
of both stars. They are probably caused by reflections and interferences in the semi-reflective
beam splitters used to separate the visible light (used for wavefront sensing)
from the infrared.
The astrometric referencing of narrow-field NACO images of a
fast-moving source such as Cen (
4'' yr-1) is not a straightforward task.
It is made all the more difficult as all direct images of the pair are heavily saturated.
As a consequence, there are no astrometric reference stars sufficiently
close to the pair to attach the NACO images to a solid astrometric reference.
We thus based our absolute astrometric calibration on the computed
positions of the Cen B star using the initial Hipparcos
position at J1991.25 to determine the ICRS astrometric position of the
barycenter. Then the positions of
Cen B are
computed at any other epoch with the orbital elements
from Pourbaix (2000) and proper motion measurements
from the Hipparcos satellite (ESA 1997).
These positions take into
account the combined effect of the parallactic apparent
displacement, proper motion, and orbital motion of the pair. It
should be noted that, even though the resulting apparent
displacement is particularly complicated, the accuracy of the
available astrometric elements is such that they do not limit the
astrometric calibration of our images. As an illustration of the
complexity of the apparent motion of
Cen A and B,
Fig. 4 shows the ICRS positions of the two stars on
the sky for the period 1999-2010.
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Figure 1: Histogram of the number of NACO images as a function of DIMM seeing (in visible light, with seeing bins of 0.2''). |
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Figure 2:
Number of NACO epochs available for the field around ![]() ![]() |
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To transfer the reference coordinates of Cen B to the detected
background sources, we used as an intermediate step the HST-ACS coronagraphic
images publicly available from the ST-ECF archive facility. These images present
the advantage of simultaneously showing an attenuated image of the occulted
Cen B, as well as several background sources that are detectable
on our NACO images. We chose the brightest of these sources, which is also the nearest
to
Cen B, numbered 167 in our catalogue (Table 7).
This ACS image was obtained on 15 June 2004, for which date we computed the ICRS
position of B to be
We would like to stress here that the wcs used for all
our images is linked to the computed position of Cen B on 15 June 2004.
Any modification of the computed astrometric position
of B for this date can be transferred to the source catalogue using a simple
translation. Given the small size of the field, we expect
an absolute astrometric accuracy better than
0.10''from this very simple astrometric reduction.
However, the relative position accuracy of the different sources within the
same NACO field is much better, with an estimated
0.03'' (2 pixels).
The orientation of true north of the S13 camera of NACO was found to be extremely
stable and accurate by Chauvin et al. (2005), with an undetectable deviation of
less than 0.1
from the true north-south direction over a period
of more than one year (Nov. 2002-Mar. 2004). The uncertainty of the scale is
estimated to be less than 0.2%, giving at most one pixel over a
field of 30 arcsec.
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Figure 3:
Extract of an HST-ACS coronagraphic image of ![]() ![]() ![]() |
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Figure 4:
Mosaic of the observed NACO fields of the environment of ![]() ![]() ![]() |
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The main limitation to the sensitivity of imaging close to bright sources is caused by diffused light. It is mostly created inside the telescope and the instrument by imperfect optics and baffling. For the preparation of adaptive optics observations of bright sources, it is important to know the properties of the diffused light to prevent saturation of the detector.
To study its profile in our images,
we considered the field located south of Cen B,
thereby avoiding the contamination by the light from
Cen A,
which is particularly strong in the northern and eastern fields.
A difficulty in measuring the diffused light is that a number of artefacts create
local biases. For instance, the large spikes produced by the secondary spider and the
ghost reflections visible in the western images should not be included in the background
estimation. We thus sampled the background level manually to avoid these
artefacts.
The result was a series of
500 samples
in each band,
with
the angular distance from
Cen B and N the camera counts (in ADUs).
These measurements were then converted to magnitudes per squared arcsec
taking into account the exposure time (
s in J, 3.5 s in Hand
), the pixel size (
), and the photometric
zero point for the night (ZP
J=23.95, ZP
H=23.85, ZP
).
In order to obtain a calibrated model that can be applied to other
sources, we normalized the resulting magnitudes to a zero magnitude source using the
apparent magnitudes
(B) of
Cen B listed in Table 1.
The expression of the measured sky-backgound contrast (in mag arcsec-2) is therefore:
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(1) |
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(2) |
The greatest difficulty in extracting sources from the diffused light of
Cen is to separate the background inhomogeneities from the
true point-like sources.
We first high-pass-filtered the combined images
using the ring median filter of IRAF (Secker 1995).
By adjusting the ring radius precisely to the radius of the PSF,
it is possible to isolate the smooth, low spatial frequency diffused light
and remove it from the image.
This filtering allows a much more robust identification
of the point-like sources.
With only 252 sources in total, a visual identification was found to be more efficient
than an automated detection algorithm.
We used the
band images for this identification,
as all sources detected in J and H were also detected in this band.
The difficulty with automated source identification is to adapt the sensitivity
to the rapidly changing background level depending on the distance to the
star. The identification of the sources was thus achieved using the blinking of the
ring median-filtered versions of the combined NACO images obtained in the J, H,
and bands. The availability of images obtained through several filters is
a big advantage, as the fixed speckle cloud scales with the observation
wavelength.
We derived aperture photometry for the detected sources using IRAF.
We also attempted the PSF-fitting technique, but due to the large perturbations
of the PSF shape by high spatial-frequency speckles close to the two stars,
the result of the star subtraction was not satisfactory. We chose tight apertures
of 24, 12, and 10 pixels in diameter (0.32'', 0.16'', and 0.13''), respectively for the J, H,
and bands, in order to reduce our sensitivity to the background fluctuations.
By using such small apertures, we became more sensitive to the quality of the AO correction;
but thanks to the brightness of the AO reference source (
Cen B) and the generally good seeing,
the Strehl ratio was relatively stable over our observations.
The background level itself was estimated from the median flux of
a ring of 50, 30, and 20 pixels in diameter (respectively for J, H, and
)
and 10 pixels in thickness (in all cases).
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Figure 5:
Model fitting of the NACO diffuse background in the ![]() |
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Table 6:
Diffused light model parameters. We considered an exponential model
,
where
is the surface magnitude
per arcsec2 of the sky background at an angular distance
of a
zero magnitude source, and (a,b,c) are the adjusted model parameters.
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Figure 6:
Apparent magnitudes of the detected objects, as a function of their
minimum angular separation with ![]() |
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The definition of the sensitivity of our search for companions around a binary star like
Cen is more difficult than for a single star.
The presence of the combined diffused light from
Cen A
and B in the NACO fields complicates the estimation of limiting magnitudes,
as they become dependent on the position relative to the
two bright stars.
We thus preferred to take advantage of the significant number of detected
sources to derive a posteriori statistical properties and estimate the true
sensitivity of our imaging survey. As shown in Fig. 6,
the magnitudes of the faintest sources at angular distances larger than 10'' from A and B
are 20 in the J band and
21 in the H and
bands.
Note that several very faint and/or close-in sources, while they were clearly
detected in the images, could not be measured by aperture photometry.
To define our practical limiting magnitude,
we chose to consider the median magnitude of the detected sources.
This definition has the advantage of giving an empirical, statistically meaningful
definition of the sensitivity, which can be expressed as a function of the distance to the
two bright stars by computing the median within angular distance bins.
Figure 6 shows the median of the detected object magnitudes in the J, Hand
bands for four angular distance bins: 6-7'', 7-10'', 10-15'', and
15'' as solid
squares. The retained angular distance is the minimum of the source distances to A and B so as to account for the "saddle'' shape of the diffused light from the two stars.
In all bands, the limiting magnitudes at large angular distances are in the 18-20 range.
They decrease to
,
,
and
at 6''.
We can compare these sensitivities with previous AO studies of the environment
of bright stars.
Using the same NACO instrument,
Chauvin et al. (2005) obtained a depth of
around HIP 6856
(
)
using an exposure of
s.
With our typical exposures of
s, we are affected by additional
readout noise, but the longer total exposure time compensates for this loss.
On the bright single star Vega (
in all bands),
Metchev et al. (2003) used the PALAO system installed at the 5 m Hale
reflector at Mount Palomar in the J, H and
bands.
With limiting magnitudes of
at 20'' and
16 at 10'',
their study is slightly less sensitive than ours, but this can be explained by the smaller aperture.
Macintosh et al. (2003) observed the same star using
the Keck AO system and reach a deeper
at 20'', 18.5 at 10'',
and 17 at 7'', using a
s exposure. These figures are comparable
to our results, although Vega is fainter by about 0.5 mag than
Cen B in
the infrared.
From these comparisons, it appears that
NACO is a well-suited instrument for studying the environment of bright stars,
as its diffused light signature is relatively low (see also Sect. 3.4).
In addition, the structure of the fixed-pattern speckle halo
created by the monolithic primary mirror of the VLT-UT4 telescope
appears smoother than with the Keck telescope's segmented primary mirror,
thus making the identification of close companions easier.
As a complement to our NACO images,
we searched the ESO/ST-ECF archive for images of Cen.
We subsequently analyzed the available data, that were obtained
using three HST instruments: ACS, NICMOS and WFPC2. In this section,
we present briefly our results.
A series of images was obtained centered on Cen A star
in September 2003 using the Advanced Camera for Surveys (ACS) onboard the
Hubble Space Telescope, and these observations were repeated in January 2004
to check for the presence of proper motion companions.
The same repeated series of images were obtained
for
Cen B behind the coronagraphic mask in June 2004 and August 2004.
In each case, eight images were recorded at eight wavelengths
between
and 1024 nm, with the FR914M broad ramp filter
wheel (bandwidth of 9%).
Using the coronagraphic mode of this instrument,
the principle of the foreseen data analysis was to use the fact that the PSF of the
instrument changes homothetically with the wavelength to remove
most of the fixed-pattern speckle noise. As the position of the potential
companions does not depend on the wavelength, their signature can
be extracted more efficiently from the speckle noise than with a single image.
This method is a particular application of the spectral deconvolution technique
developed by Sparks & Ford (2002).
However, only one star of the Cen pair
at a time can be aligned with the coronagraphic spot. This results in a
considerable amount of diffused light from the other, non-masked star, which also
scales with the wavelength but with a different homothetic center.
The application of the spectral deconvolution method is also made
difficult by the slight undersampling of the PSF and by the availability
of narrow-band filters instead of the continuous spectral coverage
provided by a dispersive spectro-imaging instrument.
The pre-processing was achieved using the automated pipeline available
at the HST archive. The images were subsequently co-added and filtered
using the same procedure as the NACO images (Sect. 3.5).
An extract of the Cen B centered co-added coronagraphic image
is presented in Fig. 3.
Over a total field comparable to our NACO images, the number of
objects detected in the ACS images is less than 10% of the NACO catalogue,
corresponding to the brightest objects. We therefore limited our use of the ACS images to the definition of an accurate astrometric coordinate
system (see Sect. 3.3).
The HST-NICMOS instrument (Thompson et al. 1998) is
based on an infrared HgCdTe
pixel array sensitive over the
0.8-2.5
m range.
Two series of exposures were taken through four
filters on 19 October 1998 (
Cen B)
and 22 October 1998 (
Cen A). The star images were positioned on the detector
surface without a coronagraphic mask, producing heavy saturation within a
radius of 2-3 arcsec around each star.
The absence of a second observation makes it impossible
to ascertain the comoving nature of potential companions. The complexity
of the HST-NICMOS PSF (Krist et al. 1998) limits
the sensitivity close to the star. Being too distant
in time, there is no overlap between our NACO fields and these HST-NICMOS
data. For these reasons, we decided not to include the NICMOS data in
the present study.
The Wide Field and Planetary Camera 2 (WFPC2) is a two-dimensional imaging
photometer that covers the spectral range between 115 to 1050 nm. Several
accepted GTO and open time proposals, in particular by Ford et al. and Henry et al.
in HST Cycles 4 to 7 resulted in a large amount of collected data. A total of 11
images centered on Cen A were obtained in 1995 over two
epochs (around May and August) in the F547M, F555W, F814W, and F850LP filters.
In 1997, another series of 10 images was recorded, this time through the F953N
and F1042M filters. The same sequences were also obtained with the WFPC2
field centered on
Cen B. As for the NICMOS data, these observations
are too far in time from our NACO images, and there is almost no overlap between
the fields. Therefore, we preferred not to include them in this study.
Table 7 lists the positions and JH
magnitudes of all the sources detected in the NACO images of
Cen.
The right ascension
and declination
refer to the ICRS and are not corrected for
possible parallax. The epoch is J2004.5, corresponding to the mean
observation time for stars observed in successive frames. As
explained earlier, the typical positional uncertainty is not larger than 0.1 arcsec.
The relatively high surface density of the detected objects can be explained
by the fact that
Cen lies almost exactly in the Galactic plane
and in a direction close to the Galactic center. This catalogue fills part of a
long-standing "hole'' in sky atlases, due to the diffused light from
Cen.
The very fast proper motion of Cen should allow its
comoving companions to be identified quickly.
However, this is also a drawback due to the particularly
dense star field around this binary star.
The identification of the companion is not a trivial task
because of the combination of the unknown orbital motion
of the putative companion with the large proper motion and parallactic
displacement of
Cen. Considering that
Cen moves an average
of approximately one NACO pixel per day, the best strategy would to observe the fields
repeatedly with a time separation of 2 to 3 weeks. Unfortunately, due to scheduling
constraints, our observations could not follow this scheme, and our first and second
epochs were separated by about 10 months. The second and third series were
separated by 5 months. Over these durations, the displacement of
Cen was
considerable, resulting in a rather poor overlap of the different fields. Moreover, the
diffused light from the two stars resulted in a moving zone of decreased sensitivity
over part of the field.
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Figure 7:
Maximum mass of possible companions to ![]() |
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In order to systematically search for statistically significant proper-motion companions, we applied the following procedure:
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(3) |
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(4) |
Massive substellar objects, as opposed to terrestrial planets, are detectable at very large
distances from their parent star, as their magnitude is set by their intrinsic emission
rather than by the reflected light.
The age of the Cen system is 5 Gyr, as determined
by Thévenin et al. (2002) and confirmed by the interferometric diameters of the
two stars (Kervella et al. 2003).
Assuming a mass of 30
,
a 5 Gyr-old giant planet has absolute
magnitudes of
MH = 18 and
MK = 20, from evolutionary models by Baraffe et et al. (2003).
At the distance of
Cen (1.3 pc), this translates into apparent
magnitudes
and mK = 18, which were within reach of our NACO
imaging search down to an angular separation of
5'' from
Cen B (Fig. 6).
Figure 7 gives the limiting sensitivity of our search in terms of
companion mass, based on model magnitudes in the K band from
Baraffe et et al. (2003), and the
median magnitudes
given in Fig. 6 (bottom). These are conservative estimates,
considering that many sources that are fainter by up to two magnitudes
have been detected in our images.
Murdoch et al. (1993) searched Cen A and B
for the radial velocity signature of BD companions with orbital
periods
days, but with a negative result.
Using improved measurements obtained over a period of 5.5 years,
Endl et al. (2001) concluded that no planet more massive than a few
Jupiter masses, in projected
value, is orbiting either
Cen A or B within 4 AU.
If we follow the conclusions of Hale (1994) that the equatorial planes
of A and B are probably coplanar with the binary orbit plane, and if we accept the hypothesis that
exoplanets orbit in the equatorial plane of their parent star, then this projected mass
value becomes a solid mass limit. The J band search with the
HST by Schroeder et al. (2000), which did not detect any companion,
was limited to a sensitivity of
mJ = 16, corresponding to 40
.
Note however that the third star of the
Cen system Proxima probably does not
host giant planets. This and other low mass stars were extensively scrutinized
for any radial velocity variation, but did not show any (Kürster et al. 2003).
Moreover, speckle-interferometry and imaging surveys (Leinert et al. 1997;
Oppenheimer et al. 2001) failed to identify
companions down to the BD masses around several low mass stars.
Within our sensitivity and coverage limitations, our negative result leads toward
the modeling results of Wiegert & Holman (1997), who
conclude that stable companion orbits may not exist beyond about 3 AU
from each component of the
Cen pair.
We have obtained deep adaptive-optics images of the close environment
of Cen A and B. From these images, we did not identify any
comoving companion, but we assembled a catalogue of 252 faint background
objects. Within the explored area, this negative result sets an upper
mass limit of 20-30
to the possible substellar companions orbiting
Cen B.
If companions of
Cen B exist, they are likely to orbit close to the star (within 5 AU)
and to be less massive than a few times Jupiter (from radial velocity surveys). They could
also be fainter than the current imaging search-limiting magnitude, but from Baraffe et al. (2003),
a 5 Gyr-old, intermediate-mass exoplanet (5
)
around
Cen has an apparent magnitude
of
,
and
,
fully out of reach of the deepest imaging searches.
Stars younger than
Cen could be more favorable targets, as the brightness of massive
exoplanets is predicted to decrease steeply with time.
However, the faintness of the detected background sources confirms the capabilities of
modern adaptive optics instruments like NACO for exploring the close environment of
very bright stars and for searching for massive exoplanets.
Acknowledgements
Based on observations made with the ESO Telescopes at Cerro Paranal Observatory under the Director's Discretionary Time (DDT) programs 272.C-5010, 273.C-5041 and 275.C-5027. We are grateful to ESO's Director General Dr. C. Cesarsky for this generous allocation of telescope time. We also wish to thank F. Namouni for fruitful discussions. This work made use of observations obtained with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555. This research made use of the SIMBAD and VIZIER databases at the CDS, Strasbourg (France), and of NASA's Astrophysics Data System Bibliographic Services.
Table 7:
Position and photometry of the sources detected
around Cen. The coordinates are for J2004.5 and
refer to the ICRS.