Issue |
A&A
Volume 515, June 2010
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|
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Article Number | A17 | |
Number of page(s) | 12 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361/200913737 | |
Published online | 01 June 2010 |
NACO-SDI imaging of known companion host stars from the AAPS and Keck planet search surveys
J. S. Jenkins1 - H. R. A. Jones2 - B. Biller3 - S. J. O'Toole4 - D. J. Pinfield2 - L. Close5 - C. G. Tinney6 - R. P. Butler7 - R. Wittenmyer6 - B. Carter8 - A. C. Day-Jones1
1 - Department of Astronomy, Universidad de Chile, Casilla 36-D, Santiago, Chile
2 - Center for Astrophysics, University of Hertfordshire, College Lane Campus, Hatfield, Hertfordshire, UK, AL10 9AB, UK
3 - Institute of Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
4 - Anglo-Australian Observatory, PO Box 296, Epping 1710, Australia
5 - Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
6 - Department of Astrophysics & Optics, University of New South Wales, NSW 2052, Australia
7 - Carnegie Institute of Washington, Department of Terrestrial
Magnetism, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA
8 - Faculty of Sciences, University of Southern Queensland, Toowoomba, 4350, Australia
Received 25 November 2009 / Accepted 2 March 2010
Abstract
Context. Direct imaging of brown dwarfs as companions to
solar-type stars can provide a wealth of well-constrained data to
``benchmark'' the physics of such objects, since quantities like
metallicity and age can be determined from their well-studied
primaries.
Aims. We present results from an adaptive optics imaging program
on stars drawn from the Anglo-Australian and Keck Planet Search
projects, with the aim of directly imaging known cool companions.
Methods. Simulations have modeled the expected contrast ratios
and separations of known companions using estimates of orbital
parameters available from current radial-velocity data and then a
selection of the best case objects were followed-up with high contrast
imaging to attempt to directly image these companions.
Results. These simulations suggest that only a very small number
of radial-velocity detected exoplanets with consistent velocity fits
and age estimates could potentially be directly imaged using the VLT's
Simultaneous Differential Imaging system and only under favorable
conditions. We also present detectability confidence limits from the
radial-velocity data sets and show how these can be used to gain a
better understanding of these systems when combined with the imaging
data. For HD32778 and HD91204 the detectabilities help little in
constraining the companion and hence almost all our knowledge is drawn
from the SDI images. Therefore, we can say that these stars do not host
cool methane objects, out to on-sky separations of 2'',
with contrasts less than 10-11 mag. However, for HD25874, HD120780
and HD145825, the contrasts and detectabilities can rule out a number
of possible solutions, particularly at low angular separations, and for
the best case, down to strong methane masses of 40
at 1'' separation. The contrast curves constructed for these five stars show 5
contrasts (
F1) of
9.2-11.5 mag at separations of
0.6'', which correspond to contrasts of
9.7-12.0 mag for companions of mid-T spectral type. Such limits allow us to reach down to 40
around
fairly old field dwarfs that typically constitute high precision
radial-velocity programs. Finally, the analysis performed here can
serve as a template for future projects that will employ extreme-AO
systems to directly image planets already indirectly discovered by the
radial-velocity method.
Key words: binaries: general - binaries: spectroscopic - planetary systems - stars: imaging - technique: radial velocities - techniques: high angular resolution
1 Introduction
The detection of over 400 planets orbiting Sun-like stars has revolutionised our knowledge of our local neighbourhood
and our position therein. Yet planets are not the sole close companions to solar-type stars. For instance,
Duquennoy & Mayor (1991b,a) and Duquennoy et al. (1992)
have examined stellar multiplicity in a series of papers.
Radial-velocity surveys have revealed few brown dwarfs orbiting
solar-type stars (e.g. Wittenmyer et al. 2009; Jenkins et al. 2009) leading to the phrase ``brown dwarf desert'' being
coined to describe this paucity (Marcy & Butler 2000). However, beyond 4 AU
one would expect few radial-velocity planetary or brown dwarf
companions to be known due to the limited temporal coverage at the
required precision levels necessary to fully sample such companions. In
addition, radial-velocity surveys also have strong biases against the
detection of long-period companions, as the radial-velocity amplitude
is a strong function of orbital period and also since this technique
requires the observation of at least half an orbit (e.g. Wright et al. 2007) to constrain companion properties. Only now are we sensitive enough
to detect solar system-like gas giant planets in solar system-like orbits (e.g. Jones et al. 2010).
Conversely, direct and coronographic imaging techniques can probe much
wider separations than current radial-velocity programs can reach. For
example, Kalas et al. (2008) and Marois et al. (2008)
have directly imaged planetary mass companions to the stars Fomalhaut
and HR 8799, located at angular separations of 14.9'' and 1.73'',
or 115 AU and 68 AU, respectively. McCarthy & Zuckerman (2004) found another deficit of brown dwarf companions between 75-1200 AU. Liu et al. (2002)
used the Gemini-North
and Keck Adaptive Optics (AO) systems to obtain three epochs of images
of the brown dwarf companion to HR 7672, which had initially been
detected by its radial-velocity signature. The flux ratio at 2.16 m
was found to be 8.6 mag at a separation of 0.79''. This level of
contrast pushed the instrumentation used in this detection to its very
limits. However the introduction of Simultaneous
Differential Imaging (SDI) on the VLT's NACO facility permits the
achievement of higher contrasts, at smaller separations, for the
coolest stellar companions. For example, contrasts on the order of
have been demonstrated at
0.5'' by Mugrauer & Neuhäuser (2005) and Biller et al. (2007).
2 Target selection
In order to guide the selection of target host stars for adaptive optics imaging of brown dwarfs and exoplanets, we have performed simulations which take the best currently available companion parameters from radial-velocity data sets, combined with host-star age estimates and brown dwarf and exoplanetary interior models, to derive predicted magnitude differences and angular separations on sky. These simulations were performed for all stars in the Anglo-Australian and Keck Planet Searches (for samples see Jones et al. 2002; Marcy et al. 2005; Butler et al. 2006 and references therein), which show a long term radial-velocity profile consistent with an orbiting low-mass companion.
2.1 Angular separation
Hipparcos distance data (van Leeuwen & Fantino 2005) is available for all these objects (which all lie at distances of less than 100 pc). It should be noted that in most cases, the radial-velocity orbital solutions are not well constrained. This is largely because the companion orbits are much longer than the monitoring baselines of the surveys, and in some cases because the companion properties have been derived with no inflection in the radial-velocity curve (often referred to in the planet searches as a ``liner''). The fits to both these classes of data produce only semi-major axis lower limits. In addition, the eccentricities of most of the companions are so poorly constrained that they are fixed to zero, causing further separation ambiguity.
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Figure 1: The solid lines are Burrows et al. (1997) models for different masses and ages. The dashed lines represent a range of masses and ages for the NextGen models of Allard et al. (1997), which are populated by our high-mass targets. The dot-dashed line marks the methane absorption boundary in cool dwarfs and provides a useful upper limit for SDI targets. The discontinuity between the models is the L to T spectral type boundary region, which is not very well modeled, thus the Burrows et al. models were truncated. |
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2.2 Contrast ratio
Infrared photometry for the primary stars were taken from the 2MASS catalogue (http://irsa.-ipac.caltech.edu/) and when combined with Hipparcos
distances we were able to generate accurate absolute
magnitudes. The
from 2MASS was converted to K using the magnitude corrections in Carpenter (2001). Absolute
magnitudes for the companions were estimated using the non-grey evolutionary tracks of Burrows et al. (1997),
the COND models (Baraffe et al. 2003) and the NextGen models of Allard et al. (1997). The masses of these simulated candidate companions were taken from the radial-velocity data and range
from 1-725
.
Ages for the systems were taken from Valenti & Fischer (2005) and Takeda et al. (2007), which limits the size of our sample but retains a high level of internal consistency.
We split the companions into two groups depending on which model
we could use to generate robust magnitudes. The split was made on the basis of companion T
values, with all the companions using the Burrows et al.
models having a
K (which for these older systems corresponds to a
)
and all other companions using the NextGen models. A custom spline
fitting procedure obtained the bolometric luminosities for all the companions (see Fig. 1).
The separation in effective temperature between the low temperature
models of Burrows et al. and Baraffe et al. and the higher
temperature models of Allard et al. is clearly apparent. Taking
the whole Butler et al. (2006) catalogue
there are 156 companions below the strong methane absorption
boundary (1200 K) (shown by the dot-dashed
vertical line on the plot), apart from their low luminosity we expect
them to be good SDI targets and a small number of these may well be
detectable. There are also 23 companions with T
values above 1200 K, 22 of which have
K. These should be observable with conventional AO methods.
![]() |
Figure 2:
A subset of our simulated candidate companions, including those imaged in this paper, taken from
Butler et al. truncated by |
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The major problem faced when attempting to image close-in companions to
bright stars, is the contrast difference. To determine the contrasts
between the
stars and their companions we had to obtain the JHK magnitudes of the companions, both from internal and reflected flux. To accomplish this the median colors and
bolometric corrections (BC) from Leggett et al. (2002) (BC
; BC
,
the subscripts denote spectral type) were employed. This correction gave the
absolute K magnitudes and, using the colors (
), we were able to generate their expected absolute H-band magnitudes. For all
planetary-mass companions Jupiter characteristics were assumed. We then
simplistically simulated the expected reflected flux in the H-band and added this component to the internal flux.
This was done by using simple geometry and assuming Jupiter's radius and albedo. The total absolute magnitudes
of the companions were then subtracted from the absolute magnitudes derived for the primaries giving the estimated contrast
.
Figure 2 shows
the contrast and separation estimates from the simulation for all the
radial-velocity companions included in this test. The simulated
companions represented by filled circles would all have strong methane
absorption. The bulk of the objects lie within 0.3'' of the primaries
due to the bias introduced by the short radial-velocity baseline. The
majority of these lower-mass companions (
)
also have high contrasts (
), putting them below the 5
NACO-SDI threshold. However, two companions have larger separations
>0.3'', approaching the separations of the already discovered objects of Marois et al. (2008), McCaughrean (2003), Chauvin et al. (2004) and Neuhäuser et al. (2005) and, combined with a
,
they could be amenable to SDI imaging. Note that another secure AO detection is Fomalhaut b but this is located far off the plot scale with an angular separation of
14.9''.
The NACO-SDI (Mugrauer & Neuhäuser 2005) and NICI (http://www.gemini.edu/sciops/instruments/nici)
sensitivities are highlighted in Fig. 2 by dotted and dashed lines respectively. Note that we can not be sure if the Mugrauer & Neuhauser detectability limits
are actual 5
limits
or some lower threshold limit. Once the masses and semimajor axes are
more precisely defined, the companion magnitudes and separations will
most likely increase giving lower contrasts
and more viable targets. This has been highlighted on the plot by the
error bars which represent the direction
in which all companions are expected to move once inclination and
eccentricity effects are considered and more RV data points acquired.
Another major source of uncertainty is age. For example, a typical 1
age uncertainty for these types of objects is
2 Gyr, which translates to a
2 mag error in
with the primary. Due to the high contrast ratios and extremely small
separations the majority of these companions are out of reach of
current instruments. However, future Extreme-AO systems which are
proposing to reach >15 mag of contrast may be able to bridge
this gap.
All companions with
K (triangles in Fig. 2 are taken from Nidever et al. 2002 and Jenkins et al. 2010) have H-band magnitudes less than 15, allowing direct imaging using normal AO
techniques. Four of these objects have separations larger than 0.35'' and
less than 8, making excellent coronographic targets. All planetary-mass companions are off the Fig. 2 plot scale since they have much larger H-band contrasts. The star in this figure
shows the position of a 6
planet in a Jupiter-like orbit as a companion to a 0.2 Gyr,
K0 star at 5 pc. The age and spectral type were chosen since
they relate to the best case scenario for one of our
objects HD120780. It shows that by adopting the lower limit to the
large errors on the age of this system that the potential exists to
detect planetary-mass objects around such stars. Even still, these
types of objects reside
extremely close to the plotted instrument thresholds, highlighting just
how difficult it is to obtain a direct image of any planetary-mass
object with the current suite
of instruments available. However, radial-velocity studies have
revealed a high number (
28% of planet hosting stars) of multiple planet systems (Wright et al. 2009),
therefore imaging planet-host stars can provide useful constraints on
any longer period, massive companions not yet revealed in the
radial-velocity dataset (e.g. Mugrauer et al. 2006; Mugrauer et al. 2007).
3 Candidate characteristics
All radial-velocity data in this section were generated using the AAPS and Keck pipelines. These pipelines are still undergoing development following the procedures and techniques described in Marcy & Butler (1992) and Butler et al. (2006,1996,2001). The Keplerian fits shown in Figs. 3-7 are performed using the Systemic algorithm (Meschiari et al. 2009), however we note that most are not very well constrained using the current radial-velocity data. Table 1 lists some relevant information for each object relating to both the radial-velocity and photometric analysis in this work. The parameters and their analysis methods can be found in van Leeuwen & Fantino (2005), Henry et al. (1996), Valenti & Fischer (2005), Wright (2005), Jenkins et al. (2006), Takeda et al. (2007) and Jenkins et al. (2008). Tables 2-6 list all radial-velocity data.
3.1 HD25874
![]() |
Figure 3:
The radial-velocity measurements for the star HD25874. The minimum-period Keplerian fit to the data yields a period of |
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Figure 4:
The best-fit Keplerian orbit to the radial-velocity measurements for
the star HD32778, taken by the Keck Planet Search. The fitted orbital
period is 6.5 years, with a circular eccentricity and a companion
minimum mass of |
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![]() |
Figure 5:
The orbital solutions for the radial-velocity dataset from the Keck
Planet Search for HD91204. All 10 data points are spread
across a period of 5 years and the best
estimate of the fit has an amplitude of |
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3.2 HD120780
The AAPS has obtained 11 radial-velocity data points over a period of 4.4 years (Fig. 3 data taken from Jenkins et al. 2010). The minimum best-fit Keplerian orbit to
this data has an amplitude of >1000 m s-1 relating to a companion period of 6.5 years, eccentricity 0.43 and a minimum mass of 66
.
However, the curvature of the fit has been generated by the algorithm
itself as within the uncertainties all the data points lie in a
straight line, known as a liner. Therefore, the orbital
solutions to this data series are lower limits. For comparison the
best-fit Keplerian with twice the orbital period would relate to a
companion minimum mass of
190
and similar
2
of 2.5. From experience we estimate the lower limit of the period
of the orbit to be around four times larger than
currently estimated. If we take the period range 6.4-25.8 years
and the Hipparcos distance of 25.91 pc, the projected separation
will be in the range 0.13-0.34''. The absolute H-band magnitude is 3.20 mag and our estimation for the absolute H-band magnitude of the companion using
the technique in Sect. 3.1 is 15.31 mag, giving a best estimate for the contrast ratio upper limit of 12.11 mag.
![]() |
Figure 6:
The radial-velocity dataset and fits for the companion to the star
HD120780. The minimum-period solution to the large trend is a companion
with a period of |
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Figure 7:
The radial-velocity dataset from the AAPS for the star HD145825.
The best-fit Keplerian orbit to the 14 data points are shown,
along with the estimated orbital solutions. Curvature can clearly be
seen in this data, and since this is not a liner the orbital solution
is better constrained than all the other four stars. The
measured semi-amplitude of 617 m s-1 indicates
the presence of a companion with a period of 7.8 years, and
eccentricity of 0.2 and a minimum mass of 44.5
|
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3.3 HD32778
Five radial-velocities over a period of 2.25 years for this object (Fig. 4) and the best-fit Keplerian orbit has a semi-amplitude of 750 m s-1. This is consistent with a companion with a minimum mass of
55
,
a period of 6.5 years and a circular eccentricity. Even though
this is not a liner the limited amount of data points and temporal
coverage means this is not very well constrained. The small amount of
curvature does help better constrain the orbit since if we look at the
best-fit for twice the orbital period we quote here, we find a minimum
mass of 137
but with a very high
2 of 186, showing such large orbits are difficult to fit well.
At a distance of 22.48 pc the estimated angular separation for this companion is 0.16''. The absolute H-band magnitude of this star is 3.71 and with an estimated maximum absolute H of 15.59 using the current fit and the lower age limit, the contrast would be
12 mag.
Table 1: Parameters for all stars in this work.
3.4 HD91204
Figure 5 shows the orbital fit to the large trend in the radial-velocity dataset and again it appears to be a liner. The estimate to
this data has an amplitude of 1000 m s-1, a period of 18 years and circular eccentricity. The minimum mass of the companion to this fit is
50
but
yet again this is a liner fit to the data and therefore the expected
orbital period will be underestimated. Again, twice the orbital period
would give rise to a companion with a minimum mass of 110
but with a fairly high
2 of 15. From the estimated orbital period above of
18 years,
and the distance to the star of 51.55 pc, we expect the lower limit on the separation to be
0.13''. The absolute H-band magnitude for HD91204
is 2.83 mag with the estimated absolute magnitude for the companion of
16.05 mag, relating to an upper limit for the contrast ratio of 13.22 mag at the given age of the system.
Table 2: HD25874 radial-velocity data.
Table 3: HD32778 radial-velocity data.
Table 4: HD91204 radial-velocity data.
Table 5: HD120780 radial-velocity data.
Table 6: HD145825 radial-velocity data.
Eight data points have been acquired over a period of 3 years and a linear fit has been plotted through the data (Fig. 6). The best minimum estimate to
the orbital solution gives an orbital period for the companion of
12 years and assuming a circular orbit, a minimum mass of
70
.
In this case the comparison
minimum mass for twice the orbital period is 145
,
with a
2 of 18, much lower than the best minimum estimate. In fact, searching the parameter space freely for
the best single companion solution to this data returns a companion with a minimum mass of 1.1
and
orbital period of over 56 years. Clearly such a companion would
manifest itself in the stellar spectra and this is not the case, which
could indicate the need for a double companion solution for this star.
However, taking the 12 year period we get a semimajor axis of
5.24 AU and at a distance of 17.01 pc, the angular separation would be
0.31''. The absolute H-band magnitude of the star is 4.3 mag and
with an estimated lower limit to the absolute H for the companion of
9.88 mag,
the estimated upper limit to the contrast ratio is 5.58 mag. This
represents the lowest contrast estimate for the five objects and arises
due to the extremely small lower age estimate of 0.2 Gyr from Valenti & Fischer (2005). Note however that this age is unconstrained
as the upper age estimate reaches as high as the age of the universe i.e. 13.4 Gyr.
3.5 HD145825
Figure 7 shows the 14 radial-velocity measurements made by the AAPS over a period of 5 years.
This allows a constrained fit to the points as the fitting algorithm
has one inflection and significant secondary inflection curvature to
describe. The best-fit Keplerian orbit finds solutions that best
reproduce the observed curve, with a semi-amplitude of 617 m s-1, described by a companion with
an orbital period of 7.8 years, an eccentricity of 0.2 and a minimum mass of 44.5
.
Since a significant percentage of the orbit has been observed no comparison fit is necessary
as this fit describes the data very well. The semimajor axis of this orbit is 4.07 AU and at a distance
of 21.55 pc the apparent separation of the companion would be
0.19''.
However, even though the solutions are better constrained than the
liner plots, they are still lower limits as only one inflection is
securely found and it is likely that, at best, only 2/3rd's of the
orbit has been mapped. The star's absolute H magnitude is 3.39 mag
and with the best estimates of the companion's absolute H set at
14.2 mag, the upper limit on the contrast ratio is estimated to be
10.8 mag.
4 NACO imaging
4.1 Observations and reduction
The observations of each of the stars chosen as primary
candidates were carried out on 02 March 2006 using the
NACO-SDI instrument mounted on the 8m ESO VLT4-Yepun telescope in
Paranal, Chile.
The average seeing throughout the observing night was 0.8''.
The NACO AO system is described in detail in Rousset et al. (2003).
Since all targets in this project are very bright (V < 8) the
star itself was chosen as the guide.
The SDI system employs a double calcite Wollaston prism to split the
incoming beam into four separate beams and then feeds them through a quad CH4 filter that is set in the focal plane. The filters are set at central wavelengths of 1.575 m (F1), 1.600
m (F2) and 1.625
m (F3a and F3b) and with bandpasses of 0.025
m, which helps to limit residuals due to speckles and calcite chromatics. The differential non-common path errors are less than 10 nm rms per Zernicke mode between the beams (Lenzen et al. 2004). In this configuration the telescope has a projected field of view of 5 arcsec square, reduced to
arcsec after the
tilt of 133
from the SDI focal plane mask during this run is considered, and has a camera plate scale
of 0.017'' pixel-1.
The observations were performed using an 8-point jitter pattern. One of the jitter frames was a pure sky-frame to better aid in sky background removal. Each jitter integration (Detector Integration Time aka DIT) ranged from 2-6 s depending on the brightness of the central PSF. Each DIT was determined by increasing the time until the central few pixels of the star were saturated, allowing very high S/N in the halo of the PSF. However, we believe that we could further increase this S/N by increasing the DIT's and saturating more of the stellar PSF, since we will gain a higher dynamic range. Each observation took around 60 min to complete, consisting of 44 jitter cycles per star.
The reduction of all the NACO-SDI raw data made use of the custom pipeline of Biller et al. (2007,2004). First, all the raw frames were cleaned for any background sky noise by subtracting out the sky image from the jitter cycle. A standard flat-field is then applied by combining the flat images into a master flat and dividing out the pixel-to-pixel variations from each individual image frame. To further clean the image a bad pixel map is created from the jittered images to flag any dead pixels on the CCD chip and these are removed from all image frames. Apertures are then extracted around each filtered image and the Airy pattern and flux is scaled. The frames are then unsharp masked by dividing through by a heavily smoothed version of the original image. A shift and subtract algorithm is used to align the jittered images, with the first frame in the series used as the reference image and all other images aligned to this first image.
5 Data analysis
Once all data frames have been fully reduced the next step is to setup the analysis procedure. This was done by adding and subtracting the various narrow-band filters across the methane feature to provide the best conditions for detecting the faint companions. The two combinations that provide the best contrasts and S/N ratios to detect cool dwarfs target both M, L-type and T, Y-type objects. The SDI instrument, by its pure design, is built to search for companions of mid-T spectral type or later, since the subtraction across the methane band suppresses the starlight and speckle pattern to highlight objects with strong methane absorption in their atmospheres. However it can also be used to search for L-dwarfs by combining all the filters to create a broadband image that would detect any L-dwarf signature. This, however, is heavily limited by the bright star and the bright super-speckles in the image. We employed both these approaches to search for faint companions around the five stars in this project.
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Figure 8:
A combined plot showing all the contrast curves ( |
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Before discussing each system individually, Fig. 8
shows the SDI reduced contrasts for all the systems on the same plot
and scale. The key in the upper right of the plot indicates which curve
represents which system. Since the SDI reduced curve represents the
limiting 5
contrast for each system this plot highlights how deep
the observations reach over the parameter space sampled. The contrast curves were estimated by defining a
square
pixel box, placing the box at the center of the images and then
calculating the standard deviation within the box whilst moving it
outwards from the center, pixel-by-pixel, in 8 different
directions, separated by 45
angles. Averaging these gives a measure of the sky background noise. To
measure the contrast ratio, these counts were divided by the peak flux
estimate for each star. Peak flux was estimated from the unsaturated
acquisition image, scaling to the appropriate exposure time for the
saturated data images. For two of these curves however, representing
the stars HD32778 (short dashed line) and HD120780 (three dot-single
dashed line), there are no unsaturated acquisition images and hence
their flux counts were estimated using the data from the other stars
with unsaturated images. The 2MASS H magnitude for each was
plotted against peak flux in the acquisition images. A polynomial was then fit to this data and by taking the H magnitude for the saturated stars we could calculate
their expected peak counts. This returns an estimate and was only employed since the observational methodology was
similar. There is a large spread in contrast limit in this data with a difference of
2.2 mag
between HD32778 and HD25874 (dotted line), with HD32778 being the least
sensitive (max contrast of 9.3 mag) and HD25874 the most sensitive
systems (max contrast of 11.5 mag). For all systems the curves are
flat beyond 0.7'' and remain so out to the edges of the scale. No
additional companions with larger separations than those indicated in
the radial-velocity datasets were found in any of these systems.
5.1 M,L-Dwarfs
![]() |
Figure 9: A broadband image of the star HD25874. The image was constructed by combining all images through the three separate filter bandpasses. Here a number of super-speckles can clearly be seen scattered across the image, the brightest of them reside close to the central star. All the possible cool dwarf companions visible in this image are ruled out as speckles when comparing the rotated images. |
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As mentioned above to extract the signal from companions that don't
exhibit strong methane in their atmospheres the best SDI approach is to
combine all the narrow-band filters employed by the SDI device and
create a broadband image that will increase the S/N of any cool
companion. This method was applied to the data
frames from all five of the target stars. Note that a targeted search
for such objects that don't exhibit strong methane absorption in their
atmospheres would benefit significantly from utilising a standard
broadband AO approach, since with the narrowband SDI filters there is a loss of efficiency and hence a loss of contrast for a given total integration time.
To help with the detection of any signals and to decipher them from residual stellar speckles all images were taken at two
different rotation angles, separated by 33
on the sky. By blinking these rotated broadband images it is possible
to spot any detection that appears at
different positions in the rotated images. Any super-speckles should
remain in the same place throughout the roll angle, therefore a real
detection can be picked
out in a field with a number of residual speckles. Figure 9 is an example of one of these broadband images for the star HD25874.
From the estimates of the companion minimum masses generated from the orbital fits we might expect three of these companions to reside in the L-dwarf regime and considering the limited data they may also have M dwarf masses. However the estimates of the contrast ratios are high and as the contrast that can be reached by using a broadband image is significantly lower than the subtracted images, it would prove extremely difficult to detect any of these L-dwarfs using this method. Indeed, the search employed for L-dwarfs around all five of these stars turned up a number of bright objects, these can be seen as the spots spread across the image in Fig. 9. However, none of these detections fulfilled the rotation roll angle criteria, indicating they are residual speckles. It is clear that without looking for roll angle modulation it is extremely difficult to differentiate between a speckle and a real object.
If the companions to these stars are L-type objects and the estimated separations from the radial-velocity curves are 0.13'' (HD25874), 0.16'' (HD32778), 0.13'' (HD91204), 0.31'' (HD120780) and 0.19'' (HD145825) then they must not have contrasts less than 6.7, 7.4, 7.0, 7.9 and 7.8 mag respectively (taken from the conventional AO (dot-dashed) curves in Figs. 10, 13 and 15). Note that the right-hand y-axes in these figures show the estimated masses at the age of these systems relating to contrast limits for methane objects only and this will be explained in Sect. 5.2. At the estimated separations the mass limits for L-dwarfs are 87, 77, 92, 72 and 72 Jupiter-masses respectively. No L-type objects with the given ages and separations can have masses above these limits. Also shown in these plots represented by the solid curves are the radial-velocity sensitivities. These confidence limits from the radial-velocity data sets are explained below and the conclusions drawn are presented there.
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Figure 10:
The 5 |
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5.2 T or Y-Dwarfs and Exoplanets
The search for T and Y-dwarfs, along with planetary mass objects,
cooler than 1200 K is accomplished by subtracting the images
inside and outside of the methane band at 1.62 m.
The filters employed for this task are the F1 filter centered at 1.575
m and the F3a filter centered at 1.625
m (Close et al. 2005). This subtraction, which we assign
F1,
helps to suppress the Airy rings from the stellar PSF, and attenuates
the speckle pattern that is a function of the stellar beam. As there is
no strong absorption at 1.62
m
in the stellar atmosphere, the subtraction removes the stellar
light leaving behind the faint methane signature from the cool
companion. Similar to the search for M, L-type objects the 33
roll angle allows one to distinguish between a genuine cool companion signal and any
residual speckles.
![]() |
Figure 11: The doppler sensitivity confidence limits in mass-angular separation space for the stars HD32778 ( top panel), HD91204 ( middle panel) and HD145825 ( bottom panel). These confidence limits have been integrated over all eccentricities and the percentage limit is shown inside each curve. The gray scale highlights the changing spacial confidence limits with dark the least constrained through to the most constrained regions being lightest. Only HD145825 has fairly well constrained doppler data out to reasonable NACO-SDI sensitive separations. Whereas for HD32778 there is no real constrained parameters below the hydrogen burning limit. |
Open with DEXTER |
For each of the five stars we performed this search on, two candidates
fulfilled the roll angle criteria, found around the stars HD25874 and
HD120780, both of which are discussed in the next section. For the
three other stars no significant companions are seen in the subtracted
images across the roll angle. The SDI reduced contrast limits in
Fig. 10
(dashed line) for the stars HD32778, HD91204 and HD145825 tell us that if the radial-velocity companions that
were targeted were T or Y-dwarfs, they can not have absolute H magnitudes <11.1, 11.5 and 13.2 mag respectively, at the estimated separations for these objects. All H,
and mass estimates are assuming T5-type status, including the right-hand y-axes,
which show the expected masses for the corresponding contrast ratios at
the estimated age of the star. We chose a T5 simply because it
represents the middle of the T-dwarf regime and since these objects are
expected to be old we might expect them to reside somewhere close to
this spectral type. A reference such as a T dwarf was necessary
due to the unique nature of the SDI observations, since they utilize
narrow-band filters across a strong absorption feature and objects with
more or less methane absorption with have more or less flux entering
the F3a filter.
Since T5 objects possess a strong methane break they provide good
references for comparison in our contrast curves. A correction must be
made to compare with model absolute H-band magnitudes and we simply apply an offset of +0.5 mag for a typical T5 dwarf (Biller et al. 2007).
5.3 Doppler sensitivities
We have performed a sensitivity analysis of each individual system to
determine the confidence limits from the radial-velocity datasets and
how these limits transfer into the contrast curve parameter space. The
sensitivities were determined using the method explained in O'Toole et al. (2009)
with a small modification. In brief, simulations of radial-velocity
curves are analysed using a two-dimensional Lomb-Scargle periodogram.
The simulations covered the mass range 3-110
,
period range 300-4800 d and eccentricities from
0.0 to 0.8 in steps of 0.1. Jitter values for each star, based on the method of Wright (2005),
were incorporated into the simulations. The resulting fits were checked
against five detection criteria: the four described in O'Toole
et al. and a fifth based on the rms of the
data, described here. If the measured period is 50% longer than the
time baseline of the dataset, a linear fit to the dataset is performed.
This fit is then subtracted from the data and the rms of the residuals
are compared to the rms before the linear trend was subtracted. A
detection is made if the rms drops by a factor two or more throughout
this process. When used in conjunction with the SDI contrast curves,
these doppler sensitivies can help to more accurately constrain the
parameter space that we have covered and help us to determine what
possible companion objects the data did not detect. Sensitivities are
shown as solid curves in Figs. 10 and 11.
The radial-velocity confidence limits for HD32778 (Fig. 10 top) show that due to the lack of data points for this star we have no strong constaint on any
sub-stellar companions. The
curve here represents the 10% confidence limit for this star (shown more clearly in the top plot of Fig. 11)
and therefore below the hydrogen burning limit we have no real
constraints on any companions in this parameter space. The gray scale
in this figure, and the following detectability curves, show the
changing confidence limits running from dark being the least
constrained to light the most constrained regions. Given the lack of
data here there is only a very small region where we can rule out any
companions with an confidence and most of the parameter space is highly
unconstrained. The 10% confidence bound barely reaches beyond the 5
(99.9% confidence limit) of the SDI reduced contrast curve and only
covers a very small part of the parameter space beyond the
AO curve. Hence, for this system the radial-velocity data does not
help to constrain the companion and any information we can extract
about the unseen companion is drawn from the contrast curves alone.
This leads to the probability that the companion is of lower mass (
)
with a fairly unconstrained orbital separation i.e >0.076'' from the baseline of the radial-velocity data.
Similar to HD32778, the confidence limits in mass-separation space for HD91204 are poorly constrained (Fig. 10 middle). However, since HD91204 has a larger database of velocities
than HD32778,
we can say to a 70% confidence level that the companion to this star is not a close by (0.15'') sub-stellar or stellar secondary. The middle
panel in Fig. 11
shows the mass-separation parameter space for this star, reaching down
onto the stars surface. Clearly very close-by companions can be ruled
out to high levels of confidence due to the larger number of data
points and the fact that this radial-velocity curve is a liner. This is
further highlighted by the large light region shown in the gray scale
and a lack of any large contrast gradient. Objects with separations
below 0.06'' and masses above around 30
can be ruled out to
90-95%
confidence and moving out to separations of 0.08'' we can still rule
out objects down to around the planetary mass limit at the 90% level of
confidence. Therefore, we can say that the companion to this star
likely has a fairly large separation and is a faint substellar
companion or, as for almost all of the imaged objects, the companion
has a longer orbital period, but the inclination of the system was such
that when we were observing the companion it was hidden behind the star
at on-sky angular separations below 0.1''.
In comparison to these other two stars, HD145825 has enough data points
and exhibits enough curvature in the timeseries that fairly high levels
of confidence from the velocities overlap with the 99.9% confidence
limits from the imaging work. The bottom panel in Fig. 10 shows that below the SDI curve we still have over 90% confidence in ruling out
close by (0.17'') objects down to low brown dwarf masses. In addition, the 95% (2
)
confidence limit can rule out a lot of possible brown dwarf/stellar
companions below the 0.1'' angular separation limit of the
SDI technique (Fig. 11
bottom). The gray scale reveals more structure than HD32778 and HD91204
due to the significant curvature in the velocities. Particularly we can
see that the region around 0.2'' separation is less constrained than
inside and outside this separation and due to the indication of
secondary curvature in the velocities, we arrive at fairly high
confidence levels beyond 0.3'' separation.
From these combined constraints we can rule out to really high levels of confidence any brown dwarf/stellar
companions with small separations (short period orbits). Also, at the 1
level we can say that there are no objects at all below a separation of around 0.34'' with masses above
40
and also no companions down into the giant exoplanet regime within 0.20''. These combined data sets argue for the companion
to HD145825 to be an extremely faint sub-stellar companion with a moderate separation.
6 HD25874 and HD120780 detections?
Out of the five stars that we searched around, two possible detections were made around the stars HD25874 and HD120780. Both of these candidates fulfilled the requirements to be considered as bona fide candidates as they were bright sources, that had counterparts at 33
![]() |
Figure 12:
An annotated image of the star HD25874 at both inverted roll angles. The lines mark the 33 |
Open with DEXTER |
Figure 12 shows the 33
roll angle of the camera and how it projects along the image through the T3 filter (F1(1.575
m)-F3a(1.625
m)) for HD25874. The detections are found at the ends of the second
arc along the projection with a separation from the central pixel of 0.29
0.01'' and a position angle of 240
.
This enhanced image highlights more of the speckles across the images
in both negative and positive formats e.g. bright spot to the extreme
left middle of the left panel and its counterpart in the corresponding
position of the right panel. Along the projected angle there is also
another bright and dark pair
that could be separated by the roll angle and these are found at the
ends of the inner arc. As these are so close and connected to the
central star we believe these to be an artifact of the PSF subtraction,
however worryingly since they are found projected along the same axis
as the potential candidate detection they may signify that the
detection is an artifact as well e.g. uncorrected residual trefoil in
the image.
![]() |
Figure 13:
The upper plot shows the contrast limits ( |
Open with DEXTER |
As mentioned, Fig. 13
(upper) shows the contrast limits that were determined for HD25874,
highlighting both the conventional AO and the SDI reduced limits. For
companion candidates such as the one here it is clear that the SDI
reduction performs significantly better than conventional AO e.g. gain
of 2.5 mag
at 0.2''. The confidence limits in this figure show a lot of structure
at separations reaching well into the imaging phase space. This is due
to the large baseline (>4 yrs) of observations, even though
they describe a liner system. 95% confidence limits are seen to rule
out objects down into the planetary mass regime with angular
separations below 0.14'', depending on where we place the boundary
between exoplanets and brown dwarfs, and we can rule out such objects
up to separations of almost 0.18'' at the 70% confidence limit. The
lower panel better highlights the structure in detectability for this
star and shows clearly that we have high levels of confidence out to
the timeseries of this data set. The gray scale shows similar structure
to that of HD145825 with a large inner region that is highly
constrained, a dark unconstrained region around 0.2'' separation from
the star and then a growing lighter constrained region out to
separations of 0.3''. This time the constrained region at larger
separations arises not from any curvature in the velocities but due to
the overall span of velocity across the data set, showing that it is
unlikely that lower mass companions at these orbital separations could
give rise to this data set. By combining the imaging data with the
radial-velocity data, we can say that with 70% confidence we can rule
out almost all companions to this star with separations below 0.17''
(4.40 AU). Again we conclude that the companion to this star is
probably a widely separated, low mass (<70
)
and therefore really faint sub-stellar object. If such is the case
then there is a fairly high possibility here that the object is a brown dwarf located in the brown dwarf desert (Grether & Lineweaver 2006). Note that there were no other objects detected around this star out to orbital distances of
52 AU.
![]() |
Figure 14: The annotated image of
the star HD120780 containing a possible detection of the companion
object. The green lines mark the projected roll angle of 33 |
Open with DEXTER |
Table 7: Summary of results from the combined SDI and radial-velocity constraints for all stars.
![]() |
Figure 15:
The upper plot shows the contrast limits ( |
Open with DEXTER |





The contrast curves created for this system (Fig. 15
upper) are extrapolated from the unsaturated images of the other
systems. The detection here is marked by the filled circle with
associated uncertainties to highlight the contrasts achieved at such
low angular separations. The detectability probability in this region
is found to be 50-70%, again showing that more data would be useful to
better constrain the companion parameters, even at such low
separations. The detectability region is better seen in Fig. 15
(lower) where again we have over 90% confidence reaching out beyond
0.1'' from the star and with fairly high brown dwarf masses, across the
stellar mass regime. The gray scale in this region shows no real dark
unconstrained regions, except for widely separated and low mass
companions. Below the SDI limits we see that we can rule out lower
mass companions with 90% confidence only out to separations of 0.14'' (
2.38 AU) at best. Yet above the 1
confidence level (70%) we can rule out a range of lower mass
companions reaching as far out as 0.25'' (4.25 AU). Finally no
other longer period methane rich companions were detected out to a
separation of
34 AU.
7 Summary
We have performed a targeted direct imaging program to detect cool companions orbiting within 2'' of their parent stars. The stars were drawn from the AAPS and Keck planet search projects and consist of objects that exhibit large radial-velocity variation over several years. These radial-velocities indicate the presence of a massive companion on a long period orbit. Five stars were examined with the NACO-SDI system on the VLT in Paranal, Chile. From the five, two possible detections were found around the stars HD25874 and HD120780. However, further analysis of these detections indicate they are probable residual artifacts since they are found to be located at the same distance from the central pixel and the same position angle in each of the images for each star. In addition we also present detectabilities for each system by analysing the radial-velocity information we have acquired. Each of these detections lie within sensitivity boundaries for these stars of between 50-70%, meaning they could not be ruled out with any high degree of certainty from the radial-velocity data.
We have summarised the results of this work in Table 7,
which shows both the broad-band AO and narrow-band SDI reduced
contrasts and mass limits at separations of 0.5'' and 1.0''. Also the
sensitivity limits from the radial-velocity data have been summarised
at confidence limits of 70, 90 and 95% for each system,
highlighting the minimum mass possibly detectable at each confidence
level. The table shows the contrasts in magnitudes and mass limits in
Jupiter-masses at 0.5'' and 1.0'' angular separations for AO and SDI
images and shows the AO performs almost as well as the SDI at
separations of 1.0''.
Also since the stars are fairly old the AO reduction tends to reach
only a few Jupiter-masses above the SDI reduction for most of the
sample, however when the star is fairly young (HD145825) the SDI
reaches far deeper than AO i.e. 13
in
this case. Also from this table we see that the radial-velocity
confidence limits are all generally the same, apart from HD32778 due
its limited number of data points and temporal coverage.
As the stars chosen are fairly old, a consequence of the
radial-velocity selection method, the companions must be sufficiently
massive to lie within the detection threshold of the instrument. Also
since three of the five stars were found to have ``liner'' trends, and
we expect these to be sufficiently massive, then we suspect either the
uncertainty in age means we are underestimating our mass thresholds,
the models are over estimating the magnitudes of the companions, the
companions are aligned such that they are found behind, or very close
to the central star (0.1'')
or the companions are sufficiently far out in the system that they are
off the 2'' contrast limit. A combination of the these mechanisms are
probably at work. We find 5
(
F1) contrasts of 11.5 mag are possible using this method around bright F-K type stars (and 5
H-band contrasts of 12 mag for mid T-like objects).
Such contrasts allow access to long period, massive (
40
)
methane objects for stars that typically constitute the bulk of
radial-velocity programmes. In the future similar analyses as those
employed here will lead to a greater understanding of the properties of
exoplanets and brown dwarfs when extreme-AO systems can gain the
contrasts necessary to directly image known planetary-mass companions
detected by ongoing Doppler programs.
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Footnotes
- ...
- Based on observations made with the ESO telescopes at the La Silla Paranal observatory under program ID 076.C-0877(B).
All Tables
Table 1: Parameters for all stars in this work.
Table 2: HD25874 radial-velocity data.
Table 3: HD32778 radial-velocity data.
Table 4: HD91204 radial-velocity data.
Table 5: HD120780 radial-velocity data.
Table 6: HD145825 radial-velocity data.
Table 7: Summary of results from the combined SDI and radial-velocity constraints for all stars.
All Figures
![]() |
Figure 1: The solid lines are Burrows et al. (1997) models for different masses and ages. The dashed lines represent a range of masses and ages for the NextGen models of Allard et al. (1997), which are populated by our high-mass targets. The dot-dashed line marks the methane absorption boundary in cool dwarfs and provides a useful upper limit for SDI targets. The discontinuity between the models is the L to T spectral type boundary region, which is not very well modeled, thus the Burrows et al. models were truncated. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
A subset of our simulated candidate companions, including those imaged in this paper, taken from
Butler et al. truncated by |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
The radial-velocity measurements for the star HD25874. The minimum-period Keplerian fit to the data yields a period of |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
The best-fit Keplerian orbit to the radial-velocity measurements for
the star HD32778, taken by the Keck Planet Search. The fitted orbital
period is 6.5 years, with a circular eccentricity and a companion
minimum mass of |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
The orbital solutions for the radial-velocity dataset from the Keck
Planet Search for HD91204. All 10 data points are spread
across a period of 5 years and the best
estimate of the fit has an amplitude of |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
The radial-velocity dataset and fits for the companion to the star
HD120780. The minimum-period solution to the large trend is a companion
with a period of |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
The radial-velocity dataset from the AAPS for the star HD145825.
The best-fit Keplerian orbit to the 14 data points are shown,
along with the estimated orbital solutions. Curvature can clearly be
seen in this data, and since this is not a liner the orbital solution
is better constrained than all the other four stars. The
measured semi-amplitude of 617 m s-1 indicates
the presence of a companion with a period of 7.8 years, and
eccentricity of 0.2 and a minimum mass of 44.5
|
Open with DEXTER | |
In the text |
![]() |
Figure 8:
A combined plot showing all the contrast curves ( |
Open with DEXTER | |
In the text |
![]() |
Figure 9: A broadband image of the star HD25874. The image was constructed by combining all images through the three separate filter bandpasses. Here a number of super-speckles can clearly be seen scattered across the image, the brightest of them reside close to the central star. All the possible cool dwarf companions visible in this image are ruled out as speckles when comparing the rotated images. |
Open with DEXTER | |
In the text |
![]() |
Figure 10:
The 5 |
Open with DEXTER | |
In the text |
![]() |
Figure 11: The doppler sensitivity confidence limits in mass-angular separation space for the stars HD32778 ( top panel), HD91204 ( middle panel) and HD145825 ( bottom panel). These confidence limits have been integrated over all eccentricities and the percentage limit is shown inside each curve. The gray scale highlights the changing spacial confidence limits with dark the least constrained through to the most constrained regions being lightest. Only HD145825 has fairly well constrained doppler data out to reasonable NACO-SDI sensitive separations. Whereas for HD32778 there is no real constrained parameters below the hydrogen burning limit. |
Open with DEXTER | |
In the text |
![]() |
Figure 12:
An annotated image of the star HD25874 at both inverted roll angles. The lines mark the 33 |
Open with DEXTER | |
In the text |
![]() |
Figure 13:
The upper plot shows the contrast limits ( |
Open with DEXTER | |
In the text |
![]() |
Figure 14: The annotated image of
the star HD120780 containing a possible detection of the companion
object. The green lines mark the projected roll angle of 33 |
Open with DEXTER | |
In the text |
![]() |
Figure 15:
The upper plot shows the contrast limits ( |
Open with DEXTER | |
In the text |
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