arXiv:0904.3936v1 [astro-ph.EP] 25 Apr 2009
Submitted to ApJ April 24, 2009
A Search for Wide Companions to the Extrasolar Planetary
System HR 8799
Laird M. Close and Jared R. Males
Steward Observatory, University of Arizona, Tucson, AZ 85721
The extrasolar planetary system around HR 8799 is the first multiplanet
system ever imaged. It is also, by a wide margin, the highest mass system with
> 27 Jupiters of planetary mass past 25 AU. This is a remarkable system with
no analogue with any other known planetary system. In the first part of this
paper we investigate the nature of two faint objects imaged near the system.
These objects are considerably fainter (H=20.4, and 21.6 mag) and more distant
(projected separations of 612, and 534 AU) than the three known planetary
companions b, c, and d (68-24 AU). It is possible that these two objects could
be lower mass planets (of mass ∼ 5 and ∼ 3 MJup) that have been scattered
to wider orbits. We make the first direct comparison of newly reduced archival
Gemini adaptive optics images to archival HST/NICMOS images. With nearly a
decade between these epochs we can accurately assess the proper motion nature
of each candidate companion. We find that both objects are unbound to HR
8799 and are background. We estimate that HR 8799 has no companions of
H< 22 from ∼ 5 − 15′′. Any scattered giant planets in the HR 8799 system are
> 600 AU or less than 3 MJup in mass. In the second part of this paper we
carry out a search for wider common proper motion objects. While we identify
no bound companions to HR 8799, our search yields 16 objects within 1 degree in
the NOMAD catalog and POSS DSS images with similar (±20 mas/yr) proper
motions to HR 8799, three of which warrant follow-up observations.
Subject headings: planetary systems, stars: individual: HR 8799
There have been several surveys to directly image extrasolar planets for the ground with
adaptive optics (AO) and from space with HST. Until very recently all surveys returned null
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results, and so it was generally assumed that wide, massive, extrasolar planets would be rare
(at least around Sun-like stars) past 20 AU (Lafreniere et al. 2007a; Nielsen et al. 2008 and
references within). However, in November 2008 Marois et al. 2008 announced the discovery
of 3 planets orbiting the A5V star HR 8799, based on Near-IR imaging at the Keck and
Gemini telescopes. Using data from Keck in 2004-2008 and Gemini in 2007-2008, they were
able to establish common-proper motions (assuming the non-common motion component of
∼ 25 mas/yr for b and c was due to orbital motion of the planets around the star). The
three planets, HR 8799b,c, and d orbit at approximately 68, 38 and 24 AU respectively.
Marois et al. 2008 estimate effective temperatures of 870, 1090, and 1090K for the three
planets, and arrive at estimates for mass of 7, 10, and 10 MJup. These estimates are based
on the estimated 30-160 Myr age of HR 8799 and hybrid theoretical cooling tracks (luminosity
vs. age) for giant planets (Marois et al. 2008).
It is possible that additional companions could be discovered during a search at wider
separations. The binary fraction of late A stars is at least 70% (Bate et al. 2009). Recently,
(Veras et al. 2009) have predicted a population of giant planets at large separations (100
AU - 10,000 AU) from stars hosting relatively close-in planets, where the distant objects are
dynamically scattered/pumped to large separations after system assembly. Such effects may
explain the very wide, low mass, companions GQ Lup b at > 100 AU (Neuhauser et al. 2005)
and/or AB Pic B at > 250 AU (Chauvin et al. 2005). There is certainly the possible ex-
istence of low mass, reddened, stellar companions that have not yet been detected. It is
even possible that HR 8799 has a small, common-proper motion group, around it. Given the
unique nature of the HR 8799 system, a search for wide (> 100 AU) companions is important
and motivated this paper.
2.Observations & Reductions
On Oct. 30 1998 UT HST/NICMOS observed HR 8799 with its coronagraph, and 2
candidate companions were identified (Lowrance et al. 2005) in the roll subtracted images.
These faint point sources where reported at 13.7′′and 15.7′′(540 AU and 619 AU) with H mag-
nitudes of 21.6 and 20.4, respectively. These are much wide separations than the 3 confirmed
planets which were not discovered in the NICMOS data at the time (Lafreniere et al. 2009).
Figure 1 shows our own roll subtraction of the otherwise already pipeline reduced NICMOS
data with the 2 objects identified. HR 8799’s high galactic latitude (b = −35o) indicates that
there is a finite chance one (or both) are not background objects (see Fig 2). However, we
need to determine if these are real common proper motion companions or simply background
objects. For lack of a better nomenclature we have elected to call the closest candidate “HR
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8799B” and the farther one “HR 8799C”, however, the use of these labels does not imply
they are physical companions.
In October 25, 2007 (Marois et al. 2008) obtained 118x30s Angular Differential Imaging
(ADI; (Marois et al. 2006)) dataset of HR 8799 on the Gemini-North telescope with the
Altair AO system and NIRI NIR camera. Typically an ADI dataset would not be the ideal
method of imaging faint companions at 13.7′′and 15.7′′since there will be a risk of rotational
blurring of the images in the azimuthal direction. However, in this HR 8799 dataset the
integrations of 30s were short enough that only minimal azimuthal blurring occurred –and
only then in the fastest rotating images near transit. Therefore, we were able to create a
new custom “ADI-like” IRAF pipeline to produce “Wide Field” ADI images (WiFi ADI)
to image faint companions at the very edge of ADI datasets.
ADI is wide companions there is no need to use more advanced ADI reductions like LOCI
(Lafreniere et al. 2007b).
Since the focus of WiFi
Our WiFi ADI pipeline is very similar to standard ADI reduction and runs in a standard
IRAF environment. In ADI the telescope rotator is disabled and so the median of the images
gives an estimate of the master PSF without “contamination” from real objects on the sky.
Then one must subtract the master PSF off each individual frame after a cross correlation
alignment of each frame as in Close et al. 2002. Once the frames have the PSF removed they
need to be rotated by the parallactic angle and median combined (the subroutine to calculate
the rotation angle is a custom script developed for Gemini data in Close et al. 2003). One
way that the WiFi ADI pipeline differs from the standard is that it is optimized to preserve
any faint off-axis objects that might fall past the edge of the IR array in the majority of
the individual ADI frames. For example, the code accurately masks many bad pixels in
the corners of the NIRI array and uses a final median combine of all 118 re-rotated (master
PSF subtracted) images with no pixel clipping or rejection (so even the corner pixels of the
individual frames are utilized in the final WiFi image). We also carefully offset each image
by the mode of the outer region of each image, this allows the final median combine to be
most sensitive in the outer regions of the image.
Our WiFi ADI pipeline when used with the NIRI detector with its 0.0219”/pixel scale
is then capable of creating a round WiFi ADI FOV of 31.71′′in diameter when there is at
least 90 degrees of field rotation during the ADI observation, and the object is centered in
the detector. In the case of the Oct. 25 2007 Gemini HR 8799 data of Marois et al. 2008
the above assumptions are all true and our pipeline produced a final image (see Fig. 3) of
all H < 22 objects within ∼ 5 − 15′′of HR 8799.
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As is clear from figure 1 and 3 there are two faint objects near HR 8799. Based on de-
tailed search of the literature and the VLT/Gemini/Subaru/HST archives we have concluded
that there has not been any published attempt to recover “B” or “C” until now. HR 8799A
has a total proper motion of 119 mas/year so the recovery of the two candidate companions
at the same separations as in 1988 wrt HR 8799A would be an unambiguous confirmation of
their physical association. These objects are both fainter than the HD8799b-d planets (they
would have masses of 3 and 5 Mjupon the 0.1 Gyr (Baraffe et al. 2002) COND tracks; which
predict reasonable luminosities at these ages for higher mass objects; Close et al. 2007b),
consistent with being scattered by the heavier, close-in, planets.
In Figure 4 and 5 the 2007 positions of these two faint companions are shown. In
both cases the current positions are much closer to the locations calculated for distant
background objects rather than physical companions. Therefore, our astrometry proves that
the NICMOS companions of Lowrance et al. 2005 appear to be faint background objects
unrelated to HR 8799A (see Table 1 for a detailed list of our astrometric measurements).
3.1. Does HR 8799b Show any Parallax Motion of a Background Object?
In the direction of Pegasus most nearby stars appear to be moving towards the East
South East. This is due to the Sun’s motion in the opposite direction wrt the LSR. In fact,
the Sun’s space motion causes a stationary object (wrt LSR) at 39.9 pc to have a measured
proper motion of 95.51 mas/yr to the East and 38.51 mas/yr to the South –based on values
for the Solar motion given by Jaschek & Valbousquet 1993. While it is clear that planets
HR 8799b and HR 8799c have similar proper motions on the sky to HR 8799A (see left side
of Fig. 6), once we subtract the Solar motion there is less agreement (see right side of Fig.
6). However, this lack of common proper motion can be explained by increased velocity due
to orbital motion of b and c around A (Marois et al. 2008).
For the planet HR 8799b (which has the largest timeline of observations), there appears
to be some “scatter” in its measured position from A from the nearly straight line expected
for b’s long period orbit. The exact solution for a stable orbit of the massive planets b, c,
and d is still somewhat uncertain (Fabrycky & Murray-Clay 2008).
In Fig. 7 we consider the question what would this “scatter” resemble if b was actually
a background object at 100 pc that had similar proper motions, by chance, to HR 8799A
(at a distance of 100 pc HR 8799b would roughly have the normal luminosity and colors of
a background L dwarf). In this model b’s position should show a “reverse” parallax w.r.t.
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HR 8799A with an amplitude of 60% that of HR 8799A’s parallax. We calculate parallax
of HR8799 in the usual manner (Biller & Close 2007) and then multiply by -0.6 to calculate
In Fig. 7 we show two models for the nature of HR 8799b motion: a background
object at 100 pc; and a simple “linear” planetary arc. The small triangles in Fig. 7 denote
time stamps in the 100 pc model to each observation date over the 9.885 yr period since
the detection of b in a LOCI analysis of the NICMOS dataset by Lafreniere et al. 2009.
We note how neither the slightly curving “linear” orbit or the 100 pc background model
fit all data points simultaneously inside the 1σ errorbars. The “linear” orbit is a simpler
model with a better fit, but it certainly is not perfect since the reduced of χ2
an ∼ 8% chance that it is the correct model –assuming no unknown systematic errors in
the astrometry. On the other hand, a background source at 100 pc model can be rejected
with 99.95% confidence (χ2
fit for b, we cannot reject it (we also do not, at this time, know what orbit to fit it to
(Fabrycky & Murray-Clay 2008)). However, we can reject the hypothesis that this scatter
in HR 8799b’s position is reverse parallax due to it being a background object at 100pc
(assuming, of course, the reported astrometric values and errors are correct).
ν∼ 2 gives
ν∼ 4.4). While there is still significant scatter in the linear orbit
3.2.A Search For Other Common Proper Motion Companions to HR 8799
Even though the “B” and “C” objects are clearly background, there exists the possibility
of other wider companions to HR 8799A that might be found by searching nearby for similar
proper motion. We searched 12875 objects within ±1oof HR 8799 in the the US Naval
Observatory Merged Astrometric Dataset (NOMAD) catalog (Zacharias et al. 2004) with
non-zero proper motions. See Figure 8 for a plot of the resulting proper motion vectors.
Most objects inside 1oof HR 8799 are background and so do not have as large a proper
motion as HR 8799, which is dominated by the Sun’s motion w.r.t the LSR. In Figure 8 we
have highlighted those objects which have proper motion vectors within a circle of radius 20
mas/yr around HR8799’s proper motion. This yields a list of 15 objects, two of which may
warrant follow up observations. We further discuss these objects below.
To increase our sensitivity to fainter objects that might have been missed by previous
surveys we manually examined the Palomar Optical Sky Survey (POSS) POSS1 (1951 August
12 08:12:00 UT) and POSS2 (1991 October 02 05:47:00 UT) Red DSS images to search for
objects within 1 square degree of HR 8799A with similar proper motions. The POSS1 image
has a plate scale of 1.7′′per pixel, whereas POSS2 has 1.0′′per pixel. We first magnified the
POSS1 image to match the POSS2 plate scale. Next 25 background stars were selected, and
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their positions measured by centroiding in each image. We then compared their positions
on the two images to determine the optimum rotation (≈ 0.7 degrees) to apply to the
POSS1 image. We then selected an additional 25 background stars for a total of 50 stars
spread across the images to build a background reference frame. Proper motions were then
measured for individual stars by determining the average relative change in offset from the
50 background stars between the 2 images
A different sample of 30 background stars was selected to test our technique. These
were each compared to the 50 reference stars in each frame, and their proper motions were
calculated from the average change in offset from the reference stars. The overall average
of these measurements was µRA= 0.81 ± 3.50 mas/yr and µDEC= 4.92 ± 3.04 mas/yr. We
subtract these offsets from the proper motion measurements made for individual stars.
For bright stars there is a ±0.3 pixel uncertainty in centroid position (estimated from
FWHM/√SNR), and for the faintest objects that we considered a ±0.95 pixel uncertainty.
Given the 40.139 year baseline between the two positions this amounts to ±10.6 mas/yr of
proper motion uncertainty for bright stars, and ±33.5 mas/yr for faint objects. We combine
these in quadrature with the standard error of our reference stars estimated from the 30
test stars above to obtain an estimated uncertainty of ≈ ±11 mas/yr and ≈ ±34 mas/yr.
The bright star case compares favorably with other POSS based proper motion surveys, e.g.
Lepine & Shara 2005 with ≈ ±8 mas/yr.
The 1oX 1oarea of sky centered on HR8799 was searched manually by blinking between
the registered POSS1 and POSS2 images. In order to compensate for subtle changes in
magnification and field distortion, the image was re-centered on convenient background stars
in each small area being searched. When a proper motion candidate was identified, its
position was measured by centroiding and then its average change in offset relative to the
50 reference background stars was calculated.
As a check on our measurements, we compared our measurements of proper motion for
2 bright high proper motion stars in the field, NLTT 55870 and NLTT 55853, with POSS
based proper motion measurements (Lepine & Shara 2005). We measured (µRA,µDEC) =
(184.7, -43.5) and (178.2, -61.4) respectively compared to the values of (185, -44) and (164,
-63) reported by Lepine & Shara 2005. These measurements are consistent with each other
within our bright star uncertainty of ≈ ±11 mas/yr.
We note that we are comparing measurements of proper motion relative to a sample of
background stars, which is not the true absolute proper motion. Lepine & Shara 2005 discuss
this systematic effect and attribute it to bulk motion of the reference stars. Taking NLTT
55870 and NLTT 55853 as an example, they apply correction offsets of ∆µRA= +4 mas/yr
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and ∆µDEC= −7 mas/yr to determine the absolute motion. We have not determined offsets
for our own sample of reference stars, but given the agreement of our relative measurements
with Lepine & Shara 2005 we apply the same offsets. The only definitive way to overcome
this systematic effect for our purposes would be to simultaneously measure the proper motion
of HR 8799 itself (which has accurate Hipparcos motions), however it is badly saturated in
the POSS images rendering measurements of its position uncertain to several pixels. We
attempted to measure the proper motion of HR 8799 by determining best fit intersection of
the horizontal and vertical diffraction spikes, obtaining a very uncertain estimate (µRA,µDEC)
= (110, -43). This is close to the Hipparcos value of HR 8799’s motion: (µRA,µDEC) =
(107.88 ± 0.75, −50.08 ± 0.64) mas/yr (Perryman et al. 1997).
Though we did identify several faint candidate objects on the POSS images with no
obvious counterparts in NOMAD, only one appears worthy of further investigation with
respect to the goals of this paper. This object is at a separation of 4.6′from HR 8799A,
with proper motions of (µRA,µDEC) = (105±34,−6±34). It is a very faint object, detected
in the POSS images with SNR ≈ 10. In the Sloan Digital Sky Survey (SDSS) database it
is designated J230716.69+210509.1. Through the SDSS we also located it in the USNO-B1
catalog (Monet et al. 2003). In USNO-B1 it is not given a proper motion measurement.
It quite clearly moves between POSS images relative to nearby background stars, and it is
close to NLTT 55870 (discussed above), which gives confidence that our detection of motion
is not due to a local data reduction artifact. It’s µDECis 1.3σ from the expected value of
a common proper motion companion of HR 8799A, however its proximity warrants further
attention. We discuss this object in more detail below in section 4.1.
The large mass of the three planets combined implies a very large initial stellar nebula,
and their large orbital radii present challenges for both the core accretion (Fortney et al. 2008)
and the gravitational instability theories of planet formation (Boss 1997). This system could
perhaps be our best evidence for a new “non-core-accretion” mode of planet formation –
hence the detection of any additional companions (planetary or higher in mass) is very
The observations of Marois et al. raise several interesting questions. The photom-
etry shows a conspicuous lack of absorption by Methane redwards of 1.6 µm, which is
expected for such relatively cool (Teff ∼ 870 K) objects like HR 8799b –by all the “hot-
start” (Baraffe et al. 2002; Burrows et al. 2003) or “core-accretion” (Fortney et al. 2008)
synthetic spectra models. Having performed photometry at several bands, they attempted
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to fit spectral energy distributions (SEDs) generated by a hybrid atmospheric modeling code.
The best fit SEDs produced effective temperatures over 1400K ((Marois et al. 2008) on-line
supplement). The authors argue that such high temperatures cannot be supported by the
observed low luminosities unless the objects are unreasonably small or each has significant
dust extinction and reddening. They claim 3 edge-on dust disks –which would be misaligned
with the plane of their orbits (which is nearly face-on)– could explain such reddening, and
so they rejected this dust reddening idea.
We find that HR 8799 appears to be reddened by no more than AH∼ 0.3 mag (from
2MASS colors in (Marois et al. 2008) compared to a ZAMS A5), making it impossible that
line of sight extinction to HR 8799A alone is the cause. However, the lack of a “reverse”
parallax in the astrometric residuals of b’s separation from A (see Fig. 7) offers strong proof
that HR8799b is a physical companion of A. Hence, it is unclear why HR8799b appears
underluminous for its best fit model temperatures. Perhaps strongly non-LTE effects sup-
press CH4in the outer atmosphere (Marois et al. 2008). In any case, the discovery of other
wide reddened (or underluminous) companions would be very interesting in this system.
Therefore we need to investigate the properties of each proper motion candidate.
4.1.Individual Objects with Similar Proper Motion to HR 8799
The NOMAD database contains 15 objects with similar (±20 mas/yr) proper motion to
HR 8799A. NOMAD #1115-0634383 (see Table 2) has a wide 48′separation from HR 8799A.
It is red, with V = 16.64 and B-V color of 1.61 from the NOMAD magnitudes. NOMAD
#1108-0634609 is a Tycho 2 star, TYC 1717-1120-1. It is noteworthy due to its relatively
bright V = 10.62. Though very likely not gravitationally bound to HR 8799A (since at
projected separations > 1x105AU from HR 8799A they would be wider than the widest
known binaries (Close et al. 1990; Close et al. 2007a)), these objects may warrant further
study to determine if they are co-moving with the HR 8799 system. The other objects in
Table 2 appear to be a combination of too faint and too widely separated to warrant such
Our 1 square degree manual search of the POSS images around HR8799 discovered
one nearby object within 1.3σ of HR8799. We will briefly discus this “common proper
motion” candidate (SDSS J230716.69+210509.1) noted in Table 3. We converted the SDSS
photometry using the relations given by Jordi et al. 2006, obtaining V = 20.83 ± 0.12 mag
and B − V = 1.78 ± 0.10 mag. It is not in the 2MASS point source catalog, however there
are ≈ 4σ detections just above the non-Gaussian noise in the J and Ks 2MASS images.
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Though our measurement of its proper motion is marginally similar to HR 8799, it
appears to be too faint/blue to have a physical association –based on its colors and magnitude
it may be more distant than 39.9 pc. We by no means claim that this is definitive, as color
based parallaxes are very uncertain. Due to its relative proximity to HR8799 and the high
interest in this system, further effort to confirm our proper motion measurement, obtain IR
photometry, and perhaps obtain a radial velocity is warranted.
In summary, a cursory inspection of the proper motions and available photometry for
these objects yields no strong candidates for a bound stellar companion to HR 8799. Nonethe-
less, given the importance of the HR 8799 system and the many astrophysical puzzles it
presents, at least these 3 objects warrant further scrutiny.
There is still a need for future work in the search for wide companions to HR 8799. Due
to the brightness of HR 8799 on survey images there exists an annulus from ≈ 15′′< r <≈ 60′′
which has not been adequately explored in this paper for stellar companions –let alone
planetary mass objects. The discovery of any such objects in this annulus could provide new
insights into the planetary companions of HR 8799.
We have made the first direct comparison of newly WiFi ADI reduced archival Gemini
AO images to archival HST/NICMOS coronagraphic images. With 9 years between these
epochs we can accurately assess the proper motion nature of each companion. We find
that both objects are unbound to HR 8799 and are background. In this paper our major
1. We estimate that no bound companions of H< 22 mag exist from ∼ 5−15′′from HR
2. Any unseen scattered giant extrasolar planets in the HR 8799 system are > 600 AU
and/or less than ∼ 3MJupin mass.
3. The residuals in the current published astrometry of HR 8799b’s orbit are not ex-
plained by forcing HR 8799b to be a background object at > 100 pc.
4. While we identify no clearly bound companions to HR 8799A in our images (beyond
the extrasolar planet HR 8799b), our search yields 16 objects within 1 degree in the USNO
catalog or POSS plates with similar (±20 mas/yr) proper motions to HR 8799A. Three of
which merit follow-up observations.
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The authors would like to thank Glenn Schneider for helpful discussions about NICMOS
coronagraphic data and Andy Skemer for helpful discussions about the LSR. This paper
utilized data from the HST and Gemini archives. This paper extensively used IRAF, the
DSS, 2MASS, SDSS and Simbad databases. LMC is supported by an NSF CAREER award
and JRM is supported by the 2008 Steward Observatory Graduate Fellowship.
Facilities: Gemmini, HST (NICMOS).
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This preprint was prepared with the AAS LATEX macros v5.2.
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Fig. 1.— Roll subtracted NICMOS F160W image from 1998, showing the 2 candidate
companions “C” (to the left) and “B” (to the right) first identified by (Lowrance et al. 2005)
– each is circled in white. Each was visible in only one roll. The red circle around the star
shows the 1.7′′radius of HR 8799b. The NE compass is correct for the positive image (HD
8799B), however for HR 8799C (negative image) the compass should be rotated by 30o
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Fig. 2.— Histogram of the H > 20 point source detections made by the HST/NICMOS
survey (Lowrance et al. 2005) and Gemini Deep Planet Survey (Lafreniere et al. 2007a),
combined, binned by galactic latitude. These results demonstrate the low density of such
background objects away from the galactic plane. We note that HR 8799’s “overdensity” of
such companions hints that there might be a co-moving group (or at least a real companion)
around HR 8799. The inset shows the total number of stars observed in each latitude bin
by the two surveys.
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Fig. 3.— Our WiFi ADI reduction of the Oct 25, 2007 Gemini Altair/NIRI dataset of
(Marois et al. 2008). Note the locations of two very faint “B” (PA = 13o; Sep=13.78′′)
and “C” (PA = 115o; Sep=14.86′′) sources. North is up East to the left. Also there is a
detection of the planet HR 8799b which is located near its nominal position reported in Keck
AO images (Marois et al. 2008). For clarity we have inserted a zoomed in box centered on
HR 8799b, the other planets (c & d) were too close and faint to be clearly detected in this
WiFi ADI reduction.
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Fig. 4.— A 1.5x1.2′′section of our reduced WiFi image of the Gemini 2007 Oct 25 dataset.
Note that the H=21.6 object “B” detected by HST/NICMOS is clearly not common proper
motion with HR 8799A. The size of the error circles is dominated by 0.5% platescale un-
certainties across the field. North up East left, 0.0219 ± 0.0001′′/pix. The object is slightly
elongated due to the significant field rotation 14.15′′off axis during these exposures near
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Fig. 5.— Same as fig 4 but for the H=20.4 object “C”. As with “B” the “C” object also
appears to be background as well. Note that the bright small pixels “clumps” are bad pixels
at the very edge of the array, there is only one real object in the image (noted by the arrow).
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Fig. 6.— Left: The proper motions of HR 8799A, and extrasolar planets candidates HR
8799b and HR 8799c plotted over a 6.14 yr period –normalized to a common starting point
in 2002 of the (Fukagawa et al. 2009) observation. Right: Same as to the Left but the Solar
reflex motion of 95.51mas/yr in RA and -38.51mas/yr DEC at the position of HR 8799A
(d=39.9pc) has been removed. Hence, this is a plot of the true motions of HR 8799A, and
extrasolar planets candidates HR 8799b and HR 8799c (all wrt the LSR). It is interesting to
note that HR 8799A, b, and c appear to have clearly different proper motions once the Solar
motion is subtracted. These differences in motion are most likely due to orbital motion of b
and c around A (Marois et al. 2008).
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Fig. 7.— Here we plot 1σ errors of ten years of HR 8799b relative astrometry of b wrt
A from Fukagawa et al. 2009 (Subaru AO; largest errors), Lafreniere et al. 2009 (NICMOS;
Oct 30 1998 top data point), and the rest of the data points from Marois et al. 2008 (Keck
AO). The dotted line traces the “reverse” parallax that should be observed if HR 8799b is a
background object at 100 pc with a similar proper motion to A. The triangles denote time
stamps to each observation date over the 9.885 yr period since the NICMOS observation.
Note how neither a slightly curving “linear” orbit or the 100 pc background object model
fits all data points simultaneously inside the 1σ errorbars. However, the fit for the 100 pc
model is quite poor, hence, we can reject the hypothesis that this “scatter” in HR 8799b’s
position wrt HR 8799A is due to the reverse parallax of a background object at 100 pc. We
do not plot our new Gemini Oct 17, 2007 position for b since there are unknown systematic
errors in the data at the ±44 mas level, hence little statistical weight would be added by
such a data point.
– 19 –
Fig. 8.— Proper motions of 12875 objects within 2 square degrees centered on HR 8799A.
The NOMAD database was searched to recover all objects in this field with proper mo-
tion measurements. Diamonds highlight the 15 objects with proper motion vectors within
±20mas/yr of HR 8799’s vector (see Table 2). We also blinked the POSS1 (1951) and POSS2
(1991) red images to search for stars with proper motions, yielding 1 interesting object (tri-
angle) with no proper motion measurement in NOMAD (see Table 3). The error bars show
our ±34 mas/yr uncertainty for that one POSS object some 280′′from HR 8799A.
– 20 –
Table 1: Relative Astrometry of the 14′′Companions w.r.t. HR 8799A
Oct. 30, 1998
Oct. 25, 2007
Oct. 30, 1998
Oct. 25, 2007
3.756 ± 0.09
3.24 ± 0.02
14.29 ± 0.09
13.42 ± 0.07
13.19 ± 0.09
13.78 ± 0.07
−6.45 ± 0.09
−6.40 ± 0.03
13.71 ± 0.08a
14.15 ± 0.07
15.68 ± 0.08a
14.86 ± 0.07
15.9 ± 0.1
13.24 ± 0.07
114.3 ± 0.1
115.5 ± 0.1
aThe HR 8799A to B (or C) separation is as given by (Lowrance et al. 2005), the errors are as given by
(Lowrance et al. 2005).
bdata from (Lowrance et al. 2005).
cdata from our WiFi ADI reduction, astrometric errors dominated by 0.5% platescale errors across the field.
– 21 –
Table 2: NOMAD Objects within ±1oWith Similar (±20 mas/yr) Proper Motions to HR
Note. — Table 2: V and R magnitudes are not always given in NOMAD. The proper motion of HR 8799
is 107.88 and -50.08 mas/yr.
aWe discuss this object in section 4.1.
bAlso known as TYC 1717-1120-1.
– 22 –
Table 3: Faint Object 280′′from A with Similar Proper Motions From POSS
105±34 mas/yr -6±34 mas/yr20.83 ± 0.121.78 ± 0.10
Note. — Table 3: Proper motion vector as measured from POSS plates in this work. V magnitude,
B-V color, and associated uncertainties were derived from the SDSS photometry using the formulas of
Jordi et al. 2006. The proper motion of HR 8799 is 107.88 and -50.08 mas/yr.
aAlso known as USNO-B1 1110-0590705.