We have used the Hubble Space Telescope to directly measure the angular size
of the large Kuiper belt object 2003 UB313. By carefully calibrating the point
spread function of a nearby field star, we measure the size of 2003 UB313 to be
34.3$\pm$1.4 milliarcseconds, corresponding to a diameter of 2400$\pm$100 km or
a size $\sim5$% larger than Pluto. The V band geometric albedo of 2003 UB313 is
$86\pm7$%. The extremely high albedo is consistent with the frosty methane
spectrum, the lack of red coloring, and the lack of observed photometric
variation on the surface of 2003 UB313. Methane photolysis should quickly
darken the surface of 2003 UB313, but continuous evaporation and redeposition
of surface ices appears capable of maintaining the extreme alebdo of this body

Barkume K. M.

C. A. Trujillo

D. L. Rabinowitz

E. L. Schaller

H. G. Roe

M. E. Brown

English

arXiv

We have used the Hubble Space Telescope to directly measure the angular size of the large Kuiper Belt object 2003 UB313. By carefully calibrating the point-spread function of a nearby field star, we measure the size of 2003 UB313 to be 34.3 ± 1.4 mas, corresponding to a diameter of 2400 ± 100 km or a size ~5% larger than Pluto. The V-band geometric albedo of 2003 UB313 is 86% ± 7%. The extremely high albedo is consistent with the frosty methane spectrum, the lack of red coloring, and the lack of observed photometric variation on the surface of 2003 UB313. Methane photolysis should quickly darken the surface of 2003 UB313, but continuous evaporation and redeposition of surface ices appears capable of maintaining the extreme albedo of this body

Brown, M. E.

Schaller, E. L.

Roe, H. G.

Rabinowitz, D. L.

Trujillo, C. A.

Caltech Authors - Main

L61
The Astrophysical Journal, 643: L61–L63, 2006 May 20
 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.
DIRECT MEASUREMENT OF THE SIZE OF 2003 UB313 FROM THE HUBBLE SPACE TELESCOPE
M. E. Brown,1 E. L. Schaller,1 H. G. Roe,1 D. L. Rabinowitz,2 and C. A. Trujillo3
Received 2006 February 8; accepted 2006 April 3; published 2006 May 4
ABSTRACT
We have used the Hubble Space Telescope to directly measure the angular size of the large Kuiper Belt object
2003 UB313. By carefully calibrating the point-spread function of a nearby field star, we measure the size of
2003 UB313 to be mas, corresponding to a diameter of km or a size ∼5% larger than34.3 1.4 2400 100
Pluto. The V-band geometric albedo of 2003 UB313 is . The extremely high albedo is consistent with86% 7%
the frosty methane spectrum, the lack of red coloring, and the lack of observed photometric variation on the
surface of 2003 UB313. Methane photolysis should quickly darken the surface of 2003 UB313, but continuous
evaporation and redeposition of surface ices appears capable of maintaining the extreme albedo of this body.
Subject headings: comets: general — infrared: solar system — minor planets, asteroids
1. INTRODUCTION
The planetary-sized scattered Kuiper Belt object 2003 UB313
was discovered in an ongoing survey at the Samuel Oschin
telescope at Palomar Observatory (Brown et al. 2005). The he-
liocentric distance of 97.5 AU and V magnitude of 18.8 at the
time of discovery implied a size larger than Pluto for any albedo
lower than 96%, but the initial discovery was incapable of pro-
viding a measurement of the albedo. The infrared reflectance
spectrum of 2003 UB313 is dominated by absorption due to
frozen methane (Brown et al. 2005), like the spectra of Pluto
and Triton, suggesting the likelihood of a high albedo for 2003
UB313 like the 50%–80% albedos of these two objects. With
such a high albedo, 2003 UB313 would have a diameter in the
range of 2500–3200 km.
To date, most Kuiper Belt objects with measured sizes have
had their diameters determined through radiometry. In this tech-
nique, the measured thermal infrared flux (or fluxes) from an
object is converted to a diameter using models in which the
thermal emission properties and surface temperature distribu-
tions of the object are assumed (Altenhoff et al. 2004; Cruik-
shank et al. 2005). Owing to the uncertainties in the appropriate
model parameters, errors in these size determinations are large.
The object (50000) Quaoar had its size directly measured by
Hubble Space Telescope (HST) observations (Brown & Trujillo
2004), which are unaffected by uncertainties in thermal emis-
sion modeling. The major source of uncertainties in these mea-
surements is the unknown center-to-limb photometric function
of Quaoar, which changes the apparent angular diameter of the
disk.
Initial Spitzer Space Telescope observations of 2003 UB313
resulted in a 70 mm upper limit of 2 mJy, consistent with a size
upper limit of 3310 km and lower limit to the albedo of 44%
(M. E. Brown et al. 2006, in preparation). More recent obser-
vations from the Institut de Radioastronomie Millime´trique
(IRAM) telescope succeeded in detecting a flux of 1.27
mJy at 1.2 mm, suggesting a size between 2600 and0.29
3400 km and an albedo between 42% and 72% (Bertoldi et al.
2006). An object of this size would have an angular diameter
seen from the Earth of between 37 and 48 mas, similar to or
larger than Quaoar, and thus should be directly resolvable from
1 Division of Geological and Planetary Sciences, California Institute of Tech-
nology, Pasadena, CA 91125; mbrown@caltech.edu.
2 Department of Physics, Yale University, New Haven, CT 06520.
3 Gemini Observatory, 670 North A‘ohoku Place, Hilo, HI 96720.
HST observations. We present here such observations and the
size and albedo of 2003 UB313 directly measured by HST.
2. OBSERVATIONS
We obtained 16 119 s exposures and 12 133 s exposures of
2003 UB313 using the High Resolution Channel (HRC) of the
Advanced Camera for Surveys in the F435W filter in two orbits
of HST. The first observations began at 8:29 on 2005 December
9 (UT), and one observation was obtained every 170 s. The second
observations began at 6:52 on 2005 December 10 and were ob-
tained every 184 s. The timing of the observations was chosen
so that 2003 UB313 would be a few arcseconds from a moderately
bright field star located at 01h35m37s.63, 053857.8, and the
telescope was tracked at the sidereal rate so that accurate point-
spread function (PSF) measurements could be obtained from the
untrailed image of the star. The motion of 2003 UB313 caused a
smearing of about 30 mas of the image of 2003 UB313 during
each observation, a factor that needed to be taken carefully into
account in the analysis of the angular size of the object.
The star had a brightness of counts s1 (58.8 0.2 Bp
) in each of the images and was single, well20.152 0.004
exposed, and not saturated; 2003 UB313 had a brightness of
counts s1 ( ) and traveled149.9 0.7 Bp 19.136 0.005
across a field uncontaminated by detectable background de-
termined from our full PSF fitting analysis, described below.
The method used to determine the size of 2003 UB313 from
these observations is almost identical to that described and
validated in detail in Brown & Trujillo (2004). Here we very
briefly summarize the procedure and particularly discuss a few
small improvements. First, for each of the 28 individual stellar
images, the PSF is modeled using the HST PSF simulator pack-
age TinyTim (Krist & Hook 2001). While TinyTim’s default
parameters provide a close match to the average HRC PSF, the
HST PSF is not absolutely stable, so a fit is performed to the
field star in each image. To obtain a best-fit PSF, we use a
downhill simplex minimization method to fit the first eight2x
Zernike terms in the aberration of the TinyTim model at the
location of the star. Figure 1 shows the best-fit focus for the
individual images, while Figure 2 gives an example of the best-
fit model for one of the images. In Brown & Trujillo and in
subsequent experiments on other images, we found that the
focus varied smoothly over the course of an orbit. Here we see
the same trend but with a few significant outliers. While some
of this scatter is likely real and caused by the increase in jitter
with HST now in two-gyro mode, we chose to discard as un-
L62 BROWN ET AL. Vol. 643
Fig. 1.—Top: Best-fit focus (in units of rms waves of aberration at 547 nm)
for the 28 images. Previous experience has shown that the focus varies
smoothly over the course of an orbit. We expect that the large jumps in derived
focus are a product of the lower signal-to-noise ratio of the PSF star and of
the increased jitter in two-gyro mode. We discard as potentially unreliable all
of the measurements marked with an “X” in which the focus appears to shift
significantly between images. Bottom: Best-fit disk size for each of the indi-
vidual images. The mean is shown as the solid line, and the standard deviation
is shown as the dashed line.
Fig. 2.—Comparison of the star and 2003 UB313 in image 21 with the best-
fit models. The data and models share identical logarithmic scales. The resid-
uals are linearly scaled from 5% of the maximum of the model image.
reliable the images in which the measured focus appears to
vary greatly between adjacent observations (although we note
that the final results are not sensitive to the removal of these
points).
With the aberrations measured for each stellar image, TinyTim
is used to construct a PSF at the location of 2003 UB313, ac-
counting for the (small) PSF changes due to the field-dependent
aberration of the HRC. This PSF is convolved with the measured
motion vector of the object and then convolved with model disks
with diameters between 10 and 50 mas. This model image is
then convolved with the pixel Gaussian kernel repre-3# 3
senting the effects of CCD charge diffusion. For each image, a
best-fit model disk diameter is found that minimizes the sum of
the square of the residuals between the modeled image and the
data. An example of a best-fit model is shown in Figure 2.
In this process, the two steps that are the largest source of
potential error are the choice of the charge diffusion kernel and
the choice of the center-to-limb brightness profile of the model
disks. TinyTim provides an estimate of the position-dependent
charge diffusion kernel on the HRC CCD, but this estimate is
based on extrapolations from measurements at other wave-
lengths (Krist 2004). Because this parameter is critical, we
chose to independently measure the charge diffusion at the
location of our star and of 2003 UB313. We used archival
images of the globular cluster M92 obtained with HRC in the
F435W filter and found several isolated stars near the positions
of the central PSF star and the positions of 2003 UB313 on
the two separate orbits. We used the procedure described above
to fit the PSFs of the globular cluster stars near the location
of the central PSF star, and then we transferred this PSF to the
locations of 2003 UB313 using TinyTim. We then performed
a minimization of the charge diffusion kernel of the globular2x
cluster stars at the locations of 2003 UB313. This procedure
does not give us a reliable measure of the absolute width of
the charge diffusion kernel (as we have to assume an initial
charge diffusion kernel for the central region) but only a mea-
sure of the difference between the center location of the PSF
star and 2003 UB313. Fortunately, our measurements are only
sensitive to relative difference and are not dependent on the
absolute values. We find that the Gaussian width of the charge
diffusion kernel at the positions of 2003 UB313 are identical
for the two measured positions and are times larger1.04 0.01
than the width at the center PSF star.
The other important factor to consider is the center-to-limb
brightness profile of the model disks. An object with a flat
center-to-limb profile will appear larger than an object whose
brightness drops steeply near the limb. In Brown & Trujillo
(2004), we considered an extremely wide range of possible
profiles as nothing was known about the surface of Quaoar at
the time. For 2003 UB313, however, we know that the surface
is covered in methane frost (Brown et al. 2005) like the surfaces
of Pluto and Triton. As we have precise measurements of the
center-to-limb profile of Triton from well-resolved Voyager
measurements, we take this profile, parameterized with a Hapke
model (Hillier et al. 1994), as our best analog for the profile
of 2003 UB313.
The size measured for 2003 UB313 in each of the 17 images
not discarded due to focus deviations is shown in Figure 1.
Taking into account the random errors and charge diffusion width
uncertainty of 1.0 mas each, we derive an angular size for 2003
UB313 of mas. At a geocentric distance of 96.4 AU,34.3 1.4
this angular diameter corresponds to a diameter of 2400
km. For an absolute V magnitude of ,100 H p 1.12 0.01v
the albedo of 2003 UB313 is .0.85 0.07
3. DISCUSSION
Although 2003 UB313 is 20% brighter than Pluto in absolute
terms, it is only 6% larger, as the albedo of 2003 UB313 is
even higher than that of Pluto. This extremely high derived
albedo gives us confidence in our choice of Triton as an analog
for the center-to-limb function. Lower albedo icy satellites have
center-to-limb profiles that are much flatter than that of Triton.
Use of an icy-satellite–like profile would give a smaller size
and thus even higher albedo for 2003 UB313, inconsistent with
No. 1, 2006 MEASUREMENT OF 2003 UB313 FROM HST L63
the low albedos of the icy satellites. We can, in fact, self-
consistently solve for the center-to-limb profile and the albedo
by assuming a Hapke model to give both the profile and the
geometric albedo. The profile gives a unique measurement of
the angular size, and thus the albedo, but the Hapke model
independently gives an albedo. The only Hapke models that
give consistent albedos have parameters very similar to Triton,
with high single-scattering albedo strongly backscattering par-
ticles and smooth surfaces.
The measured size is well below the Spitzer upper limit of
3310 km but apparently inconsistent with the reported IRAM
measured size of km (Bertoldi et al. 2006). A re-3000 400
evaluation of the IRAM measurement, however, suggests that
the measurements are indeed compatible. The IRAM size mea-
surement assumed an absolute magnitude for 2003 UB313 of
, which is about 4% brighter than the bestH p 1.16 0.1v
current measurement of (D. L. RabinowitzH p 1.12 0.01v
et al. 2006, in preparation), suggesting a smaller size. Reeval-
uating a thermal model with the fainter absolute magnitude and
assuming that 2003 UB313 is pole-on and is the largest size
allowed by our uncertainties predicts a 1.2 mm emission of
0.94 mJy, consistent at the 1.1 j level with the mJy1.27 0.29
reported by IRAM. Even an equator-on orientation would give
a flux of 0.70 mJy, consistent at the 2 j level.
The albedo of 2003 UB313 is significantly86% 7%
higher than the ∼60% average of Pluto but consistent with the
albedo of the individual brightest areas on Pluto (Young et al.
2001); 2003 UB313 is also less red than Pluto (Brown et al.
2005), and Pluto’s darkest regions appear to be a major source
of the red color (Young et al. 1999). These two characteristics
suggest that the surface of 2003 UB313 might resemble the
brightest regions on Pluto while containing few if any of the
darkest regions, a suggestion consistent with the lack of de-
tectable photometric variation on 2003 UB313 (H. G. Roe et
al. 2006, in preparation).
The geometric albedo of 2003 UB313 is higher than that of
all known bodies in the solar system with the exception of
Saturn’s satellite Enceladus, which has a geometric albedo of
103% and has active water plumes capable of providing con-
tinuous frost resurfacing (Hansen et al. 2006). Even Triton,
which has active geysers and appears freshly resurfaced, has
a slightly lower albedo of 0.77 (Hillier et al. 1994). Typical
inactive water-ice–covered satellites in the outer solar system
have albedos ranging from 0.2 to 0.4, with exceptions only
related to activity or exogenic processes. On 2003 UB313, even
highly reflective methane frosts will irreversibly darken due to
long-term photolysis; some uncommon process that continues
to maintain a high albedo on 2003 UB313 is required.
One process unique on large bodies in the Kuiper Belt with
eccentric orbits is atmospheric freezeout. At 2003 UB313’s
38 AU perihelion equilibrium temperature of ∼43 K, the vapor
pressure over a solid nitrogen surface would be ∼30 Pa, while
the pressure over a solid methane surface would be ∼0.2 Pa
(Lodders & Fegley 1998). At the current distance of 97 AU and
equilibrium temperature of 27 K, these pressures drop by 6 and
8 orders of magnitude, respectively. Any nitrogen or methane
atmosphere that exists at perihelion is essentially completely
frozen to the surface by aphelion.
This 560 yr cycle of evaporation and then freezeout of the
atmosphere of 2003 UB313 will leave fresh undarkened ices
on the top layer at aphelion. A 30 Pa nitrogen atmosphere at
perihelion corresponds to a layer of solid nitrogen several cen-
timeters thick at aphelion, while a 0.2 Pa methane atmosphere
corresponds to a layer tens of microns thick. These layers would
sit on top of any dark photolyzed methane and would maintain
a high albedo for grain sizes smaller than the surface thickness,
which appears likely. At perihelion, the dark areas would be
revealed, leading to a Pluto-like appearance. The object 2005
FY9, which also has a surface covered in methane frost (Bark-
ume et al. 2005; Licandro et al. 2006) and is currently at a
distance of 52 AU, should have a surface temperature approx-
imately midway between Pluto and 2003 UB313. The presence
of small but discernible photometric variations on 2005 FY9
(H. G. Roe et al. 2006, in preparation) and the evidence for a
small surface coverage of warm dark material (M. E. Brown
et al. 2006, in preparation) suggests that 2005 FY9 could have
undergone partial atmospheric freezeout, but that at the tem-
perature of 2005 FY9 the process is not complete as on 2003
UB313. The process of atmospheric freezeout, which has long
been debated for Pluto (i.e., Stern et al. 1988; Stansberry &
Yelle 1999; Eliot et al. 2004), can now be studied in a growing
population of methane-rich objects at a wide range of distances
and temperatures in the outer solar system.
We would like to thank the director and staff at the Space
Telescope Science Institute (STScI) for providing the oppor-
tunity to make these measurements and the assistance in making
them happen. This research has been supported by a grant from
STScI and from NASA Planetary Astronomy.
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