We present here Hubble Space Telescope NICMOS F110W and F160W observations of Haumea, and its two satellites Hi'iaka and Namaka. From the measured (F110W-F160W) colors of –1.208 ± 0.004, –1.48 ± 0.06, and –1.4 ± 0.2 mag for each object, respectively, we infer that the 1.6 μm water-ice absorption feature depths on Hi'iaka and Namaka are at least as deep as that of Haumea. The light curve of Haumea is detected in both filters, and we find that the infrared color is bluer by ~2%-3% at the phase of the red spot. These observations suggest that the satellites of Haumea were formed from the collision that produced the Haumea collisional family

Brown, M. E.

Fraser, W. C.

English

Caltech Authors - Main

The Astrophysical Journal, 695:L1–L3, 2009 April 10 doi:10.1088/0004-637X/695/1/L1
C© 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
NICMOS PHOTOMETRY OF THE UNUSUAL DWARF PLANET HAUMEA AND ITS SATELLITES
W. C. Fraser and M. E. Brown
Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
Received 2008 December 19; accepted 2009 March 2; published 2009 March 18
ABSTRACT
We present here Hubble Space Telescope NICMOS F110W and F160W observations of Haumea, and its two
satellites Hi’iaka and Namaka. From the measured (F110W–F160W) colors of −1.208 ± 0.004, −1.48 ± 0.06,
and −1.4 ± 0.2 mag for each object, respectively, we infer that the 1.6 μm water–ice absorption feature depths on
Hi’iaka and Namaka are at least as deep as that of Haumea. The light curve of Haumea is detected in both filters,
and we find that the infrared color is bluer by ∼2%–3% at the phase of the red spot. These observations suggest
that the satellites of Haumea were formed from the collision that produced the Haumea collisional family.
Key words: infrared: solar system – Kuiper Belt – planets and satellites: individual (Haumea, Hi’iaka, Namaka)
1. INTRODUCTION
The recently named dwarf planet Haumea—formerly known
as 2003 EL61—is a peculiar Kuiper Belt object; it is the primary
body of a triple system (Brown et al. 2006), the largest member
of the only known Kuiper Belt collisional family (Brown et al.
2007), and exhibits deep water–ice absorption which, in the
Kuiper Belt, has been observed only on the collisional family
members, and its largest satellite—Hi’iaka (Barkume et al.
2006, 2008; Schaller & Brown 2008).
The small angular extent of the orbit of inner and smaller
satellite, Namaka, has made a spectroscopic measure of its
reflectance impossible with ground-based facilities. The Hubble
Space Telescope (HST) has sufficient angular resolution to
separate Namaka from Haumea (see Figure 1). The F160W filter
is sensitive to the ∼1.6 μm water–ice absorption feature. Thus,
with a measure of the continuum near the water–ice absorption,
the depth of the water–ice absorption of Namaka can be inferred
from Hubble imaging photometry.
Here we present photometry of the Haumea triple system us-
ing the NICMOS camera. With these observations, we clearly
separate Haumea, Hi’iaka, and Namaka, and provide photom-
etry of these objects with the F110W and F160W filters. In
Section 2, we describe the observations and describe data re-
ductions we performed. In Section 3, we present our results. We
demonstrate that our measurements are consistent with a simple
water–ice absorption model for Haumea, and infer the water–
ice absorption of its satellites. Finally, we discuss our results in
Section 4.
2. DATA AND ANALYSIS
Observations were made in cycle 16 using NICMOS camera
1 on 2008 May 7 (UT). Images were taken in the F110W
and F160W filters with exposure times of 144 and 320 s,
respectively. Images were taken in pairs with alternating filters,
and in a five-point dither pattern to allow accurate background
removal and to avoid hot pixels in the camera. Because the time
between adjacent color pairs was short compared to the rotation
period of Haumea, the images of a pair sampled approximately
the same phase of Haumea and its satellites.
The data were initially processed through calnica, the stan-
dard HST reduction and calibration procedure (Bushouse 1997).
Residual backgrounds and nonlinearities were then removed us-
ing the pedsky and rnlincor routines from the STSDAS pack-
age.1 The background of each image did not vary significantly
from image to image. Thus, a median image was produced
from all images for each filter and removed to further subtract
any residual background left over from the previous image
reductions.
We performed aperture photometry on Haumea. Aperture
photometry was performed on the satellites using 5–6 pixel
radius apertures, after removing the best-fit amplitude scaled
tinytim point-spread function (PSF; Krist 1993) of Haumea; we
found that while the undistorted tinytim PSF could not accurately
model the core of Haumea’s image, subtraction of the PSF did
remove the wings of Haumea’s image to within the image noise
near the locations of each satellite. Infinite aperture corrections
were measured from the image of Haumea and the resultant
aperture-corrected photometry of each source is presented in
Table 1. The uncertainties presented are derived from the
statistical error image extensions provided with the calibrated
images and do not include the 5% absolute photometric
calibration error or the ∼2% relative calibration error from
the calnica data reductions (see the NICMOS Data Reductions
Handbook).
3. RESULTS
Presented in Figure 2 are the average photometry of Haumea
and its satellites with solar colors removed. As can be seen from
this figure, all three objects exhibit absorbance consistent with
water–ice.
3.1. Haumea
We find that Haumea exhibits a mean (F110W–F160W)
color of −1.208 ± 0.004. Barkume et al. (2008) found that the
spectrum of Haumea was well described by a mixture of water–
ice and a linear blue component. Using their model (Equation (1)
from Barkume et al. 2008) and best-fit parameters as input
to the synphot routine, we reproduce the color of Haumea
within the uncertainties of our measurements. This modeling
demonstrates that from these observations, we find a ∼40%
water–ice absorption depth on Haumea compatible with the
observations of Barkume et al. (2008) and can infer the relative
absorptions of the two satellites.
Rabinowitz et al. (2006) found that Haumea exhibits a double-
peaked light curve with a ∼3.9 hr rotation period. Lacerda et al.
1 STSDAS is a product of the Space Telescope Science Institute, which is
operated by AURA for NASA.
L1
L2 FRASER & BROWN Vol. 695
Figure 1. Example F110W image of Haumea Hi’iaka (left) and Namaka (right).
Table 1
Photometry
ST Magnitude
UT Filter Haumea Hi’iaka Namaka
3:40 F110W 19.212 ± 0.005 21.91 ± 0.02 23.7 ± 0.2
5:14 F110W 19.138 ± 0.006 21.84 ± 0.02 23.3 ± 0.1
5:23 F110W 19.249 ± 0.003 21.87 ± 0.02 23.10 ± 0.05
5:32 F110W 19.260 ± 0.005 21.91 ± 0.08 23.45 ± 0.05
5:41 F110W 19.32 ± 0.01 21.85 ± 0.05 23.8 ± 0.3
3:42 F160W 20.422 ± 0.005 23.41 ± 0.01 25.0 ± 0.1
5:17 F160W 20.319 ± 0.006 23.38 ± 0.03 24.6 ± 0.1
5:26 F160W 20.454 ± 0.004 23.44 ± 0.02 24.5 ± 0.1
5:35 F160W 20.490 ± 0.005 23.33 ± 0.09 24.9 ± 0.1
5:44 F160W 20.53 ± 0.01 23.25 ± 0.04 25.4 ± 0.3
Note. All observations were made on 3:40–5:44 2008 May 7 UT.
(2008) found that the color of Haumea varies, and is consistent
with, a large spot on one face of the dwarf planet which is
optically redder than the mean surface color.
We detected Haumea’s light curve in our observations, and
found that our observations are consistent in both phase and
magnitude with those of Lacerda (2008). The observed (F110W–
F160W) exhibits a variation of ∼0.03 mag from the mean
which is a >3σ deviation compared to the uncertainties in our
measurements. The deviation occurs at a light-curve phase 0.78,
consistent with the phase center of the red spot (Lacerda et al.
2008).
The variation we observe is bluer, or ∼2%–3% more negative
(F110W–F160W) at the red spot compared to the mean of the
surface. The bluer apparent color could be caused by increased
water–ice absorption or larger water–ice grains compared to the
mean surface. There is no reason to expect such an association
of ice properties with a red visible surface component, however.
A more plausible explanation is that the red spot detected by
Lacerda et al. (2008) is caused by an increase in the abundance of
irradiated organic materials (tholins, etc.) on the surface. These
materials typically appear blue in the infrared (Khare et al. 1984)
and could thus account for the observed effect.
3.2. Hi’iaka
No photometric variation of Hi’iaka was observed to the accu-
racy of our measurements. Hi’iaka exhibits a deeper absorption
than Haumea with a mean (F110W–F160W) = −1.48 ± 0.06.
The observations suggest that the ∼1.6 μm water–ice absorp-
tion is approximately 30%–40% deeper on Hi’iaka than Haumea
which is consistent with the observations of Barkume et al.
(2006).
3.3. Namaka
Namaka was found to exhibit a photometric variability of
∼ ± 0.3 mag in both filters. This variability is slightly larger
No. 1, 2009 NICMOS PHOTOMETRY OF HAUMEA AND ITS SATELLITES L3
Figure 2. Model water–ice plus flat component spectrum from Barkume et al.
(2008) (solid line). Haumea spectrum from Trujillo et al. (2007) (dotted solid).
The F110w (dotted) and F160w (dash-dotted) bandpasses multiplied by the
Solar spectrum are shown. Photometry of Haumea (squares), Hi’iaka (circles),
and Namaka (triangles) are presented. Solar colors have been removed, and the
measurements have been scaled to have equal relative flux in the F110w band.
Data for Hi’iaka and Namaka have been horizontally offset for clarity. Error bars
are only the photometric shot noise, and do not include the relative or absolute
photometric errors associated with the NICMOS filters.
Table 2
Color Measurements
(F110W–F160W) (ST Magnitude)
UT Haumea Hi’iaka Namaka
3:40 −1.209 ± 0.007(0.36) −1.50 ± 0.04 −1.25 ± 0.2
5:14 −1.182 ± 0.008 (0.78) −1.54 ± 0.04 −1.3 ± 0.2
5:23 −1.205 ± 0.005 (0.82) −1.57 ± 0.04 −1.44 ± 0.05
5:32 −1.230 ± 0.007 (0.86) −1.4 ± 0.1 −1.42 ± 0.05
5:41 −1.22 ± 0.01 (0.89) −1.40 ± 0.07 −1.6 ± 0.4
Notes. All measurements do not include the 2% relative or 5% absolute
photometric error associated with the NICMOS filters. These sources of
uncertainty, however, will cause a common offset in the colors for each
measurement and the relative differences between measurements will remain
constant. Numbers in parenthesis are the rotation phase of Haumea determined
from the published light curve of Lacerda et al. (2008).
in amplitude than the measurement uncertainties, but further
observations are required to confirm this variability. We did not
detect any variability in the absorption on Namaka to within
the accuracy of our measurements. Namaka’s absorption is
consistent with both that of Haumea and Hi’iaka, though with
larger uncertainties, and is inconsistent with a flat spectrum.
The observed mean (F110W–F160W) = −1.4 ± 0.2 suggests
that Namaka’s spectrum exhibits a water–ice absorption depth
at least that of Haumea.
4. DISCUSSION
Assuming similar infrared albedos, the observed mean flux
ratios suggest that Haumea, Hi’iaka, and Namaka have size
ratios 1 : 0.29 : 0.14. The potentially large variability of
Namaka suggests it has an elongated shape. This variability,
however, would be insignificant if the photometric uncertainty
quoted here were actually slightly underestimated. Thus, more
measurements are required to confirm this variability.
Our findings demonstrate that both Namaka and Hi’iaka ex-
hibit the deep water–ice absorption characteristic of Haumea
and its collisional family. If Hi’iaka and Namaka were dynami-
cally captured satellites, they would be expected to show colors
representative of the entire Kuiper Belt population. They, how-
ever, exhibit the deep water–ice features that are only observed
on the family members. The water–ice features exhibited by
both satellites suggest that they are made of the icy-mantle ma-
terial that covers the surface of Haumea (Brown et al. 2007).
We conclude that the satellites are a result of the disruption that
created the Haumea collisional family.
The extremely deep water–ice absorption on Hi’iaka is
consistent with a blue linear component in the water–ice mixture
approximately twice as steep as that on Haumea, assuming
similar mixture ratios. The deep absorption could also be caused
by water–ice grains ∼2–3 times larger than those observed on
Haumea. The true cause, however, cannot be determined from
our observations, and warrants further observations and spectral
modeling to understand the extremely deep absorption features
on this satellite.
The authors thank Dr Emily Schaller, and Darin Ragozzine
for their very useful discussions of, and suggestions for this
project. This material is based upon the work supported by
NASA under the grant NNG05GI02G. Support for program
HST-GO-011169.9-A was provided by NASA through a grant
from the Space Telescope Science Institute, which is operated
by the Association of Universities for Research in Astronomy,
Inc., under NASA contract NAS 5-26555.
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NICMOS Photometry of the Unusual Dwarf Planet Haumea and its Satellites

HST Calibration Workshop with a New Generation of Instruments,

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NICMOS Photometry of the Unusual Dwarf Planet Haumea and its Satellites

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