We have acquired near-infrared spectra of Kuiper belt objects 2003 UZ117,
2005 CB79 and 2004 SB60 with NIRC on the Keck I Telescope. These objects are
dynamically close to the core of the 2003 EL61 collisional family and were
suggested to be potential fragments of this collision by Ragozzine and Brown
(2007). We find that the spectra of 2003 UZ117 and 2005 CB79 both show the
characteristic strong water ice absorption features seen exclusively on 2003
EL61, its largest satellite, and the six other known collisional fragments. In
contrast, we find that the near infrared spectrum of 2004 SB60 is essentially
featureless with a fraction of water ice of less than 5%. We discuss the
implications of the discovery of these additional family members for
understanding the formation and evolution of this collisional family in the
outer solar system.Comment: 9 Pages, 2 figure

Barkume

Brown

E. L. Schaller

Lacerda

M. E. Brown

Pinilla-Alonso

Rabinowitz

Ragozzine

Stephens

Trujillo

English

arXiv

We have acquired near-infrared spectra of Kuiper Belt objects 2003 UZ117, 2005 CB79, and 2004 SB60 with NIRC on the Keck I telescope. These objects are dynamically close to the core of the 2003 EL61 collisional family and were suggested to be potential fragments of this collision by Ragozzine and Brown. We find that the spectra of 2003 UZ117 and 2005 CB79 both show the characteristic strong water ice absorption features seen exclusively on 2003 EL61, its largest satellite, and the six other known collisional fragments. In contrast, we find that the near-infrared spectrum of 2004 SB60 is essentially featureless with a fraction of water ice of less than 5%. We discuss the implications of the discovery of these additional family members for understanding the formation and evolution of this collisional family in the outer solar system

Schaller, E. L.

Brown, M. E.

Caltech Authors - Main

ar
X
iv
:0
80
8.
01
85
v1
  [
as
tro
-p
h]
  1
 A
ug
 20
08
In press ApJ Letters, 1 August 2008
Detection of additional members of the 2003 EL61 collisional
family via near-infrared spectroscopy
E.L. Schaller1, M.E. Brown1
schaller@caltech.edu
ABSTRACT
We have acquired near-infrared spectra of Kuiper belt objects 2003 UZ117,
2005 CB79 and 2004 SB60 with NIRC on the Keck I Telescope. These objects are
dynamically close to the core of the 2003 EL61 collisional family and were sug-
gested to be potential fragments of this collision by Ragozzine & Brown (2007).
We find that the spectra of 2003 UZ117 and 2005 CB79 both show the charac-
teristic strong water ice absorption features seen exclusively on 2003 EL61, its
largest satellite, and the six other known collisional fragments. In contrast, we
find that the near infrared spectrum of 2004 SB60 is essentially featureless with
a fraction of water ice of less than 5%. We discuss the implications of the dis-
covery of these additional family members for understanding the formation and
evolution of this collisional family in the outer solar system.
Subject headings: Kuiper belt — planets and satellites
1. Introduction
The only known collisional family in the Kuiper belt was detected because of the unique
spectral properties the family members (Brown et al. 2007). The near-infrared spectra of all
other non-volatile rich Kuiper belt objects (KBOs) lie on a continuum between those whose
spectra contain moderate amounts of water ice absorptions to those whose spectra are es-
sentially featureless (Brown et al. 2007; Barkume et al. 2008). In contrast, the near-infrared
spectra of 2003 EL61, its largest satellite, and five other small KBOs resemble laboratory
spectra of pure crystalline water ice (Brown et al. 1999; Barkume et al. 2006; Trujillo et al.
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
– 2 –
2007; Merlin et al. 2007; Brown et al. 2007; Barkume et al. 2008). Remarkably, these ob-
jects are also relatively clustered in orbital element space. The largest object, 2003 EL61,
had been previously suggested to have experienced a massive collision that imparted its
fast (4-hour) rotation, stripped off most of its icy mantle leaving it with a density close
to rock (2.7 g/cc), and formed its two satellites (Rabinowitz et al. 2006; Brown et al. 2006;
Lacerda et al. 2008). Brown et al. (2007) concluded that the four extremely water ice rich
KBOs and the large satellite of 2003 EL61 were in fact fragments of the icy mantle of the
proto-2003 EL61 that had been ejected during a massive collision.
While collisional families in the asteroid belt can be identified by their dynamical clus-
tering alone, families in the Kuiper belt are much harder to identify because the collisional
ejection velocities can be a significant fraction of an object’s orbital velocity. Collisional
fragments can therefore be spread out over a wide range of orbital element space with the
fastest ejected fragments having significantly different orbits from the family core. The 2003
EL61 family members were all identified on the basis of their unique surface properties: they
are the only known objects in the Kuiper belt with extremely pure water ice spectra and
neutral visible colors. It is only because of this unique surface signature that detection of
the 2003 EL61 family members was possible.
In order to determine if other known KBOs could be fragments of the 2003 EL61 colli-
sion, Ragozzine & Brown (2007) integrated the orbits of 131 high inclination KBOs to deter-
mine their proper orbital elements and minimum ejection velocities away from the 2003 EL61
family core. They determined the minimum ejection velocity a KBO must have had in order
to reach its present orbit from the modeled location of the family forming impact. In Figure
1 we show object H-magnitude vs. minimum ejection velocity (from Ragozzine & Brown
(2007)) for the known KBOs closest to the family core. For KBOs in known resonances,
we plot the ejection velocity accounting for resonance diffusion (Ragozzine & Brown 2007).
A large fraction of these objects have never been observed to determine if they have IR
spectral signatures consistent with the 2003 EL61 family members. It is important to note
that though all of the known 2003 EL61 family members have neutral visible colors, visible
color or visible spectroscopy alone (Pinilla-Alonso et al. 2008) is not sufficient for family
member identification. There are many KBOs with neutral visible colors that do not also
have strong water ice absorptions and are not members of the 2003 EL61 collisional family.
Therefore, while visible colors can be used to rule out objects as potential family members,
near infrared spectroscopy to determine water ice absorption depths is necessary for definitive
family member identification.
Identifying additional family members and characterizing the extent of the spread of
fragments throughout the Kuiper belt may provide insight into the physics of this giant im-
– 3 –
pact event in the outer solar system. In this letter we present near-infrared spectra obtained
with the Keck I telescope of KBOs 2003 UZ117, 2005 CB79 and 2004 SB60, objects that
are located 67, 97, and 221 m/s away from the modeled 2003 EL61 family core respectively
(Fig 1). 2003 UZ117 and 2005 CB79 were suggested to be the most likely additional family
candidates by Ragozzine & Brown (2007).
2. Observations and Results
Observations of 2003 UZ117, 2005 CB79, and 2004 SB60 were performed with the Near
Infrared Camera (NIRC) on the Keck I telescope. Objects were identified in the field by
their motion relative to the background stars. Targets were moved to the center of a 0.525
arcsecond wide slit and were then observed with either a J-through-H order sorting filter
and a 120 lines per millimeter grism (2003 UZ117 and 2005 CB79) or an H-through-K
order sorting filter and a 150 lines per millimeter grism (2004 SB60). In all cases, exposure
times for the individual spectra were 200 seconds. Targets were dithered along the slit in a
five position nod pattern with 5 arcseconds between each dither. During observations the
telescope tracked at each target’s predicted rate of motion.
2.1. 2003 UZ117 & 2005 CB79
Low resolution spectral observations in the H–K (1.4-2.5 micron) region are ideal for
determining the presence or absence of water ice due to two broad water ice absorption
features centered at 1.5 and 2.0 microns (Figure 2). Barkume et al. (2008) observed 45 KBOs
and centaurs in this wavelength range and parameterized all spectra with a two component
model consisting of crystalline water ice covering a fraction of the surface, f, mixed linearly
with dark continuum component with slope m, covering (1-f) of the surface. They solved
for m and f in each spectrum by using a least squares minimization scheme using the Powell
method. Family members clearly stood out because of their high fractions of water ice (f >
0.8) in this parameterization compared with all other objects in the survey (f < 0.5).
Low resolution spectral observations at H–K with NIRC are only practical for objects
with visual magnitudes less than ∼21. Spectral observations in the J–H region of the spec-
trum (1.1-1.6 microns) allow us probe the spectra of objects (vmag∼21.5) that would be
too faint to detect or obtain sufficient signal to noise in a reasonable amount of time with
H–K spectroscopy. Fortunately, the water ice absorption feature centered at ∼1.5 microns is
perfectly suited for detection with J–H spectroscopy. We can therefore observe KBOs up to
– 4 –
half a magnitude fainter than previous observations in H–K (Barkume et al. 2008) allowing
us to detect strong water ice absorptions and determine if these smaller objects are also
members of the 2003 EL61 collisonal family.
J–H spectra of two dynamically likely family members, 2003 UZ117 and 2005 CB79
(visual magnitudes of 21.3 and 21.2 respectively) were obtained. 2003 UZ117 was observed on
23 September 2007 UT for a total integration time of 4000 seconds. 2005 CB79 was observed
on 21 and 22 May 2007 UT for a total integration time of 12200 seconds. Nearby G-type
main sequence stars were observed within 0.1 airmass of the targets for telluric calibration.
Variable seeing, sky conditions, and occasionally poor tracking (especially on the 2005 CB79
observations) caused the individual spectra of each of the KBOs to be of varying quality,
thus only the highest signal-to-noise spectra were included and the rest discarded for this
analysis. The final spectra consist of total integration times of 2800 seconds for 2003 UZ117
and 5200 seconds for 2005 CB79. Spectral data reduction was carried out using standard
procedures as described in Brown (2000) and Barkume et al. (2008).
The final 2003 UZ117 and 2005 CB79 spectra were each binned over four pixels to
increase the signal to noise per pixel. We used a Gaussian weighting function with a FWHM
of 8 pixels resulting oversampled spectra with a spectral resolution of ∼33. In Figure 2 we
show these spectra with one sigma error bars along with a model of pure water ice with 50
micron grain sizes at 50 K (Grundy & Schmitt 1998). We can quantitatively parameterize the
amount of water ice using the same spectral model as Barkume et al. (2008) but considering
the wavelength region from 1.1-1.6 microns. Using a Powell minimization scheme to solve
for the best fit of the parameters, m and f, we find that the spectra of 2003 UZ117 and
2005 CB79 are fit by water ice fractions of 0.92 ±0.16 and 1.02 ±0.10 respectively. The
high fraction of water ice given by this parameterization for 2005 CB79 may be due to the
fact that the water ice grain sizes on this object are somewhat larger than those modeled by
Barkume et al. (2008). Nonetheless, the deep absorptions detected from ∼1.4-1.6 microns
on 2003 UZ117 and 2005 CB79 are clearly consistent with a high fraction of water ice (in
contrast with all other known KBOs) but comparable to the other 2003 EL61 family members
(Barkume et al. 2008).
2.2. 2004 SB60
2004 SB60, with a visual magnitude of 20.8 was bright enough to be observed in H -
K. We observed it on 23 September 2007 for a total integration time of 3000 seconds. Data
reduction was carried out as described above. A nearby G-type main sequence star was
observed at similar (within .03) airmass of the target. Division of the 2004 SB60 spectrum
– 5 –
by the star spectrum allowed us to calibrate telluric features. The resulting spectrum has a
resolution r∼160.
Using the same spectral parameterization of Barkume et al. (2008) and described above,
we find that the fraction of water ice in the spectrum of 2004 SB60 is consistent with 0%
and can be no more than 0.05 (Figure 2). The low water ice fraction seen in the spectrum
of 2004 SB60 is similar to that seen in many small KBOs (see Barkume et al. (2008)). The
spectrum of 2004 SB60 is clearly not consistent with the other members of the 2003 EL61
collisional family that have water ice fractions greater than 0.8 (Figure 2).
3. Discussion
2003 UZ117 and 2005 CB79 contain high fractions of pure water ice on their surfaces
and appear dynamically related to the other members of the 2003 EL61 collisional family.
We therefore conclude that they are members of this collisional family. Thus far, all ob-
jects within 130 m/s of the modeled collision center (Ragozzine & Brown 2007) have been
shown to have the same unique strong water ice spectral signatures. 2004 SB60, at 221
m/s from the family core, has an essentially featureless spectrum inconsistent with 2003
EL61 and its family members but comparable to many other small KBOs. The flat spec-
trum we observed is consistent with infrared photometric observations of 2004 SB60 with
HST by Stephens & Noll (2007). Infrared spectral or spectrophotometric observations of
more potential fragments with minimum ejection velocities from ∼150 to ∼300 m/s could
help constrain the extent to which fragments were scattered throughout the Kuiper belt and
could tell us about the physics of the giant impact itself. In addition to the distribution
of fragments, any collisional model would also need to explain the the presence of the two
satellites of 2003 EL61.
Though it does not have the spectral signature of the fragments of the 2003 EL61
collision, 2004 SB60 is an interesting object in its own right. Noll et al. (2008) have found
this object to be one of the few high inclination small binary objects in the Kuiper belt. It
is worth noting the possibility that not all fragments of the 2003 EL61 collision necessarily
have the unique strong water ice signature. Fragments from different locations in the initial
parent body may have had different initial compositions. However, without this unique
spectral signature, identification of an object as a family member is currently impossible.
Another object near the center of the 2003 EL61 family is the third largest KBO, 2005
FY9. 2005 FY9 has a velocity closer to the center of the collision (150 m/s) than one of the
known fragments but has a spectrum dominated by methane, not water ice absorptions. We
– 6 –
find it intriguing that two of the largest KBOs are so close to each other in orbital element
space but can think of no reason why they should be physically related.
The spectral signature of extremely pure water ice found on 2003 EL61 and its fragments
and nowhere else in the Kuiper belt is the only reason definitive identification of the collisonal
family was possible. A collision with a non differentiated parent body would likely not
produce such a distinct spectral signature in its fragments. Future large scale surveys of
the Kuiper belt such as PanSTARRS and LSST are expected to increase the numbers of
known KBOs by over an order of magnitude. With higher numbers of objects, overdensities
in certain regions may be revealed and families without unique spectral signatures may be
able to be identified by their dynamics alone.
The data presented herein were obtained at the W.M. Keck Observatory, which is oper-
ated as a scientific partnership among the California Institute of Technology, The University
of California and the National Aeronautics and Space Administration. The observatory was
made possible by the generous financial support of the W.M. Keck Foundation.
– 7 –
REFERENCES
Barkume, K. M., Brown, M. E., & Schaller, E. L. 2006, ApJ, 640, L87
—. 2008, AJ, 135, 55
Brown, M. E., Barkume, K. M., Ragozzine, D., & Schaller, E. L. 2007, Nature, 446, 294
Brown, M. E., van Dam, M. A., Bouchez, A. H., Le Mignant, D., Campbell, R. D., Chin,
J. C. Y., Conrad, A., Hartman, S. K., Johansson, E. M., Lafon, R. E., Rabinowitz,
D. L., Stomski, Jr., P. J., Summers, D. M., Trujillo, C. A., & Wizinowich, P. L. 2006,
ApJ, 639, L43
Brown, R. H., Cruikshank, D. P., & Pendleton, Y. 1999, ApJ, 519, L101
Grundy, W. M. & Schmitt, B. 1998, J. Geophys. Res., 103, 25809
Lacerda, P., Jewitt, D., & Peixinho, N. 2008, AJ, 135, 1749
Merlin, F., Guilbert, A., Dumas, C., Barucci, M. A., de Bergh, C., & Vernazza, P. 2007,
A&A, 466, 1185
Noll, K. S., Grundy, W. M., Stephens, D. C., Levison, H. F., & Kern, S. D. 2008, Icarus,
194, 758
Pinilla-Alonso, N., Licandro, J., & Lorenzi, V. 2008, ArXiv e-prints, 807
Rabinowitz, D. L., Barkume, K., Brown, M. E., Roe, H., Schwartz, M., Tourtellotte, S., &
Trujillo, C. 2006, ApJ, 639, 1238
Ragozzine, D. & Brown, M. E. 2007, AJ, 134, 2160
Stephens, D. C. & Noll, K. S. 2007, in AAS/Division for Planetary Sciences Meeting Ab-
stracts, Vol. 39, AAS/Division for Planetary Sciences Meeting Abstracts, 39.06
Trujillo, C. A., Brown, M. E., Barkume, K. M., Schaller, E. L., & Rabinowitz, D. L. 2007,
ApJ, 655, 1172
This preprint was prepared with the AAS LATEX macros v5.2.
– 8 –
0 100 200 300 400
Velocity (m/s)
10
8
6
4
2
0
Ab
so
lu
te
 M
ag
ni
tu
de
IR spectra consistent 
with 2003 EL61 family
IR spectra inconsistent
 with 2003 EL61 family
Unknown
2003 UZ117
2005 CB79
2004 SB60
2003 EL61 2005 FY9
Fig. 1.— Kuiper belt object absolute magnitude (H) vs. the minimum velocity required
for a given KBO to reach its orbit from the modeled 2003 EL61 family forming collision
(from Ragozzine & Brown 2007 Tables 1 & 2). Including 2003 UZ117 and 2005 CB79 (this
paper), all of the objects within ∼130 m/s have similar infrared spectra showing extremely
deep absorptions due to water ice on their surfaces. These are the only KBOs with such
strong absorption features. 2004 SB60 was found to have an featureless infrared spectrum
inconsistent with the other 2003 EL61 family members.
– 9 –
1.2 1.6 2.0 2.4
Wavelength (microns)
0.0
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1.0
R
el
at
iv
e 
R
ef
le
ct
an
ce
2003 EL61
2003 UZ117
2005 CB79
2004 SB60
2003 VS2
Fig. 2.— Near infrared spectra of 2003 EL61 and its collisional family members show strong
signatures of nearly pure water ice on their surfaces. Shown are the spectra of five KBOs
along with a pure crystalline water ice model (Grundy & Schmitt 1998). We observed KBOs
2003 UZ117, 2005 CB79 and 2004 SB60, objects 67, 97 and 221 m/s away from the mod-
eled 2003 EL61 family core (Ragozzine & Brown 2007) to determine if their near infrared
spectra contained the characteristic strong water ice absorptions. Spectra of 2003 UZ117
and 2005 CB79 show strong absorptions at 1.4-1.6 microns consistent with the presence of
high fractions of water ice on their surfaces. In contrast, the spectrum of 2004 SB60 is es-
sentially featureless. The flat spectrum of 2003 VS2 (from Barkume et al. (2008)), a typical
non-family member KBO, is shown for comparison.


Detection of additional members of the 2003 EL61 collisional family via near-infrared spectroscopy

Crossref

Detection of Additional Members of the 2003 EL61 Collisional Family via Near-Infrared Spectroscopy

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.144.7293