3782 Celle: Discovery of a Binary System within the Vesta Family of Asteroids
W. H. Ryan1, E. V. Ryan1, and C. T. Martinez2
1
New Mexico Institute of Mining and Technology (NMIMT), 801 Leroy Place, Socorro,
NM, 87801
2
University of New Mexico, Department of Physics and Astronomy, Albuquerque, NM,
87131
Corresponding author:
William H. Ryan
MRO/R& ED
New Mexico Institute of Mining and Technology
801 Leroy Place
Socorro, NM, 87801
bryan@nmt.edu
Phone: 505-835-6646
Fax: 505-835-6807
Keywords: asteroids, photometry, binaries, lightcurves.
1
Abstract
Photometric observations of the minor planet (3782) Celle, which has been associated
both dynamically and spectroscopically with the Vesta asteroid family, were obtained
using the 1.8-meter Vatican Advanced Technology Telescope during September 2001
and December 2002 - January 2003. Analysis of these data reveals a normal rotational
lightcurve (P=3.84 hr, amplitude = 0.10-0.15 mag). During the 2002 - 2003 run,
anomalous attenuation events were observed lasting for about 2.6 - 3.5 hrs that varied in
amplitude from 0.15 - 0.3 mag. The attenuations were of two distinct types that can
clearly be identified as primary and secondary occultation/eclipses similar to those that
have been previously observed in known minor planet binary systems (Pravec et al.
2000). We therefore interpret our data as clear evidence that (3782) Celle is actually an
asynchronous binary system with an orbital period of 36.57 +/- 0.03 hrs (Ryan et al.
2003). A preliminary model yields a primary-to-secondary diameter ratio of 0.44 +/- 0.01
and an upper limit to the average bulk density of 2.2 +/- 0.4 gm/cm3. This is indicative of
a fractured or internal rubble-pile structure for at least one, if not both, of the binary
components. Since the Vesta family is believed to have been created via a cratering
event, this finding has important implications for understanding possible ejecta
reaccumulation and satellite formation in subcatastrophic collisions.
2
I. Introduction
Although a large database of lightcurves exists for main-belt and other asteroid
populations (e.g., Near-Earth asteroids), few researchers have focussed on deriving the
photometric properties of asteroids in a particular family. One notable exception is the
study of the Koronis family initiated by Binzel (1987), and continued by Slivan, et. al.
(2003), which analyzed the uniform alignment of the family members’ spin orientations.
This study has demonstrated interesting new phenomena can be discovered when the
aggregate photometric and spin properties of the components of a single family are
examined. In an effort to better understand large-scale collisions and their outcomes, an
observational study of the Vesta family of asteroids was undertaken starting in 1999
(Ryan et al. 2000). The objective is to determine spin rates, orientations, and shapes for
as many family members as possible. It was anticipated that this will provide tighter
constraints in the modeling of collisional processes, and could possibly confirm the true
impact conditions required to create asteroid families.
Statistical analysis, spectroscopic studies, and imaging data, all uniquely point to an
impact origin for the Vesta asteroids, making it an ideal family for study. Zappala et al.
(1994) have statistically classified 64 asteroids as comprising the Vesta family, and
Bendjoya and Zappala (2002) have updated this to include 231 to 242 members. Further,
Vesta has a spectrally distinct crustal composition (basaltic) which permits any fragments
excavated from it to be easily traceable. Binzel and Xu (1993) observed 520 km diameter
Vesta and smaller asteroids (4 - 7 km) in its vicinity, and concluded that eight of the
asteroids for which they obtained spectra in the region of Vesta had been chipped from
Vesta's crust. Bus and Binzel (2002) and Burbine et al. (2001) have also obtained spectra
of some of these bodies and link them compositionally to Vesta. Images taken by the
Hubble Space Telescope of asteroid Vesta (Zellner et al. 1997) confirm that there is a
large (~450 km) impact basin across Vesta's southern hemisphere. This hints at the
possibility that this family formed as a result of a cratering collision with the observed
impact basin as the source region for the so-called Vesta “chips”. Since a compositional
database exists for a subset of the family members, this photometric investigation is
intended to provide the lightcurve data necessary to complete the picture.
The ultimate goal of this observational study, and subsequent spin vector and shape
analysis, is to address a very important issue highlighted by Binzel and Xu and raised as a
consequence of numerical modeling results. What is the mechanism for excavating these
cratering fragments off of Vesta ‘s surface? Were the Vesta chips ejected as large, intact
spall fragments (having a thin, elongated shape) or are they smaller pieces of material
that have re-accumulated into larger rubble piles (having roughly spherical, ellipsoidal, or
lumpy shapes)? Being able to answer these questions will serve to constrain numerical
models of family formation (Melosh and Ryan 1997; Asphaug 1997; Michel et al. 2001),
and uncover the basic principles of asteroid collisional evolution.
During the first phase of this survey, a binary asteroid system was fortuitously discovered
amongst the Vesta chips. An anomalous attenuation was observed in what was an
otherwise "normal" lightcurve for the Vesta asteroid family member 3782 Celle (Ryan et
3
al. 2003). The discovery of a binary system for this cratering impact-generated asteroid
family is a key piece of information to be incorporated into physical models of collisional
origin, and will certainly help address the questions raised above. A preliminary model of
the data allowed the calculation of a mean density for the components of this binary
system of 2.2.4 g/cm3. Assuming that the material comprising these bodies is basaltic,
this lower-than-normal bulk density hints at a highly fractured or rubble pile structure for
the Celle system. This paper concentrates on the details of these observations and their
implications, which are discussed in the following sections. Future papers will address
the other observational data collected for this very interesting asteroid family.
II. Observations and Data Reduction
3782 Celle was observed for a total of 15 nights using the 1.8-meter Vatican Advanced
Technology Telescope (VATT) during September 2001 and December 2002 - February
2003. The observational circumstances are given in Table 1. This table includes the UT
date of mid-observation, heliocentric, r, and geocentric () distances, solar phase angle
(), geocentric ecliptic longitudes and latitudes, and the mean R magnitude, reduced to
unit heliocentric and geocentric distances.
Observations were made tracking at sidereal rate with typical exposures in the Harris R
band of 60-120 seconds. After cleaning, aperture photometry was performed on the
asteroid and an ensemble of 5-8 comparison stars using the IRAF (Tody 1993) apphot
routine. Differential magnitudes between the program object and the ensemble were
calculated, yielding typical errors on the order of 0.005 or less. Due to the large aperture
of the VATT and the relatively bright target, non-photometric nights containing thin
clouds were routinely utilized. However, the limits of this technique were tested through
thicker clouds during the December 2002 run, resulting in somewhat noisier data.
On nights with photometric conditions during the 2002-2003 apparition, BVRI exposure
sequences were taken for Celle and VR sequences for fields from previous nonphotometric nights. This permitted the absolute calibration of the R magnitudes to ~0.01
mag or better using Landolt (1992) standards and also allowed the determination of the
following mean color indices for Celle from data taken over four nights: B-V = 0.884 
0.010, V-R = 0.491  0.004, and V-I = 0.802  0.012. The mean V-R (the color used in
the transformation of the differential R magnitudes to the standard system) of each
night’s comparison ensemble was typically within 0.15 magnitudes of the program
object, thus minimizing any systematic errors in the reported R magnitudes.
The time-series R data have been corrected for light travel time and have been reduced to
unit geocentric and heliocentric distances. For phase angles greater 8, the nightly mean
R magnitudes from Table 1 for the 2002-2003 opposition imply a linear trend of 0.029
mag/. Since this yields a variation during a single night that is smaller than the typical
noise in the data, and it is not completely clear that this trend is applicable to data taken
during the anomalous attenuations, no phase correction was applied to the data within
each night.
4
During the September 2001 run, two nights of differential photometric data were
obtained for 3782 Celle. No attenuation events were observed and it displayed what
appeared to be a normal doubly periodic lightcurve associated with the asteroid’s
rotation. Using the standard Fourier technique described by Harris and Lupishko (1989),
a period of 3.840 hours was derived and the composite lightcurve shown in in Figure 1
was generated. In the composite curves, differential magnitudes with respect to a best fit
nightly zero point are folded with the derived period and plotted versus rotational phase.
For data calibrated to the standard system and distance corrected, these nightly zero
points represent the mean R magnitudes quoted in Table 1. However, for uncalibrated
data, as was obtained during this particular opposition, these zero points are simply free
parameters in the fit. For the September 2001 run, this technique for combining data from
multiple nights yielded approximately 80 percent coverage of Celle’s rotational phase.
Since shape determination of main belt asteroids requires observations from multiple
oppositions, Celle was again observed for three nights using the VATT in December
2002, before it moved too far into the Milky Way. In the lightcurve for December 10,
2002 shown to the left in Figure 2, differential magnitudes up to twice the anticipated
lightcurve amplitude were observed during one of the sporadic clearings on an otherwise
cloudy night. The initial assumption was that the points near the end of the night were
spurious due to cloud cover. Therefore, even though the data from earlier in the night
displayed a higher than expected amplitude, an attempt was made to incorporate them
into a composite curve. However, this failed, and, supported by familiarity with the
attenuations observed during later runs, it was determined that the points at the end of the
night most likely occurred after the attenuation ended. Therefore, only these noisier
points are included in the composite curve shown to the right in Figure 2 for all three
nights of the run, allowing an estimation of the mean R magnitude for the night.
Although the data from December 10 were of marginal quality and provided very little
coverage of either the rotational curve or the attenuation event, it was these spurious
observations that prompted further analysis of Celle in 2003.
Complete coverage of an anomalous attenuation on January 5, 2003 with a duration of ~3
hours was observed, confirming suspicions regarding the December 10 data from the
previous run. The lightcurve showing this event is presented in the left hand plot of
Figure 3. Data from two other nights during this run displayed the previously observed
3.84 hour period, reinforcing the notion that the January 5 event was truly anomalous.
Therefore, these data, along with the data from January 5 excluding that taken during the
attenuation event, were combined to generate the composite lighcurve shown in the right
hand plot of Figure 3. Anomalous events were also observed on January 27, 28, 30, and
31. The lightcurves displaying these events against the background of the 3.84 hour are
presented in Figure 4. The multitude of gaps in these curves is due to interference by field
stars since, by this time, Celle had moved fully into the Milky Way. During the three
other nights of the run, Celle again displayed only the 3.84 hour variation. Therefore, a
similar analysis was performed that excludes the anomalous attenuations to generate the
composite lightcurve for January 26 - February 1, 2003 that is shown in Figure 5.
5
III. Analysis of the Anomalous Attenuations
In the initial analysis of the longer period attenuations, the Fourier fit of the 3.84 hour
period was simply subtracted from the nightly lightcurves. However, the resulting
residuals plot displayed a signature of an eclipsing binary system similar to those
observed previously (Pravec, et. al. 2000, Mottola and Lahulla 2000, Pravec, et. al. 1998,
Pravec and Hahn 1997). With the premise that the measured magnitudes were due to the
integrated sum of the light from two bodies, it was determined that the observed
lightcurve would be more correctly interpreted as the linear sum of luminosities, not
magnitudes. Therefore, to analyze the nature of the attenuations, the nightly lightcurves
and fitted curve for the 3.84 hour variation were first converted to luminosities, the 3.84
hour varying component was subtracted, and then converted back to magnitudes. The
result for the January 5, 2003 event is shown in Figure 6.
To determine the periodicity of anomalous attenuation events, the residuals from the
January 26 - February 1 run were simply folded with trial periods. Identifying two
distinct types of attenuation features, the best fit was obtained with a period of 36.57
0.03 hours and the result is shown in Figure 7. Although there are gaps in the phase plot,
coverage is complete enough to rule out any shorter periodicities for which 36.57 would
be an integer multiple. For completeness, the residuals for January 4 - 6 are folded with
the same period and plotted in Figure 8. The fact that only one feature was observed
during this three day span is consistent with the ~18.3 hour periodicity of the events. The
second feature in this plot would have occurred at ~13 hours phase, just in the gap
between the data from the nights of January 4 and January 6.
IV. Interpretation and Model
The features observed in Figures 7 and 8 display the characteristics identified in
previously studied asynchronous binary minor planet systems. Such systems, where the
orbital features occur at regular intervals, but at different place in the primary's rotational
lightcurve on each occurrence, have been proposed as the one type of binary system that
can unambiguously be identified through lightcurve analysis (Weidenschilling et al.
1989, Merline et al. 2002). Therefore, in the following discussion, 3782 Celle will be
interpreted as such a binary system and the techniques employed in the analysis of 1996
FG3 (Pravec et al. 2000) will be used to model its characteristics.
Examining the residual plots in Figure 7, two distinct types of events are observed: a
feature with a flat-bottomed plateau similar to the one observed on January 5 and a much
deeper V-shaped feature. The flat feature can be interpreted as an event in which the
smaller, or secondary component of a binary system is completely obscured by the
primary (occultation) or the primary's shadow (eclipse). This will be referred to as a
secondary occultation or eclipse event. The deeper, V-shaped minimum can be
interpreted as one in which the smaller secondary transits the primary. For binary
components of equal albedos, the depths of the two types of events should be relatively
equal at zero solar phase angle. However, the primary events in Figure 7 occurred at
6
phase angles between 9.5-10.7. In this case, the shadow of the secondary is displaced
with respect to the observer’s line of sight and allows for the possibility of detecting both
the secondary's shadow projected onto the primary's surface (eclipse) as well as an
obscuration by the secondary itself (occultation). The small inflection in the leading slope
of the primary event at approximately the same depth as the secondary event hints at a
slight gap in time between the occurrence of totality of an eclipse/occultation event and
the onset of an occultation/eclipse (i.e., the other type event). This is consistent with the
fact that the depth of the primary event is approximately twice that of the secondary.
Presumably, this is due to the total depth of the primary minimum being the combined
effect of an occultation and an eclipse by the secondary on the larger primary. In contrast,
when the larger primary or its shadow totally obscures the secondary, no further
attenuation would be detected due to the onset of the other type of event.
A first order model of the binary system can be generated by examining the intensitysubtracted January 5 event in Figure 6 in detail and by making the following postulates:
1) The primary and secondary components have similar albedos and are approximately
spherical.
2) The mutual orbit is nearly circular and has the 36.57 hour period identified in
Figure 7.
3) The bottom plateau of the January 5 event represents a total occultation/eclipse of
the secondary component and is due to a nearly equatorial transit.
Measuring the depth of the January 5 event to be 0.19  0.01, Postulates 1 and 3 imply
that the ratio of the secondary to primary diameters is ds/dp = 0.44  0.01. To estimate the
radius of the orbit, a, it is recognized that the angle that the secondary travels through
during the duration of the event, t, is   2 t/P, where P is the orbital period and zero
phase angle has been assumed. For non-zero phase angle, this would be modified as  
 + , where  is the solar phase angle expressed in radians. For the total duration of the
event, the angle may also be expressed in terms of sizes associated with the system as
  (dp + ds)/a. Therefore,
 ds 
1  d 
p
a
.
 
d p 2 t  
P
(1)
Estimating the total duration (from beginning of decline to end of rise) of the January 5
event to be 2.8  0.1 hours implies a/dp = 3.2  0.1. Similar analysis of the duration of the
bottom plateau of the event leads to the same formula as Eq. 1, but with (1+ds/dp)
replaced with (1-ds/dp) in the right hand side numerator. Estimating the duration of this
plateau to be 1.1 0.1 hours then implies a/dp = 3.4  0.3. The fact that these agree to
within observational errors lends support to the postulate that the transit was near
equatorial since the relative duration of totality to the total event length depends in part
7
on where in the primary's cross section that the transit takes place. This leads to a further
consistency check by noting that for an equatorial transit, the difference between the time
of totality and the total duration of the event should be (ds/a)P/  1.6, which agrees,
within observational errors, with the measured time difference of 1.7 hours.
With this simple model of the system, the density of the primary can then be calculated
using Kepler's Laws as
24
p 
GP 2
 a

d
 p
3

1

.
3
 




 ds  
1    s


p  d p  
 
  

(2)
For the case where s/p  1 and adopting a/dp = 3.3  0.2, this yields
 = 2.2  0.4 g/cm3. Note that this result assumes a spherical primary. Modeling the
primary more generally as an ellipsoid, the approximation that at least two of the axes are
equal is supported by the relatively small lightcurve amplitude. However, the third axis is
unconstrained since calibrated magnitudes exist for only one opposition. Therefore, the
actual errors in the calculated dimensions and density of the Celle system may be larger
than those derived from observational uncertainties.
Further, the value  = 2.2  0.4 g/cm3 is derived assuming that the primary and
secondary have the same density. It is also possible that the primary is more highly
fractured (having a bulk density even lower than 2.2 g/cm3) than the secondary, or that
the secondary is largely intact. In any event, this value for the density, when compared to
the densities measured for the Howardite, Eucrite, and Diogenite (HED) meteorites
believed to have come from Vesta, falls in the fractured to rubble-pile porosity range as
noted in Britt et al. (2001). The HED’s have densities that range from 2.9 – 3.3 g/cm3
(Kitts and Lodders 1998), which (using a mean of these values) would give
approximately 30% macroscopic porosity for the bodies in the Celle system.
V. Discussion and Conclusions
As mentioned previously, this observational study of the Vesta family is designed to
reveal whether the asteroids have dimensions similar to spalls (indicating intact
fragments from the impact) or whether they more closely resemble ellipsoidal, reaccumulated rubble-piles. If the Vesta asteroid shapes are diagnostic, we can make a
clear conclusion about the manner in which fragments are ejected in a potentially familyforming impact event (intact or re-accumulated), either confirming or refuting current
numerical results. The shape analysis will be the subject of a future paper, however the
density for the Celle system reported in this paper (placing the bodies in the fractured
regime) gives an indication independent of shape studies that some of the family
members may have fragmented interiors. This would be in line (within the error bars and
assumptions above) with the results from Michel et al. (2001) whose numerical outcomes
8
imply that large asteroid family members are formed via re-accumulation of smaller
ejecta fragments post impact. Additional observations at varying geometries are planned
at Celle’s next opposition May 2004 in order to construct a more complete model of the
binary system and to refine this density estimate. This will allow more definitive
conclusions to be made as to the competency of Celle’s internal structure.
Weidenschilling et al. (1989) discuss several plausible mechanisms to explain the
formation of asteroid binaries. Most likely, there is no single explanation that would
apply to binaries observed within the main belt and in the Near-Earth region. Celle and its
satellite are similar sized bodies, having a primary-to-secondary diameter ratio of ~2.27.
It is also a very tightly bound system with the semi-major axis of the satellite’s orbit
equal to 3.2 primary-body diameters. Therefore, the component and orbital characteristics
of the Celle system are more similar to those for NEA systems (Merline et al. 2002) than
other main belt binaries discovered, although this may likely be a selection effect of the
detection techniques utilized. The mechanism most quoted for NEA binary formation is
the tidal splitting theory of Bottke and Melosh (1996). However this would not apply to
the Celle system, or any other main belt asteroid binary. Rotational fission, where a
single spinning fragment splits into two smaller ones is a possibility. The phenomenon of
rotational splitting has been observed in laboratory impact experiments by Fujiwara and
Tsukamoto (1980) and by Giblin et al. (1998). Moreover, based on the nominal
dimensions presented in the previous section, the total angular momentum of the Celle
system is consistent with a spherical parent body rotating with a period of 2.23 hours,
which is just the critical spin rate for fission of a strengthless body with the the derived
density. Another possibility for the creation of this binary system is that Celle and its
secondary formed as a consequence of a fragment jet. Martelli et al. (1993) noted that
laboratory experiments performed in an open environment allowed them to observe that
some of the ejecta had “clumped” trajectories, which if applied to asteroidal collisions,
could result in gravitationally bound small fragments.
Finally, a preliminary analysis of the other data derived for the Vesta family members
observed thus far as part of this photometric survey seems to indicate that the Celle
binary system is not a unique occurrence. At least one other dynamical family member,
3703 Volkonskaya, has displayed lightcurve signatures indicating occultation/eclipse
events (W. Ryan, E. Ryan, and C. Martinez, unpublished data, 2003). Therefore, the
determination of whether there is a statistically significant number of binaries within the
Vesta family has now become another goal of the continuing observational study being
undertaken by the authors.
Acknowledgements
This work is supported by NASA Planetary Astronomy Grant NAG5-8734 and is based
on observations with the VATT: the Alice P. Lennon Telescope and the Thomas J.
Bannan Astrophysics Facility. We are very grateful for the generous observing time made
available by the Vatican Observatory Research Group and the telescope engineering
9
support provided by Matthew Nelson and Randall Swift. The authors also wish to thank
Lacey Stewart for observing assistance during the December 2002 run.
10
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13
Figure Captions
Figure 1.
Composite lightcurve for 3782 Celle from observations taken in September 2001.
Figure 2.
Lightcurve data (left) obtained on December 10, 2002 displaying anomalous differential
magnitudes and composite lightcurve (right), excluding anomalous data, for the three
nights of the December 2002 run.
Figure 3.
Lightcurve for January 5, 2003 (left), displaying a three hour anomalous event at
approximately mid-observation and composite lightcurve (right), that excludes data
during the anomalous event, for three days of the January 2003 run.
Figure 4.
Lightcurves from January 27, 28, 30, and 31, 2003 showing anomalous attenuation
events.
Figure 5.
Composite lightcurve for January 26 - February 1, 2003 (excluding anomalous events).
Figure 6.
Figure 6: Lightcurve for January 5, 2003 with the 3.84 hour variation removed.
Figure 7.
Residuals for January 26 - February 1, 2003 after 3.84 hour variation has been subtracted.
Figure 8.
Residuals for January 4 - 6, 2003 after 3.84 hour variation has been subtracted.
14
Tables
Date (UT)
2001 Sept
2002 Dec
2003 Jan
2003 Feb
18.4
19.3
9.4
10.4
11.4
4.3
5.3
6.2
26.3
27.3
28.3
29.2
30.3
31.2
1.2
r(AU)
(AU)
2.492
2.493
2.595
2.594
2.593
2.578
2.577
2.576
2.562
2.561
2.560
2.559
2.558
2.558
2.557
1.500
1.500
1.739
1.730
1.722
1.597
1.595
1.594
1.626
1.630
1.635
1.640
1.645
1.651
1.656
Phase
Angle
(deg)
4.8
4.5
13.2
12.8
12.5
2.1
1.7
1.3
8.6
9.0
9.5
9.9
10.3
10.7
11.2
Ecliptic
Longitude
(deg)
3.6
3.3
113.9
113.8
113.7
108.6
108.4
108.1
103.2
103.0
102.8
102.6
102.4
102.2
102.0
Ecliptic
Latitude
(deg)
8.8
8.8
-1.3
-1.3
-1.3
-2.2
-2.3
-2.3
-2.9
-2.9
-2.9
-2.9
-3.0
-3.0
-3.0
R(1,)
(mag)
-*
13.10
13.09
13.09
12.68
12.66
12.62
12.96
12.99
13.02
13.01
13.00
13.04
13.06
Table 1. Geometric Circumstances and Mean R magnitudes for 3782 Celle observations.
*
Note: For the September 2001 run, only differential photometry was obtained, and
hence, no mean R magnitudes are given.
15