NEOWISE Studies of Asteroids with Sloan Photometry: Preliminary Results

Abstract

We have combined the NEOWISE and Sloan Digital Sky Survey data to study the
albedos of 24,353 asteroids with candidate taxonomic classifications derived
using Sloan photometry. We find a wide range of moderate to high albedos for
candidate S-type asteroids that are analogous to the S-complex defined by
previous spectrophotometrically-based taxonomic systems. The candidate C-type
asteroids, while generally very dark, have a tail of higher albedos that
overlaps the S types. The albedo distribution for asteroids with a
photometrically derived Q classification is extremely similar to those of the S
types. Asteroids with similar colors to (4) Vesta have higher albedos than the
S types, and most have orbital elements similar to known Vesta family members.
Finally, we show that the relative reflectance at 3.4 and 4.6 $\mu$m is higher
for D-type asteroids and suggest that their red visible and near-infrared
spectral slope extends out to these wavelengths. Understanding the relationship
between size, albedo, and taxonomic classification is complicated by the fact
that the objects with classifications were selected from the
visible/near-infrared Sloan Moving Object Catalog, which is biased against
fainter asteroids, including those with lower albedos.

Full text

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Accepted Version 1
NEOWISE Studies of Asteroids with Sloan Photometry:
Preliminary Results
A. Mainzer1, J. Masiero1, T. Grav2, J. Bauer1,3, D. J. Tholen4, R. S. McMillan5, E.
Wright6, T. Spahr7, R. M. Cutri3, R. Walker8, W. Mo2, J. Watkins6, E. Hand1, C.
Maleszewski5
ABSTRACT
We have combined the NEOWISE and Sloan Digital Sky Survey data to study
the albedos of 24,353 asteroids with candidate taxonomic classifications derived
using Sloan photometry. We find a wide range of moderate to high albedos for
candidate S-type asteroids that are analogous to the S-complex defined by pre-
vious spectrophotometrically-based taxonomic systems. The candidate C-type
asteroids, while generally very dark, have a tail of higher albedos that overlaps
the S types. The albedo distribution for asteroids with a photometrically derived
Q classification is extremely similar to those of the S types. Asteroids with similar
colors to (4) Vesta have higher albedos than the S types, and most have orbital
elements similar to known Vesta family members. Finally, we show that the
relative reflectance at 3.4 and 4.6 µm is higher for D-type asteroids and suggest
that their red visible and near-infrared spectral slope extends out to these wave-
lengths. Understanding the relationship between size, albedo, and taxonomic
1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA
2Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 USA
3Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125 USA
4Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822-1839 USA
5Lunar and Planetary Laboratory, University of Arizona, 1629 East University Blvd., Kuiper Space
Science Bldg. #92, Tucson, AZ 85721-0092 USA
6UCLA Astronomy, PO Box 91547, Los Angeles, CA 90095-1547 USA
7Minor Planet Center, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA
02138 USA
8Monterey Institute for Research in Astronomy, Monterey, CA USA
arXiv:1110.4998v1 [astro-ph.EP] 22 Oct 2011

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classification is complicated by the fact that the objects with classifications were
selected from the visible/near-infrared Sloan Moving Object Catalog, which is
biased against fainter asteroids, including those with lower albedos.
1.Introduction
Asteroids and comets represent the leftovers from the formation of our Solar System.
By studying their compositional variation, we can begin to better understand the conditions
present at the earliest stages of planet formation as well as their subsequent evolution and
processing. Asteroids are grouped into three main categories: C type or carbonaceous aster-
oids, thought to be the most common type in the Main Belt; the S type or stony asteroids,
a spectrally diverse group, and the X types, another diverse group of asteroids that have
relatively featureless spectra but a wide range of albedos, probably representing a broad
range of mineralogies and thermal histories. Gaffey et al. (1993) give an overview of some
of the earlier taxonomic systems (e.g. Chapman, Morrison, & Zellner 1975; Bowell et al.
1978; Barucci et al. 1987; Tedesco et al. 1989a; Tedesco et al.
Lebofsky 1994). Tholen (1984) created 14 taxonomic classes based on the combinations of
colors available from the Eight-Color Asteroid Survey (ECAS; Zellner et al. 1985; Tholen
& Barucci 1989). ECAS used filter passbands with wavelengths ranging from 0.34 to 1.04
µm, since UV wavelengths were detectable by the photomultiplier available at the time.
The Tholen and Tedesco systems also used albedo to differentiate asteroids; the X-group of
asteroids have similar ECAS colors but markedly different albedos.
1989b; Howell, Merenyi &
The Tholen and Tedesco systems offer a powerful means of distinguishing unique asteroid
groups. However, when CCDs replaced the photomultipliers at most observatories, they
ushered in a factor of several improvement in quantum efficiency at red wavelengths compared
to blue; while the ECAS photomultipliers extended out to 1.1 µm, most CCDs’ responses
were diminished at this wavelength. These effects combined to make it difficult for observers
to collect the full set of UV and near-infrared bands used by ECAS. Groups such as Xu et al.
(1995), Lazzaro et al. (2004) and Bus & Binzel (2002a) obtained visible spectra of asteroids
and used them for classification without albedos. Bus & Binzel (2002b) created a new
taxonomic scheme relying solely on visible spectroscopy with 26 taxonomic types, and DeMeo
et al. (2009) revised that scheme to 24 taxonomic types based on visible and near-infrared
(VNIR) spectral signatures. Neither the Bus & Binzel (2002b) nor DeMeo et al. (2009)
systems rely upon having albedo measurements for classification. Nevertheless, systems
based solely on VNIR spectroscopy and photometry are widely used because observations at
these wavelengths have generally been far more widely available than albedo measurements

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for the ∼500,000 asteroids known today. However, as discussed in Gaffey et al. (2002), the
ability to link taxonomic classification to asteroid mineralogy is complicated by the unknown
surface properties such as particle size, the fact that some minerals have limited or no spectral
features in the wavelengths used for classification, overlapping features, etc. It is therefore
useful to try to understand how the various taxonomic types are linked to physical properties
such as albedo and density.
The fourth release of the Sloan Digital Sky Survey (SDSS) Moving Object Catalog
(MOC; Stoughton et al. 2002; Abazajian et al. 2003) provided near-simultaneous observations
of ∼100,000 known asteroids in five bands (u, g, r, i and z) (Ivezi´ c et al. 2001), and these
have been used to study the distribution of colors throughout the Main Belt (c.f. Parker et al.
2008; Nesvorn´ y et al. 2005). These studies have been conducted largely without reference to
albedo, simply because the number of asteroids in the SDSS MOC vastly outnumbers those
with well-measured albedos. To date, the Infrared Astronomical Satellite (IRAS; Tedesco et
al. 2002) has been the largest source of radiometrically measured asteroid albedos, providing
measurements for ∼2000 asteroids.
With the Wide-field Infrared Survey Explorer’s NEOWISE project (Wright et al. 2010;
Mainzer et al. 2011a), thermal observations of >157,000 asteroids are now in hand. In
Mainzer et al. (2011d), we compared the albedos derived from NEOWISE observations of
∼1900 asteroids with taxonomic types derived from VNIR spectroscopy. Here, we use the
SDSS MOC photometry to obtain approximate taxonomic classifications for asteroids with
NEOWISE observations. We initially focus on the system of Carvano et al. (2010), who define
a classification algorithm based on SDSS colors that is compatible with previous taxonomic
systems.
2.Observations
WISE surveyed the entire sky in four infrared wavelengths, 3.4, 4.6, 12 and 22 µm
(denoted W1, W2, W3, and W4 respectively). Descriptions of the pre-launch mission design
and testing can be found in Liu et al. (2008); Mainzer et al. (2005), and the post-launch
description is given in Wright et al. (2010). A series of enhancements to the WISE data
processing pipeline, known as NEOWISE, have enabled the detection of >157,000 asteroids
and comets throughout the Solar System, including the discovery of ∼34,000 new minor
planets (Mainzer et al. 2011a).
24,353 objects, including nine NEOs and ∼24,275 Main Belt asteroids (MBAs), were
detected during the fully cryogenic portion of the NEOWISE survey and had matches with

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SDSS MOC observations of sufficient quality to enable classification according to the method
described in Carvano et al. (2010). The observations of these objects were extracted from
the WISE archive using the First Pass version of the WISE data processing pipeline (Cutri
et al. 2011) following the methods and parameters given in Mainzer et al. (2011b, hereafter
M2), Mainzer et al. (2011c, M3), and Mainzer et al. (2011d, M4).
3. Preliminary Thermal Modeling
We have created preliminary thermal models for each asteroid using the First-Pass Data
Processing Pipeline described above. As described in references M2, M3 and M4, we employ
the near-Earth asteroid thermal model (NEATM) of Harris (1998). The NEATM model uses
the so-called beaming parameter η to account for cases intermediate between zero thermal
inertia Lebofsky & Spencer (the Standard Thermal Model; 1989) and high thermal inertia
Lebofsky et al. (the Fast Rotating Model; 1978); Veeder et al. (the Fast Rotating Model;
1989); Lebofsky & Spencer (the Fast Rotating Model; 1989). With NEATM, η is a free
parameter that can be fit when two or more infrared bands are available. Bands W1 and W2
typically contain a mix of reflected sunlight and thermal emission. The flux from reflected
sunlight was computed for each WISE band using the methods described in M2, M3 and
M4; when sufficient reflected sunlight was present in bands W1 and W2, it was possible
to compute the reflectivity at these wavelengths, pIR, where we make the assumption that
pIR= p3.4= p4.6. The validity of this assumption and the meaning of pIRis discussed in M4
and will be the subject of future work. As described in M2 and M3, the minimum diameter
error that can be achieved using WISE observations is ∼ 10%, and the minimum relative
albedo error is ∼ 20% for objects with more than one WISE thermal band for which η can be
fitted. For objects with large amplitude lightcurves, poor H measurements, or poor signal
to noise measurements in the WISE bands, the errors will be higher.
3.1.High Albedo Objects
We note that among the 24,353 asteroids considered here, there are ∼55 that have
pV > 0.65. Of these, 48 have W3 peak-to-peak variations >0.3 mag, indicating that they are
likely to be highly elongated. Almost all of the extremely high albedo objects have orbital
elements consistent with membership in either the Vesta family or the Hungarias. Harris
& Young (1988) and Harris et al. (1989) noted that E and V type asteroids can have slope
values as high as G ∼0.5. The assumption that we have used of G = 0.15 for an object like
this would cause an error in the computed H for observations at 20◦phase angle of ∼0.3

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magnitudes; this would drive the albedo derived using such an H value up by 0.3. These
objects would greatly benefit from an improved determination of their H and G values.
4.Discussion
The Carvano scheme uses the SDSS colors as well as their measurement uncertainties
to define nine spectral classes: Vp, Op, Sp, Ap, Lp, Dp, Xp, Qpand Cp. These are roughly
analogous to the spectroscopically defined systems of Bus & Binzel (2002b) and DeMeo et
al. (2009); the “p” indicates that the classification was derived photometrically. The nine
Carvano classes were defined based on the ability of the SDSS colors to represent the system
of Bus & Binzel (2002b). The exception to this is the Lpclass, which is a conglomeration
of the Bus K, L and Ld classes. For each SDSS observation, the probability that an object
could be associated with a particular class was computed using the five SDSS magnitudes
and their associated uncertainties; classifications and probabilities for ∼63,000 asteroids were
taken from Hasselmann et al. (2011).
Figure 1 shows pV as a function of diameter for the individual Carvano classes. The
bias of the VNIR SDSS survey against small, low albedo asteroids is evident in the dearth
of objects in this regime, despite the fact that low albedo objects dominate most of the
Main Belt (Masiero et al. 2011). As discussed in M4, this bias complicates study of the
relationship between size and pV; this relationship is best studied using the full NEOWISE
dataset as it is relatively unbiased with respect to pV. Figure 2 shows histograms of pV for
the various taxonomic types, and Table 1 summarizes their statistical properties. The Sp
types are fairly well-grouped but span a wide range of moderate to high pV; while the Cp
types tend to have significantly lower pV, roughly 15% have pV>0.1, and the Dptypes have
36% with pV>0.1. In M4 we found that spectroscopically classified C types with diameters
>30 km tended to have pV<0.1. The fact that so many Cpand Dptypes with diameters
<30 km have higher albedos suggests either that it is difficult to distinguish these objects
with the SDSS colors in the Carvano scheme, that the bias against small objects with low
pV is skewing the result, or there is real scatter within the population of Cpand Dptypes.
The bias in the SDSS observations against small, low albedo objects means that measured
pV distributions will be skewed higher. Table 1 gives the statistical properties of the various
classes both for all objects and for objects with classifications assigned with probabilities
> 50%. As can be seen in the table, removing the objects with low probabilities does not
significantly affect the results.
Some asteroids have multiple observations in SDSS, and the different SDSS observations
can each result in different classifications. Out of the 24,353 asteroids, 3924 received multiple