Main Belt Asteroids with WISE/NEOWISE: Near-Infrared Albedos
Joseph R. Masiero, T. Grav, A. K. Mainzer, C. R. Nugent, J. M. Bauer, R. Stevenson, S. Sonnett
MMain Belt Asteroids with WISE/NEOWISE: Near-Infrared Albedos
Joseph R. Masiero , T. Grav , A. K. Mainzer , C. R. Nugent , J. M. Bauer , , R. Stevenson , S.Sonnett ABSTRACT
We present revised near-infrared albedo fits of 2835 Main Belt asteroids observedby WISE/NEOWISE over the course of its fully cryogenic survey in 2010. These fitsare derived from reflected-light near-infrared images taken simultaneously with thermalemission measurements, allowing for more accurate measurements of the near-infraredalbedos than is possible for visible albedo measurements. As our sample requires re-flected light measurements, it undersamples small, low albedo asteroids, as well as thosewith blue spectral slopes across the wavelengths investigated. We find that the MainBelt separates into three distinct groups of 6%, 16%, and 40% reflectance at 3 . µ m.Conversely, the 4 . µ m albedo distribution spans the full range of possible values withno clear grouping. Asteroid families show a narrow distribution of 3 . µ m albedoswithin each family that map to one of the three observed groupings, with the (221) Eosfamily being the sole family associated with the 16% reflectance 3 . µ m albedo group.We show that near-infrared albedos derived from simultaneous thermal emission andreflected light measurements are an important indicator of asteroid taxonomy and canidentify interesting targets for spectroscopic followup.
1. Introduction
The Wide-field Infrared Survey Explorer (WISE, Wright et al. et al. . − . µ m, 4 . − . µ m, 7 . − . µ m, and 19 . − . µ m respectively, with photometriccentral wavelengths of 3 . µ m, 4 . µ m, 12 µ m, and 22 µ m respectively (Wright et al. Jet Propulsion Laboratory/Caltech, 4800 Oak Grove Dr., MS 183-601, Pasadena, CA 91109,
[email protected]; [email protected]; [email protected]; [email protected];[email protected]; [email protected] Planetary Science Institute, Tucson, AZ [email protected] Infrared Processing and Analysis Center, Caltech, Pasadena, CA a r X i v : . [ a s t r o - ph . E P ] J un q < . η ) is used to account forvariability in thermophysical properties and phase effects. NEATM provides a rapid method ofdetermining diameter from thermal emission data that is reliable to ∼
10% when the beamingparameter can be fit (Mainzer et al. can then be combined with these models to constrain the geometric albedoat visible wavelengths ( p V ). However, as these data are not simultaneous with the thermal infraredmeasurements, uncertainties due to rotation phase, observing geometry, and photometric phasebehavior instill significant systematic errors in p V determinations. The preliminary asteroid thermalfits presented in Mainzer et al. (2011e, 2012); Masiero et al. (2011, 2012a); Grav et al. (2011a,b)for the near-Earth objects, Main Belt asteroids, Hildas, and Jupiter Trojans account for theseuncertainties in the determination of the optical H magnitude resulting in a larger relative erroron albedo than is found for diameter.For objects that were observed by NEOWISE in both thermal emission and near-infrared (NIR)reflected light, we can simultaneously constrain the diameter as well as the NIR albedo. As thesedata were taken at the same time and observing conditions as the thermal data used to model thediameter, no assumptions are needed regarding the photometric phase behavior of these objects,and light curve changes from rotation or viewing geometry do not contribute to the uncertainty.These NIR albedos will thus be a more precise indicator of the surface properties than the visiblealbedos.The behavior of the NIR region of an asteroid’s reflectance spectrum can be used as a probeof the composition of the surface. Spectra of asteroids in the NIR have been used for taxonomicclassification (DeMeo et al. et al. et al. et al. et al. (2011d) use the ratio of the NIR andvisible albedos as a proxy for spectral slope, and show a correspondence between this ratio andvarious taxonomic classifications. This relation was used by Mainzer et al. (2011e) to determinepreliminary classifications for NEOs, while Grav et al. (2011b) and Grav et al. (2012) expanded et al. (2011) presented NIR albedo measurements of Main Belt asteroids assumingthat the albedos at the W1 and W2 wavelengths were identical. However, for objects that havea sufficient number of detections of reflected light in multiple NIR bands we can independentlyconstrain each albedo ( p W and p W for the W1 and W2 bandpasses respectively). In this work,we present new thermal model fits of the NIR albedos of Main Belt asteroids (MBAs), allowing p W and p W to vary independently. These albedos allow us to better distinguish different MBAcompositional classes. They are also particularly useful for investigations of collisional families seenin the Main Belt, which show strongly correlated physical properties within each family.
2. Data and Revised Thermal Fits
To fit for NIR albedos of Main Belt asteroids, we use data from WISE/NEOWISE all-skysingle exposure source table, which are available for download from the Infrared Science Archive(IRSA Cutri et al. et al. (2011) and Mainzer et al. (2011a). In particular,we use the NEOWISE observations reported to the MPC and included in the MPC’s minor planetobservation database as the final validated list of reliable NEOWISE detections of Solar systemobjects.For objects with WISE detections in all four bands, we follow the fitting technique describedin Grav et al. (2012) to independently determine the albedos in bands W1 and W2. This techniqueuses a faceted sphere with a temperature distribution drawn from the NEATM model to calculatethe predicted visible and infrared magnitudes for each object. Diameter, beaming parameter, andvisible, W1, and W2 albedos are all varied until a best-fit is found. Monte carlo simulations of thedata using the measurement errors then provide a constraint on the uncertainty of each parameter.We require at least three detections in each band above SNR=4 to use that band in our fit. MainBelt asteroids are typically closer to the Sun at the time of observation than the Trojans and Hildasdiscussed in Grav et al. (2012), and thus the measured W2 flux can have a larger contribution ofthermal emission for MBAs. Flux in the W1 band is typically dominated by reflected light forMBAs observed by WISE, although low albedo objects ( p V < .
1) at heliocentric distances of R (cid:12) < . p W ), we require that the beaming parameterbe fit by the model. The beaming parameter is a variable in the NEATM fit that consolidatesuncertainties in the model due to viewing geometries and surface thermophysical parameters, andcan be characterized as an enhancement of the thermal emission in the direction of the Sun. Changesin thermal properties or phase angles will lead to a range of possible beaming parameters for MBAs. http://irsa.ipac.caltech.edu p W . While this should be sufficient to constrain the W2 albedo in mostcases, uncertainty in the beaming parameter can lead to large uncertainties in p W . To fit W1albedo ( p W ), we followed the same procedure for p W , except now requiring the W1 reflected lightto be at least 50% of the total flux. All objects that fulfilled the above requirements had opticallymeasured magnitudes available in the literature, and thus allowed us to fit a visible albedo as well.We present our updated thermal model fits for all objects satisfying the above constraints inTable 1, where we give the object’s name in MPC-packed format, absolute H V magnitude and G photometric slope parameter from the MPC orbit file, associated family from Masiero et al. (2013)(or “...” if the object is not associated to a family), and our best fit and associated uncertainty ondiameter, beaming parameter ( η ), p W , and p W if the latter could be constrained (“...” otherwise).Objects with two epochs of coverage have each epoch listed separately. All errors are statisticaland do not include the systematic errors of ∼
10% on diameter and ∼
20% on visible albedo (cf.Mainzer et al. et al. p W and p W ; 2371 fits of 2219 unique objects with only p W constrained;63 objects that had one epoch where both NIR albedos were constrained and one epoch where only p W could be fit.We note that some of the fits for diameter and beaming parameter (and thus albedo) aredifferent from those presented in Masiero et al. (2011). Fits from NEATM using the same dataset will give different values for the diameter as the beaming parameter is varied. In this case, byindependently considering p W and p W , as opposed to averaging over both for a single value of p IR , the calculated contribution of thermal flux in W2 will vary, which will result in a refined valueof η and therefore diameter. For the majority of cases, diameters are consistent to within 10%of the previous value, visible albedos are consistent within 20%, and infrared albedo and beamingparameters are consistent within 15%. Revised beaming parameters tend to be ∼
5% smaller,making diameters ∼
3% smaller and visible albedos ∼
6% larger. W1 infrared albedos tend toincrease ∼ et al. Name diameter (km) η p V p W p W H V G Family00005 108.29 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± NIR albedo properties (cf. Grav et al. p W < . − µ m range and thus “drop out” of W2. Each of these scenarios will have adifferent implication for interpreting the distribution of W2 albedos, most notably that our dataare least sensitive to smaller, lower albedo objects, as well as objects with blue p V - p W or p W - p W spectral slopes. Interpretation of the distribution of NIR albedos or spectral slopes, particularly asa function of taxonomy or size, must thus be made with the appropriate caveats. We also explorestacking of the predicted positions of these object in W2 to recover drop-out objects in future work.
3. Discussion3.1. Albedo comparisons
Figure 1 shows p W and p W for all Main Belt asteroids with sufficient data to constrainthese parameters, compared to the fitted diameter. The W1 band is more sensitive than the W2band (single-frame 5 σ sensitivity of ∼ .
22 mJy vs ∼ .
31 mJy respectively, Cutri et al. p W for more asteroids than have p W measurements (2835 vs 679). Both data sets showa strong bias against small, low-albedo asteroids, as is expected for data that require measurementof a reflected light component. A further bias against dark objects in p W due to rising thermalemission overtaking the small reflected light component is also present. From the data availablewe see no evidence for a non-uniform distribution of p W , in contrast to p W which shows threesignificant albedo clumps at p W ∼ . p W ∼ .
16, and p W ∼ .
4. 6 –Fig. 1.— (a) W1 infrared albedo ( p W ) compared to fitted diameter, where color also indicates p W (as used in Figure 7). (b) W2 infrared albedo ( p W ) compared to diameter. As the W1 andW2 detections are a measurement of reflected light they are strongly biased by albedo. The dearthof small, low albedo objects in this plot that are observed in Masiero et al. (2011) is an artifact ofthis bias.Visible albedos for over 136 ,
000 Main Belt asteroids were presented in Masiero et al. (2011)and Masiero et al. (2012a). These measurements were based on the conversion of apparent visiblemagnitudes from a wide range of predominantly ground-based surveys to absolute H V magnitudeswhen the orbit was determined by the MPC. Absolute magnitude is then converted to a predictedapparent magnitude during the epoch of the WISE/NEOWISE observations, often after assuminga photometric G parameter (cf. Bowell et al. p V determinations beyond what would be expected from uncertaintiesin the flux measurements and from thermal modeling. As a result, the fractional error on p V istypically 50 − p W traces p V and can thus be used as an analog when p V is not available.The uncertainties of the p W measurements in our data are smaller than the errors on p V , and thusact as a better constraint of the surface properties. The relationship between p W and both p V and p W is less distinct, and varies over a large range of values for objects spanning high and low p V and p W albedos.Comparing the p V distribution to Figure 10 from Masiero et al. (2011), we see that our samplecontains significantly fewer low albedo objects than would be expected from a random sample of allMain Belt asteroids. The lack of low albedo objects is due primarily to the observational selectioneffect imprinted on our dataset by the requirement that the objects be detected in W1 and/or W2 7 –in reflected light. This bias will increase as albedo decreases, preferentially selecting objects withhigher albedos. A survey with deeper sensitivity in these wavelengths would allow us to probesmaller sizes at all albedos, but would still be subject to the same observational biases.Fig. 2.— W2 albedo vs W1 albedo for 679 Main Belt asteroids. The color of the points indicatetheir visible albedo following the colormap presented in Masiero et al. (2011) and shown in thecolorbar, while the size of the point traces the fraction of the W2 flux that was due to reflectedlight. The dotted line shows a 1-to-1 correspondence; objects below the line will have a blue spectralslope from W1 to W2, while objects above the line will have a red slope. 8 –Fig. 3.— Comparisons (top) and histograms (bottom) of asteroid albedos at visible ( p V ), W1( p W ), and W2 ( p W ) wavelengths. Blue points/lines show objects with all three albedos fit bythe thermal model (679 objects), while green crosses/dashed lines show objects with only visibleand W1 fits (2835 objects). Plots of p W do not include green crosses as these objects do not havethis parameter constrained by the model fits. Dotted black lines in the comparison plots indicate a1-to-1 relation. While visible and W1 albedos show clear clumping, W2 albedos show no separationwithin our measurement errors. 9 –Following Grav et al. (2012), we can use our albedo measurements as a proxy for spectral slopefrom visible wavelengths through the NIR. Objects that fall above the 1-to-1 relationship in thetop portion of Figure 3 will have a red spectral slope across the wavelengths plotted, while objectsbelow this relation will have a blue spectral slope. As p W is the most poorly probed of the threeparameters, there will be inherent detection biases against blue spectral slopes from objects that“drop out” and fall below our W2 detection threshold. For this reason objects with the bluestslopes, particularly low-albedo objects, will be under-represented in our fits of p W .To better compare the spectral slope information, in Figure 4 we show the difference in albedobetween p V , p W , and p W normalized to the measured p W value. Objects with positive valueshave a red spectral slope, while objects with negative values have a blue slope. High albedo objects( p W > .
1) tend to show red slopes from visible to W1 wavelengths, and then blue slopes betweenW1 and W2. This behavior is similar to what is observed for Eucrite meteorites at these wavelengths(Reddy et al. et al. p W albedo show similarvisible-W1 slopes to those with a measured p W .Low albedo objects ( p W < .
1) behave quite differently from their high-albedo counterparts.While slightly red from the visible to W1, these objects show a wide range of visible-W2 and W1-W2 slopes, from neutral in color to very red. An important caveat to this is shown by the objectswithout p W fits, which have slopes ranging from moderately red to significantly blue. Blue-slopedobjects would be much fainter in W2 than W1 and thus would drop out from detection or bedominated by thermal emission in W2. It is probable that there is a population of these objectswith blue W1-W2 slopes that are not represented in our plots. Extrapolting from the NIR spectraof low albedo objects from DeMeo et al. (2009), we associate our objects that have red visible-W1slopes with C-type and D-type objects. We can similarly associate the objects having blue-slopeswith B-type asteroids, however we note that only ∼
1% of objects studied by DeMeo et al. (2009)were identified as B-type asteroids, while ∼
10% of the asteroids in our study have low albedoand blue spectral slope. From Neese (2010) we find that the majority of our blue sloped objectsthat have Bus-DeMeo taxonomic classifications are identified as B or Ch class objects, the latter ofwhich represents a fraction of the spectroscopic sample comparable to the fraction of our sample inthis group. Our blue slope may be indicative of the presence of mineralogical absorption featuresin the spectra of low albedo objects at the wavelengths covered by W1. 10 –Fig. 4.— Spectral slope, normalized to the W1 albedo, over visible-to-W1 wavelengths (left), visible-to-W2 wavelengths (center) and W1-to-W2 wavelengths (right), compared to the W1 albedo. Thedotted line shows a neutral slope; objects with positive slope values have red spectra and objectswith negative values have blue spectra. High albedo objects tend to be red-sloped from visible toW1, and blue sloped from W1 to W2, while low albedo objects tend to be flat or red across thewhole range. Color/shape of points is the same as used in Figure 3. 11 –
Asteroids with D-type taxonomic classifications become increasingly common as distance fromthe Sun grows, from the Main Belt through the Jupiter Trojan population (DeMeo & Carry 2013).These objects, especially the Jupiter Trojans, were likely implanted from a more distant reserviorduring the early chaotic evolution of the Solar system (Morbidelli et al. et al. (2012) compare p W and p W to distinguish asteroids with D-typetaxonomic classification from those with C- and P-type, and are able to determine the overallpopulation fraction of D-type objects in the Jupiter Trojan and Hilda populations. They find thatthe majority of Jupiter Trojans are D-type at all sizes, while the Hilda population transitions froma minority of D-types at diameters D >
40 km to a majority at smaller sizes.Following Grav et al. (2012), we show in Figure 5 an expanded view of the objects with lowestinfrared albedos. We highlight the region of albedo-space that is occupied by D-type asteroids inthe Trojan and Hilda populations. The diameter and albedo fits from Grav et al. (2012) rely onthe same model and assumptions as we use here, and so comparisons between the two populationsshould only depend on the random error associated with the fits. Only 2% of all objects for whichwe measure p W and p W fall in this region; with the exception of (114) Kassandra and (267) Tirza(which are spectrally classified as T- and D-type objects, respectively), all other candidate D-typeobjects are in the outer Main Belt and have diameters between 10 km < D <
40 km, consistent withthe diameter regime where D-types dominate the Hilda asteroids. One object, (1755) Lorbach, isidentified as an S-type in Neese (2010), but this classification relies on only two optical colors. Forthe outer Main Belt, we do not see a significant population of D-type objects like what is observedin the Hildas and Trojans (DeMeo & Carry 2013), but this is expected from the lower efficiency ofdynamical implantation compared with the Hilda and Trojan populations Levison et al. (2009).We find no objects in the inner Main Belt with albedos consistent with D-type objects. Thisis in contrast to the results of DeMeo et al. (2014) who find a small population of these bodies;however, this difference can be understood through the selection effects in our survey. Although oursample probes a large number of objects with semimajor axis a < . p W for these objects and they will not appear in our analysis. 12 –Fig. 5.— W1 and W2 albedos for all measured objects. The red ellipse marks the region populatedby D-type asteroids as identified in Grav et al. (2012). This taxonomic classification shows nosignificant representation in the Main Belt objects studied here. 13 – p W /High- p W objects Figure 5 shows a group of objects with low visible and W1 albedos ( p V , p W < .
1) but high W2albedo ( p W > . et al. p W < . p W ≥ . p W < . p V and p W . The primary difference between these twogroups is that the low-high objects have significantly smaller heliocentric distances at the time ofobservation than the low-low objects, resulting in higher subsolar temperatures. The diameters ofthe low-high objects are also characteristically smaller than those of the low-low group, howeverwe cannot distinguish if this is an actual difference between the groups or is a change in sensitivityas a result of the low-high objects being closer to the Sun and telescope at the time of observation,and thus warmer and brighter.The asteroid (656) Beagle is a particularly interesting case for testing the differences betweenthese two sets of objects. NEOWISE observed this asteroid at two different epochs, both while fullycryogenic, with good sensitivity at all four bands. One epoch of observations results in a NEATMbest-fit that falls into the low-low group, while the other epoch falls into the low-high group. Thelow-high epoch data were taken when Beagle was 0 .
21 AU closer to the Sun (2 .
82 AU vs 3 .
03 AUfor the low-low case), following the trend seen for the overall population. The best-fit for NEATMin the low-high epoch has a beaming parameter of η = 1 .
46 and a diameter of D = 62 km while thelow-low epoch has best-fit values of η = 1 .
03 and D = 48 km which is the reverse of the diametertrend mentioned above. This large disagreement in diameter is not unexpected given the differencein best-fit beaming parameter which is inversely proportional to the fourth power of the subsolartemperature used in the NEATM model, and thus will change the model’s emitted flux.The observations used for our fits were visually inspected, as well as compared to the WISEall-sky atlas of stationary sources, and show no significant contamination by background stars orgalaxies. We note that (656) Beagle has a large amplitude lightcurve (A > .
035 hours (Menke 2005). Although large amplitudes can increase uncertainty in the fits, ourdata consist of 12 data points over 1 day and 15 data points over 1 .
25 days, so both epochs cover 14 –multiple rotations. As such, light curve variations should be averaged over by our fits, and shouldonly contribute a small amount to the total uncertainty in the fit.As a test of our model, we perform a NEATM fit using only bands W1, W2, and W3 asconstraints, assuming the W4 measurements are anomalously high, and a fixed beaming parameterof η = 1 .
0. When using a fixed beaming parameter we cannot adequately constrain p W , and soassume it is equal to p W . For these restricted fits, both epochs converge to diameters that agreeto within 10%, but they cannot reproduce the measured magnitudes as well as the full-fit case. Aswe are using one fewer constraint but two fewer variables, this is not surprising. The fits for (656)Beagle given by Masiero et al. (2011) are nearly identical to these restricted fits, but also cannotfully reproduce the measured magnitudes, particularly for the low-high epoch. Restricting ourmodel further and only fitting W1 and W3, we find that both epochs converge to nearly identicaldiameters, and visible and infrared albedos.We can understand these results by looking at where the best-fit model deviates from thedata. For the low-high epoch, the full NEATM fit cannot reproduce the W2, W3, and W4 fluxessimultaneously, with the W2 and W4 measurements showing excesses not observed in W3. Our fullmodel finds a best fit solution allowing W3 and W4 to determine the diameter and beaming whichunder-produces flux in W2, but corrects that by increasing p W . If we ignore the W4 measurements,the W2 and W3 fluxes still cannot be reproduced in the low-high epoch solely with thermal emissionand reflected light without resorting to extreme changes in p W .One possible explanation for the disagreement between epochs is that we are observing signif-icant differences between the thermal emission in the morning and afternoon hemispheres of theasteroid. If (656) Beagle has a relatively high thermal inertia, there may be a significant lag tothe thermal re-emission of incident light which is not accounted for in the NEATM model. Ourtwo epochs of observation are at phase angles of α ∼ ◦ , but on opposite sides of the body. (656)Beagle is on a low-inclination orbit, so if we assume the rotation pole is oriented perpendicular tothe orbital and ecliptic planes and that the rotation is prograde, then the data from the low-highepoch would correspond to the afternoon hemisphere and the data from the low-low epoch wouldcorrespond to the morning hemisphere. Future work will implement a full thermophysical model ofthis object to test if the W2 and W4 excesses can be explained by a morning/afternoon dichotomy.For all other objects in the low-high group which were only observed at a single epoch, we cannotcurrently differentiate between poor fits to the beaming parameter and actual excesses in the W2and/or W4 bands.An alternate possibility is that these fits are indicative of problems with the flux measurementof partially saturated sources in the WISE data. Cutri et al. (2012) discuss the process by whichfluxes are measured for saturated sources through PSF-fitting photometry. Flux measurementsare available for sources many magnitudes above the brightness where the central pixel saturatesthrough fitting of the PSF wings, however for very bright sources in bands W2 and W3, thereappears to be a slight over-estimation of the fluxes. None of the objects we fit here had W2 15 –magnitudes in this saturated regime, however the majority of objects with p W < . < The distributions of visible albedos for members of each asteroid family have much narrowerspread than the albedo distribution of the Main Belt as a whole (Masiero et al. et al. et al. et al. (2013) we investigate the distribution of p W for families as a more accurate tracerof the surface properties of these asteroids.Figure 6 shows the distribution of p W albedos for the 8 families where more than 20 membershad a p W albedo measurement. Asteroid families break into three clear groupings, following thethree peaks in the albedo distribution shown in Figure 3. Our dataset depends on reflected lightmeasurements, so high-albedo families are over-represented in the distribution compared with thepopulation of all known families, which is dominated by low-albedo families. The only low NIR-albedo family with more than 20 measured objects was (24) Themis, however other families such as(10) Hygiea, (145) Adeona, (276) Adelheid, (511) Davida, (554) Peraga (equivalent to other lists’Polana family), and (1306) Scythia also show low NIR albedos, but these families contain only asmall number of objects with measured p W . The families (4) Vesta, (8) Flora, (15) Eunomia, (208) 16 –Lacrimosa, (472) Roma, and (2595) Gudiachvili all have high p W and show only a small spreadin mean albedo, while (135) Hertha (equivalent to other lists’ Nysa family) and (254) Augusta jointhem at a lower significance level.The (221) Eos family is the only one of the large families to have a moderate NIR albedo, inbetween the high- and low-albedo populations, indicating that this family has surface properties thatare rare among the large Main Belt asteroids. The p W values for this family confirm the observedmoderate visible albedo as a separate grouping that could not be conclusively distinguished from thehigh p V population by Masiero et al. (2013). The Eos family parent has a K-type spectral taxonomyin the Bus-DeMeo system (DeMeo et al. µ m absorption feature typically associated with silicatessuch as olivine, but are distinct in spectroscopic principal component space from the majority ofS-class objects. Clark et al. (2009) and Hardersen et al. (2011) associate K-type objects with theparent body of carbonaceous chondrite meteorites, specifically CO chondrites, while Moth´e-Diniz(2005) show evidence that (221) Eos may have been partially differentiated. Broz & Morbidelli(2013a) calculate the time since the breakup of the (221) Eos family as 1 . − . et al. (2013) and Table 2. Families show narrowdistributions of albedos correlating with one of three major albedo groupings. 17 –We note that approximately half of the objects fit for the (298) Baptistina family had albedossimilar to the Eos family, while the remainder appear to be drawn from the high-albedo group.This result is based on only a small number of measured Baptistina members, and thus is notconclusive, however if confirmed would further impede attempts to assign a unique composition tothis family (cf. Reddy et al. et al. p W . The Main Belt is split into three regions by proper semi-major axis ( a ): the inner-Main Belt(IMB, 1 . < a < . . < a < .
82 AU), and the outer-Main Belt (OMB, 2 .
82 AU < a < . et al. (2013) and those that are members of the backgroundpopulation. The (221) Eos family stands out distinctly in the belt, although objects with similar p W are present in the background population in all three regions.These plots show the clear trend of albedo decreasing with distance from the Sun, howeverour observational bias against small, low albedo objects amplifies this effect. Broz et al. (2013b),Carruba et al. (2013), and Masiero et al. (2013) observe halos of objects beyond the limits oftypical family-identification techniques, however we do not see evidence for these halos in thebackground population in our dataset. Halos are typically associated with asteroids that havedispersed a large distance from the family center via Yarkovsky and gravitational forces, which willhave smaller diameters than objects that were above our sensitivity limit for p W determination.Our significantly smaller sample size than what is typically used in surveys investigating familyhalos may also contribute to their absence in our data. 18 –Fig. 7.— Proper orbital inclination (incl) vs eccentricity (ecc) for inner- (left), middle- (center),and outer-Main Belt populations (right), for objects associated with families (top) and backgroundobjects not linked to families (bottom). Colors of the points map the W1 albedo (from green toblack to magenta for increasing p W ), following Figure 1a. 19 –In Table 2 we present the orbital and physical properties for all families identified in Masiero et al. (2013) that had at least one member with a fitted NIR albedo. We list the name of thefamily, average proper orbital elements, largest (D max ) and smallest diameter (D min ) representedin our sample, W1 albedos with standard deviations, and number of family members with datasufficient to fit. We also provide for reference the mean p V and standard deviation from Masiero et al. (2013). For cases where only a single body had a measured p W (often but not always theparent body of the family), D min is marked with a ‘...’ entry and no standard deviation is given forthe mean W1 albedo for families with less than 10 members. Asteroids that have been incorrectlyassociated with families may have very different mineralogies and thus spectral behavior in the NIR,which could make those objects more likely to fulfill the selection requirements for measured p W .Thus, particular caution is necessary when dealing with families suffering small number statistics,especially families with only a single p W -fit object. We note that the mean p V albedos presented inMasiero et al. (2013) are based on larger numbers of objects and so will generally be more accuratethan the mean p W values given here.It is also possible to use the W1 albedo to further refine family memberships, particularlyfor confused cases such as the Nysa-Polana complex. Masiero et al. (2013) divided this complexinto a high albedo component with largest body (135) Hertha and a low albedo component withlargest body (554) Peraga which is nearly twice the diameter of (142) Polana. We use NIR albedoto reject objects from the low-albedo family that had moderate visible albedos but W1 albedoscharacteristic of the high-albedo family. Asteroids (261), (1823), (2717), and (15112) can thus berejected as members of the (554) Peraga group based on W1 albedo. We note that because of a typoin Masiero et al. (2013) (135) Hertha was mistakenly listed as associated with (554) Peraga insteadof with its own family, which we correct here. Walsh et al. (2013) present dynamical arguments todivide the (554) Peraga family into two sub-families, however we are unable to see any distinctionbetween these groups in visible or W1 albedo.
4. Conclusions
We present revised thermal model fits for Main Belt asteroids, allowing for the albedo in eachof the near-infrared reflected wavelengths to be fit independently. The 3 . µ m and 4 . µ m spectralregions covered by the WISE/NEOWISE W1 and W2 bandpasses are poorly probed in ground-based spectroscopy but can be used to provide insight into asteroid mineralogical composition byconstraining spectral slope. In total we present 3080 fits of p W and/or p W for 2835 unique MainBelt objects.The MBA population has three distinct peaks in our observed p W distribution at p W ∼ . p W ∼ .
16, and p W ∼ .
4. The high and low p W peaks correspond to the high andlow visible albedo groups observed previously, while the moderate p W peak corresponds to anintermediate visible albedo that is blended with the high p V objects in visible albedo distributions.The distribution of albedos we measure have a larger fraction of high-albedo objects than what 20 –was observed for the MBA visible albedo distribution, however this is an effect of the biases in oursample selection.Asteroid families have narrow p W distributions corresponding to one of the three observed p W peaks. The (221) Eos family represents the only significant concentration of objects nearthe peak at p W ∼ .
16, although other objects with this albedo that are not related to asteroidfamilies are scattered throughout the entire Main Belt region. This family also corresponds to anunusual ‘end member’ taxonomic classification, K-type, that has been suggested to correspond to apartially differentiated parent or olivine-rich mineralogy. NIR albedo measurements provide a wayto rapidly search the known population for candidate K-type objects in the Main Belt, and are apowerful tool that acts as a proxy for asteroid taxonomic type.Our results show that the majority of high albedo objects, believed to have surface compositionsdominated by silicates and similar to ordinary chondrite meteorites, show an overall reddening fromvisible to W1 wavelengths similar to what is seen in the NIR. The spectra become blue from W1 toW2, which is also seen in some meteorite populations, particularly the Eucrites. This overall pictureis consistent with a primarily-silicate dominated composition. Objects with moderate infraredalbedos show similar behavior across the wavelengths probed here, although the lower albedo valueat W1 may indicate subtle differences in composition from the high albedo population or even amix of different mineralogies.The low albedo objects in our sample show a much wider range of behavior in these spectralregions. Many object show red slopes across all wavelengths consistent with the NIR spectralbehavior of C/D/P-type objects. However approximately 10% of our population show a blue slopefrom visible to W1, even in spite of the biases against blue-sloped, low-albedo objects in our sample.These objects are associated with B and Ch spectral taxonomies. The blue visible-to-W1 spectralslope in the Ch class objects may be indicative of a significant absorption feature at W1 wavelengthsfrom minerals such as carbonates.The fits presented here are based on reflected light, and thus our sample will not accuratelyrepresent the true distribution of p W or p W . Small, low albedo asteroids as well as objects withblue NIR spectral slopes are more likely to be undetected in the W1 and/or W2 wavelengths andthus underrepresented in our population distributions. A larger survey with greater sensitivity inthese spectral regions is required to extend these results to a population comparable to the onewith measured diameters and visible albedos. Acknowledgments
JM was partially supported by a NASA Planetary Geology and Geophysics grant. CN, RS, andSS were supported by an appointment to the NASA Postdoctoral Program at JPL, administeredby Oak Ridge Associated Universities through a contract with NASA. We thank the referee forthe helpful comments that greatly improved this manuscript. This publication makes use of data 21 –products from the Wide-field Infrared Survey Explorer, which is a joint project of the Universityof California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology,funded by the National Aeronautics and Space Administration. This publication also makes useof data products from NEOWISE, which is a project of the Jet Propulsion Laboratory/CaliforniaInstitute of Technology, funded by the Planetary Science Division of the National Aeronautics andSpace Administration. This research has made use of the NASA/IPAC Infrared Science Archive,which is operated by the Jet Propulsion Laboratory, California Institute of Technology, undercontract with the National Aeronautics and Space Administration. 22 –Table 2: Average orbital and physical properties for asteroid family members with measured p W .Mean p V values are taken from Masiero et al. (2013) Family semimajor axis eccentricity inclination D max D min < p V > < p W > Sample(AU) (deg) (km) (km) Size00434 1.937 0.077 20.947 7.62 3.01 0.725 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
23 –
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