JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. E2, PAGES 3031-3041, FEBRUARY 25, 1993

The Enigmatic Object 2201 Oljato: Is It an Asteroid or an Evolved Comet?

Lucy A. MCFADDEN, 1'2 ANITA L. COCHRAN, 3 EDWIN S. BARKER, 4 DALE P. CRUIKSHANK, 5 AND WILLIAM K. HARTMANN 6

The study of the near-Earth object 2201 Oljato has resulted in many surprises in the decade in which it has been examined. Its orbital properties have been associated with meteor showers, and its modeled orbital evolution is chaotic, a property which might indicate a history related to comets. Telescopic observations of its visible and near-infrared spectral reflectance, broad-band visible and near-infrared photometry, infrared radiometric measurements, and radar echoes are reported here from two apparitions, 1979 and 1983. A look at all available observational data shows that this asteroid has a high radiometric albedo, a property not associated with comet nuclei. In certain wavelength regimes it is classified as an S-type asteroid, in others, an E-type, but its overall spectral reflectance is not typical of either taxonomic type, and neither type is thought of as cometlike. Unexpectedly high ultraviolet reflectance at the 1979 apparition was suggested to be the result of residual outgassing as in a comet. The UV photometric data are modeled as fluorescent emission from neutral species found in comets. The resulting calculations indicate a plausible value for OH and CN emission at 0.3085 and 0.38 grn relative to the observed range of active comets. Observations to monitor photometric variations and to search for gaseous activity using observational techniques designed specifically for this purpose were planned and executed in October 1992 to verify or refute the interpretations presented here.

INTRODUCTION

The difference between asteroids and comets is defined as

follows: asteroids have orbits predominantly between Mars and Jupiter, have no observable gas or dust coma and are detected by reflected sunlight. Comets, on the other hand, have elliptical orbits in which they travel through both the inner and outer solar system, have comas containing both gas and dust, and are detectable from both scattered sunlight from dust grains and fluorescence of dissociating molecular species. We do not know what a burned out comet looks like for two reasons: comets lose mass and

generate surfaces of unknown character as they age and gravitational perturbations of comets by both the giant and terrestrial planets result in orbital changes that may result in their mingling among the asteroids. Comets could well evolve into objects having asteroidlike orbits but with surface characteristics which we can relate to comets. Studying the orbital and surface similarities and differences between comets and asteroids is

currently a focus of scientific investigations of small bodies in the solar system. Previous studies which lend evidence to the possibility that comets may evolve toward asteroidlike orbits are reviewed below.

Opik [1963] observed that comets are constantly changing and that statistically some of them probably evolve into asteroidlike objects. Kres•k [1979] noted a striking similarity between the orbit of 2212 Hephaistos, a near-Earth asteroid (NEA) and that of Comet Encke. From dynamical evidence researchers have

•Now at Astronomy Department, University of Maryland. enow at Astronomy Department, University of Maryland, College

Park.

3Astronomy Department, University of Texas at Austin. nMcDonald Observatory, University of Texas, Ft. Davis. •NASA Ames Research Center, Moffett Field, California. aPlanetary Science Institute, Tucson, Arizona.

Copyright 1993 by the American Geophysical Union

Paper number 9ZIEO1895. 0148-0227/93/92JE-01895505.00

repeatedly concluded that a major component of NEAs, perhaps up to half the population, must be remnants of comets [(•pik, 1963; Wetherill, 1988] because no other mechanism can be shown to contribute the observed number of NEAs.

Milani et al. [1989] modeled the orbital evolution of near-Earth asteroids and defined a dynamical class of orbits with 2201 Oljato as the most representative object in the class. This class of asteroid orbits has close approaches to both Jupiter and the terrestrial planets and can, in their model, make a transition between other dynamical classes that might be related to cometary orbits. Numerical simulations of the orbital evolution of 1991 DA, for example [tlahn and Bailey, 1992], illustrate a connection between high-inclination, intermediate-period, small perihelion asteroids and Halley-type comets.

Associations of the orbits of meteor streams with orbits of

Earth-crossing asteroids [Drummond, 1982; Olsson-Steel, 1988; Drummond, 1991] are also suggestive of a cometary origin for some near-Earth asteroids. It should be noted that this association

assumes that meteor streams are derived only from comets and not from debris from asteroid-asteroid collisions.

Studies in areas not related to orbital dynamics also appear to suggest that some objects designated as asteroids may be remnants of comets. Perturbations in the magnetic field of the solar wind associated with 2201 Oljato [Russell et al., 1984], anomalous radar echoes from 2101 Adonis [Ostro eta/., 1991a], and an ultraviolet excess measured in the reflectance spectrum of 2201 Oljato analyzed in this paper have all been regarded as suggestive that these asteroids may be remnants of comets.

Until recent years, it has been impossible to observe comets and asteroids with the same instrumentation and to measure the surface

properties of both to compare their physical properties (see the review by A'Hearn [1988]). In this paper, measurements from both the 1979 and 1983 apparitions of the near-Earth asteroid 2201 Oljato are reported [McFadden et al., 1984a]. Spectral reflectance measurements acquired in 1979 [McFadden et al., 1984b] are analyzed assuming resonance fluorescence of cometary species to test the hypothesis that this asteroid might be outgassing and thus be a remnant of a comet.

3031

3032 MCFADDEN ET AL.: THE ENIGMATIC OBJECT 2201 OLJATO

BACKGROUND

The object 2201 Oljato circles the Sun on an orbit with a semimajor axis of 2.18 AU, an eccentricity of 0.71, and inclination of 2.5 ø (Figure 1). This asteroid was initially discovered by H. L. Giclas in 1947 and rediscovered by Helin in the Earth-Crossing Asteroid Survey [Helin and Shoemaker, 1979] in December 1979, when it was designated asteroid 1979 XA. In 1983, Giclas named it Oljato, after the Place of Moonlight Water near Monument Valley, Utah, on the Navajo Indian Reservation (MPC 7782). Prior to the acquisition of any physical data, Oljato had been singled out as an unusual asteroid because of its orbital elements and a possible relation to the S. • Orionid meteor shower [Drummond, 1982]. Olsson-Steel [1988] also found the orbit of Oljato to be similar to a meteor stream. Independently, Russell et al. [1984] sought an explanation for perturbations in the interplanetary magnetic field at Venus' orbit and found a statistically significant correlation between the perturbations and Oljato's crossing of Venus' orbit. Spectral reflectance measurements made in 1979 [McFadden eta/., 1984b] were not characteristic of asteroid reflectance in the ultraviolet spectral region. Instead of having constant reflectance or values which decrease with decreasing wavelength, as is characteristic of rocky bodies, Oljato's reflectance was observed to increase with decreasing wavelength in the ultraviolet (Figure 2). As a result of these findings, L.A. McFadden and J.C. Gradie coordinated an observing campaign during Oljato's July 1983 apparition with the objective of checking the high UV reflectance and learning more about the asteroid's other physical properties. By this time, 1979 XA was properly known by its number and name, 2201 Oljato, the former having been allocated upon obtaining a recoverable orbit in 1979, the latter at the discretion of the discoverer.

OBSERVATIONS

In this section we report and review the results of telescopic measurements made during the 1979 and 1983 apparitions and we discuss the implications concerning the surface composition of 2201 Oljato. A tabulation of observations appears in Table 1.

i I I I I I I I i t I

Fig. 1. Projection of the orbits of 2201 Oljato, Earth, Mars, and Jupiter.

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0 U.3

IDS Spectrograph July, 1983 _ [] Photoelectric Photometer, Dec. 1979

ß Broad-band Photomerry Nov. 1979 -

i i i i i

0.4 0.5 0.6 0.7 0.8 0.9 1.0

Wavelength (pm) Fiõ. 2. The spectra] reflectance of 2201 O]jato from 0.3 to 1.0 Ixm as measured by IDS spectroscopy (solid curve), intermediate-band spectrophotometry (squares), and UBV photometry (triangles).

Ultraviolet and Visible Measurements.

Eighteen filters spanning the visible and near-infrared region, comprising the intermediate-band spectrophotometry system, were used in December 1979 for measurements of Oljato [McFadden et al., 1984b]. These data show an excess reflectance in the ultraviolet between 0.33 and 0.46 grn (Figure 2). The procedure for reducing photometry to spectral reflectance for this data set is discussed in Appendix A because the results are unanticipated. The high ultraviolet reflectance which was observed on two different nights cannot be attributed to any instrumental or observational circumstances. During this same run the main belt asteroid 9 Metis was observed, and its measured reflectance

spectrum agrees with previous measurements. It was suggested by P. Weissman (private communication, 1981) that the apparently high reflectance measured for Oljato might actually be flux from cometary emission bands, a suggestion that led to the analysis presented in the next section.

The continuous spectrum in Figure 2 was obtained with an intensified dissector scanner (IDS) spectrograph at the McDonald Observatory's 2.7-m telescope by A.L. Cochran and E.S. Barker on July 6, 1983, using observational and reduction techniques discussed by Cochran et al. [1992]. Because the spectrum is sampled at all wavelengths simultaneously and clouds have no color dependence, observations made in nonphotometric conditions can be used to determine relative reflectance. Such was the case

for these measurements of Oljato obtained postperihelion. The spectrum is an average of measurements through two different slits first exposed in one slit followed by the other according to an A-B-B-A pattern. The agreement between spectra from both slits was 5% or better. Total integration time was 1600 s. The nonphotometric conditions necessitated the use of mean extinction coefficients for McDonald Observatory to correct flux from an airmass of 1.56. Any color dependence not accounted for using mean extinction corrections is less than the 5% repeatability of the data collected through the two different slits at this airmass. There is a 60% decrease in reflectance between 0.35 and 0.65 •tm measured by this instrument, and no cometary emission features were detected to within the 5% noise level of the data.

The results of broadband UBV photomerry measured in November 1979 by E. Bowell and A. Harris are given by Harris and Young [ 1983]. The UBV data converted to reflectance relative to the Sun are presented in Figure 2. The B-V and U-B colors of 2201 Oljato, 0.83 and 0.38 +0.02, respectively, are within the

MCFADDEN ET AL.: THE ENIGMATIC OBJECT 2201 OLJATO 3033

TABLE 1. Compilation of Data of 2201 Oljato

UT Date Instrumentation Telescope r A [5

Dec. 15, 1979 photometer (UBV) Anderson Mesa 1.22 0.30 34 Dec. 16, 1979 photometer (UBV) 1.07 m 1.21 0.29 35 Dec. 17, 1979 photometer (UBV) 1.20 0.29 38 Dec. 18, 1979 photometer (UBV) Table Mountain 1.19 0.28 40 Dec. 19, 1979 photometer (UBV) 0.6 m 1.17 0.28 42 Dec. 20, 1979 photometer (UBV) 1.16 0.28 44 Dec. 20, 1979 photometer (JHK) IRTF 3 m 1.16 0.28 44

Dec. ,28, 1979 two-beam photometer MKO 2.2 m 1.07 0.25 63 Dec. 29, 1979 (0.3-0.86 [xm) 1.06 0.25 66 June 13, 1983 radar (13 cm) Arecibo 0.93 0.20 106 June 14, 1983 radar (13 cm) 0.94 0.20 104 June 16, 1983 radar (13 cm) 0.96 0.19 98 June 17, 1983 radar (13 cm) 0.98 0.18 95 June 14, 1983 radiometer (10 •rn) IRTF 3 m 0.94 0.20 104 July 6, 1983 intensified dissector

scanner McDonald 2.7 m 1.20 0.30 46

Observer

E. Bowell and A. Harris

Harris and Young [1983] Harris and Young [1983] Harris and Young [1983] Harris and Young [1983] Harris and Young [1983] B.T. Soifer, G. Neugebauer and

E. Becklin

McFadden et al. [1984b] McFadden et al. [1984b] Ostro et al. [199 la] Ostro et al. [1984a] Ostro et al. [1991a] Ostro et al. [1991a] Veeder et al. [1989]

A J•. Cochran and E.S. Barker

i i i i i i i i U-B R

0.5

0.4

0.3

I

I 'i'-J 0.2 ' M I

I I I _1

I I I i I I I I

0.6 0.7 0.8 0.9 B-V

Fig. 3. Domains of the asteroid types in UBV colors. UBV measurements of 2201 Oljato plot in the S field. Figure modified from Zellner, [1979].

range found to be typical of S-type asteroids (Figure 3), a type commonly found in the inner asteroid belt and among Earth-approaching asteroids.

Compositional Implications of an S-type Asteroid S-type asteroids have large B-V and U-B color indices and a

moderate albedo (mean albedo of 0.15) [Zellner, 1979]. In addition, they have a moderately red reflectance spectrum, and and 1-and 2-[tm absorption bands. The inferred mineralogy of an S-type asteroid is metal, olivine, and pyroxene [Gaffey et al. 1989]. Current interpretations support a differentiated composition of the

debated [Wetherill and Chapman, 1988]. Because most S-type main belt asteroids are located in the inner belt, they probably formed in that region of the solar nebula.

Discussion of Visible Data The visible IDS spectrum does not confirm the initial

spectrophotometric results reported by McFadden et al. [1984b] but agrees with the 1979 UBV photometry of Bowell [Harris and Young, 1983]. Complete spectral overlap does not exist between the IDS and the two-beam photometer that was used in 1979, but there is sufficient overlap to detect an excess of UV flux in the IDS data if it were there. The IDS is routinely used to observe cometary emission bands [e.g., Cochran et al., 1992]. Either the spectrophotometry measurements from 1979 are erroneous or else the data reveal an event that was active at 1.06 AU preperihelion in 1979 and not at 1.16-1.20 AU when the UBV photometry was acquired approximately one month earlier. Unfortunately, the IDS spectra at the 1983 opposition were measured postperihelion and at the same heliocentric distance as the preperihelion UBV photomerry in 1979. It is therefore important that the ultraviolet spectrum of this object be monitored at future apparitions, both preperihelion and postperihelion because the physical properties may be different at these times.

Infrared Measurements Infrared photometry at 1.25, 1.6, and 2.2 [tm (JHK) was obtained

December 20, 1979, by B.T. Soifer et al. at an air mass of 1.45 at the Infrared Telescope Facility (IRTF), Mauna Kea, Hawaii. The standard star fluxes were consistent to 1.2%. Magnitudes of 13.34 +0.06, 12.95 +0.05, and 12.93 :k-0.05 were derived from the standard photometric system [Elias et al. 1982]. The infrared color indices are J-H= 0.39 +0.11 and H-K= 0.02 :k0.10. These values are plotted against taxonomic domains in JHK space in Figure 4 and fall in the region of overlap among C-, S-, and E-types [Veeder et al., 1982].

The JHK magnitudes were converted to reflectance and scaled to 1.0 at the K filter using solar color differences of J-H = 0.29 and H-J= 0.06 [Campins et al., 1985]. Figure 5 shows these and other infrared data sets plotted as reflectance. During the same run, a higher-resolution infrared spectrum

mafic silicates for most of the S-types but the subject has been (A•)• = 5%) measured over a half-hour interval relative to the

3034 MCFADDEN ET AL.: THE ENIGMATIC OBJECT 2201 OLJATO

0.9

0.8

0.7

0.6

0.4

0.3

0.2

I I I I

L x /

\ / \ /

\ /

0.0 0.0 0.1 0.2 0.3

H-K

Fig. 4. Domains of asteroid types in JHK colors provided by G. Veeder, 2201 Oljato, falls in the region that overlaps with S- and C-types and EMP- types as well, though the plot would be too confused to show the EMP fields.

2.0

1.8

1.6

o 1.4

'• 1.2

rr' 1.0

.>_ 0.8

a• 0.6

0.4

0.2

0.0

ß 2ø1o CVF spectrum

ß J,H,K photometry

ß 5% CVF spectrum A•J•.=5ø/o -- IDS spectrum

0.3' ' 0'.6' ' 019' ' 112' ' 115' ' 118' ' 211 214 Wavelength (!•m)

Fig. 5. Visible and near-infrared reflectance of 2201 Oljato acquired in 1983.

standard star BS 0813+16, spectral type F0 was acquired. A correction factor of 0.6% would convert relative flux to

reflectance, taking into account the temperature difference between F0 and G2 stars at 2.2 gan. This correction is less than observational error and has not been applied to the data. The flux is scaled to unity at 2.25 [xm. The signal to noise is not sufficiently high and the data are not sufficiently duplicated to positively identify any significant spectral absorption features in this spectrum (Figure 5).

A spectrum with 2% spectral resolution from 0.95 to 2.45 was recorded by D.P. Cruikshank and W.K. Hartmann, June 19, 1983, at the IRTF relative to a solar analog star measured at approximately the same air mass (Figure 5). Reflectance is calculated by dividing the flux of Oljato by the solar analog. The spectrum is scaled to 1.0 in the center of the spectral region since it is flat to within the experimental error of the measurements. No absorption bands or reddening are detected within the 10-20% experimental error of the data.

Discussion of IR Measurements Interpreted alone, the near-IR spectrum suggests a C- or E-type

asteroid because of its featureless spectrum and absence of increasing reflectance with increasing wavelength (reddening). The apparent absence of absorption bands at 1.0 and 2.0 [xm is not consistent with the spectral characteristics of S-type asteroids [e.g., Feierberg et al., 1982] which have three spectral features: a UV absorption edge, a 1.0-[xm and a 2.0-[xm absorption band, and a moderate to high albedo (0.15-0.25). The combined visible IDS spectrum and near-IR spectrum show spectral features not commonly found together in asteroid spectra. The high albedo and flat near-IR spectrum of E-type asteroids are not usually found with steep UV slopes such as was measured from Oljato. Of the 102 asteroids in the 52-Color Asteroid Survey (J.F. Bell et al., unpublished atlas, 1987), only two, 387 Aquitania and 422 Berolina (both classified by Tholen as type S), have the combined spectral features of a steep UV slope and a relatively flat near-IR spectrum, though Burbine et al. [1992] discuss a 2.0-gxn band in the spectrum of Aquitania. Near-IR measurements with higher signal to noise might reveal weak absorption bands.

Radiometric Measurements

Radiometric measurements of Oljato at 10 [xm were made on June 14, 1983, UT by G.J. Veeder with the IRTF [Veeder et al., 1989] The standard model [Lebofsky and Spencer, 1989] which assumes a low thermal inertia, slowly rotating, spherical body in instantaneous thermal equilibrium, and V(1,0) = 15.86 [Harris and Young, 1983] yields a radiometric albedo of 0.52 and diameter of 1.2 km with uncertainties proportional to the visual lightcurve amplitude which corresponds to about 10% in albedo (see discussion by Veeder et al. [1989]. Using a modified model for cases of rapidly rotating objects (unlike Oljato) and high thermal inertia (like that of a bare rock surface) yields an effective diameter of 1.4 km and an albedo of 0.42 [Veeder et al., 1989]. With either model, the albedo is in the high range for asteroids. If the rotation period is 24 hours or a multiple thereof [Harris and Young, 1983], one would not expect the fast rotating, high thermal inertia model to be applicable. Therefore, until the rotation period is confirmed, the appropriate radiometric model remains uncertain.

In 1983 the Infrared Astronomical Satellite (IRAS) measured the 12-, 25-, 60- and 100-[xm flux of a large number of astronomical sources including the asteroids. Eight sightings of 2201 Oljato were made on June 20, 1983. Four of the eight sightings satisfied the criteria for processing of the asteroid data published as the

MCFADDEN ET AL.: THE ENIGMATIC OBJECT 2201 OLJATO 3035

Infrared Astronomical Satellite Asteroid and Comet Survey [Matson, 1986]. Using the standard thermal model described in the catalogue, the albedo of Oljato is 0.33 _+ 0.07 and its diameter is 1.90 +0.19 kin. Systematic calibration errors between ground-based and IRAS data most likely explain the different values of albedo and diameter.

When the range of derived geometric albedos, from 0.33 based on the IRAS measurements to 0.52 with ground-based radiometry, is combined with the J-K color, 2201 Oljato plots in the region occupied by E-type asteroids (Figure 6). Its albedo is inconsistent with S, C, or D types.

0.6

0.5 IRTF

.

0.3'

0.2

0.1

STD

ROCKY

IRAS !

lO

n p

ß S ß M

o C & E

A Oljato

o o o []

El:jOG

0.0 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J-K

Fig. 6. Albedo versus J-K color showing the location of the three different albedo derivations of 2201 Oljato. All three derived albedo values result in Oljato being in the E-type asteroid domain described by these parameters.

Radar Observations

Radar observations of Oljato were carried out by Ostro et al. [1991a] (see also Ostro et al. [1985]) on June 13, 14, 16, and 17,

ß

1983, with Arecibo Observatory's dual-polarization, 13-cm system. Each date yielded echo power spectra obtained in the same circular polarization as transmitted (the SC sense) and the opposite (OC) sense. Oljato's mean circular polarization ratio (go = SC/OC -

nature as the triple peak in the OC echo from Comet Iras-Iraki-Alcock [Harmon et al., 1989; Goldstein et al., 1984]. Other near-Earth asteroids 4769 Castalia (1989 PB) [Ostro et al., 1990] and 1986 DA [Ostro et al., 1991b] also show multiple peaks and variations in their OC echo signatures as a function of rotational phase angle. This type of signature indicates structure in the centimeter-to-meter-scale range that is extremely complex. Such an echo could be caused by a significant irregularity in the surface of the object such as would be found if it had been disrupted by collision. Ostro et al. [1991b, p. 1401] describe the shapes from such radar signatures as "extremely irregular, highly nonconvex, and possibly bifurcated."

If the pole orientation of Oljato were known, more certain constraints on its maximum diameter D,,•, could be derived. The initial estimate of Oljato's rotation period from UBV photomerry [Harris and Young 1983] is not confirmed by the radar data.

Using radiometrically determined diameters from ground-based radiometry and IRAS, the radar albedo can be constrained by the expression for radar albedo, •o• = c•o•/Avm, where c• is the object's radar cross section, a measured quantity, and %oj-' •(D/2) 2 is the apparent projected area of the object. Calculation of A•j using the radiometric diameters, 1.2, 1.4, [Veeder et al., 1989] and 1.9 km [Matson, 1986] combined with the measured radar cross section of 0.65 krn 2 [Ostro et al., 1991a] results in radar albedos of 0.57, 0.42, and 0.23 respectively. The first two values are comparable to the M-type NEA, 1986 DA's value [Ostro et al., 1991b], the highest estimated so far for any asteroid or comet. Such a high radar albedo generally indicates a nearly totally metallic composition. The lowest estimate calculated with the IRAS diameter for Oljato overlaps the top of the range of main belt S-type asteroids and is less than that of 16 Psyche which is probably also a metallic asteroid. Using similar data presented and arguments discussed by Ostro et al. [1991b] (Figure 7), a radar albedo of 0.57 is consistent with either a solid surface of E- or H-type ordinary chondrite reflectivity. No powdered meteorites have radar reflectivities this high. The 0.42 albedo is consistent with a solid E-, H-, or L-ordinary chondrite surface or a powdered metallic iron assemblage with-38% porosity. The 0.23 albedo is characteristic of a solid LL ordinary chondrite or carbonaceous chondrite surface, or a stony iron with -43% porosity. It is clear that the uncertainties in the available data are too large to produce meaningful constraints on its surface composition and structure.

Discussion of Albedo Measurements

A high albedo is unexpected for an object considered to be a possible extinct comet, based on recent results for other comets. For example, Hartmann et al. [1982], Tholen et al. [1986], Jewitt and Meech [1988], A'Hearn [1988], and others have affirmed the

0.31 +0.02) is larger than values estimated for most NEAs and spectral and colorimetric similarities of comets withlow-albedo C-, reveals a high degree of centimeter-to-meter-scale roughness. The P-, and D-type asteroids but not with the higher-albedo classes S- daily echo spectra have different shapes and bandwidths, ruling out and E-types. Moreover, Hartmann et al. [1987] surveyed the best apparent spin periods commensurate with 24 hours which were available comet albedos and found an average of Pv = 0.05 _-+0.036. reported from UBV photometry [Harris & Young, 1983]. No They further found that among a sample of 10 asteroids which had periodicity sufficient to allow a determination of the rotation been dynamically classified as likely extinct comet nuclei by period is seen in the radar data. The bandwidth variations suggest that Oljato's pole-on silhouette is at least moderately elongated, by a factor > 1.4. The average bandwidth is about 1 Hz, and this result constrains the pole-on silhouette's maximum breadth, D,•, the apparent rotation period, P, and the subradar latitude, /5, to satisfy the equation D• > P/28 cos /5. A double peak in the June 17 OC echo is significantly above the noise level and is unusual by main belt asteroid standards. It is of the same general

earlier authors and for which spectral data are available, all 10 had spectra of the C-, P-, or D-type (one being type B, a low-albedo C-subtype). Of a control group of 13 Atens, Apollos, and Amors on noncometary orbits, 12 of 13 were of high albedo types.

Emerging from observations of cometary nuclei is a realization that comets and dark asteroids condensed in the outer parts of the solar system where black carbonaceous dust strongly darkens ice and refractory condensates. The observed high albedo calls into

3036 MCFADDEN ET AL.: THE ENIGMATIC OBJECT 2201 OLJATO

questions Oljato's postulated cometary behavior discussed in the next section. This question must be reconciled with future observations.

ANALYSIS OF INTERMEDIATE-BAND

SPECTROPHOTOMETRY AS FLUORESCENT EMISSION

An analysis of the UV data assuming fluorescent emission of OH and CN, is made to investigate the plausibility of cometlike activity at this asteroid. It is performed because the high ultraviolet flux is not characteristic of reflected sunlight fxom asteroidal surfaces, no systematic instrumental or calibration error seems likely to explain the measurements, and the excess is observed at wavelengths where emission occurs in comets. The filters used were not designed to detect cometary emissions consequently their band centers are not located at the emission band heads and the bandwidths are much broader than cometary emission features thus

1.0

• 0.9

0 LU 0.8

r,r' 0.7 O

•- 0.6

z 0.5

"0.4

0 o 0.3 z o •- 0.2

LU 0.1

CHONDRITES POWDERED * METEORITES

- ..

- • 1 2km

-•i '--' k•--••.•k-"m '- E •I. ----- , _ - • km•,C,,••"/•r,- ß •

[METEORITESI , ,.,,v,e C• I I

0.0 30% 40% 50% 60% POROSITY

Fig. 7. Radar reflectivity versus meteorite type and porosity from Ostro et al., [1991b].

introducing significant uncertainties into •e analysis. The results are not accurate measurements of production rates of volatile species but give an indication of the consistency of the outgassing hypothesis with the observational data. Comparison with comets is made to evaluate the plausibility of the hypothesis and to provide a basis for designing future observations of Oljato.

This analysis follows the steps described by A'Hearn [1983], including the following steps: (1) continuum subtraction, (2) conversion fxom relative reflectance to absolute flux, (3) convolution of the filter transmission and emission band profiles with the measured effective flux, and (4) calculation of column density and production rate of the species using a Haser [1957] model.

Continuum Subtraction The excess flux above a continuum is converted to absolute flux

to calculate the energy in an emission band. The reflected component of the asteroid comprises the continuum. The IDS spectrum of Oljato which has no emission features is used to derive the continuum (Figure 2). The values are listed in Table 2 and discussed in Appendix B. Propagation of errors in convolving the continuum with the filter transmission curves is negligible. The resultant continuum is subtracted from the relative reflectance and is listed in Table 3. Only one species is assumed to be emitting in the band pass of the filter. If Oljato has the traditional component of cometar), emissions, then both OH and NH are emitting in the 0.33-1•m filter. Because this calculation is highly uncertain, a correction for this fact would be meaningless.

TABLE 2. Constants for Conversion to Absolute Flux

•,, Normalized Normalization Luminosity Weighted Emissien Ixrn Continuum Factor Factor Solar Contn•

Flux

0.33 0.52 1.16 x 10 -5 1.34 x 1012 91.56 623.88 0.35 0.56 1.16 x 10 -5 1.34 x 10 -12 101.74 1100.30 0.38 0.64 1.16 x 10 -5 1.34 x 10 -12 115.57 797.41 0.40 0.66 1.16 x 10 -5 1.34 x 10 -12 153.35 524.23 0.43 0.74 1.16 x 10 -5 1.34 x 1012 180.65 696.38 0.47 0.82 1.16 x 10 -5 1.34 x 10 '12 209.23 443.45 0.59 0.90 1.16 x 10 -5 1.34 x 10 -12 197.37 651.03 0.54 0.97 1.16 x 10 -5 1.34 x 10 -12 189.95 805.97 0.57 0.98 1.16 x 10 -5 1.34 x 10 -12 185.35 608.75

TABLE 3. Conversion From Relative Reflectance to Absolute Flux

Relative

Reflectance Minus Unnormalized, x Luminosity

Continuum cps Factor Solar Flux,

erg s-lcm -2 x Emission

Filter Profiles

0.33

0.35

0.38

0.40

0.43

0.47

0.50

0.54

0.57

1.73

1.19

0.92

0.77

0.77

0.88

0.92

0.89

0.99

1.21 1.42 x 10 -5 1.89 xlO -17 1.73 X 1015 1.08 X 10 -12 0.63 7.30 x 10 -6 9.76 x10 -lg 9.93 x 10 -16 1.09 x 1012 0.29 3.33 x 106 4.45 X10 -18 5.14 X 10 -16 4.10 X 1013 0.10 1.22 x 106 1.63 x101• 2.50 x 10 -16 1.31 x 10 -13 0.02 2.86 x 10. 7 3.83 x10.19 6.91 x 10 -17 4.81 x 10 TM 0.06 6.58 x 10 -7 8.79 x10 -19 1.84 x 1016 8.16 x 10 TM 0.01 1.56 x 10. 7 2.09 x10 -19 4.12 x 10 -17 2.68 x 10 TM

-0.08 ....

0.01 1.36 x 10 -7 1.83 x10 -19 3.39 X 1017 2.06 X 10 '14

MCFADDEN ET AL.: THE ENIGMATIC OBJECT 2201 OLJATO 3037

Conversion From Relative Reflectance to Absolute Flux The conversion from relative reflectance to absolute flux is not

routinely done for the intermediate-band photometric system, yet the procedure is simple, and details are discussed in Appendix B. The multiplicative constants of the equation are listed in Table 2. The absolute flux as a function of wavelength and the intermediate values from the calculation described here are listed in Table 3.

The largest uncertainties in this calibration arise from the error in the known distance and radius to the standard star, *1 Psc. The uncertainty in the flux calibration is approximately 50%.

Convolving Filter Transmission and Emission Band Profiles This step accounts for the contribution from the anticipated

emission bands observed through the filters as described by A'Hearn [ 1983] and illustrated in Figure 8. The expression for this step is

F•. = F,ff fIx(3,)T0•)d)• (1) The details of this conversion are given in Appendix B and the constants are listed in Table 2. The uncertainty added to the calculation from this convolution is included in the 50%

uncertainty estimated above.

Column Density and Production Rate The procedure for calculating column density and production

rates of cometary species from UV and visible region photomerry has become routine from years of analysis pioneered by M.F. A'Heam, and colleagues [e.g., A'Hearn and Cowan, 1975; A'Hearn et al., 1979; Schleicher et al., 1989]. The flux is proportional to the number of molecules in the field of view assuming an optically thin coma and is expressed as column abundance N:

10

I I I , I • II I

0.30 0.35 0.40 0.45 0.50 0.55

WAVELENGTH (microns)

0.8

- 0.4 •

z

- 0.2

o 0.60

Fig. 8. Spectrum of a typical comet (Comet Tuttle). Flux scale is on the leh versus wavelength, transmission functions of the intermediate-band filters plotted with percent transmission on the right scale.

N = F•.(3,) g (2)

where g is the fluorescence efficiency of the species, a function of the transition producing the band, the heliocentric velocity of the object, object-Sun, and object-Earth distances. The column abundance times the aperture area yields M, the total number of molecules, and is listed in Table 4 for OH and CN.

The calculation of production rate is model dependent but comet physicists often use the Haser [1957] model making comparison of production rates among different comets possible. The Haser model converts the column abundance to the total number of

molecules in the coma given the scale lengths of the parent and daughter molecules and an arbitrarily assumed expansion velocity, v, of 1 km

M(p) = •-p [ Ko(y) dy + -•(1 - -•) + K,(IXx) - K,(x)] (3) Where M(p) is the total number of molecules derived from the column abundance and the aperture area. Q is production rate and v is expansion velocity. The scale lengths used here are from Schleicher et al. [1987, and references therein], scaled to heliocentric distance and heliocentric velocity in the cases of OH and CN where the Swings effect is known; x is the product of reciprocal scale length times the aperture radius, p; Ix is the ratio of the reciprocal scale lengths of the parent molecule relative to the daughter molecule. Ko and K• are modified Bessel functions of the second kind. The results of the production rate calculations for water, and the parent molecules of CN are listed in Table 4.

DISCUSSION OF PRODUCTION RATES

The derived production rate of water from 2201 Oljato is 1.3 _+ 0.7 x 1027. This value falls within the range of that observed in comet P/Neujmin-1 which averaged 7.5 x 102• over 3 days [Campins et al., 1987]. P/Neujmin-1 is considered a low-activity comet, it has a dust coma and is sometimes recognized as a comet when viewed through a telescope, although it has also been confused with being an asteroid because of its low activity.

The matter of the presence of a dust coma has not been addressed observationally for Oljato. It is assumed that there was no dust coma during the observations which enable the reflectance spectrum measured in 1983 to represent the continuum subtracted from the 1979 data. At the time of the observations, no one was

looking for a coma, no measurements with varying aperture sizes were made which would indicate a coma if the increase in

brightness was not proportional to the area of the apertures. The image on the television guider system was fuzzy but so were those of all asteroids at low light levels. Thus no evidence of a dust coma exists at present.

TABLE 4. Production Rate Parameters and Values

g•

ergs s 4 mol

X vXa M(Total), km km molecules

Q mol s 4

Qo./Q "Normal"

OIl(0-0) 0.33 1.53 x 1045 4.52 X 10 4 1.28 x 10 s 1.7 x 10 TM 1.3 +_ 0.7 x 10 27

CN(Av=0) 0.38 3.71 x 1043 1.72 X 10 4 3.31 x 10 • 3.0 X 10 29 8.9 + 4.5 x 1023 330 + 160

3038 MCFADDEN ET AL.: TIIE ENIGMATIC OBJECT 2201 OLJATO

Production rates of water in Comet Halley were significantly Appendix A: Data Reduction Procedure higher than that calculated for Oljato ranging from 2 x 102• to to Derive Relative Reflectance

6 x 103øbetween 2.7 and 0.83 AU [Feldmanetal., 1987]. Because The two-beam filter photometer [McCord, 1968] used in the the human eye is not sensitive to the wavelength of the 1970s and early 1980s for asteroid spectral reflectance fundamental OH band at 0.3085 I.tm, the object would not measurements [cf. Chapman and Gaffey, 1979] was used to study necessarily have to appear to the human eye as a comet if it were 2201 Oljato on December 28 and 29, 1979 UT. A outgassing only water. The calculated production rate of water for Oljato falls between the lowest measured value for a known comet, and an active comet when located in the middle of the asteroid belt

and is thus a plausible value for a low activity object. The relative abundance of CN to OH can be compared to a set

Gallium-Arsenide-B photomultiplier tube with uniform sensitivity between 0.3 and 0.85 !.tm was operated at 1700 V. Dark counts due to thermal noise were 10-13 counts/s. A pair of mirrors chopped with a 40-ms integration time. The aperture was approximately 10 times the seeing disk or 15 arcsec. Data

of data labelled "normal" by Schleicher et al. [1987]. This set acquisition is described in terms of a "run", which is a set of includes the mean and sigma of the mean calculated from six measured fluxes through one tum of the filter wheel (each of 18 comets with heliocentric distance less than 1.7 AU at the time of filters in this case). One run was completed in approximately 2 observation and which were reduced using uniform model min. The observations reported here include three dark count runs parameters, the same ones used for the calculations for Oljato where the dark slide covered the photomultiplier and only thermal presented here.

The CN abundance is on the low side of the range observed in comets. The resultant QoH/c• ratio, 1460, is larger than the "normal" value, 330 _+ 160, listed in Table 4 by more than a factor of 4. In the case of Comet P/Neujmin-1, in spite of its low OH activity, its Qo,/cr,• at 1.7 AU was 325 and 350 on 2 of the 3 days [Campins et al., 1987] which is within the range considered normal. The third measurement of OH production of Neujmin-1 is a 30 upper limit and the resulting ratio is 830. If the noncometary abundance of OH relative to CN in Oljato is real, then this could be a chemical marker of the source region of asteroids (assuming Oljato is an asteroid) versus comets (assuming all comets come from the same chemical reservoir). Alternatively, it could mean that the assumption of fluorescent emission is wrong.

The production rate of water is within the range observed for comets, as is that of CN, though it is at the low end of the observed values. Whereas the results here are intriguing in that Oljato could have only been outgassing water vapor, it also casts suspicion on the data or the basic assumption upon which the data were analyzed. One could conclude that if the data are correct, then the chemical reservoir of Oljato is probably not cometlike. Questions still linger: Do burned out comets retain their primary molecular ratios, Are there chemical signatures of extinct comets that are different from active comets, or Could Oljato be a chemically odd comet?

In summary, the ratio of CN to OH in Oljato, assuming fluorescent emission of common cometary species, is not consistent with the normal ratios observed in comets. This

indicates that either the observations are erroneous, the analysis is not warranted, or the composition of this object is not similar to that of any comets observed to date.

CONCLUSIONS

Further study of the physical properties of 2201 Oljato were carried out at its last apparition in October 1992. Evidence of remnant cometary activity could be sought by observing with the standard comet filters [e.g., A'Hearn, 1983]. Synodic observations of Oljato's brightness would provide information about possible transient activity. Millis et al. [1988] using multiple aperture photomerry of comet Arend-Rigaux derive the percentage of flux attributable to a coma. It would be useful to apply this method for coma detection. Observations in Oljato's orbit for evidence of ejected gas and dust using an ultraviolet telescope (IUE), or a mid-infrared imaging camera might provide evidence for residual cometary activity.

noise from the system was recorded; three runs of blank sky to check for beam inequality in the system; 15 min of observations of the asteroid followed by three or four runs of the standard star •! Pisces measured through a neutral density, ND 2.0, filter. The standard star was remeasured every 20-30 min. A total of 50 runs on the asteroid and 19 runs on the standard star were acquired on December 28 UT between 0500 and 0800 UT when the asteroid

was at an air mass of 1.00-1.5. The main belt asteroid 9 Metis was

also observed on December 28. On December 29, 1979, UT, the

same procedure was used to acquire 35 runs of Oljato and 17 runs of the same standard star between 0630 and 0830 UT at an air

mass of 1.1-1.6. Asteroids 511 Davida and 9 Metis were both

observed on December 29.

Identification of the target was made based on the ephemeris and the direction and rate of motion of the object in the aperture. There is little chance of misidentifying a fast moving asteroid moving in its predicted direction and rate of motion. Identification of main belt asteroids which have much lower relative motion is

not always easy, especially in a crowded star field. For example, we determined that we were not observing 511 Davida on December 28 because the following day, we found an object at the same position that we observed the previous day. We are confident that we were measuring flux from Oljato which was approximately V magnitude 15.2. The telescope was guided continuously by keeping the image of the asteroid in the aperture.

The standard data reduction procedure to calibrate and convert raw counts to relative reflectance was used [see Chapman and Gaffey, 1979]. Because the flux levels were low, no coincidence corrections were needed. The asteroid was located 1 hour in right ascension (RA) and 1 ø in declination (dec) from the gibbous Moon at 70-80 ø phase.

The high relative reflectance in the three shortest-wavelength filters prompted close scrutiny of all data collected during this observing run. The extinction slopes in the UV were small so there is no reason to attribute the high reflectance to poor extinction correction. Perhaps there is a clue to the problem in this high UV transparency, but we do not understand it. •1 Pisces was located approximately 30 min in RA and 7 ø in dec from the asteroid and was observed frequently enough to determine good extinction corrections. Similarly, none of the instrument calibration factors, dark and sky corrections, beam inequality, chop phasing, or neutral density colors, can be attributed to the high relative reflectance seen in the data. There was no reason to conclude that the data

were erroneous, so they were published with the standard one sigma error bars, which for the ultraviolet filters are approximately 10% of the calculated reflectanceø

MCFADDEN ET AL.: THE ENIGMATIC OBJECT 2201 OLJATO 3039

Appendix B' Conversion From Relative Reflectance to Absolute Flux

The following data reduction procedures were carried out to convert the relative reflectance values of the asteroid to absolute flux. This is the first time such a calculation has been carried out with these data.

Continuum Subtraction

The data from the IDS spectrum of Oljato measured by Cochran and Barker were used to derive a continuum for this asteroid representing the surface reflectance. Because the IDS spectrum covers the spectral region from 0.35 to 0.6 I. Lm, a linear extrapolation to 0.31 I. tm was used to cover the region in which the observed excess in the intermediate-band photometry was observed. The spectral flux as measured with the IDS was converted to that which would have been measured through the intermediate-band filters using

j••z TOO R 0•) d)• I (B1)

; T ()0 d)• Where T is the transmission function of the intermediate-band

filters and R is reflectance measured at 0.0025-I.tm intervals with the IDS spectrometer. Continuum fluxes through the filters used by McFadden were derived by numerically integrating the relative reflectance from the IDS spectrum across the transmission function of the intermediate-band filter both functions of wavelength. Because the IDS spectrum was not obtained under photometric conditions, only the relative slope of the continuum is valid. The relative continuum was subtracted from the relative reflectance

scaled to 1.0 in the 0.56 [tm region for both spectra assuming negligible slope differences due to the phase angles at which the observations were made (65 ø photometry versus 46 ø IDS).

Absolute Flux Calibration

The expression from which the absolute flux is derived is

R • d 2 rlPsc q Psc sun FOljato(•, ) = Oljato (•.) x (•.) x N x x FSu n (B2) rlPsc Sun d 2 R 2

q Psc s•

similar spectral type, according to Table 99 of Allen [1976], was used. Note should be taken of the fact that no interpolation was made for calculating this ratio and the actual values used were the flux of a K0 Ill (assumed for this purpose to be equivalent to a G8 III, q Psc) and a GO V (assumed for this purpose to be equivalent to a G2 V, the Sun). Also included in the value of N is the measured instrumental normalization factor for Oljato/q Psc, 6.11 x 104, which converts from flux scaled to 1.0 at 0.56 I.tm to the instrumental flux ratio. The value of N is thus

N = 6.11 x 10 -5 x 10 (6'28'7'ø) = 1.16 x 10 '5 (B3)

The brightness factor of rl Psc relative to the Sun is derived from

R 2 2 • Psc]ao

d 2 ps/d20 (B4)

where R is radius and d is distance, in AU, of the star and Sun, respectively. Both the distance and radius ratio of rl Psc relative to the Sun were calculated from parameters found in the literature [Keenan, 1963; Allen, 1976] and calculated to be 1.3 + 0.3 x 10 -•2.

The radius of q Psc is derived from

(R/Ro) 2 = (L/L•) 2 / Tdrl'•) 4 (B5)

•md takes into account the luminosity difference between r IPsc, a G8 III star, and the Sun, a G2 V star. The luminosity expression is

L _ 10 -0'4(m-Mø) - 44. q- 16 (B6)

where M O = +4.71 + 0.03, is the magnitude of the Sun at 10 parsecs. The absolute V magnitude of r I Picses, + 0.6, + 0.3 is derived from its measured B-V color index, + 0.97 and its spectral type according to the table of Absolute Magnitude and Color Index [Allen, 1976]. Using T e of G8 Ili type stars of 4600 + 400K [Strand, 1963] and the Te of the Sun of 5740 + 30K [also Strand, 1963], the relative flux between the two stars is

The terms in this equation will be described from left to right. Oljato/q Psc is calculated in the standard reduction procedure described in Appendix A and has photometric uncertainties of 10% for the UV data under study here. The ratio q Psc/Sun is a color ratio scaled to a value of 1.0 at 0.56 gm and was derived from a standard-star observing program. The color uncertainties are probably of order of 10%.

The absolute flux ratio of q Psc relative to Sun, N, must be calculated since it cannot be measured at the telescope (it is not possible to measure a faint star and the bright sun with the same instrument). To convert qPsc/Sun ()•) to an absolute scale, the relative flux across the 0.56-gm filter must factor into the calculation. In this case there is no measured normalization factor

for this ratio. The ratio of the flux of rl Psc relative to the Sun was derived as follows: The quantity used is the flux ratio across the V filter bandpass for a G8 III star, rl Psc, versus a G2 V star, the Sun [Allen, 1976]. The derived flux ratio of two stars of

L /L 0 R' q Psc _ q Psc

dTo•) R20 = 1.10 + 0.04 x 102 (B7)

The distance to rl Psc is 44 + 5 parsecs, or 1/44 - 0.022 arc sec Hirshfeld and Sinnott [1982]. Thus the distance factor in units of AU is

d 2 r 1Psc_. 206265/0.022 arc sec = (9.07566 x 106) 2 = 8. + 2. x 10 •3 d 2 1

(B8)

The ratio of radius to distance is 1.34 + 0.34 x10 -•2. To convert this number to in-band flux of the 24-color filters in

units of ergs/s cm 2, we multiply by the weighted solar flux transmitted through our filters. We use the solar spectrum of Arvesen et al. [1969] which has a maximum uncertainty of 5% in the UV. The flux is converted to ergs/s cm 2 I.tm, integrated over

3040 MCFADDEN ET AL.: THE ENIGMATIC OBJECT 2201 OLJATO

our filter transmission curves to calculate the weighted solar in-band flux listed in Table 2, using

f•.x• T(•) Fo(•) d)• f T (•) d•,

(B13)

Where TOO is the transmission function of each filter (Figure B 1), and F© (•,) is the solar flux from Arvesen et al. [1969]. Multiplying by the values in Table 2 for the in-band solar flux gives a value called the effective flux.

Convolving Filter Transmission and Emission Band Profiles The last step accounts for the contribution to the in-band flux

from the anticipated emission bands as observed through the filters. The total emitted flux is calculated from

JI•(•,)d•,•T(•,)d•, F•. = F,ff JI•.(•.)T(•.)d• (B14)

Where I•()•) is the emission function of a cometary emission feature across the wavelength interval of the filter. A 5% uncertainty is assigned to the emission band profile and 1% to the filter transmission function. The estimated propagated errors for the convolution is 6%. The values for this integral appear in the column 6 of Table 2. The maximum uncertainty in the entire calculation propagating the errors of each component is 50%.

Acknowledgements. We thank the staff at Mauna Kea and McDonald Observatories for their assistance with data acquisition and B.T. Soifer, G. Neugebauer and E. Becklin for the use of their data. Useful discussions with M.F. A'Heam, M.J. Gaffey, S.J. Ostro, A.L. Harris and D.G. Schleicher are appreciated as are critical reviews of the manuscript by R.L. Millis and L.A. Lebofsky. This work was supported by NASA Planetary Astronomy Program. WKH thanks comments from colleagues at PSI, which is a division of SAIC. This is PSI Publ. 302.

REFERENCES

A'Heam, M.F., and J.J. Cowan, Molecular production rates in Comet Kohoutek, Astron. J., 80, 852-860, 1975.

A'Heam, M.F., R.L. Millis, and P.V. Birch, Gas and dust in some recent periodic comets, Astron. J., 84, 570-579, 1979.

A'Heam, M.F., Photometry of Comets, in Solar System Photornetry Handbook, edited by R.M. Genet, pp. 3.1-3.33, Willman-Bell, Richmond, Va., 1983.

A'Heam, M.F., Observations of cometary nuclei, Annu. Rev. Earth Planet. Sci, 16, 273-293, 1988.

Allen, C.A., Astrophysical Quantifies, 3rd ed., 310 pp., University of London-The Athlone Press, London, 1976.

Arvesen, J.C., R.J. Griffin, Jr. and B.D. Pearson, Determination of extraterrestrial solar spectral irradiance from a research aircraft, Appl. Opt., 8, 2215-2232, 1969.

Burbine, T.H., M.J. Gaffey, and J.F. Bell, S-asteroids 387 Aquitania and 980 Anacostia: Possible fragments of the breakup of a spinel-bearing parent body with CO3/CV3 affinities, Meteoritics 27, 424-434, 1992.

Campins, H., G.H. Rieke, and M.J. Lebofsky, Absolute calibration of photometry at 1 through 5pro, Astron. J. 90, 896-899, 1985.

Campins, H., M.F. A'Heam, and L.A. McFadden, The bare nucleus of comet P/Neujmin 1, Astrophys. J., 316, 847-857, 1987.

Chapman, C.R., and M.J. Gaffey, Reflectance spectra for 277 asteroids, in

Asteroids, edited by T. Gehrels, pp. 655-687, University of Arizona Press, Tucson, 1979.

Cochran, A.L., E.S. Barker, T.F. Ramseyer, and A.D. Storrs, The McDonald Observatory faint comet survey: Gas production in 17 comets, Icarus, 98, 151-162, 1992.

Drummond, J.D., Theoretical meteor radiants of Apollo, Amor, and Aten asteroids, Icarus, 49, 143-153, 1982.

Drummond, J.D., Earth-approaching asteroid streams, Icarus, 89, 14-25, 1991.

Elias, J.H., J.A. Frogel, K. Mathews, and G. Neugebauer, Infrared standard stars, Astron. J., 87, 635-638, 1982.

Feierberg, M.A., H.P. Larson, and C.R. Chapman, Spectroscopic evidence for undifferentiated S-type asteroids, Astrophys. J., 257, 361-372, 1982.

Feldman, P.D., et al., IUE observations of comet P/Halley: Evolution of the ultraviolet spectrum between September 1985 and July 1986, Astron. Astrophys., 187, 325-328, 1987.

Festou, M.C., The density distribution of neutral compounds in cometary atmospheres, I, Models and equations, Astron. Astrophys., 96, 69-79, 1981.

Gaffey, M.J., J.F. Bell, and D.P. Cruikshank, Reflectance spectroscopy and asteroid surface mineralogy, in Asteroids H, edited by R.P. Binzel, T. Gehrels and M.S. Matthews, pp. 98-127, University of Arizona Press, Tucson, 1989.

Goldstein, R.M., R.F. Jurgens, and Z. Sekanina, A radar study of comet Iras-Araki-Alcock (1983d), Astron. J., 89, 1745-1754, 1984.

Green, S.F., A.J. Meadows, and J.K. Davies, Infrared observations of the extinct cometary candidate minor planet (3200) 1983TB, Mon. Not. R. Astron. Soc., 214, 29-36, 1985.

Hahn, G., and M.E. Bailey, Long-term evolution of 1991 DA: A dynamically evolved extinct Halley-type comet in Asteroids, Comets, Meteors 1991, edited by A. W. Harris, E. Bowell, 227-230, Lunar and Planetary Institute, Houston, 1992.

Harmon, J. K., D.B. Campbell, A.A. Hine, I.I. Shapiro, and B.G. Marsden, Radar observations of comet IRAS-Araki-Alcock (1983d), Astrophys. J., 338, 1071-1093, 1989.

Harris, A.W., and J.W. Young, Asteroid rotation, IV, 1979 observations, Icarus, 54, 59-109, 1983.

Hartmann, W.K., D.P. Cruikshank, and J. Degewij, Remote comets and related bodies: VJHK 0olorimetry and surface materials, Icarus, 52, 377-408, 1982.

Hartmann, W.K., D.J. Tholen, and D.P. Cmikshank, The relationship of active comets, "extinct" comets, and dark asteroids, Icarus, 69, 33-50, 1987.

Haser, L., Distribution d'intensit6 dans la t•te d'une comete, Bull. Cl. Sci. Acad. R. Belg., 43, 740-750, 1957.

Helin, E.F., and E.M. Shoemaker, The Palomar planet-crossing asteroid survey, 1973-1978, Icarus, 40, 321-328, 1979.

Hirshfeld, A., and R. W. Sinnott, Sky Catalog 2000.0, vol. I, 604 pp, Sky Publishing, Cambridge, Mass., 1982.

Jewitt, D., and K. Meech, Optical properties of cometary nuclei and a preliminary comparison with asteroids, Astrophys. J. 328, 1194-1209, 1988.

Keenan, P.C., Classification of stellar spectra, in Basic Astronomical Data, edited by K.A. Strand, pp. 78-122, University of Chicago Press, Chicago, 111. 1963.

Kres•k, L, Dynamical interrelations among comets and asteroids, in Asteroids, edited by T. Gehrels, pp. 289-309, University of Arizona Press, Tucson, 1979.

Lebofsky, L.A., and J.R. Spencer, Radiometry and thermal modeling of asteroids, in Asteroids II, edited by R.P. Binzel, T. Gehrels, and M.S. Matthews, pp. 128-147, University of Arizona Press, Tucson, 1989.

Matson, D.L., IRAS asteroid and comet survey, preprint version No. 1, Rep. JPL D-3698, Jet Propul. Lab., Pasadena, !986.

McCord, T.B., A double beam astronomical photometer, Appl. Opt., 7, 475-478, 1968.

McFadden, LA., S.J. Ostro, E.S. Barker, A.L. Cochran, D.P. Cruikshank, W.K. Hartmann, B.T. Soifer, and G.J. Veeder, 2201 Oljato: An asteroid, a comet, or both?, Bull. Am. Astron. Soc., 16, 691, 1984a.

MCFADDEN ET AL.: THE ENIGMATIC OBJECT 2201 OLJATO 3041

McFadden, L.A., M.J. Gaffey, and T.B. McCord, Mineralogical-petrological characterization of near-Earth asteroids, Icarus, 59, 25-40, 1984b.

Milani, A., M. Carpino, G. Hahn, and A.M. NobHi, Dynamics of planet-crossing asteroids: Cases of orbital behavior Project SPACEGUARD, Icarus, 78, 212-269, 1989.

Millis, R.I•, M.F. A'Heam, and H. Campins, An investigation of the nucleus and coma of comet P/Arend-Rigaux, Astrophys. J., 324, 1194-1209, 1988.

Olsson-Steel, D., Identification of meteoroid streams from Apollo asteroids in the Adelaide radar orbit surveys, Icarus, 75, 64-96, 1988.

'Opik, E.J., The stray bodies in the solar system, pan I, Survival of cometary nuclei and the asteroids, Adv. Astron. Astrophys., 2, 219-262, 1963.

Ostro, S.J., Radar observations of asteroids and comets, Publ. Astron. Soc. Pac., 97, 877-884, 1985.

Ostro, S.J., D.B. Campbell, and I.I. Shapiro, Radar properties of near-Earth asteroids, Bull. Am. Astron. Soc., 17, 729-730, 1985.

Ostro, S.J., J.F. Chandler, A.A. Hine, K.D. Rosema, I.I. Shapiro, and D.K. Yeomans, Radar images of asteroid 1989 PB, Science, 248, 1523-1528, 1990.

Ostro, S. J., D.B. Campbell, J.F. Chandler, I.I. Shapiro, A.A. Hine, R. Velez, R.F. Jurgens, K.D. Rosema, R. Winklet, and D.K. Yeomans, Asteroid Radar Astrometry, Astron. J. 102, 1490-1502, 1991a.

Ostro, S.J., D.B. Campbell, J.F. Chandler, A.A. Hine, R.S. Huson, K.D. Rosema, and I.I. Shapiro, Asteroid 1986 DA: Radar evidence for a metallic composition, Science, 252, 1399-1404, 1991b.

Russell, C.T., R. Aroian, M. Arghavani, and K. Nock, Interplanetary magnetic field enhancements and their association with the asteroid 2201 Oljato, Science, 226, 43-45, 1984.

Schleicher, D.G., R.L. Millis, and P.V. Birch, Photometric observations of comet P/Giacobini-Zinner, Astron. Astro., 187, 531-538, 1987.

Schleicher, D.G., R.L. Millis, D.J. Osip, M.F. A'Heam, and P.V. Birch, Thirteen years of comet photometry, Bull. Am. Astron. Soc. 21, 936, 1989.

Strand, K.A., Basic Astronomical Data, in Stars and Stellar Systems, vol. Ill, edited by G.P. Kuiper and B.M. Middlehurst, pp. 263-272, University of Chicago Press, Chicago, 1963.

Tholen, D., D. Cmikshank, W. Hartmann, N. Lark, H. Hammel, and J.

Piscitelli, A comparison of the continuum colors of P/Halley, other comets, and asteroids, in 20th ESLAB Symposium on the Exploration of Halley's Comet, vol. Ill, pp. 503-507, European Space Agency, Noordwijk, The Netherlands, 1986.

Veeder, G.J., D.L. Matson, and C. Kowal, Infrared (JHK) photomerry of asteroids, Astron. J., 87, 834-839, 1982.

Veeder, G.J., C. Kowal, and D.L. Matson, The Earth-crossing asteroid 1983 TB, Lunar Planet. Sci., XV, 878-879, 1984.

Veeder, G.J., M.S. Hanner, D.L. Matson, E.F. Tedesco, L.A. Lebofsky, and A.T. Tokunaga, Radiomerry of near-Earth asteroids, Astron. J., 97, 1211-1219, 1989.

Weissman, P.R., M.F. A'Heam, L.A. McFadden, and H. Rickman, Evolution of comets into asteroids, in Asteroids H, edited by R.P. Binzel, T. Gehrels , and M.S. Matthews, pp. 880-920, University of Arizona Press, Tucson, 1989.

Wetherill, G.W., Where do Apollo objects come from?, Icarus, 76, 1-18, 1988.

Wetherill, G.W. , and C.R. Chapman, Asteroids and Meteorites, in Meteorites and the Early Solar System, edited by J.F. Kerddge and M.S. Matthews, pp. 35-67, University of Arizona Press, Tucson, 1988.

Zellner, B., Asteroid taxonomy and the distribution of the compositional types, in Asteroids, edited by T. Gehrels, pp. 783-808, University of Arizona Press, Tucson, 1979.

E. S. Barker, McDonald Observatory, University of Texas, P.O. Box 1337, Ft. Davis, TX 79734-1337. A. L. Cochran, Astronomy Department, University of Texas at Austin

Austin, TX 78712. D. P. Cmikshank, NASA Ames Research Center, MS 245-6, Moffett

Field, CA 94035-1000.

W. K. Hartmann, Planetary Science Institute, 2421 E. 6th St., Tucson, AZ 85719.

L. A. McFadden, Astronomy Department, University of Maryland, College Park, MD 20742-2421.

(Received September 6, 1991; revised August 10, 1992;

accepted August 12, 1992.)