Edward Bowell

Photoelectric observations of asteroids 3, 24, 60, 261, and 863

Abstract Observations of five asteroids, obtained from various observatories from 1978 to 1984, are presented. A Fourier analysis method was used to derive the composite lightcurves. This solution method yields a value for the rotation period, mean absolute magnitudes on each night of observation, and Fourier coefficients defining the shape of the composite lightcurve, to any degree specified. A major advantage of the method is that it yields formal error estimates for all the quantities computed. The Fourier coefficients derived can be used for studies of the shapes and pole orientations of asteroids and to define “rotation phase” in a more formal way, for connecting observations from one apparition to another. Periods, amplitudes, and phase relations are reported for four of the five asteroids. The presence of a substantial amplitude first harmonic in the lightcurves of 3 Juno, 24 Themis, and 60 Echo suggests that these asteroids may have significant surface albedo variegation. 60 Echo exhibits a difference of 0.2 magnitude in its mean absolute magnitude from different aspects, which suggests a somewhat polar-flattened figure. High-quality magnitude vs solar phase angle data were obtained for 24 Themis from 0°.3 to 21°. These data are perhaps the best available of a dark asteroid and provide a test for light-scattering theories.

Taxonomy of asteroids

Abstract A taxonomic system was introduced by C. R. Chapman, D. Morrison, and B. Zellner [Icarus 25, 104–130 (1975)], in which minor planets are classified according to a few readily observable optical properties, independent of specific mineralogical interpretations. That taxonomy is here augmented to five classes, now precisely defined in terms of seven parameters obtained from polarimetry, spectrophotometry, radiometry, and UBV photometry of 523 objects. We classify 190 asteroids as type C, 141 as type S, 13 as type M, 3 as type E, and 3 as type R; 55 objects are shown to fall outside these five classes and are designated U (unclassifiable). For the remaining 118, the data exclude two or more types but are insufficient for unambiguous classification. Reliable diameters, from radiometry or polarimetry or else from albedos adopted as typical of the types, are listed for 396 objects. We also compare our taxonomy with other ones and discuss how classification efforts are related to the interpretation of asteroid mineralogies.

A Critical review of theoretical models of negatively polarized light scattered by atmosphereless solar system bodies

About a dozen physical mechanisms and models aspire to explain the negative polarization of light scattered by atmosphereless celestial bodies. This is too large a number for the reliable interpretation of observational data. Through a comparative analysis of the models, our main goal is to answer the question: Does any one model have an advantage over the others? Our analysis is based on new laboratory polarimetric and photometric data as well as on theoretical results. We show that the widely used models due to Hopfield and Wolff cannot realistically explain the phase-angle dependence of the degree of polarization observed at small phase angles. The so-called interference or coherent backscattering mechanism is the most promising model. Models based on that mechanism use well-defined physical parameters to explain both negative polarization and the opposition effect. They are supported by laboratory experiments, particularly those showing enhancement of negative polarization with decreasing particle size down to the wavelength of light. According to the interference mechanism, pronounced negative branches of polarization, like those of C-class asteroids, may indicate a high degree of optical inhomogeneity of light-scattering surfaces at small scales. The mechanism also seems appropriate for treating the negative polarization and opposition effects of cometary dust comae, planetary rings, and the zodiacal light.

Radiative transfer in the surfaces of atmosphereless bodies. II. Interpretation

A glass composition particularly adapted for use with ceramic materials in electronic module applications having a thermal coefficient of expansion substantially matching the thermal coefficient of expansion of ceramic material, and a low dielectric constant less than 4.5. The composition is a borosilicate glass consisting essentially of SiO2, B2O3, CaO, A12O3, Na2O, K2O, BaO, ZrO2, and MgO in relatively precise amounts.

Asteroid lightcurve observations from 1979–1981

Abstract Results of photoelectric lightcurve observations from Table Mountain Observatory are reported. Most of the observations were made from January 1980 through March 1981. Some earlier observations are included from 1979 which were previouly only partially reported ( A. W. Harris and J. W. Young, 1983 , Icarus 54, 59–109), and some observations of one asteroid in 1984 are included. In addition to the printed paper, we provide (on request to the first author) a machine readable file of the observations. About 70 different asteroids are reported, in all. Several errors in our 1983 paper are corrected. These include a few preliminary period estimates from the 1980 data, which were listed in Appendix II of that paper, which proved to be in error on careful analysis. In all about dozen new or significantly revised periods are reported. All observations gere made in the V band, with estimates of the mean and maximum reduced magnitudes given for each object. Sufficient phase angle coverage was obtained for 33 objects to obtain fits of the H - G magnitude relation ( E. Bowell et al. 1989 , in Asteroids II (R. Binzel, T. Gehrels, and M. Matthew, Eds.), Univ. of Arizona Press, Tucson, in press) to the data. From these fits, we examine the mean values of the slope parameter, G , for different taxonomic classes. These values are then applied in the analysis of less complete data sets. For the moderate albedo asteroids (classes S and M), the H - G relation appears to fit the available data well. However, for dark asteroids, the relation appears to predict more of an opposition effect than is typically present.

Evidence for Distinct Components of the Galactic Stellar Halo from 838 RR Lyrae Stars Discovered in the LONEOS-I Survey

We present 838 ab-type RR Lyrae stars from the Lowell Observatory Near Earth Objects Survey Phase I (LONEOS-I). These objects cover 1430 deg2 and span distances ranging from 3 to 30 kpc from the Galactic center. Object selection is based on phased, photometric data with 28-50 epochs. We use this large sample to explore the bulk properties of the stellar halo, including the spatial distribution. The period-amplitude distribution of this sample shows that the majority of these RR Lyrae stars resemble Oosterhoff type I, but there is a significant fraction (26%) which have longer periods and appear to be Oosterhoff type II. We find that the radial distributions of these two populations have significantly different profiles ( -->?OoI ~ R?2.26 ? 0.07 and -->?OoII ~ R?2.88 ? 0.11). This suggests that the stellar halo was formed by at least two distinct accretion processes and supports dual-halo models.

A Public-Domain Asteroid Orbit Database

We are constructing a database of J2000.0 osculating elements for all asteroids that have reasonably calculable orbits, and to make it publicly accessible via anonymous ftp. The database will be a very dynamic one; it will change on a quasi-daily basis as new observations and orbits become available. At present, an incomplete version of the database is online. In this paper, we present and discuss sample records of asteroid orbital data as they might appear in more mature form.

2060 Chiron: CCD and electronographic photometry

Abstract R -band CCD photometry of 2060 Chiron was carried out on nine nights in November and December 1986 and on 23 March 1988. The synodic rotation period is 5.9180 ± 0.0001 hr and the peak-to-peak lightcurve amplitude is 0.088 ± 0.003 mag. From the 1986 data, the absolute magnitude is H R = 6.24 ± 0.02 mag and the slope parameter is G R = 0.70 ± 0.15, though formal errors may not be realistic. The lightcurve has two pairs of extrema, but its asymmetry, as evidenced by the presence of significant odd Fourier harmonics, suggests macroscopic surface irregularities and/or the presence of some large-scale albedo variegation. The 1986 observations show no evidence of periodic or nonperiodic brightness changes that can be ascribed to comet-like activity. In contrast, the 1988 observations indicate a brightening of Chiron by 0.6 ± 0.1 mag, which confirms the finding by D. J. Tholen, W. K. Hartmann, and D. P. Cruikshank 1988, IAU Circ. 4554) and is consistent with 1978 electronographic photometry, published here for the first time. The lightcurve amplitude appears to be unchanged, however, and image profiles are indistinguishable from those of stars in both 1986 and 1988. Chiron may recently have been varying nonasteroidally on time scales greater than a month or two; but it is also possible that the asteroids intrinsic brightness has been bistable over the past decade, with an excursion of 0.56 ± 0.12 mag on a time scale of some years.

Origins for the Near-Earth Asteroids

Because of their short dynamical lifetimes, the population of near-Earth asteroids (NEAs) must be resupplied. Two sources have been hypothesized: main-belt asteroids and extinct comet nuclei. The difficulty of making physical measurements for similar sized (diameter D less than 5 kilometers) main-belt asteroids and comet nuclei has limited comparative tests for distinguishing between these alternatives. A new survey of physical properties for D < 5 kilometers main-belt asteroids reveals that their spin rate and shape distributions are similar to those of NEAs, as is fully consistent with a main-belt origin for most NEAs. Physical data on comet nuclei are limited. If the existing sample is representative of the comet population, analysis of the asteroid and comet samples constrains the fraction of comet nuclei to between 0 and 40 percent of the total NEA population.

UBV photometry of asteroid 433 Eros

Abstract UBV observations of asteroid 433 Eros were conducted on 17 nights during the winter of 1974/75. The peak-to-peak amplitude of the lightcurve varied from about 0.3 mag to nearly 1.4mmag. The absolute V mag at maximum light, extrapolated to zero phase, is 10.85. Phase coefficients of 0.0233 mag/degree, 0.0009 mag/degree and 0.0004 mag/degree were derived for V, B-V, and U-B, respectively. The zero-phase color of Eros (B−V = 0.88, U−B = 0.50) is representative of an S (silicaceous) compositional type asteroid. The color does not vary with rotation. The photometric behavior of Eros can be modeled by a cylinder with rounded ends having an axial ratio of about 2.3:1. The asteroid is rotating about a short axis with the north pole at λ0 = 15° and β0 = 9°.