Nereid: Light Curve for 1999-2006 and a Scenario for its Variations

Nereid is a small irregular moon of Neptune that displays large-, moderate-, and small-amplitude photometric variations on both fast and slow time scales. The central mystery of Nereid is now to explain the physical mechanism of these unique brightness changes and why they change with time. To characterize Nereid's variability, we have been using the SMARTS telescopes on Cerro Tololo for synoptic monitoring from 1999 to 2006. We present a well-sampled photometric time series of 493 magnitudes on 246 nights mostly in the V-band. In combination with our earlier data (for 774 magnitudes over 362 nights), our 20-year data set is the most comprehensive for any small icy body in our Solar System. Our yearly light curves show that Nereid displays various types of behaviors: large amplitude brightenings and fadings (1987 to 1990); moderate-amplitude variation about the average phase curve (1993-1997, 2003, 2005), moderate-amplitude variation and systematically brighter by roughly one-quarter magnitude throughout the entire season (2004); and nearly constant light curves superimposed on a surprisingly large-amplitude opposition surge (1998, 1999, 2000, 2006). Other than in 2004, Nereid's variations were closely centered around a constant phase curve that is well fit with a Hapke model for the coherent backscattering opposition surge mechanism with angular scale of 0.7+-0.1 degrees. In our entire data set from 1987-2006, we find no significant periodicity. We propose that the year-to-year changes in the variability of Nereid are caused by forced precession (caused by tidal forces from Neptune) on the spin axis of a nonspherical Nereid, such that cross-sectional areas and average albedos change as viewed from Earth.Comment: Icarus in press, 45 manuscript page

The Diverse Solar Phase Curves of Distant Icy Bodies. Part I: Photometric Observations of 18 Trans-Neptunian Objects, 7 Centaurs, and Nereid

We have measured the solar phase curves in B, V, and I for 18 Trans-Neptunian Objects, 7 Centaurs, and Nereid and determined the rotation curves for 10 of these targets. For each body, we have made ~100 observations uniformly spread over the entire visible range. We find that all the targets except Nereid have linear phase curves at small phase angles (< 2 deg) with widely varying phase coefficients (0.0 to 0.4 mag/deg). At phase angles > 3 deg, the Centaurs (54598) Bienor and (32532) Thereus have phase curves that flatten. The recently discovered Pluto-scale bodies (2003 UB313, 2005 FY9, and 2003 EL61), like Pluto, have neutral colors compared to most TNOs and small phase coefficients (< 0.1 mag/deg). Together these two properties are a likely indication for large TNOs of high-albedo, freshly coated icy surfaces. We find several bodies with significantly wavelength-dependent phase curves. The TNOs (50000) Quaoar, (120348) 2004 TY364 (47932), and 2000 GN171 have unusually high I-band phase coefficients (0.290+/-0.038, 0.413+/-0.064, 0.281+/-0.033 mag/deg, respectively) and much lower coefficients in the B and V bands. Their phase coefficients increase in proportion to wavelength by 0.5 - 0.8 mag/deg/um. The phase curves for TNOs with small B-band phase coefficients (< 0.1 mag/deg) have a similar but weaker wavelength dependence. Coherent backscatter is the likely cause for the wavelength dependence for all these bodies. We see no such dependence for the Centaurs, which have visual albedos ~0.05.Comment: 5 tables, 5 figures made explicit author affiliation

The Youthful Appearance of the 2003 EL61 Collisional Family

We present new solar phase curve observations of the 2003 EL61 collisional family showing that all the members have light-scattering properties similar to the bright icy satellites and dwarf planets. Compared to other Kuiper Belt objects, the five family members we observe (2003 EL61, 2002 TX300, 2003 OP32, 2005 RR43, and 1995 SM55) have conspicuously neutral color (V-I = 0.6-0.8 mag) and flat phase curves at small phase angles (phase coefficients of 0.0 - 0.1 mag deg-1). Comparing the phase curves we observe for other icy Kuiper Belt objects to the phase curves of icy satellites, we find that the flat phase curves of the 2003 EL61 family are an indication they have high albedo surfaces coated with fresh ice in the last ~100 Myr. We examine possible resurfacing processes and find none that are plausible. To avoid the influence of cosmic radiation that darkens and reddens most icy surfaces on times scales > ~100 Myr, the family members must be unusually depleted in carbon, or else the collision that created the family occurred so recently that the parent body and fragments have not had time to darken. We also find a rotation period of 4.845 (+/- 0.003) h with amplitude 0.26 (+/- 0.04) mags for 2003 OP32.Comment: 12 pages, 3 tables, 4 figure

The Diverse Solar Phase Curves of Distant Icy Bodies. II. The Cause of the Opposition Surges and Their Correlations

We collect well-measured opposition surge properties for many icy bodies orbiting the Sun (mostly from our own observations) plus for many icy moons, resulting in a data base of surface and orbital properties for 52 icy bodies. (1) We put forward four criteria for determining whether the surge is being dominated by shadow hiding (SH) or coherent backscattering (CB) based on readily measured quantities. The CB surge mechanism dominates if the surge is color dependent, the phase curve is steeper than 0.04 mag/deg, the phase curve shape matches the CB model of Hapke, or if the albedo is higher than roughly 40%. (2) We find that virtually all of our sample have their phase curves dominated by CB at low phase angles. (3) We present a graphical method to determine the Hapke surge parameters B_C0 and h_C. (4) The Kuiper Belt Objects (KBOs) and Centaurs have relatively high surge amplitudes, B_C0 > ~0.5 and widths with h_C ~ 3 deg. (5) We find highly significant but loose correlations between surge properties and the colors, albedos, and inclinations. We interpret this as young surfaces tending to have low surge slopes, high albedo, and gray colors. (6) Nereid has its surface properties similar to other icy moons and greatly different from KBOs and Centaurs, so we conclude that Nereid is likely a nearly-ejected inner Neptunian moon rather than a captured KBO.Comment: Astronomical Journal, in press, 41 pages, 5 figure

Phase Curves of Nine Trojan Asteroids over a Wide Range of Phase Angles

We have observed well-sampled phase curves for nine Trojan asteroids in B-, V-, and I-bands. These were constructed from 778 magnitudes taken with the 1.3-m telescope on Cerro Tololo as operated by a service observer for the SMARTS consortium. Over our typical phase range of 0.2-10 deg, we find our phase curves to be adequately described by a linear model, for slopes of 0.04-0.09 mag/deg with average uncertainty less than 0.02 mag/deg. (The one exception, 51378 (2001 AT33), has a formally negative slope of -0.02 +- 0.01 mag/deg.) These slopes are too steep for the opposition surge mechanism to be shadow hiding (SH), so we conclude that the dominant surge mechanism must be coherent backscattering (CB). In a detailed comparison of surface properties (including surge slope, B-R color, and albedo), we find that the Trojans have surface properties similar to the P and C class asteroids prominent in the outer main belt, yet they have significantly different surge properties (at a confidence level of 99.90%). This provides an imperfect argument against the traditional idea that the Trojans were formed around Jupiter's orbit. We also find no overlap in Trojan properties with either the main belt asteroids or with the small icy bodies in the outer Solar System. Importantly, we find that the Trojans are indistinguishable from other small bodies in the outer Solar System that have lost their surface ices (such as the gray Centaurs, gray Scattered Disk Objects, and dead comets). Thus, we find strong support for the idea that the Trojans originally formed as icy bodies in the outer Solar System, were captured into their current orbits during the migration of the gas giant planets, and subsequently lost all their surface ices.Comment: Icarus, in pres

Photometric Light Curve for Nereid in 1998: A Prominent Opposition Surge

We present a well-sampled photometric time series (57 V magnitudes on 52 nights) of Nereid over a 128-day time span in 1998. The light curve shows an amplitude of 0.52 mag (from 19.11 to 19.63 mag) with a single peak (at JD 2451019±2) that is smooth and time symmetric. The date of the peak coincides with opposition (JD 2451018) and the entire 1998 light curve varies roughly linearly with the solar phase angle. This shows that the 1998 light curve is dominated by an extreme example of opposition surge. While a fraction of the photometric data from 1987-1997 agrees with this 1998 phase function, large and significant deviations were seen by all observers on time scales from hours to days to months during the years 1987-1997. Hence, there must be at least two mechanisms capable of making Nereid change in brightness, for which only the opposition surge was operating in 1998. © 2001 Academic Press