11 research outputs found
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