43 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 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
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
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Effects of Immunization With the Soil-Derived Bacterium Mycobacterium vaccae on Stress Coping Behaviors and Cognitive Performance in a "Two Hit" Stressor Model
Previous studies demonstrate that Mycobacterium vaccae NCTC 11659 (M. vaccae), a soil-derived bacterium with anti-inflammatory and immunoregulatory properties, is a potentially useful countermeasure against negative outcomes to stressors. Here we used male C57BL/6NCrl mice to determine if repeated immunization with M. vaccae is an effective countermeasure in a "two hit" stress exposure model of chronic disruption of rhythms (CDR) followed by acute social defeat (SD). On day -28, mice received implants of biotelemetric recording devices to monitor 24-h rhythms of locomotor activity. Mice were subsequently treated with a heat-killed preparation of M. vaccae (0.1 mg, administered subcutaneously on days -21, -14, -7, and 27) or borate-buffered saline vehicle. Mice were then exposed to 8 consecutive weeks of either stable normal 12:12 h light:dark (LD) conditions or CDR, consisting of 12-h reversals of the LD cycle every 7 days (days 0-56). Finally, mice were exposed to either a 10-min SD or a home cage control condition on day 54. All mice were exposed to object location memory testing 24 h following SD. The gut microbiome and metabolome were assessed in fecal samples collected on days -1, 48, and 62 using 16S rRNA gene sequence and LC-MS/MS spectral data, respectively; the plasma metabolome was additionally measured on day 64. Among mice exposed to normal LD conditions, immunization with M. vaccae induced a shift toward a more proactive behavioral coping response to SD as measured by increases in scouting and avoiding an approaching male CD-1 aggressor, and decreases in submissive upright defensive postures. In the object location memory test, exposure to SD increased cognitive function in CDR mice previously immunized with M. vaccae. Immunization with M. vaccae stabilized the gut microbiome, attenuating CDR-induced reductions in alpha diversity and decreasing within-group measures of beta diversity. Immunization with M. vaccae also increased the relative abundance of 1-heptadecanoyl-sn-glycero-3-phosphocholine, a lysophospholipid, in plasma. Together, these data support the hypothesis that immunization with M. vaccae stabilizes the gut microbiome, induces a shift toward a more proactive response to stress exposure, and promotes stress resilience.
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Early Devonian (Late Emsian) shark fin remains (Chondrichthyes) from the Paraná Basin, southern Brazil
This is an open-access article distributed under the terms of the Creative Commons Attribution License [Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)]. The attached file is the published version of the article
Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire : an expert assessment
As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%-85% of permafrost carbon release can still be avoided if human emissions are actively reduced.Peer reviewe
Impact of Optimized Breastfeeding on the Costs of Necrotizing Enterocolitis in Extremely Low Birthweight Infants
To estimate risk of NEC for ELBW infants as a function of preterm formula and maternal milk (MM) intake and calculate the impact of suboptimal feeding on NEC incidence and costs
Large-amplitude photometric variations of Nereid
Nereid, one of the two known satellites of Neptune, was discovered in 1949 by Kuiper1. It has an extremely eccentric orbit (e ≃ 0.75), such that its distance from Neptune varies from ∼1.4 to 9.7 million km, with a period of ∼360 days. No photometric data exist for Nereid2 other than the rough estimate of photographic magnitude (mp Combining double low line 19.5) determined by Kuiper1. In June 1987 we collected photometric (UBVRI) and astrometric data on Nereid using the 0.9-m telescope and CCD camera at the Cerro Tololo Inter-American Observatory (CTIO). We obtained four astrometric positions and UBVRI reflectances spanning 8 days. From these data, an unusual reflectance spectrum for Nereid was determined. Variability of Nereid was observed in all bandpasses, with an amplitude of greater than 1.5 mag, and a probable period of between 8 and 24 h. © 1988 Nature Publishing Group