We report our latest analysis of the Oort-cloud comet C/2020 T2 (also named Palomar or T2) observed at 2.06 au from the Sun (phase angle of 28.°5) roughly two weeks before perihelion. It lacks a ...significant dust tail in scattered light, showing a strong central condensation of the coma throughout the apparition that is reminiscent of so-called Manx comets. Its spectral slope of polarized light increases and decreases in the
J
(1.25 μm) and H (1.65 μm) bands, respectively, resulting in an overall negative (blue) slope (−0.31 ±0.14% μm
−1
) in contrast to the red polarimetric color of active comets observed at similar geometries. The average polarization degree of T2 is 2.86±0.17% for the
J
and 2.75±0.16% for the
H
bands. Given that near-infrared wavelengths are sensitive to the intermediate-scale structure of cometary dust (i.e., dust aggregates), our light-scattering modeling of ballistic aggregates with different porosities and compositions shows that the polarimetric properties of T2 are compatible with low-porosity (~66%), absorbing dust aggregates with negligible ice contents on a scale of 10–100 μm (density of ~652 kg m
−3
). This is supported by the coma morphology of T2 which has a viable
β
(the relative importance of solar radiation pressure on dust particles) range of ≲10
−4
. The secular evolution of
r
-band activity of T2 from archival data reveals that the increase in its brightness accelerates around 2.4 au pre-perihelion, with its overall dust production rate that is about 100 times smaller than those of active Oort-cloud comets. We also found an apparent concentration of T2 and Manx comets toward ecliptic orbits. This paper underlines the heterogeneous nature of Oort-cloud comets, which may be investigated in the near future with dedicated studies of their dust characteristics.
We report on the detection of a population of weak metal-line absorbers in the halo or nearby intergalactic environment of the Milky Way. Using high-resolution ultraviolet absorption-line spectra of ...bright quasars (QSO) obtained with the Space Telescope Imaging Spectrograph (STIS), along six sight lines we have observed unsaturated, narrow absorption in O I and Si II, together with mildly saturated C II absorption at high radial velocities (|v LSR| = 100-320 km s-1). The measured O I column densities lie in the range N(O I) < 2 X 1014 cm-2 implying that these structures represent Lyman limit Systems and sub-Lyman limit System with H I column densities between 1016 and 3 X 1018 cm-2, thus below the detection limits of current 21 cm all-sky surveys of high-velocity clouds (HVCs). The absorbers apparently are not directly associated with any of the large high column density HVC complexes, but rather represent isolated, partly neutral gas clumps embedded in a more tenuous, ionized gaseous medium situated in the halo or nearby intergalactic environment of the Galaxy. Photoionization modeling of the observed low ion ratios suggests typical hydrogen volume densities of n H > 0.02 cm-3 and characteristic thicknesses of a several parsec down to subparsec scales. For three absorbers, metallicities are constrained in the range of 0.1-1.0 solar, implying that these gaseous structures may have multiple origins inside and outside the Milky Way. Using supplementary optical absorption-line data, we find for two other absorbers Ca II/O I column-density ratios that correspond to solar Ca/O abundance ratios. This finding indicates that these clouds do not contain significant amounts of dust. This population of low column density gas clumps in the circumgalactic environment of the Milky Way is indicative of the various processes that contribute to the circulation of neutral gas in the extended halos of spiral galaxies. These processes include the accretion of gas from the intergalactic medium and satellite galaxies, galactic fountains, and outflows. We speculate that this absorber population represents the local analog of weak Mg II systems that are commonly observed in the circumgalactic environment of low- and high-redshift galaxies.
Abstract Asteroids with low orbital perihelion distances experience extreme heating from the Sun that can modify their surfaces and trigger nontypical activity mechanisms. These objects are generally ...difficult to observe from ground-based telescopes due to their frequent proximity to the Sun. The Near-Earth Object (NEO) Surveyor mission, however, will regularly survey down to solar elongations of 45° and is well suited for the detection and characterization of low-perihelion asteroids. Here, we use the survey simulation software tools developed for mission verification to explore the expected sensitivity of NEO Surveyor to these objects. We find that NEO Surveyor is expected to be >90% complete for near-Sun objects larger than D ∼ 300 m. Additionally, if the asteroid (3200) Phaethon underwent a disruption event in the past to form the Geminid meteor stream, Surveyor will be >90% complete to any fragments larger than D ∼ 200 m. For probable disruption models, NEO Surveyor would be expected to detect dozens of objects on Phaethon-like orbits, compared to a predicted background population of only a handful of asteroids, setting strong constraints on the likelihood of this scenario.
Abstract
The known near-Earth object (NEO) population consists of over 32,000 objects, with a yearly discovery rate of over 3000 NEOs per year. An essential component of the next generation of NEO ...surveys is an understanding of the population of known objects, including an accounting of the discovery rate per year as a function of size. Using a near-Earth asteroid (NEA) reference model developed for NASA’s NEO Surveyor (NEOS) mission and a model of the major current and historical ground-based surveys, an estimate of the current NEA survey completeness as a function of size and absolute magnitude has been determined (termed the Known Object Model; KOM). This allows for understanding of the intersection of the known catalog of NEAs and the objects expected to be observed by NEOS. The current NEA population is found to be ∼38% complete for objects larger than 140 m, consistent with estimates by Harris & Chodas. NEOS is expected to catalog more than two-thirds of the NEAs larger than 140 m, resulting in ∼76% of NEAs cataloged at the end of its 5 yr nominal survey, making significant progress toward the US Congressional mandate. The KOM estimates that ∼77% of the currently cataloged objects will be detected by NEOS, with those not detected contributing ∼9% to the final completeness at the end of its 5 yr mission. This model allows for placing the NEOS mission in the context of current surveys to more completely assess the progress toward the goal of cataloging the population of hazardous asteroids.
Abstract
Polarization phase-curve measurements provide a unique constraint on the surface properties of asteroids that are complementary to those from photometry and spectroscopy and have led to the ...identification of the “Barbarian” asteroids as a class of objects with highly unusual surfaces. We present new near-infrared polarimetric observations of six Barbarian asteroids obtained with the WIRC+Pol instrument on the Palomar Hale telescope. We find a dramatic change in polarimetric behavior from visible to near-infrared for these objects, including a change in the polarimetric inversion angle that is tied to the index of refraction of the surface material. Our observations support a two-phase surface composition consisting of high albedo and high index of refraction inclusions with a small optical size scale embedded in a dark matrix material more closely related to C-complex asteroids. These results are consistent with the interpretation that the Barbarians are remnants of a population of primitive bodies that formed shortly after calcium-aluminum-rich inclusion (CAIs). Near-infrared polarimetry provides a direct test of the constituent grains of asteroid surfaces.
Thermal infrared measurements of near-Earth objects (NEOs) provide critical data for constraining their physical properties such as size. The Near-Earth Object Wide-field Infrared Survey Explorer ...(NEOWISE) mission has been conducting an all-sky infrared survey to gather such data and to improve our understanding of this population. While automated routines are employed to identify the majority of moving objects detected by NEOWISE, a subset of objects will have dynamical properties that fall outside the window detectable to these routines. Using the population of known NEOs, we have conducted a manual search for detections of these objects that were previously unreported. We report 303 new epochs of observations for 299 unique NEOs of which 239 have no previous physical property characterization from the NEOWISE Reactivation mission. As these objects are drawn from a list with inherent optical selection biases, the distribution of measured albedos is skewed to higher values than is seen for the diameter-selected population detected by the automated routines. These results demonstrate the importance and benefit of periodic searches of the archival NEOWISE data.
We present a census of z
abs 2 intrinsic (those showing partial coverage) and associated (z
abs ∼ z
em) quasar absorption-line systems detected in the Hubble Space Telescope archive of Space ...Telescope Imaging Spectrograph echelle spectra. This work complements the Misawa et al. survey of 2 < z
em < 4 quasars that selects systems using similar techniques. We confirm the existence of so-called strong N v intrinsic systems (where the equivalent width of H i Lyα is small compared to N v λ1238) presented in that work, but find no convincing cases of 'strong C iv' intrinsic systems at low redshift/luminosity. Moreover, we also report on the existence of 'strong O vi' systems. From a comparison of partial coverage results as a function of ion, we conclude that systems selected by the N v ion have the highest probability of being intrinsic. By contrast, the C iv and O vi ions are poor selectors. Of the 30 O vi systems tested, only two of the systems in the spectrum on 3C 351 show convincing evidence for partial coverage. However, there is an ∼3σ excess in the number of absorbers near the quasar redshift (|Δv| ≤ 5000 km s−1) over absorbers at large redshift differences. In at least two cases, the associated O vi systems are known not to arise close to the accretion disc of the quasar.
Abstract
The Near-Earth Object Surveyor (NEO Surveyor) mission has a requirement to find two-thirds of the potentially hazardous asteroids larger than 140 m in size. In order to determine the ...mission’s expected progress toward this goal during design and testing, as well as the actual progress during the survey, a simulation tool has been developed to act as a consistent and quantifiable yardstick. We test that the survey simulation software is correctly predicting on-sky positions and thermal infrared fluxes by using it to reproduce the published measurements of asteroids from the NEOWISE mission. We then extended this work to find previously unreported detections of known near-Earth asteroids in the NEOWISE data archive, a search that resulted in 21,661 recovery detections, including 1166 objects that had no previously reported NEOWISE observations. These efforts demonstrate the reliability of the NEO Surveyor Survey Simulator tool and the perennial value of searchable image and source catalog archives for extending our knowledge of the small bodies of the solar system.
We report results based on mid-infrared photometry of five active main belt objects (AMBOs) detected by the Wide-field Infrared Survey Explorer (WISE) spacecraft. Four of these bodies, P/2010 R2 (La ...Sagra), 133P/Elst-Pizarro, (596) Scheila, and 176P/LINEAR, showed no signs of activity at the time of the observations, allowing the WISE detections to place firm constraints on their diameters and albedos. Geometric albedos were in the range of a few percent, and on the order of other measured comet nuclei. P/2010 A2 was observed on 2010 April 2-3, three months after its peak activity. Photometry of the coma at 12 and 22 mu m combined with ground-based visible-wavelength measurements provides constraints on the dust particle mass distribution (PMD), dlog n/dlog m, yielding power-law slope values of alpha = -0.5 + or - 0.1. This PMD is considerably more shallow than that found for other comets, in particular inbound particle fluence during the Stardust encounter of comet 81P/Wild 2. It is similar to the PMD seen for 9P/Tempel 1 in the immediate aftermath of the Deep Impact experiment. Upper limits for CO sub(2) and CO production are also provided for each AMBO and compared with revised production numbers for WISE observations of 103P/Hartley 2.
We present a reanalysis of the relationship between asteroid albedo and polarization properties using the albedos derived from the Wide-field Infrared Survey Explorer. We find that the function that ...best describes this relation is a three-dimensional linear fit in the space of log (albedo)-log (polarization slope)-log (minimum polarization). When projected to two dimensions, the parameters of the fit are consistent with those found in previous work. We also define p* as the quantity of maximal polarization variation when compared with the albedo and present the best-fitting albedo-p* relation. Some asteroid taxonomic types stand out in this three-dimensional space, notably the E, B, and M Tholen types, while others cluster in clumps coincident with the S- and C-complex bodies. We note that both low albedo and small (D < 30 km) asteroids are underrepresented in the polarimetric sample, and we encourage future polarimetric surveys to focus on these bodies.
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