Following the work of Rivkin et al. (Rivkin, A.S., Thomas, C.A., Trilling, D.E., Enga, M., Grier, J.A. 2011. Icarus 211, 1294–1297) and Thomas et al. (Thomas, C.A., Rivkin, A.S, Trilling, D.E., Enga, ...M., Grier, J.A. 2011a. Icarus 212, 158–166), we investigate space weathering trends in the Koronis family using the larger sample size of the Sloan Digital Sky Survey Moving Object Catalog. We confirm the trend in spectral slope seen in our earlier work and extend our results by investigating the trend in band depth (i−z color index) to show that Koronis family asteroids smaller than 4km show the transition from fresh Q-type to weathered S-type surfaces.
Abstract Surfaces of carbonaceous asteroids (C-complex) have shown diverse, contrasting spectral variations, which may be related to space weathering. We performed laser irradiation experiments on CI ...and CM simulant material under vacuum to mimic the spectral alteration induced by micrometeorite impacts. We used in situ ultraviolet-visible and near-infrared reflectance spectroscopy to analyze spectral alterations in response to pulsed laser irradiation, as well as scanning electron microscopy and X-ray photoelectron spectroscopy to search for microstructural and compositional changes. Laser irradiation causes an increase in spectral slope (reddening) and a decrease in the albedo (darkening), and these changes are stronger in the ultraviolet-visible region. These spectral changes are likely driven by the excess iron found in the altered surface region although other factors, such as the observed structural changes, may also contribute. Additionally, while the 0.27 μ m band appears relatively stable under laser irradiation, a broad feature at 0.6 μ m rapidly disappears with laser irradiation, suggesting that space weathering may inhibit the detection of any feature in this spectral region, including the 0.7 μ m band, which has typically been used an indicator of hydration. Comparing our laboratory results with optical spectrophotometry observations of C-complex asteroids, we find that the majority of objects are spectrally red and possess colors that are similar to our irradiated material rather than our fresh samples. Furthermore, we also find that “younger” and “older” C-complex families have similar colors, suggesting that the space-weathering process is near equal or faster than the time it takes to refresh the surfaces of these airless bodies.
We report our efforts to constrain the form of the low-mass star and brown dwarf mass function via Bayesian inference. Recent surveys of M, L, and T dwarfs in the local solar neighborhood are an ...essential component of our study. Uncertainties in the age distribution of local field stars make reliable inference complicated. We adopt a wide range of plausible assumptions about the rate of Galactic star formation and show that their deviations from a uniform rate produce little effect on the resulting luminosity function for a given mass function. As an ancillary result, we calculate the age distribution for M, L, and T spectral types. We demonstrate that late L dwarfs, in particular, are systematically younger than objects with earlier or later spectral types, with a mean age of 3 Gyr. Finally, we use a Bayesian statistical formalism to evaluate the probability of commonly used mass functions in the light of recent discoveries. We consider three functional forms of the mass function, including a two-segment power law, a single power law with a low-mass cutoff, and a lognormal distribution. Our results show that at a 60% confidence level the power-law index a for the low-mass arm of a two-segment power law has a value between -0.6 and 0.6 for objects with masses between 0.04 and 0.10 M sub( ). The best-fit index is a = 0.3 c 0.6 at the 60% confidence level for a single-segment mass function. Current data require this function to extend to at least 0.05 M sub( )with no restrictions placed on a lower mass cutoff. Inferences of the parameter values for a lognormal mass function are virtually unaffected by recent estimates of the local space density of L and T dwarfs. We find that we have no preference among these three forms using this method. We discuss current and future capabilities that may eventually discriminate between mass function models and refine estimates of their associated parameter values.
Infrared Light Curves of Near-Earth Objects Hora, Joseph L.; Siraj, Amir; Mommert, Michael ...
The Astrophysical journal. Supplement series,
10/2018, Letnik:
238, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present light curves and derive periods and amplitudes for a subset of 38 near-Earth objects (NEOs) observed at 4.5 m with the IRAC camera on the the Spitzer Space Telescope, many of them having ...no previously reported rotation periods. This subset was chosen from about 1800 IRAC NEO observations as having obvious periodicity and significant amplitude. For objects where the period observed did not sample the full rotational period, we derived lower limits to these parameters based on sinusoidal fits. Light curve durations ranged from 42 to 544 minutes, with derived periods from 16 to 270 minutes. We discuss the effects of light curve variations on the thermal modeling used to derive diameters and albedos from Spitzer photometry. We find that both diameters and albedos derived from the light curve maxima and minima agree with our previously published results, even for extreme objects, showing the conservative nature of the thermal model uncertainties. We also evaluate the NEO rotation rates, sizes, and their cohesive strengths.
Abstract
Clouds are ubiquitous: they arise for every solar system planet that possesses an atmosphere and have also been suggested as a leading mechanism for obscuring spectral features in exoplanet ...observations. As exoplanet observations continue to improve, there is a need for efficient and general planetary climate models that appropriately handle the possible cloudy atmospheric environments that arise on these worlds. We generate a new 1D radiative-convective terrestrial planet climate model that self-consistently handles patchy clouds through a parameterized microphysical treatment of condensation and sedimentation processes. Our model is general enough to recreate Earth’s atmospheric radiative environment without overparameterization, while also maintaining a simple implementation that is applicable to a wide range of atmospheric compositions and physical planetary properties. We first validate this new 1D patchy-cloud radiative-convective climate model by comparing it to Earth thermal structure data and to existing climate and radiative-transfer tools. We produce partially clouded Earth-like climates with cloud structures that are representative of deep tropospheric convection and are adequate 1D representations of clouds within rocky planet atmospheres. After validation against Earth, we then use our partially clouded climate model and explore the potential climates of super-Earth exoplanets with secondary nitrogen-dominated atmospheres which we assume are abiotic. We also couple the partially clouded climate model to a full-physics, line-by-line radiative-transfer model and generate high-resolution spectra of simulated climates. These self-consistent climate-to-spectral models bridge the gap between climate modeling efforts and observational studies of rocky worlds.
A debris ring around the star HD 207129 (G0V; d = 16.0 pc) has been imaged in scattered visible light with the ACS coronagraph on the Hubble Space Telescope (HST) and in thermal emission using MIPS ...on the Spitzer Space Telescope at Delta *l = 70 Delta *mm (resolved) and 160 Delta *mm (unresolved). Spitzer IRS ( Delta *l = 7-35 Delta *mm) and MIPS ( Delta *l = 55-90 Delta *mm) spectrographs measured disk emission at Delta *l> 28 Delta *mm. In the HST image the disk appears as a ~30 AU wide ring with a mean radius of ~163 AU and is inclined by 60? from pole-on. At 70 Delta *mm, it appears partially resolved and is elongated in the same direction and with nearly the same size as seen with HST in scattered light. At 0.6 Delta *mm, the ring shows no significant brightness asymmetry, implying little or no forward scattering by its constituent dust. With a mean surface brightness of V = 23.7 mag arcsec--2, it is the faintest disk imaged to date in scattered light. We model the ring's infrared spectral energy distribution (SED) using a dust population fixed at the location where HST detects the scattered light. The observed SED is well fit by this model, with no requirement for additional unseen debris zones. The firm constraint on the dust radial distance breaks the usual grain size-distance degeneracy that exists in modeling of spatially unresolved disks, and allows us to infer a minimum grain size of ~2.8 Delta *mm and a dust size distribution power-law spectral index of --3.9. An albedo of ~5% is inferred from the integrated brightness of the ring in scattered light. The low-albedo and isotropic scattering properties are inconsistent with Mie theory for astronomical silicates with the inferred grain size and show the need for further modeling using more complex grain shapes or compositions. Brightness limits are also presented for six other main-sequence stars with strong Spitzer excess around which HST detects no circumstellar nebulosity (HD 10472, HD 21997, HD 38206, HD 82943, HD 113556, and HD 138965).
Abstract
Can multicellular life be distinguished from single cellular life on an exoplanet? We hypothesize that abundant upright photosynthetic multicellular life (trees) will cast shadows at high ...sun angles that will distinguish them from single cellular life and test this using Earth as an exoplanet. We first test the concept using unmanned aerial vehicles at a replica moon-landing site near Flagstaff, Arizona and show trees have both a distinctive reflectance signature (red edge) and geometric signature (shadows at high sun angles) that can distinguish them from replica moon craters. Next, we calculate reflectance signatures for Earth at several phase angles with POLDER (Polarization and Directionality of Earth's reflectance) satellite directional reflectance measurements and then reduce Earth to a single pixel. We compare Earth to other planetary bodies (Mars, the Moon, Venus and Uranus) and hypothesize that Earth's directional reflectance will be between strongly backscattering rocky bodies with no weathering (like Mars and the Moon) and cloudy bodies with more isotropic scattering (like Venus and Uranus). Our modelling results put Earth in line with strongly backscattering Mars, while our empirical results put Earth in line with more isotropic scattering Venus. We identify potential weaknesses in both the modelled and empirical results and suggest additional steps to determine whether this technique could distinguish upright multicellular life on exoplanets.
We report on our long-term observational campaign to characterize and monitor a select sample of 75 dynamically selected dormant comet candidates and six near-Sun asteroids. Both asteroid ...subpopulations can be considered likely to display comet-like activity. Dormant comets are currently inactive comet nuclei that can still harbor volatiles in subsurface layers, whereas near-Sun asteroids have extremely low perihelion distances and are thus prone to catastrophic disruption. As a result of our 4 yr long observing campaign, we find only dormant comet 3552 Don Quixote to show activity during our program. We furthermore find that (51 10)% of dynamically selected dormant comet candidates in near-Earth space have comet-like physical properties, as well as (56 16)% of dynamically selected dormant comet candidates in other parts of the solar system. All of our near-Sun asteroid sample targets are of nonprimitive nature, suggesting that primitive near-Sun asteroids are more likely to disrupt than nonprimitives. We furthermore find a significant fraction of our near-Sun asteroid sample to display extremely blue V-I color indices, potentially hinting at physical alterations of surface material close to the Sun.
We have begun the ExploreNEOs project in which we observe some 700 Near-Earth Objects (NEOs) at 3.6 and 4.5 Delta *mm with the Spitzer Space Telescope in its Warm Spitzer mode. From these ...measurements and catalog optical photometry we derive albedos and diameters of the observed targets. The overall goal of our ExploreNEOs program is to study the history of near-Earth space by deriving the physical properties of a large number of NEOs. In this paper, we describe both the scientific and technical construction of our ExploreNEOs program. We present our observational, photometric, and thermal modeling techniques. We present results from the first 101 targets observed in this program. We find that the distribution of albedos in this first sample is quite broad, probably indicating a wide range of compositions within the NEO population. Many objects smaller than 1 km have high albedos (0.35), but few objects larger than 1 km have high albedos. This result is consistent with the idea that these larger objects are collisionally older, and therefore possess surfaces that are more space weathered and therefore darker, or are not subject to other surface rejuvenating events as frequently as smaller NEOs.
We introduce a novel search technique that can identify trans-Neptunian objects in three to five exposures of a pointing within a single Hubble Space Telescope (HST) orbit. The process is fast enough ...to allow the discovery of candidates soon after the data are available. This allows sufficient time to schedule follow-up observations with HST within a month. We report the discovery of 14 slow-moving objects found within 5? of the ecliptic in archival data taken with the Wide Field Channel of the Advanced Camera for Surveys. The luminosity function of these objects is consistent with previous ground-based and space-based results. We show evidence that the size distribution of both high and low inclination populations is similar for objects smaller than 100 km, as expected from collisional evolution models, while their size distribution differs for brighter objects. We suggest that the two populations formed in different parts of the protoplanetary disk and after being dynamically mixed have collisionally evolved together. Among the objects discovered there is an equal-mass binary with an angular separation ~053.
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