A series of blends of the biodegradable polymers poly(
d,
l-lactic acid) and poly(
ε-caprolactone) were prepared by varying mass fraction across the range of compositions. Tensile testing was ...performed at room temperature using an extensometer and the elastic modulus was calculated for each blend. The blends were also tested to failure, and the strain-at-failure and yield stress recorded. While the blend has been shown to have a lower critical solution temperature, the mechanical properties were insensitive to the annealing conditions. Scanning electron microscopy was used to characterize the blend microstructure and poor adhesion was observed at the interface between blend components. Differential scanning calorimetry was performed but the results were somewhat variable, indicating this blend may have complex phase behavior that depends sensitively on the method of preparation. However, nuclear magnetic resonance data indicate the two components are phase separated. A percolation model is used to explain the observed mechanical data and the results are consistent with the predictions of the Kerner–Uemura–Takayangi model. The results of these experiments demonstrate the utility of polymer blending in tuning material properties.
•A method suitable for comparisons of observed asteroids with N-body simulations is presented, which does not rely on the hierarchical clustering (HCM).•Size-frequency distribution of the synthetic ...family and background matches the observed SFD. Colours and albedos are used to decrease the contamination.•The Eos family shape in the space of proper elements is fully explained, including the distribution of pole latitudes.•Traces of the original velocity field are detectable even 1.3 Gyr after the impact.
In order to fully understand the shapes of asteroids families in the 3-dimensional space of the proper elements (ap, ep, sin Ip) it is necessary to compare observed asteroids with N-body simulations. To this point, we describe a rigorous yet simple method which allows for a selection of the observed asteroids, assures the same size-frequency distribution of synthetic asteroids, accounts for a background population, and computes a χ2 metric. We study the Eos family as an example, and we are able to fully explain its non-isotropic features, including the distribution of pole latitudes β. We confirm its age t=(1.3±0.3)Gyr; while this value still scales with the bulk density, it is verified by a Monte-Carlo collisional model. The method can be applied to other populous families (Flora, Eunomia, Hygiea, Koronis, Themis, Vesta, etc.).
In the Nice model, the late heavy bombardment (LHB) is related to an orbital instability of giant planets which causes a fast dynamical dispersion of a trans-Neptunian cometary disk. We study effects ...produced by these hypothetical cometary projectiles on main belt asteroids. In particular, we want to check whether the observed collisional families provide a lower or an upper limit for the cometary flux during the LHB. We present an updated list of observed asteroid families as identified in the space of synthetic proper elements by the hierarchical clustering method, colour data, albedo data and dynamical considerations and we estimate their physical parameters. We selected 12 families which may be related to the LHB according to their dynamical ages. We then used collisional models and N-body orbital simulations to gain insight into the long-term dynamical evolution of synthetic LHB families over 4 Gyr. We account for the mutual collisions between comets, main belt asteroids, and family members, the physical disruptions of comets, the Yarkovsky/YORP drift in semimajor axis, chaotic diffusion in eccentricity/inclination, or possible perturbations by the giant-planet migration. Assuming a “standard” size-frequency distribution of primordial comets, we predict the number of families with parent-body sizes DPB ≥ 200 km – created during the LHB and subsequent ≃4 Gyr of collisional evolution – which seems consistent with observations. However, more than 100 asteroid families with DPB ≥ 100 km should be created at the same time which are not observed. This discrepancy can be nevertheless explained by the following processes: i) asteroid families are efficiently destroyed by comminution (via collisional cascade), ii) disruptions of comets below some critical perihelion distance (q ≲ 1.5 AU) are common. Given the freedom in the cometary-disruption law, we cannot provide stringent limits on the cometary flux, but we can conclude that the observed distribution of asteroid families does not contradict with a cometary LHB.
The Eos family halo Brož, M.; Morbidelli, A.
Icarus (New York, N.Y. 1962),
April 2013, 2013-04-00, Letnik:
223, Številka:
2
Journal Article
Recenzirano
Odprti dostop
► We study K-type asteroids which form a ‘halo’ around the Eos family. ► We explain its origin as bodies escaping from the family ‘core’. ► Major processes are the Yarkovsky effect and interactions ...with resonances. ► Our N-body model allows us to estimate the age of the family 1.5–1.9Gyr.
We study K-type asteroids in the broad surroundings of the Eos family because they seem to be intimately related, according to their colours measured by the Sloan Digital Sky Survey. Such ‘halos’ of asteroid families have been rarely used as constraints for dynamical studies to date. We explain its origin as bodies escaping from the family ‘core’ due to the Yarkovsky semimajor-axis drift and interactions with gravitational resonances, mostly with the 9/4 mean-motion resonance with Jupiter at 3.03AU. Our N-body dynamical model allows us to independently estimate the age of the family 1.5–1.9Gyr. This is approximately in agreement with the previous age estimate by Vokrouhlický et al. (2006) based on a simplified model (which accounts only for changes of semimajor axis). We can also constrain the geometry of the disruption event which had to occur at the true anomaly f≃120–180°.
Context.
Asteroid families that are less than one million years old offer a unique possibility to investigate recent asteroid disruption events and test ideas about their dynamical evolution. ...Observations provided by powerful all-sky surveys have led to an enormous increase in the number of detected asteroids over the past decade. When the known populations are well characterized, they can be used to determine asteroid detection probabilities, including those in young families, as a function of their absolute magnitude.
Aims.
We use observations from the Catalina Sky Survey (CSS) to determine the bias-corrected population of small members in four young families down to sizes equivalent to several hundred meters.
Methods.
Using the most recent catalog of known asteroids, we identified members from four young families for which the population has grown appreciably over recent times. A large fraction of these bodies have also been detected by CSS. We used synthetic populations of asteroids, with their magnitude distribution controlled by a small number of parameters, as a template for the bias-corrected model of these families. Applying the known detection probability of the CSS observations, we could adjust these model parameters to match the observed (biased) populations in the young families.
Results.
In the case of three families, Datura, Adelaide, and Rampo, we find evidence that the magnitude distribution transitions from steep to shallow slopes near 300 to 400 meters. Conversely, the Hobson family population may be represented by a single power-law model. The Lucascavin family has a limited population; no new members have been discovered over the past two decades. We consider a model of parent body rotational fission with the escaping secondary tidally split into two components (thereby providing three members within this family). In support of this idea, we find that no other asteroid with absolute magnitude
H
≤ 18.3 accompanies the known three members in the Lucascavin family. A similar result is found for the archetypal asteroid pair Rheinland–Kurpfalz.
Context.
The Ch-type asteroid (130) Elektra is orbited by three moons, making it the first quadruple system in the main asteroid belt.
Aims.
We aim to characterise the irregular shape of Elektra and ...construct a complete orbital model of its unique moon system.
Methods.
We applied the All-Data Asteroid Modelling (ADAM) algorithm to 60 light curves of Elektra, including our new measurements, 46 adaptive-optics (AO) images obtained by the VLT/SPHERE and Keck/Nirc2 instruments, and two stellar occultation profiles. For the orbital model, we used an advanced
N
-body integrator, which includes a multipole expansion of the central body (with terms up to the order
ℓ
= 6), mutual perturbations, internal tides, and the external tide of the Sun acting on the orbits. We fitted the astrometry measured with respect to the central body and also relatively, with respect to the moons themselves.
Results.
We obtained a revised shape model of Elektra with the volume-equivalent diameter (201 ± 2) km. Of two possible pole solutions, (
λ
,
β
) = (189; −88) deg is preferred, because the other one leads to an incorrect orbital evolution of the moons. We also identified the true orbital period of the third moon S/2014 (130) 2 as
P
2
= (1.642112 ± 0.000400) days, which is in between the other periods,
P
1
≃ 1.212days,
P
3
= 5.300 days, of S/2014 (130) 1 and S/2003 (130) 1, respectively. The resulting mass of Elektra, (6.606
-0.013
+0.007
) ×10
18
kg, is precisely constrained by all three orbits. Its bulk density is then (1.536 ± 0.038) g cm
−3
. The expansion with the assumption of homogeneous interior leads to the oblateness
J
2
= −
C
20
≃ 0.16. However, the best-fit precession rates indicate a slightly higher value, ≃0.18. The number of nodal precession cycles over the observation time span 2014–2019 is 14, 7, and 0.5 for the inner, middle, and outer orbits.
Conclusions.
Future astrometric or interferometric observations of Elektra’s moons should constrain these precession rates even more precisely, allowing the identification of possible inhomogeneities in primitive asteroids.
Infrared radiation emitted from an asteroid surface causes a torque that can significantly affect rotational state of the asteroid. The influence of small topographic features on this phenomenon, ...called the YORP effect, seems to be of utmost importance. In this work, we show that a lateral heat diffusion in boulders of suitable sizes leads to an emergence of a local YORP effect which magnitude is comparable to the YORP effect due to the global shape. We solve a three-dimensional heat diffusion equation in a boulder and its surroundings by the finite element method, using the freefem++ code. The contribution to the total torque is inferred from the computed temperature distribution. Our general approach allows us to compute the torque induced by a realistic irregular boulder. For an idealized boulder, our result is consistent with an existing one-dimensional model. We also estimated (and extrapolated) a size distribution of boulders on (25143) Itokawa from close-up images of its surface. We realized that topographic features on Itokawa can potentially induce a torque corresponding to a rotational acceleration of the order of 10−7 rad d−2 and can therefore explain the observed phase shift in light curves.
•We constructed a new collisional model of the main belt (MB) consisting of 6 parts.•We use size–frequency distributions (SFD) from WISE and up-to-date list of families.•Monolithic asteroids seem to ...better match the observational data than rubble-piles.•Scaling laws much different from Benz and Asphaug (1999) cannot be used for the MB.•Scalings of fragment SFDs should be verified by SPH simulations for D∼1km bodies.
In this work, we construct a new model for the collisional evolution of the main asteroid belt. Our goals are to test the scaling law of Benz and Asphaug (Benz, W., Asphaug, E. 1999. Icarus, 142, 5–20) and ascertain if it can be used for the whole belt. We want to find initial size–frequency distributions (SFDs) for the considered six parts of the belt (inner, middle, “pristine”, outer, Cybele zone, high-inclination region) and to verify if the number of synthetic asteroid families created during the simulation matches the number of observed families as well. We used new observational data from the WISE satellite (Masiero et al., 2011) to construct the observed SFDs. We simulate mutual collisions of asteroids with a modified version of the Boulder code (Morbidelli, A., et al. 2009. Icarus, 204, 558–573), where the results of hydrodynamic (SPH) simulations of Durda et al. (Durda, D.D., et al. 2007. Icarus, 498–516) and Benavidez et al. (Benavidez, P.G., et al. 2012. 219, 57–76) are included. Because material characteristics can significantly affect breakups, we created two models — for monolithic asteroids and for rubble-piles. To explain the observed SFDs in the size range D=1 to 10km we have to also account for dynamical depletion due to the Yarkovsky effect. The assumption of (purely) rubble-pile asteroids leads to a significantly worse fit to the observed data, so that we can conclude that majority of main-belt asteroids are rather monolithic. Our work may also serve as a motivation for further SPH simulations of disruptions of smaller targets (with a parent body size of the order of 1km).
Context.
Asteroid (16) Psyche is the largest M-type asteroid in the main belt and the target of the NASA Psyche mission. It is also the only asteroid of this size (
D
> 200 km) known to be metal ...rich. Although various hypotheses have been proposed to explain the rather unique physical properties of this asteroid, a perfect understanding of its formation and bulk composition is still missing.
Aims.
We aim to refine the shape and bulk density of (16) Psyche and to perform a thorough analysis of its shape to better constrain possible formation scenarios and the structure of its interior.
Methods.
We obtained disk-resolved VLT/SPHERE/ZIMPOL images acquired within our ESO large program (ID 199.C-0074), which complement similar data obtained in 2018. Both data sets offer a complete coverage of Psyche’s surface. These images were used to reconstruct the three-dimensional (3D) shape of Psyche with two independent shape modeling algorithms (
MPCD
and
ADAM
). A shape analysis was subsequently performed, including a comparison with equilibrium figures and the identification of mass deficit regions.
Results.
Our 3D shape along with existing mass estimates imply a density of 4.20 ± 0.60 g cm
−3
, which is so far the highest for a solar system object following the four telluric planets. Furthermore, the shape of Psyche presents small deviations from an ellipsoid, that is, prominently three large depressions along its equator. The flatness and density of Psyche are compatible with a formation at hydrostatic equilibrium as a Jacobi ellipsoid with a shorter rotation period of ∼3h. Later impacts may have slowed down Psyche’s rotation, which is currently ∼4.2 h, while also creating the imaged depressions.
Conclusions.
Our results open the possibility that Psyche acquired its primordial shape either after a giant impact while its interior was already frozen or while its interior was still molten owing to the decay of the short-lived radionuclide
26
Al.
In this work, we analyse Jovian Trojans in the space of suitable resonant elements and we identify clusters of possible collisional origin by two independent methods: the hierarchical clustering and ...a so-called randombox. Compared to our previous work, we study a twice larger sample. Apart from Eurybates, Ennomos and 1996 RJ families, we have found three more clusters – namely families around asteroids (20961) Arkesilaos, (624) Hektor in the L
4 libration zone and (247341) 2001 UV209 in L
5. The families fulfill our stringent criteria, i.e. a high statistical significance, an albedo homogeneity and a steeper size–frequency distribution than that of background. In order to understand their nature, we simulate their long term collisional evolution with the Boulder code and dynamical evolution using a modified SWIFT integrator. Within the framework of our evolutionary model, we were able to constrain the age of the Hektor family to be either 1–4 Gyr or, less likely, 0.1–2.5 Gyr, depending on initial impact geometry. Since (624) Hektor itself seems to be a bilobed-shape body with a satellite, i.e. an exceptional object, we address its association with the D-type family and we demonstrate that the moon and family could be created during a single impact event. We simulated the cratering event using a smoothed particle hydrodynamics. This is also the first case of a family associated with a D-type parent body.
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