Hirayama Families Research Articles

Japanese asteroid studies in the century after the discovery of the Hirayama families

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Rotation periods of binary asteroids with large separations – Confronting the Escaping Ejecta Binaries model with observations

Durda et al. (Durda, D.D., Bottke, W.F., Enke, B.L., Merline, W.J., Asphaug, E., Richardson, D.C., Leinhardt, Z.M. [2004]. Icarus 170, 243–257), using numerical models, suggested that binary asteroids with large separation, called Escaping Ejecta Binaries (EEBs), can be created by fragments ejected from a disruptive impact event. It is thought that six binary asteroids recently discovered might be EEBs because of the high separation between their components (∼100>a/Rp>∼20).However, the rotation periods of four out of the six objects measured by our group and others and presented here show that these suspected EEBs have fast rotation rates of 2.5–4h. Because of the small size of the components of these binary asteroids, linked with this fast spinning, we conclude that the rotational-fission mechanism, which is a result of the thermal YORP effect, is the most likely formation scenario. Moreover, scaling the YORP effect for these objects shows that its timescale is shorter than the estimated ages of the three relevant Hirayama families hosting these binary asteroids. Therefore, only the largest (D∼19km) suspected asteroid, (317) Roxane, could be, in fact, the only known EEB.In addition, our results confirm the triple nature of (3749) Balam by measuring mutual events on its lightcurve that match the orbital period of a nearby satellite in addition to its distant companion. Measurements of (1509) Esclangona at different apparitions show a unique shape of the lightcurve that might be explained by color variations.

Collisional Evolution of Asteroid Families

Most asteroid dynamical families are thought to be the outcomes of collisional disruption of parent asteroids destroyed by high-velocity impacts with other astroids. However, subsequent collisions modify both the sizes and the orbits of family members, so the distributions that we see today may be very different from those following the breakup of the parent body. We study the postbreakup evolution of family asteroids with a numerical model which keeps track of both the sizes and the orbits of the fragments as they collisionally interact with the field population of asteroids. Using this model we visualize how the family appears at different evolutionary stages. In particular we find that the size distribution of a family becomes less steep with time. We have simulated the possible evolutionary history of the three most populous Hirayama families, Koronis, Eos, and Themis. By matching the distribution of sizes and orbits with those observed for the families, we obtain significant constraints on the properties of their parent bodies and on some collisional response parameters, together with the evolutionary ages of the families. The Themis family appears as the outcome of the catastrophic disruption of one of the largest asteroids, probably a unique event over the history of the Solar System. On the other hand, the Koronis and Eos families appear to have been formed from smaller parent bodies, but peculiar features may require specific processes or events. Koronis' size distribution has several bodies of comparable size at the large diameter end, which can be explained if the largest fragment of the initial breakup underwent subsequent fragmentation. The "anisotropic" orbital distribution of the Eos family requires either a peculiar fragment velocity field or the action of poorly understood dynamical processes on the orbits of its members. For both the Koronis and the Themis families we derive an estimate of the age of the order of 2 Byr. The uncertainties affecting our estimates of family ages and of the properties of the parent bodies are mainly due to the present limited understanding of collisional breakup processes for bodies hundreds of kilometers in size and to the poor knowledge of the size distribution of small asteroids.

Some peculiarities in the Asteroidal Belt

The analysis of the fine structure of the Asteroidal Belt evidenciates a group of asteroids next to the resonance 4/9 with Jupiter. In this group and in other groups associated to the Hirayama families there are indications that their orbital parameters can be represented by quantum numbers as defined here and in two of our previous works. Together with this the distribution of the eccentricities and inclinations of the orbital planes of short period comets and diverse type of asteroids indicates that they can be classified as objects with e > sin i and objects with e > sin i with a limit e = sin i which determinates geometrical properties of the orbits related with discrete states in the solar system. This study lets open the possibility of following studies in order to confirm the quantum characteristics of the Asteroidal Belt being these characteristics common to all the solar system and depending of the same fundamental constant of action per mass unit H0 = 1/2π φ0 × T0 (potential × time) because only a small part of all the available data in the Asteroid Belt is used here.

Collisional history of asteroids: Evidence from Vesta and the Hirayama families

Collisional evolution studies of asteroids indicate that the initial asteroid population at the time mean collisional velocities were pumped up to ∼5 km/sec was only modestly larger than it is today; i.e., the asteroid belt was already depleted relative to the mean surface density elsewhere in the planetary region. Numerical simulations of the collisional evolution of hypothetical initial asteroid populations have been run, subject to three constraints: they must (a) evolve to the present observed asteroid size distribution, (b) preserve Vesta's basaltic crust, and (c) produce at least the observed number of major Hirayama families. A “runaway growth” initial asteroid population distribution is found to best satisfy these constraints. A new model is presented for calculating the fragmental size distribution for the disruption of large, gravitationally bound bodies in which the material strength is increased by hydrostatic self-compression. This model predicts that large asteroid behave as intrinsically strong bodies, even if they have had a history of being collisionally fractured. This model, when applied to the breakup of the Themis and Eos family parent bodies, gives size distributions in reasonably good agreement with those observed.

Review of asteroidal dynamics

It is a major goal of research in the field of asteroidal dynamics to relate the distribution of orbital elements to the evolutionary history of the asteroidal belt. The distribution of orbital elements shows characteristics which are presumably of cosmogonic significance. In particular the distribution of the mean eccentricities and inclinations of asteroidal orbits, the gaps and clusterings of asteroidal semi-major axes are related to the dynamical evolution of the asteroidal belt. Gaps are associated with resonance phenomena. The clustering of asteroidal semi-major axes is associated with resonances or like in the case of Hirayama families with an early break-up of a parent body. Another dynamical problem present the planet-crossers, in particular the Earth-crossers since their low collisional lifetimes require a source. Presumably, the Earth-crossers are a mixture of asteroidal collisional fragments and of extinct cometary nuclei. For this reason, Earth-crossers are of high interest as targets for spaceflights.

The spin periods of masteroids

In an attempt to verify the suggestion by Harris and Burns (1979) that M-type asteroids in general have high spin rates we are currently involved in a programme of photoelectricUBV observations of M and CMEU asteroids. Recently we have thus determined rotation periods, light-curve amplitudes andUBV colour indices for the asteroids 201 Penelope and 250 Bettina, both of which are situated in the (C,M,E) domain of the two-colour diagram. The resulting periods are very short (cf., IAUC 3523 and 3527). In general, we find that both the M and CMEU asteroid groups appear to be characterized by fast rotations, large amplitudes and an avoidance of membership in Hirayama families.

Resonant structure of the asteroid belt

Statistical techniques are applied to asteroid orbital data in an attempt to define the characteristics of the Kirkwood gaps in the distribution of the asteroids. A significant tendency is found for the eccentricities and inclinations of asteroid orbits to increase away from the gaps. These effects are observed throughout the distribution of main-belt asteroids. They are not merely confined to the immediate vicinities of the resonances and are not caused by observational selection effects. There is no evidence that the magnitude–frequency distribution of the asteroids changes near the gaps. Evidence is given for the existence of gaps in Hirayama families, possibly of a different nature from the classical Kirkwood gaps at locations corresponding to high order commensurabilities with Jupiter. These results support the theory that the Kirkwood gaps were formed by gravitational processes acting on individual asteroids throughout their lifetimes.

10. A Statistical Investigation of Asteroid Families: Preliminary Results

The correct identification of asteroid families is a prerequisite for understanding their nature, their orbital evolution and their physical origin. For this reason, a statistical investigation of asteroid families has been carried out, using a new clustering technique developed by A. I. Gavrishin. Proper elements for 2764 asteroids (1810 numbered and 954 Palomar-Leiden-Survey (PLS) asteroids) have been computed. Using these data, the Gavrishin method gives only ten significant classes. Five of them are coincident with the Hirayama families 1, 2, 3, 5, and the Flora group, that cannot be univocally subdivided. The PLS families are recognized. Furthermore many small-sized families reported by several authors lose their statistical relevance.

Asteroid Families: Observational Evidence for Common Origins

Colors of minor planets in the UBV system indicate compositions quite distinct from those of the field population in each of three Hirayama families. The Eos and Koronis families apparently originated from the collisional fragmentation of undifferentiated silicate bodies, and the Nysa group from a geochemically differentiated parent body.

New contributions to the problem of capture

We present a treatment of libration-point capture in the restricted three-body problem. Examples of capture are given, and a long-term numerical integration is presented, to illustrate major features of orbits arising from capture. A theory of lifetimes is given, providing order-of-magnitude (though rather conservative) estimates of the time a body remains captured. A general capture criterion, giving bounds on admissible values of the postcapture semimajor axis, for given values of eccentricity and inclination. This criterion is used to demonstrate that, in general, direct postcapture orbits lie outside retrograde ones. We also emphasize the importance of mass-change, of one or both primaries, in producing capture. This phenomenon is shown to give rise to a new type of capture, “pull-down capture,” which produces retrograde orbits. The effects of nebular drag also are noted.These results suggest the improbability of a capture origin for Jupiter's outer satellites within the last 4+ billion years, or since the solar system reached its present dynamical configuration. Computations indicate, however, that either mass-change or nebular drag could have been effective in producing capture. The outer satellite groups are shown to resemble Hirayama families physically, thus supporting a hypothesis of capture followed by collisional fragmentation.

Asteroids as meteorite parent-bodies: the astronomical perspective

A review of astronomical evidence suggests that asteroids are the parent-bodies for most meteorites. The surfaces of most asteroids are like carbonaceous chondrites while a significant minority are of stony-iron composition. Other meteorite types are recognized in the belt but are rare. Plausible dynamical processes are known that can deliver meteorites from the main belt to Earth. Asteroid compositions tend to correlate with solar distance, suggesting that the boundary between ordinary and carbonaceous chondritic nebular condensates was near the inner edge of the present belt. The size distribution of stony-iron asteroids implies they are remnant cores of ~ 100 differentiated bodies subjected to collisional fragmentation by carbonaceous objects initially ~ 300 times more numerous than now. Incomplete evidence on parent-body collisions exists in data on Hirayama families, asteroid lightcurves, and the compositional homogeneity of individual asteroids. Modern-day asteroid regoliths are thin and cannot have been environments for formation of most brecciated, gas-rich meteorites; such meteorites formed during early accretion of the asteroids. A scenario for the origin and evolution of meteorite parent-bodies is presented which includes: 1. (1) interruption of planet-formation by processes due to Jupiter; 2. (2) substantial asteroidal collisions during the first 0.5 b.y.; 3. (3) formation of most meteorite types within the differentiated bodies.

Asteroids - Spectral reflectance and color characteristics. II

view Abstract Citations (41) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Asteroids spectral reflectance and color characteristics. II. McCord, T. B. ; Chapman, C. R. Abstract New spectrophotometry for 31 asteroids, and improved data for nine previously observed, is presented, raising the total sample to 98. Several important new spectral types have been found. Asteroid 349 Dembowska is the first large main-belt asteroid found to resemble ordinary chondritic meteorites in spectral properties (it is similar to LL6 chondrites in pyroxene/olivine content). The first two measured Trojan asteroids show unusual spectra not compatible with carbonaceous chondrites or other known meteorites. The spectrum of Mars-crosser 887 Alinda is compatible with unequilibrated chondrites. Most fainter asteroids (especially those in the outer half of the belt) have flat spectra indicating probable carbonaceous composition. Compositional heterogeneity of Hirayama families is common among the 16 families studied to date. But individual asteroids seem to have remarkably uniform surface compositions, indicated by the usual lack of spectral changes with rotation. Spectra of a preliminary sample of proposed meteorite source-bodies are consistent with derivation of meteorites by proposed mechanisms, but further observations are needed. Publication: The Astrophysical Journal Pub Date: May 1975 DOI: 10.1086/153565 Bibcode: 1975ApJ...197..781M Keywords: Asteroids; Astronomical Photometry; Colorimetry; Spectral Reflectance; Chemical Composition; Meteoritic Composition; Orbital Elements; Spectrophotometry; Lunar and Planetary Exploration full text sources ADS |

Descriptive Survey of Families, Trojans, and Jetstreams

The word “group” is so general that I would like to suggest that here it is used in its most general way: a group of asteroids is a collection of minor planets that have some feature in common. If we agree on this use of the word “group,” then we can discern the following asteroidal groups:(1) Groups that have a dynamical cause:(a) Trojans(b) Commensurability groups:(i) Hungaria group(ii) Hilda group(iii) Thule(2) Groups that probably have no dynamical cause:(a) Hirayama families(b) Brouwer groups(c) Jetstreams

Fragmentation history of asteroids

The absolute magnitude distribution of asteroids between 2.0 and 2.6 a.u. resembles a Gaussian curve for the brighter members, grading into a logarithmic curve for the fainter ones. When the asteroids belonging to Hirayama families are reassembled into their parent asteroids, the Gaussian portion is enhanced at the expense of the logarithmic one. If this operation is repeated once (to allow for dispersed and unrecognized families), the logarithmic branch disappears entirely, leaving what may be the original, Gaussian distribution. The following tentative conclusions can be drawn from a comparison of the present distribution between 2.15 and 2.6 a.u. with the “original” one: 1. 1.|The asteroid belt is not in a highly fragmented state. The present distribution is only a few steps removed from the original one. 2. 2.|The collision half-life of asteroids, for a loss of one-half their original mass, is 6.1 Æ. 3. 3.|The cross-sectional area of all asteroids brighter than g = 14 has increased by only 6% during the last 4.5 Æ. 4. 4.|The average crushing strength of asteroids seems to ne about 2 × 10 8 dynes/cm 2. 5. 5.|The half-life for dispersal of destruction of Hirayama families is ≥2.2 Æ. 6. 6.|The original distribution peaked at about g = 10, the same as the apparent present distribution of the Trojans. The original magnitudes of 13 reconstituted Hirayama families range from g = 7.4 to g = 10.1. One of the four families crossing the orbit of Mars seems to have originated in a collision 400 M yr ago.

Origin, age, and composition of meteorites

Origin, age, and composition of meteorites