Towards Initial Mass Functions for Asteroids and Kuiper Belt Objects

Our goal is to understand primary accretion of the first planetesimals. The primitive meteorite record suggests that sizeable planetesimals formed in the asteroid belt over a period longer than a million years, each composed entirely of an unusual, but homogeneous, mixture of mm-size particles. We sketch a scenario in which primary accretion of 10-100km size planetesimals proceeds directly, if sporadically, from aerodynamically-sorted mm-size particles (generically "chondrules"). These planetesimal sizes are in general agreement with the currently observed asteroid mass peak near 100km diameter, which has been identified as a "fossil" property of the pre-erosion, pre-depletion population. We extend our primary accretion theory to make predictions for outer solar system planetesimals, which may also have a preferred size in the 100km diameter range. We estimate formation rates of planetesimals and assess the conditions needed to match estimates of both asteroid and Kuiper Belt Object (KBO) formation rates. For nebula parameters that satisfy observed mass accretion rates of Myr-old protoplanetary nebulae, the scenario is roughly consistent with not only the "fossil" sizes of the asteroids, and their estimated production rates, but also with the observed spread in formation ages of chondrules in a given chondrite, and with a tolerably small radial diffusive mixing during this time between formation and accretion (the model naturally helps explain the peculiar size distribution of chondrules within such objects). The scenario also produces 10-100km diameter primary KBOs. The optimum range of parameters, however, represents a higher gas density and fractional abundance of solids, and a smaller difference between keplerian and pressure-supported orbital velocities, than "canonical" models of the solar nebula. We discuss several potential explanations for these differences.Comment: Icarus, in pres

Origin of the Near-Ecliptic Circumsolar Dust Band

The zodiacal dust bands are bright infrared (IR) strips produced by thermal emission from circumsolar rings of particles. Two of the three principal dust bands, known as β and γ, were previously linked to the recent asteroid collisions that produced groups of fragments, so-called asteroid families, near the orbits of (832) Karin and (490) Veritas. The origin of the third, near-ecliptic α band has been unknown until now. Here we report the discovery of a recent breakup of a >20 km diameter asteroid near α's originally suspected source location in the Themis family. Numerical modeling and observations of the α-band thermal emission from the Spitzer Space Telescope indicate that the discovered breakup is the source of α-band particles. The recent formation of all principal dust bands implies a significant time variability of the circumstellar debris disks

ON A SCATTERED-DISK ORIGIN FOR THE 2003 EL61 COLLISIONAL FAMILY - AN EXAMPLE OF THE IMPORTANCE OF COLLISIONS ON THE DYNAMICS OF SMALL BODIES

International audienceThe recent discovery of the 2003EL61 collisional family in the Kuiper belt (Brown et al., 2007) is surprising because the formation of such a family is a highly improbable event in today's belt. Assuming Brown et al.'s estimate of the size of progenitors, we find that the probability that a Kuiper belt object was involved in such a collision since primordial times is less than roughly 0.001. In addition, it is not possible for the collision to have occurred in a massive primordial Kuiper belt because the dynamical coherence of the family would not have survived whatever event produced the currently observed orbital excitation. Here we suggest that the family is the result of a collision between two scattered disk objects. We show that the probability that a collision occurred between two such objects with sizes similar to those advocated in Brown et al. (2007) and that the center of mass of the resulting family is on an orbit typical of the Kuiper belt can be as large as 47%. Given the large uncertainties involved in this estimate, this result is consistent with the existence of one such family. If true, this result has implications far beyond the origin of a single collisional family, because it shows that collisions played an important role in shaping the dynamical structure of the small body populations that we see today

Debiased population of very young asteroid families

We use observations from the Catalina Sky Survey (CSS) to determine the bias-corrected population of small members in four very young families down to sizes equivalent to several hundred meters. 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. 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 $H\leq 18.3$ accompanies the known three members in the Lucascavin family. A similar result is found for the archetypal asteroid pair Rheinland--Kurpfalz.Comment: 32 pages, 27 figures, accepted for publication in Astronomy and Astrophysic

Candidates for asteroid dust trails

The contribution of different sources to the circumsolar dust cloud (known as the zodiacal cloud) can be deduced from diagnostic observations. We used the Spitzer Space Telescope to observe the diffuse thermal emission of the zodiacal cloud near the ecliptic. Several structures were identified in these observations, including previously known asteroid dust bands, which are thought to have been produced by recent asteroid collisions, and cometary trails. Interestingly, two of the detected dust trails, denoted t1 and t2 here, cannot be linked to any known comet. Trails t1 and t2 represent a much larger integrated brightness than all known cometary trails combined and may therefore be major contributors to the circumsolar dust cloud. We used our Spitzer observations to determine the orbits of these trails and were able to link them to two ("orphan" or type II) trails that were discovered by the Infrared Astronomical Satellite (IRAS) in 1983. The orbits of trails t1 and t2 that we determined by combining the Spitzer and IRAS data have semimajor axes, eccentricities, and inclinations like those of the main-belt asteroids. We therefore propose that trails t1 and t2 were produced by very recent (<~100 kyr old) collisional breakups of small, <~10 km diameter main-belt asteroids

Observed Binary Fraction Sets Limits on the Extent of Collisional Grinding in the Kuiper Belt

The size distribution in the cold classical Kuiper belt can be approximated by two idealized power laws: one with steep slope for radii R>R* and one with shallow slope for R<R*, where R*~25-50 km. Previous works suggested that the SFD roll-over at R* can be the result of extensive collisional grinding in the Kuiper belt that led to the catastrophic disruption of most bodies with R<R*. Here we use a new code to test the effect of collisions in the Kuiper belt. We find that the observed roll-over could indeed be explained by collisional grinding provided that the initial mass in large bodies was much larger than the one in the present Kuiper belt, and was dynamically depleted. In addition to the size distribution changes, our code also tracks the effects of collisions on binary systems. We find that it is generally easier to dissolve wide binary systems, such as the ones existing in the cold Kuiper belt today, than to catastrophically disrupt objects with R~R*. Thus, the binary survival sets important limits on the extent of collisional grinding in the Kuiper belt. We find that the extensive collisional grinding required to produce the SFD roll-over at R* would imply a strong gradient of the binary fraction with R and separation, because it is generally easier to dissolve binaries with small components and/or those with wide orbits. The expected binary fraction for R<R* is <0.1. The present observational data do not show such a gradient. Instead, they suggest a large binary fraction of ~0.4 for R=30-40 km. This may indicate that the roll-over was not produced by disruptive collisions, but is instead a fossil remnant of the KBO formation process.Comment: The Astronomical Journal, in pres