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Register a new userThe heart of the swarm: K2 photometry and rotational characteristics of 56 Jovian Trojan asteroids
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We present fully covered phased light curves for 56 Jovian Trojan asteroids as acquired by the K2 mission of the Kepler space telescope. This set of objects has been monitored during Campaign 6 and represents a nearly unbiased subsample of the population of small Solar System bodies. We derived precise periods and amplitudes for all Trojans, and found their distributions to be compatible with the previous statistics. We point out, however, that ground-based rotation periods are often unreliable above 20h, and we find an overabundance of rotation periods above 60h compared with other minor planet populations. From amplitude analysis we derive a rate of binarity of 20$pm$ 5%. Our spin rate distribution confirms the previously obtained spin barrier of ~5h and the corresponding ~0.5 g cm$^{-3}$ cometary-like density limit, also suggesting a high internal porosity for Jovian Trojans. One of our targets, asteroid 65227 exhibits a double rotation period, which can either be due to binarity or the outcome of a recent collision.
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The Jovian Trojans are two swarms of small objects that share Jupiters orbit, clustered around the leading and trailing Lagrange points, L$_4$ and L$_5$. In this work, we investigate the Jovian Trojan population using the technique of astrocladistics, an adaptation of the `tree of life approach used in biology. We combine colour data from WISE, SDSS, Gaia DR2 and MOVIS surveys with knowledge of the physical and orbital characteristics of the Trojans, to generate a classification tree composed of clans with distinctive characteristics. We identify 48 clans, indicating groups of objects that possibly share a common origin. Amongst these are several that contain members of the known collisional families, though our work identifies subtleties in that classification that bear future investigation. Our clans are often broken into subclans, and most can be grouped into 10 superclans, reflecting the hierarchical nature of the population. Outcomes from this project include the identification of several high priority objects for additional observations and as well as providing context for the objects to be visited by the forthcoming textit{Lucy} mission. Our results demonstrate the ability of astrocladistics to classify multiple large and heterogeneous composite survey datasets into groupings useful for studies of the origins and evolution of our Solar system.
The Trojan asteroids provide a unique perspective on the history of Solar System. As a large population of small bodies, they record important gravitational interactions and dynamical evolution of the Solar System. In the past decade, significant advances have been made in understanding physical properties, and there has been a revolution in thinking about the origin of Trojans. The ice and organics generally presumed to be a significant part of Trojan compositions have yet to be detected directly, though low density of the binary system Patroclus (and possibly low density of the binary/moonlet system Hektor) is consistent with an interior ice component. By contrast, fine-grained silicates that appear to be similar to cometary silicates in composition have been detected, and a color bimodality may indicate distinct compositional groups among the Trojans. Whereas Trojans had traditionally been thought to have formed near 5 AU, a new paradigm has developed in which the Trojans formed in the proto-Kuiper Belt, and they were scattered inward and captured in the Trojan swarms as a result of resonant interactions of the giant planets. Whereas the orbital and population distributions of current Trojans are consistent with this origin scenario, there are significant differences between current physical properties of Trojans and those of Kuiper Belt objects. These differences may be indicative of surface modification due to the inward migration of objects that became the Trojans, but understanding of appropriate modification mechanisms is poor and would benefit from additional laboratory studies. Many open questions remain, and the future promises significant strides in our understanding of Trojans. The time is ripe for a spacecraft mission to the Trojans, to turn these objects into geologic worlds that can be studied in detail to unravel their complex history.
Aims. We investigate the influence of the Yarkovsky force on the long-term orbital evolution of Jupiter Trojan asteroids. Methods. Clones of the observed population with different sizes and different thermal properties were numerically integrated for 1 Gyr with and without the Yarkovsky effect. The escape rate of these objects from the Trojan region as well as changes in the libration amplitude, eccentricity, and inclination were used as a metric of the strength of the Yarkovsky effect on the Trojan orbits. Results. Objects with radii $Rleq$1 km are significantly influenced by the Yarkovsky force. The effect causes a depletion of these objects over timescales of a few hundred million years. As a consequence, we expect the size-frequency distribution of small Trojans to show a shallower slope than that of the currently observable population ($R$ $gtrsim$ 1 km), with a turning point between $R$ = 100 m and $R$ = 1 km. The effect of the Yarkovsky acceleration on the orbits of Trojans depends on the sense of rotation in a complex way. The libration amplitude of prograde rotators decreases with time while the eccentricity increases. Retrograde rotators experience the opposite effect, which results in retrograde rotators being ejected faster from the 1:1 resonance region. Furthermore, for objects affected by the Yarkovsky force, we find indications that the effect tends to smooth out the differences in the orbital distribution between the two clouds.
We analyze 1187 observations of about 860 unique candidate Jovian Trojan asteroids listed in the 3rd release of Sloan Digital Sky Survey (SDSS) Moving Object Catalog. The sample is complete at the faint end to r=21.2 mag (apparent brightness) and H=13.8 (absolute brightness, approximately corresponding to 10 km diameter). A subset of 297 detections of previously known Trojans were used to design and optimize a selection method based on observed angular velocity that resulted in the remaining objects. Using a sample of objects with known orbits, we estimate that the candidate sample contamination is about 3%. The well-controlled selection effects, the sample size, depth and accurate five-band UV-IR photometry enabled several new findings and the placement of older results on a firmer statistical footing. We find that there are significantly more asteroids in the leading swarm (L4) than in the trailing swarm (L5): N(L4)/N(L5)=1.6$pm$0.1, independently of limiting objects size. The overall counts normalization suggests that there are about as many Jovians Trojans as there are main-belt asteroids down to the same size limit, in agreement with earlier estimates. We find that Trojan asteroids have a remarkably narrow color distribution (root-mean-scatter of only $sim$0.05 mag) that is significantly different from the color distribution of the main-belt asteroids. The color of Trojan asteroids is correlated with their orbital inclination, in a similar way for both swarms, but appears uncorrelated with the objects size. We extrapolate the results presented here and estimate that Large Synoptic Survey Telescope will determine orbits, accurate colors and measure light curves in six photometric bandpasses for about 100,000 Jovian Trojan asteroids.
We have used the XSHOOTER echelle spectrograph on the European Southern Obseratory (ESO) Very Large Telescope (VLT) to obtain UVB-VIS-NIR (ultraviolet-blue (UVB), visible (VIS) and near-infrared (NIR)) reflectance spectra of two members of the Eureka family of L5 Mars Trojans, in order to test a genetic relationship to Eureka. In addition to obtaining spectra, we also carried out VRI photometry of one of the VLT targets using the 2-m telescope at the Bulgarian National Astronomical Observatory - Rozhen and the two-channel focal reducer. We found that these asteroids belong to the olivine-dominated A, or Sa, taxonomic class. As Eureka itself is also an olivine-dominated asteroid, it is likely that all family asteroids share a common origin and composition. We discuss the significance of these results in terms of the origin of the martian Trojan population.
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