Four new binary minor planets: (854) Frostia, (1089) Tama, (1313) Berna, (4492) Debussy

Abstract

Aims. We present evidence that four minor planets of the main belt are binary systems. Methods. These discoveries are based on CCD photometric measurements made by many observers coordinated in a network of observatories. Results. Orbital and physical properties are derived from a total of 134 partial light curves involving 26 stations. (854) Frostia, (1089) Tama, (1313) Berna, and (4492) Debussy show mutual eclipses features on their light curves. In all cases, rotation and revolution are synchronous. Synodic periods are 37. 728, 16.444, 25.464 and 26.606 h respectively. From a simple model, we have derived their bulk densities as follows: 0.89 ± 0.14, 2.52 ± 0.30, 1.22 ± 0.15 and 0.91 ± 0.10 g cm −3 respectively. Uncertainties in the bulk densities, arising from scattering and shadow effects are not taken into account. These could increase the density estimates by a factor up to 1.6. Our method of determining bulk density is completely independent of their mass and their diameter estimates. The low rotational periods and the low bulk densities clearly imply a collisional process to explain this kind of binary asteroid. Based on our database of a few thousand light curves of minor planets, the population of similar-sized objects in the main belt is estimated to 6 ± 3 percent in the 10-50 km diameter class.Key words. planets and satellites: formation -minor planets, asteroids -techniques: photometric IntroductionThe first confirmed satellite of a minor planet was Dactyl around (243) Ida, discovered in 1993 on Galileo spacecraft images (Belton & Carlson 1993). Ten years later, about fifty asteroids are suspected or confirmed to be multiple bodies. These asteroids are very useful in determining the physical and dynamical properties of small bodies, in particular the bulk density, which is important in understanding the collisional evolution of this kind of object.Various techniques are used to detect satellites for minor planets: occultation of stars, light curves, direct imaging by spacecraft, radars or adaptive optic (see review by Merline et al. 2002). Binary studies allows the derivation of dimensions, shape, bulk densities, rotation and orbital properties of the bodies. Asteroid densities are known for only a few of them Based on observations performed with the T1 M and T60 cm at the Pic du Midi, TAROT at the Calern observatory, Faulkes-North at Hawaii, T50 cm at Brorfelde, and by many amateur observatories coordinated by Geneva Observatory.(see the review by Britt et al. 2002). We distinguish two types of multiple systems. Those consisting of comparable size bodies are designated as binary. The others are designated as primaries with satellites. When the orbital plane of a binary system is seen edge-on, mutual eclipses can be detected from light curves. That allows to discover binary systems that cannot be found by other methods. Based on binary system light curves anlysis, we present a method to derive the bulk density independently of size and mass determinations. From single opposition measurement...

Density of asteroids

“…Indirect determination of the density are also possible by modeling the mutual eclipses of a binary system (e.g., Behrend et al 2006) or the non-gravitational forces on a comet nucleus (e.g., Davidsson et al 2007). This study aims at deriving constraints on the intrinsic physical properties of small bodies by searching for relationships between, the dynamical and taxonomic classes, size, and density.…”

Physical properties of the ESA Rosetta target asteroid (21) Lutetia

“…Eight lightcurves come from the CDR-CDL group led by Raoul Behrend at the Geneva observatory 3 . The aim of this group is to organize photometric observations (including those from many amateurs) for selected asteroids and to search for binary objects (Behrend et al 2006 ) and 2009 (new data presented here). See Table 2 for a detailed listing of the observations, including the mid-observing time (Date), duration (D), sampling (number of frames N), phase angle (α), and ecliptic coordinates (longitude λ, latitude β), and observer of each observation.…”

Angular momentum of binary asteroids: Implications for their possible origin

“…The last objects incorporated in the dataset are the four small synchronous binary systems (854) Frostia, (1089) Tama, (1313) Berna, and (4492) Debussy reported by Behrend et al (2006). However, the authors confused the semi-major axis with the semi-separation 1 .…”