Binary asteroid population. 2. Anisotropic distribution of orbit poles of small, inner main-belt binaries (original) (raw)
Abstract
Our photometric observations of 18 main-belt binary systems in more than one apparition revealed a strikingly high number of 15 having positively re-observed mutual events in the return apparitions. Our simulations of the survey showed that it cannot be due to an observational selection effect and that the data strongly suggest that poles of mutual orbits between components of binary asteroids in the primary size range 3–8
km are not distributed randomly: The null hypothesis of an isotropic distribution of the orbit poles is rejected at a confidence level greater than 99.99%. Binary orbit poles concentrate at high ecliptic latitudes, within 30° of the poles of the ecliptic. We propose that the binary orbit poles oriented preferentially up/down-right are due to either of the two processes: (i) the YORP tilt of spin axes of their parent bodies toward the asymptotic states near obliquities 0° and 180° (pre-formation mechanism) or (ii) the YORP tilt of spin axes of the primary components of already formed binary systems toward the asymptotic states near obliquities 0° and 180° (post-formation mechanism). The alternative process of elimination of binaries with poles closer to the ecliptic by dynamical instability, such as the Kozai effect due to gravitational perturbations from the Sun, does not explain the observed orbit pole concentration. This is because for close binary asteroid systems, the gravitational effects of primary’s irregular shape dominate the solar-tide effect.
Highlights
► Parameters of 18 binary inner main-belt asteroids estimated/constrained. ► Orbit poles of small binary asteroids concentrate at high ecliptic latitudes. ► The YORP tilt of spins of binary parent bodies or primaries proposed as the cause.
Introduction
Orientation of the orbital plane of components in a binary asteroid can be estimated from photometric observations of their mutual events—occultations/eclipses. It requires observations taken over a range of geometries of the system with respect to Earth and Sun. Scheirich and Pravec (2009) derived or constrained orbit poles of five near-Earth asteroid binaries, taking advantage of rapid changes of viewing geometries of the near-Earth binaries during their approaches to Earth. Binary systems in the main belt of asteroids show a limited change of observing geometry during one apparition1 and observations over 2–3 apparitions are typically needed to estimate the orientation (ecliptic longitude and latitude of the pole) of the mutual orbit for a main-belt asteroid (MBA) binary.
We run a photometric survey for binaries among small asteroids since 2005. Among 477
MBAs surveyed until May 2011, we found 45 binaries. Of them, 18 were re-observed in their return apparitions. Using the technique of Pravec et al., 2006, Scheirich and Pravec, 2009, we analysed the data and estimated or constrained mutual orbits of the 18 binaries observed in 2–3 apparitions. An interpretation of the sample of derived binary parameters must take into account existing observational biases, see a theory of the selection effects of the photometric technique of binary detection presented in Section 2. A direct estimation of the biases present in the discovered sample of binaries is complicated by a limited probability of covering the mutual event in a binary with a priori unknown orbit period with a given set of survey observations. This complication is overcome with analysis of the statistics of re-detections of mutual events in the binaries in their return apparitions. The key advantage is that a time distribution of the planned follow-up observations of the binaries in the return apparitions was matched to their orbit periods determined in the discovery apparition, which made our simulations of the observational selection effects feasible.
Section snippets
Probability of photometric detection of a binary asteroid
The probability of the photometric detection of a binary asteroid is formulated as follows:Pdet=pmepcovpres,where _p_me is a probability of occurrence of a mutual event (occultation or eclipse) between the components of the system, _p_cov is a probability of covering the mutual event with a given set of observations, and _p_res is a probability of resolving the mutual event with the given photometric observations.
The probability of occurrence of a mutual event depends on the parameters of the system:p
Observations and binary parameters estimations
We run a long-term project of photometric observations of binary systems among small asteroids called “Photometric Survey for Asynchronous Binary Asteroids” since 2005. The collaborating station Palmer Divide Observatory runs a parallel survey project aimed at describing rotations and binary systems in the Hungaria asteroids group (Warner et al., 2009a, Warner et al., 2009c). Both surveys used similar observing techniques and strategies, and they actually cooperated and coordinated their
Simulations of the survey
The rate of 15 positive re-detections of the 18 binaries in their return apparitions is strikingly high. We simulated the survey, tested the null hypothesis of isotropic distribution of binary orbit poles and found that it is rejected at a high confidence level. We found that poles of mutual orbits of small binaries concentrate at high ecliptic latitudes around the poles of the ecliptic. We present results of the simulations in this section.
The model of the binary survey is analogous to that we
Interpretation and discussion
Binary systems among small asteroids (primary diameters _D_1
≲
10
km) appear to form from parent bodies spinning at a critical rate by some sort of fission or mass shedding process (Scheeres, 2007, Pravec and Harris, 2007, Walsh et al., 2008, Jacobson and Scheeres, 2011). A mechanism to spin the parent asteroid up to its critical rotation frequency is provided by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect (e.g., Bottke et al., 2006). While spinning it up, the YORP effect also changes
Acknowledgments
The work at Ondřejov and the Charles University Prague was supported by the Grant Agency of the Czech Republic, Grants 205/05/0604 and 205/09/1107, and by the Research Programs MSM0021620860 and MEB0810085 of the Czech Ministry of Education. A.W.H. was supported by NASA Grant NNX 09AB48G and by National Science Foundation Grant AST-0907650. Observations at the Carbuncle Hill Observatory and the Hunters Hill Observatory were supported by a Gene Shoemaker NEO Grant from the Planetary Society. The
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The binary nature of this Hungaria asteroid was discovered by Warner et al. (2007). Observations from the first three apparitions 2007, 2009 and 2011 were published in Pravec et al. (2012), and we have obtained additional data in the apparition 2012. A model of the binary will be presented in Scheirich et al. (in preparation).
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As a last alternative, Bigbee’s formation could have been coeval to Holden. Indeed, recent studies show that binary asteroids represent 1/6 of >300 m large body among near Earth asteroids (e.g., Pravec et al., 2006, 2012). Observations at Venus surface confirm the accuracy of these estimations (Cook et al., 2003).
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