New modeling of 4492 Debussy eclipsing binary asteroid

Introduction

The main belt asteroid 4429 Debussy belongs to a unique group among binary asteroids in the Solar System which are known as synchronous binary, the systems with two bodies having comparable sizes and with the same rotational and orbital period. Photometric observations of these objects are still the main source of their physical properties. Lightcurves for synchronous binary systems are not quasi-sinusoidal as for most asteroids, but have very characteristic U-V shape due to the rotation of nonspherical bodies and caused by mutual eclipses. The collection of data from apparitions over several years (evenly distributed along the ecliptic longitudes), allows to determine key parameters of such binary systems (such as a non-convex shape solution and the object's spin-axis orientation) (Bartczak et al., 2014, Bartczak et al., 2017, Kryszczyńska et al., 2008).

The ultimate goal of this kind of sophisticated spin-shape modeling is to determine the asteroid's density. It is indicative of the internal structure of the body and also puts constraints on its composition. However, the estimation of density is based on mass and volume. The determination of the volume of an asteroid requires a detailed shape solution, and information about the absolute size. Our main goal of modelling Debussy is calculating a new non-convex model with its uncertainties, which are crucial in further analysis (like radiometric studies) and for the estimation of other parameters (like thermal surface properties or bulk density).

4492 Debussy

The binary nature of Debussy was discovered in 2002 (Behrend et al., 2004), when photometric observations showed a lightcurve with typical features for eclipsing binaries - the amplitude of light changes (of about 0.5 mag) corresponding to the rotation of two nonspherical bodies, and two minima (about 0.6 mag deep) due to their mutual eclipses. Further observations showed that the rotation period is P = 26.5811 ± 0.0002 h (Polińska et al., 2008). Since 2002 Debussy has been observed at almost all ecliptic longitudes evenly distributed along the orbit. All gathered data present lightcurves with deep minima caused by mutual eclipses. This means that the edge of the orbit of the binary system always points towards the observer.

Modelling

The spin-shape model of the Debussy system is calculated in the same way as for 90 Antiope (Bartczak et al., 2014) and 809 Lundia (Bartczak et al., 2017, Kryszczyńska et al., 2008), using the genetic-algorithm-based modelling method SAGE (Shaping Asteroids with Genetic Evolution). The SAGE method is based on photometric observations from several apparitions. The modelling process works best for  observations obtained with different and evenly distributed ecliptic longitudes. It allows to create non-convex shape, spin axis orientation and rotational period of synchronous binary asteroids. With the new model we would like to demonstrate the recently developed shape uncertainty method for binary objects in the same way as it was done for single asteroids and described in Bartczak & Dudziński (2019).

4492 Debussy was detected in the AKARI infrared survey and also during the cryogenic phase of WISE (calibrated fluxes were taken from the Database for thermal IR observations of small bodies, see Szakats et al. 2020). In addition, there is a dedicated Spitzer-IRS spectrum available (Marchis et al. 2012). We used the available thermal measurements to determine the size (size of an equal-volume sphere), the geometric albedo and the thermal inertia of Debussy via a well-tested thermophysical model (see e.g., Müller et al. 2017). For the radiometric study, we considered Debussy's derived spin properties, as well as different simple, convex, and non-convex shape solutions.

 

References

Behrend et al., 2004, IAU Circ., No. 8354

Bartczak et al., 2014, MNRAS, 443, 1802

Bartczak et al., 2017, MNRAS, 471, 1, 941-947

Bartczak P. & Dudziński G., 2019, MNRAS, 485, 2, 2431-2446

Kryszczyńska et al., 2008, A&A 501, 769-776

Marchis et al. 2012, Icarus 221, 1130

Müller et al. 2017, A&A 599, A103

Polińska et al., 2008, ACM paper id. 8134

Szakats et al. 2020, A&A 635, A54