Letter to the Editor

(3200) Phaethon: Bulk density from Yarkovsky drift detection

¹ Institute of Astronomy, Charles University, Prague, V Holešovičkách 2, 18000 Prague 8, Czech Republic
e-mail: hanus.home@gmail.com
² Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
³ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁴ Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 0076100, Israel
⁵ Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 1, 25165 Ondřejov, Czech Republic
⁶ Center for Solar System Studies, 11355 Mount Johnson Court, Rancho Cucamonga, CA 91737, USA
⁷ Center for Solar System Studies, 446 Sycamore Ave., Eaton, CO 80615, USA

Received: 12 September 2018
Accepted: 23 November 2018

Abstract

Context. The recent close approach of the near-Earth asteroid (3200) Phaethon offered a rare opportunity to obtain high-quality observational data of various types.

Aims. We used the newly obtained optical light curves to improve the spin and shape model of Phaethon and to determine its surface physical properties derived by thermophysical modeling. We also used the available astrometric observations of Phaethon, including those obtained by the Arecibo radar and the Gaia spacecraft, to constrain the secular drift of the orbital semimajor axis. This constraint allowed us to estimate the bulk density by assuming that the drift is dominated by the Yarkovsky effect.

Methods. We used the convex inversion model to derive the spin orientation and 3D shape model of Phaethon, and a detailed numerical approach for an accurate analysis of the Yarkovsky effect.

Results. We obtained a unique solution for Phaethon’s pole orientation at (318 ° , − 47 ° ) ecliptic longitude and latitude (both with an uncertainty of 5°), and confirm the previously reported thermophysical properties (D = 5.1 ± 0.2 km, Γ = 600 ± 200J m⁻² s^−0.5 K⁻¹). Phaethon has a top-like shape with possible north-south asymmetry. The characteristic size of the regolith grains is 1 − 2 cm. The orbit analysis reveals a secular drift of the semimajor axis of −(6.9 ± 1.9)×10⁻⁴ au Myr⁻¹. With the derived volume-equivalent size of 5.1 km, the bulk density is 1.67 ± 0.47 g cm⁻³. If the size is slightly larger ∼5.7 − 5.8 km, as suggested by radar data, the bulk density would decrease to 1.48 ± 0.42 g cm⁻³. We further investigated the suggestion that Phaethon may be in a cluster with asteroids (155140) 2005 UD and (225416) 1999 YC that was formed by rotational fission of a critically spinning parent body.

Conclusions. Phaethon’s bulk density is consistent with typical values for large (> 100 km) C-complex asteroids and supports its association with asteroid (2) Pallas, as first suggested by dynamical modeling. These findings render a cometary origin unlikely for Phaethon.