The combination of adaptive optics, photometric and Spitzer mid-IR observations of the 121 Hermione binary asteroid system allowed us to confirm the bilobated nature of the primary and derive a bulk density of 1.4 g/cc, implying a rubble-pile interior

Berthier, J.

Descamps, P.

Durech, J.

Emery, J.P.

Harris, A.W.

Kaasalainen, M.

Marchis, F.

Institute of Transport Research:Publications

THE CYBELE BINARY ASTEROID 121 HERMIONE REVISITED  F. Marchis1,2,3, P. Descamps3, J. Durech4, J.P Emery5, A.W. Harris6, M. Kaasalainen7, and J. Berthier3, 1SETI Institute, Carl Sagan Center, 515 N. Whisman Road, Mountain view CA 94043, USA, fmarchis@seti.org. 2UC-Berkeley, Dept of Astronomy, 601 Campbell Hall, Berkeley CA 94720, USA. 3Institut de Mécanique Céleste et de Calcul des Éphémérides, Observatoire de Paris, France, descamps@imcce.fr. 4Astronomical Institute, Charles University, Prague, Czech Republic 5University of Tennessee, Knoxville, USA. 6DLR, Berlin, Germany. 7Dept of Mathematics, University of Helsinki,  Finland.  Introduction:  (121) Hermione is a Cybele group binary asteroid with a km-size moonlet discovered in 2002 [1]. The ~200-km primary was conjectured to be bilobated [2] based on Adaptive Optics (AO) observa-tions. Because of the limited angular resolution of the AO observations (~60 mas) its size and shape were poorly constrained, impacting its bulk density estimate which  ranged between 0.8 and 2.0 g/cm3. From the determination of the orbital characteristics of Hermi-one’s satellite [2] we predicted that the system would be seen in equatorial aspect in June 2007. Shape of Hermione’s primary: We organized a campaign to record with small aperture telescopes the photometric variations a.k.a lightcurve of Hermione’s primary in 2007 before and after the asteroid equinox. From the 19 lightcurves recorded, we derived a spin period of 5.55096±0.00015 h and confirmed that Hermione is elongated (Δm=0.60-0.75). Additional AO observations were recorded with the Keck tele-scope in September 2008. A general nonconvex shape and spin model was constructed by combining the lightcurve and the limb/shadow profil [3]. The final shape model confirmed the bifurcated nature of Hermione’s primary [4].  Size of Hermione’s primary: The Keck AO sys-tem was recently improved and the last observations taken in September 2008 have an angular resolution almost twice that of the previous observations (~35 mas). Combining the shape and pole model with the recent AO images, we derived an equivalent radius Req= 94±2 km. This radius is ~10% smaller than the IMPS (IRAS) radius of 104.5±2.5 km [5]. However, re-analysis of the IRAS data with the NEATM [6], and correction for the different aspect angles, leads to ex-cellent agreement between the new AO and IMPS sizes. Similarly, taking account of the different aspect angles leads to very good agreement between the new AO size and that derived from Spitzer/IRS observa-tions performed on September 30 2007 (see Fig 1): Using the NEATM model, we derived Reff=82.4 (+4.3 -4.1), pv=0.078 (-0.016 +0.020) with H=7.31 and a beaming factor  η=0.98 (+0.03 -0.01). Using the pole solution, nonconvex shape model, and equivalent size (Req=94 km) of the primary, we calculated an effective radius of 80.7 km for the time of the Spitzer observa-tions. Satellite of Hermione: Additional observations of the satellite were recorded using the Very Large Tele-scope AO system in May 2006 and the Keck telescope in September 2008. The size of the satellite is re-estimated to a diameter of ~32 km. The orbit solution was improved but does not vary significantly from that previously published [2]. The satellite orbits around the primary in 2.5632±0.0021 days describing a circu-lar orbit (e<0.02) with a radius of 747±11 km.  Bulk density of Hermione: Combining the new size estimate and the mass derived using the 3rd Kepler law from the satellite orbit, we derived a bulk density value of 1.4±0.2 g/cm3. Hermione’s taxonomic class is C-type with a low albedo making the CI and CM car-bonaceous chondrites the best meteoritic analog. With a grain density of 2.1 g/cm3 [7] we derived a macropo-rosity of 33±5% for Hermione’s interior, very close to the low porosity of rubble.  Dryer or Fe-rich carbona-ceous meteorites will imply a higher porosity.  Tidal evolution: Dissipation by tides between the satelllite and the primary of a binary asteroid system force the orbital elements to evolve. [8] describe the conditions of stability and evolution of eccentricity and semi-major axis of various binary asteroid systems Using the new bulk density and improved orbital solu-tion, with a rigidity µ = 1010 dynes/cm2 [9] and a dissi-pation parameter Q=100 [8], we derived a timescale for evolution by tidal interaction of ~20 Myr for 121 Hermione, significantly younger than other C-group binary systems such as  45 Eugenia (>4 Byrs), 87 Sylvia (110 Myrs), or 107 Camilla (920 Myrs) [8,10]. Figure 2 shows that  because of the new size  meas-urements of primary and secondary, the Hermione bi-nary system is  now above the limit of excitation ap-proximated assuming Qs=Qp, ks=kp and ρs=ρp. There-fore the orbit of the satellite was most likely circular-ized by the tides. Conclusion: The combination of AO, photometric and mid-IR observations of the Hermione binary aster-oid system is key to deriving accurate insight into the system’s components and evolution. We confirmed  the bilobated nature of the primary already suggested in [1], derived a low and accurate bulk density imply-ing a rubble-pile interior.  Considering an evolution by tidal interaction, the Hermione system seems to be younger than other binary asteroid systems. With (624) 1336.pdf40th Lunar and Planetary Science Conference (2009)Hektor [11] and  (216) Kleopatra [12], (121) Hermione is the third confirmed bifurcated asteroid with a satel-lite. Spectroscopic studies, combining visible, near-infrared reflectances and mid-IR spectroscopy emissiv-ity spectra could help to better estimate the composi-tion and thus infer an accurate porosity estimate of the asteroid. Acknowledgements: FMA’s work was supported by the National Science Foundation Science and Technol-ogy Center for Adaptive Optics, and managed by the University of California at Santa Cruz under coopera-tive agreement No. AST-9876783 and by the National Aeronautics and Space Administration issue through  the Science Mission Directorate Research and Analysis Programs number NNX07AP70G. Part of these data was obtained at the W.M. Keck observatory, which is operated as a scientific partnership between the Cali-fornia Institute of Technology, the University of Cali-fornia and the National Aeronautics and Space Ad-ministration. The observatory and its AO system were made possible by the generous financial support of the W. M. Keck Foundation. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. JD’work was supported by the grant GACR 205/07/P070 of the Czech grant agency and by the Research Program MSM0021620860 of the Ministry of education. References:  [1] Merline et al. (2002) IAU 7980. [2] Marchis et al. (2005) Icarus 178, 450-464 [3] Kaasalainen (2009), submitted to Inverse Problems and Imaging [4] Des-camps et al. (2009) Icarus, submitted [5] Tedesco et al. (2002) AJ 123, 1056-1085 [6] Harris (2008), Icarus, 131, 291[7] Britt and Consolmagno, (2003) Meteor. & Plan. Sc 38, 1161-1180 [8] Marchis et al., (2008) Ica-rus 195, , 295-316 [9] Yoder (1982) Icarus 49, 327-326. [10] Marchis et al. (2008) 196, 1, 97-118 [11] Marchis et al. (2006) DPS #38, 65.07 [12] Marchis et al., (2008) IAU Circ 8980, Ed. D.W. Green  Figure 1: Low- and high-resolution Spitzer/IRS spectra of 121 Hermione taken on September 30 2007 at 23:56 UT. We derived the effective radius, albedo and beaming factor using the NEATM model [6].    Figure 2: : Evolution of binary asteroid mutual or-bits due to tidal dissipation. A binary asteroid system above the synchronous stability line, such as 90 An-tiope, does not evolve via tidal interaction. The satel-lite of a system below the excitation limit curve will have a slightly eccentric orbit, such as we observed for Emma & Elektra [7], due to tidal excitation. Previously derived characteristics of Hermione (labeled 2005Hermione in the figure) from [2] were not in agreement with the model since the satellite orbit is known to be circular. New measurements of the den-sity and size of Hermione’s components now place the asteroid slightly above the limit, in agreement with the theory. 1336.pdf40th Lunar and Planetary Science Conference (2009)

The Cybele binary asteroid 121 Hermione revisited

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