2009 KD5: Photometric Signature of a Binary Near-Earth Asteroid?

ATel #2709; M. Hicks (JPL/Caltech), H. Rhoades (JPL/Caltech), J. Somers (Moorpark), J. Foster (CSULA), T. Troung (CSULA), K. Garcia (CSULA)
on 30 Jun 2010; 22:42 UT
Credential Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

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2009 KD5 was discovered May 26 2009 by the Observatorio Astronomico de Mallorca LSSS (La Sagra Sky Survey) Near-Earth Object (NEO) survey (MPEC 2009-K55) and identified as a Potentially Hazardous Asteroid (PHA) by the Minor Planet Center. The object's 2010 closest approach to the Earth is quite distant (delta=0.24 AU on June 29 2010). However, the apparition allows the NEO to be observed under slowly varying illumination and viewing geometry.

We obtained four nights of Bessel BVRI observations at the JPL Table Mountain Observatory (TMO) 0.6-m telescope, summarized in Table 1. The PHA's averaged broad-band colors (B-R=1.300+/-0.034 mag; V-R=0.491+/-0.027 mag; R-I=0.409+/-0.027 mag) were found to be most compatible with an L-type spectral classification (Bus & Binzel 2002) [Figure 1 and Table 2]. L-types asteroids are members of the S-family of asteroids and may represent surfaces highly processed by collisional gardening and space weathering.

Normalized to unit heliocentric and geocentric distance, the R-band photometry allowed the construction of a solar phase curve [Figure 2]. With the caveat that our observations cover a restricted solar phase angle range, we found a remarkably high phase parameter g=0.84, suggesting a very high geometric albedo. With our H-G fit and V-R color, we determined an Absolute Magnitude H_V = 19.18+/-0.03 mag, significantly fainter than the H_V=18.39 listed in the JPL HORIZONS database. After converting our reduced R magnitudes to flux units, we performed a period search using standard Fourier techniques. Figure 3 plots the Fourier model misfit as a function of rotation period for 0 < P_syn < 5 hr. Assuming a double-peaked lightcurve we found the a fit-period P_syn = 2.66+/-0.02 hr. The local minima near 3.78 hr and 3.95 hr require triple-peaked lighcurves. 2009 KD5 is rotating not far from the rotational break-up speed near 2.3 hr. Figure 4 plots the lighcurve assuming g=0.84 and P=2.66 hr. The dispersion in the phased lightcurve suggests that 2009 KD5 may be a binary system, with variations in observed flux caused by an unresolved, tidally locked secondary companion, similar to the 1999 KW4 system (Ostro et al., 2006). 2009 KD5 should remain brighter than V=18 until late July 2010. We plan to obtain additional R-band photometry to confirm the solar phase behavior and resolve the lightcurve contribution of the hypothetical companion. Collaborations with other small-body observers would be very welcomed.

Copyright 2010. All rights reserved. The research described in this telegram was carried out at the Jet Propulsion Laboratory, under a contract with the National Aeronautics and Space Administration. The student participation was supported by the National Science Foundation under REU grant 0852088 to Cal State LA.

  
Table 1:   Observational circumstances.  
  
                           Solar  Solar  
  UT Date       r    delta Phase  Elong.   V    Observer(s)  
               [AU]  [AU]  [deg]  [deg]  [mag]  
2010 06 15.29  1.219 0.248  31.4  141.3  17.1   Rhoades  
2010 06 16.33  1.216 0.246  32.1  140.6  17.1   Rhoades  
2010 06 19.29  1.207 0.242  34.3  138.1  17.2   Somers, Hicks  
2010 06 20.30  1.204 0.241  35.1  137.1  17.2   Foster, Truong, Garcia  

  
Table 2:  Best-fit SMASS II spectral analogs.  
  
                         Taxonomic Class  
Misfit  Object Name      (Tholen)  (Bus)  
  
0.822  980 Anacostia         SU      L  
1.222 2448 Sholokhov                 L  
1.367 4607 Seilandfarm               L  
1.871  443 Photographica     S       Sl  
1.937  642 Clara             S       L