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ATEL # 2372; M. Hicks (JPL/Caltech), J. Somers (Moorpark College)
on 5 Jan 2010; 23:00 UT
Password Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)
Subjects: Optical, Asteroids, Planets, Planets (minor), Solar System Objects
The Near-Earth Asteroid (NEA) 2007 MK13 was discovered by the Catalina Sky Survey on June 21 2007 (MPEC 2007-M37). With a likely diameter between 200 m < D < 600 m, and with a Minimum Orbit Intersection Distance MOID = 0.032 AU, the NEA has been designated as a Potentially Hazardous Asteroid (PHA) by the Minor Planet Center. We took advantage of the object's well-placed 2009 apparition to obtain Bessel BVRI colors and rotational light-curve information over the course of two photometric nights at the JPL Table Mountain 0.6-m telescope (TMO), as illustrated in Figures 1-2  and summarized in Table 1.
Our data constrains 2007 MK13's size, composition, shape, and rotational state. The NEA's colors (B-R=1.095+/-0.020; V-R=0.387+/-0.016; R-I=0.337+/-0.010) are most compatible with a C-type taxonomy. Though less likely, an X-type classification is also possible. This association was obtained through a comparison of our colors with the 1341 asteroid spectra in the SMASS II database as archived on the PDS node (Bus & Binzel 2002) [Figure 3 and Table 2].
Light-time corrected photometry was converted to reduced magnitude assuming a phase parameter G=0.0, consistent with a C-type classification, and the BVI data were registered to R using the measured colors. After converting the photometry from magnitude to flux units, we performed a rotational period search using standard Fourier techniques. Figure 4 plots chi-squared 5th and 6th-order Fourier model misfit as a function of assumed rotation period. We found a best-fit synodic period P_syn = 5.286+/-0.005 hr, as shown in Figure 5 . Two additional minima in our periodogram, P_syn =6.07 hr and P_syn = 7.06 hr, require triple-peaked and quadruple-peaked lightcurves, respectively, which can be rejected for an asteroid with such a large lightcurve amplitude.
Assuming G=0.0, our photometry yields an absolute magnitude H_v=19.94+/-0.01 mag, implying an effective diameter D=575+/-20 m for a C-type geometric albedo rho=0.05. The lightcurve amplitude (2.00+/-0.05 mag) at modest solar phase angle suggests that the shape of 2007 MK13 is highly elongated, with an triaxial ellipsoid a:b ratio of at least 2:1. We conclude that 2007 MK13 is likely a contact binary.
2007 MK13 remains at high declination and brighter than V=19 through Jan 31 2010. We welcome collaborations with other minor planet observers: Additional R-band photometry should allow a direct measurement of the object's solar phase behavior in order to better constrain albedo and size.
Copyright 2009. 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.
Table 1: Observational circumstances. UT DATE r delta phase V Num. FILTER OBSERVER [AU] [AU] [deg] [mag] Exp. 2009 12 26.28 1.033 0.059 32.3 15.1 55 BVRI Hicks 2009 12 27.28 1.035 0.061 30.5 15.2 63 R Somers, Hicks
Table 2: Best-fit SMASS II spectral analogs. TAXONOMIC CLASS MISFIT OBJECT NAME (THOLEN) (BUS) 0.61 93 Minerva CU C 0.97 3192 A'Hearn C 1.10 1017 Jacqueline C 1.13 1041 Asta C 1.18 5294 Onnetoh X