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Broadband Photometry of the Potentially Hazardous Asteroid 1999 MN: Suggestive of YORP and/or Tidal Spin-Up?

ATEL # 2706; M. Hicks (JPL/Caltech), D. Mayes (JPL/Caltech), A. McAuley (CSULA), J. Foster (CSULA)
on 29 Jun 2010; 21:43 UT
Password Certification: Michael D. Hicks (

Subjects: Optical, Asteroids, Planets, Planets (minor), Solar System Objects

1999 MN was discovered by Carl Hergenrother on behalf of the Catalina Sky Survey on June 22 1999 (MPEC 1999-M27) and identified as a Potentially Hazardous Asteroid (PHA) by the Minor Planet Center. The object's low inclination and perihelion distance allows for frequent gravitational encounters with Mercury, Venus, and Earth. 1999 MN passed within 0.033 AU of the Earth on June 4.5 2010. Our Bessel BVRI observations at the JPL Table Mountain Observatory 0.6-m telescope, summarized in Table 1, were scheduled to support Arecibo radar observations obtained in 2004 and 2005.

The plane-of-sky proximity of 1999 MN to the galactic center complicated our analysis. Approximately 1/3 of the exposures were rejected due to stellar contamination. The object's rotationally averaged colors (B-R=1.206+/-0.047 mag; V-R=0.432+/-0.049 mag; R-I=0.221+/-0.056 mag) were found to be most compatible with an Sq spectral classification, an association obtained through a comparison of our colors with the 1341 asteroid spectra in the SMASS II database (Bus & Binzel 2002) [Figure 1 and Table 2].

Figure 2 illustrates our observed R-band photometry. 1999 MN's solar phase curve [Figure 3] was best fit to first order with absolute magnitude H_R=20.57 mag and solar phase parameter g=0.32, suggesting a moderately high albedo consistent with the Sq taxonomic classification. Our photometry yielded an absolute magnitude H_v=21.00+/-0.06 mag, implying an effective diameter D~160m for an albedo rho=0.25.

After converting our reduced R magnitudes to flux units, we performed a period search using standard Fourier techniques. Figure 4 plots the Fourier model misfit as a function of rotation period 0 < P_syn < 12 hr, with three principle minima near 5.5/8.2/9.3 hours, respectively [Figure 5]. Assuming a double-peaked lightcurve we measured a synodic period P_syn = 5.482+/-0.007 hr [Figure 6], slightly faster than the period found by Hergonrother & Whiteley (2005) P = 5.495 hr and suggesting the possibility of YORP and/or tidal spin-up. Our current data is insufficient to resolve a synodic versus sidereal period determination and we invite collaboration with other observers.

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       -Galactic-     UT Date       r    delta  Phase   V   Long.  Lat. Filter(s)  Observer                  [AU]  [AU]   [deg] [mag] [deg] [deg]   2010 06 05.40  1.046 0.034  21.5  15.2   15    -2     BVRI     McAuley   2010 06 06.36  1.051 0.037   9.4  15.0    8     8     BVRI     Foster   2010 06 08.25  1.060 0.046  11.4  15.6  357    23     BVRI     Mayes   2010 06 09.29  1.065 0.052  18.6  16.1  352    28     BVRI     Mayes   2010 06 10.33  1.069 0.059  24.4  16.6  348    32       R      Mayes   

   Table 2:  Best-fit SMASS II spectral analogs.                              Taxonomic Class   Misfit  Object Name     (Tholen)  (Bus)   0.536   4261 Gekko                  Sq   0.647   1977 Shura                  Sq   0.716     33 Polyhymnia     S       Sq   0.886   2427 Kobzar                 Sq   0.891   7728 Giblin                 Sq   0.899    180 Garumna        S       Sq   

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