6478 Exhibits a Rotational Period of 3.360 h and might be double

ATel #12663; Ignacio Ferrin, Institute of Physics, University of Antioquia, Medellin, Colombia; Agustin Acosta, Observatory Costa Tequise, Z39
on 15 Apr 2019; 19:37 UT
Distributed as an Instant Email Notice Comets
Credential Certification: Ignacio Ferrin (ignacio.ferrin@udea.edu.co)

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We used the 24 cm telescope of Z39 to take 178 observations of asteroid-comet 6478 Gault and processed them with software Canopus and Maxim-DL to measure the rotational period of this object. Previously this period had been estimated by Kleyna et al. (2019) who found a value ~2h. However phasing and smoothing of their data did not reveal any obvious light curve, suggesting that the periodic signal was buried in aperiodic, non-Gaussian noise. We observed 6478 from January 12th to April 8th during 8 nights and used a 5 data point running mean to diminish the scattering of the data and to increase the signal to noise ratio by a factor of 2.2. Since this average operates only in the vertical direction there is no way it can modify the period. The NASA Exoplanet periodogram tool was used to calculate the period with the Lomb-Scargle option. To our surprise the periodogram found a period around ~0.07 days corresponding to rotational light curve with one peak and Prot(1)=1.680+-0.002 and amplitude Amp(1) = 0.18+-0.02. If we double the period we find two peaks and Prot(2)=3.360+-0.005 and amplitude Amp(2) = 0.16+-0.02. It is interesting to point out that we did not expect that the 24 cm telescope could find a period for this 17-magnitude target, but it did. There is no way in which this result could have been falsified. Each observation corresponds to a phase in the phase curve, and the observer does not have any way to know the phase for any observing night. The only thing he can do is secure the observations and hope for the best. So if the software finds a period, it must be real. As a confirmation, the light curves produced look quite robust. The first period with one rotational peak violates the rotational limit for disruption of a rubble pile asteroid ( ~2.2 h), so we adopt the second one with two rotational peaks. A 0.16 mag amplitude corresponds to a ratio of axis a/b=1.08. The minimum density needed to ensure the material at the tip of a prolate body in rotation about its minor axis that is gravitationally bound is approximately given by Jewitt et al. (2014). If we use the observed period, 3.360 h and the observed ratio a/b=1.08 then the minimum density comes out to be ~1.13 gm/cm3, a very reasonable value expected for a comet mainly made of water ice. Surprisingly the phased light curve diagram shows evidence of an eclipse in the form of a flat bottom V, with the following de-trended values T1(phase,mag) = (0.53,0.0) = 190 deg, T2 = (0.62,0.13) = 223 deg, T3 = (0.74,0.13) = 266 deg, T4 = (0.80,0.0) = 288 deg, T4-T1 = 0.27 =97 deg, T3-T2 = 0.12 = 43 deg, (T1+T2+T3+T4)/4 = 0.673, m3-m1 = 0.13 mag. A preliminary analysis depends on the absolute magnitude value of the object. Ye et al. (2019) give Hg = 14.81+-0.04 based on observations at aphelion. However the Secular Light Curve (being prepared for publication) shows that the comet is active at aphelion and a more reliable absolute magnitude is mv(1,1,0) = 16.07+-0.05. Using these results we find two components with preliminary diameters 3.86 and 1.4 km in synchronous rotation. We request observations to confirm or deny these results. References: (1) Jewitt, D., et al. (2014). Hubble space telescope investigation of Main Belt comet 133P/Elst-Pizarro. An. J., 147, 117-129. (2) Kelyna, J. T. et al. (2019) The Sporadic Activity of (6478) Gault: A YORP driven event. https://arxiv.org/pdf/1903.12142v1.pdf (3) Ye, Q., et al. (2019). Multiple Outbursts of Asteroid (6478) Gault. https://arxiv.org/pdf/1903.05320.pdf.

6478 Gault Exhibits a Period of 3.360 h and it might be double