We report on accurate BVR_c observations of (6478) Gault, a 5–6 km diameter inner mainbelt asteroid in the Phocaea family, notable for its sporadic, comet-like ejection of dust. This curious behaviour has been mainly interpreted as reconfigurations after YORP spin-up, although merging of a contact binary system cannot be fully excluded. We collected optical observations along the 2019 March–April period, at orbital phase angles between 12^o and 21^o, to search for direct evidence of asteroid quick spinning rotation. A prevailing period value of 3.34 ± 0.02 h is supported by our and other photometric observations. In the YORP spin-up hypothesis, this period points to a bulk density ρ ~1 g/cm³. The mean colours are B−V = +0.82±0.3, V−R_c = +0.28±0.06, and B−R_c = +1.11±0.4, but we have observed a trend towards bluer colour during the April session, with about (B−V) ~ 0.35±0.09 mag. This colour change can be due to asteroid rotation and support the hypothesis that there is a bluer surface under the Gault’s dust.

A 14×11 arcmin picture of (6478) Gault with its tail taken from OAVdA on 2019 March 23, about 20:20 UT (α = 10h 04m 25.2s; δ = −01^o 08' 06".7, J2000.0). North is up, east to the left. The main tail length is 5.5' at position angle PA ~ 272^o. Its also visible a fainter 12"-extended antitail at PA ~ 91^o. The image is a stack of 38 frames, each with 180 sec exposure time.

The expected PE versus ρ relationship according to Richardson et al. (2005). Spin-barrier critical period PE in case of a ‘cigar-shaped’ triaxial ellipsoid is obtained from the spherical case modulated by the shape factor F of equation (2), fully depending on the body’s eccentricity. In addition to the spherical geometry (e = 0) two cases are displayed in the plot, respectively, with e = 0.6 and 0.4, with a slower critical period increasing with body’s eccentricity, at fixed bulk density ρ, as labelled on the plot. The reference bulk-density figures for C- and S-type asteroids, according to Carbognani (2017) are reported as yellow and red bands, respectively. The prevailing estimate of Gault’s rotation period of P = 3.34 h is marked in the plot, together with the implied range for asteroid’s density (arrows) ρ ~ 1 g/cm³. See text for a discussion.

R_c-band isophotal contour plot of a Gault’s illustrative image from the Loiano data set, along the night of 2019 April 15. Esposure time is 240 sec with telescope tracked at non-sidereal rates to follow Gault’s motion. The displayed field of view is about 60×45 arcsec across, with North up and East to the left. Coordinate axes are labelled in pixel scale (1 px = 0.58 arcsec). Gault is the ‘rounded’ object about (x, y) = (362, 591) coordinates. The vertically elongated object to the left of the image is a saturated star distorted by on-target tracking. Seeing on the image is about 2.2 arcsec FWHM. A bright full Moon, only 12^o apart was strongly affecting the sky background, here estimated in μR ~ 16.8 mag/arcsec². From the image, we can however rule out at an S/N ≥ 3 confidence level any activity signature around the asteroid, brighter than 21.9 mag/arcsec².

Gault’s colours variations over time.

Gault’s observing sessions of 2019 March 23–27 from OAVdA (the mean uncertainty for the three sessions are, respectively, 0.020, 0.022, and 0.023 mag) and 2019 April 15 from Loiano (mean uncertainty 0.09 mag), are summarized in the upper and lower panels, respectively. The R_c magnitude scale is reproduced, throughout, from the local CMC-15 calibration, according to equation (4). Along the OAVdA observations, the asteroid was about its Earth opposition, at orbital phase angle α ~12.9^o, a figure that increased to α ~21.4^o for the Loiano data. The B, V magnitude sampling from Batch #1–3 observations is marked on the plot. Note a substantial difference in light-curve amplitude and shape between the two observing sessions. See text for a discussion.

Gault’s observing sessions of 2019 March 26 and 30 from Sanchez et al. (2019) are summarized in the upper and lower panels, respectively.

The Lomb-Scargle periodogram of spectral power versus period (between 0.5 and 10 hours) for OAVdA’s sessions and Sanchez 30 March. The best period is 3.34±0.02 h.

The phased lightcurves of the March 23, 26, 27 OAVdA and March 30 Sanchez according to the Falc algorithm implemented in MPO Canopus. The best period is 3.36 h very near to the 3.34±0.02 h of Fig. 7.

Like Fig. 8, according to the ~2× best period values. This phased lightcurve is compatible with a contact binary system with equal components (Descamps 2008).

The expected P vs. ρ relationship for a close binary system with the asteroid consisting of two components of similar size and mass, according to eq. (7). The component distance is parameterized in terms of multiple "n" of the body’s reference radius, RG, as in eq. (6). Accordingly, a contact system is obtained for n = 2, while for n = 3 the two asteroid components are orbiting one RG apart. The nominal periodicity of case P =7 h is singled out in the plot, with an implied density for Gault of ρ~1.0 g/cm³, in case of a contact binary system.