Disrupted Asteroid P/2016 G1. II. Follow-up Observations from the Hubble Space Telescope

Asteroid P/2016 G1 (Panstarrs) was discovered by R. Weryk and R. J. Wainscoat on CCD images acquired on 2016 April 1 UT with the 1.8 m Pan-STARRS1 telescope (Weryk & Wainscoat 2016). Its Tisserand parameter with respect to Jupiter (Kresák 1982) can be calculated as T_J = 3.38, so that the object belongs dynamically to the main asteroid belt, yet showing cometary appearance. The first object of this type, and the best characterized so far, was discovered by Elst et al. (1996), currently designated as 133P/Elst–Pizarro. This object constitutes the target of a proposed European Space Agency Mission called Castalia (Snodgrass et al. 2017). This new class of objects in the solar system today comprises about twenty members. The proposed activation mechanisms for these objects are very diverse, ranging from sublimation-driven to rotational instabilities. Jewitt et al. (2015) give an excellent review of the different objects discovered so far, their orbital stability, and their activation mechanisms.

In a previous paper (Paper I, Moreno et al. 2016), disrupted asteroid P/2016 G1 was observed with instrumentation attached to the GTC, from 2016 late April to early June, and a Monte Carlo dust tail model of the ejecta was applied to obtain the dust physical properties. In this paper, we report follow-up observations of P/2016 G1 acquired with the HST, during two epochs (2016 late June and early July) and apply the same dust model to the images to assess the validity of the model parameters over a longer temporal baseline.

Observations of P/2016 G1 were performed using the Wide Field Camera 3 (WFC3) with the wide-band filter F350LP, which has an effective wavelength of 584.6 nm and a width of 475.8 nm. The object was observed on two epochs, 2016 June 28 and July 11. In the first observing run, five frames of 420 s exposure time each, were acquired. In the second, four images were taken of 580 s exposure time each. Table 1 shows the log of the observations, where the observation times refer to the starting UT time at each observing date. Table 1 displays the relevant geometric parameters, namely the geocentric (Δ) and heliocentric (R) distances, the phase angle (α), the position angle of the extended Sun-to-asteroid radius vector (PsAng), and the angle between the Earth and target orbital plane (PlAng). This latter parameter shows values close to the latest GTC observation on 2016 June 8, so that the appearance of the object is similar to the GTC image of that date (see Paper I, Figure 1, and compare with Figure 1).

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Our target was placed on the UVIS2 chip of WFC3, which provides an image scale of 004/px, giving pixel sizes of 46 km and 49 km at the asteroid on 2016 June 28 and July 11, respectively. At each epoch, our dithered images were median-combined, resulting in the rejection of background sources, cosmic rays, and bad pixels. As the object was so faint, the resulting combined images were binned 8× in order to increase the signal-to-noise ratio (S/N), which becomes ∼10 when averaged over the object. As no nucleus or other spatial reference exists, the alignment procedure of the images was difficult, so that some blurring cannot be ruled out.

For the F350LP filter, we used a flux calibration factor (the so-called PHOTFLAM parameter) of 5.297 × 10⁻²⁰ erg cm⁻² s⁻¹ Å⁻¹. The calibration formula, relating the $C={{DNs}}^{-1}$ values of the image to surface brightness S is given by $S=(\mathrm{PHOTFLAM}/{\rm{\Omega }})C$, where Ω = 3.76 × 10⁻¹⁴ sr is the solid angle subtented by a pixel. The surface brightness values are finally converted to solar disk intensity units ($i/{i}_{0}$), which are the output units of the Monte Carlo dust tail code. To do this, we need to convolve the intensity of the mean solar disk spectrum (Cox 2000) with the filter response, which gives i₀ = 2.23 × 10⁶ erg cm⁻² s⁻¹ Å⁻¹ sr⁻¹. We finally get $i/{i}_{0}=6.31\times {10}^{-13}\,S$.

The resulting combined images of the asteroid are shown in Figure 1. As one can see, there are no apparent condensations on the images that could be attributed to an asteroid fragment. The object presents a very diffuse structure, becoming much more diluted than in the images taken previously with the GTC (Paper I), although retaining the main features, the inverted C-shaped structure near the head with an inner darker region near the predicted nucleus position relative to the surrounding material, and a slight westward lobe, more clearly apparent in the June 28 image. The increasing diffuseness is clearly a consequence of the expected outward expansion of the disrupted material and the effect of radiation pressure. This will be tested in the modeling of the ejecta as described in the next section.

As a result of the disruption event, in Paper I we found no fragments larger than ∼50 m (assuming a geometric albedo of p_v = 0.15). In these observations, the limiting magnitude (considering an S/N = 3) of the HST combined images obtained with the F350LP in both observing dates would be $V\sim 27.2$ for a G2V star according to the HST exposure time calculator (http://etc.stsci.edu/etc/input/wfc3uvis/imaging/). We compute the absolute magnitudes as ${H}_{v}=V-5\mathrm{log}(R{\rm{\Delta }})-\beta \alpha$ where β = 0.03 mag deg⁻¹ is the adopted linear phase coefficient. For the geometric conditions of the observations (see Table 1), the absolute magnitudes become H_v = 23.7 and H_v = 23.5 on June 28 and July 11, respectively. Adopting the empirical equation $D=\tfrac{1329}{\sqrt{{p}_{v}}}{10}^{-{H}_{v}/5}$ (Harris & Lagerros 2002), where D is the fragment diameter, those absolute magnitudes can be translated into fragment radii of 31 m and 34 m, respectively. Hence, no fragments larger than ∼30 m in radius would remain from the asteroid disruption that produced the observed activity. However, because the limiting magnitude refers to a dark background and not to a source located within a faint coma, this is actually an optimistic size limit.

The Monte Carlo dust tail model is described in Paper I, and will not be repeated here. The input parameters of the model were the dust-loss rate as a function of time, and the particle velocities. A half-Gaussian function was adopted for the dust-loss rate, which is defined by a peak activity ${\dot{M}}_{0}$, located at the event onset t₀, and having a half-width at half-maximum (denoted by HWHM), which is a measure of the effective time span of the event. For the particle velocities, we adopted a random function of the form $v={v}_{1}+\zeta {v}_{2}$, where ζ is a random number in the [0, 1] interval, and v₁ and v₂ were the fitting parameters. The remaining dust parameters were set to the following values: the particles were assumed to be distributed in a broad range of sizes from r = 1 μm to r = 1 cm and following a power-law differential size distribution with index κ = −3. The particles had a density of ${\rho }_{p}=1000$ kg m⁻³ and a geometric albedo of p_v = 0.15. The phase function correction was performed using a linear phase coefficient of β = 0.03 mag deg⁻¹, as given above, which is in the range of comet dust particles in the $1^\circ \leqslant \ \alpha \leqslant \ 30^\circ$ phase angle domain (Meech & Jewitt 1987). The particle ejection was assumed isotropic, except at the beginning of the event, where it was set to occur along a privileged direction for a short time interval. This ad-hoc assumption was made in order to explain the westward extension on the head, very clearly seen in the GTC images (see Paper I, Figure 4) but very diffuse on these HST images. This direction was found to be given by ${u}_{r}\sim \ 0.98$, ${u}_{\theta }\sim \ 0.18$, ${u}_{z}\sim \ 0.08$, where $({u}_{r},{u}_{\theta },{u}_{z})$ are unit vectors defining a cometocentric reference system, with u_r pointing away from the Sun, u_θ is perpendicular to u_r in the orbital plane and opposite to the comet motion, and u_z is perpendicular to the orbital plane.

The application of those input model parameters to the HST images resulted in a good agreement, although the fits are still improved by increasing the peak dust-loss rate from 7.6 to 10.5 kg s⁻¹. Figure 2 displays the measured and modeled contours, showing excellent agreement. This demonstrates that the model parameters derived from the previous GTC images are compatible with the follow-up HST images, thereby providing stronger confidence on the validity of the results.

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From the combined GTC and follow-up HST observations and the application of the dust tail modeling to the disrupted asteroid P/2016 G1, the following conclusions can be drawn.

(1) Asteroid P/2016 G1 was activated ${350}_{-30}^{+10}$ days before perihelion, i.e., around 2016 February 10. The activity had a duration of ${24}_{-7}^{+10}$ days (HWHM). The total dust mass emitted was at least ∼2 × 10⁷ kg, with a maximum level of activity of ∼8–11 kg s⁻¹. The dust-loss mass rate parameters were estimated assuming a power-law size distribution of particles between 1 μm and 1 cm, with power index of κ = −3.0, geometric albedo of 0.15, and being emitted isotropically.

(2) The isotropic ejection model is able to reproduce approximately the observed tail evolution of the disrupted target. An impulsive, short-duration ejection event in a privileged direction pointing approximately away from the Sun has been invoked to produce a dust feature near the head of the asteroid in the west direction. We hypothesize that this could be attributed to an impact that triggered the disruption of the asteroid.

(3) The ejection velocities inferred from the dust model are very small, being in the range from 0.015 to 0.14 m s⁻¹, with an average value of ∼0.08 m s⁻¹, corresponding to the escape velocity of an object of 35 m radius and 3000 kg m⁻³ density. On the other hand, from the HST observations, we find that the upper limit to the fragment sizes in the image is ∼30 m. These results are consistent and explain why we were not able to find any sizable fragment.

We are indebted to the anonymous reviewer for his/her useful comments that allowed us to improve our paper.

This paper is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program #14524.

This work was supported by contracts AYA2015-67152-R and AYA2015-71975-REDT from the Spanish Ministerio de Economía y Competitividad (MINECO, Spain). J.L. gratefully acknowledges support from contract AYA2015-67772-R (MINECO, Spain). N.P.-A. acknowledges support for Program number HST-GO-14524 provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. F.J.P. is supported by Marie Curie CO-FUND fellowship, co-founded by the University of Liège and the European Union.