Velocity of Dust Ejected from Interstellar Comet 2I/Borisov

The discovery of interstellar comet 2I/Borisov (C/2019 Q4) provides an opportunity to study a planetesimal formed beyond our solar system. de León et al. (2019) measured the spectrum of 2I/Borisov in the visible and near-IR, which appears to be dominated by continuum, suggesting it is the result of elastic scattering by dust particles. The spectrum is often characterized by either its color index CI or color slope S' (e.g., A'Hearn et al. 1984). de León et al. (2019) found S' ≈ (10 ± 1)% per 0.1 μm over the wavelength range λ = 0.55–0.9 μm. Zubko et al. (2019) analyzed this color slope and demonstrated that the 2I/Borisov coma cannot be dominated by particles having water–ice, Mg-rich silicate, and/or amorphous-carbon composition. Instead, it could be populated by particles made of either Mg–Fe silicates or organic refractory residue as processed in the diffuse interstellar medium.

On 2019 September 22, 01:37 UT, we obtained images of the comet 2I/Borisov using the 1.3 m telescope (TSP) at the Skalnaté Pleso Observatory (MPC code 056) using broadband Bessel filters (Bessell et al. 1998) V (λ_eff = 0.5477 μm and Δλ  = 0.0991 μm) and R (λ_eff = 0.6349 μm and Δλ  = 0.1066 μm). The comet was at heliocentric distance r_h = 2.64 au, at a geocentric distance of Δ = 3.22 au, and phase angle α = 163. From the logarithmic image of 2I/Borisov taken with the R filter that is shown at the top of Figure 1 we infer a stand-off distance of its dust particles. This is an important parameter corresponding to a projected distance from the nucleus to the location where dust particles ejected in the sunward direction come to a stop by the solar-radiation pressure (e.g., Zubko et al. 2015). At this point, their motion is turned back. We estimate the stand-off distance to be (10,340 ± 660) km.

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We integrate the radiation flux from the comet over the aperture radius of ~11,300 km (shown with a red ring in Figure 1) and find the apparent magnitude of comet 2I/Borisov to be V = 18.16 ± 0.04 and R = 17.69 ± 0.03. This suggests a color index (V − R) = 0.46 ± 0.07. We further compensate for the color index of solar radiation (V − R)_Sun = 0.383 ± 0.01 (Bessell et al. 1998) and obtain a true color index CI = 0.087 ± 0.08. We convert the true color index CI into the color slope S', using Equation (2) of Ivanova et al. (2017), and obtain S' = (9.2 ± 8.4)% per 0.1 μm. Using the approximation of the 2I/Borisov spectrum inferred by Zubko et al. (2019) from the data reported by de León et al. (2019), we compute the color slope for the V and R filters, and obtain S' ≈ 14.9% per 0.1 μm. Thus, both values of S' appear in agreement within the error bars. This suggests similar microphysical properties of 2I/Borisov dust during both epochs. As reasoned in Zubko et al. (2019), the 2I/Borisov coma could be populated by particles made of either Mg–Fe silicates or organic refractory residue processed as expected in the diffuse interstellar medium. Interestingly, the effect of the solar-radiation pressure on small particles made of these materials is similar, with β parameters equal to 1.0 ± 0.1 (Zubko et al. 2015).

Using a time-domain technique (e.g., Zubko et al. 2015; Luk'yanyk et al. 2019), we simulate the motion of dust particles with β = 1 ejected from the comet's nucleus. We search for an ejection velocity that may reproduce an extension of the coma in the sunward direction as seen from Earth (10,340 km). The ejection is assumed to be along a normal to the surface with the latitude of ejection points being ±30° or ±60° (Figure 1, bottom) as measured from the subsolar point. We find that the stand-off distance can be fit with ejection velocities of V₀ = 92 m s⁻¹ and 69 m s⁻¹, respectively. Taking into account the dependence of the expansion velocity of cometary gases on the heliocentric distance r_h (au), ${V}_{\mathrm{gas}}=850/\sqrt{{r}_{{\rm{h}}}}$ (m s⁻¹) (e.g., Budzien et al. 1994), we find that the dust particles have 13.2%–17.6% of the velocity of the expanding gas V_gas = 523 m s⁻¹. This efficiency of momentum transfer appears consistent with solar system comets. For instance, Zubko et al. (2015) found that dust particles of comet C/2012 S1 (ISON) had from 8.2% to 16.5% of the velocity of the expanding gas.