Dust Activity Evolution and Constraints on Icy Grains in the Interstellar Comet 3I/ATLAS

Abstract

We present Zwicky Transient Facility observations of the interstellar comet 3I/ATLAS to characterize its dust-activity evolution before and after perihelion. Before perihelion, the effective scattering cross section $C_{\mathrm{e}}$ brightened across the inner $2\times 10^4$ km as $C_{\mathrm{e}}\propto r_{\mathrm{H}}^{-\gamma }$, with $\gamma$ increasing from $1.63\pm 0.14$ to $1.88\pm 0.34$ with aperture size. After perihelion, the heliocentric index decreased from $2.44\pm 0.09$ in the $5\times 10^3$ km aperture to $1.54\pm 0.09$ in the $8\times 10^4$ km aperture, indicating faster fading of the inner coma. Under adopted grain-size and residence-time assumptions, the implied dust mass-loss rates are $\sim 123$--$203\mathrm{kg}{\mathrm{s}}^{-1}$ before perihelion and $\sim 596$--$1412\mathrm{kg}{\mathrm{s}}^{-1}$ after perihelion. The morphology yields a sunward ejection-speed scale of $v_{\mathrm{ej}}\sim (190\pm 3)a_{\mu \mathrm{m}}^{-1/2}\mathrm{m}{\mathrm{s}}^{-1}$ and an effective perpendicular velocity of $v_{\perp }=(79\pm 6)a_{\mu \mathrm{m}}^{-1/2}\mathrm{m}{\mathrm{s}}^{-1}$. Post-perihelion surface-brightness profiles transiently steepened to $k\sim -2.7$ in late November and early December, then became shallower and approached $k=-1.5$ by January. Matched $g-r$ profiles show that this phase coincided with an anomalously blue outer coma that disappeared by January. We characterize the fading component with a projected destruction length of ${\ell }_{\mathrm{DE}}\sim (6.4$--$7.2)\times 10^4$ km. Thermophysical sublimation calculations indicate that this scale, together with the order of magnitude of the water production near the main imaging epoch, is consistent with $a_0\sim 5$--$10\mu \mathrm{m}$ icy grains containing a small absorbing carbonaceous fraction of $f_{\mathrm{C}}\sim 0.1\%$. These results suggest that the early post-perihelion coma likely contained a short-lived, volatile-bearing icy-grain component whose fading contributed to the outer-coma brightness and color evolution

19 pages. Published in The Astrophysical Journal Letters

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