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Interstellar Interloper 3I/ATLAS: Nucleus Size, Photometry in RGB, Af(rho) and Antitail Structure Analysis

Toni Scarmato

TL;DR

This study reconciles disparate nucleus-size constraints for the interstellar comet 3I/ATLAS by unifying high-resolution imaging, non-gravitational dynamics, and thermophysical models with new ground-based RGB observations of the dust anti-tail. A PSF–δ photometric decomposition on deep imaging constrains the unresolved nucleus flux, yielding a radius of about $R_n\approx0.25$ km, in line with dynamical inferences of $R_n\sim0.26$–$0.37$ km. Ground-based RGB imaging reveals a tightly collimated sunward anti-tail extending to $L_R\approx5\times10^4$ km (dust-dominated, strongest in $R$ and $G$ with gas emissions shaping the $G$ channel), while the $B$ channel shows a compact inner coma. A simple one-dimensional dust-dynamics model shows that the anti-tail extents require initial speeds of tens to ~100 m s$^{-1}$ for grains with plausible radiation-pressure parameters $\beta$, implying a dynamically extreme, high-latitude jet accelerating large grains. Overall, the nucleus is constrained to $0.16\lesssim R_n\lesssim2.8$ km, with a preferred sub-kilometre size, and the dust anti-tail morphology and colours provide new, independent constraints on dust production and dynamics in this first-known interstellar visitor.

Abstract

Interstellar comet 3I/ATLAS (C/2025 N1) exhibits an unusual, tightly collimated dust feature in the sunward hemisphere which has been widely described as an anti-tail. At the same time, precise constraints on the nucleus size have been derived from a combination of high-resolution imaging and non-gravitational dynamics. In this work I present a unified analysis that combines existing constraints on the nucleus radius with new ground-based RGB imaging of the dust anti-tail obtained with a 0.25 m telescope at the Toni Scarmato Astronomical Observatory (MPC L92).

Interstellar Interloper 3I/ATLAS: Nucleus Size, Photometry in RGB, Af(rho) and Antitail Structure Analysis

TL;DR

This study reconciles disparate nucleus-size constraints for the interstellar comet 3I/ATLAS by unifying high-resolution imaging, non-gravitational dynamics, and thermophysical models with new ground-based RGB observations of the dust anti-tail. A PSF–δ photometric decomposition on deep imaging constrains the unresolved nucleus flux, yielding a radius of about km, in line with dynamical inferences of – km. Ground-based RGB imaging reveals a tightly collimated sunward anti-tail extending to km (dust-dominated, strongest in and with gas emissions shaping the channel), while the channel shows a compact inner coma. A simple one-dimensional dust-dynamics model shows that the anti-tail extents require initial speeds of tens to ~100 m s for grains with plausible radiation-pressure parameters , implying a dynamically extreme, high-latitude jet accelerating large grains. Overall, the nucleus is constrained to km, with a preferred sub-kilometre size, and the dust anti-tail morphology and colours provide new, independent constraints on dust production and dynamics in this first-known interstellar visitor.

Abstract

Interstellar comet 3I/ATLAS (C/2025 N1) exhibits an unusual, tightly collimated dust feature in the sunward hemisphere which has been widely described as an anti-tail. At the same time, precise constraints on the nucleus size have been derived from a combination of high-resolution imaging and non-gravitational dynamics. In this work I present a unified analysis that combines existing constraints on the nucleus radius with new ground-based RGB imaging of the dust anti-tail obtained with a 0.25 m telescope at the Toni Scarmato Astronomical Observatory (MPC L92).
Paper Structure (23 sections, 43 equations, 1 figure, 1 table)

This paper contains 23 sections, 43 equations, 1 figure, 1 table.

Table of Contents

  1. Introduction
  2. Global constraints on the nucleus size
  3. Observational and dynamical methods
  4. PSF--$\delta$ decomposition and bicubic resampling
  5. Comparative summary
  6. Detailed photometric derivation of the 18 December nucleus radius
  7. Ground-based RGB imaging of the dust anti-tail
  8. Observations and data processing
  9. Morphology and colour dependence
  10. Coma colours and spectral slope from RGB photometry
  11. Dust production proxy from the $R$-band Af$\rho$
  12. Simple dynamical model for the dust anti-tail
  13. One-dimensional nucleus-centred framework
  14. Application to the observed extents
  15. Discussion
  16. ...and 8 more sections

Figures (1)

  • Figure 1: Multi-channel view of the inner coma of 3I/ATLAS from the 2025 December 15 dataset. Top row: orientation of the field with projected North and East and the two main axes drawn at PA $\simeq 110^\circ$ (green) and PA $\simeq 290$--$300^\circ$ (yellow; left), followed by the $R$, $G$, and $B$ channels obtained from the ASI294MC PRO colour camera. The axis at PA $\simeq 110^\circ$ corresponds to the dust anti-tail; along this direction the inner coma is elongated and collimated in the $R$ and $G$ channels, while the $B$ channel appears more compact and nearly symmetric. Bottom row: colour Larson--Sekanina rotational-gradient image and three monochromatic reconstructions centred near $\lambda\simeq 659$, 530, and 445 nm. A strong, tightly collimated anti-tail jet is visible at 659 nm and 530 nm, extending up to $\sim 7\times 10^4$ km from the nucleus towards PA $\simeq 110^\circ$, whereas no comparable extended jet is present in the 445 nm image. This behaviour indicates that the anti-tail is a predominantly dust-dominated structure, with gas emission contributing mainly to the compact blue inner coma.