Physical and Physicochemical Analysis of Comet 3I/ATLAS (C/2025 N1)

 

Physical and Physicochemical Analysis of Comet 3I/ATLAS (C/2025 N1)
J.P.Navarro Pina
Observatorio Astronomico Vega del Thader MPC J70/IAU

This work presents a photometric-based physical characterization of the interstellar comet 3I/ATLAS (C/2025 N1), using CCD and visual light-curve data obtained between July and December 2025. The analysis derives dust and gas production rates, active surface area, and constraints on the nucleus size.

1. Observational Data

The dataset consists of CCD (green crosses) and visual (white crosses) magnitudes compiled by COBS, covering heliocentric distances from ~3.8 au inbound to ~1.4 au near perihelion. A simple photometric fit yields H0 = 8.0 and activity index n = 8.0.

2. Photometric Model

The total apparent magnitude is modeled using the standard cometary law:

m = H0 + 5 log10(Δ) + 2.5 n log10(r)

where r is the heliocentric distance in astronomical units and Δ is the geocentric distance.

3. Dust Production (Afρ)

From the reduced magnitude m(1,1,0) ≈ 8.0, the dust proxy Afρ is estimated to be:

Afρ ≈ 250–350 cm

with a representative mean value of ~300 cm near r = 2–3 au, consistent with published CCD photometry.

4. Gas Production Rate

The water production rate is estimated using the empirical relation (Jorda et al. 2008):

log Q(H2O) = 30.675 − 0.245 m(1,1,0)

yielding:

Q(H2O) ≈ 5 × 10^28 molecules s−1

This value represents an upper effective limit; UV measurements suggest actual production rates of ~10^27–10^28 molecules s−1 due to extended sources in the coma.

5. Active Area and Nucleus Size

Assuming a sublimation rate of Z(H2O) ≈ 10^20 molecules m−2 s−1 at 2–3 au, the active surface area is:

A_act ≈ 15–20 km^2

Assuming an active fraction f ≈ 0.2, the equivalent nucleus diameter is:

D ≈ 4–5.5 km

in agreement with observational upper limits from HST.

6. Conclusions

Comet 3I/ATLAS is characterized as a highly active, volatile-rich object with a steep heliocentric brightness dependence (n ≈ 8). The dominance of gas over dust production and the inferred nucleus size support its classification as a dynamically new, possibly interstellar comet.