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.
