Spectral Characteristics of Interstellar Object 3I/ATLAS from SOAR Observations

TL;DR

This study presents spectroscopic observations of interstellar object 3I/ATLAS, revealing a red continuum and lack of gas emission at 4.4 AU, providing insights into its surface composition and activity mechanisms.

Contribution

First spectroscopic characterization of 3I/ATLAS, highlighting its surface properties and activity state at large heliocentric distance, with implications for interstellar object behavior.

Findings

Red continuum similar to trans-Neptunian objects

No detectable gas emission at 4.4 AU

Early dust activity without sublimation evidence

Abstract

Interstellar objects (ISOs) provide unique insights into the building blocks and conditions of extrasolar planetary systems. The newly discovered object, 3I/ATLAS (C/2025 N1), represents the third known ISO after 1I/'Oumuamua and 2I/Borisov. We present initial spectroscopic characterizations of 3I using observations from the Goodman High Throughput Spectrograph on the 4.1 m SOAR Telescope in Chile during the night of July 3rd. The reflectance spectrum of 3I, covering 3700-7000{\AA} reveals a red continuum, comparable to extreme trans-Neptunian objects, with a weak UV-optical turnover indicative of complex carbonaceous and irradiated organics. At the time of observation, when 3I was at a heliocentric distance of 4.4 AU, we detected no discernible gas emission from canonical cometary species (CN, C$_3$, C$_2$, CO$^+$, [OI]). This is in agreement with expectations from our thermal-evolution model, which indicates sublimation-driven activity should commence once 3I/ATLAS approaches smaller heliocentric distances. Nonetheless, the paradoxical situation of early onset coma without evidence of sublimation tracers calls for other dust-liberating mechanisms that ancient ISOs may be subjected to at large heliocentric distances.