Origin of the 3.4 micrometer hydrocarbon feature in IC 348 objects

Determine the origin of the unknown hydrocarbon absorption feature near 3.4 micrometers in the spectra of the IC 348 star-forming-region objects, which contaminates the F335M bandpass and complicates the identification of methane absorption.

Background

The IC 348 objects are extremely young, low-gravity substellar objects whose spectra overlap the temperature range of transiting exoplanets and brown dwarfs. Their F335M photometry is affected by an absorption feature near 3.4 micrometers that was identified as an unknown hydrocarbon feature in prior observations.

Because the feature overlaps the methane-sensitive F335M bandpass, it can artificially increase the F335M−F410M color and mimic methane absorption. The paper masks the affected wavelength interval and interpolates across it, but this removes the contaminant empirically rather than identifying its physical carrier. Additional observations are therefore needed to establish whether the feature is common and to determine whether delayed methane onset is universal among low-gravity objects.

References

This feature, whose origin remains unknown, can artificially enhances the apparent F335M$-$F410M color and could be mistaken for CH$_4$ absorption (Figure \ref{spectra}).

— JWST color sequence of transiting exoplanets and directly imaged substellar objects  (2608.25993 - Fu et al., 26 Aug 2026) in Section 3.4, subsection “IC 348 star forming region objects”