  Linking atmospheric characteristics of planets to their formation pathways is
a central theme in the study of extrasolar planets. Although the 12C/13C
isotope ratio shows little variation in the solar system, the atmosphere of a
super-Jupiter was recently shown to be rich in 13CO, possibly as a result of
dominant ice accretion beyond the CO snowline during its formation. In this
paper we aim to measure the 12CO/13CO isotopologue ratio of a young, isolated
brown dwarf. While the general atmospheric characteristics of young, low mass
brown dwarfs are expected to be very similar to those of super-Jupiters, their
formation pathways may be different, leading to distinct isotopologue ratios.
We analyse archival K-band spectra of the L dwarf 2MASS J03552337+1133437 taken
with NIRSPEC at the Keck telescope. A free retrieval analysis is applied to the
data to determine the isotopologue ratio 12CO/13CO in its atmosphere. The
isotopologue 13CO, is detected in the atmosphere through the cross-correlation
method at a signal-to-noise of ~8.4. The detection significance is determined
to be ~9.5 sigma using Bayesian model comparison between two retrieval models.
We retrieve an isotopologue ratio 12CO/13CO of 97(+25/-18), marginally higher
than the local interstellar standard. Its C/O ratio of ~0.56 is consistent with
the solar value. Although only one super-Jupiter and one brown dwarf have now a
measured 12CO/13CO ratio, it is intriguing that they are different, possibly
hinting to distinct formation pathways. Albeit spectroscopic similarities,
isolated brown dwarfs may experience a top-down formation via gravitational
collapse, which resembles star formation, while giant exoplanets favorably form
through core accretion, which potentially alters isotopologue ratios in their
atmospheres depending on the material they accrete from protoplanetary disks.
(abridged)

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics