Location of carbon dioxide ice in NGC
6302. The image shows HST/WFC3 observations featuring filter F656N, which
traces hydrogen-alpha emission. The JWST MIRI mosaic is indicated by the white
frame. Contours show the column density of gas-phase carbon dioxide, with
corresponding log N values (cm−2) provided in the lower left. Credit: arXiv (2026). DOI: 10.48550/arxiv.2602.22366
An international team of
astronomers has employed the James Webb Space Telescope (JWST) to observe a
complex planetary nebula known as NGC 6302. The observations, detailed in a
paper published Feb. 25 on the arXiv pre-print server, resulted in
the discovery of dry (carbon dioxide) ice in this nebula. This is the first
time dry ice has been detected in a planetary nebula.
Planetary nebulae (PNe) are
expanding shells of gas and dust that have been ejected from a star during the
process of its evolution from a main sequence star into a red giant or white
dwarf. They are relatively rare, but are important for astronomers investigating
the composition of the interstellar medium (ISM).
Complex chemistry of the Butterfly
NGC 6302, dubbed the Butterfly
Nebula or the Bug Nebula, is a bipolar type PN located some 3,400 light years
away in the constellation Scorpius. The nebula has a radius of at least 1.5
light years and exhibits bright east-west oriented bipolar lobes bisected by a
massive dusty torus.
Previous observations of NGC 6302
have detected the presence of methyl cation (CH3+) in this nebula, which is a key
driver of organic chemistry. Moreover, some studies found a widespread presence
of polycyclic aromatic hydrocarbons (PAHs) in NGC 6302.
These two findings suggest that the
environment of NGC 6302 supports rich chemical processes, and therefore makes
it a particularly intriguing laboratory for exploring some of the complex
chemical pathways in PNe.
That is why a group of astronomers
led by Charmi Bhatt of the University of Western Ontario, Canada, decided to
further investigate the chemical composition of NGC 6302. For this purpose,
they used JWST's Mid-Infrared Instrument (MIRI).
"This work utilizes JWST
MIRI/MRS observations of NGC 6302 covering the central star, torus, and
innermost region of the bipolar lobes," the paper states.
Icy planetary nebula
Observations conducted with the
MIRI medium-resolution spectrometer (MRS) revealed clear absorption features in
the 14.8–15.2 µm range corresponding to gas-phase carbon dioxide. Further
investigations unveiled the two key signatures of dry ice in the dusty torus of
NGC 6302: a shallow, broad absorption between 14.9–15.15 µm, and a second
absorption between 15.2–15.3 µm.
The astronomers underline that the
detection of carbon dioxide ice in NGC 6302 represents the first identification
of an ice species more volatile than water in any planetary nebula. They note
that although molecular ices are abundant in cold, shielded environments,
including dense molecular clouds, envelopes of young stellar objects (YSOs) and
protoplanetary disks, the environments of PNe are generally hostile to fragile
molecular species and ices due to intense ultraviolet irradiation. This makes
their detection unique.
According to the paper, the
gas-to-ice ratio in NGC 6302 differs markedly from that observed in YSOs. This
indicates a distinct ice formation or processing mechanism in evolved stellar
environments.
Summing up the results, the authors of the study underscore the need for high spatial resolution observations of PNe that would constrain their chemical pathways, temperature structure, and ice processing mechanisms. This will be essential to establish whether ice chemistry is common in dense PN tori.
Source: Dry ice detected in a planetary nebula for the first time
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