Astronomers used the James Webb Space Telescope (JWST) to observe dry ice inside a complex planetary nebula known as NGC 6302.
This is the first time that dry ice has been observed inside a planetary nebula.
A planetary nebula (PNe) is an expanded shell of gas and dust ejected from a star during its evolution from a main sequence star to a red giant or white dwarf.
Although they are relatively rare, they are important to astronomers studying the composition of the interstellar medium (ISM).
The complex chemistry of rare planetary nebulae
NGC 6302, also known as the Butterfly Nebula or Bug Nebula, is a bipolar PN located approximately 3,400 light-years away in the constellation Scorpius.
The nebula has a radius of at least 1.5 light-years and has bright east-west bipolar lobes bisected by a huge dusty torus.
Previous observations of NGC 6302 detected methyl cations (CH3+), an important driving force in organic chemistry, within the nebula.
Additionally, several studies have found extensive polycyclic aromatic hydrocarbon (PAH) emissions in NGC 6302.
These two findings suggest that the environment of NGC 6302 supports rich chemical processes and, therefore, NGC 6302 is a particularly interesting laboratory for exploring complex chemical pathways in PNe.
“This study utilizes JWST MIRI/MRS observations of NGC 6302 covering the innermost region of the central star, torus, and bipolar lobe,” the authors said.
A unique discovery of dry ice
Observations made with the MIRI Medium Resolution Spectrometer (MRS) revealed distinct absorption features in the 14.8–15.2 μm range, corresponding to gas-phase carbon dioxide.
Further investigation revealed two important features of dry ice in NGC 6302’s dusty torus. a shallow and broad absorption band from 14.9 to 15.15 μm and a second absorption band from 15.2 to 15.3 μm.
Astronomers stress that the detection of carbon dioxide ice in NGC 6302 marks the first time an ice species more volatile than water has been identified in any planetary nebula.
They point out that while molecular ice is abundant in cold, shielded environments such as dense molecular clouds, the envelopes of young stellar objects (YSOs), and protoplanetary disks, PNe environments are generally harsh for fragile molecular species and ice due to intense ultraviolet radiation.
This makes the detection unique.
Higher quality observations of planetary nebulae are needed
According to the paper, the gas-to-ice ratio in NGC 6302 is significantly different from that observed in YSO.
This indicates a distinct ice formation or processing mechanism in an evolved stellar environment.
Summarizing their findings, the study authors highlight the need for high spatial resolution observations of PNe to constrain its chemical pathways, thermal structure, and ice processing mechanisms.
This is essential to establish whether ice chemistry is common in dense PN tori.
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