A brief introduction of chemistry in the disks of protoplanetary nebulae

<p indent="0mm">Stars with an initial mass between approximately 0.8 and 8 solar masses enter the asymptotic giant branch (AGB) phase during the late stages of their evolution. The AGB phase lasts for about 0.1–20 million years, during which the star’s radius can expand to approximately 1 astronomical unit, and its luminosity can increase to thousands of times the current solar luminosity. The temperature in the outer layers becomes very low, leading to the formation of substantial dust that obscures the central star. The strong radiation pressure acting on the dust drives gas outward in the form of stellar winds. These ejected materials gradually form an expanding circumstellar envelope around the star. Subsequently, the winds continue to strip the star’s outer envelope, and once the remaining envelope’s mass is reduced to about 1% of the star’s total mass, the star evolves into the post-asymptotic giant branch (post-AGB) phase. This phase is relatively short, lasting for only a few thousand years. During this time, the temperature of the star gradually increases, and when the central star’s temperature reaches approximately <sc>30000 K,</sc> it begins to ionize the circumstellar envelope. Eventually, the envelope becomes fully ionized, transforming into a planetary nebula (PN) with a hot central core. Objects that evolve from the post-AGB phase to the PN phase are referred to as protoplanetary nebulae (PPNs). With advancements in observational techniques, high-resolution observations in recent decades have shown that during the PPN phase, most circumstellar envelopes undergo morphological changes and generally exhibit non-spherical symmetry. These structures range from small-scale high-density clumps to large-scale bipolar, spiral, or disk-like formations, suggesting that the radiation pressure driving the ejection of material from the stellar surface is not isotropic and involves more complex physical processes. These changes occur over very short timescales, and the high mass-loss rates at the end of the AGB phase increase the optical depth of the circumstellar envelope, obscuring the internal structure and making it more difficult to investigate the mechanisms behind the asymmetry. Currently, various hypotheses have been proposed to explain this asymmetry in the envelope, but the roles these mechanisms play in shaping the nebula remain highly debated. Disk-like structures are a typical form of envelope morphology during this transitional phase. Relatively well-defined evolutionary timescales and compositional components make them ideal laboratories for studying molecular chemical processes in the circumstellar envelopes of evolved stars. This paper reviews the formation and observations of circumstellar disks in PPNs, the molecules detected in typical circumstellar disks, and recent chemical modeling. Additionally, we compare the physical and chemical differences between PPN disks and protoplanetary disks. Our report highlights the presence of rich chemical processes within PPN disks and summarizes the unresolved issues in current studies of these disks, which warrant further observational and theoretical investigations.