Searching for corannulene with ALMA observations of the Red Rectangle Nebula

Polycyclic Aromatic Hydrocarbons (PAHs) are organic molecules responsible for the Aromatic Infrared Bands (AIBs), observedacross a multitude of astrophysical environments. Despite their ubiquity, the precise formation mechanisms of PAHs remainunclear. One of the possible way for PAHs to form is in the outflows of evolved stars, such as HD 44179, which produces the RedRectangle nebula – a known emitter of AIBs. However, no specific PAH molecules have been detected in such environments,complicating the understanding of PAH formation and evolution. This study aimed to detect the PAH molecule corannulene(C₂₀ H₁₀ ), a viable candidate for radio detection due to its large dipole moment of 2.07 D. We analysed high-resolution band 4ALMA observations of the Red Rectangle nebula, collected over almost 9 h. Although corannulene emission was not detected,we estimated a firm upper limit on its abundance compared to hydrogen (5 × 10⁻¹³ ) and we discuss the lack of detection in thecontext of our current understanding of PAH formation and destruction mechanisms. Additionally, we report tentative detectionof signals at 139.612, 139.617, and 139.621 GHz, potentially originating from cyclopropenyledine (c-C₃ H₂ ) and the 140 GHzH₂ O maser.