Searching for corannulene with ALMA: observations of the Red Rectangle Nebula

ABSTRACT Polycyclic Aromatic Hydrocarbons (PAHs) are organic molecules responsible for the Aromatic Infrared Bands (AIBs), observed across a multitude of astrophysical environments. Despite their ubiquity, the precise formation mechanisms of PAHs remain unclear. One of the possible way for PAHs to form is in the outflows of evolved stars, such as HD 44179, which produces the Red Rectangle 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 (C$_{20}$H$_{10}$), a viable candidate for radio detection due to its large dipole moment of 2.07 D. We analysed high-resolution band 4 ALMA 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 $\times$ 10$^{-13}$) and we discuss the lack of detection in the context of our current understanding of PAH formation and destruction mechanisms. Additionally, we report tentative detection of signals at 139.612, 139.617, and 139.621 GHz, potentially originating from cyclopropenyledine (c-C$_3$H$_2$) and the 140 GHz H$_2$O maser.