Searching for Corannulene with ALMA: Observations of the Red Rectangle Nebula

TL;DR

This work probes whether the bowl-shaped PAH corannulene (C20H10) resides in the Red Rectangle nebula by conducting high-sensitivity ALMA Band 4 observations and stacking six rotational lines. No corannulene emission is detected, enabling a robust upper limit on its abundance relative to hydrogen and its share of PAH carbon, while photodissociation analyses suggest corannulene would be relatively stable under the local radiation field. The study also reports tentative detections near 139.6 GHz that may arise from cyclopropenylidene (c-C3H2) and a 140 GHz water maser, refining the inventory of hydrocarbon chemistry in the circumbinary disk/PDR. Collectively, these results constrain PAH formation and destruction scenarios in post-AGB environments and motivate further multi-band observations and astrochemical modeling to understand PAH evolution around evolved stars.

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 C20H10, a viable candidate for radio detection due to its large dipole moment of 2.07D. We analyzed high-resolution band 4 ALMA observations of the Red Rectangle nebula, collected over almost 9 hrs. Although corannulene emission was not detected, we estimated a firm upper limit on its abundance compared to hydrogen (5x10^-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 GHz, 139.617 GHz, and 139.621 GHz, potentially originating from cyclopropenyledine c-C3H2 and the 140 GHz H2O maser.

Figures (8)

• Figure 1: The photodissociation rate for the loss of hydrogen directly from corannulene cation (in red) or via an isomerized state of corannulene (in orange). The infrared emission rate of the corannulene neutral is shown in black.
• Figure 2: Calculation of the internal energy distribution of the corannulene molecules in the Red Rectangle radiation field.
• Figure 3: The Red Rectangle as observed at a frequency of 139.617 GHz. The blue contour shows the $3\sigma$ and $5\sigma$ line emission region, while the green contour shows the $3\sigma$ continuum emission region. The blue ellipse in the bottom-left corner shows the image beam.
• Figure 4: The spectrum corresponding to the 139.617 GHz emission region, shown in figure \ref{['fig:unknown_emission_image']}. The $1\sigma$, $3\sigma$, and $5\sigma$ levels are shown, with the brightest emission peak reaching above $5\sigma$, while four other peaks span ranges of $\sim 3\sigma - 5\sigma$. The origin of two of these emission peaks remains unknown. The other the peaks could be a result of the 140 GHz water maser and cyclopropenyledine (c-C$_3$H$_2$), whose rest frequencies are indicated.
• Figure 5: The ALMA images at the rest frequencies of corannulene, where blue contours denote 3$\sigma$ regions and the green contour shows the 3$\sigma$ continuum emission region. The blue ellipse in the bottom-left corner shows the image beam. We also report the corresponding rms noise.