Millimeter Wave Studies of Biogenic Elements in the Interstellar Medium

Abstract

The unique conditions within the circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars facilitate the formation of radical species and other complex molecules. While some of these molecules are destroyed within the envelope, their remnants contribute to the broader molecular lifecycle. Molecules containing the NCHOPS elements, those essential for life, are of particular interest in astrochemistry. This work presents millimeter-wave observations of CSEs and molecular clouds conducted using the Arizona Radio Observatory (ARO) 12-m and Submillimeter Telescope (SMT) to identify new interstellar molecules containing NCHOPS and other astrochemically relevant elements, as well as to examine elemental abundance gradients. Phosphorus monoxide (PO) was identified for the first time in the star-forming region Orion-KL through analysis of a 3 mm spectral line survey obtained with the ARO 12-m. By examining the line profile and LSR velocity of PO, the molecule was localized to the Plateau region of Orion-KL, providing additional evidence that shock chemistry plays a role in the formation of phosphorus molecules. Additionally, 3, 2, and 1 mm spectral line surveys were conducted towards the carbon-rich envelope of the AGB star IRC+10216 using both the 12-m and SMT telescopes. These sensitive surveys allowed for the detection of very weak rotational lines, leading to the identification of two new interstellar molecules: SiP, a phosphorus-bearing radical, and FeC, the first metal carbide (in the chemist’s sense) detected in the interstellar medium. Both molecules were found in shell distributions within the envelope of IRC+10216, with formation mechanisms likely tied to shock chemistry. To explore the molecular content of the Outer Galaxy, observations were conducted towards Galactic Edge Clouds using the ARO 12-m and IRAM 30-m telescopes. PO and PN were identified in the Edge Cloud WB89-621, located 22.6 kiloparsecs (kpc) from the Galactic Center. This discovery represents the furthest detection of phosphorus in the Galaxy and confirms the presence of all NCHOPS elements in the Outer Galaxy. Furthermore, c-C3H2 was identified in 20 Edge Clouds with galactocentric distances ranging from 12.9 to 23.5 kpc, and nitric oxide (NO) was identified in 16 of these sources. The abundances of c-C3H2 and NO appear to remain constant with respect to galactocentric distances, suggesting a more widespread distribution of these molecules than previously believed and lending further support to the extension of the Galactic Habitable Zone into the Outer Galaxy.