JCMT observations of CO, SO₂, and a U-line in circumstellar envelopes of four O-rich AGB stars

The circumstellar envelopes (CSE) of asymptotic giant branch (AGB) stars are abundant in molecular emissions, offering valuable insights into the physical and chemical conditions of these evolving stars. In this paper, we report observations of two molecules (CO and SO₂) toward four O-rich AGB stars using the James Clerk Maxwell Telescope (JCMT). We detected an unusual SO₂ spectral feature comprising both broad and narrow components in IK Tau and AP Lyn. The broad line profiles may originate from thermal molecular emission, while the narrow profiles could come from other species (or masers) or astrophysical phenomena occurring within the CSEs of the AGB stars, such as episodic mass loss, bipolar outflows, or emissions associated with the complex physical processes near the central star. The narrow lines of SO₂ may also arise from vibrationally excited emissions. Additionally, we observed the same U-line in both TX Cam and IK Tau, which may originate from the molecule N¹⁷O. We analyzed the identified molecular lines using rotational diagrams to determine their excitation temperatures, column densities, and fractional abundances. This information aids in the constructing of reliable astrochemical models for a more detailed examination of the target stars. The narrow component of the SO₂ line suggests unusual astrophysical phenomena, making IK Tau and AP Lyn particularly intriguing for further investigation to fully understand the physical processes at play in these sources.