Probing Stellar Chemistry in the Envelope of Evolved Stars: An Experimental and Observational Approach

Abstract

Pure rotational spectroscopy is one of the best tools for studying molecules in the gas phase, as itprovides high-resolution spectral data, enables the calculation of structural parameters, and aids in understanding weak interactions resulting from angular momenta couplings. This technique is particularly valuable for investigating open-shell molecules, which are commonly found in interstellar regions characterized by non-local thermodynamic equilibrium (non-LTE) conditions. The high-resolution data obtained from rotational spectroscopy serves as a powerful means of detecting such molecules in the interstellar medium using ground-based telescopes. Direct absorption rotational spectroscopy was employed to examine the ground electronic state of the radicals SiP (X 2Πi), FeH (X 4Δi), and FeD (X 4Δi). These highly unstable radicals were synthesized in situ within a single-pass glass cell coupled with RF frequency AC discharge. The acquired measurements were analyzed using an effective Hamiltonian incorporating rotational, fine, and hyperfine terms. The rotational transitions of SiP were also instrumental in its initial astronomical detection in the envelope of the carbon-rich star IRC+10216, using the Arizona Radio Observatory's Sub-Millimeter Telescope (SMT). Rotational transitions of molecules also provide crucial information about the evolution of evolved stars. During their late stages, these stars shed mass from their atmospheres, resulting in the formation of massive envelopes of gas and dust known as circumstellar envelopes. The thermodynamic conditions within these envelopes facilitate the creation of a rich chemical inventory that carries essential information about stellar nucleosynthesis and the history of mass loss. Ground-based telescopes such as ALMA and ARO's SMT can be utilized to detect these gases in the stellar envelopes, particularly in the millimeter region, which experiences less obstruction from atmospheric water vapor. Observations of hypergiant stars NML Cyg and VY CMa were conducted in the 1 mm region using SMT and ALMA. Radiative transfer modeling was employed, utilizing the emissions of various molecules such as CO, SO, SO2, SiO, HCN, NaCl, and AlOH, to calculate their abundances and spatial distributions within the envelope of NML Cyg. Similar calculations were carried out for VY CMa using emissions of CO and HCN obtained from ALMA. Furthermore, interferometric data from ALMA were utilized to generate a molecular map of VY CMa's envelope with a high spatial resolution (0.2 arcseconds). The absence of obscuration at millimeter wavelengths allowed for the detection of the most extensive image of VY CMa's envelope, with a radius of 12 arcseconds.