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Description
One of the biggest unsolved mysteries of modern astrochemistry is understanding chemical formation pathways in the interstellar medium (ISM) and circumstellar environments (CSEs). The detections (or even nondetections) of molecules composed of low-abundance atomic species (such as S, P, Si, and Mg) may help to constrain chemical pathways. Thioacetamide (CH_3_CSNH_2_) is the sulfur analog to acetemide (CH_3_CONH_2_) and it is a viable candidate to search for in astronomical environments - specifically toward regions where other S-bearing molecules have been found and, if possible, that also contain a detection of CH_3_CONH_2_. If detected, it would not only continue to expand the view of molecular complexity in astronomical environments, but also help to better elucidate the possible formation pathways of these types of species in these environments. Our aim is to expand the frequency range of the measured rotational spectrum of CH_3_CSNH_2_ beyond 150GHz and then to use those measurements to extend the search for this species in the ISM. The new laboratory measurements and expanded search cover more parameter space for determining under what conditions CH_3_CSNH_2_ may be detected, leading to possible constraints on the formation of large S-bearing molecules found in the ISM. The rotational spectrum of CH_3_CSNH_2_ was investigated up to 650GHz. Using the newly refined spectrum of CH_3_CSNH_2_, as well as additional spectroscopic data on the chemically related species CH_3_CONH_2_, a variety of astronomical sources were searched including data from the following large surveys: Prebiotic Interstellar Molecule Survey (PRIMOS) conducted with the Green Bank Telescope (GBT); Exploring molecular complexity with ALMA (EMoCA) conducted with the Atacama Large Millimeter/submillimeter Array (ALMA); and Astrochemical Surveys At IRAM (ASAI) conducted with the Institut de Radioastronomie Millim'etrique (IRAM) 30m Telescope. A total of 1428 transitions from the v_t_=0 state with maximum values J=47 and K_a_=20 in the range up to 330GHz, and J=95 and K_a_=20 in the range from 400-660GHz were assigned. We also assigned 321 transitions from the v_t=1 state with the maximum values J=35 and K_a_=9 up to 330GHz. We achieved a final fit with a root-mean-square deviation of 43.4kHz that contains 2035 measured lines from our study and the literature for v_t_=0 and v_t_=1 states of A and E symmetries. The final fit is based on the rho-axis- method (RAM) Hamiltonian model that includes 40 parameters. An astronomical search for CH_3_CSNH_2_ was conducted based on all the new spectroscopic data. No transitions of CH_3_CSNH_2_ were detected toward any of the sources contained in our survey. Using the appropriate telescope and physical parameters for each astronomical source, upper limits to the column densities were found for CH_3_CSNH_2_ toward each source.
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