Context. One of the biggest unsolved mysteries of modern astrochemistry is understanding chemical formation pathways in the interstellar medium (ISM) and circumstellar environments. 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 (CH3CSNH2) is the sulfur analog to acetemide (CH3CONH2) 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 CH3CONH2. 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. Aims. Our aim is to expand the frequency range of the measured rotational spectrum of CH3CSNH2 beyond 150 GHz 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 CH3CSNH2 may be detected, leading to possible constraints on the formation of large S-bearing molecules found in the ISM. Methods. The rotational spectrum of CH3CSNH2 was investigated up to 650 GHz. Using the newly refined spectrum of CH3CSNH2, as well as additional spectroscopic data on the chemically related species CH3CONH2, a variety of astronomical sources were searched including data from the following large surveys: Prebiotic Interstellar Molecule Survey conducted with the Green Bank Telescope; Exploring molecular complexity with ALMA conducted with the Atacama Large Millimeter/submillimeter Array; and Astrochemical Surveys at IRAM conducted with the Institut de Radioastronomie Millimétrique 30 m Telescope. Results. A total of 1428 transitions from the vt = 0 state with maximum values J = 47 and Ka = 20 in the range up to 330 GHz, and J = 95 and Ka = 20 in the range from 400–660 GHz were assigned. We also assigned 321 transitions from the vt = 1 state with the maximum values J = 35 and Ka = 9 up to 330 GHz. We achieved a final fit with a root-mean-square deviation of 43.4 kHz that contains 2035 measured lines from our study and the literature for vt = 0 and vt = 1 states of A and E symmetries. The final fit is based on the rho-axis-method Hamiltonian model that includes 40 parameters. An astronomical search for CH3CSNH2 was conducted based on all the new spectroscopic data. No transitions of CH3CSNH2 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 CH3CSNH2 toward each source.
The detection of many complex organic molecules (COMs) in interstellar space has sparked the study of their origins. While the formation of COMs detected in hot cores is attributed to photochemistry on warming grain surfaces followed by recombination of radicals and desorption, the formation routes in colder regions are still a debated issue with a number of theories such as cosmic ray bombardment on interstellar ice mantles or nondiffusive surface chemistry. Here we present another method with reactions involving metastable atomic oxygen in the O(1D) state, which is initially produced by photodissociation of oxygen-containing species in interstellar ices. As a first example, we study the reactions of metastable oxygen atoms and methane in ices to form both formaldehyde and methanol. The reaction is studied incorporating two different surface processes: diffusive and nondiffusive chemistry. The formation of methanol and formaldehyde via metastable oxygen atoms is compared with well-known formation routes of both to understand the O(1D) contributions at different temperatures.
The formation of molecules in the interstellar medium (ISM) remains a complex and unresolved question in astrochemistry. A group of molecules of particular interest involves the linkage between a carboxyl and amine group, similar to that of a peptide bond. The detection of molecules containing these peptide-like bonds in the ISM can help elucidate possible formation mechanisms, as well as indicate the level of molecular complexity available within certain regions of the ISM. Two of the simplest molecules containing a peptide-like bond, formamide (NH2CHO) and acetamide (CH3CONH2), have previously been detected toward the star-forming region Sagittarius B2 (Sgr B2). Recently, the interstellar detection of propionamide (C2H5CONH2) was reported toward Sgr B2(N) with Atacama Large Millimeter/submillimeter Array (ALMA) observations at millimeter wavelengths. Yet, this detection has been questioned by others from the same set of ALMA observations as no statistically significant line emission was identified from any uncontaminated transitions. Using the Prebiotic Interstellar Molecule Survey (PRIMOS) observations, we report an additional search for C2H5CONH2 at centimeter wavelengths conducted with the Green Bank Telescope. No spectral signatures of C2H5CONH2 were detected. An upper limit for C2H5CONH2 at centimeter wavelengths was determined to be N T < 1.8 × 1014 cm−2 and an upper limit to the C2H5CONH2/CH3CONH2 ratio is found to be <2.34. This work again questions the initial detection of C2H5CONH2 and indicates that more complex peptide-like structures may have difficulty forming in the ISM or are below the detection limits of current astronomical facilities. Additional structurally related species are provided to aid in future laboratory and astronomical searches.
In the interstellar medium (ISM), the formation of complex organic molecules (COMs) is largely facilitated by surface reactions. However, in cold dark clouds, thermal desorption of COMs is inefficient because of the lack of thermal energy to overcome binding energies to the grain surface. Non-thermal desorption methods are therefore important explanations for the gas-phase detection of many COMs that are primarily formed on grains. Here we present a new non-thermal desorption process: cosmic ray sputtering of grain ice surfaces based on water, carbon dioxide, and a simple mixed ice. Our model applies estimated rates of sputtering to the 3-phase rate equation model Nautilus-1.1, where this inclusion results in enhanced gas phase abundances for molecules produced by grain reactions such as methanol (CH3OH) and methyl formate (HCOOCH3). Notably, species with efficient gas phase destruction pathways exhibit less of an increase in models with sputtering compared to other molecules. These model results suggest that sputtering is an efficient, non-specific method of non-thermal desorption that should be considered as an important factor in future chemical models.
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