Exploring molecular complexity with ALMA (EMoCA): Alkanethiols and alkanols in Sagittarius B2(N2)

Context. Over the past five decades, radio astronomy has shown that molecular complexity is a natural outcome of interstellar chemistry, in particular in star forming regions. However, the pathways that lead to the formation of complex molecules are not completely understood and the depth of chemical complexity has not been entirely revealed. In addition, the sulfur chemistry in the dense interstellar medium is not well understood. Aims. We want to know the relative abundances of alkanethiols and alkanols in the Galactic center source Sagittarius B2(N2), the northern hot molecular core in Sgr B2(N), whose relatively small line widths are favorable for studying the molecular complexity in space. Methods. We investigated spectroscopic parameter sets that were able to reproduce published laboratory rotational spectra of ethanethiol and studied effects that modify intensities in the predicted rotational spectrum of ethanol. We used the Atacama Large Millimeter Array (ALMA) in its Cycles 0 and 1 for a spectral line survey of Sagittarius B2(N) between 84 and 114.4 GHz. These data were analyzed by assuming local thermodynamic equilibrium (LTE) for each molecule. Our observations are supplemented by astrochemical modeling; a new network is used that includes reaction pathways for alkanethiols for the first time. Results. We detected methanol and ethanol in their parent C-12 species and their isotopologs with one C-12 atom substituted by C-13; the latter were detected for the first time unambiguously in the case of ethanol. The C-12/C-13 ratio is similar to 25 for both molecules. In addition, we identified (CH3OH)-O-18 with a O-16/O-18 ratio of similar to 180 and a (CH3OH)-C-13/(CH3OH)-O-18 ratio of similar to 7.3. Upper limits were derived for the next larger alkanols normal- and iso-propanol. We observed methanethiol, CH3SH, also known as methyl mercaptan, including torsionally excited transitions for the first time. We also identified transitions of ethanethiol (or ethyl mercaptan), though not enough to claim a secure detection in this source. The ratios CH3SH to C2H5SH and C2H5OH to C2H5SH are greater than or similar to 21 and greater than or similar to 125, respectively. In the process of our study, we noted severe discrepancies in the intensities of observed and predicted ethanol transitions and propose a change in the relative signs of the dipole moment components. In addition, we determined alternative sets of spectroscopic parameters for ethanethiol. The astrochemical models indicate that substantial quantities of both CH3SH and C2H5SH may be produced on the surfaces of dust grains, to be later released into the gas phase. The modeled ratio CH3SH/C2H5SH = 3.1 is lower than the observed value of greater than or similar to 21; the model value appears to be affected most by the underprediction of CH3SH relative to CH3OH and C2H5OH, as judged by a very high CH3OH/CH3SH ratio. Conclusions. The column density ratios involving methanol, ethanol, and methanethiol in Sgr B2(N2) are similar to values reported for Orion KL, but those involving ethanethiol are significantly different and suggest that the detection of ethanethiol reported toward Orion KL is uncertain. Our chemical model presently does not permit the prediction of sufficiently accurate column densities of alkanethiols or their ratios among alkanethiols and alkanols. Therefore, additional observational results are required to establish the level of C2H5SH in the dense and warm interstellar medium with certainty.