摘要：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
12C species and
their isotopologs with one 12C atom substituted by 13C; the latter were detected for
the first time unambiguously in the case of ethanol. The 12C/13C ratio is ~25 for both molecules. In addition, we
identified CH318 OH with a 16O/18O ratio of ~180 and a 13CH3OH/CH318
OH ratio of ~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 ≳21 and ≳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 ≳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.
