| Abstract
| - Abstract. We present the results of a multiline survey at millimetre wavelengths of the Cepheus A star-forming region. Four main flows have been identified: three pointing in the southwest, northeast and southeast directions and accelerating high-density CS clumps. The fourth outflow, revealed by high-sensitivity deuterated water (HDO) observations, is pointing towards the south and is associated with conditions particularly favourable to a chemical enrichment. At the CepA-East position the emissions due to the ambient clump and to the outflows coexist and different molecules exhibit different spectral behaviours. Some species (C13CH, C3H2, CH2CO, CH3C2H, HC18O+) exhibit relatively narrow lines at ambient velocities (ambient peak). Other molecules (CO, CS, H2S, SiO, SO, SO2) show extended wings tracing the whole range of the outflow velocities. Finally, OCS, H2CS, HDO and CH3OH are associated with wings and, indeed, show wings, and also reveal a bright high-velocity redshifted spectral peak (outflow peak) which can be used to investigate the southern outflows. At ambient velocities the gas is dense (>105 cm−3) and different components at distinct temperatures coexist, ranging from the relatively low kinetic temperatures (≤50 K) measured with H2S, CH3OH, H2CS and CH3C2H, to definitely higher-temperature conditions, ∼100-200 K, obtained from the SiO, SO and SO2 spectra. For the outflow peak we derive densities of between ∼104 and ∼107 cm−3 and high temperatures, ≃100-200 K, indicating regions compressed and heated by shocks. The analysis of the line profiles shows that the SiO molecule dominates at the highest velocities and at the highest excitation conditions, confirming its close association with shocks. H2S, SO2 and SO preferentially trace more quiescent regions than SiO, and, in particular, a lack of bright H2S emission at the highest velocities is found. OCS and H2CS emit at quite high velocities, where the abundances of three shock tracers such as SiO, CH3OH and HDO are higher. These results may indicate that H2S is not the only major sulphur carrier in the grain mantles, and that OCS and H2CS may probably play an important role on the grains, or that alternatively they rapidly form once the mantle is evaporated after the passage of a shock. Finally, the outflow peak emission has been compared with recent time-dependent sulphur chemistry models: the results indicate that, if associated with accurate measurements of the physical conditions, the CH3OH/H2CS column density ratio can be used as an effective chemical clock to date the age of shocked gas.
|
