First hyperfine structure resolved OH FIR spectrum of a star-forming region
Résumé
Embedded protostars interact with their natal cloud through shocks and irradiation. The ambient interstellar medium warms up, allowing icy grain mantles to evaporate and making different chemical routes in the gas phase available. Water then becomes one of the most abundant molecular species in the gas phase. The Herschel key program 'Water in Star-Forming Regions with Herschel (WISH)' studies the excitation and chemistry of water around protostars. Hydroxyl (OH) is of the cornerstone species in the water chemistry network, because it is closely linked to both the formation and destruction of water through the OH + H2 leftrightarrow H2O + H reactions and photodissociation processes. This poster presents the first OH observation with resolved hyperfine structure at 163 μm of a star-forming region obtained using HIFI on Herschel. The OH triplet from the high-mass star-forming region W3 IRS5 is in emission, with the line profile revealing a narrow component on top of a broad feature. The broad component is attributed to outflow emission based on comparison with molecular lines of other species, whereas the narrow component is in agreement with radiative transfer results for a spherically symmetric envelope model. The resolved hyperfine structure allows us to constrain the excitation temperature and the OH column density in our models simultaneously. The derived OH/H2O ratios in the envelope are consistent with the current picture of the water chemistry. In the outer envelope (T < 100 K), where OH and H2O are released into the gas phase by photodesorption from the ice mantles of dust grains, we find a ratio of about unity. Laboratory work by Öberg et al. (2009) demonstrated that similar amounts of OH and water are released with an expected OH/H2O ratio of 0.5-1. This ratio is also in agreement with the theoretical work by Andersson & van Dishoeck (2008). In the inner envelope (T > 100 K), water is efficiently formed from OH and the OH/H2O ratio is therefore expected to drop significantly, which is consistent with the derived value of the order of 10-4. For the outflow, a lower limit of OH/H2O > 0.025 is obtained and can be explained with either a fast J-type shock or a slower UV irradiated C-type shock.