WATER ABSORPTION IN GALACTIC TRANSLUCENT CLOUDS: CONDITIONS AND HISTORY OF THE GAS DERIVED FROMHERSCHEL/HIFI PRISMAS OBSERVATIONS

Abstract

ABSTRACT2 O for the lower energy level of each transition observed. The total column density of water in translucent clouds is usually about a few 10 13 cm −2 . We find that the abundance of water relative to hydrogen nuclei is 1 × 10 −8 in agreement with models for oxygen chemistry in which high cosmic ray ionization rates are assumed. Relative to molecular hydrogen, the abundance of water is remarkably constant through the Galactic plane with X(H 2 O) = 5 × 10 −8 , which makes water a good traced of H 2 in translucent clouds. Observations of the excited transitions of H 2 O enable us to constrain the abundance of water in excited levels to be at most 15%, implying that the excitation temperature, T ex , in the ground state transitions is below 10 K. Further analysis of the column densities derived from the two ortho ground state transitions indicates that T ex 5 K and that the density n(H 2 ) in the translucent clouds is below 10 4 cm −3 . We derive the water ortho-to-para ratio for each absorption feature along the line of sight and find that most of the clouds show ratios consistent with the value of 3 expected in thermodynamic equilibrium in the high-temperature limit. However, two clouds with large column densities exhibit a ratio that is significantly below 3. This may argue that the history of water molecules includes a cold phase, either when the molecules were formed on cold grains in the well-shielded, low-temperature regions of the clouds, or when they later become at least partially thermalized with the cold gas (∼25 K) in those regions; evidently, they have not yet fully thermalized with the warmer (∼50 K) translucent portions of the clouds.

Water deuterium fractionation in the high-mass star-forming region G34.26+0.15 based on Herschel/HIFI data

“…The hot core of G34 has been the target of many Herschel/HIFI observations for the past four years, including water line emission (Flagey et al 2013) and its deuterated couterparts. We present in Section 3 the H 18 2 O and HDO transitions observed from the ground as well as the Herschel/HIFI observations.…”

“…Flagey et al (2013). (8) The integrated flux of the emission component is ∼ 1.80 K km s −1 , whereas it is ∼ −0.13 K km s −1 for the absorbing component.…”

“…assuming a sideband gain ratio of unity) was added to the spectrum of the 1 1,1 -0 0,0 fundamental line. To constrain the HDO/H 2 O ratio, we also used two H 18 2 O transitions observed in the framework of the PRISMAS program and previously published by Flagey et al (2013). We refer to this paper for the data reduction of these two lines.…”

“…It seems therefore possible that the constant inner abundance can be reached only at ∼200 K. Some tests assuming a gradual abundance increase were attempted in Section 4.2.5 and this explanation seems to hold here. Flagey et al (2013) are not well suited to measure abundances, because of their large opacities. With its excitation level, the para-H 18 2 O 3 1,3 -2 2,0 transition observed at 203 GHz with the IRAM-30m telescope is suitable to probe the hot core and derive the HDO/H 2 O ratio in the warm inner region.…”

“…With its excitation level, the para-H 18 2 O 3 1,3 -2 2,0 transition observed at 203 GHz with the IRAM-30m telescope is suitable to probe the hot core and derive the HDO/H 2 O ratio in the warm inner region. The H 18 2 O fundamental lines previously detected with Herschel/HIFI by Flagey et al (2013) are also used to constrain the HDO/H 2 O ratio in the envelope, as these lines combine both emission and absorption. Note that we only use here the ortho 1 1,0 -1 0,1 transition at 548 GHz and the para 1 1,1 -0 0,0 transition at 1102 GHz.…”