Astronomy and Astrophysics – Astrophysics – Galaxy Astrophysics
Scientific paper
2010-07-08
Astronomy and Astrophysics
Astrophysics
Galaxy Astrophysics
Accepted for publication in A&A, HIFI Special Issue; 5 pages, 3 figures
Scientific paper
We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H$_2^{16}$O and H$_2^{18}$O in absorption towards Sagittarius~B2(M) and W31C. The ortho/para ratio in water in the foreground clouds on the line of sight towards these bright continuum sources is generally consistent with the statistical high-temperature ratio of 3, within the observational uncertainties. However, somewhat unexpectedly, we derive a low ortho/para ratio of $2.35 \pm 0.35$, corresponding to a spin temperature of $\sim$27~K, towards Sagittarius~B2(M) at velocities of the expanding molecular ring. Water molecules in this region appear to have formed with, or relaxed to, an ortho/para ratio close to the value corresponding to the local temperature of the gas and dust.
Bell Thomas A.
Bergin Edwin A.
Black John Harry
Blake Geoffrey A.
Boogert Adwin
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