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Six rotational transitions of hydrogen peroxide, H2O2, were searched for with the APEX 12 m radio telescope toward ρ Oph A by
P. Bergman, B. Parise, R. Liseau, B. Larsson, H. Olofsson, K. M. Menten, and R. Güsten,
Detection of Interstellar Hydrogen Peroxide
Astron. Astrophys. 531, Art. No. L8 (2011).
The JK<sub>a,Kc</sub> = 30,3 – 21,1 and 50,5 – 41,3 of the τ = 4 – 2 ladder near 219.2 and 318.2 GHz, respectively, as well as the 61,5 – 50,5 and 40,4 – 31,2 of the τ = 2 – 4 ladder near 251.9 and 269.0 GHz, respectively, were detected in emission; albeit the latter one only barely with significance.
The non-observation of the 11,0 – 00,0, τ = 3 – 1 transition near 670.6 GHz was attributed to subthermal excitation.
A column density of 8 (3) × 1012cm–2 and a rotational temperature of 22 ± 3 K were derived. Since the column density of H2 is rather uncertain, the fractional abundances of H2O2 with respect to H2 should be viewed with great caution.
The number of observed transitions may seem a bit small, but the low rotational temperature and the corresponding small number of lines detected in that source suggest the observation to be rather secure. Moreover, the strongest evidence for molecular oxygen in the ISM stem from observations of this source.

Contributor(s): H. S. P. Müller; 06, 2011

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