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 J_{K_a,K_c} = 3_0,3 – 2_1,1 and 5_0,5 – 4_1,3 of the τ = 4 – 2 ladder near 219.2 and 318.2 GHz, respectively, as well as the 6_1,5 – 5_0,5 and 4_0,4 – 3_1,2 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 1_1,0 – 0_0,0, 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