Importance of collisional rates for anomalous absorption in H2CC molecule

Abstract

Formaldehyde (H2CO) is the first organic molecule identified in a number of galactic and extragalactic sources. It was identified through its transition 110–111 at 4.829GHz in absorption. Later on, this transition of formaldehyde was found as showing anomalous absorption in the direction of four dark nebulae. Anomalous absorption is an unusual phenomenon. Structure of H2CC is very similar to that of H2CO and H2CS. Both H2CO and H2CS have already been identified in a number of cosmic objects. Since cosmic abundance of carbon is 20 times larger than that of sulfur, we propose that H2CC may be identified in cool cosmic objects through its transition 110–111 at 4.85GHz. Keeping in view of the similarity of energy levels of H2CC with H2CO, in the present investigation we have performed calculations for three sets of collisional rates for rotational transitions in ortho-H2CO due to collisions with (i) He atom, (ii) ortho-H2 (J=1), and (iii) para-H2 (J=0). All the three sets of calculations showed the anomalous absorption of 110–111 transition of H2CC. However, the anomalous absorption of 211–212 transition is found to be weak.