Importance of collisional rates for anomalous absorption in H2CO molecule

Formaldehyde (H2CO) is the first organic molecule identified in a number of galactic and extragalactic radio sources through its transition 110-111 at 4.830 GHz in absorption. Later on, this transition was found in anomalous absorption. In some cosmic objects, this transition however was found in emission and even as a maser radiation. Since the transition 110-111 of ortho-H2CO is considered as a unique probe of high density gas at low temperature, the study of H2CO has always been of great importance for astrophysicists as well as for spectroscopists. In view of the availability of better input data required for such investigation, it is worth while to investigate again about the radiations from ortho-H2CO. In the present study, we have investigated anomalous absorption of 110-111, 211-212 and 312-313 transitions of ortho-H2CO. The present results are more reliable as compared to those obtained earlier.

► Accurate rotational levels and A-coefficients for H2CO are calculated. ► Transitions 110-111, 211-212 and 312-313 show anomalous absorption. ► Anomalous absorption is found to increase with kinetic temperature. ► Anomalous absorption may be found for nH2≈104cm−3. ► Colliding partner para-H2 may be approximated as He atom.