Suresh Chandra, P.G. Musrif and S.V. Shinde
School of Physical Sciences, S.R.T.M. University, Nanded 431 606,
India
Abstract. Snyder et al. (1969) detected H2 CO through its transition 110 - 111 at 4.829 GHz in absorption in the interstellar medium in a number of galactic and extragalactic sources (M17, W3, W3(OH position), W49, NGC 2024, DR 21, W43, W44, W51, Sgr A, Sgr B2, W33, NGC 6334, Cas A, and 3C 123). This transition of H2 CO was found in anomalous absorption by Palmer et al. (1969) in the direction of four dark nebulae. In some objects, this transition has however been detected in emission and even as a maser line (Forster et al. 1980; Whiteoak et al. 1983). Evans et al. (1970) reported detection of H2 CO molecule through its transition 211 - 212 at 14.488 GHz in absorption in some cosmic objects. This transition was also found in anomalous absorption by Evans et al. (1975). Since the transition 110 - 111 is considered as a unique probe of high density gas at low temperature, the study of H2 CO in cosmic objects is of great importance. Garrison et al. (1975) investigated the problem of anomalous absorption of 110 - 111 and 211 -212 transitions of H2 CO where they accounted for 8 energy levels connected by 10 radiative transitions and considered a kinetic temperature of 5 - 20 K. They found weak anomalous absorption of 110 - 111 and 211 - 212 transitions of H2 CO.
Here, we have investigated the transfer of radiation in H2 CO accounting 22 rotational energy levels connected by 47 radiative transitions in ground vibrational and ground electronic state. Further, we considered a kinetic temperature of 10 - 40 K, as the kinetic temperature in some regions may be rather high. Thus, the present work may be considered as an extension of the work of Garrison et al. (1975) where the investigation is carried out in more detail. We have obtained remarkable anomalous absorption of 110 - 111, 211 - 212 and 312 - 313 transitions of H2 CO. In order to include a large number of cosmic objects, where H2 CO may be found, parameters in our investigation are varied over wide ranges.
Keywords: Interstellar material - H2 CO - anomalous absorption