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6cm and 2mm HCO Absorption Toward Compact Extragalactic mm-wave Continuum Sources

Liszt, R. Lucas
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, USA 22903-2475
Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France

Abstract: We have observed galactic 6cm and 2mm ortho- and para-HCO absorption along lines of sight toward and around compact extragalactic mm-wave continuum sources (Fig. 6). From comparison of on and off-source 6cm profiles it is possible to derive the anomalous excitation temperature, which is surprisingly close to the temperature of the cosmic microwave background in several cases. The 2mm absorption profiles show that the ortho/para ratio is 3-4, which (taken alone) favors formation either on grains at K or at comparable temperatures in the gas phase. However the abundance of HCO , as with other species, is probably too high at low extinction to be explained by any known process; we find for .



Figure: 2mm para () and ortho absorption line profiles toward two sources.

Robert Lucas
Thu Mar 9 12:14:01 MET 1995