Arecibo Observations of Formaldehyde in L1551

Abstract

We report observations of the formaldehyde (H2CO) 6 cm (4.8 GHz) line toward L1551. The observations were conducted with the Arecibo Telescope (beam FWHP ~ 1') to verify the tentative detection of H2CO thermal emission reported by Duncan and collaborators in 1987. The H2CO emission lines were expected to be present with a signal-to-noise ratio of 10 in our spectra. However, we did not detect H2CO emission; i.e., our data rule out their tentative detection. The absence of H2CO emission is also confirmed by the fact that the H2CO line profiles at the two positions of expected emission are well fitted by a single absorption component (accounting for the hyperfine structure of the line) in one of the positions and by a single absorption line plus a red-wing absorption component in the second position. The Orion BN/KL region remains the only H2CO 6 cm thermal emitter known. Our observations also demonstrate that the H2CO 6 cm absorption line traces not only the quiescent molecular cloud but also the kinematics associated with the star formation process in L1551-IRS 5.