2006AJ....132.1851A

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.