Fig. 2

Parts of the 3 mm line survey of IRC +10216 showing the pairs of lines tentatively
assigned to the N = 9−8 and N = 10−9 doublets of MgCCH. The
rms
noise levels per 1 MHz channel are 0.5 and 0.7 mK in the bottom and top
panels, respectively. Thick gray lines correspond to the calculated line
profiles under LTE, assuming that MgCCH extends between 5 and 20′′ with a homogeneous
fractional abundance and rotational temperature of 20 K.
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