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In view of both the size of its gap and the previously reported asymmetriesand near-infrared spiral arms, the transition disk of the Herbig Fe star HD142527 constitutes a remarkable case study. This paper focuses on themorphology of the outer disk through ALMA observations of $^{12}$CO J=2-1,$^{12}$CO J=3-2 and $^{13}$CO J=2-1. Both $^{12}$CO J=2-1 and $^{12}$CO J=3-2show spiral features of different sizes. The innermost spiral arm (S1) is aradio counterpart of the first near-infrared spiral observed by Fukagawa et al.(2006), but it is shifted radially outward. However, the most conspicuous COspiral arm (S2) lies at the outskirts of the disk and had not been detectedbefore. It corresponds to a cold density structure, with both brightness andexcitation temperatures of order 13$\pm$2 K and conspicuous in the $^{12}$COJ=2-1 peak-intensity map, but faint in $^{12}$CO J=3-2. There is also a faintcounterarm (S3), point-symmetrical of S2 with respect to the star. These threespirals are modelled separately with two different formulae that approximatethe loci of density maxima in acoustic waves due to embedded planets. S1 couldbe fit relatively well with these formulae, compared to S2 and S3. Alternativescenarios such as gravitational instability or external tidal interaction arediscussed. The impact of channelization on spectrally and spatially resolvedpeak intensity maps is also briefly addressed.

Spiral arms in the disk of HD 142527 from CO emission lines with ALMA