In view of both the size of its gap and the previously reported asymmetries and near-infrared spiral arms , the transition disk of the Herbig Fe star HD 142527 constitutes a remarkable case study .
This paper focuses on the morphology 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=3-2 show spiral features of different sizes .
The innermost spiral arm ( S1 ) is a radio counterpart of the first near-infrared spiral observed by Fukagawa et al .
( 20 ) , but it is shifted radially outward .
However , the most conspicuous CO spiral arm ( S2 ) lies at the outskirts of the disk and had not been detected before .
It corresponds to a cold density structure , with both brightness and excitation temperatures of order 13 \pm 2 K and conspicuous in the ^ { 12 } CO J=2-1 peak-intensity map , but faint in ^ { 12 } CO J=3-2 .
There is also a faint counterarm ( S3 ) , point-symmetrical of S2 with respect to the star .
These three spirals are modelled separately with two different formulae that approximate the loci of density maxima in acoustic waves due to embedded planets .
S1 could be fit relatively well with these formulae , compared to S2 and S3 .
Alternative scenarios such as gravitational instability or external tidal interaction are discussed .
The impact of channelization on spectrally and spatially resolved peak intensity maps is also briefly addressed .