Radial profile of the fractional abundance of o-NH₃  with respect to H₂, X(o-NH₃), in L1544. The blue profile has been deduced from observations carried out by Crapsi et al. (2007) assuming an ortho-to-para NH₃ abundance ratio of 1 (dashed curve) and 0.7 (solid curve); the red profile is the result of a chemical model calculation by Sipilä et al. (2015a,b). The black profile refers to the total (ortho+para) abundance of H₂O from Keto et al. (2014).