Three dimensional projection effects on chemistry in a Planck galactic
  cold clump
  Offsets of molecular line emission peaks from continuum peaks are very common
but frequently difficult to explain with a single spherical cloud chemical
model. We propose that the spatial projection effects of an irregular three
dimensional (3D) cloud structure can be a solution. This work shows that the
idea can be successfully applied to the Planck cold clump G224.4-0.6 by
approximating it with four individual spherically symmetric cloud cores whose
chemical patterns overlap with each other to produce observable line maps. With
the empirical physical structures inferred from the observation data of this
clump and a gas-grain chemical model, the four cores can satisfactorily
reproduce its 850 $\mu$m continuum map and the diverse peak offsets of CCS,
HC$_3$N and N$_2$H$^+$ simultaneously at chemical ages of about $8\times
10^5\sim 3\times 10^6$ yrs. The 3D projection effects on chemistry has the
potential to explain such asymmetrical distributions of chemicals in many other
molecular clouds.
