Chemical differentiation in a prestellar core traces non-uniform
  illumination
  Dense cloud cores present chemical differentiation due to the different
distribution of C-bearing and N-bearing molecules, the latter being less
affected by freeze-out onto dust grains. In this letter we show that two
C-bearing molecules, CH$_3$OH and $c$-C$_3$H$_2$, present a strikingly
different (complementary) morphology while showing the same kinematics toward
the prestellar core L1544. After comparing their distribution with large scale
H$_2$ column density N(H$_2$) map from the Herschel satellite, we find that
these two molecules trace different environmental conditions in the surrounding
of L1544: the $c$-C$_3$H$_2$ distribution peaks close to the southern part of
the core, where the surrounding molecular cloud has a N(H$_2$) sharp edge,
while CH$_3$OH mainly traces the northern part of the core, where N(H$_2$)
presents a shallower tail. We conclude that this is evidence of chemical
differentiation driven by different amount of illumination from the
interstellar radiation field: in the South, photochemistry maintains more C
atoms in the gas phase allowing carbon chain (such as $c$-C$_3$H$_2$)
production; in the North, C is mainly locked in CO and methanol traces the zone
where CO starts to freeze out significantly. During the process of cloud
contraction, different gas and ice compositions are thus expected to mix toward
the central regions of the core, where a potential Solar-type system will form.
An alternative view on carbon-chain chemistry in star-forming regions is also
provided.
