摘要：Dense cloud cores present chemical differentiation because C- and N-bearing molecules are
distributed differently, the latter being less affected by freeze-out onto dust grains. In
this letter we show that two C-bearing molecules, CH3OH and c-C3H2, present a strikingly different
(complementary) morphology while showing the same kinematics towards the prestellar core
L1544. After comparing their distribution with the large-scale H2 column density N(H2) map from the Herschel
satellite, we find that these two molecules trace different environmental
conditions in the surrounding of L1544: the c-C3H2 distribution peaks close to the southern part of the
core, where the surrounding molecular cloud has an N(H2) sharp edge, while
CH3OH mainly
traces the northern part of the core, where N(H2) presents a shallower tail. We conclude that this is
evidence of chemical differentiation driven by different amounts 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-C3H2) 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 towards
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.
