Chemical differentiation in a prestellar core traces non-uniform illumination^⋆,⋆⋆

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: spezzano@mpe.mpg.de
² Department of Physics, PO Box 64, University of Helsinki, 00100 Helsinki, Finland
³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 6 April 2016
Accepted: 7 July 2016

Abstract

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.