High-sensitivity Molecular Line Observations toward the L1544 Prestellar Core Challenge Current Models^*

Abstract The increased sensitivity and spectral resolution of observed spectra toward the prestellar core L1544 are challenging the current physical and chemical models. With the aim of further constraining the structure of L1544 as well as assessing the completeness of chemical networks, we turn to radiative transfer modeling of observed molecular lines toward this source. We obtained high-sensitivity and high spectral resolution observations of HCO + ( J = 1–0), CS ( J = 2–1), C 34 S ( J = 2–1), H 2 CO ( J <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> = 2 1,2 –1 1,1 ), c-C 3 H 2 ( J <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>a</mml:mi> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> = 2 1,2 –1 0,1 ), SO ( N , J = 2, 3–1, 2), and 34 SO ( N , J = 2, 3–1, 2) with the IRAM 30 m telescope toward the dust peak of L1544. A non–local thermodynamic equilibrium radiative transfer code is coupled to the Markov Chain Monte Carlo method to model the observations. We find that with just one transition for each isotope, the modeling cannot find a global minimum that fits the observations. The derived fractional abundance profiles are compared to those computed with chemical models. The observed transitions trace gas components with distinct dynamics at different distances along the radius of the core. Moreover, the results evidence a poor reproduction of sulfur chemistry by chemical modeling and stress the need to include a more consistent S-depletion process to accurately reproduce the S-chemistry toward dense cores.