[Paper Review] On the internal structure of starless cores. II. A molecular survey of L1498 and L1517B

M. Tafalla, J. Santiago-García+4 more|ArXiv.org|May 19, 2006

Astrophysics and Star Formation StudiesPhysics and Astronomy70 references96 citations

TL;DR

This study conducts a molecular survey of the starless cores L1498 and L1517B using radiative transfer modeling to derive self-consistent radial abundance profiles for 13 species. By combining previously established physical structures with a spherically symmetric Monte Carlo radiative transfer code, the authors reveal that most molecules—like CO, CS, SO, and CH₃OH—exhibit sharp central holes due to depletion, while N₂H⁺ and NH₃ remain abundant in the core centers, indicating their resilience to freeze-out. The findings highlight SO, C₂S, and CH₃OH as the most sensitive tracers of depletion and reveal significant discrepancies between observed abundances and current chemical models.

ABSTRACT

[Abridged] We present a molecular survey of the starless cores L1498 and L1517B. These cores have been selected for their relative isolation and close-to-round shape, and they have been observed in a number of lines of 13 molecular species (4 already presented in the first part of this series): CO, CS, N2H+, NH3, CH3OH, SO, C3H2, HC3N, C2S, HCN, H2CO, HCO+, and DCO+. Using a physical model of core structure and a Monte Carlo radiative transfer code, we determine for each core a self-consistent set abundances that fits simultaneously the observed radial profile of integrated intensity and the emergent spectrum towards the core center (for abundant species, optically thin isopologues are used). From this work, we find that L1498 and L1517B have similar abundance patterns, with most species suffering a significant drop toward the core center. This occurs for CO, CS, CH3OH, SO, C3H2, HC3N, C2S, HCN, H2CO, HCO+, and DCO+, which we fit with profiles having a sharp central hole. The size of this hole varies with molecule: DCO+, HCN, and HC3N have the smallest holes while SO, C2S and CO have the largest holes. Only N2H+ and NH3 are present in the gas phase at the core centers. From the different behavior of molecules, we select SO, C2S, and CH3OH as the most sensitive tracers of molecular depletion. Comparing our abundance determinations with the predictions from current chemical models we find order of magnitude discrepancies. Finally, we show how the ``contribution function'' can be used to study the formation of line profiles from the different regions of a core.

Motivation & Objective

• To determine the radial abundance profiles of 13 molecular species in two isolated, round starless cores, L1498 and L1517B, to understand chemical inhomogeneity.
• To correct for biases in molecular line observations caused by selective depletion of species in dense core interiors.
• To test current chemical models against observed abundance profiles by comparing predictions with radiative transfer simulations.
• To identify the most sensitive molecular tracers of chemical depletion in low-mass starless cores.
• To use the contribution function to analyze how line profiles originate from different regions within the core, revealing observational biases.

Proposed method

• A spherically symmetric Monte Carlo radiative transfer code was used to model line emission and intensity profiles from the cores.
• The physical structure (density, temperature, turbulence, velocity) of L1498 and L1517B, derived from prior continuum and line data, was used as a fixed input.
• Abundance profiles for each molecule were fitted simultaneously to observed radial intensity profiles and central spectra, assuming a radial dependence with a central hole for most species.
• The contribution function was calculated to assess the spatial origin of line emission and understand which core regions dominate the observed line profiles.
• Model fits were validated against literature data, including C₂S and H₂CO observations from Wolkovitch et al. (1997) and Young et al. (2004), with adjustments to envelope parameters to improve fit for low-excitation transitions.
• Chemical models (e.g., Aikawa et al. 2005) were compared with derived abundances to assess model accuracy.

Experimental results

Research questions

• RQ1 How do the radial abundance profiles of key molecular species vary in starless cores like L1498 and L1517B?
• RQ2 Which molecules are most sensitive to chemical depletion, and how can they be used to trace core evolution?
• RQ3 To what extent do current chemical models predict the observed abundance patterns in these cores?
• RQ4 How do molecular line profiles reflect the internal structure of the core, and which regions contribute most to the observed emission?
• RQ5 What role does the low-density envelope play in shaping the observed line profiles, especially for optically thin transitions?

Key findings

• Most molecules, including CO, CS, CH₃OH, SO, C₃H₂, HC₃N, C₂S, HCN, H₂CO, HCO⁺, and DCO⁺, exhibit a sharp central hole in their abundance profiles, indicating strong depletion in the dense core center.
• The size of the central hole varies by molecule: DCO⁺, HCN, and HC₃N have the smallest holes, while SO, C₂S, and CO have the largest, reflecting differences in freeze-out susceptibility.
• Only N₂H⁺ and NH₃ remain significantly abundant in the gas phase at the core center, confirming their status as robust tracers of dense, quiescent gas.
• SO, C₂S, and CH₃OH are identified as the most sensitive tracers of molecular depletion due to their strong central depletion and clear radial profiles.
• The observed abundance profiles show order-of-magnitude discrepancies with predictions from current chemical models, particularly Aikawa et al. (2005), indicating a need for model refinement.
• The 1 11–1 10 transition of H₂CO shows strong absorption against the cosmic background, and its profile is highly sensitive to low-density gas outside the core; the model underestimates absorption unless a backside envelope is included, highlighting limitations in core-only modeling for such transitions.