Chemical study of two starless cores in the B213/L1495 filament

TL;DR

This study investigates the chemical evolution of two nearby starless cores, C2 and C16, in the B213/L1495 Taurus filament by conducting two complete 7 mm spectral surveys. Using LTE rotational diagrams and a neural-emulator version of the NAUTILUS astrochemical model, the authors derive molecular inventories, rotational temperatures, and fractional abundances, and then explore a broad parameter space to identify the best-fit chemical ages and physical conditions. They find both cores are best described by early-time chemistry around $t \sim 0.1$ Myr, with C2 being denser and more sulfur-depleted than C16, suggesting a more advanced evolutionary stage for C2. While the models reproduce many molecules (including COMs and long cyanopolynes), species such as CCS and C$_{3}$O are poorly matched, highlighting gaps in sulfur and carbon-chain chemistry and the need for more sophisticated, dynamically coupled models to capture environmental effects and accretion history.

Abstract

The chemical evolution of pre-stellar cores during their transition to a protostellar stage is not yet fully understood. Detailed chemical characterizations of these sources are needed to better define their chemistry during star formation. Our goal is to characterize the chemistry of the starless cores C2 and C16 in the B213/L1495 filament of the Taurus Molecular Cloud, and to understand how it relates to the environmental conditions and the evolutionary state of the cores. We made use of two complete spectral surveys at 7 mm of these sources, carried out using the Yebes 40-m telescope. Derived molecular abundances were compared with those of other sources in different evolutionary stages and with values computed by chemical models. Including isotopologs, 22 molecules were detected in B213-C2, and 25 in B213-C16. The derived rotational temperatures have values of between $\sim$ 5 K and $\sim$ 9 K. A comparison of the two sources shows lower abundances in C2, except for l-C$_{3}$H and HOCO$^{+}$, which have similar values in both cores. Model results indicate that both cores are best fit assuming early-time chemistry, and point to C2 being in a more advanced evolutionary stage, as it presents a higher molecular hydrogen density and sulfur depletion, and a lower cosmic-ray ionization rate. Our chemical modeling successfully accounts for the abundances of most molecules, including complex organic molecules and long cyanopolynes (HC$_{5}$N, HC$_{7}$N), but fails to reproduce those of the carbon chains CCS and C$_{3}$O. Chemical differences between C2 and C16 could stem from the evolutionary stage of the cores, with C2 being closer to the pre-stellar phase. Both cores are better fit assuming early-time chemistry of t $\sim$ 0.1 Myr. The more intense UV radiation in the northern region of B213 could account for the high abundances of l-C$_{3}$H and HOCO$^{+}$ in C2.

Figures (9)

• Figure 1: B213-C2 and B213-C16 molecular hydrogen column density maps derived by Palmeirim, reconstructed at an angular resolution of 18.2$"$. Contour levels are $(5, 10, 15 \ \text{and} \ 20) \times 10^{21} \ \text{cm}^{-2}$ in the left panel and $(5, 10, 15, 20 \ \text{and} \ 25) \times 10^{21} \ \text{cm}^{-2}$ in the right panel. The beam of the Yebes telescope in the Q band (HPBW$\approx$42.5$"$) is plotted in each panel.
• Figure 2: Left: Velocity components observed in CS overlapped with the two velocity components observed in C$^{34}$S for the detected transitions in B213-C2. Right: Superposition of the emission lines detected in B213-C2 for C$\mathrm{H}_{3}$OH with both one and two visible velocity components.
• Figure 3: Rotational diagrams for the detected molecules in B213-C2. The calculated values for the rotational temperature, $T_{rot}$, and column density, $N_{tot}$, are indicated for each molecule.
• Figure 4: Rotational diagrams for the detected molecules in B213-C16. The calculated values for the rotational temperature, $T_{rot}$, and column density, $N_{tot}$, are indicated for each molecule.
• Figure 5: Fractional abundance ratios between B213-C2 and B213-C16, for all the detected species. Lower limits of the ratios, corresponding to molecules detected in B213-C16 but not in B213-C2, are marked with downward triangles. Conversely, upper limits of the ratios, for molecules detected in B213-C2 but not in B213-C16, are indicated with upward triangles.