Carbon chain diversity in L1544 and IRAS 16293-2422: an astrochemical pathfinder study for the SKAO

TL;DR

This work presents a high-resolution, low-frequency spectral survey of the prestellar core L1544 and the IRAS 16293-2422 protostellar envelope with the GBT, uncovering a rich inventory of carbon-chain species and cyanopolyynes. LTE analyses and updated rest frequencies refine abundances and trace a chemical dichotomy: L1544 is richer in carbon chains than IRAS 16293, though both share similar abundance ratios among long chains, consistent with an underlying growth pathway from small to large carbon chains. Astrochemical modeling with a revised reaction network reproduces some trends but underestimates longer cyanopolyynes, pointing to missing ion–molecule formation routes and the need for network expansion. The results motivate high-angular-resolution SKAO observations to resolve spatial origins and inform a more complete understanding of carbon-chain chemistry relevant to prebiotic molecular synthesis.

Abstract

Astrochemical observations have revealed a surprisingly high level of chemical complexity, including long carbon chains, in the earliest stages of Sun-like star formation. The origin of these species and whether they undergo further growth, possibly contributing to the molecular complexity of planetary systems, remain open questions. We present recent observations performed using the 100-m Green Bank Telescope of the prestellar core L1544 and the protostellar system IRAS 16293-2422. In L1544, we detected several complex carbon-bearing species, including $\mathrm{C_2S}$, $\mathrm{C_3S}$, $\mathrm{C_3N}$, $\mathrm{c\text{-}C_3H}$, $\mathrm{C_4H}$, and $\mathrm{C_6H}$, complementing previously reported emission of cyanopolyynes. In IRAS 16293-2422, we detected $\mathrm{c\text{-}C_3H}$ and, for the first time, $\mathrm{HC_7N}$. Thanks to the high spectral resolution, we refine the rest frequencies of several $\mathrm{c\text{-}C_3H}$ and $\mathrm{C_6H}$ transitions. We perform radiative transfer analysis, highlighting a chemical difference between the two sources: IRAS 16293-2422 shows column densities 10-100 times lower than L1544. We perform astrochemical modeling, employing an up-to-date chemical network with revised reaction rates. The models reproduce the general trends, with cyanopolyyne and polyynyl radical abundances decreasing as molecular size increases, but they underestimate the abundances of cyanopolyynes longer than $\mathrm{HC_5N}$ by up to two orders of magnitude. Current models, which include the dominant neutral-neutral formation routes, cannot account for this discrepancy, suggesting that the chemical network is incomplete. We propose that additional ion-molecule reactions are crucial for the formation of these species. Developing a more comprehensive chemical network for long carbon chains is essential for accurately interpreting present and future observations.

Figures (18)

• Figure 1: Overview of the L1544 (upper panel) and IRAS 16293--2422 (lower panel) regions as traced by the Herschel continuum maps at 350 $\mu$m. The white contours correspond to [5,7,9,12,14]$\sigma$, where $\sigma$ is 12 MJy sr$^{-1}$ in L1544, and 1.1 MJy sr$^{-1}$ in IRAS 16293--2422, respectively. The FoVs of the GBT observations are shown as blue circles -- solid line for Ku-band and dashed line for X-band observations (see Sect. \ref{['sec:obs']}). Green diamonds identify the continuum ($\sim$1.3 mm) peak position in L1544 WardThompson99 and the position of the A/B, and E objects in IRAS 16293--2422 Kahle2023. The size of the Herschel beam ($25\arcsec$) is shown in the bottom left corner, while the scale bar is shown in the bottom right.
• Figure 2: Full continuum-subtracted spectra (in main-beam temperature, T$_{\rm MB}$) of the IRAS 16293-2422 envelope (Upper panel) and the L1544 prestellar source (Lower panel) observed in Ku Band (13.9 -- 15.41 GHz). The brightest lines associated with the detected species (see Tab. \ref{['Tab:lines']} for the list of all the detections) are labelled. The L1544 spectrum has a spectral resolution of 1.4 kHz ($\sim$ 30 m s$^{-1}$), while the IRAS 16293 spectrum has been here smoothed to 12 kHz ($\sim$240 m s$^{-1}$) to increase the S/N of the emission lines. In the L1544 spectrum, the H$_2$CO line at $\sim$14.5 GHz is observed in absorption.
• Figure 3: (Panel a): Full spectrum (in main-beam temperature, T$_{\rm MB}$) of the L1544 prestellar source observed in X Band (7.8 -- 11.6 GHz). The brightest lines associated with the detected species (see Tab. \ref{['Tab:lines']} for the list of all the detections) are labelled. The spectral resolution is 2.8 kHz ($\sim$ 60 m s$^{-1}$). (Panel b, c, d):. Zooms in to highlight selected lines of C$_4$H, C$_6$H and C$_3$N, respectively. The cyan solid lines correspond to 3$\sigma$.
• Figure 4: C$_2$S, C$_{\rm 3}$S, and C$_{\rm 3}$N spectra (in T$_{\rm MB}$ scale) observed towards L1544. The vertical dashed lines mark the ambient LSR velocity (+7.2 km s$^{-1}$, Tafalla1998).
• Figure 5: c--C$_3$H spectra (in T$_{\rm MB}$ scale) observed towards L1544. The vertical dashed lines mark the ambient LSR velocity (+7.2 km s$^{-1}$, Tafalla1998). When reported, the frequencies (in MHz) are different from those extracted from the JPL catalogue Pickett1998, and refer to the values needed to center the spectra to the L1544 systemic velocity.