Deuteration of HC3N and CH3CCH in the pre-stellar core L1544

TL;DR

This study maps the deuteration of HC3N and CH3CCH across the pre-stellar core L1544 using IRAM 30 m observations of HC3N, HCC13CN, DC3N, CH3CCH, CH2DCCH, and CH3CCD. It combines non-LTE radiative-transfer modelling (LOC) at the dust peak with LTE column-density maps to derive pixel-by-pixel deuteration fractions and link them to physical structure. The results reveal DC3N/HC3N deuteration of about $0.04-0.07$ that is broadly homogeneous and traces intermediate-density gas in the outer layers, while CH2DCCH/CH3CCH reaches $0.09-0.15$ with a NE peak, and CH3CCD/CH3CCH reaches $0.07-0.09$ with central enhancements. The data favor gas-phase formation and deuteration for both carbon chains but suggest additional grain-surface deuteration for CH2DCCH, especially near the CH3OH peak, highlighting the role of density, external radiation, and microstructure in shaping deuteration patterns. These results refine our understanding of deuteration as a tracer of core evolution and motivate richer chemistry networks incorporating grain-surface processes.

Abstract

Deuterated molecules are a useful diagnostic tool to probe the evolution and the kinematics in the earliest stages of star formation. Due to the low temperatures and high densities in the centre of pre-stellar cores, the deuterium fraction is enhanced by several orders of magnitude. We study the distribution of the emission and the deuteration of the two carbon chains HC3N and CH3CCH throughout the pre-stellar core L1544. We analyse emission maps of CH3CCH, CH2DCCH, CH3CCD, HC3N, HCC13CN, and DC3N, observed with the IRAM 30m single-dish radio telescope. We use non-LTE radiative transfer calculations, combined with chemical modelling of the molecular abundances, to constrain physical parameters of the observed species. Following this, we derive the column density and deuteration maps. We find D-fractions of N(DC3N)/N(HC3N)=0.04-0.07, N(CH2DCCH)/N(CH3CCH)=0.09-0.15, and N(CH3CCD)/N(CH3CCH)=0.07-0.09. The deuteration of HC3N appears homogeneous across the core, with widespread D-fraction values above 0.06, tracing intermediate-density gas in the outer layers of the core. CH3CCD is most efficiently formed in the higher-density regions towards the core centre, while the D-fraction of CH2DCCH traces a local density enhancement in the north-east of the core, coinciding with the CH3OH emission peak. The results suggest that gas-phase reactions dominate the formation and deuteration of both HC3N and CH3CCH in L1544, with spatial variations driven by physical structure, density and external radiation. The significantly higher D-fraction of CH2DCCH compared to CH3CCD and a tentative gradient with higher values in the north suggest different deuteration mechanisms for the two functional groups. Similarities between the CH2DCCH emission and CH2DOH might indicate an additional deuteration pathway of CH3CCH on the surfaces of dust grains, as observed for H2CO.

Figures (13)

• Figure 1: Integrated intensity maps of the observed transitions. The grey solid line contours indicate the 30%, 70%, and 90% level of the peak integrated intensity. The dashed line contours represent 30%, 50%, and 90% of the H$_2$ column density peak derived from $Herschel$ maps Spezzano2016. The markers in blue represent the dust peak (triangle) and the molecular emission peaks of CH$_3$OH (diamond), CH$_3$CCH (star), and c-C$_3$H$_2$ (plus sign), where emission spectra (shown in Fig. \ref{['fig:CH3CCHmoleculepeaks']}) are extracted within a circular aperture with a diameter corresponding to the telescope beam. The white circle in the bottom-left corner indicates the beam size of the IRAM 30 m telescope (31").
• Figure 2: Observed spectra (black) of the CH$_3$CCH (top) and the CH$_3$CCD (bottom) K=0 and K=1 transitions extracted towards the three molecular peaks in L1544 and the dust peak, using a circular aperture with diameter 31". The extraction locations are indicated in Fig. \ref{['fig:IntegratedIntensityMaps']}. Shown in red are synthetic spectra produced with the LTE model generator of the python package pyspeckit. The input column densities are [4, 2, 5, 8]$(\pm0.3)\times10^{13}$ cm$^{-2}$ for CH$_3$CCH and [3, 1, 4, 5]$(\pm0.3)\times10^{12}$ cm$^{-2}$ for CH$_3$CCD at the dust peak, CH$_3$OH peak, CH$_3$CCH peak, and c-C$_3$H$_2$ peak, respectively. The input excitation temperature is set to a constant value of 10 K for both molecules.
• Figure 3: Profiles of the gas temperature (red), H$_2$ number density (orange, in logarithmic scale), and infall velocity (blue, in units of 0.1 km s$^{-1}$) for the Keto-Caselli model of L1544 Keto2015. The velocity in the model is negative but is shown here as positive to improve readability.
• Figure 4: Fractional abundance profiles of HC$_3$N (solid) and CH$_3$CCH (dashed) of the best-fit results produced with LOC. For HCC$^{13}$CN, the radial abundance profiles derived with chemical modelling (green, red) correspond to the profiles of the main isotopologue, scaled down by the isotopic ratio, 68.
• Figure 5: Comparison of the observed spectra (grey) of HC$_3$N, and HCC$^{13}$CN, extracted towards the dust peak of L1544, to the synthetic spectra (red, blue, green) produced with LOC. For the constant abundances, a depletion radius of 1800 au is assumed.