Cosmic-ray ionisation rate in low-mass cores: the role of the environment

TL;DR

This work delivers a uniform set of cosmic-ray ionisation rate measurements, $zeta_2$, for 17–20 low-mass starless cores embedded in different parental clouds. Using the Bovino20 analytical framework and new APEX line data together with Herschel-derived $N(H_2)$ and $T_{dust}$, it links $zeta_2$ to environment rather than simply to column density, finding $zeta_2$ spanning ~1.3×10^-18 to 8.5×10^-17 s^-1 and correlating positively with $T_{dust}$. The analysis shows no strong correlation between $zeta_2$ and $N(H_2)$, while warmer environments, likely with enhanced star-formation activity, show higher ionisation rates consistent with local re-acceleration of cosmic rays by nearby protostars. LVG modelling supports sub-thermal excitation of key molecules and indicates modest underestimation of central densities; overall, the results imply that ionisation states in dense cores are not universal but are shaped by local and environmental factors, with implications for chemistry and collapse timescales in star-forming regions.

Abstract

Context: Cosmic rays drive several key processes for the chemistry and dynamical evolution of star-forming regions. Their effect is quantified mainly by means of the cosmic-ray ionisation rate $ζ_2$. Aims: We aim to obtain a sample of $ζ_2$ measurements in 20 low-mass starless cores embedded in different parental clouds, to assess the average level of ionisation in this kind of sources and to investigate the role of the environment in this context. The warmest clouds in our sample are Ophiuchus and Corona Australis, where star formation activity is higher than in the Taurus cloud and the other isolated cores we targeted. Methods: We compute $ζ_2$ using an analytical method based on the {column density} of ortho-$\rm H_2D^+$, the CO abundance, and the deuteration level of HCO$^+$. To estimate these quantities, we analysed new, high-sensitivity molecular line observations obtained with the Atacama Pathfinder EXperiment (APEX) single-dish telescope and archival continuum data from Herschel. Results: We report $ζ_2$ estimates in 17 cores in our sample and provide upper limits on the three remaining sources. The values span almost two orders of magnitude, from $1.3 \times 10^{-18}\, \rm s^{-1}$ to $8.5 \times 10^{-17}\, \rm s^{-1}$. Conclusions: We find no significant correlation between $ζ_2$ and the core's column densities $N\rm (H_2)$. On the contrary, we find a positive correlation between $ζ_2$ and the cores' temperature, estimated via Herschel data: cores embedded in warmer environments present higher ionisation levels. The warmest clouds in our sample are Ophiuchus and Corona Australis, where star formation activity is higher than in the other clouds we targeted. The higher ionisation rates in these regions support the scenario that low-mass protostars in the vicinity of our targeted cores contribute to the re-acceleration of local cosmic rays.

Figures (31)

• Figure 1: $N\rm (H_2)$ maps towards each core in the sample (labelled in the top-left corner of every panel). The scalebar shown in the bottom-right corner represents a length of $0.05\,\rm pc$. The white contours show the 20, 40, and 60% levels of the peak value within the central Herschel beam. The solid circles show the APEX pointing and the $N\rm (H_2)$ map beam size, whilst the dashed circles show the beam sizes and pointings of APEX for the o$\rm H_2D^+$ line. Note the small shift present between the two positions for Oph D, where the o$\rm H_2D^+$ beam is not the central LAsMA beam, but one of the external ones (see Main Text). The shift is however smaller than the continuum and APEX resolutions.
• Figure 2: $T\rm _{ex}$ of $\rm H^{13}CO^+$ (blue circles) and $\rm DCO^+$ (red triangles) for all cores where two transitions of the species are available, as a function of $T\rm _{dust}$ values. The dashed black line shows the 1:1 relation. For cores where two velocity components are identified and fitted, we show the parameters of both.
• Figure 3: The black histograms show the spectra collected towards CrA 151. The transitions are labelled in the top-left corner of each panel. The red histograms show the best-fit solution of the spectral fit performed as described in Sect. \ref{['sec:Ncols']}.
• Figure 4: Same as Fig. \ref{['fig:cra_151']}, but for Oph 4. In this core, two velocity components are seen in all the transitions, and we fit them separately. The red/blue curves show the individual best-fit solutions, whilst the green curve represents the total fit.
• Figure 5: The orange points show the measured flux ratio between the $\rm H^{13}CO^+$ and $\rm HC^{18}O^+$$(2-1)$ lines, in those cores where both transitions are detected (labelled on the y-axis). Errorbars show the $3\sigma$ level. The vertical dashed orange line is the weighted average of the sample. The expected value $557/68=8.2$ is shown with the vertical green line, and the shaded green area shows a variation of a factor $2$ around it.