Invited Talk - Splinter CosmicRays

Monday, 13 September 2021, 09:45   (virtual Cosmic)

The cosmic-ray ionisation rate in the prestellar core L1544

E. Redaelli, O. Sipilä, M. Padovani, P. Caselli, D. Galli, A. V. Ivlev
Max Planck Institut für Extraterrestrische Physik, INAF - Osservatorio astronomico di Arcetri, Florence

As known, Cosmic rays (CRs) play an important role in the chemistry and dynamics of the interstellar medium. In particular, in dense environments, CRs represent the main ionising agent, hence driving the rich chemistry of molecular ions. Furthermore, they determine the ionisation fraction, which regulates the degree of coupling between the gas and the interstellar magnetic fields, and the heating of the gas. Estimates of the CR ionisation rate of molecular hydrogen (CRIR) span several orders of magnitude, depending on the targeted sources and on the used method. Recent theoretical models have characterised the CR attenuation with increasing density. We aim to test these models for the attenuation of CRs in the low-mass pre-stellar core L1544, a prototypical source due to its high and centrally-peaked density (n~10^6 cm-3) and low temperatures (T<10K). We used a state-of-the-art gas-grain chemical model, which accepts the CRIR profile as input, to predict the abundance profiles of four ions: N2H+, N2D+, HC18O+, and DCO+. Using the non-local thermodynamic equilibrium radiative transfer code MOLLIE, we produced synthetic spectra based on the derived abundances, and compared them with previous observations obtained with the IRAM 30m telescope. I will show how our results indicate that a model with high CRIR (> 10^-16 s-1) is excluded by the observations. Also the model with the standard CRIR = 1.3x10-17 s-1 produces a worse agreement with respect to the attenuation model based on Voyager observations, which is characterised by an average CRIR = 3x10-17 s-1 at the column densities typical of L1544. I will show how the single-dish data, however, are not sensitive to the attenuation of the CR profile, which changes only by a factor of two in the range of column densities spanned by the core model. Interferometric observations at higher spatial resolution, combined with observations of transitions with lower critical density —hence tracing the low-density envelope— are needed to observe a decrease of the CR ionisation rate with density.