Probing the physics of star formation (ProPStar)

I. First resolved maps of the electron fraction and cosmic-ray ionization rate in NGC 1333^★,★★

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: jpineda@mpe.mpg.de
² Department of Astronomy, The University of Texas at Austin, 2500 Speedway, Austin, TX 78712, USA
³ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin d’Hères, France
⁴ Department of Astronomy, University of Virginia, Charlottesville, VA, 22904, USA
⁵ IPAG, Université Grenoble Alpes, CNRS, 38000 Grenoble, France
⁶ Department for Physics, Engineering Physics and Astrophysics, Queen’s University, Kingston, ON, K7L 3N6, Canada

Received: 17 September 2023
Accepted: 15 February 2024

Abstract

Context. Electron fraction and cosmic-ray ionization rates in star-forming regions are important quantities in astrochemical modeling and are critical to the degree of coupling between neutrals, ions, and electrons, which regulates the dynamics of the magnetic field. However, these are difficult quantities to estimate.

Aims. We aim to derive the electron fraction and cosmic-ray ionization rate maps of an active star-forming region.

Methods. We combined observations of the nearby NGC 1333 star-forming region carried out with the NOEMA interferometer and IRAM 30 m single dish to generate high spatial dynamic range maps of different molecular transitions. We used the DCO⁺ and H¹³CO⁺ ratio (in addition to complementary data) to estimate the electron fraction and produce cosmic-ray ionization rate maps.

Results. We derived the first large-area electron fraction and cosmic-ray ionization rate resolved maps in a star-forming region, with typical values of 10⁻⁶⁵ and 10^−16.5 s⁻¹, respectively. The maps present clear evidence of enhanced values around embedded young stellar objects (YSOs). This provides strong evidence for locally accelerated cosmic rays. We also found a strong enhancement toward the northwest region in the map that might be related either to an interaction with a bubble or to locally generated cosmic rays by YSOs. We used the typical electron fraction and derived a magnetohydrodynamic (MHD) turbulence dissipation scale of 0.054 pc, which could be tested with future observations.

Conclusions. We found a higher cosmic-ray ionization rate compared to the canonical value for N(H₂) = 10²¹−10²³ cm⁻² of 10⁻¹⁷ s⁻¹ in the region, and it is likely generated by the accreting YSOs. The high value of the electron fraction suggests that new disks will form from gas in the ideal-MHD limit. This indicates that local enhancements of ζ(H₂), due to YSOs, should be taken into account in the analysis of clustered star formation.