Speaker
Description
During the last decade, Voyager spacecrafts have measured the very local interstellar spectra of cosmic-ray (CR) particles down to energies of about 3 MeV. These measurements represent unique information on unmodulated CR spectra below a few GeV. Otherwise, properties of such low-energy CRs can only be probed indirectly. A universal parameter characterizing their impact on a medium is the ionization rate (CRIR), which is usually estimated from observed abundances of certain ions that can only be produced by CRs. Because of its particularly simple chemistry, H3+ is often considered as the most reliable tracer of CRIR in diffuse molecular gas. Available H3+ observations in the local Galactic environment (within 1 kpc) are also the most numerous, which makes it possible to evaluate how CRIR varies in space. All previous estimates of CRIR rely on model-dependent assessments of the gas density along the probed sight lines [1,2], and the resulting values of CRIR typically exceed the values derived for the Voyager spectra by an order of magnitude.
We revisited data from H3+ measurements, by utilizing the recently developed 3D dust extinction maps that enable direct reconstruction of the 3D gas density distribution [3]. High-resolution maps developed for the local Galactic environment allow us to precisely identify the location of molecular clouds probed in each H3+ measurement, and also derive the gas density in these clouds. By performing numerical simulations of the 3D physical structure of the clouds and comparing the results with measured H3+ and H2 column densities, we were able to evaluate CRIR in each cloud without involving any model-dependent assumption about the environment [4].
Our results indicate that (i) values of CRIR probed in individual diffuse molecular clouds in the local Galactic environment may vary by an order of magnitude from cloud to cloud, and (ii) the average CRIR value is a factor of ~10 smaller than that derived previously. We will present details of the preformed analysis, discuss correspondence of our results to the Voyager measurements, and highlight the profound implications for understanding the origins of low-energy CRs.
[1] Indriolo, N. McCall, B. J. 2012, ApJ, 745, 91
[2] Albertsson, T., Indriolo, N., Kreckel, H., et al. 2014, ApJ, 787, 44
[3] Edenhofer, G., Zucker, C., Frank, P., et al. 2024, A&A, 685, A82
[4] Obolentseva, M., Ivlev, A. V., Silsbee, K., et al. 2024, ApJ, 973, 142
