Cosmic rays are a key ingredient in many fields of astrophysics and in particular in star formation, but despite their great relevance our understanding is still relatively incomplete. Thanks to the data delivered by the new generation of infrared, radio, and (sub)millimeter telescopes, we have now the opportunity of attaining a comprehensive knowledge about the role of cosmic rays in the physics and chemistry of the interstellar medium, hence about the processes leading to star and planet formation. Observations are needed to constrain the multiple aspects of the proposed theoretical models, and models are required to properly interpret observations. Given the multidisciplinarity of the topics where cosmic rays play an important role, we propose a workshop to bring together for the first time the observational and theoretical communities working on cosmic rays in star-forming environments to present their recent results and to trigger active discussions which will establish a solid basis about the key challenges in cosmic-ray related physics and chemistry for the next decade.

The cosmic-ray ionisation rate, defined as the number of ionisation of hydrogen molecules per unit time, is the key-brick parameter which governs many processes of the physics and chemistry of star-forming regions. It is one of the main parameters used in chemical codes to interpret the observed abundances of molecules, and in non-ideal magnetohydrodynamic simulations. It determines the microscopic resistivities (ambipolar diffusion, Hall, Ohm) affecting the timescale of the collapse of a molecular cloud core and the formation of a protostellar disc. The value of the cosmic-ray ionisation rate strongly depends on the environment, decreasing from diffuse clouds to dense cores, to protostellar discs. This fundamental parameter can be derived observationally using different methods which rely on the detection of molecular specie through rovibrational spectroscopy in the infrared, radio, and (sub)mm wavelength ranges, using telescopes such as IRAM-30m, NOEMA, ALMA, APEX and Effelsberg as well as at infrared wavelengths (Herschel, CRIRES/VLT, MMT). Moreover, cosmic-ray electrons can also be responsible for the synchrotron emission in protostellar jets, protostars, and the diffuse medium. This non-thermal emission is detected with increasing accuracy with the last-generation radiotelescopes such as LOFAR, VLA, and GMRT. These phenomena are still poorly understood, but are very relevant, for example, to the irradiation of protoplanetary discs and thus in planet formation.

Cosmic rays could be responsible for the formation of short-lived radionuclei contained in the calcium-aluminium inclusions of carbonaceous meteorites, such as ¹⁰Be. This finding would support the idea that ¹⁰Be was formed in situ by spallation reactions rather than moved from the solar atmosphere. Planetary atmospheres can also be affected by photon and particle radiation causing huge air showers, ionising the atmosphere and creating secondary particles and photons. The higher degree of ionisation alters the chemical composition, for instance, impacting on the ozone layer of the terrestrial atmosphere and enhancing the biological radiation dose.

Despite the importance of cosmic rays in many fields of astrophysics, the interplay between observers, chemical modellers, and theoreticians is still rather scarce. Therefore, the second edition of this workshop has the goal of bringing together experts in theory and simulations of cosmic-ray propagation, astrochemists, and observers to share ideas, discuss about recent and present results, and identify the key challenges regarding the chemistry and the physics of cosmic rays for the near future.

The discussion arising from this workshop will settle the ground for a more efficient exploitation of the current facilities (ALMA, VLA, NOEMA, IRAM 30m, Yebes, APEX, Effelsberg, LOFAR, VLBI, MeerKAT). We stress that we expect not only participants who already work on cosmic rays, but also experts on different aspects of star formation with the aim of make everyone aware about the relevance of cosmic rays in their respective research. Besides, all these works will contribute to design the next challenges in the field and to prepare future ambitious projects making use of powerful facilities available in the following years: SKA, ALMA (bands 1-2), JWST, and the next generation VLA.

PhD students and Postdoctoral Fellows will be encouraged to attend the workshop and give contributed talks or present posters. They will have the opportunity to interact with international experts in different fields (star formation, astrochemistry, particle physics). The interdisciplinary environment will be particularly beneficial for students and young postdocs, who might find new directions in their research by linking the different fields that coexist in the workshop.