Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today.This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.

Context. Helium, the second most abundant element in the Universe, with a relative abundance of He/H ∼ 1/10, has never been observed in any other form than that of a neutral atom (He) or an ion (He + ) in the interstellar medium. Since He is a noble gas its non-observation as part of neutral molecular systems is understandable, but it is very surprising for a positively charged species such as HeH + that is a stable diatomic ion whose spectral signatures are well known in the laboratory. Aims. This non-observation, even in hydrogen rich regions, could imply that HeH + is not a proper target and that alternatives have to be considered, such as small HeH + n clusters. The present study aims at finding whether the leading term HeH + 3 fulfills the conditions required. Methods. We addressed the question with state-of-the-art numerical simulations. We determined a two-dimension ab initio potential energy surface (PES) of the HeH