Hands-on Activities

Abstracts for the hands on activities

The ortho-to-para ratios of interstellar molecules

The ortho-to-para (OPR) ratio of interstellar molecules with identical nuclei, e.g. H 2 O, is commonly believed to reflect a “formation temperature”. Transitions between the ortho and para states by radiative and collisional (inelastic) processes are indeed strongly forbidden. The OPRs should therefore preserve information on the formation conditions. In thermal  equilibrium, the OPR is a function of temperature only. At high temperature (e.g. T ≥60 K for H 2 O), the ratio approaches the statistical weigths of the ortho and para states (e.g. 3 for H2O), and it increases or decreases exponentially at low  temperature, depending on the relative energy of the ortho and para ground states. In the interstellar medium (ISM), the reported OPRs are in general consistent with statistical equilibrium, but exceptions exist. In particular, non-statistical OPRs have been measured e.g. for H2CO in molecular clouds.

Do astrochemical clocks exist ?

One of the most important question in Astronomy is to determine the time scale during star formation. However, our current understanding is limited to theoretical and chemical modeling; and observational results still remain sparse. Nevertheless, the latter are important because one way to access this issue is to study the molecular content together with the evolution of the material that is associated with the different stages of star formation. Thus, we need to search for molecular probes that can quickly evolve with time and be observable (i.e abundant) by our instruments (ground based and satellite).

The mystery of the methanol in the ISM

Methanol has been observed in large quantities (up to about 10% of elemental carbon) in the solid and gaseous phases of the ISM, from molecular clouds to prestellar cores and towards star forming regions. Despite its detection has been known for decades, there is still a debate on what are the routes of formation and destruction of methanol and what processes release methanol in the gas phase from the solid one.

The reservoir of interstellar nitrogen

Nitrogen is, with carbon and oxygen, the most abundant and highly reactive heavy atom. Nitrogenated species (e.g. NH3, HCN) are among our best tracers of the dense interstellar medium, and especially of prestellar cores prior to the protostellar stage. However, our understanding of the abundances of these species remains elusive. One salient problem is that the dominant reservoir of nitrogen, presumably N or N2, is not securely known. Unfortunately, N and N2 are not observable directly in the dense ISM. Their abundance derives from trace species, through chemical models.

Bibliography

Starting bibliography for each hands-on are provided here:

• interstellar nitrogen reservoirs (P. Hily-Blant)
• spin-state chemistry (C. Rist & A. Faure)
• do astrochemical clocks exist ? (C. Favre)
• the methanol mystery (C. Ceccarelli)

Preliminary bibliography for the laboratory experiment (E. Quirico, E. Dartois). The laboratory will take place in OSUG-D.