Interstellar heritage and isotopic ratios

The prestellar phase is a pivotal phase in star formation. Prestellar cores are condensations of gas and dust characterised by very low temperatures which represent the initial conditions for star formation. These objects collapse under their own gravity, giving rise to protostars, which evolve to form stars surrounded by planetary systems. A major timely question is to what extent the composition of planetary systems is influenced by the composition in the early stages of star formation.

One of the methods to address this question is to observe gas tracers, determine isotopic ratios of these tracers, and follow these isotopic ratios (for example 12C/13C) at key phases of protostellar evolution. For carbon, different 12C/13C ratio values have been identified in the interstellar medium (Lucas and Liszt 1998, Sakai et al. 2007). One of these values is observed in molecules such as CO or formed from CO is close to 70, while molecules with carbon chains (e.g. CCH) are very depleted in 13C (Furuya et al. 2011) and have much larger 12C/13C ratios. Recent measurements at later stages of star and planet formation seem to give similar results (Bergin et al. 2024, Zhang et al. 2021).

The isotopic ratios depend on the formation mechanisms of the species making up the gas, which in turn depend on the physical conditions. However, the inhomogeneity of the physical conditions in the objects considered complicates their determination, in particular because of the presence of gradients in physical conditions. As a result, the studies carried out to date have only looked at isotopic ratio values averaged along the line of sight.

The proposed thesis work consists of analysing the spectra of abundant gas-phase tracers in the interstellar medium, formaldehyde (H2CO) and methanol (CH3OH), in a sample of pre-stellar cores, in order to determine their 12C/13C isotopic ratios and to study their variation with physical conditions (density, temperature, radiation field). The observations available to us are spectral maps (from the IRAM 30m telescope) of different rotational transitions. These species make it possible to trace both dense, cold regions (prestellar cores) and the less dense, hotter filaments that surround them, without emitting in the outermost regions of the molecular clouds. The maps already obtained cover one prestellar core and its filament, and offer the possibility to probe a whole range of physical conditions. The work will involve modelling the maps to recover information along the line of sight (in the 3rd dimension), in order to obtain local values of the isotopic fractionation. The analysis will then be extended to more evolved sources from the star formation sequence (protostars), and comparisons will be made with measurements in protoplanetary disks from an ALMA large programme and chemistry models (Hily-Blant et al. 2018, Loison et al. 2020).
Some of the tools needed for this analysis, such as radiation transfer codes, are already available. In addition, the methods for determining the physical and chemical structure along the line of sight will be optimised. In particular, artificial intelligence techniques, which are currently being developed within the thesis team, could be improved and integrated in this approach.