Audrey Coutens

The star formation process starts with the formation of cold and dense cores in molecular clouds. Some of these cores undergo gravitational collapse and disks form following the accretion of material onto the protostar. Planets are expected to form in these disks. Physical changes are known to affect the chemistry. It raises the question of the evolution of the chemical composition during the star formation process as well as the question of the chemical composition of protoplanetary disks. Cold cores are also known to show a diversity of chemical compositions. It remains to know if disks inherit the chemical composition of cold cores or if the physical evolution during the collapse plays a major role in the disk composition.

To answer these questions, we carried out 3D dense core collapse magneto-hydrodynamic calculations with the adaptative-mesh refinement RAMSES code. The simulation accounts for resistivity. Among the 10^6 tracer particles introduced in the simulations, 15,000 end in the disk at the final time of simulations (t = 5.8x10^4 yrs, i.e. 8x10^3 yrs after the formation of the first hydrostatic core). We calculated the chemical evolution for these 15,000 particles with the gas-grain code Nautilus (Ruaud et al. 2016). We assumed two different sets of initial abundances A and B that are characteristic of two cold cores with different history. These abundances are taken from smooth particle hydrodynamics simulations of dense core formation (Ruaud et al. 2018).

We studied the spatial distribution of molecules in the disk. The disk presents spiral arms and has a diameter of about 100 au. From the outer region to the inner region, the temperature goes from ~10 to ~800 K, while the density ranges between ~10^9 and ~10^13 cm-3. Molecules show different spatial distributions, but the same kind of spatial distributions is usually seen for the sets of initial abundances A and B. The spatial distribution of molecules is sensitive to the temperature distribution. Molecules such as water and complex organic molecules (COMs) are found to be abundant in the gas-phase in the warm inner regions of the disks. It is due to the desorption of the icy grain mantles at high temperature (>100K). Other molecules such as CCH, H3+ and CH5+ are abundant in the colder spiral arms.

Regarding the reservoirs of the chemical elements, we found that the main carriers in the disk are usually the same ones as in the cold core. For example, for oxygen, water dominates. No change is seen for N, Si, Cl, F. For S, H2S3 becomes a major carrier instead of HS. In the case of cloud B, the contributions of HCO and CH2OH decrease significantly for O and C and the contributions of PO, HCP, and CP increase for P.

We classified more than 70 species according to their sensitivity to the initial abundances. Some molecules show similar initial and final abundances. It includes H2O, HNCO, H2CO and complex species such as CH3OH, CH3CN, NH2CHO and HCOOH. There is consequently an inheritance of the cold core composition by the disk. Other species show, however, significant changes in abundances between the initial and final times of the simulations. The abundances of radicals decrease significantly, while the abundances of COMs such as CH3CHO and HCOOCH3 increase significantly. This shows that the collapse play a key role in the increase of molecular complexity. More details can be found in Coutens et al. (submitted).

References :
Ruaud, Wakelam, Gratier, & Bonnell, A&A, 611 A96 (2018)
Ruaud, Wakelam, & Hersant, MNRAS, 459, 3Ionospheric and Solar Terrestrial (2016)
Coutens, Commerçon & Wakelam, submitted to A&A