SFB 1601

Massive star formation is still poorly understood. Since young massive stars are rare and deeply embedded in their parental molecular clumps, this mechanism is difficult to observe. Simulations have tried to bypass these difficulties but one major issue remains, star formation spans a large range of space scale which leads to the impossibility of resolving stars with a reasonable amount of numerical resources. They are instead described as sink particles (SP), point like objects accreting mass. This SPs are only describing single stars without discs.

During my PhD I created a python code called Disc modEl for Sink paRtIclE post procEssing (DESIREE) which computes the disc evolution in SPs of already existing core collapse simulations, with different initial parameters, from Zimmermann et al. 2025. We aim to compare my model with single star SP to understand what would be the impact of taking into account the disc if DESIREE was run on-the-fly with FLASH simulations. The figure shows the proportion of mass ending up in the star compared to the total mass of the SP (i.e disc+star system) and the lifetime of the SPs in color. The values are taken at t_HII which correspond to the time when the HII region starts forming, i.e when the UV radiation from massive stars becomes relevant before completely dispersing the cloud. The formation of the HII region corresponds more or less to the end of the accretion phase.

First thing we can notice on the figure is that none of the SP has 100% of its mass in the star, it means that some mass is still stored in the disc even at the end of the accretion phase. The second thing we can note is that there is a small positive correlation between the “final” mass of the star and the mass of the SP. This could be explained by, more mass in the disc leads to more mass available to be accreted onto the star. However, we can also see that there is a large spread, which could mean that the mass of the SP is not the only factor acting on the final mass of the star but also, how and when the mass is accreted onto the disc, could have a large impact. For example matter with angular momentum opposite to the disc rotation axis could produce a rapid and important accretion phase onto the star (episodic accretion events).

In conclusion, with DESIREE we confirm that discs act as mass reservoirs and slow down the growth of stars which results in a lower SFE. The evolution of the star+disc system is also strongly dependent on how and where the infalling matter is accreted onto the disc, that is to say, knowing the mass of one SP does not immediately gives the final distribution of mass in the star and in the disc. Taking into account the disc physics inside the SPs in 3D simulations would lower the star masses, delay the formation of the HII region and give weaker feedback as lower mass stars produce weaker feedback.