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Some of the youngest stars (age $\lesssim 10$ Myr) are clustered, while manyothers are observed scattered throughout star forming regions or in completeisolation. It has been intensively debated whether the scattered or isolatedstars originate in star clusters, or if they form truly isolated, which couldhelp constrain the possibilities how massive stars are formed. We adopt theassumption that all stars form in gravitationally bound star clusters embeddedin molecular cloud cores ($\Gamma$-$1\;$ model), which expel their natal gas,and compare the fraction of stars found in clusters with observational data.The star clusters are modelled by the code nbody6, which includes stellar andcircumbinary evolution, gas expulsion, and the external gravitational field oftheir host galaxy. We find that small changes in the assumptions in the currenttheoretical model estimating the fraction, $\Gamma$, of stars forming inembedded clusters have a large influence on the results, and we present acounterexample as an illustration. This calls into question theoreticalarguments about $\Gamma$ in embedded clusters, and it suggests that there is nofirm theoretical ground for low $\Gamma$ in galaxies with lower star formationrates (SFRs). Instead, the assumption that all stars form in embedded clustersis in agreement with observational data for the youngest stars (age $\lesssim10$ Myr). In the $\Gamma$-$1\;$ scenario, the observed fraction of the youngeststars in clusters increases with the SFR only weakly; the increase is caused bythe presence of more massive clusters in galaxies with higher SFRs, whichrelease fewer stars to the field in proportion to their mass. The$\Gamma$-$1\;$ model yields a higher fraction of stars in clusters for olderstars (age between $10$ and $300$ Myr) than what is observed. This discrepancycan be caused by interactions with molecular clouds.

Do the majority of stars form as gravitationally unbound?