Context : An exciting recent finding regarding scaling relations among globular clusters is the so-called ’ blue tilt ’ : clusters of the blue sub-population follow a trend of redder colour with increasing luminosity .

Aims : In this paper we evaluate to which extent field star capture over a Hubble time changes the photometric properties of star clusters .
Given that field stars in early type giant galaxies are very metal-rich , their capture will make blue GCs redder and may in principle explain the ’ blue tilt ’ .

Methods : We perform collisional N-body simulations to quantify the amount of field star capture occuring over a Hubble time to star clusters with 10 ^ { 3 } to 10 ^ { 6 } stars .
In the simulations we follow the orbits of field stars passing through a star cluster and calculate the energy change that the field stars experience due to gravitational interaction with cluster stars during one passage through the cluster .
The capture condition is that their total energy after the passage is smaller than the gravitational potential at the cluster ’ s tidal radius .
By folding this with the fly-by rates of field stars with an assumed space density as in the solar neighbourhood and a range of velocity dispersions \sigma ( 15 to 485 km s ^ { -1 } ) , we derive estimates on the mass fraction of captured field stars as a function of environment .

Results : We find that integrated over a Hubble time , the ratio between captured field stars and total number of clusters stars is very low ( \lesssim 10 ^ { -4 } ) , even for the smallest field star velocity dispersion \sigma = 15 km s ^ { -1 } .
This holds for star clusters in the mass range of both open clusters and globular clusters .
We furthermore show that tidal friction has a negligible effect on the energy distribution of field stars after interaction with the cluster .
We note that field star capture at the time of cluster formation , when the cluster potential increases with time , is more efficient .
However , it can not explain the trend that more massive star clusters are redder .

Conclusions : Field star capture is not a probable mechanism for creating the colour-magnitude trend of metal-poor globular clusters .