The starburst cluster westerlund 1: The initial mass function and mass segregation

Westerlund 1 is the most important starburst cluster in the Galaxy due to its massive star content. We have performed BVI_C and JK_S photometry to investigate the initial mass function (IMF). By comparing the observed color with the spectral-type-intrinsic-color relation, we obtain the mean interstellar reddening of 〈E(B-V)〉 = 4.19 ± 0.23 and 〈E(J-K_S )〉 = 1.70 ± 0.21. Due to the heavy extinction toward the cluster, the zero-age main sequence fitting method based on optical photometry proved to be inappropriate for the distance determination, while the near-infrared photometry gave a reliable distance to the cluster - 3.8 kpc from the empirical relation. Using the recent theoretical stellar evolution models with rotation, the age of the cluster is estimated to be 5.0 ± 1.0 Myr. We derived the IMF in the massive part and obtained a fairly shallow slope of Γ = -0.8 ± 0.1. The integration of the IMF gave a total mass for the cluster in excess of 5.0 × 10⁴ M⊙. The IMF shows a clear radial variation indicating the presence of mass segregation. We also discuss the possible star formation history of Westerlund 1 from the presence of red supergiants and relatively low luminosity yellow hypergiants.