The Starburst Cluster Westerlund 1: The Initial Mass Function and Mass Segregation

Abstract

Westerlund 1 is the most important starburst cluster in the Galaxy due to its massive star content. We have performed BV I 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 interstellarto 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 ± 1.0Myr. 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 × 10 4 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 lowluminosity yellow hypergiants.

The Sejong Open Cluster Survey (Sos). Iv. The Young Open Clusters NGC 1624 and NGC 1931

“…The V transformation with respect to V I -shows a plateau for the reddest stars (V I 1.55 -> ). The TiO band spectral features in late-type stars are related to this plateau as seen in other studies Lim et al 2009Lim et al , 2013. The B transformation was approximated by a combination of two straight lines, and the coefficients are very small.…”

“…The form of the spatial variations can be represented by a second-order polynomial. The pattern may relate to the shape of the focal plane of the Kuiper 61" telescope (see also Magnier & Cuillandre 2004;Sung et al 2008;Lim et al 2013). In most cases, the amount of the variation from the CCD center was less than 0.03 mag.…”

“…Many efforts have been made to study the structure of open clusters using the techniques of surface density distribution and minimum spanning tree (Sung et al 1999;Sharma et al 2007;Koenig et al 2008;Gutermuth et al 2009;Kook et al 2010;Jose et al 2011;Lim et al 2013;Pandey et al 2013a;Sung et al 2013b, etc). In this section, we describe the apparent structure of NGC 1624 and NGC 1931 using the surface density distribution, with additional information from previous studies.…”

“…The author attributed the cause of the scatter to dynamical effects and Poisson noise. It used to be believed that young open clusters were too young to have undergone dynamical mass segregation; however, a few young open clusters show clear signs of mass segregation, e.g., Arches cluster (Habibi et al 2013), NGC 3603 (Pang et al 2013), NGC 6231 (Raboud & Mermilliod 1998;Sung et al 2013b), Orion Nebula Cluster (ONC) (Hillenbrand & Hartmann 1998), Westerlund 1 (Gennaro et al 2011;Lim et al 2013), etc. Several studies, through multiple sets of numerical simulation, suggested that the mass segregation found in young clusters has a dynamical origin (McMillan et al 2007;Allison et al 2009Allison et al , 2010Moeckel & Bonnell 2009).…”

Sejong Open Cluster Survey (Sos). 0. Target Selection and Data Analysis

“…Open clusters can be classified into three groups according to their age (Sung 1995): young (age ∼ < 10 7 yrs), intermediate-age (age: 10 7 -7 × 10 8 yrs), and old (age ∼ > 7 × 10 8 yrs). Young open clusters can give information on the stellar evolution of massive stars (Conti et al 1983;Massey 2003;Kook et al 2010;Lim et al 2013) as well as on lowmass pre-main sequence (PMS) stars (Sung et al 1997;Luhman 2012). As massive stars are still in the main sequence (MS) or in evolved stages, young open clusters are ideal targets for studying the stellar IMF in a wide mass range ).…”