The Initial Mass Function and the Surface Density Profile of NGC 6231

Abstract

We have performed new wide-field photometry of the young open cluster NGC 6231 to study the shape of the initial mass function (IMF) and mass segregation. We also investigated the reddening law toward NGC 6231 from optical to mid-infrared color excess ratios, and found that the total-to-selective extinction ratio is R V = 3.2, which is very close to the normal value. But many early-type stars in the cluster center show large color excess ratios. We derived the surface density profiles of four member groups, and found that they reach the surface density of field stars at about 10 , regardless of stellar mass. The IMF of NGC 6231 is derived for the mass range 0.8-45 M . The slope of the IMF of NGC 6231 (Γ = −1.1 ± 0.1) is slightly shallower than the canonical value, but the difference is marginal. In addition, the mass function varies systematically, and is a strong function of radius-it is very shallow at the center, and very steep at the outer ring suggesting the cluster is mass segregated. We confirm the mass segregation for the massive stars (m 8 M ) by a minimum spanning tree analysis. Using a Monte Carlo method, we estimate the total mass of NGC 6231 to be about 2.6 (±0.6) × 10 3 M . We constrain the age of NGC 6231 by comparison with evolutionary isochrones. The age of the low-mass stars ranges from 1 to 7 Myr with a slight peak at 3 Myr. However, the age of the high-mass stars depends on the adopted models and is 3.5 ± 0.5 Myr from the non-rotating or moderately rotating models of Brott et al. as well as the non-rotating models of Ekström et al. But the age is 4.0-7.0 Myr if the rotating models of Ekström et al. are adopted. This latter age is in excellent agreement with the timescale of ejection of the high-mass runaway star HD 153919 from NGC 6231, albeit the younger age cannot be entirely excluded.

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

“…Recently, Brott et al (2011) and Ekström et al (2012) published new stellar evolution models with stellar rotation. The results (age and the IMF) are very similar to each other (see for details Hur et al 2012;Sung et al 2013).…”

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

“…Since Sung et al (1997) achieved the successful detection of many PMS stars in NGC 2264, this efficient technique has been used to search for PMS members in a series of studies (e.g., Sung et al 1998Sung et al , 2008Sung et al 2013b;Park et al 2000;Park & Sung 2002;Lim et al 2014aLim et al , 2014b. In order to identify Hα emission stars photometrically we have defined the Hα index as Hα − (V I + )/2 .…”

“…Previous studies on NGC 1624 have adopted the normal reddening law (R V = 3.1, Chini & Wink 1984;Jose et al 2011), while Pandey et al (2013a showed, from photometric and polarimetric data, that the reddening law toward NGC 1931 is slightly different from that found in the general ISM. In order to check the reddening law toward these clusters we investigated various color excess ratios in the optical and IR passbands as performed in the series of studies (Kook et al 2010;Lim et al 2011;Hur et al 2012;Sung et al 2013b;Lim et al 2014aLim et al , 2014b. Lim et al (2014a) have noted that several mid-late-B-type MS stars in the young open cluster IC 1848 have a bluer color than that of normal B-type MS stars.…”

“…The various color excesses E V ( ) l -in the optical and IR passbands were computed by comparing the observed colors with the intrinsic relations from Paper 0 and Sung et al (in preparation). The R V is expressed as a linear relation for each color excess ratio Guetter & Vrba 1989;Sung et al 2013b). We present the color excess ratios of the cluster members in Figure 9.…”

“…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).…”