The Star Clusters of the Small Magellanic Cloud: Structural Parameters

Abstract

We present structural parameters for 204 stellar clusters in the Small Magellanic Cloud derived from fitting King and Elson, Fall, & Freeman model profiles to the V-band surface brightness profiles as measured from the Magellanic Clouds Photometric Survey images. Both King and EFF profiles are satisfactory fits to the majority of the profiles although King profiles are generally slightly superior to the softened power-law profiles of Elson, Fall, and Freeman and provide statistically acceptable fits to ~90% of the sample. We find no correlation between the preferred model and cluster age. The only systematic deviation in the surface brightness profiles that we identify is a lack of a central concentration in a subsample of clusters, which we designate as "ring" clusters. In agreement with previous studies, we find that the clusters in the SMC are significantly more elliptical than those in the Milky Way. However, given the mean age difference and the rapid destruction of these systems, the comparison between SMC and MW should not directly be interpreted as either a difference in the initial cluster properties or their subsequent evolution. We find that cluster ellipticity correlates with cluster mass more strongly than with cluster age. We identify several other correlations (central surface brightness vs. local background density, core radius vs. tidal force, size vs. distance) that can be used to constrain models of cluster evolution in the SMC.Comment: 14 pgs., accepted for publication in A

Structural Parameters of Seven Small Magellanic Cloud Intermediate-Age and Old Star Clusters

“…In the SMC, Hill & Zaritsky (2006) found the distribution of cluster core sizes to be broader than in the MW, which they argue is due to a prevalence of surviving low-concentration clusters in the SMC. In her analysis of the LMC star clusters, Elson (1991) noted that for clusters of a given age, there appears to exist an upper limit for their core size.…”

“…This method works well until γ ≈ 2 is reached, because then the total luminosity formally becomes infinite. The EFF profiles were used in this study, because all recent studies (Kontizas et al 1982(Kontizas et al , 1986(Kontizas et al , 1990Kontizas & Kontizas 1983;Mackey & Gilmore 2003b;Hill & Zaritsky 2006;Carvalho et al 2008) of structure parameters of SMC star clusters used EFF profiles. This choice facilitates the comparison of our results with the earlier studies.…”

“…These authors studied surface photometry of 25 SMC star clusters, of which four overlap with our sample. Hill & Zaritsky (2006) used data from the Magellanic Clouds Photometric Survey (MCPS) (Zaritsky et al 2002). Structure parameters were Mackey & Gilmore (2003b) with data from Kontizas et al (1982) and Kontizas & Kontizas (1983).…”

“…The distances were adopted from Paper I and Crowl et al (2001). The core radii for NGC 152, NGC 361, and NGC 411 are taken from Mackey & Gilmore (2003b), while for Kron 28 and Kron 44 the values are taken from Hill & Zaritsky (2006). (A color version of this figure is available in the online journal.)…”

“…Age vs. ellipticity showing the clusters from our sample (crosses) and five clusters from the literature (circles). The ellipticities for NGC 152, NGC 411, Kron 28, Kron 44, and BS90 are taken from Hill & Zaritsky (2006) and the ages from Alves & Sarajedini (1999), Crowl et al (2001), Piatti et al (2001), and Sabbi et al (2007). It seems that the younger clusters tend to be flatter than the older clusters, although NGC 121 is the oldest and the flattest cluster.…”