III – MASS FUNCTIONS

1. AddClus: a tool to model star clusters and tidal tails Basílio Santiago1,4, Eduardo Balbinot1,4, Leandro Kerber2, Bruno Rossetto3,4, Luiz A. N. da Costa3,4, Marcio A. G. Maia3,4, Ricardo Ogando3,4, P. S. S. Pellegrini3,4 1- Departamento de Astronomia, Instituto de Física, UFRGS; 2 – Universidade Estadual de Santa Cruz, UESC/BA; 3- MCT/Observatório Nacional; 4 – DES-Brazil consortium I - INTRODUCTION IV – TIDAL TAILS • Star clusters may often be modelled as simple stellar populations → useful tools to constrain the star formation history of their host galaxies (refs). • They are also important labs for studying multi-body gravitational problems and stellar dynamics in general (refs). • We have a long past experience in modelling colour-magnitude diagrams (CMDs) of star clusters (Kerber, Javiel & Santiago 2001, A&A, 365, 424; Kerber et al 2002, 390, 121). Unbound cluster stars will slowly detach from the cluster and form an extended and low density structure roughly tracing the cluster orbit around the Galaxy. These tidal tails tend to have a density distribution that falls with 1/R, where R is the distance from the centre (refs). Figure 3 – The radial surface density profile of the model cluster shown in the previous figure. The solid line is the King model used to generate the stellar distribution. III – MASS FUNCTIONS • Even if stars form according to a unique, and possibly universal, initial mass function (IMF), stellar encounters and energy equipartition make more massive stars sink to the cluster centre. • Low mass stars, as they gain orbital from these encounters, energy, may acquire escape velocity and evaporate from the cluster. • The external tidal field may also remove less bound stars from the cluster, especially if this field varies fast along the cluster orbit. • As a consequence of these different effects, the present day mass function (PDMF) varies as a function of position within the cluster. We model this effect using power-law PDMFs of variable slopes. Figure 5 – On-sky distribution of stars from the same model cluster as in the previous figures, but now with a long tidal tail added by AddClus. The tail contains 20% of the cluster stars, spread along a straight line in space out to 3000 pc and a density falling with 1/R. The tail makes an angle of 20 deg relative to the sky and has a position angle of 45 deg counted along the N-E-S-W direction. Image generated in the Brazilian DES/LineA Science Portal. Figure 1 –Right panel: an observed HST CMD from NGC 2249, a rich LMC cluster; middle panel: a model CMD that reproduces very well the observed one, according to an objective statistical assessment. The fiducial lines from both data and model are superposed t each other on the right panel. From Kerber, Santiago & Brocato 2007, A&A, 462, 139. V – CONCLUSIONS We have developed a tool to simulate star clusters, with different density profiles, ages, metallicities, and following different stellar evolutionary models. AddClus also includes the effects of unresolved binarism and mass segregation. AddClus is currently being used or will be used in the near future for the following purposes: To test and validate algorithms to detect star clusters and tidal tails in simulations of the stellar sample of the Dark Energy Survey (DES). In particular, AddClus is being used to validate Sparse, a code that applies a matched filter in order to identify tidal tails. Sparse is also being developed within the DES-Brazil collaboration (see poster ### by Balbinot et al). To detect new clusters and associated tidal tails in the BOSS photometric imaging survey around the Southern Galactic cap. Evaluate selection effects on the sampling of star clusters and tidal tails as a function of their mass, size, distance and dinamical state in future wide angle photometric surveys such as DES, PANStars and LSST. • We here present AddClus. It is a new tool, that also models the cluster density distribution, including a tidal tail. It also allows for a variable stellar mass function (PDMF), therefore accommodating the commonly observed effect of mass segregation. II – DENSITY PROFILES • The density structure of star clusters is well described by simple profile laws. Different light profiles have been successfully used to describe them. AddClus allows us to create model star clusters following a Hubble profile, a modified Hubble profile and a King profile. Figure 4 – PDMFs for 3 different radial distances from the centre of a model cluster 107 years old generated with AddClus. The PDMFs follow power-laws with slopes α = dlog φ(m) / dlog m = -1.60 (inner region), -2.35 (middle region), -2.80 (outer region). At subsolar masses the outer region PDMF is made very shallow (α=+0.2), to mimick the evaporation of low-mass stars. A Salpeter line, α= -2.35, is also shown for reference. Figure 2 – A model globular cluster created with AddClus. The stars are seen projected on the sky according to their equatorial coordinates. The model used a 3 parameter King profile with a core radius rc=2.5 pc and a tidal radius rt = 100 pc. A total of 90.000 stars were simulated. Image generated in the Brazilian DES/LineA Science Portal.