The Low-Mass Stellar Content of Westerlund 1

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The low-mass stellar content of Westerlund 1 Morten Andersen (Gemini South) Galactic young massive star clusters • Connection between local and global SF. • Can resolve the stellar populations to low masses • Star counts instead of integrated properties • Directly measure the IMF The IMF in resolved massive clusters Only scale-free part probed Westerlund 1 • Distance of ~4 Kpc, age 3-5 Myr • Total mass estimated to be 50000 Msun • High foreground extinction • Best opportunity for resolving the low mass content in a young massive cluster • HST J (F125W) and H (F160W) band imaging Westerlund 1 from ground 5pc 4.5’ SOfI JHK, Brandner et al. 2007, Gennaro et al. Early lessons • Probe down to ~3 Msun, normal (Salpeter) IMF • Total mass of ~50000 Msun assuming standard IMF • Mass segregated, at least for high masses • Elliptical cluster • Currently difficult to improve from the ground Westerlund 1 with HST 5pc 4.5’ WFC3 F125W, F160W. Andersen et al. 2016 Color-magnitude diagrams Andersen et al. 2016 Foreground population Andersen et al. 2016 Red Clump Andersen et al. 2016 Background contamination Andersen et al. 2016 Cluster main sequence MS/PMS turn-on Andersen et al. 2016 Field star subtraction Andersen et al. 2016 Mass Functions Log-normal fit below 1 Msun to the 50% completeness limit. Power-law fit above 1 Msun (Siess 4 Myr isochrone) Change of fit parameters as a function of radius M < 1 Msun, log-normal fits Comparable peak mass as the field. More narrow distribution Change of fit parameters as a function of radius M > 1 Msun Evidence for mass segregation out to 1.5-2 pc Andersen et al. 2016 The IMF of Westerlund 1: • Reach down to 0.2 Msun in outer parts • Log-normal fit provides results similar to the field • Evidence for M > 1 Msun mass segregated • Total mass of cluster (star counts) 50000 Msun Is Westerlund 1 bound? • Gas expulsion has already occurred • The most massive stars exploded. • Has this disrupted the cluster or will it survive? • Is it a light-weight globular cluster progenitor? • Radial vel. measurements provide vel. dispersion • Multi-epoch Magellan R~20000 spectra Radial velocity dispersion B supergiants Yellow hyper giants Yellow hyper giants Low velocity dispersion 2.1+3.4-0.9km/s Cottaar, Meyer, Andersen & Espinoza, A&A 2012 Consistent with cluster being bound Gemini GeMS/GSAOI K band: NGS configuration on the observed mosaic GSAOI (2.1 micron) vs HST (F125W) FOV 45”x60” Source detection: GSAOI (left), HST (right) Increased source counts mainly due to resolution.

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