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