Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11708a&link_type=abstract
HST Proposal ID #11708. Cycle 17
Astronomy and Astrophysics
Astronomy
Scientific paper
Despite over 50 years of active research, a key question in galactic astronomy remains unanswered: is the initial mass function {IMF} of stars and sub-stellar objects universal, or does it depend on initial conditions? The answer has profound consequences for the evolution of galaxies as well as a predictive theory of star formation. Work to date suggests that certain environments {high densities, e.g. Elmegreen 2004; low metallicity, e.g. Larson 2005} should produce a top-heavy IMF, and there are hints from unresolved star-bursts that this might be the case. Yet, there is no clear evidence for an IMF that differs from that characterizing the Galactic field stars in a resolved stellar population down to one solar mass. Westerlund 1 is the most massive young star cluster known in the Milky Way. With an estimated mass of 5x10^4 Msun, an age of 3-5 Myr, and located at a distance of 3-4 kpc, it presents a unique opportunity to test whether the IMF in such a cluster deviates from the norm well down into the brown dwarf regime. We propose WFC3 near-IR imaging to probe the IMF down to 40 Jupiter masses. The data will enable use to: 1} provide a stringent test of the universality of the IMF under conditions approximating those of star-bursts; 2} search for primordial or dynamic mass segregation in the clusters; and 3} assess whether the cluster is likely to remain bound {as a massive open cluster} or disperse into the field. We will obtain images in the F125W, F160W, and F139M filters. The F139M filter covers a strong water absorption feature and the color F125W/F139M is a powerful temperature diagnostic in the range 2800-4000 K. This information will enable us to: a} confirm membership for low mass stars suspected on the basis of their position in the color-magnitude diagram; b} place the members in the HR diagram; and c} estimate the masses and ages of cluster members for low-mass stars and sub-stellar objects. This new capability offered with the WFC3 {through a novel combination of filter complement, high spatial resolution, and large field of view} will enable us to make a fundamental test of whether the IMF is universal on a unique resolved stellar population, as well as assess the clusters structure, dynamics, and ultimate fate.;
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