Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21715209d&link_type=abstract
American Astronomical Society, AAS Meeting #217, #152.09; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astronomy
Scientific paper
Exciting recent developments in theory and observations call for renewed scrutiny of the early evolution of star clusters spanning a large range of parameter space. First, using a large ensemble of N-body simulations of moderately sized (N=1000), cool, fractal clusters, we find that cool, clumpy clusters dynamically mass segregate on a short timescale. This implies that the notion of ‘primordial mass segregation’ may be obsolete. The cluster properties also change rapidly on very short timescales: young clusters may undergo core collapse on timescales of < 1 Myr, when a dense core containing massive stars is hardened because of energy losses to a halo of lower-mass stars. In addition, we use high-resolution Hubble Space Telescope imaging observations of the young ( 15-25 Myr-old) star cluster NGC 1818 in the Large Magellanic Cloud to derive an estimate for the binary fraction of F stars (1.3 < m*/M¤ 1.6). Our new study provides the strongest constraints yet on the (close to) initial binary fraction in a low-metallicity environment ([Fe/H] -0.4 dex). We find that our novel artificial-star-test method is sensitive to binaries with mass ratios, q≥ 0.4. For binaries with F-star primaries and mass ratios q> 0.4, the binary fraction is 0.35. This suggests a total binary fraction for F stars of 0.55 to unity, depending on assumptions about the form of the mass-ratio distribution at low q, which is consistent with the field and lower-density clusters. This suggests that, at least among intermediate-mass stars, metallicity down to [Fe/H] -0.4 dex does not suppress fragmentation and binary formation, and the binarity of these stars is at least as high as at solar metallicity. We therefore strongly argue for inclusion of realistic binary fractions in simulations aimed at following early star cluster evolution.
Allison James Richard
de Grijs Richard
Deng Licai
Goodwin Simon P.
Hu Yan
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