Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

The Formation and Evolution of Rich Star Clusters in the LMC:
NGC 1818

Rebecca A. W. Elson1, Gerard Gilmore1, Sverre Aarseth1, Melvyn Davies1,
Steinn Sigurdsson1, Basilio Santiago2, and Jarrod Hurley1

1Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
2Depto. de Astronomia, Inst. de Fisica, UFRGS, Av. Bento Goncalve s,
9500, Agronomia, Porto Alegre, RS, CEP:91501-970, Brazil

Received 02nd February 1998

An extensive series of observations of rich star clusters in the Large Magellanic Cloud, totalling 95 orbits, will be carried out with the Hubble Space Telescope in Cycle 7. The program (Project 7307) will make use of WFPC2, NICMOS, and STIS (in imaging mode) to obtain images of four pairs of clusters with different ages: NGC 1805 and NGC 1818 at ∼107 years, NGC 1831 and NGC 1868 at ∼108 years, NGC 2209 and H14 at ∼109 years and NGC 2210 and H11 at ∼1010 years. These clusters are ideal for exploring the complex dynamical structure of N-body systems. They exhibit the long-term effects that can be important in classical stellar dynamics, as well as the more detailed effects of stellar evolution, close encounters between small numbers of stars including the interactions of binaries, tidal interactions, and physical collisions of stars with each other. They may also be used to investigate the universality of the stellar initial mass function. The data will be interpreted with the help of state-of-the-art numerical simulations of rich N-body systems using a special purpose HARP computer.

As a pilot project we investigate the binary population of one of the clusters in our sample, NGC 1818, using WFPC2 data from the HST Archive. Images are in the F336W (≡ U336) and F555W (≡ V555) bands, and exposure times are 960 seconds and 880 seconds respectively. NGC 1818 is ∼2×107 years old and has mass ∼3×104 Msun (cf. Hunter et al. 1997). Figure 1 shows a colour-magnitude diagram for the central regions of the cluster. A binary sequence is clearly visible. The binary fraction increases towards the cluster centre, from ∼20% in the outer parts, to ∼35% inside the core. A comparison of the cluster age and local relaxation time suggests that this trend is consistent with dynamical mass segregation and need not be primordial. Two-body encounters would already have destroyed many binaries, and the primordial binary fraction may be greater than the value we measure by a factor of two. Modelling suggests that the binaries in our sample have mass ratios Msecondary/Mprimary≥0.7. This bias towards near-equal mass companions is not due to dynamical selection, and must be a result of the binary formation process.

We have identified a candidate young (∼105 yr) luminous (MV ∼ 0) white dwarf in NGC 1818 on the basis of its extremely blue U336-V555 colour. This discovery strongly constrains the boundary mass, Mc, at which stars stop forming neutron stars as their evolutionary end product and start forming white dwarfs. We estimate Mc≥7.8±0.2 Msun, assuming the presence of convective overshooting. Without convective overshooting, Mc could be as high as 9 Msun. The determination of Mc is critical for understanding stellar evolution. It also has important implications for the chemical evolution of galaxies, in that it affects the rate of enrichment of the interstellar medium. Spectroscopic follow-up observations will confirm the identity of the candidate. We plan to obtain deeper U-band images with HST of NGC 1818 and other LMC clusters with similar ages, to increase the sample size of candidate white dwarfs, to determine this critical mass more precisely, and possibly for a range of metallicities.

[Click here to see Fig. 1!]

References

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First version: 19thFebruary,1998
Last update: 29thSeptember,1998
Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|richtler@astro.uni-bonn.de)