CCD photometry of eclipsing binary star systems in the Large and Small Magellanic Clouds
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Double lined, eclipsing binaries are an important probe of stellar structure and evolution. Their study provides the most accurate data on the defining fundamental properties of stars, namely stellar masses, radii and luminosities. Observations made at the Mount John University Observatory (MJUO) have yielded high precision, six colour CCD light curves and calibrated standard system Strömgren uvby and Cousins VI photometry for three Magellanic Cloud eclipsing binaries; HV982 and HV2241 in the Large Magellanic Cloud (LMC) and HV 1620 in the Small Magellanic Cloud (SMC). The masses, radii and luminosities for HV 1620 and HV 2241 have been determined by analysis of the light curves and standard system photometry in conjunction with IUE ultraviolet spectrophotometry and spectroscopic radial velocity curves obtained by other investigators. The masses, radii and luminosities for HV 982 have been estimated in conjunction with IUE ultraviolet spectrophotometry by adopting a distance for the system. No spectroscopic radial-velocity curves are available for HV 982 at this time. Analysis of the calibrated standard system photometry, light curves and IUE spectrophotometry for the well-detached, 5.55335 d period HV 982 permits the determination of the effective temperatures of the two components of HV 982; Teff,₁= 24 000 ± 5 000 K and 23 400 ± 5 000 K. The large uncertainties result from the poorly defined reddening which in turn is primarily due to large uncertainty in the (b-y) photometry resulting from observations obtained on only two nights, both of questionable quality. The fact that no spectroscopic radial-velocity curves are available means that neither the mass ratio nor the physical size of the system can be determined directly. The derived temperatures are nonetheless insensitive to reasonable ranges of the mass ratio (as deduced from the the form of the light curve) and of the physical size of the system (in order to be consistent with the distance to the LMC). Adopting a value of unity for the mass ratio and a distance modulus of (m- M)₀ = 18.35 ± 0.2mag allows the estimation of masses, radii and thus luminosities of the individual components of HV982. They are M₁ = M₂ = 9.1 ± 3.2M⊙, R₁ = R₂ = 7.0 ± 1.1 R⊙ and log[L₂/ L⊙] = 4.2±0.5, log[L₂/ L⊙ = 4.1±0.5. These properties correspond well to the expected properties of normal single stars as predicted by modern theoretical stellar evolution models. Moreover the position of the components of HV 982 in the Hertzsprung-Russell Diagram (HRD) implies the components are main sequence stars, although not far from the Terminal Age Main Sequence (TAMS), i.e. HV 982 is still relatively young, which is consistent with the fact that HV 982 is an eccentric orbit system. Apsidal motion with a period of 206±6 yr has also been discovered and an improved ephemeris, including the effects of the apsidal motion has been derived. Combined analysis of the MJUO photometry and light curves, the IUE spectrophotometry and the published spectroscopic radial-velocity curves permits a complete analysis and direct determination of the properties of the 3.62642d period HV 1620. The analysis yields the following properties for the components of HV 1620: Teff,₁ = 33 000±4 500 K, M₁ = 20.9±0.4 M⊙, R₁ = 6.27±0.10 R⊙ and log[L₁/ L⊙] 4.62±0.25; Teff,₂ = 24 400±3 500 M₂ = 14.3±0. 7 ⊙, R₂ = 11.3±0.2 R₂ and log[L₂/ L⊙] 4.53±0.25. The system is found to be in a semi-detached configuration, with the cooler component filling its Roche lobe, while the hotter component is well-detached from its Roche lobe. Despite this evolved binary state, the properties of both components correspond well to those of normal single stars, implying that any mass transfer currently in progress is occuring at a rate slow enough that both components can retain at least the appearance of normal single stars. Comparison of the the observed spectral flux distribution with model flux distributions also yields the distance modulus, (m M)₀ = 18.6±0.3 mag, which is in agreement with other measurements of the distance modulus for the SMC. An improved ephemeris has been determined. Similarly, the combined analysis of the MJUO photometry and light curves, the IUE spectrophotometry and the published spectroscopic radial-velocity curves permits a complete analysis and direct determination of the properties of the 4.3426241 d period HV 2241. The effective temperatures, masses, radii and luminosities of the components of HV 2241 are Teff,₁ = 27 000±3 000 K, M₁ = 36.82±0.10 M⊙, R₁ 16.1±0.2 R⊙ and log[L₁/L⊙] = 5.10±0.20 and Teff,₂ = 20200±1500K, M₂ = 19.4±0.6M⊙, :R₂ 13.9±0.1 R⊙ and log[L₂/ L⊙] = 4.49±0.13. The distance modulus is 18.50±0.16 mag, likewise in good agreement with other measurements of the distance modulus for the LMC. Like HV 1620, this system is found to be in a semi-detached configuration with the cooler component filling its Roche lobe. However the hotter component of HV 2241 is very close to filling its Roche lobe also. The above properties give stars that are somewhat under-luminous in comparison to normal single-star models and are not consistent with binary-star evolution models for very massive, short period systems. The derived properties can however be brought into agreement with current theory if there is a large (~12 percent) systematic error in the radial velocity data for this star. (If there is a systematic error in one of the data sets, it seems most probable that it would be in the radial velocities since this data was obtained photographically on a 1-m telescope and a Mv = 13.5mag star like HV2241 must surely be close to the practical limit of such a system.) If on the other hand the properties derived here are accurate then it would appear that HV 2241 has been caught in an unprecedented evolutionary phase. A new analysis procedure has been employed in order to investigate the nature of a selection of eight stars from the recently published EROS catalogue of eclipsing binary stars in the bar of the Large Magellanic Cloud. All eight stars are well-detached systems with obviously eccentric orbits. The results of the analysis show seven of the eight to be composed of young (i.e. main sequence), coeval components, in accord with theoretical models for binary star formation and evolution. Consideration of the position of the components of eighth system, EROS 1061, in the HRD leads to the suggestion that this system is composed of two pre-main sequence stars in the final stages of contraction down onto the zero-age main sequence.