Extragalactic Eclipsing Binaries

Fundamental Properties and Distances of the Large Magellanic Cloud From Eclipsing Binaries. II. HV 982

1 January 2002, ApJ, 564, 260

E.L. Fitzpatrick, I. Ribas, E.F. Guinan, L.E. DeWarf,
F.P. Maloney & D. Massa

Abstract:

We have determined the distance to a second eclipsing binary (EB) system in the Large Magellanic Cloud, HV 982 (~B1 IV-V + ~B1 IV-V). The measurement of the distance - among other properties of the system - is based on optical photometry and spectroscopy and space-based UV/optical spectrophotometry. The analysis combines the "classical" EB study of light and radial velocity curves, which yields the stellar masses and radii, with a new analysis of the observed energy distribution, which yields the effective temperature, metallicity, and reddening of the system plus the distance "attenuation factor," essentially (radius/distance)². Combining the results gives the distance to HV 982, which is 50.2 ± 1.2 kpc. This distance determination consists of a detailed study of well-understood objects (B stars) in a well-understood evolutionary phase (core H burning). The results are entirely consistent with - but do not depend on - stellar evolution calculations. There are no "zero-point" uncertainties as, for example, with the use of Cepheid variables. neither is the reult subject to sampling biases, as may affect techniques that utilize whole stellar populations, such as red giant branch stars. Moreover, the analysis is insensitive to stellar metallicity (although the metallicity of the stars is explicitly determined), and the effects of interstellar extinction are determined for each object studied. After correcting for the location of HV 982, we find an implied distance to the optical center of the LMC's bar of d_LMC = 50.7 ± 1.2 kpc. This result differs by nearly 5 kpc from our earlier result for the EB HV 2274, which implies a bar distance of 45.9 kpc. These results may either reflect maginally compatible measures of a unique LMC distance or, alternatively, suggest a significant depth to the stellar distibution in the LMC. Some evidence for this latter hypothesis is discussed.

The Large Magellanic Cloud Eclipsing Binary HV 2274: Fundamental Properties and Comparison with Evolutionary Models

10 January 2000, ApJ, 528, 692

I. Ribas, E.F. Guinan, E.L. Fitzpatrick, L.E. DeWarf, F.P. Maloney,
P.A. Maurone, D.H. Bradstreet, A. Giménez & J.D. Pritchard

Abstract:

We are carrying out an international, multiwavelength program to determine the fundamental properties and independent distance estimates of selected 14th and 15th magnitude eclipsing binaries in the Large and Small Magellanic Clouds (LMC and SMC). Eclipsing binaries with well-defined double-line radial velocity curves and light curves provide valuable information on orbital and physical properties of their component stars. These properties include, among other characteristics, stellar mass and radius. These can be measured with an accuracy and directness unachievable by any other means. The study of stars in the LMC and SMC, where the metal abundances are significantly lower than solar (by one-third to one-tenth) provides an important opportunity to test opacities and stellar atmosphere, interior, and evolution models. For the first time, we can also measure direct mass-luminosity relations for stars outside our Galaxy. In a previous paper we demostrated how a precise distance to the LMC - corresponding to (V₀ - M_V) = 18.30 ± 0.07 mag.- could be determined using the 14th magnitude LMC eclipsing binary HV 2274. In this paper we concentrate on the determination of the orbital and physical properties of HV 2274 and its component stars from analyses of light curves and new radial velocity curves formed from Hubble Space Telescope (HST)/Goddard High-Resolution Spectrograph observations. HV 2274 (B1-2 IV-III + B1-2 IV-III; V_max

+14.2; P = 5.73 days) is a particularly appealing star becuae it is a detached binary that has an eccentric orbit (ε = 0.136) and shows rapid apsidal motion. The results of these analyses yield reliable masses and absolute radii, as well as other physical and orbital properties of the stars and the system. From UV/optical spectrophotometry (1150-4820 Å) of HV 2274 obtained with the HST Faint Object Spectrograph, the temperatures and the metallicity ([Fe/H] = -0.45 ± 0.06) of the stars were found, as well as the interstellar extinction of the system. The values of the mass, absolute radius, and effective temperature for the primary and secondary stars are 12.2 ± 0.7 M, 9.9 ± 0.2 R, 23000 ± 180 K and 11.4 ± 0.7 M, 9.0 ± 0.2 R, 23110 ± 180 K, respectively. The age of the system (τ = 17 ± 2 Myr), helium abundance (Y = 0.26 ± 0.03), and a lower limit of the convective core overshooting parameter of α_OV

0.2 were obtained by fitting the stellar data with evolution models of Claret & Giménez. HV 2274 has a relatively well determined (and fast) apsidal motion period of U = 123 ± 3 yr. From an analysis of apsidal motion, additional information and constraints on the structure of the stars can be obtained. The apsidal motion analysis corroborates that some amount of convective core overshooting (α_OV between 0.2 and 0.5) is needed.

The Distance to the Large Magellanic Cloud From the Eclipsing Binary HV 2274

10 December 1998, ApJ, 509, L21

E.F. Guinan, E.L. Fitzpatrick, L.E. DeWarf, F.P. Maloney, P.A. Maurone,
I. Ribas, J.D. Pritchard, D.H. Bradstreet & A. Giménez

Abstract:

The distance to the Large Magellanic Cloud (LMC) is crucial for the calibration of the cosmic distance scale. We derive a distance to the LMC based on an analysis of ground-based photometry and Hubble Space Telescope (HST)-based spectroscopy and spectrophotometry of the LMC eclipsing binary system HV 2274. Analysis of the optical light curve and the HST/Goddard High-Resolution Spectrograph, radial velocity curve provides the masses and radii of the binary components. Analysis of the HST/Faint Object Spectrograph, UV/optical spectrophotometry provides the temperatures of the component stars and the interstellar extinction of the system. When combined, these data yield a distance to the binary system. After correcting for the location of HV 2274 with respect to the center of the LMC, we find d_LMC = 45.7 ± 1.6 kpc or (V₀ - M_V) = 18.30 ± 0.07 mag. This result, which is immune to the metallicity-induced zero-point uncertainties that have plagued other techniques, lends strong support to the "short" LMC distance scale as derived from a number of independent methods.