Marek Górski

An eclipsing-binary distance to the Large Magellanic Cloud accurate to two per cent.

In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better. At present, its uncertainty is dominated by the uncertainty in the distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale. Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately. The eclipsing-binary method was previously applied to the LMC, but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future.

THE ARAUCARIA PROJECT. INFRARED TIP OF THE RED GIANT BRANCH DISTANCES TO THE CARINA AND FORNAX DWARF SPHEROIDAL GALAXIES

We present distance determinations for two Local Group dwarf spheroidal galaxies, Carina and Fornax, based on the near-infrared magnitudes of the tip of the red giant branch (TRGB). For Carina we derive true distance moduli of 20.09 and 20.13 mag in the J and K bands, respectively, while for Fornax the same distance modulus of 20.84 mag was derived in both filters. The statistical errors of these determinations are of the order of 0.03-0.04 mag, whereas the systematic uncertainties on the distances are 0.12 mag in the J band and 0.14 mag in the K band. The distances obtained from the near-infrared TRGB method in this paper agree very well with those obtained for these two galaxies from optical calibrations of the TRGB method, their horizontal branches, RR Lyrae variables, and the near-infrared magnitudes of their red clumps.

THE ECLIPSING BINARY CEPHEID OGLE-LMC-CEP-0227 IN THE LARGE MAGELLANIC CLOUD: PULSATION MODELING OF LIGHT AND RADIAL VELOCITY CURVES

We performed a new and accurate fit of light and radial velocity curves of the Large Magellanic Cloud (LMC) Cepheid—OGLE-LMC-CEP-0227—belonging to a detached double-lined eclipsing binary system. We computed several sets of nonlinear, convective models covering a broad range in stellar mass, effective temperature, and chemical composition. The comparison between theory and observations indicates that current theoretical framework accounts for luminosity—V and I band—and radial velocity variations over the entire pulsation cycle. Predicted pulsation mass—M = 4.14 ± 0.06 M ☉—and mean effective temperature—Te = 6100 ± 50 K—do agree with observed estimates with an accuracy better than 1σ. The same outcome applies, on average, to the luminosity amplitudes and to the mean radius. We find that the best-fit solution requires a chemical composition that is more metal-poor than typical LMC Cepheids (Z = 0.004 versus 0.008) and slightly helium enhanced (Y = 0.27 versus 0.25), but the sensitivity to He abundance is quite limited. Finally, the best-fit model reddening—E(V – I) = 0.171 ± 0.015 mag—and the true distance modulus corrected for the barycenter of the LMC—μ0, LMC = 18.50 ± 0.02 ± 0.10 (syst) mag—agree quite well with similar estimates in the recent literature.

THE ARAUCARIA PROJECT. POPULATION EFFECTS ON THE V- AND I-BAND MAGNITUDES OF RED CLUMP STARS

We present measurements of the V- and I-band magnitudes of red clump stars in 15 nearby galaxies obtained from recently published homogeneous Hubble Space Telescope photometry. Supplementing these results with similar data for another eight galaxies available in the literature, the populational effects on the V- and I-band magnitudes of red clump stars were investigated. Comparing red clump magnitudes with the I-band magnitude of the tip of the red giant branch in a total sample of 23 galaxies possessing very different environments, we demonstrate that population effects strongly affect both the V- and I-band magnitude of red clump stars in a complex way. Our empirical results basically confirm the theoretical results of Girardi & Salaris, and show that optical (V – I) photometry of red clump stars is not an accurate method for the determination of distances to nearby galaxies at the present moment, as long as the population effects are not better calibrated, both empirically and theoretically. Near-infrared photometry is a much better way to measure galaxy distances with red clump stars given its smaller sensitivity to population effects.

The Araucaria Project: accurate stellar parameters and distance to evolved eclipsing binary ASAS J180057-2333.8 in Sagittarius Arm

We have analyzed the double-lined eclipsing binary system ASAS J180057-2333.8 from the All Sky Automated Survey (ASAS) catalogue . We measure absolute physical and orbital parameters for this system based on archival

THE ARAUCARIA PROJECT. INFRARED TIP OF THE RED GIANT BRANCH DISTANCES TO FIVE DWARF GALAXIES IN THE LOCAL GROUP

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THE ARAUCARIA PROJECT: ON THE TIP OF THE RED GIANT BRANCH DISTANCE DETERMINATION TO THE MAGELLANIC CLOUDS

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THE ARAUCARIA PROJECT: THE DISTANCE TO THE CARINA DWARF GALAXY FROM INFRARED PHOTOMETRY OF RR LYRAE STARS*

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A Precision Determination of the Effect of Metallicity on Cepheid Absolute Magnitudes in VIJHK Bands from Magellanic Cloud Cepheids

-band ASAS photometry, as well as on high-resolution spectroscopic data obtained with ESO 3.6m/HARPS and CORALIE spectrographs. The physical and orbital parameters of the system were derived with an accuracy of about 0.5 - 3%. The system is a very rare configuration of two bright well-detached giants of spectral types K1 and K4 and luminosity class II. The radii of the stars are

The Araucaria Project: High-precision Cepheid Astrophysics from the Analysis of Variables in Double-lined Eclipsing Binaries

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