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Register a new userThe RR Lyrae distance scale from near-infrared photometry: current results
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M. Dallora
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We present new observational results on the RR Lyrae K-band Period-Luminosity relation (PLK). Data on the Galactic globular clusters NGC 3201 and NGC 4590 (M68), and on the Large Magellanic Cloud cluster Reticulum are shown. We compare the observed slopes of the PLK relations for these three clusters with those predicted by pulsational and evolutionary models, finding a fair agreement. Trusting on this finding we decided to adopt these theoretical calibrations to estimate the distance to the target clusters,finding a good agreement with optical-based RR Lyrae distances, but with a smaller formal scatter.
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We have obtained deep infrared J and K band observations of nine 4.9x4.9 arcmin fields in the Small Magellanic Cloud (SMC) with the ESO New Technology Telescope equipped with the SOFI infrared camera. In these fields, 34 RR Lyrae stars catalogued by the OGLE collaboration were identified. Using different theoretical and empirical calibrations of the infrared period-luminosity-metallicity relation, we find consistent SMC distance moduli, and find a best true distance modulus to the SMC of 18.97 +/- 0.03 (statistical) +/- 0.12 (systematic) mag which agrees well with most independent distance determinations to this galaxy, and puts the SMC 0.39 mag more distant than the LMC for which our group has recently derived, from the same technique, a distance of 18.58 mag.
We have obtained deep infrared $J$ and $K$ band observations of five fields located in the Large Magellanic Cloud (LMC) bar with the ESO New Technology Telescope equipped with the SOFI infrared camera. In our fields, 65 RR Lyrae stars catalogued by the OGLE collaboration were identified. Using different theoretical and empirical calibrations of the period-luminosity-metallicity relation, we find consistent LMC distance moduli values. Since the observed fields are situated very close to the center of the LMC, the correction for the tilt of the LMC bar with respect to the line of sight is negligible. Our adopted best true distance modulus to the LMC of $18.58 pm 0.03$ (statistical) $pm$ 0.11 (systematic) mag agrees very well with most independent determinations to this galaxy.
We have obtained single-phase near-infrared (NIR) magnitudes in the J- and K-bands for 77 RR Lyrae (RRL) stars in the Fornax Dwarf Spheroidal Galaxy. We have used different theoretical and empirical NIR period-luminosity-metallicity calibrations for RRL stars to derive their absolute magnitudes, and found a true, reddening-corrected distance modulus of 20.818 +/- 0.015 (statistical) +/- 0.116 (systematic) mag. This value is in excellent agreement with the results obtained within the Araucaria Project from the NIR photometry of red clump stars (20.858 +/- 0.013 mag), the tip of the red giant branch (20.84 +/- 0.04 +/- 0.14 mag), as well as with other independent distance determinations to this galaxy. The effect of metallicity and reddening is substantially reduced in the NIR domain, making this method a robust tool for accurate distance determination at the 5 percent level. This precision is expected to reach the level of 3 percent once the zero points of distance calibrations are refined thanks to the Gaia mission. NIR period-luminosity-metallicity relations of RRL stars are particularly useful for distance determinations to galaxies and globular clusters up to 300 kpc, that lack young standard candles, like Cepheids.
We present new absolute trigonometric parallaxes and proper motions for seven Pop II variable stars: five RR Lyr variables; RZ Cep, XZ Cyg, SU Dra, RR Lyr, UV Oct; and two type 2 Cepheids; VY Pyx and {kappa} Pav. We obtained these results with astrometric data from Fine Guidance Sensors, white-light interfer- ometers on Hubble Space Telescope. We find absolute parallaxes in milliseconds of arc: RZ Cep, 2.12pm0.16 mas; XZ Cyg, 1.67pm0.17 mas; SU Dra, 1.42pm0.16 mas; RR Lyr, 3.77pm0.13 mas; UV Oct, 1.71pm0.10 mas; VY Pyx, 6.44pm0.23 mas; and {kappa} Pav, 5.57pm0.28 mas; an average {sigma}{pi}/{pi} = 5.4%. With these parallaxes we compute absolute magnitudes in V and K bandpasses corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Using these RRL absolute magnitudes, we then derive zero-points for MV -[Fe/H] and MK -[Fe/H]-Log P relations. The technique of reduced parallaxes corroborates these results. We employ our new results to determine distances and ages of several Galactic globular clusters and the dis- tance of the LMC. The latter is close to that previously derived from Classical Cepheids uncorrected for any metallicity effect, indicating that any such effect is small. We also discuss the somewhat puzzling results obtained for our two type 2 Cepheids.
We have obtained single-phase near-infrared magnitudes in the J and K bands for a sample of 78 RR Lyrae stars in the Sculptor dSph galaxy. Applying different theoretical and empirical calibrations of the period-luminosity-metallicity relation for RR Lyrae stars in the infrared, we find consistent results and obtain a true, reddening-corrected distance modulus of 19.67 $pm$ 0.02 (statistical) $pm$ 0.12 (systematic) mag for Sculptor from our data. This distance value is consistent with the value of 19.68 $pm$ 0.08 mag which we obtain from earlier V-band data of RR Lyrae stars in Sculptor, and the V magnitude-metallicity calibration of Sandage (1993). It is also in a very good agreement with the results obtain by Rizzi (2002) based on tip of the red giant branch (TRGB, 19.64 $pm$ 0.08 mag) and horizontal branch (HB, 19.66 $pm$ 0.15 mag).
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